The reptile assemblage from the Moenkopi Formation (Middle Triassic) of New Mexico

DOI: 10.1127/0077-7749/2009/0030 0077-7749/09/0030 $ 6.25© 2009 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart

N. Jb. Geol. Paläont. Abh. 2010, vol. 255/3, p. 345–369, Stuttgart, March 2010, published online 2009

The reptile assemblage from the Moenkopi Formation(Middle Triassic) of New Mexico

Rainer R. Schoch, Stuttgart, Sterling Nesbitt, New York, Johannes Müller, Berlin,Spencer G. Lucas, Albuquerque and Jürgen A. Boy, Mainz

With 8 figures

SCHOCH, R. R., NESBITT, S., MÜLLER, J., M., LUCAS, S. G. & BOY, J. A. (2010): The reptileassemblage from the Moenkopi Formation (Middle Triassic) of New Mexico. – N. Jb. Geol. Paläont.Abh., 255: 345–369; Stuttgart.

Abstract: Focused collecting and excavation in the Moenkopi Formation (Anton Chico Member)of north-central New Mexico yielded a large quantity of tetrapod bones. Most of the finds werecollected from intraformational conglomerates, and consist of isolated bones or bone fragments. Themost abundant large members of the assemblage, the archosaurs, include at least three taxa: (1) aprimitive suchian or archosauriform, (2) a primitive poposauroid (Arizonasaurus babbitti), and (3) ataxon similar to shuvosaurids. Less abundant remains are tentatively referred to archosauromorphswith rhynchosaur affinities. An analysis of the tetrapod Lagerstätten reveals that primary depositsformed in lakes that were located on floodplains. In these lakes, autochthonous conchostracans,actinopterygians, coelacanths, and temnospondyls were evidently preserved. Fluvial reworking oflacustrine deposits resulted in a secondary deposition of bones, teeth, coprolites, and wood inchannel-borne conglomerates. However, the large amount of elements from terrestrial tetrapodsindicates that these conglomerates acquired bones from additional primary deposits (?channels,paleosols?) that are still unknown.

Key words: Reptiles, diversity, faunal assemblages, preservation, Triassic, U.S.A.

1. Introduction

The Moenkopi Formation is a widespread and diverseLower to Middle Triassic sequence of sedimentsthat formed under epicontinental, shallow marine toterrestrial conditions (MCKEE 1953; STEWART et al.1972). It is characterized by purple to reddish-brownsandstones, siltstones, and mudstones across most ofits outcrop area, which spans eastern California,Nevada, Utah, Arizona, Colorado, Wyoming, and NewMexico. Superficially similar to the Buntsandstein(Bunter) in Central Europe, and the Otter Sandstone inEngland, the Moenkopi has yielded vertebrate taxa

that have also been found in Europe. Among these,capitosauroid temnospondyls have been used as anargument to biochronologically correlate certainmembers of the Moenkopi Formation with theGerman Buntsandstein (WELLES & COSGRIFF 1965;LUCAS & MORALES 1985; LUCAS & SCHOCH 2002).

WELLES (1947, 1967) and PEABODY (1948) con-ducted the pioneering studies on the Moenkopi verte-brate assemblages, complemented by MCKEE’s (1954)detailed sedimentological and stratigraphic study ofthe formation. The most impressive fossils fromArizona were skulls of large temnospondyls, whichwere reported by WELLES (1967), WELLES & COSGRIFF

346 R. R. Schoch et al.

(1965), and MORALES (1987). The latter provided acomprehensive account of the Moenkopi fauna andflora in northern Arizona, which by that time hadproduced the richest and most diverse localities; seealso NESBITT (2001; 2005a; 2005b) and HECKERT etal. (2005) for updates on the vertebrate assemblage.Recently, the temnospondyl fauna has attractedadditional attention (WELLES 1993; SCHOCH 2000;WARREN & MARSICANO 2000).

Outside northern Arizona, the Moenkopi Formationhad not yielded many vertebrate fossils thus far;exceptions were an impressive but still undescribedtemnospondyl skull from the Torrey Member ofsouthern Utah (WELLES 1967) and some fragmentarymaterial discovered in north-central New Mexico(LUCAS & MORALES 1985; LUCAS & HUNT 1987). Thedeposits in New Mexico were later revisited by two ofus (SGL and RRS) in order to find more localities,supported by a local resident, ALEX VELAZQUEZ ofAnton Chico, who had found bones in several places.

Initially, we expected to find primarily temno -spondyls, which had been known since the 1940s from

the Moenkopi Formation of Arizona and of which afew finds had already been made in New Mexico at asite within our study area (LUCAS & MORALES 1985).It was therefore surprising to discover numerousbones of large reptiles, particularly archosaurs. Thiswas unexpected because remains of terrestrial tetra-pods are extremely rare in the Moenkopi of northernArizona, where only recent collecting has yieldedsome material, including a partial skeleton of thepseudosuchian Arizonasaurus (NESBITT 2003, 2005a)and a range of other very fragmentary remains (NES-BITT 2005b; NESBITT & ANGIELZYK 2002; NESBITT &WHATLEY 2004).

The project we report on here was initiated bySPENCER LUCAS and RAINER SCHOCH and subse -quently organized by JÜRGEN BOY, who receivedfunding by the DFG in 2000-2004. BOY et al. (2001)reported the first results on the stratigraphy of theAnton Chico Member of the Moenkopi Formation andits vertebrate assemblage in central New Mexico.After two additional collecting campaigns in the area,we describe the reptile assemblage in detail here. The

Fig. 1. Map showing outcrop areas in the region under study (east-central New Mexico, USA).

objectives are twofold: (1) provide a concise de -scription with useful illustrations of the identifiedspecimens of reptiles, and (2) describe the depo -sitional features of the vertebrate Lagerstätten, withfocus on the rich conglomerate localities.

2. Localities and stratigraphy

Most localities were discovered in the course of thisproject, such as the most productive area north ofColonias. These sites are mostly located within theAnton Chico and Preston Beck Land Grants, but insome cases encompass adjacent areas (Fig. 1). Thestratigraphy was described by BOY et al. (2001), whofound eight, vertebrate-bearing horizons, particularlyconglomerates (Fig. 2). The conglomerates are allintraformational and many yield reworked bonefragments, coprolites, and small pieces of petrifiedwood; teeth are much rarer and confined to localhorizons. The richest deposits were found at twolevels: (1) a thick and regionally widespread conglo-merate (C3), which is at least present in the AntonChico and Colonias regions, but probably extendssouth towards Santa Rosa and Fort Sumner (Fig. 1);(2) a single and locally restricted siltstone deposit,which yielded disarticulated skeletons. The main

localities are listed below, with abbreviations referringto larger groupings of localities.

AC: A large area south-west of Anton Chico. Thesource is a 1-2 m thick conglomerate (C3) thatpro duced numerous vertebrae and limb bones. Thepreservation is excellent, and even includes an articu-lated, small temnospondyl skull. The most commontaxon is Arizonasaurus. Rhynchosaurs are absent,whereas small diapsids are unusually common.

COL: The richest and largest locality by area,located along the northern rim of the junction betweenthe Gallinas and Pecos rivers, northwest of Colonias.The fossil bed is an up to 2 m thick conglomerate (C3)rich in large bones. Remarkable are the high quantityof bones and coprolites, the unusual number of largeand complete bones, and the excellent preservation.

DIL: South of Dilia. The only primary deposit inwhich skeletons of terrestrial tetrapods were found(NESBITT et al. 2006). Coprolites are abundant in thebeds that produced the archosauriform skeletons, andthere is a sequence of conchostracan-bearing bedsbelow the vertebrate-bearing horizon. This depositis unusual in that it is embedded in a thick sequenceof mudstones and siltstones (lacustrine deposit), asituation only observed in one other place in the New

The reptile assemblage from the Moenkopi Formation of New Mexico 347

Fig. 2. Standardized section of the Moenkopi Formation (Anton Chico Member) of east-central New Mexico (modidifiedfrom BOY et al. 2001).


Mexico Moenkopi, near the canyon west of the Mesade la Chamizalosa (south of Anton Chico) (RRS andJAB, personal observation 2000). However, the Diliasequence grades into typical Anton Chico facieslaterally, and there is no doubt that the locality fallswithin the Anton Chico Member of the MoenkopiFormation.

MES: North of the large table mountain Mesita delos Ladrones northwest of Tecolotito, a small redmesita has produced mostly small bone fragments anda piece of a procolophonoid jaw (NMMNH-P 55801).The source is a conglomerate (C2) near the base of theAnton Chico Member.

TEC: North of Tecolotito. This locality yieldsalmost exclusively one taxon (Eocyclotosauruswellesi), deposited in a small channel formed by silt-stones and fine-grained sandstones. Skulls and post-cranial elements are abundant and closely packed in achannel-fill in chaotic fashion. Remains of reptiles areconfined to teeth and a few postcranial elements ofsmall diapsids.

Puer to de Luna: This locality is very similar toColonias in yielding numerous large bones, and aproportionally large quantity of identified elements.However, the fossiliferous outcrop area is muchsmaller than the ones at Colonias and Anton Chico.The stratigraphic position of the conglomerate isunknown, but probably falls within the lower AntonChico Member.

3. Material

All material described here is housed in the New MexicoMuseum of Natural History and Science (NMMNH). Castsof some of the most interesting specimens are kept in theStaatliches Museum für Naturkunde Stuttgart (SMNS). Inthe course of this study, Moenkopi material from northernArizona was also examined for comparison; these speci-mens are housed in the Museum of Paleontology at theUniversity of California at Berkeley (UCMP) and the MesaSouthwest Museum (MSM).

The material described in this study falls into thefollowing clusters (all numbers are NMMNH-P).

1. Procolophonoid: 55801 (jaw fragment).

2. Small diapsids: 2 dorsal vertebrae (55671, 55789),humerus (55778).

3. Archosauromorphs: proximal portions of femora(55705, 55711, 55611, 55774); distal portions of femoratype A (55612); distal portions of femora type B (55705,55711); caudal vertebrae (55588, 55667, 55783).

4. Archosauriforms or primitive suchian archosaurs: 27269(two partial skeletons), maxilla (55712, 55717); largepostorbital (55687); cervical vertebrae (55646, 55723,55736); dorsal vertebra (55674); humerus (55790); ilium(55686); pubis (55779); ischium (55657, 55768).

5. Indeterminate pseudosuchians: small postorbital(55679); undetermined cranial material (55695, 55696);quadrates (55766, 55767); dorsal vertebrae (55571, 55606,55627, 55688, 55697, 55726, 55764, 55795); caudal verte-brae (55631, 55641, 55650, 55739, 55747); scapula(55648); humerus (55644); tibia (55762); fibula (55769);calcaneum A (55602); calcaneum B (55742).

6. Arizonasaurus babbitti: maxilla (55689, 55710); cervi-cal vertebrae (55577, 55593, 55630, 55698, 55719, 55728,55794); dorsal vertebrae (55606, 55688, 55697, 55739,55793-55795); femora (55592, 55633, 55714, 55737,55763, 55777); Ilia (55652, 55683-55685), pubes (55643,55651), ischia (55648, 55715).

7. Moenkopi poposauroid: cervical vertebrae (55660,55661, 55699); femora (55708, 55720, 55731); ilia (55732,55740); ischia (55744, 55771, 55791).

4. Descriptions

The exclusively isolated nature and fragmentary stateof the finds only permits an assignment to more in -clusive clades rather than species-level taxa. Becauseof the absence of articulated skeletons in the con -glomerate deposits, elsewhere in the Moenkopi For-mation, or other early reptiles from the MiddleTriassic it is further unknown which particularelements pertain to the same taxon. Throughout thedescriptive part of the study, we have therefore strictlyavoided any inference about which of any two dif -ferent bones might stem from the same taxon. Weshall discuss the different alternatives of assignmentof single elements and taxonomic diversity in aseparate section (conclusions) in cases where synapo-

Fig. 3. Isolated elements of diapsid reptiles from the Anton Chico Formation. A-F: humerus (NMMNH 55778); A-B:ventral; C-D: dorsal; E-F: anterior view; G-H: femur, proximal head (NMMNH 55611); G: ventrolateral, H: posterior view;I-L: femur, distal end type A (NMMNH 55612); I-J: dorsal; K-L: ventral view; M-O: femur, distal end type B (NMMNH55705); M-N: ventral; O: posterior; P: anterior view.


Fig. 3 (Legend see p. 348)


morphies are available. The single case in which acloser assignment can be made is the recently de -scribed partial skeleton of Arizonasaurus babbitti(NESBITT 2003, 2005a) from the Holbrook Member ofthe Moenkopi Formation in northern Arizona. How -ever, even here we have only referred bones to thattaxon in unequivocal cases (e.g., cervical vertebrae,femur). Because of the many uncertainties, we haveconsequently not named any new taxon, althoughthere is substantial evidence of previously unknownspecies in the sample given the rarity of archosaursfrom the Middle Triassic. We believe that withoutadditional articulated material, especially good cranialremains, new taxa should not be named, and theexisting ones that are based on fragmentary materialshould be treated with great caution.

Archosauria is used here in the restricted, crown-group sense as defined by GAUTHIER (1986). Weemploy the clade names Pseudosuchia instead ofCrocodylotarsi or Crurotarsi (e.g., NESBITT 2005a),Suchia sensu KREBS (1974), and Poposauroideasensu WEINBAUM & HUNGERBÜHLER (2007). Whenspeaking of ‘rauisuchian’ suchians more primitivethan (meaning outside the clade of) poposauroids, weemploy the term ‘primitive suchians’. When using thisterm, we explicitly refer to the taxa Batrachotomus,Saurosuchus, and Fasolasuchus; among these, Batra-chotomus is most completely preserved, and becausethe complete material of that taxon was available tous, we refer to it as a standard in comparison. Further-more, we rank Stagonosuchus among the ‘primitivesuchians’ because of overall similarity and the plesio-morphic condition of most of its postcranial features.

Comparisons with other taxa are based on V. HUENE

(1911: Erythrosuchus, 1938a: Stenaulorhynchus,1938b: Stagonosuchus, 1942: Prestosuchus, Rauisu-chus), YOUNG (1964: Shansisuchus, Fenhosuchus),KREBS (1965: Ticinosuchus; 1975: pseudosuchians ingeneral), SILL (1974: Saurosuchus), BONAPARTE

(1981: Fasolasuchus), CHATTERJEE (1985: Postosu-chus kirkpatricki partim!), PARRISH (1992: Erythro -suchus, Garjainia), LONG & MURRY (1995: Trilo -phosaurus, Otischalkia, phytosaurs, Postosuchus kirk-

patricki, Poposaurus, Shuvosaurus), ALCOBER &PARRISH (1997: Sillosuchus), GOWER (1999: Batra-chotomus, 2000: ‘rauisuchians’, 2003: Erythro -suchus), GOWER & SCHOCH (in press: Batrachotomuspostcranium), ALCOBER (2000: Saurosuchus), GOWER

& SENNIKOV (2000: Garjainia), GEBAUER (2004:Stagonosuchus), NESBITT (2005a: Arizonasaurus,2007: Effigia), WEINBAUM & HUNGERBÜHLER (2007:Poposaurus), and personal examination of material indifferent collections (Protorosaurus, Amotosaurus,Tanystropheus, Stagonosuchus, Batrachotomus, Pre-stosuchus, Arizonasaurus, and Ticinosuchus).

4.1. Parareptiles

A detailed description and analysis of this single find(NMMNH-P 55801), which represents a procolo -phonoid jaw fragment with teeth, will be given else-where (MÜLLER et al., in preparation).

4.2. Small indeterminate reptiles

Relatively few small elements of indeterminatereptiles have been collected, probably because of bothsorting and some extent of collection bias.

Ver tebrae: Two isolated trunk vertebrae in the rangeof 0.5-3 cm length were collected in the conglo -merates. They are consistent with most Mesozoicdiapsids in general morphology and dimensions anddo not permit any further assignment.

Humerus: A single distal half of a small humerus(Fig. 3A-F) is consistent with the morphology inprotorosaurs, Prolacerta, and basal archosauromorphs(V. MEYER 1856; GOW 1974; WILD 1978).

4.3. Archosauromorphs

A small fraction of the sample can be assigned torather primitive archosauromorphs. The most com-mon limb element is the femur, readily identified byits diagnostic proximal and distal regions. Other

Fig. 4. Isolated elements of large archosauriform reptiles from the Anton Chico Formation. A-C: postorbital (NMMNH55687); A: medial; B: lateral; C: dorsal view; D-G: humerus (NMMNH 55790); D-E: ventral; F-G: medial; H-J: ilium(NMMNH 55686); H-I: lateral; J: anterior view; K-N: cervical vertebra (NMMNH 55646); K-L: posterior; M-N: lateralview.


remains (vertebrae) are hardly diagnostic, but appearmost similar to taxa falling within the archosauro-morph grade.

Proximal por t ion of femur (Fig. 3G-H): Thesample includes numerous fragments of proximalportions of the femora with pronounced internaltrochanters, all of which indicate the presence ofarchosauromorphs. Most of these finds stem fromanimals in the 1-2 m body length range, but they differsubstantially in the degree to which the internaltrochanter is offset from the femoral head area. Noneof these specimens allows any further assignment, buttheir resemblance to Tanystropheus, Trilophosaurusand small rhynchosaurs is noteworthy GREGORY 1945;LONG & MURRY 1995).

Distal por t ion of femur, Type A (Fig. 3I-L):Two specimens of different size preserve the distalends of archosauromorph femora. They closelyresemble femora of rhynchosaurs, particularly Stenau-lorhynchus (V. HUENE, 1938a). The dorsal side of thedistal end is faintly concave, whereas the ventral sideis strongly curved medially, with two pronouncedcondyles separated by a wide notch that continuesproximally into a depression. The lateral condyle isoffset from the lateral margin by a deep groove withwell-defined dorsal and ventral margins; the distal endof this groove is framed by small bosses. The distalend is an feature shared with rhynchosaurs, whereasthe proximal portion can only be assigned to arela tively large archosauromorph. The proximal endis much more robust than in protorosaurs and Pro -lacerta, with a markedly offset and very large internaltrochanter. However, the five specimens differ quiteconsiderably from one another, suggesting that morethan one archosauromorph taxon might be resent inthe assemblage.

Distal end of femur, Type B (Fig. 3M-P): Alarger femur, again represented by a distal end, differsfrom the smaller specimens in not having such amarkedly curved posterior side, having less pro -nounced condyles, and lacking the groove borderingthe lateral condyle. By analogy with rhynchosaurs, wepresume that larger specimens should have morepronounced condyles and a stronger curvature. Theclosest resemblance we could find is with the erythro-suchid Shansisuchus (YOUNG, 1964), but the Moen -kopi fossil differs clearly from Erythrosuchus (GOWER,2003) and pseudosuchians (GEBAUER 2004). Among

all the conglomerate finds, this specimen probablycomes nearest to the condition in erythrosuchid archo-sauriforms, but differs from all these taxa by itsrelatively small size.

NESBITT & WHATLEY (2004) reported rhynchosaurjaw fragments from the Holbrook Member of theMoenkopi Formation in northern Arizona, and on thismaterial they based the new taxon Ammorhynchusnavajoi. The New Mexico material does not includeany cranial elements diagnostic of rhynchosaurs. Thetwo distal femora of type A are quite similar toStenaulorhynchus but we are still reluctant to viewthis as unequivocal evidence for rhynchosaurs in theAnton Chico Member.

4.4. Large archosauriforms or ‘primitivesuchians’

The mostly isolated bones reported in this section allhave in common that they differ substantially fromArizonasaurus and the Moenkopi poposauroid. It istherefore safe to consider each of them as belonging toa third, much more basal taxon. However, as thenumber of taxa represented by this part of the materialis uncertain, and the morphology itself provides someconflicting evidence (e.g., non-archosaurian archo-sauriforms versus primitive suchians) when con -sidered one taxon, we shall list and describe thematerial separately.

Dil ia skeletons: Recently, NESBITT et al. (2006)reported incomplete remains of two partial, largearchosauriform skeletons that possibly pertain to asingle taxon (NMMNH-P 27269). These were foundin a lacustrine deposit near Dilia, in the upper partof the Anton Chico Member. The larger animal re -presented by one of the two partial skeletons musthave been huge (6-7 m body length), as assessed bythe size of the dorsal vertebrae and the limbs; bones ofskeletons with this size are only rarely found in theMoenkopi conglomerate deposits in New Mexico. TheDilia skeletons are too fragmentary to permit a well-founded diagnosis, so they were not referred to a newtaxon, but clearly represent a taxon not previouslyknown from the Moenkopi Formation. There are afew points to be added or focussed at with respect toNESBITT et al. (2006), which we list as follows. (1) Thepossession of an extensive set of osteoderms is quiteinteresting, as evidenced by more than 50 diverselyshaped dermal plates found with the skeletons. Theseplates are unusually thick and lack any transverse

curvature or arching; likewise, they have no keel oreminence. By that they differ not only from dorsalosteoderms of most pseudosuchians (including phyto-saurs), but also from those non-archosaurian archo-sauriforms known to have possessed paramedian

osteoderms (e.g., Euparkeria). A further feature of theDilia taxon is the presence of overlap areas betweenplates. (2) The ilium is equally consistent with ery -throsuchids, Euparkeria, phytosaurs, and primitivesuchians. (3) The cervical vertebrae are short unlike


Fig. 5. Isolated maxillae of large archosauriforms and archosaurs from the Anton Chico Formation. A-E: large specimen 1(NMMNH 55712); A, D: lateral; B, E: medial view; C: anterior; F-G, I-J: medium-sized specimen, probably Arizonasaurusbabbitti (NMMNH 55689); H: large specimen 2 (NMMNH 55717).


Tanystropheus, but not as anteroposteriorly compres-sed as in Shansisuchus (YOUNG, 1964); their generalmorphology is more similar to Batrachotomus andphytosaurs). (4) The teeth that were found with theskeleton do not have serrated edges; in fact, they areoval to almost circular in cross-section; by that theydiffer from erythrosuchids as well as most pseudo -suchians, although these teeth may not belong to thesame taxon. In sum, there is no positive evidence(apomorphies) that these finds stem from erythro -suchids, whereas some features are more consistentwith pseudosuchians than with non-archosaurianarchosauriforms. We shall refer to this taxon as the‘Dilia archosauriform’ because this term includes bothalternatives (erythrosuchid, phytosaur or primitivesuchian-grade) and it is the least exclusive clade tobear the majority of the synapomorphies of the taxon.

Conglomerate bones: These finds were allderived from intraformational conglomerates thatprobably include bones from reworked lacustrinedeposits of the Dilia type (see discussion). Given theoccurrence of both Lagerstätten in the same for -mation, it is conceivable that remains of the large Diliaarchosauriform may also occur in the conglomeratesat Anton Chico and Colonias. Most of the non-popo-sauroid remains in the conglomerates are from ani-mals in the size range of Batrachotomus and Sauro -suchus, which spans some 4-5 m body length. Thetwo exceptions are a very impressive postorbital (Fig.4A-C) and an anterior portion of a maxilla representedby three pieces (Fig. 5A-E). These pertain to animalsthat were considerably larger than the aforementioned,approaching the size of the Dilia archosauriform.Although there are no contradictions between bonesfrom the conglomerates and the Dilia skeleton, thenumber of taxa represented by the material ultimatelyremains an open question.

Maxil la (Fig. 5A-E): We found three maxillary frag-ments in the same place at the Colonias deposit (twofragments still fit when put together: Fig. 4A-E,NMMNH 55712, the third one is isolated: Fig. 4H,NMMNH 55717). Although they do not fit along thebroken edges, they match precisely in size and generalmorphology. They belong to (a) specimen(s) similarin size to the largest Batrachotomus skull (50 cm)(GOWER 1999). Because of the missing overlap, it isimpossible to know for certain whether these fragmentsare from the same or different individuals, althoughtheir occurrence at the same locality suggests they are

from a single individual. These fragments differ fromArizonasaurus in the following features: (1) theanterior margin is rounded, as in Batrachotomus,but does not have an equally large, anteromedially-projecting palatal process like the latter and Arizona-saurus (but this make be due to weathering); (2) thereis an elongated, but shallow dorsal process on themedial side, clearly offset from the main body by agroove, producing a long suture with the premaxilla;(3) the anterior margin does not have a notch like inBatrachotomus; (4) the lateral flank bears a prominentand wide ridge below the dorsal process, and a uniquefeature (5) is the lack of any groove at the lateralmargin of the antorbital fenestra; instead, the baseof the ascending process is strongly concave on itsunusually broad posterior side. The maxillary frag-ments have in common that they are heavily ossifiedeven relative to their size, and especially the ascendingprocess is thicker than in most suchians, resemblingthe condition in Erythro suchus and Garjainia. Thereare two teeth preserved within their thecae, but theirmorphology is similar to that of the majority ofprimitive suchians and erythrosuchids.

Postorbi ta l (Fig. 4A-C): Two postorbitals of verydifferent size have been identified. They are generallysimilar in having a laterally convex dorsal orbitalmargin that forms a marked projection. In the largerspecimen, this projection is proportionally larger andmore rugose. While the postorbital of poposauroids(e.g., Arizonasaurus, Moenkopi poposauroid) arequite different, some suchians (Batrachotomus, Sauro-suchus, Postosuchus kirkpatricki) have similar post -orbitals. However, similar morphologies are also pre-sent in erythrosuchids (Erythrosuchus, Shansisuchus).Both specimens are fragmentary, lacking the ventralportion of the ventral (descending) process; the pro-cess is relatively broad and unlike Batrachotomus, andit does not have a recess on its posterior side. Becauseof its incompleteness, it is unknown whether thepostorbital bar was kinked (like in Batrachotomus)or straight. The larger Colonias specimen (NMMNH-P 55687) indicates a skull length beyond that of thelargest Batrachotomus skull.

Ver tebrae (Fig. 4K-N): A small sample of largevertebrae provides further evidence for the presence ofan archosaur taxon more basal than Arizonasaurus.These include three large cervical vertebrae and onesmaller dorsal vertebra. The largest cervical is similarto anterior cervicals of Shansisuchus (YOUNG, 1964)

in being anteroposteriorly short and having pro -nounced anterior and posterior margins. However,similar cervicals are present in Stagonosuchus, Batra-chotomus, and even Postosuchus kirkpatricki. Byanalogy with the latter, NMMNH-55646 could re -present either one of cervicals 3-5, as judged from thesize, position, and distance of the two rib facets (basedon Postosuchus kirkpatricki specimen TTUP 9235, seeLONG & MURRY 1995). A further characteristic of thisspecimen is its deeply concave flank region at thelevel of the diapophysis, and the lack of a ventralkeel; instead, the ventral side is concave. The second,slightly smaller cervical (NMMNH-55723) is weath -ered and does not preserve the precise morphology ofthe rib facets, but its flank is not as strongly concaveas in the last specimen. This probably represents aposterior cervical (position 7-8, by analogy withPostosuchus kirkpatricki), and it bears a well-definedkeel as do most cervicals of archosauriforms. Thethird, fragmentary cervical (NMMNH-55736) falls inbetween the former two in the morphology of its ribfacets. The dorsal vertebra is smaller but generallysimilar in surface texture and the heavy appearance; itdiffers from all other dorsals in the more massively-bulging margins. The described vertebrae are similarto but much smaller than those of the Dilia archo -sauriform (which agrees in the relative length of thecervicals, their proportions, their bulging margins andconcave flanks, and the morphology of the rib facets).

Humerus: A single proximal fragment of a largehumerus probably stems from a large non-archo -saurian archosauriform or primitive pseudosuchian,but is weathered and incomplete in the diagnosticregions. It differs from the other large humerus (seenext section) by having a deltopectoral crest that is notaligned perpendicular to the main axis of the proximalhead, but that is markedly flexed (Fig. 4D-G). Thismorphology is more similar to that of erythrosuchidsthan pseudosuchians, but the specimen is too frag-mentary to permit closer comparison. However, themassive and wide proximal heads of erythrosuchidhumeri look rather different than the present speci-men.

I l ium: A good portion of a large ilium from Colonias(NMMNH-P 55686, Fig. 4H-J) preserves a remark -ably plesiomorphic morphology: the acetabulum hasa pronounced dorsal margin, and there is no verticalridge dorsal to the supra-acetabular rim as with ‘raui-suchians’. Instead, the region adjacent to the dorsal

margin is smooth and even. The element is veryheavily built, forming a mostly planar and thick bone.There is no evidence of a transverse curvature as inBatrachotomus, as seen in dorsal view, but this maybe the result of imperfect preservation of the dorsalportion. The acetabular margin continues antero -ventrally, forming part of the anterior margin of theelement. In all these features, NMMNH-P 55686 ismore similar to erythrosuchids (especially Shansi -suchus) than to pseudosuchians. However, the in -complete dorsal portion of this bone does not permitfurther comparison, which would be required forconstraining a more precise taxonomic assignment.

Pubis : Two specimens of different size preserve thedistal region of the pubis (NMMNH-P 55779). Theyare generally similar to the pubic boot of Batracho -tomus, but more massive. This type of pubic bootdiffers from that of Arizonasaurus in being laterallymore expanded and having a more robust distal endthat is oval in posteroventral view. The lateral side isconcave and bears numerous pits and ridges.

Ischium: A single, fragmentary ischium differsclearly from that of Arizonasaurus and the Moenkopipoposauroid in having a triangular transverse sectionand not being fused ventrally (NMMNH 55657).Instead of meeting in a suture or ridge, the ischia arenot completely attached ventrally, a narrow slit isretained. A similar condition is preserved in Batracho-tomus, but not in poposauroids (LONG & MURRY

1995; WEINBAUM & HUNGERBÜHLER 2007).

Conclusion: We emphasize that most of these‘primitive’ archosaur remains, including some ofthe largest bone fragments, cannot be attributed to aparticular archosaur clade, but have features mostconsistent with either large non-archosaurian archo-sauriforms (stem-group archosaurs) or primitivesuchians. The only pseudosuchians that can be ruledout are aetosaurs, so we can at least rather safelyrestrict the assignment to suchians (for the pseudo -suchian alternative). Furthermore, the absence oferythrosuchid synapomorphies permits us only toconstrain the second alternative to non-erythrosuchidarchosauriforms.

It is very difficult to say which alternative is themore plausible, because there is some conflictingevidence as to which taxa the single bones from theconglomerates resemble most closely. For instance,the ilium and postorbitals are more similar to erythro-



suchids, whereas the maxilla, humerus, pubis, andcervical vertebrae agree more with primitive suchians.Interestingly, we found no diagnostic paramedianosteoderms or other osteoderms in the conglomeratedeposits, whereas the Dilia skeletons were associatedwith numerous osteoderms. On the other hand, there isno conflicting evidence between the Dilia and conglo-merate bones that would indicate the presence of morethan one taxon. The size difference between theskeletons and most of the conglomerate bones isreadily explained by sorting. However, the morpho -logy of the ilium and cervical vertebrae is not entirelyconsistent. The ilium from Colonias appears moreplesiomorphic in the following features: the supra-acetabular crest forms part of the anterior face of theilium, and the anterior margin is only gently curved(like in Shansisuchus) rather than forming a deepnotch between the pubic process and the anteriorprocess; apparently, the anterior process was short inthe Colonias ilium.

4.5. Indeterminate suchians

The elements reported in this section may either gowith the taxon or taxa mentioned in the last section(primitive suchians or non-archosaurian archo -sauriforms), or alternatively with poposauroids likeArizonasaurus. However, these bones are not pre -served in the associated Arizonasaurus material fromMoenkopi Formation of Arizona, and as these bodyregions are also poorly diagnostic, we have notattempted further assignment of these elements. Limbbones (the complete forelimb, the lower leg, thetarsus, and the foot), and caudal vertebrae are notpresent in the associated skeleton of Arizonasaurus(NESBITT 2003; 2005a).

Cranial e lements: A range of skull elements isavailable that are generally well-preserved, but tooincomplete to permit even an unequivocal deter -mination. This includes a piece of the paramedianseries (?frontals), and fragments of the orbit rim

(?frontal, ?postfrontal). A single small, but highlyinformative nasal fragment was found at Colonias.The nasal fragment bears a distinct rugose, lateralridge that is exclusively found in Postosuchus kirk -patricki kirkpatricki, Batrachotomus, and Rauisuchus.Furthermore, the rugose ridge bifurcates anteriorlyand a small narial fossa forms between the two ridges– this is clearly present in the nasal of Batrachotomus(GOWER, 1999).

Quadrate: Two ventral quadrate fragments of dif -ferent size are present (NMMNH 55766, 55767).These specimens cannot be assigned to any taxon withcertainty because they lack the dorsal portions of thebones.

Neural arches: Although the elongated neuralarches of the taxon are the most peculiar feature ofArizonasaurus, remains of these cannot not bepositively identified in the New Mexico Moenkopisample, although many fragments are consistent withportions of the tall neural spines in that taxon.Throughout the conglomerate finds, neural spines arepreserved as small pieces.

Scapula (Fig. 6L): A small fragment of a scapula,preserving only the glenoid region, is consistent withthe morphology in Arizonasaurus (NESBITT 2005a)and other suchians (Postosuchus kirkpatricki, Batra-chotomus).

Humerus (Fig. 6A-D): A well-preserved proximalhalf of a large humerus (NMMNH 55644) is almostidentical to that of Batrachotomus. It has a well-expanded and fairly symmetrical proximal head, arobust shaft, and a deltopectoral crest aligned per -pendicular to the long axis of the proximal humeruscondyle. In these features it differs not only markedlyfrom the humerus described above (indeterminatelarge archosauriform), but also, and more sub -stantially from the humerus of Postosuchus kirk -patricki, which is asymmetrical with a more slender

Fig. 6. Isolated postcranial elements of large suchian archosaurs from the Anton Chico Formation. A-D: large left humerus(NMMNH 55644); A-B: anterior, C: posterior, D: dorsal view; E-H: femur (NMMNH 55714); E-F: medial, G-H: lateralview; I-N: probably Arizonasaurus babbitti, I-J, M: ilium (NMMNH 55677, 85737), I-J: lateral, M: dorsal view;K, N (NMMNH 55681): proximal part of pubis, K: dorsal, N: ventral view; L: scapulaocoracoid in ventrolateral view(NMMNH 55648); O: cervical vertebra (above: lateral, below: ventral view) (NMMNH 55630); P: cervical vertebra, lateralview (NMMNH 55698); Q: dorsal vertebra (NMMNH 55697).


shaft (LONG & MURRY 1995). Given the close re -lationship between Arizonasaurus and Postosuchuskirkpatricki put forward by NESBITT (2003), we countthis as evidence against the possibility that thishumerus belongs to Arizonasaurus. Rather, it mightpertain to a taxon similar to Stagonosuchus and Batra-chotomus.

Femur (Fig. 6E-H): Proximal femoral heads attri -buted to Arizonasaurus (NESBITT 2005a) are verycommon in the conglomerates. This type of femur issimilar to the femora of Postosuchus kirkpatricki andBatrachotomus in general shape. The distal end hastwo prominent condyles merging into narrow bulges.The slender and thin-walled shaft is crushed in mostspecimens. In distal view, the distal end is gentlysemilunar, bearing a central depression of similarshape, surrounded by a continuous bulge. NESBITT

(2005a) reported femora documenting a tenfoldincrease in length (3-30 cm total length), and oursample has a similarly broad size range. This suggeststhat this taxon – presumably Arizonasaurus – was themost abundant archosaur preserved from the Moen -kopi Formation.

Tibia: Both proximal and distal portions of tibiae areamong the most frequent finds of limb bones in theconglomerates. The largest is a distal end, which musthave formed part of a tibia more than 50 cm long (incomparisons with a phytosaur); this would correlatewith an animal in the 5-6 m body length range (ascompared with dimensions in Batrachotomus). Mostother specimens fall in the same, much smaller sizerange, giving tibiae in the 20-25cm range.

Calcanea (Fig. 7): We identified three well-pre -served, but somewhat weathered specimens and onequestionable fragmentary find that may representdifferent types of calcanea. The two best-preservedspecimens are slender and generally similar to thoseof poposauroids (LONG & MURRY 1995). They seemto represent two different types. Type A (NMMNH-55602, Fig. 7A-D) bears a tuber that is not widerthan the central region of the element and is generallysimilar to the calcanum of Postosuchus kirkpa -tricki (LONG & MURRY 1995: Fig. 142A-E). Type B(NMMNH-55742; Fig. 7E-H) has a more prominenttuber with a large dorsal process and a less markedastra galus peg. As both specimens are somewhatweathered, it is possible but not very likely that theyrepresent two versions of the same calcaneum.

4.6. Arizonasaurus babbitti

Arizonasaurus babbitti was erected by WELLES (1947)for a fragmentary maxilla from the Holbrook Memberof the Moenkopi Formation in northern Arizona.NESBITT (2003; 2005a) reported the find of a partialarchosaur skeleton with a skull from another localitynear Holbrook that closely resembles the type of A.babbitti, and consequently he referred the find to thattaxon. Based on the skeleton, A. babbitti was dia -gnosed and described in detail by NESBITT (2005a),who showed its affinities with poposauroids, parti -cularly the sail-bearing ctenosauriscids, which werepresent in various regions across Pangaea (ZHANG

1975; KREBS 1969; GALTON & WALKER 1996;ALCOBER & PARRISH 1997; EBEL et al. 1998).

The large New Mexico sample includes numerousbones from many body regions that are consistent withthe Arizonasaurus skeleton described by NESBITT

(2005a). We shall concentrate exclusively on bonesthat are definitely known from that associated ske -leton, and will focus on features that are preserved inmore detail or that have not been mentioned before.There are many additional bones from body regionsnot preserved in NESBITT’s (2005a) material, that arelikely to also stem from Arizonasaurus. However, thepresence of a second poposauroid with shuvosauridaffinities makes it difficult to refer these elements toArizonasaurus at present.

Maxil lae (Fig. 5F-G, I-K):Two maxillae of differentsize are consistent with WELLES’ (1947) and NESBITT’s(2005) descriptions of Arizonasaurus. They representabout the same portions of the bone, comprising abouthalf of the element each. This includes the base of theascending process and the largest, central part of thetooth arcade. The most diagnostic feature of Arizona-saurus (NESBITT 2003, 2005a) is preserved in both,albeit it is much more pronounced in the larger speci-men: the antorbital fossa ends in a marked concavityat the posterior face of the ascending process; thisgroove is not visible in lateral or medial views. Unlikein the large maxilla described above, the antorbitalfossa is very clearly defined and offset from the lateralside of the maxilla. The element was described indetail by NESBITT (2005a).

Ver tebrae (Fig. 6O-Q): BOY et al. (2001) suggestedthat elongated cervical vertebrae indicated the pre -sence of prolacertiforms or protorosaurs. Althoughgenerally similar to Protorosaurus (V. MEYER, 1856)and Prolacerta (GOW, 1974), these elements turned

out to be exclusively referable to Arizonasaurus, afterNESBITT’s (2003) find of a partial skeleton. This high-lights the need for cautious examination of elongatedcervical vertebrae throughout Archosauromorpha.Trunk vertebrae that probably stem from Arizona -saurus were found in larger quantities (Fig. 6Q).

I l ium (Fig. 6I-J, L-M):. The ilium of Arizonasauruswas described in detail by NESBITT (2005a), and ourfragmentary material adds no new information. Theanterior portion, including the pubic and anteriorprocesses, is lacking in both specimens. The posterioriliac wing is also incomplete, but aligned in straightfashion, by which it differs from Batrachotomus. As inother poposauroids and Postosuchus kirkpatricki, thesupra-acetabular rim terminates slightly posterior tothe supra-acetabular buttress, which gives the aceta -bulum an ill-defined posterior margin as comparedwith Batrachotomus and Saurosuchus. As NESBITT

(2005a) emphasized, the ischium and pubis must havebeen well separated by a broad and concave ventralmargin of the ilium. The posterior iliac wing becomesposteriorly broader and is massive; isolated posteriorwings count among the most frequent finds ofArizonasaurus in the conglomerates. They can beeasily distinguished from other isolated, less massiveposterior iliac wings – the taxonomic affinities of thespecimens are unclear.

Ischium and pubis (Fig. 6K, N): Elements iden -tical to the ischium of Arizonasaurus have beenrepeatedly found at Colonias. The proximal region iscompletely fused and bears a narrow ventral ridgealong the midline. The pubis was medially fused, withtwo prominent proximal buttresses, and a markedventral keel. The distal end was apparently roundedand not expanded.


Fig. 7. Isolated tarsal elements of suchian archosaurs from the Anton Chico Formation. A-D: calcaneum type A (NMMNH55602), A-B: medial, C-D: proximal view; E-F: calcaneum type B (NMMNH 55742), E-F: medial, G-H: proximal view.


4.7. Moenkopi poposauroid

The material we report here adds important infor -mation about the enigmatic taxon that, like Arizona-saurus, provides evidence for pseudosuchian diversityby early Anisian time. NESBITT (2005b) reportedpeculiar poposauroid specimens collected by him innorthern Arizona, including several fragmentaryfemora and ilia. LUCAS et al. (2007) referred thismaterial to “Archosauria indet.” in a list of Moenkopivertebrates without any discussion, clarification, orobserving the material first hand. We agree withNESBITT (2005b) and we shall refer to this taxon asthe ‘Moenkopi poposauroid’, because it shows a mixof plesiomorphic poposaurid and shuvosaurid (sensuNESBITT 2007) features. Yet the structure of the iliumshares poposauroid synapomorphies (see below) thatpermit assignment at least to this larger clade if not amore specific clade within Poposauroidea.

In the small sample of tetrapods described byWELLES (1947), there was a peculiar femur with amarked depression on the medial side and a pro -nounced lateral tuber. NESBITT (2005b) described twomore femora and an associated specimen with twoilia and a sacrum in conglomerates of the HolbrookMember in northern Arizona. He tentatively referredthese finds to the “Chatterjeeidae”, a peculiar smallgroup of bipedal suchians formerly known only fromthe Norian of the United States. In the meantime, thediscovery of a new “chatterjeeid” in plaster jacketstaken from the famous Coelophysis Quarry at GhostRanch in northern New Mexico, Effigia okeeffeae, hasprompted a reconsideration of two other Norian taxa:with the complete anatomy of Effigia at hand, ‘Chat-terjeea’ (a partial postcranium) and Shuvosaurus (askull) turned out to most probably pertain to the sametaxon, which is to be referred to as Shuvosaurus.Shuvosauridae (sensu NESBITT 2007) would be theappropriate name for this clade.

Femora and ilia of the Moenkopi poposauroididentical to those reported by NESBITT (2005b) havebeen found in the Anton Chico Member, particularlyat the Colonias locality. These finds add informationto those of NESBITT (2005b) in two respects: some ofthem are much better preserved and more complete

than the material from Arizona, and the whole samplespans a broad range of sizes, suggesting the presenceof animals of different age classes. In addition, wefound several cervical vertebrae that closely resemblethose of Effigia and Shuvosaurus, and thereforedescribe these remains with the other material of theMoenkopi poposauroid.

Cervical ver tebrae (Fig. 8M-N): Two neck verte-brae that closely resemble those of Effigia, Shuvo -saurus, and Sillosuchus were found at Colonias. Theyare very low and elongated, with the transversesections of the anterior and posterior ends circular.The centrum is markedly constricted at about mid-length, with the rib facets borne by a crest that is offsetfrom the centrum proper; this crest rises from thelateral wall of the centrum in its anterior third. Theneural arches and dorsal spine are not preserved. Thecentra have about the same length as in Arizona -saurus, but are distinctly lower, and they are notkeeled ventrally (Most of the cervicals of Shuvosaurusand Sillosuchus have a ventral keel on the anteriorportion of the ventral side). Similar vertebrae havebeen described and illustrated as Effigia (NESBITT,2007) and Sillosuchus (ALCOBER & PARRISH, 1997).

I l ium (Fig. G-K): Partial ilia were briefly describedby NESBITT (2005b) from northern Arizona. The findsfrom Colonias add to that study by providing muchbetter-preserved material and information on the sizerange. The ilium is the most characteristic element ofthe Moenkopi poposauroid in having the followingfeatures: (1) the element is very low in lateral view,especially ventral to the acetabular rim, (2) the pubicand ischiadic processes are bulbous and are separatedby a deep notch, (3) the supra-acetabular rim forms athin sheet with a gently sigmoidal cross-section inlateral view, (4) the supra-acetabular crest bifurcatesanteriorly, merging into the raised margin of theanterior process, (5) the anterior process and the pubicprocess are not clearly separated, (6) the supra-acetabular crest forms a pronounced “superior iliacbutress” (ALCOBER & PARRISH 1997) pointing antero-dorsally, there forming the highest elevation, (7)

Fig. 8. Isolated postcranial elements of poposauroid archosaurs from the Anton Chico Formation. A-F: femur (NMMNH55734), A-B: medial, C: anterior, D: posterior, E-F: lateral view; G-K: ilium (NMMNH 55716), G-H: dorsal, I-J: lateral,K: anterior view; L: ischium, ventral view (NMMNH 55791); M-N: cervical vertebra (NMMNH 55699), M: ventral,N lateral view.


there is a convex dorsomedial process medial to thesuperior iliac buttress. The general morphology issimilar to the ilium of Sillosuchus, Shuvosaurus, andEffigia, and there is no danger of confusing this typewith the ilia of other poposauroids or suchians ingeneral. The anterodorsal (= preacetabular) process ofthe Moenkopi poposauroid could be homologous tothe large, dorsally-extended crest or blade of Effigia,Shuvosaurus, and Sillosuchus (NESBITT 2005).

Ischium (Fig. 8L): Two fragments of fused ischiapreserve the central region of the shaft, revealingthat the elements were excessively robust and welldifferentiated. Quite different from most suchians, thecompletely fused ischia form a massive bone with acomplicated, basically quadrangular cross-section.The ventral side bears paired paramedian ridgesseparated by a central groove, and the larger fragmentpreserves a section where the groove becomes widerproximally, suggesting the ridges diverged to runtowards the iliadic facets. The dorsal surface bears araised, wide ridge created by the combination of theleft and right ischium. This raised ridge is exclusivelypresent in Effigia and Shuvosaurus (NESBITT 2007)and not in Sillosuchus.

Femur (Fig. 8A-F): WELLES (1947) and NESBITT

(2005b) reported characteristic femora from Arizonathat have an anteromedial depression and a pro -nounced lateral tuber. New finds from Anton Chico,Colonias, and Puerto de Luna span a large size range(10-30 cm). These femora are very distinct from thoseattributed to Arizonasaurus in having a kinked pro -ximal region and a slender proximal head. Also con-spicuous is a large, oval depression emplaced withinan anteromedial excursion of the shaft (part of theattachment of the m. caudifemoralis). In larger speci-mens this feature is more pronounced. The lateral sideof the proximal region bears a small, well-definedtuber that was already noticed in a specimen from theHolbrook quarry (WELLES 1947); the proximal lateraltuber is absent in Shuvosaurus and Effigia but is pre-sent in Sillosuchus. There are no unequivocal distalends of the femora, at least there are no such elementsin the sample that would sufficiently differ from theputative Arizonasaurus femora to indicate thembelonging to a different taxon.

Dorsal ver tebra: A very unusual find is a dorsalvertebral centrum with fragments of a neural arch thathas an extremely narrow ventral portion. In transverse

view, this gives an almost triangular cross-section,with the pointed end facing ventrally. The flanks aredeeply concave. We consider this dorsal centrum toprobably belong to the same taxon.

Conclusion: We suggest that the material describedin this section – with the exception of the enigmaticdorsal vertebra – probably belongs to one taxon,which we have referred to as the Moenkopi popo -sauroid. The finds reported by NESBITT (2005b) areconsistent with our material, and there is no need toassume the presence of two such taxa. It is certain thatthe cervical vertebrae, ilia, ischia, and pubes all do notstem from Arizonasaurus, because these elements areknown from the associated skeleton and look all verydifferent. Furthermore, each of the different elementsshares derived characters known from poposauroids,some of which occur only in the more inclusive cladeformed by Sillosuchus, Shuvosaurus, and Effigia(NESBITT 2007). These are: (1) the low ilium with thesheet-like acetabular rim, (2) the massive ischiumwith the ventral ridges, (3) the articulation of the pubiswith the ilium, and (4) the long and low cervical verte-brae. All these features are only found together, so weregard it as justified to assign the four listed elementsto the same putative taxon. The femur, which forms anabundant element, is similar to that of Shuvosaurus inthe possession of a pronounced lateral tuber and adeep anteromedial depression located on a convexprojection on the anterior side of the element.

In conclusion, we agree with NESBITT (2005b) thatthe Moenkopi poposauroid is a close relative of theshuvosaurids (a still undefined clade, here understoodto include Sillosuchus, Shuvosaurus, and Effigia).However, as most of the skeleton is still unknown andthe ultimate proof that the described elements reallypertain to the same taxon, we do not consider it ashuvosaurid sensu stricto. Unfortunately, among themany cranial fragments that we collected, there are nobones similar to those of shuvosaurids.

5. Discussion

5.1. The vertebrate assemblage

The vertebrate fauna of the Anton Chico Member, aspreserved in the conglomerates, certainly involves amix of aquatic and terrestrial faunal components. Thetemnospondyls, coelacanths, and probably the bulk ofthe coprolites are derived from deposits of lakes orother water bodies in which the aquatic vertebrateslived. The terrestrial taxa, some of which we know of

very little, are probably also derived from diverseprimary deposits (see below). Therefore, it would beinaccurate to speak of a single vertebrate “fauna,”when some of the taxa evidently inhabited differentenvironments. Even the terrestrial fauna might con -stitute a taphonomically-produced mixture of taxafrom different environments, because the conglo -merates might well have “collected” bones fromdifferent terrestrial deposits as well.

Composi t ion of the ter restr ia l assemblage.– Before we provide a minimal list of vertebrates inthe Anton Chico Member, we wish to stress that suchreasoning, albeit parsimonious, may be misleading.Experience with other deposits (e.g., recent collectingin the Lower Keuper of Germany) revealed an un -expectedly high number of reptiles, especially archo-saurs (SCHOCH 2002, 2006).

A minimalistic approach to the reptile assemblagegives the following list of diapsids that were definitelypresent in the Anton Chico Member of New Mexico:

(1) an archosauromorph with some rhynchosaurfeatures (1 m): similar in size and morphology toStenaulorhynchus;

(2) a primitive suchian, in the preserved regions mostsimilar to the ‘rauisuchians’ Stagonosuchus, Batra-chotomus, and Saurosuchus (5-6m). This suchian isprobably identical to the archosauriform from theDilia lake deposit (NESBITT et al. 2006), but there areno shared derived characters preserved to supportthat;

(3) the poposauroid Arizonasaurus babbitti (2-4 msize range preserved);

(4) a more derived taxon (Moenkopi poposauroid)that has some shuvosaurid features (2-3 m size rangepreserved).

In contrast to our former suggestion (BOY et al.2001) and to NESBITT’s (2001) list of the Arizonafauna, we could not identify any diagnostic pro -lacertiform/protorosaurian elements. These specimensare not diagnostic to any “prolacertiforms” given theprobable paraphyly of the group. However, the variousmorphotypes of archosauromorph femora we identi-fied might well include those of “prolacertiforms.”Small diapsids were probably more frequent in theprimary deposits, but are clearly less common than allaforementioned taxa; this may be equally likely theresult of sorting or simply a sampling bias.

Relat ive frequency. – Among the identified ele-ments, remains of Arizonasaurus babbitti are parti -cularly frequent. Their abundance may have been evengreater, because numerous limb bones that may stemfrom that taxon have been found. The abundance ofArizonasaurus bones is similar to that of the Moen -kopi Formation of northern Arizona. However, theoccurrence of a more basal Batrachotomus-like suchi-an does not permit assignment of isolated limb bonesto any particular taxon. Remains of the Moenkopipoposauroid and the archosauriforms/primitivesuchians are about equally frequent, whereas identi-fied bones of archosauromorphs and small, non-archosaur diapsids are much rarer. From an ecologicalperspective, this almost certainly indicates a samplingbias, if not some kind of sorting by the events thatproduced the intraformational conglomerates. Sortingby weight and robustness must have played a majorrole, as revealed by the following observation. Arizo-nasaurus possessed some robust bones (femoral head,vertebrae, maxillae) that are very common in the con-glomerates, whereas the Moenkopi poposauroidappears not quite so abundant when the same elementsare counted. However, the ilium and ischium, whichare more robust in the Moenkopi poposauroid than inany other taxon, are frequent finds. Therefore, relativefrequencies of taxa (as assessed by single bones) areto be looked at with caution, and palaeoecologicalconclusions simply cannot be drawn from suchsamples.

Cal ibrat ion with t rack record. – The ichnologyof the Moenkopi Formation has been studied in greatdetail by PEABODY (1948) and was summarized byMORALES (1987). Now that a substantial terrestrial“fauna” has been found, it is interesting to take a closelook at the available footprint evidence. These aremostly derived from northern Arizona and Utah(PEABODY 1948), and from various stratigraphicalmembers that probably range from Olenekian throughlower Anisian ages (MORALES 1987). In the HolbrookMember, which is believed to be coeval with theAnton Chico Member of north-central New Mexico, adiverse terrestrial ichnofauna has been reported: (1)Rhynchosauroides indicates the presence of 0.5-1 mlong diapsids, probably archosauromorphs, (2) Chiro-therium barthii probably pertain to 4-6 m long archo-sauriforms and pseudosuchians (SOERGEL 1925), (3)bipedal chirotheriid tracks (C. sickleri) have also beenreported, which may have been produced by occa -sional or facultative bipeds (?Arizonasaurus) or



obligate bipeds (shuvosaurids), and (4) Therapsipushas been interpreted as dicynodont tracks (HUNT et al.1993; NESBITT & ANGIELCZYK 2002).

The calibration of fossil tracks and bones gives arelatively good match, with dicynodonts, archosauro-morphs, archosauriforms, and suchians representedin both “faunas”. In this particular case, the conglo-merate bone “faunas” are no more precise with respectto the number of taxa involved than the ichnofauna is;both permit only very gross assessments. The problemof bipedalism in suchians has only recently receivedmore attention after the far-reaching convergencesbetween shuvosaurids and theropods have been de -tected (NESBITT & NORELL 2006). Based on materialfrom the Upper Buntsandstein of southern Germany,EBEL et al. (1998) have argued that ctenosauriscids(e.g., Arizonasaurus) were facultative bipeds that mayhave produced tracks of the Chirotherium sickleri type(see also HADERER et al. 1995). As long as the pedalmorphology remains unknown in both Arizonasaurusand the Moenkopi poposauroid, it is impossible to testthis hypothesis at least for the upper MoenkopiFormation. A further, still more elusive question is theidentity of the Rotodactylus producer, for which thereis still no skeletal evidence available in North Americaor Europe.

5.2. Origin of the vertebrate deposits

Among the conglomerates studied in detail, thoseyielding rich bone samples appear to have formed inmeandering river channels (Anton Chico, Colonias,Tecolotito). We gathered this from analysis of thesurrounding beds and their architecture. The conglo-merates often grade into channel sandstones laterally,an observation we made at Anton Chico as well asColonias. These channels must have frequently erodedsilty overbank deposits, which are preserved invarious places (e.g., at Pecos river east of Dilia, northof Mesita de los Ladrones, west of Mesa de Chami -zalosa near Anton Chico). There, the channels ac -quired the intraformational components, and theabsence of exotic components indicates that transportdid not exceed larger distances. In some places, flood-plains housed lakes or residual ponds in which con-chostracans, small bivalves, and actinopterygianslived (Dilia, Tecolotito quarry). Sediments of suchlakes have been found in several places, but these arenot very common in the channel-dominated AntonChico Member, at least in the study area. Such lakeand pond deposits include fine silt- and mudstone

sequences which in certain horizons yield plant andvertebrate fossils. Because of limited outcrops, it isnot clear how large these lakes were and how longthey persisted, but the dominance of conchostracans(estherians) suggests that most of them were ephe -meral. Larger, more permanent water bodies wereprobably inhabited by the 2-3 m long temnospondylEocyclotosaurus and the 1-2 m long coelacanthMoenkopia. Such a long-lived lake is preserved southof Dilia, where the abundant remains of concho -stracans and vertebrates (coprolites) are preserved(NESBITT et al. 2006). Similar coprolites have beenfound reworked in the conglomerates, often bearingactinopterygian scales. Their size and shape suggestthey may stem from temnospondyls, by analogy withcoprolites in the Permian Rotliegend facies of CentralEurope, where such coprolites have been found inassociation with temnospondyls (BOY & SUES 2000).

Our general interpretation of the aforementionedobservations is as follows. Floodplain lake and ponddeposits with previously fossilized bones and copro -lites were eroded by channels. This resulted in a localenrichment of conglomeratic channel-fills with bones,coprolites, and fossil wood, among other intrafor -mational components. (Our prospecting field workand analysis of numerous conglomerate horizonsrevealed that highly fossiliferous conglomerates are infact rather exceptional cases, although they are muchmore common than primary deposits). The dominancein the conglomerates of temnospondyls over coela-canths might have been caused by sorting (coelacanthbones appear to have been less durable) as well as bydifferent ecological characteristics of these twogroups: whereas coelacanths were dependent onpermanent water bodies (which were probably rare),temnospondyls were capable of land excursions andcould eventually escape desiccated ponds. Capito -sauroid tracks found in the Moenkopi Formation ofnorthern Arizona confirm that (PEABODY 1948;MORALES 1987). Thus, we have gained a certainunderstanding of the composition of the vertebratefauna in stillwater bodies of the New Mexican Moen-kopi, although undoubtedly only the larger inhabitantsare preserved in the conglomerates, while smallactinopterygians are usually even disarticulated insmall pond deposits (Tecolotito quarry). However,these data permit a first comparison with deposits inArizona: there, the amphibian fauna was much richer,at least during upper Moenkopi (Holbrook) time,where two or more large temnospondyls have beenpreserved in lake deposits (WELLES 1947, 1967;

SCHOCH 2000). Probably the lakes in Arizona werelarger and deeper with a more differentiated fauna thatsuspended several large predators; this is confirmedby the occurrence of hybodont sharks and lungfishesin the Holbrook Member, of which we found no tracesin the Anton Chico Member (MORALES 1987;HECKERT et al. 2005). The conglomerates often havesmall limestone clasts in abundance, which are mostlikely rip-ups of calcrete nodules from floodplainpaleosols.

The different conglomerate localities vary withrespect to the overall number, composition, and size ofbones, coprolites, and teeth. From this we concludethat the reworked deposits were rather different fromeach other, possibly diverse small water bodies withendemic faunas. The only constant feature is temno -spondyl bones, which are abundant in all conglo -merate localities, sometimes comprising up to 50 % ofthe finds. However, the large quantity of reptile bonesin all conglomerates indicates that lake deposits maynot have been the exclusive source for the vertebratefossils within conglomerates. In fact, the large lakedeposit at Dilia also yielded the skeletons of a large?pseudosuchian archosaur, which confirms that rep -tiles were preserved in such deposits. Yet, the extentto which fossils of terrestrial tetrapods were present insuch sites is not known, and it is difficult to conceivewhy terrestrial reptiles should have been accumulatedin deposits of small lakes. Also conceivable asprimary localities of reptiles are river banks, flood-plains, fans, or a deposition of fresh bones in channelconglomerates. In the presumably hot climate, decayof carcasses was probably fast, and isolated freshbones were readily deposited in channels.

Our analysis of bone surfaces revealed that theywere not subject to conditions of long subaerial ex -posure. Indeed, cracks within the periosteal bonesurface are absent or very minor, according to the cri-teria of BEHRENSMEYER (1978) and SMITH (1993).From this we conclude that fresh bones were depo -sited either quickly within the sediment or came to lieunder water after a short time. Reworking of channel,fan, or floodplain sediments may thus have contri -buted a mix of fresh and fossilized material to theconglomerates. However, while this is a plausiblehypothesis, we found little evidence for bones that hadthe conglomerates as their primary deposit. Amongthese, a pair of associated caudal vertebrae of apseudosuchian (? Arizonasaurus) in the thick conglo-merate C3 at Colonias is the most convincing. Furtherevidence comes from bone fragments embedded in the

conglomerates with rugged edges, but these are rare.Instead, many have straight edges and are coveredwith a matrix different from that of the conglomerate,both of which indicates reworking of fossilized bonesthat had already been embedded in a cemented siltyor sandy matrix. In most cases, similar to the lakedeposits, reptile skeletons were probably not accu -mulated, as indicated by the very sporadic occurrenceof vertebrate fossils and even coprolites in the Dilialake horizons.

In conclusion, the rich occurrences of reptile bonesin conglomerates must have had numerous differentsources. The richer localities, such as Colonias, mighthave received their bones from highly fossiliferousdeposits in close proximity, which is indicated bycomplete skulls of Eocyclotosaurus and large skullelements of the coelacanth Moenkopia co-occuringwith numerous Arizonasaurus bones. At Colonias, theprimary deposit may still be characterized to someextent because the original sedimentary matrix isoften preserved with larger bone fragments. Sur -prisingly, this proves that the same primary depositsincluded the aquatic and terrestrial taxa. Instead ofmudstones, which were to be expected if the primarydeposits had been larger lakes, the matrix mostlyconsists of fine-grained sandstones and siltstones,quite similar to the channel-fill deposit at the Teco -lotito quarry, which contained numerous remains ofEocyclotosaurus at one place. Therefore, we interpretthe Colonias localities as involving reworked channel-fill deposits in which large bones and skulls of manydifferent vertebrates had been accumulated. (We ex -cavated a similar situation in a quarry near Tecolotito,which yielded Eocyclotosaurus and small diapsids).We cannot rule out that some faunal components atColonias were derived from lake deposits, but wefound no evidence for that either.

However, the situations we studied are still morecomplicated upon closer inspection. Some of thethickest conglomerate sequences (1-3 m) have beenfound to bear the most diverse tetrapod faunas andalso contain the largest and best-preserved bones(Anton Chico, Colonias, south of Santa Rosa, Puertode Luna). At Tecolotito, Anton Chico, and Colonias,we had the opportunity to study these sequences ingreat detail. We found that bones were confined tovery well-defined horizons that were only locallyenriched in bones, with each of the fossiliferous bedsseparated from one another. Each of these conglo -merates has a different composition in terms ofrelative quantities of coprolites, temnospondyls, and



terrestrial tetrapods. In addition, they differ in therelative quantities of poorly preserved (= transported)and well preserved bones. These findings indicatethat: (1) the bones were gathered in separate events,(2) probably from different primary deposits, and (3)with different transport distances. This indicates thatfor the richest conglomerates, the primary sources ofbones were probably diverse.

Teeth are generally very rare in the conglomerates,probably because they were readily broken in trans-port; at Colonias, the few finds of teeth show straightedges at their bases, suggesting that they were em -bedded in a pre-fossilized condition. (A single conglo-merate locality yielded larger quantities of small dia -psid teeth, which probably formed by reworking anearby primary locality). In addition, in the thick con-glomerates, there was evidently no sorting betweencylindrical and planar elements. The only unexpectedpattern is the absence of archosaur osteoderms, whichshould be present, given that at least one largearchosauriform or pseudosuchian is known to havepossessed such osteoderms (see the Dilia skeletons,NESBITT et al. 2006). In places, especially at Colonias,temnospondyl and coelacanth bones are markedlyaffected by transport (stronger fragmentation, roundededges, eroded bone surfaces), while archosaur bonesare often preserved in a better state. However, we lackquantitative data to support this preliminary pattern.The thinner, locally restricted conglomerates are oftenenriched in pedogenic pebbles and small, heavilyworn bone fragments that are difficult to identify(Mesita, Tecolotito). Occasionally, such conglome -rates are enriched in teeth (Tecolotito). This indicatesthat some vertebrate deposits also formed in orbetween pedogenic horizons, a situation also knownfrom the Upper Buntsandstein of Germany (ORTLAM

1967). What remains unclear is how accumulations of

terrestrial tetrapods, such as the 3-4 m long carnivoreArizonasaurus, could have formed in the first place,considering the facts we know of the Moenkopi en -vironment. Conceivably, their formation might haveinvolved predation on carcasses of aquatic animals,which in this arid, probably seasonal climate couldhave suffered from droughts. But unless such a case ispreserved, this explanation remains highly speculativeand somewhat simplistic. In addition, except forArizonasaurus and the enigmatic large pseudo -suchian, we know next to nothing of the feeding habitsof all the other terrestrial taxa. At this point we need tocome back to the problem of assessing the signi -

ficance of the terrestrial fauna preserved in the Moen-kopi Formation. This problem exists in New Mexicoas well as in northern Arizona, where in recent timesimilar faunal compositions have been reported(NESBITT 2001). When collecting in the conglo -merates, we noticed that at first sight, large terrestrialcarnivores seem to dominate. Apart from the temno -spondyls, most of the terrestrial taxa pertained to‘rauisuchians’ of some sort. However, our analysis ofbone quantities and the actually identified fraction ofthis (which is unfortunately less than 10 %) tell adifferent story. In fact, the large pseudosuchianscomprise one of the rarest elements, similar to thearchosauromorphs. They might represent a probablecarnivore (possibly a basal pseudosuchian) and aherbivore (e.g., a rhynchosaur, as concluded from thedistal femora). Dicynodonts are extremely difficult totrace, and we cannot confirm any diagnostic find fromthe New Mexican Moenkopi. The Moenkopi poposau-roid is puzzling: its closest relatives were beakededentuluous bipeds that might well have been herbi -vores (NESBITT 2005b). The relative abundance of thattaxon and the rarity of rhynchosaurs thus must not beinterpreted as evidence of a sorting bias at the expenseof herbivores; if the Moenkopi poposauroid wasindeed a herbivore, it could simply mean that it wasthe dominant plant eater in that environment. How -ever, as this is an open question, we simply cannotmake any further assessment of the ecological situ -ation of New Mexico during Moenkopi time. Thelogical next step in order to get a better understandingof the actual faunas needs to be the discovery ofprimary deposits in the Moenkopi, preferably lakehorizons. Unfortunately, our search for such depositsin several larger areas (Anton Chico and Preston BeckLand Grants, Santa Rosa region, Puerto de Lunaregion, Fort Sumner) yielded mostly negative results.

Acknowledgements

The late ALEX VELAZQUEZ of Anton Chico first discoveredsome of the localities on the Anton Chico Land Grant anddonated the bones to the NMMNH. The authorities of theLand Grant were very helpful in permitting access to thearea. We thank NORBERT ADORF, MARKUS POSCHMANN,MICHAEL FASTNACHT and PETE RESER for their engagementin field work, and NORBERT ADORF, ACHIM LEHMKUHL andMICHAEL MAUS for preparation. The German team is thank-ful to the New Mexico Museum of Natural History andScience for support, providing a working place, and forloaning the complete material over a period of 3-7 years.The senior author thanks STERLING NESBITT for showinghim his material from northern Arizona in 2001 (now at

MSM) and KEVIN PADIAN and PAT HOLROYD for making theMoenkopi material at UCMP accessible, and he is alsoindebted to SAMUEL WELLES for valuable information andinviting him to work on the Moenkopi temnospondyls fromArizona in 1995. DIETER SEEGIS was helpful in various dis-cussions on vertebrate deposits. The Deutsche Forschungs-gemeinschaft is thanked for funding this project by DFGgrant BO-553/11-2. The senior author has much benefitedfrom meetings with DAVID GOWER, FRANK HADERER,HARTMUT HAUBOLD, RANDALL IRMIS, ROBERT REISZ, andJONATHAN WEINBAUM. We finally thank HANS SUES andan ano nymous reviewer for their helpful comments, andGÜNTER SCHWEIGERT for advice.

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Manuscript received: November 5th, 2008.Revised version accepted by the Stuttgart editor: January16th, 2009

Addresses of the authors:

Dr. RAINER SCHOCH, Staatliches Museum für Naturkunde,Rosenstein 1, D-70191 Stuttgart, Germany;e-mail: [email protected]

Dr. STERLING NESBITT, Lamont-Doherty Earth Observatory,Columbia University, 61 Rt. 9W Palisades, NewYork 10964,USA.

Dr. JOHANNES MÜLLER, Museum für Naturkunde, Leibniz-Institut für Evolutions- und Diversitätsforschung an derHumboldt-Universität zu Berlin, Invalidenstraße 43,D-10015 Berlin, Germany.

Dr. SPENCER LUCAS, New Mexico Museum of NaturalHistory and Science, 1801 Mountain Road NW,Albuquerque, 87104 NM, U.S.A.

Prof. Dr. JÜRGEN BOY, Institut für Geowissenschaften,Becherweg 10, D-55099 Mainz, Germany.


The reptile assemblage from the Moenkopi Formation (Middle Triassic) of New Mexico

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