Introduction

The order Pantodonta is of uncertain relationships within the Mammalia. Pantodonts were mostly large herbivorous or omnivorous quadrupeds that are known from many Paleogene terrestrial faunas across the Holarctic (Simons, 1960). Eight Paleogene pantodont families are recognized, each restricted to Asia or North America but not occurring on both continental areas except for one genus, Coryphodon, of the family Coryphodontidae (McKenna & Bell, 1997), which is Holarctic in distribution. The other coryphodontid genera are only Asian, although no pantodonts are known from southern Asia or Africa. In North America, coryphodontids are known from Alabama and Mississippi on the Gulf Coast and southern Texas (Westgate, 2001; Beard & Dawson, 2009) to the Eureka Sound Group on central Ellesmere Island, Nunavut, Canada, reported here.

Systematic palaeontology

Family Coryphodontidae (Marsh, 1876)

Genera of coryphodontids are: in North America, Coryphodon, late Paleocene (mid-Clarkforkian)-early middle Eocene (Bridgerian); in Europe, Coryphodon, early Eocene (Sparnacian); in Asia, Coryphodon, late Paleocene (Gashatan)-early Eocene (Bumbanian) Wutucoryphodon (including C. dabuensis), late Paleocene-early Eocene, Heterocoryphodon, early-middle Eocene, Asiocoryphodon, early-middle Eocene, Hypercoryphodon, middle Eocene, Metacoryphodon, middle Eocene, and Eudinoceras, middle-late Eocene.

Coryphodon (Owen, 1845)

Stratigraphic and geographic distribution: Late Paleocene (middle Clarkforkian)-early Eocene (Wasatchian), early middle Eocene (Gardnerbuttean) of North America; early Eocene (MP 7-9) of Europe; late Paleocene (Gashatan)—early Eocene (Bumbanian) of Asia.

Terminology for dental structures follows the basic plan outlined by Simpson (1929) with modifications related to M3 following Uhen and Gingerich (1995).

Coryphodon pisuqti new species

  • Coryphodon lobatus (Lucas, 1984, p. 39; 1998, p. 475).

  • Coryphodon anthracoideus (Lucas, 1984, p. 37; 1998, p. 475).

  • Coryphodon subquadratus (Lucas, 1984, p. 39; 1998, p. 475).

Holotype: Canadian Museum of Nature (CMN) 32420, four associated large concretions: one containing right P1, alveoli of P2–4, and left P2–M2; one containing left M3; one containing left P2–M1; one containing right M2–3; and associated loose teeth and bones including an upper canine, and left M2–3 (locality 76-34).

Referred specimens CMN 30802, incomplete right mandible with dP3–4; CMN 30803, left P3, CMN 30811, left M3/3, canine, other tooth fragments; CMN 30961, partial skull; CMN 30965, right and left mandibles with teeth; CMN 30969, edentulous right mandible; CMN 30973, left mandible fragment, M3, skull fragments; CMN 30975, mandible fragment; CMN 32421, edentulous mandibles; CMN 32422, edentulous posterior part of mandible; CMN 32423, skull and mandible fragments, upper and lower teeth; CMN 32424, upper molar and other dental fragments; CMN 32431, right and left partial mandibles; CMN NUFV24, incomplete maxilla. Other unnumbered tooth fragments and bones.

Horizon and localities Wasatchian; Margaret Formation (Miall, 1986; West & Dawson, 1977, member IV), Eureka Sound Group (Ellesmere Island, Nunavut: Mattthew and Troelson peninsulas in Bay Fiord-Strathcona Fiord area; Stenkul Fiord).

Diagnosis Large species of Coryphodon (M3 anteroposterior, 47.3 mm). M3 long with distinct hypoconulid and supporting section of talonid root clearly set off by buccal and lingual grooves; M3 differs from that of C. proterus, in which the hypoconulid is even more pronounced, in having hypoconulid lobe relatively more linguad in position and thus closer to the cuspate entoconid; small metastylid on lingual wall anterior to entoconid. I1 largest upper incisor. M3 having long lingual side, distinct paraconule on protoloph situated at about transverse midpoint, postprotocrista extending obliquely posterobuccally from protocone, and complete anterior to lingual cingulum that posterolingually bears a distinct swelling (“hypocone”), as in C. proterus. Lower incisors wide transversely, flattened anteroposteriorly. Talonid of P4 relatively short anteroposteriorly.

Etymology Inuktitut, pisuqti (pee-shuk-tee), one who walks around most of the time, in reference to the Eocene wanderings of Coryphodon.

Description The following incorporates morphological details from the holotype, CMN 32420, and referred specimens from the Margaret Formation in the Bay Fiord-Strathcona Fiord area as well as from equivalent-aged deposits at Stenkul Fiord.

Skull No complete skull is known. CMN 30961 is a shattered partial skull including some of the skull roof. As in other Coryphodon the dorsal part of the skull is massive, having a width across the supraorbital crests estimated to be 18.3 cm. The compact, heavy premaxillae, represented by several specimens, seem to have been relatively loosely attached to the maxilla. A small foramen occurs medially in line with I1 and a second, presumably the anterior palatine foramen, is higher, above the space between I1 and I2. The nasal incision slopes strongly posterodorsally, as is typical for the genus.

Mandible Only incomplete mandibles are represented (Fig. 1), exhibiting some size variation, as is usual in species of Coryphodon (Uhen & Gingerich, 1995). On the lateral side of the jaw, two mental foramina occur in a line below the alveolus of the anterior premolar, the larger anterior foramen at about mid-depth of the ramus and slightly more ventral than the posterior foramen. Variably there may be another foramen in a line below P4. On the medial side of the ramus, the posterior edge of the symphysis is in a line between P2 and P3. CMN 32422 is the posterior part of a jaw with the alveolus for M3, part of the ascending ramus and the base of the condyloid process: the ascending ramus rises at about a right angle with the horizontal ramus, and the coronoid process is higher than the presumed position of the condyle, which is still well above occlusal level. On the essentially undivided medial surface of the ascending ramus there is a deep excavation within which is the opening (9 mm dorsoventrallly) of the inferior dental foramen. The lateral surface of the ascending ramus is concave with no separation of temporal and masseteric fossae.

Dentition There are no upper incisors in place in a premaxilla, so their relative size can be approximated only from an edentulous premaxilla. Based on alveolar size, the I1 root is larger than I2 and the I3 root is the smallest. The incisor crowns are slightly convex buccally, flattened lingually, and have a very narrow lingual cingulum. The upper canine in CMN 32420 is very large and roughly triangular in broken cross section. In CMN NUFV24, which retains broken canines of both sides preserved in the maxilla, C1 has a flattened medial side, and reaches its greatest width a little dorsal to mid-height on the buccal side. The latter is not as large as the tusk in the holotype but still has a massive root, extending far up into maxilla. Presumably both these specimens are males.

Of the upper premolars (Fig. 2), the crown of P1 is not preserved. In CMN NUFV24 part of the P1 root is present, showing that the diastema between it and the canine alveolus is short (12.6 mm); on the other side a piece of maxilla suggests a single-rooted P1. The premolars increase in size from P2–4, all of which are three-rooted. P2 is transversely narrow at the parastyle, which extends less buccad than the metastyle; the protocone is separated by a groove from paracone-metacone. On the triangular, transversely elongated P2–4, there are anterior and posterior cingula but neither continues around the lingual side of the protocone.

Fig. 1
figure 1

Coryphodon pisuqti, CMN 30969, right mandible. a Dorsal view showing tooth roots, mandible reversed; b lateral view; c enlargement of root of M3 showing elongated hypoconulid lobe

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Fig. 2
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Coryphodon pisuqti, CMN 32420, Holotype, left P2–M3, occlusal view

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The upper molars (Figs. 2, 3) increase in size from M1 to M3. Other than size, little can be determined of the morphology of any known M1, all specimens of which are either broken or too worn to show significant details of the occlusal surface. M2, a larger tooth than M1, has an anterior cingulum that continues around the lingual side to the “metaconule.” The parastyle is large and elongated posteriorly. A swollen area (“paraconule”) is situated at about the midpoint of the protoloph. The high, rounded paracone lacks a preparacrista and connects by a continuous ridge (postparacrista and premetacrista) to the metacone. This ridge, the metacone and the postmetacrista enclose a V-shaped valley. There is no metastyle. The large M3 is wide lingually and has continuous anterior and lingual cingula. A rounded cuspule (“hypocone”) is on the well developed posterolingual part of the cingulum. The parastyle is strong and has a ridge extending posteriorly from it. About at the midpoint of the long protoloph there is a small swelling (“paraconule”). Paracone and metacone are large cusps, connected by the postparacrista-premetacrista ridge, which is concave posterobuccally. There is no preparacrista. The large protocone is anterior to the buccolingual midline of the tooth and has a postprotocrista extending posterobuccally to form a ridge bounding the buccal side of the posterolingual cingular shelf.

Fig. 3
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Coryphodon pisuqti, CMN 32420, Holotype, right M2–3. a Occlusal view; b lingual view; c buccal view

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The lower incisors are flattened and spatulate. None are preserved in place so it cannot be determined whether or not the I1 > I2 > I3 size relationship of the upper incisors applied also to the lower teeth. The lower canine, variable in size, has wear facets that appear to be related to wear with I3 and the upper canine. Of the lower premolars, the only P1 represented is small and appears to be single rooted. Its trigonid is broken and possibly crushed, obscuring most of its morphology. There is a short diastema between it and P2. The talonid differs from those of P2–4 (Fig. 4) in having two narrow crests that converge at the posterior end of the tooth. P2–4, each two-rooted and having an anterior cingulid, increase in width toward the posterior. P2 has a shallower lingual valley than on P3–4. There is a more anterolingually protruding paraconid on P2–3 than on P4. The metaconid is a more prominent cusp on P4 than on the more anterior premolars, and the tooth is relatively short anteroposteriorly due to its more abbreviated talonid.

Fig. 4
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Coryphodon pisuqti, CMN 32420, Holotype, left mandible with P2–M2. a Lingual view; b occlusal view; c buccal view, jaw reversed

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As with the upper molars, the lower molars (Figs. 4, 5) increase in size from M1–3. Each molar has anterior and posterior cingulids. Although the surface of M1 is somewhat damaged in the holotype, essential details are preserved, showing the narrow cristid obliquid on M1 intersecting the metalophid approximately medially whereas on M2–3 the contact is more buccad. M3 is expanded posteriorly by the large hypoconulid, which is linguad of the midline. The single very large root that supports the entire talonid has well defined vertical grooves setting off the portion of the root below the hypoconulid. Anterior to the entoconid a small rounded metastylid occurs in the lingual edge of the talonid valley. The hypoconulid and entoconid are much better developed than in C. eocaenus. In the Clarkforkian C. proterus, the M3 hypoconulid is still more elongated posteriorly and is more medial in position than in C. pisuqti.

Fig. 5
figure 5

Coryphodon pisuqti, CMN 30811, left M3. a Occlusal view; b lingual view; c buccal view

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CMN 30802 (Fig. 6) is a partial right mandible of an immature C. pisuqti with dP3–4 in place. A round mental foramen occurs low, in a line below the talonid alveolus of dP2. The dorsal surface of the mandible anterior to dP3 is broken away, and the lateral side of the jaw shows an oval depression, possibly a bite mark, below the alveolus of dP2. The robust symphysis extends back to below the presumed alveolus for dP1. As preserved the anterior-most alveolus is a small, rounded, anterolateral facing recess, presumably for the lower deciduous canine. An elongate depression in the broken area may represent the alveolus for a single rooted dP1; posterior to this only the transversely elongated alveolus for the talonid root of dP2 is clearly preserved. Where unworn, the enamel of dP3–4 is distinctly wrinkled. DP3 is worn on the anterior surface of the paracristid and on the anteroposterior lingual lophid in the talonid. The trigonid of dP3 is high, with protoconid slightly higher than metaconid, and has anterior and lingual cingulids. The relatively small, low talonid, slightly broken posterobuccally, has a hint of a cuspate hypoconid. An anteroposterior ridge, possibly representing a cristid obliquid, extends between the middle of the metacristid and the posterior edge of the tooth. Medial to this ridge there is only a faint suggestion of a posterolingual cuspid. Compared to Coryphodon sp. (Simons, 1960, Fig. 2) dP3 of C. pisuqti has a straighter metacristid, more open trigonid, and anteroposteriorly much shorter talonid. The completely unworn dP4 is molariform, having five well developed cusps. The tooth has anterior and posterior cingulids and a small lingual cingulid on the paraconid. The talonid is slightly wider than the trigonid. Compared to previously illustrated dP4 of Coryphodon sp. (Simons, 1960; Lucas & Schoch, 1990), the metacristid is straighter and there is a short posterior spur on the posterior side of the metaconid. Posterior to dP4 the specimen preserves a large, empty crypt for M1.

Fig. 6
figure 6

Coryphodon pisuqti, CMN 30802, partial right mandible with dP3–4. a Occlusal view; b lingual view; c buccal view

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Fig. 7
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Map of Ellesmere Island, Nunavut, Canada, showing main location near Bay Fiord of the Margaret Formation, Eureka Sound Group, and similarly aged deposits near Stenkul Fiord

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Isolated postcranial bones are known for C. pisuqti but none are associated, nor are any complete enough for meaningful measurements.

Comparisons and relationships. So far as can be determined from the known specimens, Coryphodon pisuqti is the only pantodont taxon present in the Margaret Formation. All of the specimens having M3 exhibit the main characteristics of this taxon. Coryphodon pisuqti can serve as an index fossils within the Margaret Formation, being both widely distributed and morphologically distinctive, and exhibiting the ubiquity and presumed short geologic range that characterizes a useful index fossil (Simons, 1960). Postcranial bones lacking association with any dental remains are considered to represent the same taxon.

Coryphodon pisuqti is a large animal, only slightly smaller than the Clarkforkian Coryphodon proterus, the most primitive member of the genus known from well represented dental remains (Simons, 1960; Uhen & Gingerich, 1995). It also resembles C. proterus in having a large hypoconulid on M3 that is distinctly set off from the remainder of the talonid, although relatively less elongated than in the Clarkforkian species. The hypolophid of M3 is weak and the hypoconulid is more linguad than in C. proterus. The cristid obliqua on M1–2 are stronger and longer in C. pisuqti and a metastylid is present only on M3, rather than on all three lower molars as in C. proterus. Several of the relatively primitive species of coryphodontid including C. eocaenus, Wutucoryphodon xianwui, and W. dabuensis have relatively long hypoconulids on M3 but less so than in C. proterus and C. pisuqti. This cusp has moved lingually and anteriorly in more derived species (Uhen & Gingerich, 1995). The trends in coryphodontid dental evolution include reduction of the hypoconulid, as in the North American and European representatives (Hooker, 2010) of C. eocaenus, and increased bilophodonty of upper and lower molariform teeth, which occurs in more derived species.

Coryphodon was the dominant terrestrial herbivore of the Margaret Formation at a time of warm, humid temperate conditions with relatively mild summers and winters and a cold-month mean of about 5°C (Estes & Hutchison, 1980). It must have been able to maintain significant population levels due both to a presumed scarcity of carnivores and an abundance of vegetation. In the warm temperate climate, even coupled with the dark Arctic winter, it could presumably browse on remaining vegetation, as do musk oxen even under Holocene severe winter conditions. Southward migration during the dark intervals across the large river valleys that later were flooded into fiords and sounds may also have been a possibility.

Remarks on the genus Coryphodon The northernmost record of Coryphodon is now from early Eocene (Wasatchian) deposits of the Margaret Formation, Eureka Sound Group, at localities about 78° north latitude (Figs. 7, 8), in the Bay-Strathcona and Stenkul Fiord areas of Ellesmere Island, Nunavut, Canada (West & Dawson, 1977; Miall, 1986), where it occurs as the largest and relatively well represented of the fossil mammals in these localities. Biostratigraphic evidence indicates a Wasatchian, probably mid-Wasatchian age for the Margaret Formation (Dawson, 2001). Coryphodon is one of the most widely represented mammals in the Margaret Formation. This record is based in large part on numerous fragments, but enough single teeth, dentitions and postcranial bones are present to show that a new species is represented. The most complete dental remains are enclosed in large siderite or limonite concretions, within which teeth may be well preserved, but the bone tends to merge into the surrounding matrix. Some specimens consist of well preserved bone, but all too frequently the teeth are broken away and only roots remain, probably as a result of Holocene permafrost action.

Fig. 8
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Detailed map of the Bay Fiord-Srathcona Fiord area showing location of deposits of the Euraka Sound Group

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For the most part, dental characters distinguish species within Coryphodon (Simons, 1960; Uhen & Gingerich, 1995), with some emphasis on the size and structure of M3/3. Historically, species definitions within the genus have been rather unsettled, due in part to differing interpretations of individual variation. Thirty-five species of North American Coryphodon have been named since the first, Bathmodon radians Cope, 1872, but current work has restricted this to five species (Uhen & Gingerich, 1995).

Holarctic distribution of Coryphodon

Coryphodon has long been recognized as having a wide geographic distribution, occurring in the early Eocene of both North America and Europe (Marsh, 1876; Owen, 1878). Recognition that the family occurred also in Asia came later (Osborn & Granger, 1931), when Eudinoceras was recognized to be a pantodont rather than a uintathere. The North American and European records are shorter and the coryphodontids less taxonomically diverse (late Paleocene to early middle Eocene, one genus, in the former, and early Eocene, one genus, in the latter) than the Asian, where the record spans the late Paleocene through the entire Eocene, during which time seven genera have been recognized (McKenna & Bell, 1997; Tong & Wang, 2006). Stratigraphic distribution supports the premise of an Asian origin for coryphodontids. Now, recognition of a relatively primitive species of Coryphodon in the high Arctic documents their movement across high latitudes in the Holarctic.

Definitive evidence on the phylogenetic position of C. pisuqti is absent. Perhaps the most parsimonious explanation is evolution of this species from the older and less derived C. proterus as its lineage followed cooler temperatures, typical of the later Paleocene, to higher latitudes during the time of the Paleocene-Eocene Thermal Maximum (PETM) at mid-latitudes. Persistence of C. pisuqti into the mid-Wasatchian at high latitudes might thus have been relictual. Another possibility is that it was an Arctic endemic. A more dynamic phylogenetic role could be postulated for C. pisuqti if it led to the origin of C. eocaenus, which is known from the earlier Eocene in mid-latitudes of both North America and Europe, presuming that the species is indeed synonymous in both places.

Though relatively common, Coryphodon itself appears to be poorly understood. Clearly, as the only migrant in its family, its habits differed from those of other coryphodontids. Early Eocene climatic factors did not impede its distribution across the Holarctic. By the end of the early Eocene in Europe and the earliest middle Eocene in North America coryphodontids were gone. Their span in Asia was both much longer, until the end of the Eocene, and exhibited more taxonomic diversity, but without any more evidence of migrations between the current continents. Whether this was in response to cooling climates or to biotic factors remains to be discovered.