Introduction
Hyaenodonts are a significant component of Eocene carnivorous guilds across the Holarctic and Africa (Gunnell, 1998;Rose, 2006;Lewis & Morlo, 2010). Along with other “creodonts” (e.g., Oxyaenidae), hyaenodonts are distinguished from modern carnivorans and their fossil relatives (Carnivoraformes) by the presence of multiple carnassial pairs in the dentition, which results in alternating shearing and crushing/grinding areas in the dentition, rather than regional separation of the molar series into mesial shearing and distal crushing/grinding areas. The latter innovation in Carnivoraformes, and convergently in Viverravidae (Zack, 2019), may have facilitated the ecological diversification of carnivorans (Friscia & Van Valkenburgh, 2010) ultimately allowing carnivorans to displace hyaenodonts over the course of the Paleogene in the northern continents and Miocene in Africa (Wesley-Hunt, 2005;Friscia & Van Valkenburgh, 2010;Borths & Stevens, 2017).
In North America, hyaenodont diversity was greatest during the earlier half of the Eocene, particularly the Wasatchian and Bridgerian North American Land Mammal Ages (NALMAs) (Gunnell, 1998;Van Valkenburgh, 1999;Wesley-Hunt, 2005;Friscia & Van Valkenburgh, 2010). In the subsequent Uintan NALMA, hyaenodont diversity declined dramatically. Only four genera,Limnocyon,Mimocyon,Oxyaenodon, andSinopa, have been described from Uintan faunas (Matthew, 1899;Matthew, 1909;Peterson, 1919;Gustafson, 1986), although an additional, small hyaenodont taxon is known but undescribed (Rasmussen et al., 1999; S Zack, pers. obs., 2019). This mid-Eocene decline of hyaenodont and other “creodont” diversity corresponds with an increase in the diversity of carnivorans and their immediate relatives (Carnivoraformes) (Van Valkenburgh, 1999;Wesley-Hunt, 2005;Friscia & Van Valkenburgh, 2010), a pattern suggesting some form of replacement of hyaenodonts by carnivoraform taxa. Understanding the nature of that replacement requires a detailed record of the diversity of both groups.
Reexamination of existing collections is one key to refining the record of carnivorous mammals across this critical period, as overlooked or misidentified specimens can shift the temporal and geographic ranges of known taxa and allow recognition of new forms. MCZ VPM 19874, the specimen that forms the focus of the present study, is an example of significant discoveries that can be made in existing collections. The specimen, a dentary with M2−3, was collected by a Harvard University expedition to the Uinta Basin, Utah in 1940 (Fig. 1) and has not been described or mentioned in the literature in almost 80 subsequent years. It documents a new hyaenodont taxon from the late Uintan that differs substantially from known Uintan hyaenodonts, particularly in its possession of a strongly hypercarnivorous morphology, greater than previously known in Wasatchian through Uintan North American hyaenodonts. In fact, the affinities of the new taxon appear to lie withPropterodon, a genus previously known only from eastern Asian faunas correlated with the Chinese middle Eocene Irdinmanhan and Sharamurunian stages (sensuWang et al., 2019). The new taxon increases Uintan hyaenodont diversity and disparity while providing evidence for interchange of Asian and North American carnivores during this critical interval in the divergent histories of Hyaenodonta and Carnivoraformes.
Figure 1:Stratigraphic and geographic position of Leota Quarry.
(A) Generalized stratigraphic section of middle Eocene Uinta Formation in the west-central Uinta Basin showing the position of Leota Quarry along with biochron boundaries (Prothero, 1996) and geomagnetic polarity chrons (Murphey et al., 2018). (B) Map of Utah, United States showing the location of Uintah County and map of Uintah County showing the position of Leota Quarry (as indicated byPeterson & Kay, 1931). Orange shading in (B) indicates outcrop of the Uinta Formation (afterHintze, 1980). Abbreviations: BB Mbr, Brennan Basin Member of the late middle Eocene Duchesne River Formation; Gr Fm, early middle Eocene Green River Formation. Drawings by Shawn P. Zack.
Materials & Methods
Dental terminology followsRana et al. (2015), with two exceptions. “Mesiobuccal cingulid” is used followingZack (2011) instead of “buccal cingulid”, as this structure is mesially restricted in the new species. FollowingKay (1977), “hypocristid” is used rather than “postcristid” for the crest connecting the hypoconid and hypoconulid. Measurements followGingerich & Deutsch (1989, fig. 1) andBorths & Seiffert (2017, fig. 1e), with the addition of a measurement of maximum talonid height. Dental measurements taken are illustrated inFig. 2. Mandibular depth was measured lingually below M3. All measurements were taken to the nearest tenth of a millimeter with Neiko digital calipers. MCZ VPM 19874 was whitened using ammonium chloride prior to being photographed.
Figure 2:Measurements of hyaenodont lower molars.
Schematic drawing of a hyaenodont lower molar in (A) occlusal and (B) buccal views to show measurements taken for this study. Abbreviations: L, maximum length; TrL, maximum trigonid length; TrW, maximum trigonid width; TrH, maximum trigonid height; TaL, maximum talonid length; TaW, maximum talonid width; TaH, maximum talonid height. Drawings by Shawn P. Zack.
The electronic version of this article in Portable Document Format (PDF) will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefixhttp://zoobank.org/. The LSID for this publication is: urn:lsid:zoobank.org:pub:CDA777EE-C052-4922-90DD-AAFD41D3F345. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central and CLOCKSS.
Phylogenetic Methods—To test the taxonomic affinities of the new species, it was added to a substantially modified version of the character taxon matrix used byRana et al. (2015). The dental sample used byRana et al. (2015) was modified to eliminate non-independent characters (e.g., removing a character describing the number of P3 roots, which reflects development of a P3 protocone lobe), following the recommendations of recent authors who have argued that inclusion of non-independent characters can mislead phylogenetic analyses that rely heavily on mammalian dental morphology (Sansom, Wills & Williams, 2017;Billet & Bardin, 2019). Overall, several dental characters were revised, replaced, combined, or deleted, and one additional character describing the number of upper incisors was added fromBorths & Stevens (2019a). Numerous individual scorings were modified to improve scoring consistency, with particular emphasis placed on ensuring scoring consistency across geographic regions.
While the dental character sample fromRana et al. (2015) was used, the non-dental character sample used byRana et al. (2015) which, in turn was derived fromPolly (1996), was largely replaced by the cranial, mandibular, and postcranial character sample used byBorths & Stevens (2019a), and Borths and Stevens’ scorings were used with some additions (e.g., postcranial scorings were added forGalecyon chronius andPrototomus martis). One character fromRana et al. (2015) describing mandibular symphysis depth was retained because this variation was not captured by Borths and Stevens’ characters.
In addition to the inclusion of the new species, several changes were made to the taxonomic composition of the matrix. First, the compositePropterodon spp. OTU used byRana et al. (2015) was replaced with separate OTUs forP. morrisi andP. tongi. Reflecting newly published material, the African“Sinopa” OTU included inRana et al. (2015) was replaced byBrychotherium ephalmos, scored from descriptions inBorths, Holroyd & Seiffert (2016) and accompanying 3D models. Scorings ofAkhnatenavus were updated to includeA. nefertiticyon described in the same work, while scorings forMasrasector were updated based on material ofM. nananubis described byBorths & Seiffert (2017). ThePterodon spp. OTU was restricted toP. dasyuroides and rescored, given that new evidence indicatesPterodon, as traditionally defined, is likely polyphyletic (Solé et al., 2015a;Borths & Stevens, 2019a;Borths & Stevens, 2019b). Three additional taxa were added to the matrix,Boritia duffaudi,Preregidens langebadrae, andMatthodon menui. These three taxa are either newly described or newly identified as hyaenodonts, and they significantly enhance the documentation of early European hyaenodonts (Solé, Falconnet & Yves, 2014;Solé, Falconnet & Vidalenc, 2015).
In addition, six OTUs included in theRana et al. (2015) matrix were excluded from the present analysis. As withPterodon, monophyly ofMetapterodon, as used byRana et al. (2015), now appears dubious (Morales & Pickford, 2017;Borths & Stevens, 2019b), but, unlike the well-documentedPterodon dasyuroides, individual species ofMetapterodon are fragmentary and poorly known, contributing little to the broader structure of hyaenodont interrelationships. Until the composition ofMetapterodon is better understood, the genus is better excluded. A second taxon,Eoproviverra eisenmanni, was removed over concerns about the permanent versus deciduous status of the type and most informative specimen, MNHN.F.RI 400. Described as an M2 (Godinot, 1981;Solé et al., 2015b), MNHN.F.RI 400 shows several features that suggest the tooth may instead represent dP4, including a low paraconid, open trigonid, small talonid, and generally tall, delicate cusp construction. If this is the case, MNHN.F.RI 400 would likely represent a larger taxon than the remainder of the hypodigm.
Finally,Tinerhodon disputatum and the three species that have been referred to Koholiinae (Boualitomus marocanensis,Koholia atlasense,Lahimia selloumi) were excluded. As briefly noted byRana et al. (2015), the hyaenodont status of these taxa remains to be clearly demonstrated. Referral of all four taxa to Hyaenodonta appears to have been made based on the presence of multiple carnassial pairs and retention of three molars. As discussed byZack (2019), thisde facto definition of Hyaenodonta combines two eutherian symplesiomorphies (molar homodonty and three molars) with a trait found in all carnivorous clades (carnassials). Given this weak evidence, the possibility that some or all these taxa are not hyaenodonts must be considered. In fact,Tinerhodon disputatum has not been consistently recovered as a hyaenodont in analyses that do not constrain the ingroup to monophyly (e.g.,Borths & Stevens, 2019b). Among members of the potentially polyphyletic Koholiinae, two species known exclusively from lower dentitions (Boualitomus marocanensis andLahimia selloumi) lack P1, a feature that is unusual for Hyaenodonta but typical for members of Tenrecoidea (Gheerbrant et al., 2006;Solé et al., 2009). Combined with the small size of both species, this raises the possibility that koholiines may actually represent an endemic African carnivorous radiation prior to an Eocene immigration of hyaenodonts to Africa. The third koholiine,Koholia atlasense, is known only from a fragmentary upper dentition, and recent phylogenetic analyses have not recovered it in a clade withB. marocanensis andL, selloumi (Borths, Holroyd & Seiffert, 2016;Borths & Seiffert, 2017;Borths & Stevens, 2017;Borths & Stevens, 2019a;Borths & Stevens, 2019b). The M1 ofK. atlasense has a paracone that is distinctly lingual to the metacone, although this may be exaggerated by damage to the metacone (Crochet, 1988). This morphology is not characteristic of hyaenodonts but occurs in the early tenrecoidsSperrgale minutus andArenagale calcareus (Pickford, 2015). Other aspects of the morphology ofK. atlasense are also unusual for a hyaenodont including the elongate P4 metastyle, strong M1 prevallum shear, and massive M1 parastyle connected to the preparacrista at its mesial margin. The overall morphology ofK. atlasense is distinctive enough to cast doubt on its hyaenodont status.
The final matrix includes 48 ingroup taxa and two outgroups scored for 115 characters. The list of characters and specimens examined are available in theSupplemental Information. The full matrix is also available on MorphoBank as project P3489 (http://morphobank.org/permalink/?P3489). The matrix was analyzed using parsimony in TnT version 1.5 (Goloboff & Catalano, 2016). Initial analyses used the Sectorial Search algorithm under the New Technology search dialog. The matrix was analyzed until trees of the same minimum length were recovered by 100 replicates of the algorithm, each beginning from a different starting topology. If a particular replicate identified a tree shorter than the existing minimum length trees, the process restarted until 100 replicates had recovered trees of the new minimum length. Novel minimum length trees from each replicate were retained, up to 10,000. Once this process was completed, resulting trees were then submitted for branch swapping in the Traditional Search dialog to ensure that all most parsimonious trees were identified, again with a limit of 10,000 trees in total.
Results
Systematic paleontology
| MAMMALIALinnaeus, 1758 |
| EUTHERIAHuxley, 1880 |
| HYAENODONTAVan Valen, 1967 (sensuSolé, 2013) |
| HYAENODONTIDAELeidy, 1869 |
| HYAENODONTINAE (Leidy, 1869) |
| PROPTERODONMartin, 1906 |
Comments—Propterodon was named byMartin (1906) without designation of a type species. In 1925, Matthew and Granger named a new species that they referred toPropterodon,P. irdinensis. In the absence of any prior referral of a species toPropterodon,P. irdinensis became, by default, the type species, a situation that spawned considerable taxonomic confusion and was ultimately resolved byPolly & Lange-Badré (1993).Matthew & Granger (1925) namedPropterodon irdinensis based on jaw fragments, not certainly associated, from Inner Mongolian exposures of the middle Eocene Irdin Manha Formation (Irdinmanhan stage) (Fig. 3). The previous year,Matthew & Granger (1924) had describedParacynohyaenodon morrisi from the same beds, and most recent workers have regarded the two species as conspecific, withPropterodon morrisi the appropriate name for this taxon (Dashzeveg, 1985;Polly & Lange-Badré, 1993;Morlo & Habersetzer, 1999).Dashzeveg (1985) named an additional hyaenodont taxon,Pterodon rechetovi, for two maxillae from the Irdin Manha-equivalent Khaichin Ula 2 fauna from the Khaichin Formation of Mongolia. This species was subsequently made the type species of a new genus,Neoparapterodon, byLavrov (1996), butMorlo & Habersetzer (1999), noting that the upper dentition ofPropterodon morrisi is essentially identical to that ofN. rechetovi, placed the latter genus and species in synonymy with the former. In addition toP. morrisi, three other species ofPropterodon have been named.Propterodon pishigouensis was named byTong & Lei (1986) for a dentary preserving P4-M1 from the Hetaoyuan Formation (Irdinmanhan), Henan Province, China (Fig. 3). As is discussed below, the affinities ofP. pishigouensis, do not appear to lie with eitherPropterodon or with Hyaenodonta generally. An additional Chinese species,P. tongi was named byLiu & Huang (2002) for a dentary with P1-M3 from the Huoshipo locality, Yuli Member of the Hedi Formation (Irdinmanhan), Shanxi Province. This species differs fromP. morrisi in being slightly smaller and in having a more strongly hypercarnivorous morphology, with metaconids lacking at least on M2−3, trigonids more open, and talonids more reduced, especially on M3. Most recently,Bonis et al. (2018) namedPropterodon panganensis for a dentary preserving P4-M1 from the Sharamurunian equivalent Pondaung Formation of Myanmar (Fig. 3). This species has some unusual features (symmetric P4 protoconid, P4 and M1 similar in size, very reduced M1 talonid) that suggest its relationship to otherPropterodon requires confirmation, but it is clearly a hypercarnivorous hyaenodont.
Figure 3:Temporal distribution of significant taxa discussed in this work.
Geomagnetic polarity chrons followOgg, Ogg & Gradstein (2016). North American Land Mammal Age (NALMA) boundaries followTsukui & Clyde (2012) andMurphey et al. (2018). Chinese stage boundaries followWang et al. (2019). Age ranges for hyaenodont and oxyaenodont taxa followProthero (1996),Gunnell et al. (2009),Liu & Huang (2002),Tomiya (2013),Zaw et al. (2014),Solé, Falconnet & Vidalenc (2015),Solé et al. (2016),Wang et al. (2019), and personal observation ofPyrocyon spp. Abbreviations: Ar, Arshantan; Br, Bridgerian; Du, Duchesnean; Ir, Irdinmanhan; Li, Lingchan; Sh, Sharamurunian; Ui, Uintan; Wa, Wasatchian. Drawings by Shawn P. Zack.
PROPTERODON WITTERI, sp. nov. urn:lsid:zoobank.org:act:4D88F815-E7BE-4997-890F-59BC65A06A28
Figure 4:Holotype ofPropterodon witteri sp. nov. (MCZ VPM 19874).
Right dentary with M2−3 in (A) buccal, (B) lingual, and (C) occlusal views. Scale bars are 10 mm. Photographs by Shawn P. Zack.
Holotype—MCZ VPM 19874, left dentary preserving M2−3, the back of the horizontal ramus and almost all of the ascending ramus.
Etymology—Named for R. V. Witter, whose party collected the type and only known specimen in 1940.
Type Locality—Leota Quarry, Uinta Basin, Uintah County, Utah (Fig. 1B).
Stratigraphy and Age—Myton Member of the Uinta Formation (Uinta C,Fig. 1A), late Uintan (Ui3) North American Land Mammal Age (NALMA), late middle Eocene (Prothero, 1996) (Fig. 3).
Diagnosis—Largest known species ofPropterodon, with M2 and M3 lengths approximately 11 and 13 mm, respectively, and dentary depth approximately 25 mm beneath M3. Talonid on M3 relatively large, comparable to M2 talonid. Metaconids on M2−3 present but extremely reduced.
Differential Diagnosis—Differs fromP. panganensis in substantially larger size, with dentary more than 100% deeper. Differs fromP. morrisi in larger size, approximately 40% longer M2−3, more reduced metaconids on M2−3, and a relatively larger talonid on M3. Differs fromP. tongi in larger size, approximately 50% longer M2−3, retention of rudimentary metaconids on M2−3, larger talonids on M2−3, and a less recumbent M3 protoconid.
Description—The preserved portion of the horizontal ramus of the dentary is deep and transversely compressed beneath M3 (Figs. 4A–4B). Posterior to the tooth row, the coronoid process forms an approximately 60-degree angle with the alveolar margin. The process is elongate and extends well above the tooth row, although its dorsal extremity is lacking. The posterior margin of the coronoid process is concave, and the process appears to have overhung the mandibular condyle. On the ventral margin of the dentary, there is a slight concavity between the horizontal ramus and the angular process. The angular process itself is directed posteriorly, with no meaningful ventral or medial inflection. The process is relatively thick, with no medial excavation between the angular process and condyle. The tip of the process extends posterior to the mandibular condyle and has a slight dorsal curvature. The mandibular condyle is positioned at the level of the alveolar border. The condyle is flush with the ascending ramus, with no development of a neck. The visible portion of the condyle is deepest at its medial margin, tapering dorsolaterally. The bone of the ascending ramus is thickest in a low, broad ridge extending anteriorly and somewhat ventrally from the condyle. Just inferior to this ridge, near mid-length of the ascending ramus is the opening of the mandibular canal.
| Specimen Number | Locus | L | TrL | TrW | TrH | TaL | TaW | TaH |
|---|---|---|---|---|---|---|---|---|
| MCZ VPM 19874 | M2 | 11.5 | 7.8 | 5.4 | 9.7 | 3.8 | 4.0 | 4.8 |
| M3 | 13.5 | 10.3 | 6.2 | 12.2 | 3.2 | 3.7 | 4.8 | |
| Dentary depth | 24.7 | |||||||
Notes:
Abbreviations as inFig. 2.
M2 is complete, aside from slight damage to the apex of the paraconid and the buccal base of the talonid (Figs. 4A–4C). The trigonid is much longer and more than twice the height of the talonid. It would likely have been taller, but a large, vertical wear facet on the buccal surface of the paracristid has removed the apex of the protoconid and likely the paraconid. The facet extends nearly to the base of the crown and, occlusally, has exposed dentine of both cusps.
The protoconid is the largest and tallest trigonid cusp. The paracristid descends relatively steeply and directly mesially from its apex to meet the paraconid portion of the paracristid in a deep carnassial notch that is continued lingually as a horizontal groove between the paraconid and protoconid. At the distolingual corner of the protoconid, the vertical protocristid is indistinct near the apex of the cusp, becoming better-defined basally and meeting the metaconid in a small carnassial notch.
Mesially, the paraconid is approximately two-thirds the height of the protoconid. The paraconid portion of the paracristid forms an angle of approximately 45 degrees to the long axis of the crown. From its junction with the protoconid portion, it rises slightly towards the paraconid apex. At the mesial margin of the tooth, the paraconid forms a mesial keel that helps define a flattened, diamond-shaped lingual surface. Lingually, the paraconid and protoconid are fused to a level close to three quarters the height of the former cusp. Buccally, the paraconid supports a strong, vertical mesiobuccal cingulid that extends distally, even with the carnassial notch and projects further mesially than the mesial keel. Together, the cingulid and mesial keel form a well-defined embrasure for the back of the talonid of M1.
The metaconid of M2 is a tiny but distinct cusp positioned high on the protoconid, just below the level of the paraconid apex. The metaconid is fused with the protoconid to a level above the level of fusion of the paraconid and protoconid. The apex of the metaconid is directed slightly distally as well as lingually and bears a distinct crest that meets the protoconid portion of the protocristid.
The talonid is dominated by the hypoconid. The apex of the cusp is worn away but was likely flat topped, as in M3. Buccally, the talonid falls away steeply from the apex of the hypoconid and a wear facet occupies most of the buccal surface of the talonid. Lingually, there is a gentler slope, forming a flat, inclined surface. The cristid obliqua is nearly longitudinal in orientation, meeting the base of the trigonid in a small carnassial notch. The contact is buccal to the level of the metaconid, but still well lingual of the buccal margin of the protoconid, resulting in a shallow hypoflexid.
Near the distal margin of the lingual side of the talonid is a shallow groove that appears to separate the hypoconid from a much smaller, lower hypoconulid. There is no entoconid or entocristid. Aside from the mesiobuccal cingulid, there is no development of cingulids. Buccal enamel extends slightly more basally than lingual enamel.
M3 is larger than M2 and almost unworn but is otherwise quite similar in gross morphology (Figs. 4A–4C). The unworn protoconid of M3 is slightly recumbent and the protoconid portion of the paracristid is modestly more elongate than the paraconid portion. The mesial keel of the paraconid is stronger than on M2 and projects further than the mesiobuccal cingulid. The M3 metaconid is even smaller than on M2, reduced to a projection at the end of the almost vertical protocristid. Even in this rudimentary state, a tiny carnassial notch still separates the cusp from the protoconid, but there is no distal projection of the metaconid, unlike M2.
The talonid is shorter than on M2 and, unlike on the latter tooth, is noticeably narrower distally, with its lingual margin running distobuccally from the lingual base of the protoconid. As on M2, the largest cusp on the M3 talonid is the hypoconid. The unworn M3 hypoconid is flat-topped, but the lingual enamel appears to be thickest near its distal margin, indicating a distal position for the hypoconid apex. As on M2, the cristid obliqua meets the trigonid in a small carnassial notch buccal to the level of the metaconid. From that point, the cristid obliqua continues briefly as a vertical crest that ascends the trigonid, reaching approximately one third of the height of the protoconid. The hypoconulid of M3 is small but better defined than on M2, being separated from the hypoconid by a carnassial notch. At the lingual margin of the talonid, opposite the apex of the hypoconid, is a linear thickening of enamel that suggests the presence of a very weak entocristid.
Comparisons—The strongly hypercarnivorous morphology ofP. witteri distinguishes the new species from known Uintan and older North American hyaenodonts. Among named Uintan hyaenodonts (Matthew, 1899;Matthew, 1909;Hay, 1902;Peterson, 1919;Gustafson, 1986),Mimocyon longipes andSinopa major differ dramatically from the new species, with relatively low, closed trigonids, unreduced metaconids, and large, deeply basined talonids. The limnocyoninesLimnocyon potens andOxyaenodon dysodus show greater carnivorous adaptation thanMimocyon orSinopa, but both have more closed trigonids, larger metaconids, and broader, better-developed talonids thanP. witteri.
WasatchianPyrocyon and BridgerianTritemnodon (Fig. 3) more closely approach the morphology of the new species, but with less developed hypercarnivorous adaptation. M2−3 in species ofPyrocyon (P. dioctetus,P. strenuus) and inTritemnodon agilis resemblesPropterodon witteri in having open trigonids (that is, with the paraconid apex well mesial to the apices of the protoconid and, if present, metaconid) with elongate prevallid shearing blades, reduced metaconids, strong mesiobuccal cingulids (particularly inT. agilis), small, narrow talonids, and reduced hypoconulids. However, in all of these features, the morphology ofP. witteri is more extreme, with more open trigonids with more elongate prevallids, much more reduced metaconids, mesiobuccal cingulids that are stronger and more vertical, and more simplified talonids with a very weak to absent entoconid/entocristid complex, which is retained in bothPyrocyon andTritemnodon. In addition, in bothPyrocyon andTritemnodon, M3 is subequal to M2, while inP. witteri, it is substantially larger.Tritemnodon agilis further differs fromP. witteri in having a shallower, more gracile dentary and a more inclined (less vertical) coronoid process.
The temporal gap betweenPropterodon witteri and species ofPyrocyon andTritemnodon is also problematic (Fig. 3).Pyrocyon is well-known known from mid-Wasatchian faunas (Gingerich & Deutsch, 1989) but does not appear to persist until the end of the interval. In the Willwood Formation of the Bighorn Basin,Pyrocyon disappears from the record during Wa6, well before the end of the densely sampled portion of the Willwood record (Chew, 2009), and the genus is unknown from Wa7 through Uintan faunas.Tritemnodon is well-documented from the earlier portion of the Bridgerian, particularly Br2, but has a limited record from Br3 and no record from the earlier portions of the Uintan (Ui1−2) (Eaton, 1982;Gunnell et al., 2009). A close relationship ofP. witteri to either genus would imply substantial gaps in the hyaenodont record.
Hypercarnivorous hyaenodonts are also present in mid-Eocene faunas from Africa (Furodon), Asia (Propterodon), and Europe (Oxyaenoides) (Matthew & Granger, 1924;Matthew & Granger, 1925;Lange-Badré & Haubold, 1990;Lavrov, 1996;Liu & Huang, 2002;Solé et al., 2014;Solé, Falconnet & Vidalenc, 2015;Solé et al., 2016;Godinot et al., 2018) (Fig. 3). UnlikePyrocyon orTritemnodon, M3 is distinctly larger than M2 in these taxa, a similarity shared withP. witteri. A link to one or more of these taxa would have implications for the origins of the Uinta form and for intercontinental dispersals of hyaenodonts more generally.
Compared toPropterodon witteri the M2−3 trigonids of species of EuropeanOxyaenoides (O. bicuspidens,O. lindgreni,O. schlosseri) are more closed, with a shorter paraconid portion of the paracristid (Lange-Badré & Haubold, 1990;Solé, Falconnet & Yves, 2014;Solé, Falconnet & Vidalenc, 2015;Godinot et al., 2018) (Figs. 5C–5D).Oxyaenoides has completely lost metaconids on all molars, whileP. witteri retains small metaconids on M2−3. InOxyaenoides, the protoconid and paraconid are separated to a level close to the base of the crown, contrasting withP. witteri, where these cusps are fused to approximately mid-height. Both taxa have a distinct mesiobuccal cingulid, but it is much lower inOxyaenoides. While both have reduced talonids, the hypoconulid is relatively larger inOxyaenoides and a more distinct entoconid/entocristid complex is retained, even in the derivedO. schlosseri.Oxyaenoides talonids are also much shorter relative to their width than inP. witteri. Overall,Propterodon witteri displays a mixture of more derived morphologies (open trigonids, trenchant talonids) and less derived morphologies (retained metaconids, elongate talonids) in comparison toOxyaenoides. This pattern is suggestive of parallel developments in lineages assembling a hypercarnivorous morphology independently.
Figure 5:Comparison of M2−3 ofPropterodon witteri sp. nov. with other middle Eocene hypercarnivorous hyaenodonts.
Left M2−3 ofPropterodon witteri, MCZ VPM 19874, in (A) lingual and (B) occlusal views. Right M2−3 (reversed) ofOxyaenoides schlosseri, MNHN.F.ERH 429, in (C) lingual and (D) occlusal views. Left M2−3 ofFurodon crocheti, HGL 50bis-56, in (E) lingual and (F) occlusal views. Right M2−3 (reversed) ofPropterodon morrisi, AMNH FM 21553, in (G) lingual and (H) occlusal views. Left M2−3 ofPropterodon tongi, IVPP V12612, in (I) lingual and (J) occlusal views. All scale bars are 10 mm. Drawings by Shawn P. Zack. (A–B) and (G–H) drawn from photographs by Shawn P. Zack. (C-D) drawn fromSolé, Falconnet & Vidalenc (2015, fig. 4). (E–F) drawn fromSolé et al. (2014, fig. 2). (I–J) drawn from photographs provided by M. Borths.
AfricanFurodon crocheti has more closed trigonids thanPropterodon witteri (Solé et al., 2014) (Figs. 5E–5F). However, the length of the paraconid portion of the prevallid blade is similar, resulting in the paraconid overhanging the lingual margin of the crown inF. crocheti. The metaconid is larger inF. crocheti than inP. witteri. However, whereas inP. witteri, the metaconid is positioned high on the protoconid, almost at the same height as the paraconid apex, it is positioned much lower inF. crocheti. As a result, despite its size, the metaconid apex is substantially lower than the paraconid apex. The talonids ofF. crocheti are relatively larger than inP. witteri, particularly on M2, and the M2 talonid is much wider as well. The M2 hypoconid has a mesial apex inF. crocheti, with a subequal cristid obliqua and hypocristid. InP. witteri, the apex of the hypoconid is distal and there is no hypocristid to speak of. While the hypoconulid appears to be small inF. crocheti, the entoconid/entocristid complex remains prominent, contrasting with the trenchant morphology present inP. witteri. Finally, on the dentary ofF. crocheti, the ventral margin of the angular process grades smoothly into the horizontal ramus, lacking the distinct inflection that occurs inP. witteri.
Some of the features that distinguishF. crocheti fromP. witteri are shared with other, less hypercarnivorous taxa from Africa and South Asia. The paraconid overhang is present in AfricanBrychotherium and South Asian Indohyaenodontinae (Kumar, 1992;Egi et al., 2005;Rana et al., 2015;Borths, Holroyd & Seiffert, 2016), while the low placement of the metaconid is shared with these taxa as well as AfricanGlibzegdouia andMasrasectorSolé et al., 2014;Borths & Seiffert, 2017). A mesially positioned hypoconid apex occurs inGlibzegdouia,Masrasector, and the indohyaenodontinesKyawdawia andYarshea (Egi et al., 2004;Egi et al., 2005;Solé et al., 2014;Borths & Seiffert, 2017). These similarities are consistent with phylogenetic analyses that linkFurodon to African and South Asian hyaenodonts (Rana et al., 2015;Borths, Holroyd & Seiffert, 2016;Borths & Seiffert, 2017;Borths & Stevens, 2019a;Borths & Stevens, 2019b). Their absence inPropterodon witteri indicate that its affinities lie elsewhere.
The morphology of the two best known species of AsianPropterodon,P. morrisi (senior synonym of the type species,P. irdinensis) (Figs. 5G–5H) andP. tongi (Figs. 5I–5J), is quite similar to that ofP. witteri (Matthew & Granger, 1924;Matthew & Granger, 1925;Liu & Huang, 2002). Trigonid proportions of M2−3 inP. morrisi (e.g., AMNH FM 21553) are nearly identical toP. witteri, whileP. tongi has slightly more open trigonids than either species. InP. morrisi, the metaconids of M2−3 are reduced but remain slightly larger than inP. witteri. The opposite is true ofP. tongi, with both M2 and M3 lacking defined metaconids. InP. morrisi, the metaconids are positioned high on the protoconid, comparable toP. witteri. Both Asian species have well-developed, vertical mesiobuccal cingulids that extend high up on the paraconid. Talonid structure is also closely comparable, at least on M2. The Asian species have small talonids (smaller inP. tongi) with distal hypoconid apices, rudimentary hypoconulids positioned directly distal to the hypoconid, and no entoconid/entocristid complex, all identical to the morphology on M2 ofP. witteri. The M3 talonid is more reduced in the Asian forms than in the North American taxon. In the case ofP. tongi, it is reduced to a cuspule on the distal end of the trigonid. The talonid is larger inP. morrisi, but still smaller than inP. witteri. As in the North American form, there does appear to be a trace of an entocristid on the M3’s of AMNH FM 20128 and 21553. Taken together, the morphology ofPropterodon witteri is closely comparable toP. morrisi andP. tongi, particularly the former. The most significant morphological distinction is the relative size of the M3 talonid, which is relatively larger inP. witteri than in either Asian species. Despite this contrast, AsianPropterodon species are clearly the closest matches toP. witteri among relevant taxa, and referral of the new species toPropterodon can be made with confidence.
Figure 6:Phylogenetic position ofPropterodon witteri sp. nov.
Majority rule consensus of 145 most parsimonious trees (L = 510, CI = 0.294, RI = 0.615) showing the inferred phylogenetic position ofPropterodon witteri sp. nov. Numbers below branches indicate percent support, where less than 100 percent. Subfamilies mentioned in the text are labelled. Taxa included in Proviverrinae followsSolé et al. (2015). Abbreviations: Apt, Apterodontinae; Hyd, Hyaenodontinae; Hyl, Hyainailourinae; Ind, Indohyaenodontinae; Lim, Limnocyoninae; Prov, Proviverrinae; Ter, Teratodontinae. Drawings by Shawn P. Zack.
Phylogenetic Results—Analysis of the matrix described in Materials & Methods produced 145 most parsimonious trees (L = 510, CI = 0.294, RI = 0.615), the majority rules consensus of which is shown inFig. 6. Resolution is poor, even using the majority rules rather than a strict consensus. The largest clade unites a paraphyletic Indohyaenodontinae with the three primary African subfamilies (Hyainailourinae, Apterodontinae, Teratodontinae). A second major clade comprises most members of Proviverrinae along withArfia, which is unexpectedly deeply nested within Proviverrinae as the sister taxon ofProviverra andLeonhardtina. Smaller groupings include Limnocyoninae, Hyaenodontinae, and groupings of the North AmericanSinopa andGazinocyon and the European hypercarnivorous generaOxyaenoides andMatthodon. All of these clades form a massive polytomy at the base of the ingroup, along with numerous genera and species of early and middle Eocene hyaenodont.
While disappointing, the poor resolution of the consensus tree is consistent with a lack of clarity in other recent analyses of hyaenodont phylogeny. While the consensus topology is better resolved, most clades recovered byRana et al. (2015) have poor bootstrap support. This is also true in other recent analyses using parsimony (Borths, Holroyd & Seiffert, 2016;Borths & Seiffert, 2017). Most nodes in Bayesian trees recovered by Borths and colleagues (Borths, Holroyd & Seiffert, 2016;Borths & Seiffert, 2017;Borths & Stevens, 2017;Borths & Stevens, 2019a;Borths & Stevens, 2019b) have similarly low posterior probabilities, and there are substantial topological differences between analyses with different assumptions concerning character evolution (e.g., Prionogalidae inBorths & Stevens, 2019a, supplementary fig. 1 versus 2). Simply put, many relationships within Hyaenodonta are neither stable nor well-resolved.
With regard toPropterodon witteri, two conclusions can be made. First, all trees recover a clade linking the new species toPropterodon morrisi,P. tongi, andHyaenodon. Monophyly ofPropterodon is not recovered, with a majority of trees linkingP. tongi andP. witteri more closely toHyaenodon than toP. morrisi on the basis of greater metaconid and entoconid reduction in the former species. These results indicate thatPropterodon is paraphyletic and is likely to be directly ancestral toHyaenodon, although further support would be desirable, particularly as metaconid and entoconid reduction have occurred convergently in many different lineages of carnivorous mammal (e.g.,Muizon & Lange-Badré, 1997).
In addition, the position of Hyaenodontinae within Hyaenodonta is not well-resolved. While hyaenodontine monophyly is supported in all shortest trees, the subfamily is recovered in the large polytomy at the base of the ingroup. This contrasts with recent analyses that have consistently supported some form of a link to European hyaenodonts (Rana et al., 2015;Borths, Holroyd & Seiffert, 2016;Borths & Seiffert, 2017;Solé & Mennecart, 2019;Borths & Stevens, 2019a;Borths & Stevens, 2019b), particularly the hypercarnivorousOxyaenoides. The implications of this aspect of the topology are discussed below
One other result that warrants brief comment is that the two recently described European hyaenodont genera, both described as potential proviverrines (Solé, Falconnet & Yves, 2014;Solé, Falconnet & Vidalenc, 2015),Boritia andPreregidens, are not recovered in proximity to Proviverrinae. Instead, many individual trees recover these genera in positions proximate to species ofPrototomus (specificallyP. martis andP. minimus) andPyrocyon. This includes trees in which the European genera are successive sister taxa toPyrocyon and trees in whichPreregidens is the sister taxon ofPrototomus minimus (withP. martis as sister taxon to this clade). Consistent with this result, both genera lack the distinctive enlarged, bulbous entoconid typical of proviverrine molar talonids (e.g.,Solé, 2013). Of the two,Boritia is very similar to several early Eocene North American hyaenodonts (Prototomus martis,Pyrocyon spp.), and it may represent a parallel development from an early European species ofPrototomus (e.g.,P. girardoti). Alternatively, it may document evidence of faunal exchange between North America and Europe after the Paleocene-Eocene Thermal Maximum, consistent with evidence from the Abbey Wood fauna (Hooker, 2010).
| OXYAENODONTAVan Valen, 1971 |
| OXYAENIDAECope, 1877 |
| MACHAEROIDINAEMatthew, 1909 |
| APATAELURUSScott, 1937 |
| APATAELURUS PISHIGOUENSISTong & Lei, 1986, comb. nov. |
| (Fig. 7) |
| ?Propterodon pishigouensisTong & Lei, 1986:212,Fig. 2, pl. 1.3 |
| ?Propterodon shipigouensisTong, 1997:6 (lapsus calami) |
Holotype—IVPP V7997, left dentary preserving P4-M1.
Type Locality—Shipigou, Liguanqiao Basin, Xichuan County, Henan Province, China.
Stratigraphy and Age—Hetaoyuan Formation, Irdinmanhan stage (Wang et al., 2019).
Revised Diagnosis—Smallest known species ofApataelurus, with P4 and M1 lengths approximately 10 and 9 mm, respectively.
Comparisons and Discussion—Tong & Lei (1986) described IVPP V7997 as a new species ofPropterodon,P. pishigouensis. Compared to other species referred toPropterodon, the most distinctive feature of“P”. pishigouensis is the shape of the dentary, which is ventrally deflected anteriorly, beginning below the anterior root of P4 (Tong & Lei, 1986), indicating the presence of an anterior flange (Fig. 7A). In contrast, the symphysial region is shallow inP. morrisi andP. tongi and tapers anteriorly. In fact, an anterior dentary flange has not been documented in any hyaenodont. The only middle Eocene carnivorous mammals known to possess such a flange are machaeroidines (Scott, 1938;Matthew, 1909;Gazin, 1946;Dawson et al., 1986), a small clade of North American Wasatchian through Uintan carnivores recently supported as oxyaenids (Zack, 2019).
Figure 7:Comparison ofApataelurus pishigouensis comb. nov. withA. kayi.
(A)Apataelurus pishigouensis, IVPP V7997, left dentary with P4-M1; (B)Apataelurus kayi, CM 11920, right dentary with P3-M2 (reversed). Both images show the dentary in buccal view. Arrows indicate the ventral deflection of the dentaries of both specimens. Note that the apparently greater height of the protoconids on P4 and M1 and paraconid on M1 inA. pishigouensis reflects much heavier wear inA. kayi. All scale bars are 10 mm. Drawings by Shawn P. Zack. (A) drawn fromTong & Lei (1986, pl. 1). (B) drawn from a photograph by Shawn P. Zack.
Machaeroidines, particularly the UintanApataelurus kayi, share substantial similarities with the type specimen of“Propterodon” pishigouensis, including features that distinguish the latter species from otherPropterodon (Fig. 7). On P4, bothA. kayi andpishigouensis have a well-developed paraconid that is nearly as tall as the talonid (Scott, 1938;Tong & Lei, 1986). The paraconid is absent on P4 inP. tongi (Liu & Huang, 2002). InP. panganensis it is low and weakly developed (Bonis et al., 2018). While all relevant species have simple P4 talonids dominated by a tall hypoconid, inpishigouensis andA. kayi, the talonid is distinctly broader than the remainder of the crown (Scott, 1938;Tong & Lei, 1986). In contrast, P4 width is uniformly narrow inP. panganensis andP. tongi (Liu & Huang, 2002;Bonis et al., 2018). InPropterodon tongi and, to judge the roots of P4,P. morrisi, P4 is enlarged relative to M1 (Matthew & Granger, 1925;Liu & Huang, 2002). Inpishigouensis andA. kayi, along withP. panganensis, the two teeth are subequal in size (Scott, 1938;Tong & Lei, 1986;Bonis et al., 2018).
On M1, a defined metaconid is lacking inpishigouensis andA. kayi (Scott, 1938;Tong & Lei, 1986), again along withP. panganensis (Bonis et al., 2018), but retained inP. morrisi (e.g., AMNH FM 21553), with M1 ofP. tongi too worn to assess. The primary difference in M1 morphology is in the talonid. The talonids ofP. morrisi,P. tongi, andP. panganensis are short and much lower than the paraconid (Matthew & Granger, 1925;Liu & Huang, 2002;Bonis et al., 2018; pers. obs. of AMNH FM 21553). Inpishigouensis andA. kayi, the talonid is relatively elongate and nearly as tall as the paraconid (Scott, 1938;Tong & Lei, 1986). Talonid morphology is simplified in bothpishigouensis andA. kayi, with both taxa only retaining a hypoconid. InP. morrisi andP. tongi, some lingual structure is retained, although the extremely reduced talonid ofP. panganensis is also simplified.
Taken together, the mandibular and dental morphology of“Propterodon” pishigouensis differs substantially from other species ofPropterodon, particularlyP. morrisi andP. tongi, but closely matches the morphology of the North American machaeroidineApataelurus kayi. Accordingly,Propterodon pishigouensis is recombined asApataelurus pishigouensis. As a species ofApataelurus,A. pishigouensis differs fromA. kayi primarily in its somewhat smaller size. The talonid ofA. pishigouensis may be smaller than that ofA. kayi, but this is complicated by heavier wear in the type and only described specimen of the North American form. Referral ofpishigouensis to Machaeroidinae represents the first clear record of a machaeroidine in Asia.
There may be an additional, older Asian machaeroidine, also initially described as a hyaenodont.Isphanatherium ferganensis was named for an isolated upper molar from the Andarak-2 fauna (Lavrov & Averianov, 1998). The morphology ofI. ferganensis is strikingly derived for an early hyaenodont, with an extremely elongate, longitudinally oriented postvallum blade and a strongly reduced protocone. Both of these features would be consistent with a machaeroidine identity. The overall morphology of the type ofI. ferganensis is closely comparable to M1 ofMachaeroides spp. from the early and middle Eocene of North America (Gazin, 1946;Dawson et al., 1986). They share development and orientation of the metastylar blade, protocone reduction without mesiodistal compression, fusion of the paracone and metacone to a point close to their apices, with the metacone taller than the paracone, and the presence of a low but distinct parastyle that is continuous with a buccal cingulum that is restricted to the mesial portion of the crown. A specific similarity shared byI. ferganensis andM. simpsoni (S Zack, pers. obs., 2019 of CM 45115) is the presence of contrasting compression of the paracone and metacone, with the former compressed mesiodistally while the latter is compressed transversely. More material is needed to be certain, but the age and morphology ofIsphanatherium ferganensis supports the tentative reidentification of the species as a machaeroidine and of the holotype as an M1 rather than an M2.
Discussion
Hyaenodontine Origins—Recent assessments of hyaenodont biogeography (Borths, Holroyd & Seiffert, 2016;Borths & Stevens, 2017) have supported a European divergence of Hyaenodontinae fromOxyaenoides, which was recovered as the sister taxon of Hyaenodontinae in both analyses. This grouping is nested within a broader assemblage of European hyaenodonts comprising taxa referred to Proviverrinae bySolé (2013) andSolé et al. (2015). More recent studies (Borths & Stevens, 2019a;Borths & Stevens, 2019b;Solé & Mennecart, 2019) complicate this scenario slightly by recovering Prionogalidae andThereutherium within the clade defined byOxyaenoides and Hyaenodontinae, but the basic biogeographic scenario is unchanged, with Hyaenodontinae deeply nested within a clade of European hyaenodonts. As was noted byBorths & Stevens (2019a) with regard to the position of Prionogalidae, the character support unitingOxyaenoides,Thereutherium, Prionogalidae, and Hyaenodontinae consists primarily of features associated with hypercarnivory, specifically reduction of the metaconids and talonids on lower molariform teeth. Hypercarnivory has evolved iteratively in diverse carnivorous mammalian clades and homoplasy in features associated with hypercarnivory is well-documented (Muizon & Lange-Badré, 1997;Holliday & Steppan, 2004;Solé & Ladevèze, 2017). Accordingly, support for a close relationship betweenOxyaenoides and Hyaenodontinae should be regarded cautiously, despite its recovery in several analyses.
In contrast to the analyses just discussed, results of the current phylogenetic analysis do not place Hyaenodontinae phylogenetically proximate toOxyaenoides, nor do the results of theRana et al. (2015) analysis. While the position of Hyaenodontinae is not consistently resolved in the present study, a sister taxon relationship toOxyaenoides is not present in any most parsimonious tree. Some most parsimonious trees (MPTs) do recover Hyaenodontinae as the sister taxon of Proviverrinae, as used bySolé (2013) andSolé et al. (2015b). However, other MPTs recover Hyaenodontinae as the sister taxon of North American and EuropeanGalecyon or to a clade comprisingGalecyon plus HolarcticArfia. Still other MPTs place Hyaenodontinae at the base of a diverse grouping that includes all sampled taxa exceptingArfia and Proviverrinae, with Asian and North American Limnocyoninae the next diverging clade. There is no particular support in this analysis for a European origin for Hyaenodontinae.
In fact, a European origin appears unlikely. UnlikeOxyaenoides, which shares some distinctive dental features with other proviverrines, including a double-rooted P1 and molar talonids with three, more or less equally developed and equidistantly spaced cusps, hyaenodontine dental morphology has little in common with proviverrines. The relatively large P1 remains single-rooted inP. morrisi andP. tongi (Matthew & Granger, 1925;Liu & Huang, 2002), while the entoconid and hypoconulid are weakly developed in all species ofPropterodon. With the exception of a reduced metacingulum on M1−2, other distinctive proviverrine dental features enumerated bySolé (2013) (entoconids on P3−4, prominent paraconids on P2−3 and parastyle on P4, M1−2 with metacones taller than paracones) are absent inPropterodon (Matthew & Granger, 1925;Lavrov, 1996;Liu & Huang, 2002).
Biogeographic evidence also suggests that derivation of hyaenodontines from within the European Eocene hyaenodont radiation is unlikely. From the late early Eocene through the Eocene/Oligocene transition, Europe was an island isolated from the rest of Holarctica (e.g.,Meulenkamp & Sissingh, 2003), resulting in the evolution of a diverse endemic mammalian fauna (Hooker, 1989;Badiola et al., 2009;Danilo et al., 2013). This period encompasses the radiation of proviverrine hyaenodonts (sensuSolé, 2013), which formed the dominant carnivorous element of this endemic European fauna. There is little evidence of mammalian dispersal from Europe to Asia during this interval.
In fact, there is some evidence from the fossil record consistent with an earlier Asian record of Hyaenodontinae. The ?Arshantan fauna from Andarak-2, Khaichin Formation, Kyrgyzstan, includes a fragmentary hyaenodont dentition (ZIN 34494) described byLavrov & Averianov (1998) as similar toNeoparapterodon rechetovi, the latter a likely synonym ofPropterodon morrisi according toMorlo & Habersetzer (1999). If correctly identified, this would extend the Asian record of Hyaenodontinae back to the early part of the middle Eocene and would support an Asian origin for the subfamily. Unfortunately, the hyaenodont record from both the Arshantan and the preceding Lingchan (equivalent to the Bumbanian) is very poor. Aside from ZIN 34494, the published hyaenodont record from the Arshantan is limited to the type specimen ofIsphanatherium ferganensis (Lavrov & Averianov, 1998), which may not be a hyaenodont (see above). Lingchan hyaenodont records comprise two specimens referred to distinct species ofArfia and two specimens referred to?Prototomus sp. (Lavrov & Lopatin, 2004;Tong & Wang, 2006;Morlo et al., 2014;Solé, Gheerbrant & Godinot, 2013). Until early and early middle Eocene hyaenodonts from Asia are better documented, it is difficult to determine what role, if any, Asia played in the origin of Hyaenodontinae.
Late Uintan Carnivore Dispersals—In addition toPropterodon, several other carnivorous taxa that first appear in the late Uintan (Ui2−3) have a potential origin outside western North America. Among hyaenodonts, the limnocyonineOxyaenodon dysodus is quite distinct fromLimnocyon potens, the only limnocyonine known from the early Uintan. Compared toL. potens,O. dysodus is smaller and more hypercarnivorously adapted, with smaller, less basined talonids and a longer M2 prevallid blade.Oxyaenodon dysodus also retains a full complement of relatively uniform incisors, whileL. potens has enlarged I2 and lost I3 (Denison, 1938). WhileMorlo & Gunnell (2005) recoveredO. dysodus andL. potens as sister taxa in a phylogenetic analysis of limnocyonines, an earlier analysis of a nearly identical matrix (Morlo & Gunnell, 2003) recoveredO. dysodus as the sister taxon of BridgerianThinocyon medius, outside of a monophyleticLimnocyon (note that the consensus tree shown inMorlo & Gunnell (2005, fig. 1) is in error; all four shortest trees found by analyzing the published matrix without modification recoverThinocyon medius rather than BridgerianLimnocyon as the sister taxon ofL. potens plusO. dysodus). BothMorlo & Gunnell (2003) andTong & Lei (1986) have noted similarities to the Irdinmanhan Chinese taxonProlaena parva. Taken together, it is plausible that the appearance ofOxyaenodon in the late Uintan reflects immigration from Asia, similar to the pattern hypothesized forP. witteri. A full assessment of the affinities ofOxyaenodon is beyond the scope of this study. Published descriptions and illustrations of material ofO. dysodus are inadequate to confidently score the species, and substantial additional material remains unpublished (Friscia & Dunn, 2016).
The affinities of another late Uintan hyaenodont, the small undescribed taxon or taxa referenced above are unclear at present, but small hyaenodontid material from the Mission Valley Formation appear to document a non-limnocyonine with a narrow M1 talonid (S Zack, pers. obs., 2019), very divergent from bothLimnocyon orSinopa, the only hyaenodont genera known from the early Uintan.
Other carnivorous groups show a similar pattern. At least two machaeroidine taxa are present in late Uintan faunas (Scott, 1937;Scott, 1938;Rasmussen et al., 1999;Wagner, 1999;Zack, 2019), but none is known from Ui1. Among miacids, several taxa appear in the late Uintan without obvious Ui1 antecedents, includingTapocyon spp.,“Miacis” uintensis, and“M.” hookwayi (Wesley & Flynn, 2003;Spaulding & Flynn, 2009;Tomiya, 2013). Finally, the enigmatic carnivorous mammalSimidectes first appears in the late Uintan, again without obvious early Uintan relatives (Coombs, 1971).
The lack of an early Uintan ancestry for some taxa may reflect limited data from the Ui1 interval, which remains relatively poorly sampled. With this caveat, the discovery ofPropterodon witteri is evidence of a potential Asian origin for many of the carnivorous taxa that first appear in the late Uintan. Referral ofPropterodon pishigouensis toApataelurus documents an additional tie between the carnivorous faunas of the Irdinmanhan and Uintan. In addition, both the hyaenodontSinopa and the mesonychidHarpagolestes are shared by Irdinmanhan and Uintan faunas (Jin, 2005;Jin, 2012;Morlo et al., 2014;Robson et al., 2019). The Huadian Formation fauna containingS. jilinia was considered post-Irdinmanhan in age byMorlo et al. (2014) based on the stage of evolution of the omomyidAsiomomys, but the presence ofZelomys, a genus otherwise known from the Irdinmanhan Yuli Member of the Hedi Formation (Dawson et al., 2003) suggests an older age. Carnivore dispersals from Asia to North America during the later Uintan would be concordant with evidence for dispersal of other mammals from Asia to North America during this interval, including the chalicotheroid perissodactylGrangeria and the omomyid primateMacrotarsius in Ui2 (Woodburne, 2004). Ui3 sees additional dispersals including several brontotheriid perissodactyls, andMytonolagus, the oldest known North American lagomorph (Woodburne, 2004;Mihlbachler, 2008).
A complicating factor is the poor quality of the Asian middle Eocene carnivore record. As discussed above, the Lingchan and Arshantan record of hyaenodonts is extremely poor, and other carnivorous clades are also poorly sampled in both intervals. The Irdinmanhan record is somewhat better but remains inadequate. Among non-mesonychians, Irdinmanhan hyaenodonts include two species ofPropterodon,P. morrisi andP. tongi, the sinopanineSinopa jilinia, and the limnocyonineProlaena parva (Matthew & Granger, 1924;Matthew & Granger, 1925;Xu et al., 1979;Tong & Lei, 1986;Lavrov, 1996;Liu & Huang, 2002;Morlo et al., 2014). In addition to the machaeroidineApataelurus pishigouensis, the last recorded oxyaenine,Sarkastodon hetangensis, occurs in the Irdinmanhan (Tong & Lei, 1986). Finally, Irdinmanhan miacoids are represented by three species, all questionably referred toMiacis:M. boqinghensis,M. invictus, andM. lushiensis (Matthew & Granger, 1925;Chow, 1975;Tong & Lei, 1986;Qi, Zong & Wang, 1991;Huang, Tong & Wang, 1999). Of these, onlyPropterodon morrisi andMiacis lushiensis are represented by multiple specimens (this may be in error forM. lushiensis as the size and morphology of referred material suggests the presence of multiple species).
Considering the limited nature of the Asian record, the presence of four genera shared between Uintan and Irdinmanhan faunas (Harpagolestes,Apataelurus,Sinopa,Propterodon) constitutes clear evidence for substantial exchange of carnivorous mammals during this interval. As noted above,Prolaena can be potentially added to this list althoughMorlo & Gunnell (2003) were skeptical of a relationship between AsianProlaena and North AmericanOxyaenodon. Despite the assignment of species on both continents to a wastebasket“Miacis”, there is less obvious overlap between miacoids, although“Miacis” lushiensis has been compared with Bridgerian“M”. hargeri (Tong & Lei, 1986). Further study will be required to confirm this possibility and assess the potential for North American connections for other Irdinmanhan“Miacis”. For the present, it is clear that investigations into the decline in North American hyaenodont diversity and coincident rise in carnivoraform diversity must consider the role of immigration in shaping the North American carnivore guild during the Uintan.
Conclusions
The new species described in this work,Propterodon witteri, is the first known North American representative of the genusPropterodon. Comparisons of the new species with other early and middle Eocene hypercarnivorous hyaenodonts support a link to AsianPropterodon and Hyaenodontinae more generally, a conclusion supported by the results of the phylogenetic analysis. The broader relationships of Hyaenodontinae are not well-resolved. Despite being supported by several phylogenetic assessments, a link to EuropeanOxyaenoides is unlikely. An Asian origin for Hyaenodontinae is more likely, but better material of poorly known Linchan and Arshantan hyaenodonts is needed to test this hypothesis. Recognition of a Uintan hyaenodontine and an Irdinmanhan machaeroidine increases the evidence for dispersal of carnivorous mammals between Asia and North America during the late middle Eocene. Much of the apparent shift in North American carnivorous guilds, from “creodont” to carnivoramorphan dominated, may ultimately reflect the effects of this immigration rather than intrinsic processes within North American faunas.