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Differences in the tooth eruption sequence in Hyaenodon ('Creodonta':Mammalia) and implications for the systematics of the genusKatharina Bastla; Michael Morlob; Doris Nagela; Elmar Heizmannc

a Department of Paleontology, Vienna University, Vienna, Austria b Forschungsinstitut Senckenberg,Frankfurt am Main, Germany c Staatliches Museum für Naturkunde, Stuttgart, Germany

Online publication date: 08 February 2011

To cite this Article Bastl, Katharina , Morlo, Michael , Nagel, Doris and Heizmann, Elmar(2011) 'Differences in the tootheruption sequence in Hyaenodon ('Creodonta': Mammalia) and implications for the systematics of the genus', Journal ofVertebrate Paleontology, 31: 1, 181 — 192To link to this Article: DOI: 10.1080/02724634.2011.540052URL: http://dx.doi.org/10.1080/02724634.2011.540052

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Journal of Vertebrate Paleontology 31(1):181–192, January 2011© 2011 by the Society of Vertebrate Paleontology

ARTICLE

DIFFERENCES IN THE TOOTH ERUPTION SEQUENCE IN HYAENODON (‘CREODONTA’:MAMMALIA) AND IMPLICATIONS FOR THE SYSTEMATICS OF THE GENUS

KATHARINA BASTL,*,1 MICHAEL MORLO,2 DORIS NAGEL,1 and ELMAR HEIZMANN3

1Department of Paleontology, Vienna University, Althanstraße 14, A-1090 Vienna, Austria, katharina.bastl@inode.at;doris.nagel@univie.ac.at;

2Forschungsinstitut Senckenberg, Senckenberganlage 25, D-60325 Frankfurt am Main, Germany, Michael.Morlo@senckenberg.de;3Staatliches Museum fur Naturkunde, Rosenstein 1, D-70191 Stuttgart, Germany, heizmann@gmx.at

ABSTRACT—Specimens that give new information on the tooth eruption sequence in European Hyaenodon are described.These specimens show a marked difference from the tooth eruption sequence established for North American Hyaenodon.The eruption of p4 after p3 and m3 and the earlier eruption of the permanent canine in European specimens versus theeruption of p4 before p3 and m3 and the eruption of the permanent canine as the last tooth in North American specimensindicate divergent evolutionary pathways for New and Old World lineages. In conjunction with other skeletal differences,described in the literature, our findings support the need for a reanalysis of the taxonomy of Hyaenodon.

INTRODUCTION

Hyaenodon is the eponymic genus for the family Hyaenodon-tidae (‘Creodonta,’ Mammalia). Monophyly of ‘Creodonta’ isdoubted (Polly, 1996; Sole et al., 2009) and high-level system-atics are in flux (Morlo et al., 2009). Hyaenodon is one of thebest known creodonts due to its wide temporal and geographi-cal range (Tsubamoto et al., 2008): The genus is distributed inthe late middle Eocene to Oligocene of North America (Mellett,1977), in the late Eocene to Oligocene of Europe (Lange-Badre,1979), and in the late middle Eocene to early Miocene of Asia(Lange-Badre and Dashzeveg, 1989; Wang et al., 2005). Today,Hyaenodon comprises a large number of species (Table 1), in-cluding 11 European (Lange-Badre, 1979), 10 North American(Mellett, 1977; Gustafson, 1986), and 8 Asian (Wang et al., 2005;Morlo and Nagel, 2006; Tsubamoto et al., 2008) species. Mel-lett (1977) revised the North American species of Hyaenodon,whereas species of European Hyaenodon were revised by Lange-Badre (1979). Due to what is known regarding sexual dimor-phism and intra-specific size variation in North American (Mel-lett, 1977) and Asian (Morlo and Nagel, 2006; Tsubamoto et al.,2008) taxa, it is highly probable that there are fewer actual speciesthan have been proposed.

Origin and descent of the genus is not clear; however, Hyaen-odon appeared in North America suddenly, represented by twospecies (Mellett, 1977), supporting the hypothesis of immigrationof this taxon into the New World. As Hyaenodon arrived evenlater in Europe (Lange-Badre, 1979), we assume Asia to be themost likely center of the origin of Hyaenodon.

In the last few years, various authors have become inter-ested in tooth eruption sequences because of their potentialto provide useful phylogenetic signals (e.g., Morlo and Haber-setzer, 1999; Smith, 2000; Asher and Lehmann, 2008). Tootheruption sequences in Hyaenodon have already been studied byMellett (1977) for North American taxa, whereas Lange-Badre(1979) provided some information for European specimens. TheAsian material has not yet been studied. In this contribution,we re-assess the realm of juvenile Hyaenodon mandibles from

*Corresponding author.

Europe with the aims (1) to test the sequence given by Lange-Badre (1979) with hitherto unstudied material and (2) to comparethe resulting sequence with the sequence established by Mellett(1977) to uncover possible phylogenetic information.

Institutional Abbreviations—AM, American Museum of Nat-ural History, Department of Vertebrate Paleontology, NewYork, U.S.A.; BSPG, Bayerische Staatssammlung fur Palaon-tologie und Geologie, Munchen, Germany; FAM, Frick Col-lection, American Museum of Natural History, New York,U.S.A.; GMM, Geomuseum, Westfalische Wilhelms-UniversitatMunster, Germany; MNHN, Museum d’histoire naturelle, Paris,France; NHMW, Naturhistorisches Museum Wien, Austria;YPM-PU, Princeton University collection moved to YalePeabody Museum, New Haven, Connecticut, U.S.A.; Qu, QuercyCollection at the Museum d’histoire naturelle, Paris, France;SMNS, Staatliches Museum fur Naturkunde, Stuttgart, Germany;USNM, United States National Museum of Natural History,Washington, D.C., U.S.A.; YPM, Yale Peabody Museum, NewHaven, Connecticut, U.S.A.

Anatomical Abbreviations—C/c, upper/lower canine; D/d, up-per/lower deciduous tooth; I/i, upper/lower incisor; M/m, up-per/lower molar; P/p, upper/lower premolar.

MATERIALS AND METHODS

New specimens have contributed to the information now avail-able on the tooth eruption sequence in Hyaenodon (Bastl, 2008).Further new findings are discussed herein: Five specimens rep-resent early stages and are housed in the Naturhistorisches Mu-seum Wien (Austria). These are three left and two right lowerjaws. They are all broken and from Lot (Department Tarnet Garonne, France) except for one (NHMW 2009z0084/0002),which is from Mouillac (at Caylux, Department Tarn et Garonne,France). These sites bear fossils from the late Eocene and earlyOligocene (BiochroM’97, 1997). Further specimens from Mouil-lac are a right mandible (GMM A5.103) of Oligocene age anda left mandible (BSPG 1879 XV 18a). The former is housed inthe Geomuseum (Westfalische Wilhelms-Universitat Munster)and the latter is housed in the Bayerische Staatssammlung fur

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TABLE 1. List of species of Hyaenodon.

Europe North America Asia

H. leptorhynchus Laizier and Parieu, 1838H. brachyrhynchus Blainville, 1841H. minor Gervais, 1848–1852H. requieni Gervais, 1848–1852H. dubius Filhol, 1873H. exiguus Gervais, 1873H. cayluxi Filhol, 1876H. heberti Filhol, 1876H. filholi Schlosser, 1887H. gervaisi Martin, 1906H. rossignoli Lange-Badre, 1979

H./N. horridus Leidy, 1853H./P. crucians Leidy, 1853H./P. mustelinus Scott, 1895H./N. montanus Douglass, 1901H./N. vetus Stock, 1933H./P. brevirostrus Macdonald, 1970H./N. megaloides Mellett, 1977H./P. venturae Mellett, 1977H./P. microdon Mellett, 1977H. raineyi Gustafson, 1986

H. eminus Matthew and Granger, 1925H. yuanchensis Young, 1937H. mongoliensis Dashzeveg, 1964H./N. incertus Dashzeveg, 1985H. chunkhtensis Dashzeveg, 1985H./N. gigas Dashzeveg, 1985H. weilini Wang, Qiu and Wang, 2005H. pervagus Matthew and Granger, 1924 (including

H. neimongoliensis [Huang and Zhu, 2002] afterMorlo and Nagel, 2006)

Follows mainly for Europe Lange-Badre (1979), for North America Mellett (1977), for Asia Wang et al. (2005) and Tsubamoto et al. (2008).Abbreviations: H., Hyaenodon; N., Neohyaenodon; P., Protohyaenodon.

Palaontologie und Geologie (Munchen). One specimen is froma different site: a left mandible (SMNS 47259) was found in thecleft filling Liptingen, northern Hegau in the Western SwabianAlb (Germany). This specimen is of lower Oligocene age, datedat MP 21 (Scherzinger et al., 2005) and housed in the StaatlichesMuseum fur Naturkunde (Stuttgart, Germany).

X-rays were taken with a Siemens Polidoros 800 X-ray ap-paratus with an X-ray tube from Optilix and a small fo-cal spot was used. X-rays were recorded using 77 kV volt-age, 56 mAs, and 501 ms (GMM A5.103, SMNS 47259, andBSPG 1879 XV 18a) or with 85 kV, 80 mAs, and 837 ms(NHMW 2009z0082/0001, 2009z0082/0001/0002, 2009z0083/0001,2009z0084/0001, and 2009z0084/0001/0002) on a Kodak mam-mogram film. The X-ray images were digitally reversed withAdobePhotoshop (Fig. 1).

Tooth nomenclature and orientation is given after Van Valen(1994). In Hyaenodon, the first premolar has no predecessor orsuccessor. In the only detailed study on tooth eruption sequencein Hyaenodon thus far (Mellett, 1977), this tooth is called a p1.However, because it cannot be ascertained whether it is devel-oped by the first or second tooth lamellae, some authors (e.g.,Lange-Badre, 1979) have regarded it as dp1. Because the time oferuption of this tooth fits well with that of the other permanentpremolars, we refer to it as p1 (sensu Mellett, 1977).

Measurements were taken with a caliper to the nearest 0.1 mm.They are given in mm and are summarized in Table 2.

The tooth eruption sequences in this contribution conform tothe time of the beginning eruption of single teeth. For this, weregard the beginning of the eruption to be important and not themoment when the tooth is fully erupted, because this cannot bereconstructed with certainty.

Although the affiliation of the discussed specimens to Hyaen-odon is clear, determination of juvenile specimens on thespecies level is problematic, as the intra-specific variability ofdp4 for example is largely unknown. Moreover, differences ofEuropean taxa on the species level are not fully understood.

TABLE 2. Measurements of the specimens (in mm).

Specimens L dp4 W dp4 L m1 W m1 D dp4

NMHW 2009z0082/0001 7.8 3.2 — — 11.7NMHW 2009z0084/0001 8.7 3.4 — — 11.4NMHW 2009z0082/0002 8.0 3.0 6.9 3.3 12.2NMHW 2009z0083/0001 8.7 2.9 — — 13.3GMM A5.103 11.8 4.5 9.9 5.7 24.8SMNS 47259 12.7 5.1 7.6 4.2 18.8BSPG 1879 XV18a 6.5 2.8 — — 11.4

Abbreviations: L, length; W, width; D, jaw depth.

We therefore discuss Hyaenodon without delving into speciessystematics.

HISTORY

The first part of the name Hyaenodon means ‘hyena’, whereasthe second part refers to ‘tooth’. This is because the robust pre-molars of Hyaenodon resemble the premolars of hyenas. Thetype species is Hyaenodon leptorhynchus erected by Laizier andParieu (1838). Thus, the scientific history of the genus beganin Europe. Major descriptive work was done by Martin (1906).Lange-Badre revised the European species (1979). Neohyaen-odon was proposed by Thorpe (1922) as a new genus, separatingH. horridus from other North American species of Hyaenodon.The next indication for separating features in the genus followedwith the establishment of the subgenus Protohyaenodon (Stock,1933). However, the species of North American Hyaenodon wererevised by Mellett (1977) and differences within this group are re-garded as divergent lineages. As recently discussed in Morlo andNagel (2006) and Tsubamoto et al. (2008), the taxonomy of theAsian species of Hyaenodon is complicated: Hyaenodon incertusDashzeveg (1985) was assigned to the genus Neohyaenodon byLavrov (1999). The term Neohyaenodon was originally createdfor the North American species H. horridus by Thorpe (1922) toseparate this species on the genus level. Hyaenodon is widely ac-cepted as the proper generic designation for all hitherto knownspecies, despite the discussion and according to other authors(McKenna and Bell, 1997; Morlo and Nagel, 2006; Tsubamotoet al., 2008).

SYSTEMATIC PALEONTOLOGY

Order ‘CREODONTA’ Cope, 1875Family HYAENODONTIDAE Leidy, 1869

HYAENODON Lazier and Parieu, 1838

Cuvier figured the first specimens that belong to the genus(1825:269, fig. 3, pl. 68 and figs. 2-4, pl. 69) and assigned themto the Procyonidae. They were found in Montmartre (France)and were mentioned as maxilla and mandible fragments. The firstspecimen described as Hyaenodon (Laizier and Parieu, 1838) wasa complete mandible found in Cournon (Departement Puy-de-Dome, France). At first, a relationship to marsupials was consid-ered (Gaudry, 1878) but this was contrasted by Filhol (1876) andSchlosser (1887) who favored a placental descent partly becauseof the manner of tooth replacement.

Generic diagnosis commonly includes the tooth formula3142/3143, emphasizing the missing upper third molar, carnas-sials mainly M2/m3, but also M1/m2, and earlier in life, DP3/dp4and DP4/m1 with shearing function, reduced protocone on upper

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FIGURE 1. A–H, X-ray images, digitally reversed. A, Hyaenodon sp., NMHW 2009z0082/0001; B, Hyaenodon sp., NHMW 2009z0084/0001; C,Hyaenodon cf. brachyrhynchus, GMM A5.103; D, Hyaenodon dubius, SMNS 47259; E, Hyaenodon sp., NMHW 2009z0082/0002; F, Hyaenodon filholi,BSPG 1879 XV 18a; G, Hyaenodon sp., NMHW 2009z0083/0001; H, Hyaenodon sp., NMHW 2009z0084/0002.

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molars, reduced talonid on m3, and a digitigrade stance (Savage,1965; Mellett, 1977; Lange-Badre, 1979; Dashzeveg, 1985).

SPECIMENS

General Description

The symphysis reaches up to (d)p3. A mental foramen is placedunder p1 and under (d)p3.

The p1 is double-rooted, small, not elongated and has no ac-cessory cusps; the p2 is elongated and possesses a nub-like cuspidposterior to the main cusp. The permanent premolars p3 and p4possess no anterior accessory cusps and the posterior accessorycusps are compressed to the main cusp. The paraconid is lowerthan the protoconid on all molars. The lingual cingulid on the firstmolar is weak. The talonid is small and distant. The parastylid isreduced. The second molar has an even weaker lingual cingulidthat the first molar. The parastylid is reduced. The m2 shows asmall talonid.

The dp3 possesses an anterior and posterior accessory cuspwith the anterior one higher. The anterior accessory cusp of dp4exceeds the talonid in height but is clearly lower than the maincusp. The buccal-posterior part of the tooth is concave and themain cusp points distally. The blade built by the anterior acces-sory and the main cusp of dp4 is oriented along the long axis ofthe mandible. The talonid has a straight and short cutting bladethat is placed more buccal. From an occlusal view, the tooth has asmaller width at the main cusp compared to the proximal or dis-tal parts. The tooth is proximally angled to give guidance for theanterior tooth.

HYAENODON sp. (small)

As diagnostic features of the dp4 have hitherto not been de-scribed, taxonomic determination of a juvenile specimen is diffi-cult. Therefore, these specimens are provisionally designated toHyaenodon sp. of small size.

Material—NMHW 2009z0082/0001, NMHW 2009z0084/0001,NHMW 2009z0084/0002, NMHW 2009z0082/0002, NMHW2009z0083/0001.

Description—The lower left jaw fragment (NMHW2009z0082/0001) holds an erupting p1, the dp4, and the alveoliof dp2, dp3, and m1 (Fig. 2A–C). The first premolar is erupting.There seems to be just one alveolus for this tooth. The anterioraccessory cusp of the dp4 shows a small wear facet. Its tip isslightly flattened. The cutting blades are not worn. The X-rayshows no germs (Fig. 1A).

The right mandible (NMHW 2009z0084/0001) is broken andholds an erupting p1, the dp4, and the alveoli of dp2, dp3, and m1(Fig. 3A–C). On the dp4 the tip of the anterior accessory cusp isplaced more lingual relative to the tip of the main cusp. This re-sults in a bowed cutting blade that extends buccally. From a buc-cal view, the main cusp is distally not concave. The talonid doesnot have a clear blade but rather a crest that is built by two acces-sory cusps, with the anterior one higher. These lead into a crenu-lated cingulid that runs distally and lingually. The dp4 is moreangled proximally than in the other specimens described hereinand has a strong buccal bulge. The anterior accessory cusp andthe main cusp of dp4 show distinct wear facets. These wear facetscan be described as flattening of the cusps. On the main cusp, thiswear is stronger, although the tip or part of it might be broken.The cutting blades show no sign of a wear facet. The X-ray showsa small germ of the p2 (Fig. 1B). The morphology of the dp4 ofthis specimen differs from the other specimens. The roots of thep1 show a tendency toward fusion.

The right mandible (NMHW 2009z0082/0002) is broken infront of the alveolus of dc and the process coronoideus is bro-ken (Fig. 2D–F). The lower jaw holds p1, dp4, m1, and the alve-oli of dc, dp2, dp3, and m2. The blade on the talonid of the dp4

is not just a simple cutting edge, but built by a small accessorycusp that does not lead into a crenulated cingulid like in NHMW2009z0084/0001. The main cusp of dp4 is more worn than the an-terior accessory cusp. Both show a flattening. No trace of a wearfacet can be seen on the cutting blades. On the m1, no wearfacet is present, except for a small dot-like facet on the para-conid. The X-ray (Fig. 1E) shows the germs of p2 and the canine.The morphology of the dp4 is not exactly the same as in NMHW2009z0082/0001, but it is quite similar.

The fragment of a left mandible (NMHW 2009z0083/0001)holds a permanent incisor in crypt, dp4, and the alveoli of p1,dp2, dp3, m1, and m2 (Fig. 3G–I). There are two alveoli of p1,with the posterior one much stronger than the anterior one. Thetalonid on the dp4 shows two small cuspids that lead into aslightly crenulated cingulid. The latter is weaker than in NMHW2009z0084/0001. The dp4 is proximally angled, but weak and pos-sesses no bulge. The incisor is placed very buccally and probablyis an i3. The anterior accessory cusp of dp4 is flattened. This wearfacet is on the occlusal surface. The main cusp shows no wearfacet. The X-ray (Fig. 1G) shows germs of the canine and p2 andsmaller germs of p3 and p4. This specimen has two distinct alve-oli for p1 (in contrast to the other previous described specimens).This difference can be noted, although it is not diagnostic, as sim-ilar variation is known in H. rossignoli (Lange-Badre, 1979).

The left mandible (NHMW 2009z0084/0002) is broken ante-rior of an erupting canine and posterior to the erupting p3 (Fig.3D–F). It holds the erupting c, the erupting p3, and the alveoli ofp1 and p2. The canine is quite large in relation to the depth of themandible, as can also be seen in the X-ray (Fig. 1H). The p3 has adistinct posterior accessory cusp, and distal to it, a smaller secondaccessory cusp. The presence of the alveoli of p1 and p2, as wellas the absence of germs, indicate that the erupting third premo-lar belongs to the permanent dentition. The teeth in this speci-men are erupting and no wear facets are present. No germs canbe seen in the X-ray (Fig. 1H). The mandible belongs to a smallsized Hyaenodon, although it seems to be larger than the previ-ously described specimens, probably due to its more advancedontogenetic stage.

HYAENODON FILHOLI Schlosser, 1887

Holotype—Qu 8661 (MNHN) Schlosser, 1887 (figured in Fil-hol, 1876:pl. 34, figs. 167, 172). The specimen is a left mandiblefragment with p3, p4, m1, m2, and m3 present. Locality is un-known.

Material—BSPG 1879 XV 18a.Description—The mandible (Fig. 4I–L) is broken in front of c

(in crypt) and behind the last milk premolar. The first premolar isbroken, p2 is erupting, and dp3 and dp4 are present. The symph-ysis reaches up to dp4. The p2 is near complete eruption. The dp4possesses a buccal bulge. The tips of the main cusp and the ante-rior accessory cusp of dp4 are worn and flattened. As in previ-ously described specimens, wear on dp4 is occlusal. On this spec-imen, a slight wear facet on the crest from the anterior accessorycusp to the main cusp is also present. The dp3 shows the samepattern: small punctuated wear facets on the tips, although theyare weak. The erupting p2 and the c in crypt do not show wearfacets. In the X-ray (Fig. 1F), the permanent premolars p3 andp4 are similar and possess a posterior accessory cusp compressedto the main cusp. No anterior accessory cusps are present. Thisspecimen is assigned to H. filholi due to its small size, the double-rooted p1, and the presence of a nub-like cuspid on p2.

HYAENODON BRACHYRHYNCHUS Blainville, 1841

Holotype—Skull with mandible with incomplete and damageddentition (housed in the collection of the Geologic Laboratory of

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FIGURE 2. A–C, Hyaenodon sp., NMHW 2009z0082/0001: A, buccal, B, occlusal, and C, lingual views. D–F, Hyaenodon sp., NMHW2009z0082/0002: D, buccal, E, occlusal, and F, lingual views.

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FIGURE 3. A–C, Hyaenodon sp., NHMW 2009z0084/0001: A, buccal, B, occlusal, and C, lingual views. D–F, Hyaenodon sp., NMHW2009z0084/0002: D, buccal, E, occlusal, and F, lingual views. G–I, Hyaenodon sp., NMHW 2009z0083/0001: G, buccal, H, occlusal, and I, lingualview.

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FIGURE 4. A–D, Hyaenodon cf. brachyrhynchus, GMM A5.103: A, buccal, B, occlusal, and C, lingual views, and D, drawing. E–H, Hyaenodondubius, SMNS 47259: E, buccal, F, occlusal, and F, lingual views, and H, drawing. I–L, Hyaenodon filholi, BSPG 1879 XV 18a: I, buccal, J, occlusal,and K, lingual views, and L, drawing. M–N, Hyaenodon cayluxi: M, buccal view and N, drawing, redrawn from Filhol (1876:figs. 147–149).

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the University Paul Sabatier in Toulouse, France). It was foundin the locality Rabastens (Tarn-et-Garonne).

Material—GMM A5.103. The specimen (Fig. 4A–D) wasfound in Mouillac at Caylux (department Tarn et Garonne,France). It is of Oligocene age.

Description—The right lower jaw is broken in the position ofp3. The mandible has a clear curvature. The process coronoideusis broken. The fossa masseterica is deep and reaches under thelast molar. There is no sign of the symphysis. The p3 and dp4,m1, m2, and m3 are preserved. The third molar is erupting. Itsprotoconid is visible. The tips of the dp4 are strongly worn. Boththe anterior accessory cusp and the main cusp are flattened. Thetalonid too has a small wear facet. These wear facets can be seenfrom an occlusal view, unlike the wear facets on the molars, whichare situated buccad. On the m1, the paraconid is more worn thanthe protoconid. The cutting blade shows a wear facet. The m2is broken at about the middle of its cutting blade and only themesial part is preserved. The paraconid is worn as well as thecutting blade. These wear facets are not as distinctive as on them1. The erupting teeth (p3 and m3) show no wear facets. Thisspecimen is assigned to H. brachyrhynchus due to its size and themorphology of p3 and p4 (Fig. 1C).

HYAENODON DUBIUS Filhol, 1873

Holotype—Mandible fragment holding p2, p3, p4, m1, and m2(MNHN Qu 8627; Filhol, 1873:figs. 39–41) from Saint-Antonin.

Material—SMNS 47259. A third lower molar, m3 (SMNS47276), was found together with the mandible (Scherzinger et al.,2005).

Description—The right lower jaw (Fig. 4E–H) is broken infront of the alveoli of p3. The mandible has a curvature, pos-sesses a low process coronoideus, and a deep fossa masseterica.The bone was damaged in the area of m3, which was found inthe cleft filling near the mandible. There is no sign of the symph-ysis. There is a mental foramen on the lingual side close to theprocess articularis. Alveoli of p3 and the p4, m1, and m2 are pre-served. The fourth premolar is erupting. No wear facets can beseen on the erupting p4. There are distinct wear facets on thefirst molar. The tip of the paraconid is worn. The protoconid alsoshows wear, although the facets are not as pronounced as on theparaconid. A wear facet is present on the cutting blade. The samefacets are present on m2, with a more distinctive facet at the para-conid compared to that on the protoconid. The cutting blade alsoshows wear, although wear on m2 is weaker than on m1. TheX-ray confirms that (Fig. 1D) the main cusp on p4 is dominant.There is an anterior accessory cusp, which is lower than the pos-terior accessory cusp. First and second molars are not worn. Asmall mediolingual parastylid is present, but weaker on m1 thanon m2. The lingual cingulid is well developed but the buccal oneis weak. The specimen is assigned to H. dubius because of thepresence of an anterior accessory cusp on p4, the posterior acces-sory cusp, which is detached from the main cusp on p4, distinctparastylids, and well-developed lingual cingulids on the molars(Lange-Badre, 1979).

HYAENODON CAYLUXI Filhol, 1876

Holotype—Left mandible fragment holding the alveoli ofall premolars and the molars itself (MNHN, Qu 8622) fromBach (Lot). The specimen cited herein was described by Filhol(1876:figs. 147–149) and determined as H. cayluxi.

Description—The right mandible (Fig. 4M–N) is broken infront of the third premolar. The process coronoideus is broken.The lower jaw has a slight curvature. The p3, dp4, alveoli of m1,and the m2 and p4 and m3 in crypt are preserved.

TOOTH ERUPTION SEQUENCE IN HYAENODON

Tooth Eruption Sequence in European Hyaenodon

The tooth eruption sequence of European Hyaenodon isshown in Figure 5. In the ontogenetically youngest stage, the den-tition consists of the milk dentition: di, dc, dp2, dp3, dp4, and m1.Thus, the first molar belongs functionally to the first dentitionand erupts very early. The p1 erupts in the second stage. Thisis represented by the specimens NHMW 2009z0082/0001 (Figs.1A, 2A–C) and NHMW 2009z0084/0001 (Figs. 1B, 3A–C)—bothsmall-sized forms of Hyaenodon. The mandible fragments showan erupting p1, dp4, and alveoli for dp2, dp3, and m1. Anotherspecimen, Qu 8660, referred to in Lange-Badre (1979:123), holdsan erupting p1, dp2, and dp3. In the third stage, the m2 erupts.A mandible holding p1, dp2, dp3, dp4, m1, and m2 is mentionedby Lange-Badre (1979:123). In the fourth stage, the incisors arereplaced. NHMW 2009z0083/0001 (small sized; Figs. 1G, 3G–I)shows a permanent incisor in crypt and m2 has already erupted.In the fifth stage, p2 erupts and so replaces the first milk pre-molar. BSPG 1879 XV 18a (small sized; Figs. 1F, 4I–L) holdsc in crypt, p1, a nearly erupted p2, dp3, and dp4. In the sixthstage, c, p3, and m3 simultaneously start to erupt. In NHMW2009z0084/0002 (small sized; Figs. 1H, 3D–F), c and p3 are erupt-ing (with alveoli of p1 and p2 present) and in GMM A5.103 (largesized; Figs. 1C, 4A–D), p3 and m3 are erupting (with dp4, m1,and m2 present). However, teeth such as the canine and the thirdmolar need more time for complete eruption. This is achieved inthe sixth stage. In the seventh stage, the dentition is the perma-nent dentition except for one milk premolar, the dp4. Thus, thep4 erupts as the last permanent tooth, as can be seen in SMNS47259 (large sized; Figs. 1D, 4E–H). In the eighth stage, tooth re-placement is complete and the permanent dentition is shown.

Tooth Eruption Sequence in North American Hyaenodon

The tooth eruption sequence of North American Hyaenodonwas described by Mellett (1977) and is shown in the same wayfor comparison in Figure 5. The youngest specimen mentionedby Mellett holds p1, dp2, dp3, dp4, and m1 (1977:50, fig. 26B;FAM 75667). Thus, it cannot be determined whether p1 or m1erupts earlier. In the second stage, m2 erupts. FAM 75653 holdsdi, dc, p1, dp2, dp3, dp4, m1, and m2 (Mellett, 1977:38, fig. 5B). Inthe third stage, the incisors are replaced. In the fourth stage, p2erupts as in AM 39439, which shows dc, dp3, dp4, m1, m2, and anerupting p2 (Mellett, 1977:36, fig. 17A). In the fifth stage, p4 startseruption. In the sixth stage, m3 erupts. YPM-PU 11082 holds dc,p1, p2, p4, m1, m2, m3, and an erupting p3 (Mellett, 1977:46, fig.24B). As the last tooth, the canine is replaced, shown in the sev-enth stage. In USNM 214643, the permanent dentition, the p1, p2,p3, p4, m1, m2, and m3, is present and c is erupting (Lavrov andEmry, 1998:754, fig. 1). In the eighth stage, the tooth replacementis complete.

DISCUSSION

The European specimens of Hyaenodon described here pro-vide new information, including that the germs of p2 and c de-velop simultaneously; there is a stressful stage in the tooth re-placement, when c, p3, and m3 start eruption; and the p4 is thelast tooth to start eruption in European Hyaenodon.

The tooth eruption sequence indicates a significant difference(Fig. 5) between European and North American Hyaenodon(Bastl, 2008). The tooth eruption sequence was originally de-scribed by Mellett (1977) on the basis of North American speci-mens as follows: m2-i-p2-p4-m3-p3-c. This was interpreted differ-ently by Lange-Badre (1979) as follows: m1-dp1-m2-p2-m3-p4-p3-c. NMHW 2009z0082/0001 and NMHW 2009z0084/0001 showthat p1 erupts after the m1. NHMW 2009z0082/0002, NMHW2009z0083/0001, NMHW 2009z0084/0002, and BSPG 1879 XV

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BASTL ET AL.—TOOTH ERUPTION SEQUENCE IN EUROPEAN HYAENODON 189

FIGURE 5. Illustration of the tooth eruptionsequence. A, European Hyaenodon; B, NorthAmerican Hyaenodon. Milk teeth are grey. Ar-rows indicate the differences, the earlier eruptionof p3 and c in the European Hyaenodon.

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FIGURE 6. Occurrence of the species of Hyaenodon through time on the three continents: North America (Mellett, 1977; Lavrov and Emry, 1998),Europe (Lange-Badre, 1979), and Asia (Huang and Zhu, 2002; Wang et al., 2005; Morlo and Nagel, 2006; Tsubamoto et al., 2008). Abbreviation: H.,Hyaenodon.

18a show that the development of c and p2 is simultaneous.NMHW 2009z084/0002 also indicates that the canine erupts earlyand begins eruption about the same time as p3. This is differentin relation to the North American pattern: in North Americanspecies, the permanent canine is the last tooth to erupt. The othersignificant difference described by Bastl (2008) was that the p4starts eruption after p3 and m3. GMM A5.103 shows the erupt-ing p3 and m3 together with the presence of dp4. SMNS 47259indicates the same, presenting the next stage: presence of m3 andp3 and an erupting p4. Furthermore, a specimen figured by Fil-hol (1876) is pertinent here and should be mentioned. The leftmandible holds dp4, m2, and shows p3, p4, and m3 in crypt withp3 erupting (Fig. 4M–N). These specimens (Figs. 1A–H, 2A–F,3A–I, 4A–N) show that p4 erupts after p3 contrary to the NorthAmerican specimens (Mellett, 1977) and the earlier eruption ofthe canine in European forms. The m3 erupts earlier in Euro-pean forms as already indicated by Lange-Badre (1979). Thus,the tooth eruption sequence for European Hyaenodon (Fig. 5) ism1-dp1-m2-i-p2-c/p3/m3-p4.

It should be noted that no European specimen is known to fitthe tooth eruption sequence documented by Mellett (1977) forNorth American taxa and that no North American specimen isknown that fits the sequence given in this study for Europeantaxa. Despite the small number of juvenile specimens, we canconfidently regard tooth eruption sequences in European andNorth American Hyaenodon juveniles to differ significantly.

Life History—Conclusions concerning the life history ofHyaenodon from the wear facets are as follows: in the on-togenetic stages of GMM A5.103 (stage 6) and SMNS 47259(stage 7), it can be assumed that Hyaenodon already processed

flesh. The cutting blades show wear facets and have been used.Thus, lactation was likely finished before this period of tootheruption. In all younger specimens (NMHW 2009z0082/0001,NMHW 2009z0084/0001, NHMW 2009z0084/0002, NMHW2009z0082/0002, NMHW 2009z0083/0001, BSPG 1879 XV 18a),the dp4 shows the same pattern: the tips of the anterior accessorycusp, and in some, the tip of the main cusp as well are flattened,whereas the cutting blades show no trace of a wear facet. Thus,meat slicing was not probable in this early ontogenetic stages(stages 2 and 3). The dp4 was obviously used, but the punctuatedwear facets cannot be interpretated at this time.

Differences within the Genus—Hitherto valid species ofHyaenodon are listed (Table 1) and show the diversity of thegenus (Fig. 6). Scott (1895) described differences between Euro-pean and North American Hyaenodon. The features concernedare located in the skull, the humerus, the carpals, and tarsals.The presence of an alisphenoid canal was originally noted in theEuropean species, but not in North American species (Scott,1895; Martin, 1906). Later, this potential difference turned out tobe invalid because North American Hyaenodon does possess analisphenoid canal (Mellett, 1977). Another difference is apparent:in European Hyaenodon, the scaphoid and lunatum are fused toform a scapholunatum (Scott, 1895; Martin, 1906), whereas thereis no trend to fusion in North American Hyaenodon (Scott, 1895;Mellett, 1977). Wortman (1902) reported a fused scapholunatumin North American Hyaenodon (YPM 10076) but Mellett (1977)stated that this was a misidentification and that this specimen ac-tually belonged to Daphoenus, an Oligocene ‘Fissiped.’ All otherdifferences described by Scott (1895) have hitherto not been re-viewed.

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As documented here, the tooth eruption sequence suggestsmajor differences between European and North Americanspecies of Hyaenodon. We therefore propose divergence withinthe genus Hyaenodon. In depth study of other morphologicalstructures is needed in order to clarify the taxonomy of New andOld World Hyaenodon.

SUMMARY

The genus Hyaenodon has a long history, with two major re-visions (Mellett, 1977; Lange-Badre, 1979) that clarified NorthAmerican and European species. Differences between the Eu-ropean and North American forms were described early on byScott (1895), but these differences have not been further exam-ined as a whole or concerning their relevance for the taxonomyof Hyaenodon. There is a strong indication for divergence be-tween European and North American species of Hyaenodon. Asrecently described, there are significant differences in the tootheruption sequence within the genus (Bastl, 2008), as examinedherein in more detail. The issue needs further research and inclu-sion of the Asian species to clarify genus-level relationships of allHyaenodon species.

ACKNOWLEDGMENTS

We gratefully thank G. Roßner (Ludwig-Maximilians-Universitat, Munchen) for entrusting us with the specimenBSPG 1879 XV 18a, M. Bertling (Geomuseum der Westfalis-chen Wilhelms-Universitat, Munchen) for entrusting us withthe specimen GMM A5.103, and U. Gohlich (Naturhis-torisches Museum Wien) for entrusting us with the specimensNHMW 2009z0082/0001, 2009z0082/0001/0002, 2009z0083/0001,2009z0084/0001, and 2009z0084/0002. J. Galkin (AmericanMuseum of Natural History, New York) and P. Brewer (BritishMuseum of Natural History, London) gave access to the collec-tions. G. Withalm (Institut fur Palaontologie, Universitat Wien)X-rayed the specimens. R. Gold (Institut fur Palaontologie, Uni-versitat Wien) took the photographs (except for the specimen ofH. cayluxi, which was photographed by one of us, M.M., in theMusee d’histoire naturelle, Paris) and arranged them. M. Schat-tleitner (Bibliothek der Erdwissenschaften, Universitat Wien),H. Schmitz (Naturhistorisches Museum Wien), W. Brunnbauer(Naturhistorisches Museum Wien), and M. Grillitsch (Naturhis-torisches Museum Wien) gave access to literature. F. Bastlhelped with the illustrations. G. Semprebon (Bay Path College,Longmeadow, Massachusetts) helped improve the paper withproofreading. K.B. was supported by a student member travelgrant of the SVP (Society of Vertebrate Paleontology) to presentthe topic at the 69th Conference in September 2009 in Bristoland by a grant of the University of Vienna. T. Martin, JVPeditor, gave valuable comments. We wish to thank G. Gunnelland H. Smith for their critical reading and detailed review, whichimproved this publication.

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Submitted April 26, 2010; accepted October 12, 2010.Handling editor: Thomas Martin.

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