The phylogeny and evolution of Cretaceous-Paleogene metatherians: New cladistic analysis and...

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The phylogeny and evolution of Cretaceous–Palaeogenemetatherians: cladistic analysis and description ofnew early Palaeocene specimens from the NacimientoFormation, New MexicoThomas E. Williamson a , Stephen L. Brusatte b c , Thomas D. Carr d , Anne Weil e & BarbaraR. Standhardt fa New Mexico Museum of Natural History and Science, 1801 Mountain Rd, NW, Albuquerque,NM, 87104, USAb Division of Paleontology, American Museum of Natural History, New York, NY, 10024c Department of Earth and Environmental Sciences, Columbia University, Palisades, NY,10964, USAd Department of Biology, Carthage College, 2001 Alford Park Drive, Kenosha, WI, 53140, USAe Oklahoma State University Center for Health Sciences, Department of Anatomy and CellBiology, 1111 West 17th St., Tulsa, OK, 74107-1898, USAf 14700 FM 307, Stanton, TX, 79782, USAVersion of record first published: 05 Dec 2012.

To cite this article: Thomas E. Williamson , Stephen L. Brusatte , Thomas D. Carr , Anne Weil & Barbara R. Standhardt(2012): The phylogeny and evolution of Cretaceous–Palaeogene metatherians: cladistic analysis and description of new earlyPalaeocene specimens from the Nacimiento Formation, New Mexico, Journal of Systematic Palaeontology, 10:4, 625-651

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Journal of Systematic Palaeontology, Vol. 10, Issue 4, December 2012, 625–651

The phylogeny and evolution of Cretaceous–Palaeogene metatherians: cladisticanalysis and description of new early Palaeocene specimens from the Nacimiento

Formation, New MexicoThomas E. Williamsona∗, Stephen L. Brusatteb, Thomas D. Carrc, Anne Weild and Barbara R. Standhardte

aNew Mexico Museum of Natural History and Science, 1801 Mountain Rd, NW, Albuquerque, NM 87104, USA; bDivision ofPaleontology, American Museum of Natural History, New York, NY 10024 and Department of Earth and Environmental Sciences,

Columbia University, Palisades, NY 10964, USA; cDepartment of Biology, Carthage College, 2001 Alford Park Drive, Kenosha, WI53140, USA; dOklahoma State University Center for Health Sciences, Department of Anatomy and Cell Biology, 1111 West 17th St.,

Tulsa, OK 74107-1898, USA; e14700 FM 307, Stanton, TX 79782, USA

(Received 4 January 2011; accepted 24 May 2011; printed 5 December 2012)

Metatherian mammals were the most diverse mammalian clade in North America through the Late Cretaceous, but theyunderwent a severe extinction at the Cretaceous–Palaeogene (K-Pg) boundary. In order to clarify the origin of Palaeogenemetatherians and the pattern of metatherian survivorship across the K-Pg boundary we conducted an inclusive species-level phylogenetic analysis of Cretaceous and early Palaeogene metatherian taxa. This analysis includes information fromnew Palaeocene specimens from south-western North America. Both the phylogenetic topology and information from newspecimens support the validity of the genus Thylacodon and justify the recognition of a new species, T. montanensis.Thylacodon is closely related to Swaindelphys and Herpetotheriidae, which must have diverged by the latest Cretaceousdue to its close relationship with late Campanian Ectocentrocristus. Pediomyidae and ‘Peradectidae sensu lato’ togethercomprise a major metatherian clade. Maastrichtidelphys, from the Late Cretaceous of the Netherlands, is the oldest memberof ‘Peradectidae sensu lato’, indicating a Cretaceous origination for this group. Therefore, the major groups Herpetotheriidaeand ‘Peradectidae sensu lato’, represented almost completely by Palaeocene taxa, must have originated in the Late Cretaceous.The lineages leading to these clades include at least four lineages that must have crossed the K-Pg boundary and thereforeconfirm that the K-Pg boundary marked a profound extinction event for metatherians and suggests that Palaeogene taxaoriginated from only a few clades of Cretaceous species, all of which were relatively minor or very rare components ofknown Cretaceous mammalian faunas.

Keywords: Metatheria; Marsupialia; K-Pg boundary; systematics; cladistics; Palaeocene

Introduction

The two dominant extant mammal lineages, Metathe-ria (including extant marsupials) and Eutheria (includingextant placentals) diverged from a common ancestor priorto the Early Cretaceous (c.125 Ma). Nearly complete skele-tons of the basal metatherian Sinodelphys (Luo et al. 2003)and basal eutherian Eomaia (Ji et al. 2002) from the LowerCretaceous Yixian Formation of China provide conclusivefossil evidence for the presence of these two lineages by thistime and may suggest that Asia was an important centrefor the initial diversification of these clades (Luo et al.2003). Most of the fossil evidence for these lineages forthe remainder of the Cretaceous, an interval of about 60million years, consists almost exclusively of jaws and teeth(Kielan-Jaworowska et al. 2004). Cretaceous metatherians,including the basal metatherian clade Deltatheroida, areknown only from North America and other Laurasian areas.

∗Corresponding author. Email: [email protected]

A specimen once claimed to be a Cretaceous metatherianfrom South America, Alphadon austrinum Sige, 1972, ataxon later referred to Peradectes by Crochet (1980), hasrecently been shown to be latest Palaeocene–earliest Eocenein age (Sige et al. 2004).

Metatheria underwent a significant radiation duringthe Late Cretaceous in North America (Cifelli & Davis2003; Luo et al. 2003; Cifelli 2004). They exhibited awide range of body sizes, spanning nearly three ordersof magnitude, from about 20 g for the smallest taxa toover 1700 g for Didelphodon vorax (Gordon 2003a), thelargest therian of the Late Cretaceous. Based on toothmorphology and comparison to living taxa, Cretaceousmetatherians were characterized by a diverse array of dietsthat included insectivory (especially among smaller taxa),carnivory (especially among deltatheroidans, pediomyidsand stagodontids; Fox & Naylor 2006; Davis 2007; Wilsonet al. 2010), possible frugivory (Glasbius; Gordon 2003b),

ISSN 1477-2019 print / 1478-0941 onlineCopyright C© 2012 The Natural History Museumhttp://dx.doi.org/10.1080/14772019.2011.631592http://www.tandfonline.com

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626 T. E. Williamson et al.

durophagy (some stagodontids including Eodelphis andDidelphodon; Fox & Naylor 2006), and omnivory amongmany taxa, perhaps indicating opportunistic feeding as inmany extant didelphid marsupials (Gordon 2003b).

Through the latter half of the Late Cretaceous of NorthAmerica, metatherians were the most diverse mammalclade (Clemens 2002; Cifelli & Davis 2003; Cifelli 2004),and they attained their highest diversity at the very endof the Cretaceous (c.20 species). The uppermost Creta-ceous (Lancian) Hell Creek Formation of eastern Montanahas yielded 11 metatherian taxa and these comprise over40% of the total mammal species and over 60% of therianmammal species in that fauna (Clemens 2002). In contrast,Palaeogene metatherians of North America were taxonom-ically and morphologically less diverse than Late Creta-ceous forms. Approximately one dozen metatherian specieshave been reported from the entire Palaeocene of NorthAmerica. These species, which fall within a relativelynarrow range of morphology (Korth 2008), are placed intwo clades: Peradectidae and Herpetotheriidae. The largestof these forms, Mimoperadectes from the early Eocene,was less than 350 g in mass (calculated using regressionof body size on tooth size after Gordon 2003a), approxi-mately one fifth the size of the largest known Late Creta-ceous species. Therefore, it is clear that North Ameri-can metatherian faunas underwent dramatic changes acrossthe Cretaceous–Palaeogene (K-Pg) boundary, and it seemsas if they experienced a sudden and drastic decline atthe end of the Cretaceous, associated with the devastat-ing Cretaceous–Palaeogene mass extinction that may haveeradicated between 53 and 64% of terrestrial vertebratespecies (Archibald and Bryant 1990; Archibald 1996) and60% of all animals (Schulte et al. 2010).

Understanding the patterns of metatherian extinctionacross the K-Pg boundary, as well as other major macroevo-lutionary trends in diversity, lineage evolution and morpho-logical evolution, is a sought-after goal. This is hindered,however, by the unsettled phylogenetic relationships ofCretaceous–Palaeocene metatherians (Fig. 1). Previousstudies usually have included only a few of the mostcomplete Cretaceous and Palaeogene taxa, and have oftencombined species-level taxa into supraspecific terminalsthat are problematic from a methodological standpoint(Wiens 1998; Prendini 2001; Brusatte 2010). For exam-ple, the recent analysis of Horovitz et al. (2009) includedonly two Mesozoic metatherian taxa (Deltatheridium andAsiatherium) and only three North American Palaeogenemetatherian taxa (Herpetotherium, Peradectes and Mimop-eradectes). Other studies also included only a relativelysmall number of metatherian taxa (e.g. Johanson 1996;Martin et al. 2005; Davis 2007; Vullo et al. 2009; Averianovet al. 2010; Fig. 1).

Aside from their problematic taxonomic sampling,these studies have produced different topologies thatsupport different hypotheses of metatherian macroevo-

lution (Fig. 1). In particular, the interrelationships of anumber of major metatherian subclades are unresolved,which is problematic because different phylogenies positdrastically different estimates for the divergence dates ofmajor clades, the number of lineages that passed throughthe K-Pg, and the membership of crown clade Marsupialia.For instance, in a recent phylogenetic analysis that includeddental, cranial and postcranial characters, Sanchez-Villagraet al. (2007) concluded that Herpetotheriidae (including theeponymous genus Herpetotherium) is the sister group tothe crown clade Marsupialia. In a subsequent analysis thatincluded the skull and upper dentition of a new species ofMimoperadectes, M. houdei, Horovitz et al. (2009; Fig. 1C)expanded on this analysis and concluded that Peradectidaeis the sister group of didelphids and thus are a memberof crown clade Marsupialia. They also concluded thatHerpetotheriidae originated in the Cretaceous based onthe understanding that Nortedelphys, a Cretaceous taxonknown only by its dentition (see Case et al. 2005) is abasal member of Herpetotheriidae. Similarly, Martin et al.(2005; Fig. 1A) concluded that the latest Cretaceous taxonMaastrichtidelphys was a basal herpetotheriid based on aphylogenetic analysis that included Nortedelphys, but noPalaeogene members of Herpetotheriidae, many of whichare among the best represented members of the clade.

These findings are significant as they would indicate thatHerpetotheriidae is one of the very few therian mammallineages that survived across the Cretaceous–Palaeogeneboundary. Furthermore, if herpetotheriids are the sistertaxon to crown Marsupialia, ghost range extension wouldrequire that the lineage leading to the crown originated inthe Cretaceous, and survived the K-Pg extinction. However,the close relationship between the purported Cretaceousherpetotheriids Nortedelphys and Ectocentrocristus and themorphologically distinct Palaeogene herpetotheriids suchas Copedelphys and Herpetotherium has not been estab-lished through numerical cladistic analysis. Also, becauseMartin et al. (2005) did not include undisputed Palaeogeneherpetotheriids in their phylogenetic analysis, the identityof Maastrichtidelphys as a basal member of Herpetotheri-idae has not been adequately established.

Another problem which plagues study ofCretaceous–Palaeocene metatherians is disagreements anduntested assumptions regarding alpha level taxonomy.For instance, Horovitz et al. (2009) included Peradectesin their phylogenetic analysis, and indicated that thisgenus is present in the earliest Palaeocene (Puercan;see Lofgren et al. 2004) of western North America.Indeed, metatherian taxa have been reported from severalearly Palaeocene (Puercan) localities of western NorthAmerica (e.g. Matthew & Granger 1921; Standhardt 1980;Archibald 1982; Middleton 1983; Williamson 1996; Eberle& Lillegraven 1998; Clemens 2006). However, the genericidentity of many of these reported specimens is in doubt,in part due to questions regarding the taxonomic validity

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Phylogeny and evolution of Cretaceous–Palaeogene metatherians 627

Figure 1. Previous metatherian phylogenies after original publications redrawn to include only Cretaceous and Palaeogene metatheriansand selected outgroup and extant taxa. A, Martin et al. (2005); B, Vullo et al. (2009); C, Horovitz et al. (2009); D, Luo et al. (2003); E,Kielan-Jaworowska et al. (2004); F, Rougier et al. (2004); G, Averianov et al. (2010).

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of the problematic taxon, Thylacodon pusillus Matthew &Granger, 1921, whose fragmentary type specimen was oneof the only known early Palaeocene metatherian specimensfor decades (e.g. Krishtalka & Stucky 1983a; Johanson1996a; Clemens 2006).

Since the description of the type of Thylacodon pusil-lus, numerous Puercan metatherian specimens recoveredfrom western North America have been referred, or tenta-tively referred, to Thylacodon pusillus or Peradectes pusil-lus (e.g. Standhardt 1980, Archibald 1982; Middleton 1983,Lofgren 1995; Williamson 1996; Eberle and Lillegraven1998; Clemens 2006). Clemens (2006) recently establisheda new smaller metatherian taxon, Peradectes minor, fromthe middle and/or late Puercan (Pu2/3) Tullock Formationof north-eastern Montana. One or more additional metathe-rians have also been reported from the Puercan of theNacimiento Formation (Standhardt 1980; Williamson 1993,1996) but until now they are undocumented or unpublished.In this study consideration of these taxa in a comparativeand phylogenetic context will help resolve the problem-atic alpha level taxonomy of Palaeocene metatherians fromNorth America.

The goals of this paper are four-fold. First, we conducta thorough parsimony analysis of a broad range of Creta-ceous and Palaeogene metatherians. Most previous stud-ies include only a small subset of Cretaceous and Palaeo-gene species, usually restricted to only a few of the mostcompletely known genera. By contrast, the current study isthe first to include a nearly complete, species-level coverageof all Cretaceous and early Palaeogene metatherian taxa.We accomplished this by using a dental character matrix.Second, we document new metatherian specimens from theearly Palaeocene (Puercan) of the Nacimiento Formation(New Mexico). Third, using information from the phylo-genetic analysis, the new specimens, and new anatomicalobservations, we revise the alpha level taxonomy of somecritical Puercan metatherians of North America. Fourth, andfinally, we discuss major patterns in Cretaceous–Palaeogenemetatherian evolution using our phylogeny as a guide.

Materials and methods

Stratigraphy and geological ageSeveral new metatherian specimens are described fromthe Nacimiento Formation of the San Juan Basin, NewMexico. These are included in the phylogenetic analysisand described fully in the discussion. The NacimientoFormation contains vertebrate faunas that are Puercanand Torrejonian in age (Williamson 1996; Lofgren etal. 2004). Puercan faunas are restricted to just a fewlocales on the south-western portion of the San Juan Basinnear the confluences of Betonnie-Tsosie and KimbetoWashes and in and near the Bisti/De-na-zin Wilderness

Figure 2. Geologic map of the San Juan Basin showing the loca-tions of microfossil sites that yielded early Palaeocene (Puercan)metatherians included in this study.

area (Fig. 2). These two areas produce distinct faunasthat are from different stratigraphic horizons and that areconsidered middle (Pu2) and late (Pu3) Puercan in age,respectively (Williamson 1996; Lofgren et al. 2004). Thesetwo horizons are fossiliferous and superposed in only theBisti/De-na-zin Wilderness Area (Williamson et al. 2011).

Matthew and Granger (1921) named Thylacodon pusillusbased on a dentary fragment (AMNH 16414) collected from“two miles above Ojo Alamo” in the Bisti/De-na-zin Washarea. This area yields a sparse fauna from a lower fossilif-erous horizon that is believed to be Pu2 in age (Williamson1996) and a more fossiliferous ‘upper level’, where theholotype was collected (Matthew & Granger 1921, p. 2)that is Pu3 in age (Williamson 1996; Lofgren et al. 2004).New specimens described here are from microvertebratelocalities that are middle (Pu2) and late (Pu3) in age. Spec-imens were collected from NMMNH&S localities 4723and 4725 from the head of Willow Wash that are part of theSplit Lip Flats local fauna (Pu3; Williamson & Weil 2002;Williamson et al. 2011); 6254, De-na-zin Wash (Pu3); 6387,Black Toe locality of the West Flank of Kimbeto Wash (Pu2;Standhardt 1980); and 646, the Mammalon Hill locality and844 of Betonnie-Tsosie Wash (Pu2) (Fig. 2).

Dental measurementsTooth nomenclature follows Van Valen (1966) and Szalay(1969). All measurements are in mm and were made to thenearest 0.05 mm using a WildTM measuring reticule and aLeicaTM MZ 6 microscope.

Abbreviations for descriptive statisticsDW = distal width; L = length; MW = mesial width.

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Phylogenetic definitionsFour inclusive clades are commonly used in classificationsof Cretaceous–Palaeogene metatherians: Stagodontidae,Herpetotheriidae, Peradectidae and Pediomyidae. Differ-ent authors, however, use these taxon names in differentways and sometimes classify individual species as belong-ing to these clades based on vague or unstated criteria.In order to clarify the definitions of these taxa, providenomenclatural stability and enable objective classificationof individual species, we provide a phylogenetic definitionfor each clade. Phylogenetic definitions delineate cladesbased on ancestry instead of the possession of ‘essential’characters (see de Queiroz & Gauthier (1990, 1992) andSereno (2005) for more details). In other words, cladesare defined either as encompassing all taxa that descendedfrom a certain common ancestor or as all taxa that aremore closely related to a certain taxon (usually a species)than another taxon. This is distinctly different from thetraditional practice of defining higher-level taxa based ona simple list of included species or a set of anatomicalfeatures that are held to be shared by all members of thegroup. Compared to this tradition, phylogenetic definitionsare held to be more stable, clear and objective (de Queiroz& Gauthier 1990, 1992), and therefore, it is no surprise thatthey are commonly used by mammalian palaeontologists(e.g. Rowe 1987, 1988; Novacek et al. 1997; Luo et al.2002; Sereno 2006). With that being said, however, suchdefinitions have not previously been applied to the fourmetatherian clades we are concerned with here.

Herpetotheriidae was coined by Trouessart (1879), andwe here propose a phylogenetic definition (stem-based)where Herpetotheriidae is the most inclusive clade contain-ing Herpetotherium fugax Cope, 1873, but not Peradecteselegans Matthew & Granger, 1921, Pediomys elegans(Marsh, 1889), Didelphodon vorax Marsh, 1889 or Didel-phis virginiana (Kerr, 1792).

Peradectidae was named by Crochet (1979), and wehere propose a phylogenetic definition for Peradectidae(stem-based) where it is the most inclusive clade contain-ing Peradectes elegans, but not Herpetotherium fugax,Pediomys elegans or Didelphis virginiana.

The family Pediomyidae was named by Simpson (1927).Although Davis (2007) utilized a cladistic analysis toexamine the relationships of Pediomyidae, he defined theclade based on a list of included species, not a phylo-genetic definition. We propose a phylogenetic definition(stem-based) where Pediomyidae is the most inclusiveclade containing Pediomys elegans, but not Peradecteselegans, Herpetotherium fugax, Didelphis virginiana orDidelphodon vorax.

We propose a phylogenetic definition for StagodontidaeMarsh, 1889 (stem-based) where it is the most inclusiveclade containing Didelphodon vorax, but not Glasbius intri-catus Clemens, 1966, Dakotadens morrowi Eaton, 1993,Turgidodon praesagus (Russell, 1952), Herpetotherium

fugax, Peradectes elegans, Pediomys elegans or Didelphisvirginiana.

We also provide a cautionary note. As explained below,when the phylogenetic definition for Peradectidae is appliedto our strict consensus topology it only refers to a restrictedclade of two taxa. Many other taxa, including numerousspecies of Peradectes, fall into a basal polytomy withthis restricted clade and the speciose clade Pediomyidae.Many of these taxa within the polytomy form a clade withPeradectes elegans (the name-bearing taxon for our phylo-genetic definition of Peradectidae) in many of the individualmost parsimonius trees, and therefore would be consideredmembers of Peradectidae when the phylogenetic definitionis applied to these trees. High levels of homoplasy resultin the polytomy in the strict consensus, but we suspect thatfuture work will resolve this portion of the tree and recovermany species of Peradectes (as well as other taxa) withina phylogenetically defined Peradectidae. In the meantime,in this paper we use ‘Peradectidae sensu lato’ to refer tothe individual species of Peradectes and other taxa thatcomprise this basal polytomy (i.e. all members of this poly-tomy that are not members of Pediomyidae).

Institutional abbreviationsAMNH: American Museum of Natural History, New York,USA; KU: University of Kansas, Lawrence, USA; MNA:Museum of Northern Arizona, Flagstaff, USA; NMMNH:New Mexico Museum of Natural History and Science,Albuquerque, USA; OMNH: Oklahoma Museum of Natu-ral History, Norman, USA; UALP: University of AlbertaLaboratory of Paleontology, Edmonton, Canada; UCM:University of Colorado, Boulder, USA; UCMP: Universityof California, Museum of Paleontology, Berkeley, USA;UNM: University of New Mexico, Albuquerque, USA;USNM: National Museum of Natural History, SmithsonianInstitution, Washington, DC, USA.

Phylogenetic analysis

Previous phylogenetic analysesSeveral recent cladistic analyses have examined thehigher-level phylogenetic relationships of mammals, andthey include some Cretaceous–Palaeogene metatherians asexemplars (e.g. Luo et al. 2002; Luo, Ji, Wible & Yuan2003 (Fig. 1D); Kielan-Jaworowska et al. 2004 (Fig. 1E)).Other studies have examined, in detail, the relationshipswithin Metatheria and the position of Metatheria withinMammalia (e.g. Rougier et al. 1998, 2004 (Fig. 1F);Horovitz & Sanchez-Villagra 2003; Sanchez-Villagra et al.2007; Horovitz et al. 2008, 2009 (Fig. 1C)). Yet other anal-yses have explored the relationships of a small numberof Mesozoic taxa within Metatheria (e.g. Johanson 1996;Martin et al. 2005 (Fig. 1A); Davis 2007; Vullo et al. 2009

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(Fig. 1B)). However, most of these studies include a smallsample of Cretaceous and Palaeogene Metatheria that islimited to the most completely known species. Anotherproblem is where multiple species were combined into acomposite supraspecific terminal (Fig. 1). This is prob-lematic methodologically because a composite terminal isa hypothetical construction that is several steps removedfrom the actual species-level anatomical data, and it oftendoes not include the primitive character conditions for theclade it purportedly represents (e.g. Wiens 1998; Prendini2001; Brusatte 2010).

Methodology of new phylogenetic analysisDataset. We studied the phylogenetic relationships of 92Mesozoic and Palaeogene metatherian species with a newanalysis of 78 dental characters that includes one stemCretaceous eutherian (Prokennalestes), six deltatheroidans,a clade generally considered to represent basal metatherians(see Davis et al. 2008), 58 Cretaceous non-deltatheroidanmetatherians and 26 Palaeogene metatherians. Most non-deltatheroidan metatherian taxa are from western NorthAmerica, but we have included one non-deltatheroidanmetatherian taxon from Asia, Asiatherium, and one Creta-ceous metatherian taxon from Europe, Maastrichtidelphys.This represents over 95% of all valid Cretaceous metathe-rian taxa and about 95% of all valid Palaeocene throughmiddle Eocene metatherian taxa from North America.Rationale for excluded taxa is presented below.

We also note that our dataset includes onlydental characters. The vast majority (about 90%) ofCretaceous–Palaeocene metatherians are represented bylittle more than teeth, and by including non-dental char-acters our matrix would be swamped by an enormousamount of missing data (approximately 75% or more ofall entries would be missing). This is extreme comparedto the levels of missing data in most published vertebratephylogeny datasets. Although some missing data is usuallynot a problem in cladistic datasets, an excessive numberof missing scores can greatly increase the number of mostparsimonious trees, decrease resolution in consensus trees,and return inaccurate results (Wiens 1998, 2003, 2005,2006). We plan on incorporating non-dental informationinto future, more focused phylogenetic studies of metathe-rian subgroups, but for the present broad-scale analysis, itshould be remembered that our results are based on dentalcharacters only.

Independence of dental characters. Phylogenetic analy-sis requires that each morphological character used repre-sents a single evolutionary trait that varies independentlyof every other trait occurring in the dataset. Establishingcharacter independence is particularly necessary when thematrix is principally composed of characters describing themammalian dentition. Features of teeth long believed to beindependent, and previously treated that way in phyloge-

netic analyses, have been shown to be related by in vivodevelopmental studies. In particular, Kangas et al. (2004)showed that in mice the number of cusps, cusp shape andposition, and number of teeth all vary in coordinated fashionas the result of differences in the overall level of expressionof a single cell signalling protein.

In the absence of similar studies conducted on extantmarsupials, we tested an early iteration of our scored charac-ters for statistical independence. The character states beingnominal data, we used a contingency table in JMP (2009)to determine whether states in suites of characters werecorrelated with one another. We paid particular attentionto characters previously identified as being developmen-tally linked in rodents (Kangas et al. 2004; Kassai et al.2005) but did not find strong correlations. We did, however,find a correlation within our dataset between the absenceof stylar cusp D and the absences of stylar cusps B and C:when D is absent, neither B nor C will be present; whenD is present, any combination of B and C is possible. Wedeleted and combined characters as appropriate to reducethe probability of scoring a single evolutionary trait morethan once.

Taxon sampling. In this analysis, our primary goals areto recover the relationships of Mesozoic metatherians,and use the resulting phylogeny to illuminate the originand evolution of early Palaeogene metatherians of NorthAmerica. We have included nearly all known Cretaceousmetatherian taxa that are represented by teeth, but haveexcluded some taxa that are poorly known (see below).As no Palaeogene metatherians are known from outsideof North and South America prior to the early Eocene,we have limited our Palaeogene sample to only North andSouth American taxa. We have included all Palaeocene andearly Eocene North American taxa, except for taxa that wedeem to represent probable synonyms of other included taxa(e.g. Peradectes pauli), or that are known solely by lowerteeth (Esteslestes). We have included one unnamed taxon,the ‘Goler Formation Taxon’ from the middle Palaeocene(Tiffanian) of Southern California, and also included theOligocene North American taxon Herpetotherium fugaxbecause it is unusually complete and has been included insome recent phylogenetic analyses examining the originof crown-clade Marsupialia (Horovitz et al. 2009). OurSouth American Palaeogene sample consists of three taxa:Pucadelphys, Szalinia and Roberthoffstetteria. Pucadel-phys is a substantially complete metatherian taxon fromthe middle Palaeocene of South America that was includedin several recent phylogenetic analyses (e.g. Rougier et al.2004), which place it near the base of the Palaeogenemetatherian radiation. We included Szalinia because it hasa relatively complete dentition and it is considered to be abasal didelphoid (de Muizon & Cifelli 2001). We includedRoberthoffstetteria because several workers have proposeda close relationship between it and the latest Cretaceous

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North American taxon Glasbius (Marshall & de Muizon1988; de Muizon 1992; Case et al. 2005; Goin et al. 2003).

Excluded taxa. We have excluded some published taxafor several reasons. First, we do not include taxa withpoorly known dentitions (e.g. Sinodelphys) or that are rela-tively incomplete, including those represented only by lowerdentitions or are very incomplete (e.g. Alphadon eatoni,Esteslestes, Arcantiodelphys; see Online SupplementaryMaterial Appendix 1). When included in a preliminaryversion of our analysis, any one of these taxa acted as awildcard taxon resulting in a greatly reduced resolution inour strict consensus tree. Second, we excluded a handfulof taxa with dubious phylogenetic affinities, because theylikely do not belong to Metatheria. Pappotherium, a taxonfound to be a metatherian by some phylogenetic analyses(e.g. Kielan-Jaworowska et al. 2004), was not included inthis analysis because more recent analyses concluded thatPappotherium is either nested within Eutheria (e.g. Vulloet al. 2009) or is positioned outside of Theria (e.g. Rougieret al. 2004; Averianov et al. 2010). We also excluded Holo-clemensia, a relatively poorly known taxon based on theholotype, a maxilla fragment with a partial penultimate andultimate molars and a lower molar later tentatively referredto Holoclemensia by Butler (1978). At least two analyses(Vullo et al. 2009; Averianov et al. 2010) found Holo-clemensia to be outside of Methatheria. However, severalphylogenetic analyses found Holoclemensia to be a basalmetatherian (e.g. Luo et al. 2003). When included in apreliminary version of our analysis, it acted as a wildcardtaxon resulting in greatly reduced resolution in our strictconsensus tree.

In addition, we have excluded some taxa (see OnlineSupplementary Material Appendix 1 for a complete list)because they are of doubtful validity (e.g. Didelphodonpadanicus (Cope 1892); see Cifelli and de Muizon, 1998),are probably synonyms of other taxa (e.g. Peradectespauli is here considered a synonym of Peradectes elegans;Mimoperadectes sowasheensis is excluded because webelieve that it may be a synonym of M. houdei), or arebased on specimens not represented by teeth (e.g. Khuduk-lestes, Oxlestes; Kielan-Jaworowska et al. 2004) and maynot represent metatherians (Averianov & Archibald 2005).

Problematic taxa. Ectocentrocristus remains problematicbecause it may represent a deciduous P3, possibly of aTurgidodon-like taxon (Kielan-Jaworowska et al. 2004;Cifelli in Beck et al. 2008). We find that it lacks somefeatures typical of dP3s of other metatherians such as Turgi-dodon rhaister (see Clemens 1966, fig. 11), including astylar shelf that narrows mesially. Furthermore, if it is not adeciduous tooth, the identification of its position is unclear(e.g. Sahni 1972; Rigby & Wolberg 1986; Case et al. 2005).We are also uncertain that a lower molar (m4; AMNH77371) is correctly referred to this taxon (Sahni 1972; Rigby

& Wolberg 1986; Case et al. 2005). As discussed by Fox(1979), Sahni (1972) originally referred this tooth to thesame taxon as the upper tooth (‘Alphadon cf. A. rhais-ter’) based on the supposed resemblance to lower teethof Alphadon cf. A. rhaister from the Lance Formation, aconclusion that Fox (1979) regarded as puzzling since nolower teeth of this taxon had been reported from the LanceFormation. Nevertheless, Fox (1979) found that the refer-ral was reasonable based on the narrowness of the talonidof the lower tooth position for occlusion with the narrowprotocone of the holotype upper molar. Therefore, we haveincluded Ectocentrocristus with the holotype scored as anM1 and including the referred lower tooth. We emphasizethat the evidence for the referral of the lower molar to Ecto-centrocristus is incomplete and without direct association,and therefore it may change in light of new data.

The extant didelphid opossum Didelphis is included insome recent phylogenetic analyses that examine the rela-tionships of Cretaceous and Palaeogene metatherian taxa(e.g. Rougier et al. 2004; Fig. 1F). However, extant taxa arenot highly relevant to the current study. For example, Didel-phis first appears in the Miocene of South America and itis greatly derived relative to the more basal species thatare the focus of our analysis (Marshall 1978). Our studyfocuses on the earliest phases of the metatherian radia-tion during the Cretaceous and Palaeogene, and essentiallytakes the middle Eocene as a culling horizon, after whichsubsequently evolving species, with the exception of thelate Eocene Peradectes californicus and early OligoceneHerpetotherium fugax, are not included. Members of allnested subclades that include Didelphis, however, areincluded in this analysis if these clades have Cretaceous-Palaeogene members. In other words, basal members ofthe lineage leading to Didelphis (and all modern metathe-rians) are included (after Horovitz et al. 2009; Fig. 1C).This taxon selection strategy is analogous to that employedin phylogenetic analyses of Mesozoic dinosaurs, whichusually exclude the much younger and morphologicallyaberrant crown group birds (Aves/Neornithes), even thoughthey descended from theropod dinosaurs (e.g. Turner et al.2007; Csiki et al. 2010).

Nevertheless, as an experimental exercise, we addedDidelphis virginiana (Kerr, 1792) to our dataset, based onscores obtained from specimens in the NMMNH osteologycollection. When the revised dataset was analysed using thesame protocols for the original dataset, as outlined below,this resulted in considerable loss of resolution among non-deltatheroidan metatherians. This is probably due to thefact that a wealth of additional character data, pertinent toDidelphis and other Neogene–Recent metatherians, is notincluded in our character list, which was created with theexpress purpose of encapsulating phylogenetically infor-mative anatomical variety in older and basal metatherians.It is clear that Didelphis is highly derived with respect tothe Cretaceous–Palaeogene taxa included in this study; for

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example, it possesses well-developed penultimate and ulti-mate premolar cingulae, larger lower molar precingulidsthat may be related to enhanced postvallum/prevallid shear,and enlarged and bulbous stylar cusps. These charactersmust have evolved after the Palaeogene–Neogene bound-ary, and are therefore irrelevant to the phylogeny of oldermetatherians. Furthermore, it must be pointed out thatDidelphis virginiana is the largest didelphid and is signifi-cantly larger than other taxa included in this analysis.

Analytical protocols. We subjected our dataset (OnlineSupplementary Material Appendices 1, 2) to a parsimonyanalysis in TNT v. 1.1 (Goloboff et al. 2008). As an initialstep, we analysed the matrix under the ‘New Technologysearch’ option, using sectorial search, ratchet, tree drift,and tree fuse options with default parameters. The mini-mum length tree was found in 10 replicates, which aimedto sample as many tree islands as possible. The 20 recov-ered trees were then analysed under traditional TBR branchswapping, to more extensively explore each tree island,which resulted in 412 most parsimonious trees of length462 (consistency index = 0.214; retention index = 0.709).The strict consensus of these trees is presented in Fig. 3.Bremer branch supports (Fig. 3) were calculated from apool of 50,000 suboptimal trees of up to 10 steps longerthan the shortest trees obtained.

Results of phylogenetic analysisOur strict consensus tree (Fig. 3) is well resolved (especiallythe relationships of more derived taxa), although individualsupport values for many clades are low. Specifically, mostclades fall apart in the strict consensus of all trees oneor two steps longer than the most parsimonious trees, andonly slightly more than half of the clades have a Bremersupport of two or greater. Judging by the consistency andretention indices, it is clear that homoplasy is rampant in theanalysis (low CI), likely explaining the low support valuesof individual clades, but much of this homoplasy (characterreversals and independent gains) is acting as synapomorphyfor different subclades (high RI), likely explaining why thestrict consensus remains well resolved.

Thylacodon pusillus and T. cf. T. pusillus are in a poly-tomy with Swaindelphys, a clade containing Glasbius andRoberthoffstetteria, Pediomyidae and a clade composedof ‘Peradectidae sensu lato’ (all species of Peradectes,Armintodelphys, Mimoperadectes, Maastrichtidelphys,Szalinia, and Pucadelphys) and Pediomyidae. This anal-ysis supports the generic validity of Thylacodon as ataxon distinct from Peradectes as suggested previously (e.g.Krishtalka & Stucky 1983a; Johanson, 1996a).

Herpetotheriidae, or the clade consisting of Ectocen-trocristus, the Goler Formation taxon, Copedelphys andHerpetotherium, is supported by one character (Charac-ter 44; Online Supplementary Material Appendices 1, 3):the centrocrista is deflected buccally so that it is V-shaped.

The clade composed solely of species of Herpetotheriumis supported by two characters (Characters 30, 34; OnlineSupplementary Material Appendices 1, 3): stylar cusp Dis positioned mesiobuccal to the metacone and closer tothe deepest part of the ectoflexus and the upper molarpreparacrista runs to a position mesial to the apex ofstylar cusp B or toward stylar cusp A if stylar cusp B isabsent.

The genus Peradectes is found to be polyphyletic,as its various species do not form their own distinctsubclade. Peradectes minor falls within a basal poly-tomy with a new species of Peradectes, Maastrichtidel-phys, Szalinia plus Pucadelphys, a clade composed of twospecies of Peradectes (P. elegans and P. californicus), aclade composed of Peradectes protinnominatus, P. ches-teri, Mimoperadectes, Armintodelphys and Pediomyidae.Also, Peradectidae as originally conceived is not found tobe monophyletic, as some supposed peradectids fall into apolytomy with a monophyletic Pediomyidae. Therefore, onour topology, our phylogenetic definition of Peradectidaerefers solely to the clade of P. elegans and P. californicus,although this is a labile result and future studies may likelyrecover a more inclusive phylogenetically-defined Peradec-tidae (which may also include many of the species that fallinto the polytomy in our strict consensus, many of whichdo group with P. elegans and P. californicus in many ofthe individual most parsimonious trees). Pediomyidae issupported by three characters (Characters 22, 46 and 54;Online Supplementary Material Appendices 1, 3): molarstylar cusp B is positioned mesiobuccal to the paracone;the molar paraconule is positioned relatively closer to theparacone; and the postprotococrista merges with the post-metaconule crista and extends buccally to near the buccalmargin of the tooth.

The clade consisting of Pediomyidae and Peradecti-dae sensu lato (including Maastrichtidelphys, Szalinia andPucadelphys) is united by a two characters (Characters 6and 77; Online Supplementary Material Appendices 1, 3):the DP3 mesial stylar shelf is present as an ectocingulumand ultimate molar cristid obliqua joins the distal wall ofthe trigonid below the protocristid notch. Two characterssupport a clade that includes Herpetotheriidae, ‘Peradecti-dae sensu lato’, Maastrichtidelphys, Szalinia, Pucadelphysand Pediomyidae: the mesial portion of the penultimatemolar stylar shelf is not reduced and the paracone and theestimated body mass is small.

Horovitz et al. (2009) identified six synapomorphiessupporting a clade composed of Herpetotherium, peradec-tids and other marsupials, of which one was a dentalcharacter: a reduced to absent metaconule on the uppermolars. However, our analysis includes basal members ofHerpetotheriidae and basal members of ‘Peradectidae sensulato’ (including the genotype, P. elegans), which all haveunreduced metaconules. In our analysis the character stateis a homoplastic character within these clades.

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Figure 3. Strict consensus of 412 trees of 462 steps calculated using TNT (Goloboff et al. 2008) based on a taxon character matrix of92 taxa and 78 dental characters (CI = 0.214; RI = 0.709). Analysis conducted using a New Technology Search with a Driven Search(Sectorial Search, Ratchet, Drift and Tree Fusing), finding minimum length 10 times. All characters unweighted, 20 characters additive.Numbers to the left of each node correspond to nodes in Online Supplementary Material Appendix 3 listing the synapomorphies commonto the 412 shortest trees. Numbers to the right of each node correspond to Bremer branch supports calculated from a pool of 50,000suboptimal trees of up to 10 steps longer than the shortest trees obtained.

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Other salient results of this analysis include the recoveryof several well-supported clades including a Roberthoffstet-teria plus Glasbius clade, a stagodontid clade that includesa Pariadens species (P. kirtlandi) as a basal member, anda Deltatheroida clade that includes Nanocuris, a result inagreement with the recent referral of this taxon by Wilsonand Riedel (2010). Deltatheroida is supported by five char-acters (Characters 36, 48, 54, 58 and 59; Online Supplemen-tary Material Appendices 1, 3) a carnassial notch is presentalong the postmetacrista; the protocone is shorter than theparacone/metacone; the postprotocristae extends from theprotocone, or merges with the postmetaconule crista, tothe base of the metacone; the paraconid is longer thanthe metaconid; and the paraconid is taller than the meta-conid. Three characters support Stagodontidae (Characters58, 59 and 60; Online Supplementary Material Appen-dices 1, 3): the paraconid is longer than the metaconid;the paraconid is taller than the metaconid; and the mesi-olingual face of the paraconid is strongly keeled. Finally,three characters support the clade composed of Glasbiusand Roberthoffstetteria (Characters 37, 49 and 54; OnlineSupplementary Material Appendices 1, 3): the penulti-mate molar ectoflexus is shallow, the upper molar (allbut last) protocone possesses a basal distal expansion, andthe upper molar postprotocrista merges with the postmeta-conule crista to extend to near the buccal margin of thetooth.

Species of Nortedelphys are not closely allied withHerpetotheriidae or other Palaeogene metatherian taxa,disagreeing with Case et al.’s (2005) referral of Nort-edelphys to Herpetotheriidae. Nortedelphys lacks severalcharacters that have traditionally been used to uniteHerpetotheriidae (see Krishtalka & Stucky 1983a), includ-ing a longer and higher metacone than paracone; a buccallyshifted cristid obliqua, and a shelf-like hypoconulid posi-tioned lingually, distal to the entoconid; and a short post-metaconule crista that terminates near the lingual base ofthe metacone.

The possession of a V-shaped centrocrista, which Caseet al. (2005) used as a character to assign Nortedel-phys to Herpetotheriidae, is present in several metatherianclades, and based on our phylogeny it evolved at leastfive times independently and it is present in numerousgenera that are usually not considered to be members ofHerpetotheriidae, including Protalphadon (based on P. foxi;see Johanson 1996b) and the Palaeogene South Americantaxa Roberthoffstetteria, Pucadelphys and Szalinia. There-fore, it is a homoplastic character that cannot be used asunequivocal evidence, on its own, to refer isolated teethto a specific metatherian subclade. However, this analy-sis supports a close relationship between Ectocentrocristusand Herpetotheriidae. Depending on the resolution of thepolytomy between Ectocenrocristus, Herpetotheriidae andother taxa, this may extend the origins of Herpetotheriidaeto the Late Cretaceous (late Campanian).

Anatomical revisions and taxonomicimplicationsThe phylogenetic analysis, as well as observations ofnew specimens, require some taxonomic amendments andprovide new information on the dental morphology ofbasal metatherians from western North America. Here,we describe new material of Thylacodon and Peradectesfrom the early Palaeocene (Puercan) of New Mexico andprovide revised diagnoses for these genera. We show thatThylacodon pusillus is a valid taxon and not referable toPeradectes and that a second taxon (P. cf. P. pusillus) isneither assignable to Peradectes nor T . pusillus, but belongsto a distinct species. Finally, we provide an alpha taxonomicrevision of the genus Nortedelphys and formally reassignAlphadon jasoni to Nortedelphys jasoni.

Systematic palaeontology

Metatheria Huxley, 1880Herpetotheriidae Trouessart, 1879

Genus Thylacodon Matthew & Granger, 1921

1937 Thylacodon Matthew & Granger; Matthew: 297.1982 Peradectes Matthew & Granger; Archibald: 130.2006 Thylacodon nomen dubium; Clemens: 21.

Type species. Thylacodon pusillus Matthew & Granger,1921.

Included species. Thylacodon pusillus and T. montanensissp. nov. (below).

Revised diagnosis. Differs from Alphadon and Nortedel-phys in possessing a cristid obliqua that intersects the trigo-nid wall buccal to the protocristid notch, an upper molarpostmetaconule crista that terminates near the distolingualbase of the metacone, and a metacone that is larger thanthe paracone. Differs from Swaindelphys in possessing anentoconid that is significantly larger than the hypoconulidand upper molars that lack V-shaped centrocristae. Differsfrom Peradectes and other peradectids in having moredistinct conules and stylar cusps, and an entoconulid thatis relatively larger than the hypoconulid (M1-3), bladelikerather than conical, and positioned relatively farther fromthe hypoconulid, leaving a distinct notch between the twocusps.

Nomenclatural note. Our phylogenetic analysis does notresult in a monophyletic Thylacodon if we include T. pusil-lus and the new species T . montanensis (see below). Instead,these are found to be successive outgroups to Swain-delphys and a traditional ‘Herpetotheriidae’. However,previous workers (e.g. Archibald 1982; Lofgren 1995;Clemens 2006) have tentatively referred T. montanensis to

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T. pusillus (as ‘P. cf. P. pusillus’) and we suspect that theyprobably do form a clade, but we did not recover it due to thehigh amount of homoplasy in the analysis. Therefore, weconservatively refer this new species to Thylacodon ratherthan introduce a new genus based on what are presentlypoorly supported phylogenetic results. We are comfortable,however, in later naming a new genus should future phylo-genetic analyses corroborate our results and find that the twoputative Thylacodon species cannot be united in a single,well-supported clade.

Discussion. Matthew and Granger (1921) erected thegenus and species Thylacodon pusillus based on a dentaryfragment that included a portion of the talonid of m1and m2-3 that have their paraconids broken off and miss-ing (AMNH 16414). Although he would later dismissThylacodon as a nomen dubium in 2006, Clemens (1979)regarded Thylacodon to be a synonym of Peradectes, agenus erected for Peradectes elegans Matthew & Granger,1921 from the upper Palaeocene Animas Formation,San Juan Basin, south-western Colorado (Matthew &Granger 1921). Following Clemens (1979), and citing anunpublished manuscript by Clemens, Archibald (1982)and Lofgren (1995) referred several isolated teeth fromthe lower Palaeocene Tullock Formation of Montana toPeradectes cf. P. pusillus.

Krishtalka and Stucky questioned the synonymy ofThylacodon with Peradectes (1983a, b). As acknowledgedby them, this conclusion was based on examination offigures of specimens referred to Peradectes cf. P. pusil-lus in Archibald (1982, figs 42, 43) rather than of thetype or topotypic material. Additional questions regardingthe generic referral of Thylacodon pusillus were raised byJohanson (1996a, b), who described a new early Palaeocenetaxon, Swaindelphys cifellii, and referred it to Herpetotheri-idae. She noted that T. pusillus possesses characters of thelower teeth, particularly regarding the morphology of theentoconid and hypoconulid, which are not present in otherspecies of Peradectes.

Therefore, there is ongoing debate about whether Thyla-codon is a valid genus or synonymous with Peradectes. Theresults of our phylogenetic analysis, which includes the newfossil material described below, helps clarify this puzzle.Our phylogenetic analysis indicates that the holotype of T .pusillus and the referred specimens from Montana are notparticularly closely related to the holotype of Peradectesand other specimens referred to this genus. In other words,T . pusillus, the Montana specimens, and Peradectes do notform their own clade, which is a requirement for them tobe formally synonymized to the same genus. Also there areseveral discrete differences in dental morphology betweenT . pusillus and the referred Montana specimens, on the onehand, and Peradectes on the other. Therefore, neither theanatomical character data nor the results of the phyloge-netic analysis support the synonymy of Thylacodon and

Peradectes. Consequently, Thylacodon is a valid taxon,in agreement with previous assessments (Johanson 1996a;Krishtalka & Stucky 1983a, b).

Archibald (1982) noted that based on an unpublishedmanuscript, Clemens held the view that Thylacodon wasa synonym of Peradectes because they both possessed asimilar talonid cusp morphology. However, we found differ-ences in the entoconid and hypoconulid between T. pusillusand P. elegans. In T. pusillus, the entoconid is large andmesiodistally long in contrast to all species of Peradectes(including P. minor, described below), so that the entoconidforms a dorsally projecting blade that is triangular in lingualview, whereas the entoconid is cylindrical or spire-shapedin herpetotheriids or it is peg like and subequal in size tothe hypoconulid in species of Peradectes.

Johanson (1996a) noted that the entoconids andhypoconulids of lower molars of Swaindelphys cifellii fromthe lower Palaeocene Fort Union Formation of Wyomingresemble those of Thylacodon pusillus more closely thanthose of the genus type Peradectes elegans. As in T. pusil-lus, the two talonid cusps are separated by a notch and thehypoconulid is positioned distobuccally rather than directlydistal to the entoconid. S. cifelli was distinguished by beingsmaller and having a relatively shorter talonid (Johanson1996a). The lower molars of T. pusillus closely resembletwo recently described species of Swaindelphys: S. johan-soni and S. encinensis (Williamson & Taylor 2011). These,like S. cifelli and T. pusillus, have trenchant entoconids andhypoconulids that are positioned buccal to the distolingualcorner of the talonid. These teeth are subequal in size to T.pusillus, but they are readily distinguished by their robusttrigonid cusps and lower entoconids. Based on this, theholotype of Thylacodon pusillus, AMNH 16414, is diag-nostic and it can be easily distinguished from similar taxa.

Thylacodon pusillus Matthew & Granger, 1921(Figs 4, 5, Table 1)

1921 Thylacodon pusillus Matthew & Granger: 2.1937 Thylacodon pusillus Matthew & Granger; Matthew:

298, fig. 82.1980 Peradectes pusillus (Matthew & Granger); Stand-

hardt: 51, fig. 11 (in part).1993 Thylacodon pusillus Matthew & Granger; Williamson

& Lucas: 117.1993 Thylacodon pusillus Matthew & Granger;

Williamson: 100.1996 Thylacodon pusillus Matthew & Granger;

Williamson: 34.2006 Peradectes pusillus (Matthew & Granger); Clemens:

24, text-fig. 1, table 1.

Holotype. AMNH 16414, partial left dentary with partialm1, m2–3.

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Table 1. Measurements of specimens of Thylacodon pusillusfrom the early Palaeocene Nacimiento Formation, New Mexico.All measurements are in mm.

L MW DW

AMNH 16414 (holotype of T. pusillus)m2 2.55 1.47 1.61m3 2.49 1.46 1.34AMNH 58385A∗

m2 2.46 1.54 1.65m3 2.36 1.52 1.53m4 2.42 1.50 1.33M1NMMNH P-47329 2.65 2.35 2.65NMMNH P-47330 — 1.90 —NMMNH P-47331 2.80 2.50 2.55M2NMMNH P47333 2.45 2.95 2.90M3NMMNH P-47327 2.75 2.95 2.95NMMNH P-47328 2.45 — —NMMNH P-47335 — — 3.00M4NMMNH P-47334 2.40 — —m1NMMNH P-34808 2.40 1.20 1.10NMMNH P-41142 2.25 1.00 1.10NMMNH P-53929 2.60 1.15 1.35m2 or 3NMMNH P-08699 2.50 1.30 1.40NMMNH P-47322 2.50 1.40 1.45NMMNH P-47323 2.30 1.25 1.40NMMNH P-47326 2.25 1.35 1.40NMMNH P-47501 — — 1.50NMMNH P-51546 — — 1.60NMMNH P-54382 2.65 1.45 1.60NMMNH P-56649 2.65 1..85 1.60

∗Measurements from Clemens (2006).

Referred specimens. From locality L-0646, AMNH58230, right m2 or 3, left M1, partial M2 or 3; 58378,left M3; 58379, left M2; 58385a, left partial dentary withm2–m4; 58385, right m1, right partial m1; 58518, leftM2; 59897, right partial dentary with m4; NMMNH P-08699, left m2 or 3 (missing portion of hypoconid); fromNMMNH locality L-0844, NMMNH P-62554, partial leftM3; from NMMNH locality L-4723, NMMNH P-34808,right abraded m1; 38459, partial left M3, missing lingualportion of tooth, buccal to paracone and metacone and toothdistal to apex of metacone; 41142, right m1; 41199, rightM3, missing portion of tooth on mesiobuccal corner andwith abraded buccal margin; 51546, right abraded m2 or 3;53303, right partial M2; 53929, right m1; 54382, right m2or 3; 56649, left m2 or 3; from NMMNH locality L-6387,NMMNH P-47321, partial right upper molar consistingof the lingual part of the tooth, including the protoconeand conules; 47322, right m2 or 3; 47323, right abradedm2 or 3; 47324, partial right upper molar including meta-cone and portion of stylar shelf; 47325, right partial M2or 3 consisting of paracone and mesiobuccal portion of

tooth; 47326, right partial m2 or 3, missing distobuccalportion of talonid; 47327, right M3; 47328, partial rightM3, missing the lingual portion of the tooth including theprotocone; 47329, left M1 with abraded buccal margin;47330, left partial M1, missing distobuccal corner of tooth;47331, left M1; 47332, partial left M1, missing the disto-buccal corner of tooth; 47333, right M2; 47334, partial leftM4, missing protocone and lingual portion of tooth; 47335,partial left M3, missing paracone and mesiobuccal cornerof tooth; from locality L-6254 (De-na-zin Wash), NMMNHP-47501, left partial m talonid.

Distribution. From early Palaeocene (Puercan) of NewMexico.

Revised diagnosis. Differs from Thylacodon montanensissp. nov. (below) in its larger average size; upper molars(M1–3) possess a buttress-like ridge that extends from themetacone across the stylar shelf toward the region of theectoflexus; and the m4 entoconulid is not reduced in contrastto those of the preceding molars.

Description. Thylacodon pusillus is represented primar-ily by isolated teeth and also by two specimens that includepartial dentaries with lower molars. Upper molars are repre-sented by isolated teeth identified as M1s, M2s, M3s and asingle partial M4. We present a revised and detailed descrip-tion here.

M1. M1 is known from a single complete tooth (NMMNHP-47331; Fig. 4A), a nearly complete tooth that is wornalong the buccal margin (47329; Fig. 4C), and two partialteeth, each missing a portion of the distobuccal corner(47330; Fig. 4B, 47332; Fig. 4D). Only 47331 has acomplete stylar shelf. Stylar cusps A–D are distinct andthey are in a row. The stylar shelf is wide distally andnarrows mesially. Stylar cusp A (parastyle) is the smallestof the stylar cusps and originates close to stylar cusp B.Stylar cusp B (stylocone) is the largest of the stylar cusps,but it is smaller and lower than the paracone. It is long andpositioned buccal to the paracone. Stylar cusps C and Dare subequal in size. Stylar cusp C is buccal to the posi-tion between the paracone and metacone. Stylar cusp D isbuccal to the metacone.

The paracone is smaller and lower than the metaconeand the two cusps are well separated at their bases. Thecones have rounded lingual surfaces, but they are nearlyflat buccally. The metacone has a distinct ridge or buttressat its base buccally that extends to the base of stylar cuspD; we have not observed this feature in any other metathe-rian tooth. The centrocrista is nearly straight or it has aslight V-shape with the apex directed toward stylar cuspC, especially in 47331. The preparacrista extends fromthe paracone to the mesiolingual base of stylar cusp B.Both the preparacrista and the postmetacrista lack carnas-sial notches.

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Figure 4. Thylacodon pusillus upper molars. A, NMMNH P-47331, left M1 (stereopair); B, NMMNH P-47330, partial left M1 (stereopair);C, NMMNH P-47329, left M1 (stereopair); D, NMMNH P-47332, partial left M1 (stereopair); E, NMMNH P-47328, left M2 (stereopair);F, NMMNH P-47327, partial right M2 (stereopair); G, NMMNH P-47333, right M3 (stereopair); H, AMNH 58518, left M3 (stereopair);I, NMMNH P-38459, partial left M3 (stereopair); J, NMMNH P-41199, right M3 (stereopair); K, NMMNH P-47334, partial left M4(stereopair).

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The protocone lacks lingual and buccal cingula and isasymmetrical, with the apex positioned mesially and themesial surface is steeper than the distal surface. The post-protocrista is directed distally and the preprotocrista extendsmesiobuccally. The paraconule and metaconule are bothpositioned on the pre- and postprotocrista, respectively andthey are positioned closer to the apex of the protocone thanto the paracone/metacone. They are nearly subequal in sizeon some specimens, with the metaconule clearly larger thanthe paraconule on most specimens. Internal conular cristaeare present. The postparaconule crista extends distobuccallyto the base of the paracone and the premetaconule cristaextends buccally to the base of the metacone. However, thestrength of the premetaconule crista is variable. It is clearlypresent in some specimens (e.g. 47331), but it is absent inothers (e.g. 47330). The talon basin is deep and wide as isthe sulcus separating the bases of the paracone and meta-cone. The postmetaconule crista descends basally to thebase of the tooth lingual to the metacone. The preparaconulecrista extends buccally to stylar cusp A. On two specimens,the preparaconule crista or precingulum supports a cuspulemesial to the paracone (47330, 47331). However, on 47332,the precingulum has a sinuous curve mesial to the base ofthe paracone and it has a constant width.

M2. M2 is represented by one complete tooth, 47328 (Fig.4E) and a partial tooth that is missing the protocone andconules (47327; Fig. 4F). The M2 resembles the M1, butthe stylar shelf is wider mesially. Specimen 47328 lacksan ectoflexus but the other specimen has a mesiodistallywide and shallow ectoflexus centred on stylar cusp C. Thestylar cusps are distinct in 47328, but they are less distinctin 47327. Stylar cusp D is larger than stylar cusp C and itis mesiodistally longer.

M3. Four specimens almost certainly represent M3s:NMMNH P-47333, a right M3 (Fig. 4G), AMNH 58518(Fig. 4H), a nearly complete tooth missing only a portionof the base of the protocone, NMMNH P-38459 (Fig. 4I), atooth missing the protocone, the conules, and a portion ofthe tooth distal to the metacone, and NMMNH P-41199, anabraded right M3 (Fig. 4J). On these teeth, the stylar cuspsare less distinct than on the more mesial upper molars andthe stylar shelf has a nearly constant width buccal to theparacone and metacone. All the teeth distal to M1 haveextensive wear on the paracingulum that obliterated somefeatures. The buttress buccal to the metacone makes a talland wide crest on M1 that becomes increasingly mesiodis-tally wider in succeeding teeth and it projects mesiallytoward the base of stylar cusp C. On specimen 38459,this portion of the tooth appears to be damaged, but thestylar shelf buccal to the metacone is raised as an irregularhummocky surface.

M4. A partial left M4 is represented by a single tooth,NMMNH P-47334 (Fig. 4K) missing the protocone andparaconule. The metastylar region and metacone arereduced in size compared to M3. Stylar cusp A is largerthan stylar cusp B and the preparacrista is directed towardstylar cusp A, but it stops before reaching the cusp.

m1. A complete isolated tooth, NMMNH P-41142 (Fig.5A–C), represents a right m1. The tooth is narrower thansucceeding molars. The trigonid is narrower buccolinguallythan the talonid and the paraconid projects mesiolingually.The trigonid cusps are markedly taller than those of thetalonid. The metaconid is lower than the protoconid, andthe paraconid is lower than the metaconid. However, inocclusal view, the paraconid is larger than the metaconid.The metaconid is positioned distolingual to the protoconid.The paracristid and protocristid are notched. The hypoconidis the largest of the talonid cusps. The entoconid andhypoconulid are twinned along the distolingual margin ofthe talonid basin. The entoconid is mesiodistally long andblade-shaped and it is nearly mesiodistally symmetrical.The hypoconulid is smaller and lower than the entoconidand inclined distally. The cristid obliqua intersects the distalwall of the trigonid below the apex of the protoconid. Amesial cingulid is present.

m2–m4. Several isolated teeth represent either the m2 orm3 loci (Fig. 5D–O). Clemens (2006, fig. 1B–D) referreda partial dentary with m2–4 (AMNH 56385a) from theNacimiento Formation to Peradectes pusillus (here Thyla-codon pusillus). These teeth differ from the m1 in havingwider trigonids that are subequal in buccolingual width tothe talonids and also paraconids that are in a buccolingualrow. The metaconid is positioned mesially relative to theprotocone so that the distal surface of the trigonid is trans-versely oriented. A postcingulid descends buccally fromthe hypoconulid to the base of the tooth. An ectocingulid isabsent.

For m2–3, the talonid is subequal in length and widthto the trigonid. The hypoconid is the largest of the talonidcusps. The entoconid is smaller than the hypoconid, but it islarger than the hypoconulid. The entoconid is tall, reachingover 40% of the height of the metaconid. It is mesiodis-tally elongate, forming a blade that is keeled mesially, butit is rounded distally. The hypoconulid is positioned nearthe entoconid so that it is ‘twinned’ with it. However, arounded notch, sometimes referred to as the ‘entoconidnotch’ (Krishtalka & Stucky 1983a, b), separates them. Thehypoconid is not positioned at the distolingual corner of thetooth, directly distal and nearly distal to the hypoconid as isalso seen in Peratherium or Herpetotherium, but it is posi-tioned buccally. A postcingulid descends from the buccalmargin of the hypoconulid close to the base of the toothbuccally.

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Figure 5. Thylacodon pusillus lower molars. A–C, NMMNH P-41142, right m1 in occlusal (A, stereopair), B, buccal, and C, lingualviews; D–F, NMMNH P-08699, left m2 or 3 in occlusal (D, stereopair), E, buccal, and F, lingual views; NMMNH P-47322, right m2 or3 in occlusal (G, stereopair), H, buccal, and I, lingual views; NMMNH P-47326, right incomplete m2 or 3 in occlusal (J, stereopair), K,buccal, and L, lingual views; NMMNH P-51546, right m2 or 3 in occlusal (M, stereopair), N, buccal, and O, lingual views.

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Figure 6. An example of extreme plastic deformation in specimen, NMMNH P-01684, a left dentary with a partial p2, p4-m3 of the‘triisodontid’ ‘condylarth’ Eoconodon coryphaeus from the late Puercan (Pu3) fossiliferous zone of the Nacimiento Formation in theBisti/De-na-zin Wilderness area in A, buccal and B, occlusal views compared to an undeformed partial left dentary with m1-2 of E.coryphaeus (C; AMNH 3354; holotype of Triisodon buculminatus Cope, 1888).

The m4 is represented by the single tooth preserved inthe partial dentary AMNH 56385a (Clemens 2006, text-fig. 1B–D). It is similar to m2–3, but it has a relativelylonger and narrower talonid. The entoconid is unreduced ascompared to m2–3 and a postcingulid is present.

Discussion. Clemens (2006) included the first publishedimage of the holotype of Thylacodon pusillus (AMNH16414; Clemens 2006, text-fig. 1a). He noted that the m2–3preserved in AMNH 56385a are shorter and wider thanthose of the type. AMNH 16414 is plastically deformed sothat the shape of the m2 talonid is distorted. This damageexplains some or all of the differences in size and shapebetween the holotype and 56385A. Similar deformationis commonly observed in specimens from the De-na-zinWash/Barrel Springs area (Fig. 6).

Other specimens tentatively referred to T. pusillus includea lower partial left dentary with p3–m3 and a partial m4(UCM 35070) from the Denver Formation, Denver Basin,Colorado (Middleton 1983). Several of the lower molars aredamaged, but the m3 includes an undamaged talonid with acomplete entoconid and hypoconulid. It closely resemblesthat of Thylacodon, where the entoconid is tall, narrow,bladelike and triangular in lingual view. Also, the teeth ofUCM 35070 fall within or close to the size range of T. pusil-lus from the San Juan Basin. This specimen is significant inpreserving the p3, which otherwise has not been described

for T. pusillus. As described by Middleton (1983, p. 155),the p3 is a tall, trenchant tooth. It has one main cusp andone talonid cusp.

One additional question that remains: is Thylacodonpusillus conspecific with ‘P. cf. P. pusillus’, reported fromthe lower Palaeocene Tullock Formation of eastern Montana(Archibald 1982). The tooth sizes for T . pusillus reportedhere (Table 1) largely overlap with those reported for ‘P.cf. P. pusillus’ from V-74111 of the Tullock Formation byClemens (2006, table 2). However, a few teeth from theNacimiento Formation referred to T. pusillus fall outsideof this size range. Also, several upper molars from theNacimiento Formation have a distinct ridge or buttress thatextends from the buccal base of the metacone over thestylar shelf to the ectoflexus. This is evident on M1 and 2(see Fig. 4A, E). A similar ridge or buttress was not seenin corresponding upper molars of teeth referred to ‘P. cf.P. pusillus’ from Montana (TEW pers. obs.). We concludethat these differences warrant recognition of two distinctspecies: Thylacodon pusillus and Thylacodon montanensissp. nov. (below).

Thylacodon montanensis sp. nov.

1982 Peradectes cf. P. pusillus Archibald: 130, figs 42–44,tables 24, 25.

1995 Peradectes cf. P. pusillus Lofgren: 109, table 29.2006 Peradectes cf. P. pusillus Clemens: 24, table 2.

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Table 2. Measurements of specimens of Peradectes minor fromthe early Palaeocene Nacimiento Formation, New Mexico. Allmeasurements are in mm.

L MW DW

M1NMMNH P-21802 — — 1.60NMMNH P-34819 — — 1.65NMMNH P-42011 1.75 1.55 1.80NMMNH P-47288 1.70 1.75 1.80M2NMMNH P-42006 1.70 — —M3NMMNH P-47293 1.60 2.05 2.00NMMNH P-47301 1.40 1.75 1.75NMMNH P-47489 1.50 — —NMMNH P-55404 1.40 1.80 1.85NMMNH P-41199 1.80 — —m1NMMNH P-21807 1.55 0.80 0.90m2 or 3NMMNH P-21804 1.75 1.00 1.00NMMNH P-21806 — 0.90 —NMMNH P-34822 1.75 0.85 0.85NMMNH P-47302 1.60 1.00

∗1.05

m4NMMNH P-47294 1.55 0.90 0.75

∗Measurements from Clemens (2006).

Holotype. UCMP 117770, left M3 from UCMP local-ity V74111, Tullock Formation, Garfield County, easternMontana.

Referred specimens. UCMP 117744, right M1; 117764,right M2; 117771, left M4; 117776, right m1; 117791, leftm2; 117796, right m3; 117801, right m4; 117806, partialleft DP3; 117807, left dp3; 132299, associated M1–2;132300, m1.

Etymology. The species name Montana is in reference tothe state from where the type and referred specimens havebeen collected.

Distribution. From early Palaeocene (Puercan; Pu1,Pu2/3) of eastern Montana (Clemens 2006).

Diagnosis. Similar to Thylacodon pusillus, but differs inits smaller average size and by the absence of a ridge fromthe upper molar (M1–M3) stylar shelf to the stylar cusp Carea.

Description. This taxon was thoroughly described byArchibald (1982) and several additional features were alsonoted by Lofgren (1995) and Clemens (2006).

Discussion. Archibald (1982) referred numerous speci-mens from UCMP locality V74111 to ‘Peradectes cf. P.

pusillus’, a taxonomic assignment that was followed byClemens (2006). Lofgren (1995) and Clemens (2006) notedvariability in features of the upper teeth of Thylacodonsp. nov., including one stylar cusp C which on occasionis replaced by two smaller cusps and the variable pres-ence and absence of M3 stylar cusp C (absence is morecommon). These differences may be of taxonomic valueand may comprise additional features that separate the twospecies of Thylacodon, because stylar cusp C is presentin all M3s of Thylacodon pusillus. However, T. pusillus isrepresented by a much smaller sample size and so the distri-bution of this character in T. pusillus might be affected bysampling bias.

Peradectidae Crochet, 1979Genus Peradectes Matthew & Granger, 1921

Peradectes minor Clemens, 2006(Figs 7, 8, Table 2)

1980 ‘Peradectes cibolensis’ Standhardt: 55, fig. 12.1993 Peradectes? n. sp. A Williamson & Lucas: 1171993 Peradectes? n. sp. A Williamson: 101.1996 Peradectes? n. sp. A Williamson: 34.2006 Peradectes minor Clemens: 26, text-fig. 2, table 3.

Holotype. UCMP 115386, left M3 from UCMP localityV-74122, Tullock Formation, Garfield County, Montana.

Referred specimens. From NMMNH locality L-646,NMMNH P-21802, right M1, missing mesiobuccal cornerof tooth; 21803, left m2 or 3; 21804, right m2 or 3; 21806,broken left m2 or 3; 21807, right m1; 42011, left M1, miss-ing portion of the stylar shelf mesobuccal to the paracone;from NMMNH locality L-6254, NMMNH P-47489, partialright M3, missing protocone and lingual portion of tooth;from NMMNH locality L-6387, NMMNH P-47288, leftM1; 47293, right M3; 47294, partial right dentary withpartial m4, missing paraconid; 47301, right M3; 47489,partial right M3, missing the lingual portion of the toothincluding the protocone; from NMMNH locality 4723,NMMNH P-34819, partial left M1; 34822, left m2 or 3;42006, left M2; 47302, partial right m2 or 3, missing portionof metaconid and protoconid; 55404, left M3.

Distribution. Peradectes minor is known from middle tolate Puercan localities of the Tullock Formation of easternMontana (Clemens 2006) and from middle and late Puercanlocalities of the Nacimiento Formation, New Mexico.

Revised diagnosis. Differs from other species ofPeradectes in that the shape of the buccal margin of theupper molars in occlusal view is described as severalstraight lines that intersect at distinct angles, rather than

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as a series of gentle curves; the buccal margin of the ulti-mate molar and the cristid obliqua meets the trigonid belowthe apex of the protoconid, buccal to the protocristid notch(Character 75, Online Supplementary Material Appendices1, 3).

Description. Numerous isolated teeth from several locali-ties are referred to P. minor.

M1. Four M1s are referred to Peradectes minor: NMMNHP-21802 (Fig. 7B), 34819, 42011 (Fig. 7A) and 47288.These closely resemble the M1s of P. minor described andillustrated by Clemens (2006) from the early Palaeocene(Puercan) of the Tullock Formation, eastern Montana. Thestylar shelf narrows mesially. Stylar cusps C and D aresubequal in size and mesiodistally long. The paracone andmetacone are completely separated at their bases. The para-cone is lower and mesiodistally shorter than the metacone.

The buccal surfaces of the para- and metacones are flatbuccally and convex lingually. All M1s show damage to themesiobuccal portion of the tooth.

M2. A partial tooth, NMMNH P-42006 (Fig. 7C), is almostcertainly an M2 and is missing the protocone and conules.It lacks an ectoflexus. Instead, the stylar cusps are in a row.Stylar cusp B is the largest of the stylar cusps and cuspsC and D are nearly subequal. The centrocrista is straight.The paracone is mesiodistally shorter and lower than themetacone. The preparacrista is short and stops abruptly at anotch that separates it from the mesiolingual base of stylarcusp B. As in M1, the distal margin of M2 expands abruptly,buccal to the buccal end of the postmetaconule crista, givingthese teeth a waisted appearance.

M3. A single complete M3 (NMMNH P-47293; Fig. 7D)and a well-preserved partial M3 missing the protocone

Figure 7. Peradectes minor upper molars. A, NMMNH P-42011, incomplete left M1 (stereopair); B, NMMNH P-21802, incomplete rightM1 (stereopair); C, NMMNH P-42006, partial left M2 (stereopair); D, NMMNH P-47293, right M3 (stereopair); E, NMMNH P-47489,partial right M3 (stereopair).

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and conules (47489, Fig. 7E) show that the paracone issmaller, both lower in height and mesiodistally shorter,than the metacone. Both cones are convex lingually andhave nearly flat buccal surfaces. They are well separatedat their bases and are connected by a straight centrocrista.The preparacrista is strong, lacks a carnassial notch, and itis directed to a position between stylar cusps A and B. Thepostmetacrista descends from near the apex of the meta-cone and it lacks a carnassial notch. The stylar shelf is wideand the para- and metastylar portions are similar in width,so that the tooth is nearly symmetrical in occlusal view.Stylar cusps A, B, C and D are similar in size, with stylarcusp B being the largest, followed by stylar cusp A. Stylarcusp C is smaller than stylar cusp D. An ectoflexus is deepand wide and it is nearly symmetrical, and centred on stylarcusp C. The buccal margin of the tooth is distinctly angularin occlusal view, deviating from a rounded outline. Thismargin is more smoothly rounded bordering the metastylarlobe in 47489. The protocone of 47293 is V-shaped andthe conules are distinct and subequal in size, but internalconular wings are not evident. The postmetaconule cristastops near the distolingual base of the metacone.

Lower molars. Several isolated teeth and tooth fragmentsrepresent lower molars of P. minor. These include a singlenearly complete m1 (P-21807, Fig. 8A–C) that is damagedand missing some of the enamel from the mesiobuccal faceof the trigonid. The trigonid is narrower than the talonid,with a mesially projecting paraconid and a small meta-conid.

Several specimens represent either m2 or 3 (Fig. 8D–I).Nearly all of them are damaged. Several lower molarspreserve the mesial margin, which includes the area thatreceives the hypoconulid of the preceding tooth. This area isbuccolingually narrow and it is defined lingually by a flangethat is present below the apex of the paraconid, and buccallyby the lingual edge of the precingulid. The paraconids aredamaged on all teeth, but the base of the paraconid showsthat it extended mesiolingually. The trigonid cusps are wellpreserved on P-34822 and this shows that the metaconidwas smaller and lower than the protoconid. The paraconidis smaller and lower than the metaconid. The para- andprotocristid are separated by a deep carnassial notch. Onm2–3, the talonid is subequal in length and width to thetrigonid. The cristid obliqua intersects the distal wall of thetrigonid buccal to the protocristid notch. The hypoconid isthe largest cusp of the talonid cusps. The entoconid andhypoconulid are twinned near the distolingual corner of thetooth. An m1 (P-21807, Fig. 8A–C) preserves a tall andbladelike entoconid that is similar to that typically seenin lower molars of T. pusillus. As in T. pusillus, the ento-conid is approximately triangular in lingual view and it isdistinctly taller than the erect hypoconulid. Lower molarsrepresenting the m2 or 3 (e.g. NMMNH P-21804, 47302;

Figs 8D–F and 8G–I, respectively) have low and conicalentoconids that are subequal in height to the hypoconulids.Distally, a postcingulid descends from the hypoconulid. Anectocingulid is not present.

The single m4 (NMMNH P-47294, Fig. 8J–L) is nearlycomplete, but it is missing the paraconid. It is similarto the preceding lower molars, but it has a longer andnarrower talonid. The entoconid is distally positioned onthe talonid that is widely separated from the metaconid.An entocristid forms a low wall along the lingual marginof the talonid basin. The entoconid is smaller and lowerthan the hypoconulid. A low ridge connects the two cusps.A postcingulid is present buccal to the hypoconulid. Anectocingulid, comprised of discontinuous short segments,is present as a distal extension of the precingulid, whichextends distally to extend below the protoconid and as anindistinct ridge within the hypoflexid, but it is completelyabsent buccal to the hypoconid.

Discussion. Clemens (2006) reported Peradectes minor inmiddle and late Puercan, undifferentiated (Pu2/3) localitiesof eastern Montana and he also reported this taxon in NewMexico. We document the presence of P. minor in Pu2-3localities of the Nacimiento Formation, San Juan Basin,New Mexico. Most specimens are within the size rangeof teeth reported from the Tullock Formation of Montana(Clemens 2006, table 3), but they are closer to the uppersize range of that sample. Some teeth from the NacimientoFormation exceed the maximum observed size reportedfrom the Tullock Formation, but only by a small amountthat we consider to be taxonomically insignificant.

We observed one difference in tooth morphologybetween the Montana and New Mexico samples ofPeradectes minor related to the relative sizes of the m4entoconid and hypoconulid. Clemens (2006) reported thatthe entoconid is taller than the hypoconid (we concludethat Clemens mistakenly substituted the word hypoconidfor hypoconulid in his description; Clemens 2006, p. 29)on m4 of P. minor of the Tullock Formation. In the singlem4 known for P. minor from the Nacimiento Formation,the entoconid is lower and smaller than the hypoconulid.For a number of species referred to Peradectes, the m4entoconid is larger than the hypoconulid. The presence ofan entoconid that is smaller than the hypoconulid on m4 isconsidered to be a synapomorphy of two peradectid genera,Didelphidectes and Armintodelphys (Korth 1994; 2008;Krishtalka & Stucky 1983b), and it is also present in at leastsome specimens of Swaindelphys encinensis (Williamson& Taylor 2011). Unfortunately, we were unable to gaugethe variability of this feature for P. minor of the NacimientoFormation. Therefore, we do not recognize the differencesin relative entoconid and hypoconulid sizes between theMontana and New Mexico specimens as firm ground fortaxonomic separation, at least at this time.

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Figure 8. Puercodelphys minor lower molars. A–C, NMMNH P-21807, right m1 in occlusal (A, stereopair), B, buccal, and C, lingualviews; D–F, NMMNH P-21804, left m2 or m3 in occlusal (D, stereopair), E, buccal, and F, lingual views; G–I, NMMNH P-47302, rightm2 or m3 in occlusal (G, stereopair), H, buccal, and I, lingual views; J–L, NMMNH P-47294, partial right dentary with incomplete m4in occlusal (J, stereopair), K, buccal, and L, lingual views.

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Incertae sedisGenus Nortedelphys Case et al., 2005

Nortedelphys jasoni comb. nov. (Storer, 1991)

1969 Alphadon marshi Lillegraven: 33, figs 14–16 (in part).1984 Alphadon sp. Johnston & Fox: 209, pl. 14, fig. 2.1989 Alphadon sp. Fox: 21, pl. 2, fig. 1.1991 Alphadon jasoni Storer: 356, fig. 7, tables 6, 7.1996b Alphadon jasoni Storer; Johanson: 163, pl. 9, figs

A–K, pl. 10, figs A–D, pl. 11, figs A–D7 (in part).1996b Alphadon cf. A. jasoni Storer; Johanson: 163, pl. 7,

fig c, pl. 12.2005 Nortedelphys intermedius Case et al.: 237, fig. 7.

Discussion. As discussed by Johanson (1996b), severalworkers had suggested that some specimens that had beenreferred to Alphadon marshi have a V-shaped centrocristaand therefore they are referable to a new taxon (Cifelli1990; Krishtalka & Stucky 1983a; Russell 1975, 1984).Storer (1991) named Alphadon jasoni based on isolatedteeth from the upper Cretaceous Frenchman Formation ofsouthern Saskatchewan. The upper molars of this taxonpossess a V-shaped centrocrista. Johanson (1996b) referrednumerous specimens that had been referred to Alphadonmarshi and Alphadon wilsoni, but possessed a V-shapedcentrocrista and differed in other upper molar features, toA. jasoni. Johanson (1996b) demonstrated that specimensfrom the Scollard Formation including UALP 2846, anassociated upper and lower dentition originally referred toA. marshi by Lillegraven (1969), were significantly largerthan A. jasoni and therefore they might represent a newtaxon. She referred these to A. sp. cf. A. jasoni.

Recently, Case et al. (2005) erected the genus Nortedel-phys and named three species: N. magnus, N. intermediusand N. minimus. Case et al. (2005) diagnosed the genusbased on a number of features, including the possession ofa ‘highly invasive’ centrocrista that approaches the stylarshelf. Case et al. (2005) did not note that the holotypeof N. magnus, UALP 2846, had previously been referredto A. sp. cf. A. jasoni by Johanson (1996b). In addition,at least one specimen that Johanson (1996b) referred toA. jasoni, UCMP 51385 (Johanson, 1996b, pl. 9, fig. A),was subsequently referred to Nortedelphys intermedius byCase et al. (2005) without referencing Johanson’s previousreferral.

Based on the identical morphology of the two taxa andoverlap of referred specimens, both possessing a suite ofuniquely combined characters including a V-shaped centro-crista, a cristid obliqua that intersects the distal trigonid wallbelow the protocristid notch, and a postmetaconule cristathat extends buccally past the lingual base of the meta-cone, and a nearly identical size range, we conclude that N.intermedius is a junior subjective synonym of A. jasoni andrecognize the new combination, N. jasoni (Storer 1991).The fact that the specimens originally referred to N. inter-

medius and ‘A.’ jasoni are from the same temporal range,and overlap in geographic range provides additional supportfor the proposed synonymy.

Discussion

Macroevolutionary implicationsOur phylogenetic analysis allows us to examine three impor-tant macroevolutionary questions: the number of metathe-rian lineages that crossed the K-Pg boundary, changes inNorth American metatherian diversity from the Late Creta-ceous to the Palaeocene, and the estimated origination agesfor various metatherian clades.

Previous examination of these questions has been limitedbut thought-provoking. Most earliest Palaeocene mammalslack possible ancestors or closely related sister groupsin Lancian local faunas of the northern Western Interior,and therefore likely represented immigrant taxa (Weil &Clemens 1998; Weil 1999; Clemens 2002). Clemens (2010)examined this question by focusing on mammalian faunascrossing the K-Pg boundary in the relatively geograph-ically restricted area of Garfield County, north-easternMontana. Based on this record, among latest CretaceousMetatheria, all 11 species were either locally extirpatedor became extinct at the boundary. In the same area, onlyone taxon, Thylacodon montanensis, is known from earli-est Palaeocene (Pu1) local faunas. Clemens (2010) tenta-tively classified T. montanensis ( = ‘Peradectes cf. P. pusil-lus’) as a resident taxon, provisionally assuming Alphadonand ‘Peradectes’ (here Thylacodon) to be sister taxa orhave an ancestor–descendant relationship. This provisionalassumption was largely based on results of a phyloge-netic analysis of Johanson (1996a) that showed the cladePeradectes plus Peratherium to be in a polytomy with cladesincluding several species of Alphadon and Turgidodon. Ouranalysis, however, does not support a close relationshipbetween Alphadon spp. and Thylacodon spp. or any Palaeo-gene metatherian taxa.

Boundary crossings and diversity changes across the K-Pg. Based on our analysis a minimum of four, a maximumof 13 and a median of 8.5 lineages crossed the K-Pg bound-ary. First, at least one boundary crossing is required leadingto the various species of Thylacodon and Swaindelphys.Second, the ‘intermediate’ position of the Cretaceous Ecto-centrocristus within the Herpetotheriidae clade indicates atleast one boundary crossing of this clade. Third, the well-supported clade consisting of Glasbius and Roberthoffstet-teria requires at least one lineage crossing of the boundary.Finally, read literally, at least one boundary crossing isrequired within ‘Peradectidae sensu lato’ leading from thepolytomy that includes Maastrichtidelphys and Pediomyi-dae to the numerous Palaeogene members of ‘Peradectidae’.

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Relationships within ‘Peradectidae’ sensu lato are poorlyresolved, and several taxa collapse into a basal polytomy,so it is possible that several boundary crossings occurred.

In contrast to the Cretaceous, the diversity of metatheri-ans in the Palaeogene is depauperate. There were at least18 metatherians present in North America during the LateCampanian and 20 during the Late Maastrichtian. However,there is only one metatherian taxon present in the earliestPalaeocene (Pu1) of North America (Thylacodon monta-nensis) and no more than seven taxa through the entire earlyPalaeocene. Both specimen-level diversity and consider-ation of phylogenetic lineages strongly demonstrate thatthere was a considerable extinction of metatherians acrossthe K-Pg boundary. Including calculations of lineage cross-ings, a minimum of 24, a maximum of 33 and a median of28.5 metatherian taxa were present at the end of the Creta-ceous resulting in a survival rate of 20, 39.4 and 29.8%,respectively.

Clemens (2010) noted that the holotypes of Nortedel-phys intermedius (here considered a junior synonym of N.jasoni, comb. nov.) and N. minimus are isolated molars fromthe Bug Creek Anthills locality and the Tedrow QuarryD (UCMP locality V 87072), respectively, localities thatpreserve time-averaged Lancian and Pu1 fossil assemblages(Lofgren 1995). Clemens (2010) suggested that these taxamight be of Pu1 age. We dismiss this suggestion and notethat both species are clearly Lancian because both taxa arefound in other Lancian faunas (Case et al. 2005) and havenot been identified in earliest Palaeocene faunas that prob-ably lack reworked Cretaceous fossils.

Clade originations and ghost lineages. Our analysisproposes a close relationship between Palaeogene metathe-rians and the Cretaceous Pediomyidae, a relationship thathas not previously been proposed. Unfortunately, this partof the tree is poorly resolved, but the topology indicatesthat there is at least one, and perhaps several (dependingon resolution of the large polytomy), ghost lineages within‘Peradectidae sensu lato’ that extend to at least the earlyCampanian. Approximately 10 million years separate theoldest pediomyid from the oldest member of the peradec-tid clade, meaning that at least one lengthy ghost lineageis required regardless of how the peradectid polytomy isresolved.

Two major clades that are almost entirely representedby Palaeogene taxa, Herpetotheriidae and ‘Peradectidaesensu lato’, almost certainly originated in the Late Creta-ceous. In each case, a single fragmentary taxon from theCretaceous constrains the minimum divergence date forthese clades and provides evidence for the presence ofthese groups in North America and/or Europe prior tothe K-Pg extinction. The lineage leading to Herpetotheri-idae falls into a basal polytomy with the Late CretaceousEctocentrocristus, whereas the lineage leading to ‘Peradec-tidae sensu lato’ is part of a polytomy that includes the

Late Cretaceous Maastrichtidelphys. The exact resolutionof these polytomies makes little difference to the largerpicture: either Herpetotheriidae/‘Peradectidae sensu lato’,or a slightly more inclusive clade, must have originatedbefore the Palaeogene and endured the K-Pg extinction ifour phylogenetic analysis is correct.

Based on our phylogenetic analysis, only two clades,one containing Glasbius and Roberthoffstetteria and theother containing Ectocentrocristus and other species ofHerpetotheriidae, show well-resolved sister-taxon relation-ships between Cretaceous and Palaeogene metatherian taxa.Ectocentrocristus is a rare taxon, represented with certaintyonly by the holotype, an isolated tooth, and is separated fromthe oldest Palaeogene herpetotheriid representative by ∼18Ma.

The Glasbius plus Roberthoffstetteria clade is the onlyclade of Cretaceous and Palaeogene metatherians withrepresentatives that closely bracket the K-Pg boundary.However, the representatives are found on different conti-nents. Glasbius has a wide geographic distribution in west-ern North America during the latest Cretaceous and isknown from specimens from Montana, Wyoming and NewMexico (Williamson & Weil 2008). Glasbius first appearsin Montana Lancian faunas coincident with an increase inglobal temperatures near the end of the Cretaceous (Wilson2005). Wilson (2005) suggested that Glasbius represented awarm-adapted species from southerly regions that expandedor shifted its geographic range northward during warmingconditions near the end of the Cretaceous. The relativegreater abundance of Glasbius in the more southerly SanJuan Basin, New Mexico during the Lancian is consistentwith this hypothesis. We suggest that the Glasbius plusRoberthoffstetteria clade is one representative of a poorlyknown clade of metatherians that resided in North America,but is rarely sampled in North American Lancian faunas.

Biogeography and sampling. In contrast with many of theclades posited by our study, the close relationship betweenGlasbius and Roberthoffstetteria is robustly supported andits origination estimate does not depend on the resolutionof a polytomy. This clade must also have arisen prior tothe Palaeogene and crossed the K-Pg boundary, but whatis particularly interesting is that the Palaeogene taxon isfound on a different continent than its Cretaceous counter-part, which emphasizes the importance of intercontinentalinterchange of mammals at or near the K-Pg boundary.However, sampling is a confounding issue. It is possiblethat Glasbius and Roberthoffstettaria, although recoveredas sister taxa here, are merely the only known examplesof more cosmopolitan clades or major subclades that arerestricted to North America and South America, respec-tively.

Cretaceous metatherians are unknown outside of Laura-sia, and the majority of sampled taxa are from NorthAmerica. In fact, only two taxa are known from Europe

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(Arcantiodelphys and Maastrichtidelphys) and only twonon-deltatheroidan metatherian taxa are known from Asia(Asiatherium and Sinodelphys). An unnamed marsupial wasreported from the Late Cretaceous of Madagascar (Krause2001) based on a fragmentary lower molar. However, itsidentification as a ‘marsupial’ was later questioned (Averi-anov et al. 2003) and we do not consider it further. Basedon a literal reading of the fossil record, North Americawas a centre of metatherian origins and diversificationduring the Cretaceous, and it was the ancestral area forPalaeocene lineages. However, sampling must be consid-ered. The dearth of Cretaceous European metatherians maybe an artefact of sampling, and single new discoveriescould falsify current hypotheses about metatherian evolu-tion. The recent discovery of Maastrichtidelphys from thelatest Cretaceous of the Netherlands, for instance, indi-cates some major Palaeocene clades may have originated inEurope. Even in North America, the fossil record is incom-plete. Nearly all North American metatherians are knownonly from western North America, from terrain that wasnext to the Western Interior Seaway. However, metatherianswere also present in the Late Campanian of eastern NorthAmerica, although they are comparatively rare (Grandstaffet al. 1992, 2000).

There is geological evidence for latest Cretaceous globalclimate shifts, and these may have affected metatherianevolution and biogeography. Wilson (2005) documentedchanges in the relative abundances of species, mean indi-vidual body size, and taxonomic composition in successivemammalian communities during the last 1.8 million years ofthe Cretaceous from the Hell Creek Formation of Montanathat he interpreted as ‘normal’ mammalian responses toclimate changes immediately preceding the K-Pg bound-ary. Late Maastrichtian warming, at approximately 66 Ma,is indicated by oxygen isotopes from marine benthic andplanktonic foraminifera at middle and high latitude drillingsites and from leaves from North Dakota (Wilf et al. 2003).This warming was accompanied by a regression in the LateMaastrichtian (Haq et al. 1987), which might have removedthe marine barriers between eastern and western NorthAmerica and possibly also between North America andEurope. If this occurred, metatherians may have dispersedwidely, at least across Laurasia, during the waning years ofthe Cretaceous. In an ideal situation with complete or near-complete fossil sampling, this could be tested by examiningthe phylogenetic relationships of latest Cretaceous metathe-rians, because cosmopolitan clades that include taxa fromEurope and eastern and western North America would beexpected if this hypothesis was true. However, this is diffi-cult because of the patchy fossil record of eastern NorthAmerica and Europe, although it should become possibleif sample sizes from these areas increase.

The biogeography of early Palaeogene metatherians,at least those from North America, is characterized byendemism. There is currently no evidence of intercon-

tinental dispersal of metatherians into or out of NorthAmerica between the first appearance of Thylacodon andPeradectes in earliest Palaeocene deposits of western NorthAmerica and the early Eocene. The taxonomic diversityof Palaeocene metatherians was relatively low during thatinterval, with the presence of only two to three generathrough an interval spanning nearly 10 million years. Addi-tional taxa were present as indicated by ghost lineages,but even counting these, Palaeogene metatherian diver-sity remains markedly lower than latest Cretaceous diver-sity. The early Eocene is marked by a modest increasein metatherian diversity coincident with the first appear-ance of derived herpetotheriids such as Copedelphys andmultiple species of Herpetotherium. Peradectids also expe-rience a modest increase in taxonomic and morphologi-cal diversity with the first appearance of Mimoperadectes,the largest Palaeogene metatherian, near the end of thePalaeocene (Woodburne et al. 2009). This early Eocene risein diversity coincides with the transient climate shift of thePETM, followed by the middle Eocene climate optimum(Woodburne et al. 2009), and was also accompanied byrampant intercontinental dispersal (Bowen et al. 2002).

Biostratigraphic implications. Despite uncertaintyconcerning the validity of Thylacodon, the first evolution-ary appearance of the genus Peradectes was used to definethe onset of Puercan time by Archibald et al. (1987) andthe Peradectes/Ectoconus Interval-Zone (Pu1). The genusProtungulatum replaced Peradectes as the first appearancedatum in Lofgren et al. (2004). However, Lofgren et al.(2004) listed Peradectes as a First Appearance Taxon forthe Puercan land-mammal ‘age’. Based on our taxonomicrevision, this should be modified so that Thylacodonis listed as a First Appearance Taxon for the Puercanland-mammal ‘age’. Peradectes, represented by P. minor,first appears in the middle and late Puercan of westernNorth America (Clemens 2006).

Conclusions

An inclusive species-level phylogenetic analysis of Creta-ceous and early Palaeogene metatherian taxa and newspecimens from North America has allowed us to betterresolve relationships among the oldest and most prim-itive metatherians and correct the taxonomy of severalproblematic taxa. The Cretaceous–Palaeogene metatherianphylogeny is well resolved but individual clades are gener-ally poorly supported. Both the phylogenetic topology andinformation from new specimens support the validity ofthe genus Thylacodon and justifies the recognition of anew species, T . montanensis, from the early Palaeoceneof Montana. Thylacodon is closely related to Swaindel-phys and Herpetotheriidae, which must have diverged bythe Late Cretaceous due to its close relationship with

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Ectocentrocristus. Pediomyidae and ‘Peradectidae sensulato’ are closely related and together comprise a diversemetatherian clade. Maastrichtidelphys, from the Late Creta-ceous of the Netherlands, is the oldest member of ‘Peradec-tidae sensu lato’, indicating a Cretaceous origin forthis lineage. Therefore, the clades Herpetotheriidae and‘Peradectidae sensu lato’, although represented almostcompletely by Palaeocene taxa, must have originated in theLate Cretaceous. The lineages leading to these clades areamong at least four clades that crossed the K-Pg boundary,a fraction (about 20%) of the number of lineages present inthe Late Cretaceous. This analysis, therefore, confirms thatthe K-Pg boundary marked a profound extinction event formetatherians and suggests that Palaeogene taxa arose fromat least four clades of Cretaceous species, all of which wererelatively minor or very rare components of known Creta-ceous mammalian faunas.

Acknowledgements

We express gratitude to Pat Hester and Sherri Landon ofthe Bureau of Land Management for providing permits andfield assistance. We also thank L. Becenti, J. Benally, G.Briggs, U. Denetclaw, C. Hughes, S. Libed, J. Meserve, W.Slade, K. T. Smith, K. S. Smith, K. Tremaine, W. Tsosie,S. Williams, R. T. Williamson and T. E. Williamson forfield and lab assistance. M. Spaulding provided advice forthe phylogenetic analysis and the use of TNT and R. Beckand C. Kammerer provided nomenclatural advice. Numer-ous individuals provided access to specimens includingJ. Galkin and J. Meng (AMNH); D. Bohaska (NMNH);F. Jenkins, Jr. and J. Cundiff (MCZ); A. Henrici and A.Tabrum (CMNH); W. Clemens and P. Holroyd (UCMP).We also thank W. Clemens for providing access to unpub-lished manscripts. D. Lofgren supplied specimens that formthe basis of the Goler Formation taxon. We are grateful toI. Horovitz and G. Wilson for their reviews and suggestionsto improve our manuscript. This research was supportedby NSF grants EAR 0207750 to TEW and EAR 0207732(EAR 0654096) to AW.

Supplementary material

Supplementary material is available online DOI:10.1080/14772019.2011.631592

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The phylogeny and evolution of Cretaceous-Paleogene metatherians: New cladistic analysis and...

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