INTRODUCTION

The Duchesnean North American Land Mammal Age (NALMA), ca. 41–38 million years ago (Ma; Kelly et al., 2012), marks a pivotal early stage of the Eocene–Oligocene terrestrial faunal transition in North America (Prothero, 1994; Rasmussen et al., 1999; Robinson et al., 2004; Figueirido et al., 2012). It is also an important period for understanding the demise of early carnivoraform lineages (i.e., close relatives of carnivorans) and the rise of crown-group carnivorans (cf. Tomiya, 2011; Tomiya and Tseng, 2016). Although a diverse array of carnivorous mammals is known from Duchesnean-age deposits of what is now the southwestern U.S.A., many of them are represented by few specimens (Gustafson, 1986; Tomiya, 2013). As such, careful documentation of all available materials is essential for clarifying the patterns of Duchesnean carnivore diversity in time and space. The purpose of this paper is to document a hitherto uncited and undescribed specimen that evidently formed the basis for an occurrence of the carnivoraform Uintacyon in the Duchesnean portion of the Galisteo Formation of New Mexico (Stearns, 1943). If that previous taxonomic identification were correct, this specimen would represent the last surviving member of the oldest known carnivoraform genus (cf. Solé et al., 2016). However, on the basis of a close inspection of the dental morphology, we identify it instead as one of the earliest amphicyonid carnivorans, or the “beardogs.”

METHODS

Anatomical terminology follows primarily Tomiya and Tseng (2016) and in part Van Valen (1966). We tentatively accept Carnivoraformes and Carnivoramorpha (the latter consisting of viverravids and carnivoraforms) as successively more inclusive laurasiatherian clades that include crown-group Carnivora (Flynn et al., 2010), while recognizing the need for additional testing of the hypothesized monophyly of Carnivoramorpha (cf. Zack, 2019). Measurements were taken with digital calipers to the nearest 0.01 mm.

Institutional Abbreviations. CM, Carnegie Museum of Natural History, Pittsburgh, Pennsylvania, U.S.A.; FMNH, Field Museum of Natural History, Chicago, Illinois, U.S.A.; MCZ, Museum of Comparative Zoology, Cambridge, Massachusetts, U.S.A.; TMM, Texas Memorial Museum (in collection of the Vertebrate Paleontology Laboratory, University of Texas at Austin), Austin, Texas, U.S.A.

RESULTS

Figure 1.

Occurrences of Duchesnean amphicyonids in North America (crossed circles for Angelarctocyon). Abbreviations: B, Badwater Creek area (Wagon Bed Formation, Wyoming); L, Lac Pelletier lower fauna (Cypress Hills Formation, Saskatchewan, Canada); P, Porvenir local fauna (Chambers Tuff, Texas); T, Tonque local fauna (Galisteo Formation, New Mexico); ?H, Halfway Hollow area (Duchesne River Formation, Utah; possible Brachyrhynchocyon). See Table 3. Relief map from U.S. Geological Survey National Center for EROS (2005).

Figure 2.

Dentaries of Angelarctocyon. A, F, right partial dentary of FMNH PM 423 (holotype of A. australis) in lateral (A; inverted) and medial (F) views. B–E, right partial dentary of MCZ VPM-4225 (Angelarctocyon cf. A. australis) in lateral (B; inverted), occlusal (C; close-up of teeth in D), and medial (E) views. In D, hatched area indicates breakage on entoconid, and black triangles point to minor cuspulids along posterior margin of talonid basin. Same 10-mm scale bar applies to A–C, E, F. Images in B–E courtesy of Museum of Comparative Zoology, Harvard University (©President and Fellows of Harvard College). Abbreviations: aci, anterior cingulid; ap, angular process; hyd, hypoconid; mc, mandibular condyle; msy, mandibular symphysis; nac, notch anterior to cristid obliqua; pac, posterior accessory cuspulid; pbr, posterior basal ridge; pci, posterior cingulid; pcr, paracristid.

Table 1.

Measurements (mm) of MCZ VPM-4225 (Angelarctocyon cf. A. australis).

Table 3.

Distributions of carnivoraforms from late Uintan (Ui3 subage) to Duchesnean of North America

DISCUSSION

MCZ VPM-4225 was previously identified on the specimen label (Fig. 3A) and in the MCZ collections database (as of 20 September 2017) as the noncarnivoran carnivoraform Uintacyon. However, it clearly differs from Uintacyon in having: (1) a slender dentary that tapers more gradually toward its anterior end; (2) labiolingually more compressed lower premolars of less disparate sizes (in contrast to the much smaller sizes of p2 and p3 relative to p4 in Uintacyon); (3) a more dorsally positioned posterior accessory cuspulid on p4; (4) a relatively longer m1 trigonid, in which the prevallid is appreciably longer than the postvallid; and (5) a labiolingually broader m1 talonid and talonid basin (the talonid basin is typically only about half as wide as the trigonid in Uintacyon). It further differs from middle Eocene species of Uintacyon in having a more concave m1 talonid basin and likely an m2 with a more anteroposteriorly compressed trigonid (based on the shape of the anterior alveolus). Instead, this carnivoraform most closely resembles the early amphicyonid carnivoran A. australis, which thus far is known from just two specimens (a set of jaws and a dentary fragment with m2) from the lower portion of Chambers Tuff in Texas (Gustafson, 1986; Tomiya and Tseng, 2016).

Figure 3.

Specimen label (A) and field label (B) for MCZ VPM-4225 (color and contrast were digitally modified to enhance legibility). Courtesy of Museum of Comparative Zoology, Harvard University (©President and Fellows of Harvard College).

Indeed, the specimen from New Mexico exhibits all the differentially diagnostic traits of Angelarctocyon so far as the lower dentition is concerned (Tomiya and Tseng, 2016). In comparison with other small Eocene amphicyonids, it differs from: (1) Daphoenus lambei in being >30% smaller (in m1 anteroposterior length; cf. Hunt, 1996); (2) Daphoenus demilo (which may be con-specific with D. lambei; Hunt, 1996) in having a posteriorly more gracile dentary (dentary depth below m1 is roughly 150% of the height of m1 compared with ∼200% in the holotype CM 15596; Dawson, 1980: fig. 2), relatively taller lower premolars, and a single-rooted m3 (double-rooted in CM 15596); (3) Cynodictis lacustris (genotypic species of Cynodictis) in having less robust premolars and a less open m1 trigonid. Direct comparison with the Chadronian Gustafsonia cognita is not possible, as it is known only from the holotype cranium, but the linear dimensions of the teeth of MCZ VPM-4225 are roughly 20% larger than expected for the holotype of G. cognita, TMM 40209-200, whereas they closely match those of A. australis (cf. Gustafson, 1986).

Although the specimen is here confidently referred to the currently monospecific genus Angelarctocyon, its species-level referral to A. australis is tentative considering the dental morphological differences from FMNH PM 423 as described above. These differences, although subtle, are all suggestive of more plesiomorphic conditions, which are consistent with the potentially older geologic age of MCZ VPM-4225 (see below). Alternatively, these minor differences could be ascribed to geographic variations over a potentially biogeographically significant distance (∼600 km at present) that separated the New Mexico locality from the holotype locality of A. australis in Texas. In any case, a close affinity between MCZ VPM-4225 and FMNH PM 423 is indubitable, and it would be premature to propose a separate species on the basis of the very limited data at hand.

The specimen metadata preserved at MCZ, combined with information in the published literature, help illuminate the long-obscure history of MCZ VPM-4225 (Table 2), which we reconstruct as follows (see below for justification). In the summer of 1938, Theodore E. White, then a research associate working with Alfred S. Romer at MCZ (Lyman, 2016), conducted fieldwork at Sweet's Ranch near Los Cerrillos, New Mexico and collected the specimen (Stearns, 1943). It was recorded as a “Daphoenus jaw” on the field label and later updated as Daphoenus vetus (Fig. 3B). It is not entirely certain whether these identifications were made by White himself, who also listed it as “Cynodictus” [sic] in his field notes (Lucas, 1982). The latter identification may have been referring to the Eurasian amphicyonid Cynodictis or the North American canid Hesperocyon (which was sometimes classified as Cynodictis; cf. Wang, 1994). Then, on the basis of this specimen, Stearns (1943:310) reported an occurrence of “Uintacyon sp.” at Sweet's Ranch, attributing that taxonomic identification to White. Stearns did not, however, cite or describe the voucher specimen for this purported occurrence. In reviewing the vertebrate fossil assemblages from the Galisteo Formation, Lucas and Kues (1979) were unable to locate the pertinent material at MCZ and suggested that the specimen might have been lost. Subsequently, Lucas (1982:18) discovered the 1938 field notes of White at MCZ and determined that the jaw of Cynodictus [sic] listed in them likely corresponded to Uintacyon sp. of Stearns (1943). Still, the specimen itself was nowhere to be found, and in consultation with the then-MCZ curatorial associate Charles Schaff, he concluded that “it never was entered into the MCZ collection” (Lucas, 1982:19). Being unable to confirm the identity of Uintacyon, Lucas (1982) listed it as an indeterminate carnivoran in his faunal list for the upper portion of the Galisteo Formation (Tonque local fauna).

Table 2.

Synoptic history of MCZ VPM-4225.

Here, MCZ VPM-4225 is cited for the first time as the basis for the occurrence of Uintacyon reported by Stearns (1943). Even though the presumed original specimen label (Fig. 3A) lists the year of collection as 1938, not 1939 as noted by Stearns (1943:310), there is little doubt about their correspondence because, as far as we are aware, it is the only MCZ specimen from Sweet's Ranch that was at one point identified on its specimen label as Uintacyon. In fact, it is the only carnivoramorphan specimen from Sweet's Ranch in the MCZ collections catalog. Furthermore, one of us (ZSM) searched the MCZ archive but did not find field notes or records by White from any trip in 1939 to Sweet's Ranch. Therefore, it is unlikely that he collected there in 1939, and Stearns (1943) must have incorrectly reported the collecting year, as suspected by Lucas (1982). We are also confident in linking this specimen to Cynodictus [sic] from Sweet's Ranch mentioned in White's unpublished 1938 field notes (cf. Lucas, 1982) because it is the only carnivore he lists, and the dentary of MCZ VPM-4225 matches the description as a jaw.

The relatively primitive morphology of Angelarctocyon compared with better-known later amphicyonids has generated much confusion and delayed its recognition as one of the earliest beardogs (Tomiya and Tseng, 2016). MCZ VPM-4225 appears to have undergone the same fate. At least five different taxonomic identities—including as a relative of procyonids and an undescribed species of the early carnivoramorphan Miacis—have been suggested for this specimen since its discovery 82 years ago (Table 2). In retrospect, it is interesting that the initial, never-published identification as either Daphoenus or Cynodictis (assuming that the latter referred to the Eurasian amphicyonid and not Hesperocyon) proved to be the most perspicacious. It should be noted that the recognition of A. australis as an amphicyonid benefited from: (1) the preservation of several upper teeth in the holotype, which was collected in 1946 but remained undescribed for 40 years (Gustafson, 1986); and (2) computed-tomographic data on the internal cranial morphology of its close relative, G. cognita (Tomiya and Tseng, 2016). Without a web of such information, accurate family-level classification of an isolated dentary like MCZ VPM-4225 had to be difficult.

MCZ VPM-4225 represents one of the earliest records of the family Amphicyonidae. Prothero and Lucas (1996) correlated an upper portion of the Galisteo Formation in the vicinity of Los Cerrillos, which had yielded elements of the Tonque local fauna, with Chrons C18n.1n to C17n.3n (39.55–37.92 Ma; Kelly et al., 2012). Thus, MCZ VPM-4225 is potentially older than the holotype FMNH PM 423 of A. australis (from a portion of the Chambers Tuff, Texas, which was correlated with C17n.3n by Kelly et al. [2012]) by as much as ∼1.6 million years. If future work shows the Tonque local fauna to be indeed older than the Porvenir local fauna, it will make Angelarctocyon the second-oldest amphicyonid genus after Daphoenus, which first appears at the early-Duchesnean Locality 20 in the Badwater Creek area, Wyoming (Dawson, 1980; Hunt, 1996).

The rediscovery of Stearns' Uintacyon is significant because if MCZ VPM-4225 actually belonged to Uintacyon, it would represent the last known—and the only post-Uintan—occurrence of the genus (cf. Flynn, 1998). With our reidentification of this specimen, the last occurrence of Uintacyon is pushed back to the Uintan NALMA and is represented by the holotype YPM VP 013101 of Uintacyon acutus from the Uinta Formation of Utah (Thorpe, 1923). This last appearance date happens to agree with the temporal range of the genus reported by Heinrich et al. (2008), although U. acutus was not included in their list of species. The timing of the demise of Uintacyon, which first appeared in the late Paleocene (Gingerich, 1983), accentuates the pattern of almost complete genus-level replacement of early-diverging carnivoraforms by crown-group carnivorans between the late Uintan and Duchesnean NALMAs (Table 3; see Tomiya [2011], Solé et al. [2014], and Tomiya and Tseng [2016] for phylogenetic hypotheses). This transition apparently coincided with major climatic fluctuations between ∼42 and 40 Ma, which culminated in the middle Eocene climatic optimum (Bohaty and Zachos, 2003; Doria et al., 2011 and references therein). The only ancient carnivoraform lineage that clearly survived beyond the Uintan was Miocyon. It is securely known from the Duchesnean of Saskatchewan, Canada, by an aberrantly large species Miocyon magnus (Bryant, 1992), underscoring that taxa with more typical adaptations to the environmental conditions of the late Uintan were selected against during the Uintan–Duchesnean turnover event. Taken together, these observations suggest the possibility that the rise of crown-group carnivorans in North America was catalyzed by the late middle Eocene global climatic instability.