Introduction Pleistocene vertebrate faunas from Beringia – the unglaciated region between northeast Asia and northwest North America straddling the Bering Isthmus – are of particular importance for our understanding of intercontinental dispersals (Hopkins et al., 1982). These faunas record the movement, persistence, and extinction during the Pleistocene and establishment of present-day guilds and food webs. Members of the family Hyaenidae (part of Order Carnivora, which also includes wolves, cats, seals, weasels, and relatives) have an extensive fossil record across the Old World beginning in the early Miocene, with >60 extinct hyaenid taxa having been described. They are often the most abundant predators at fossil localities across Eurasia, and are considered to have played key ecological roles as mesopredators and scavengers (Turner et al., 2008). They were, however, absent from North America until dispersal (presumably via the Beringian land bridge) during the Blancan North American Land Mammal Age (NALMA, Figure S1), ~4.7 Ma (mega-annum) ago. The hitherto northernmost record of the hyaenid Chasmaporthetes in North America is from Meade County, Kansas at approximately 37°N latitude. In this report, we describe the first known fossil records of hyaenids in the Arctic latitudes of Beringia, represented by two isolated teeth of Chasmaporthetes cf. C. ossifragus, a member of the cursorial hyaenid clade, adding to the exclusively southern United States and Mexico fossil records of hyenas in North America. Geographic Region and Geologic Context—The Old Crow Basin, in the unglaciated region of northern Yukon Territory, Canada, has been the focus of paleontological, paleoenvironmental, and stratigraphic investigations for over a century (Harington, 1989, Harington, 2011, Irving and Beebe, 1984). Numerous exposures of unconsolidated, perennially frozen alluvial sediments are present along the Old Crow River and its tributaries, and reveal rich records of Quaternary biotic communities and environmental change. However, most fossils are recovered from numerous alluvial point bars, where admixture of anachronistic fossils from various Pleistocene and Holocene stratigraphic units makes geologic age constraint of the re-worked fossils difficult. One particular point bar near the downstream end of the basin known as Locality CRH 11A has been a focus for palaeontologists and is, by far, the richest and most diverse (Harington, 2011). Stratigraphic investigations on river bluffs, including biochronology, tephrochronology, and magnetostratigraphy suggest that exposed sediments in the basin are no older than early Pleistocene, ~1.4 Ma; thus, these re-worked fossils can be considered to span the Irvingtonian to Rancholabrean NALMA (Figure S1) (Westgate et al., 2017, Kuzmina et al., 2014). Over 50,000 vertebrate fossils, representing at least 80 species from over 200 bluffs and point bar localities along Old Crow River, have been amassed in the collections of the Canadian Museum of Nature and Yukon Government Palaeontology Program. These faunas are dominated by common Quaternary Holarctic herbivores such as Mammuthus, Bison, Equus and Rangifer, whereas remains of large carnivorans including the bear Arctodus, and the felids Homotherium and Panthera are relatively rare (Harington, 2011). However, it is the exceedingly rare specimens from the Old Crow Basin that have proved most important. For example, a single fossil of the flat-headed peccary Platygonus compressus extended the known range for this species northward by over 3000 km (Beebe, 1980). Rare specimens of the canid Xenocyon lycaonoides provide a glimpse of a brief dispersal of this typically Eurasian carnivoran into the North American Arctic (Tedford et al., 2009). Rare specimens of the large bodied camel cf. Paracamelus provide an intermediary fossil locale for this taxon, which ranged from the Canadian High Arctic to western Europe (Rybczynski et al., 2013). Although the chronological uncertainties with these re-worked faunas are difficult to resolve, these proverbial “needle-in-a-hay-stack” rare fossils from the Old Crow Basin have provided major advances in our knowledge of Quaternary biotic history and biogeography.

Institutional Abbreviations CMN, Canadian Museum of Nature, Ottawa, Canada; F:AM, Frick fossil mammals collection, Division of Paleontology, American Museum of Natural History, New York, USA; IGM, Geological Institute, Mongolian Academy Sciences, Ulan Baatar, Mongolia; IPM, Paleontological Institute, Russian Academy of Sciences, Moscow, Russia; IVPP, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China; MAN, Museo Archeologico, Nuoro, Sardinia, Italy; MNHN, Muséum National d’Histoire Naturelle, Paris, France; NMT, National Museum of Tanzania; OGU, Odessa I.I. Mechnikov State University, Ukraine; PIU, Paleontology Collection, Museum of Evolution, Uppsala University, Sweden; PPM, Panhandle Plains Historical Museum, Canyon, Texas, USA; SAM, Iziko South African Museum, Cape Town, South Africa; Sk, Swartkrans Collection, Ditsong National Natural History Museum, Pretoria, South Africa; TM, Toros-Menalla locality, National Museum of Chad; UCB, Claude Bernard University, Lyon, France; UF, University of Florida Museum of Natural History, Gainesville, Florida, USA; USNM, United States National Museum (Smithsonian Institution)/National Museum of Natural History, Washington D.C., USA. Systematic Palaeontology

Order CARNIVORA Bowdich, 1821

Family HYAENIDAE Gray, 1869

CHASMAPORTHETES Hay, 1921

CHASMAPORTHETES cf. C. OSSIFRAGUS Hay, 1921

(Figure 1A–1F 1

Adcrocuta sp. Harington, 1989 “Hyaena”, Hyaenidae Harington, 2011

Specimen No. CMN 24958 CMN 38053 Element Rp3 Lm1 Length (mm) 19.11 29.08 Width (anterior, mm) 9.94 13.12 Width (posterior, mm) 11.58 8.24 (partial) L, mac (mm) 4.97 – L. main cusp (mm) 10.38 – Height, main cusp (mm) 13.34 – L, dac (mm) 4.92 – L. trigonid (mm) – 22.25 L, talonid (mm) – 6.41 H, paraconid (mm) – 13.4 H, protoconid (mm) – 15.55 HSB (b, m, t) u, a, ? u, a, a Referred specimens—CMN (Canadian Museum of Nature, Ottawa, Canada) 24958, isolated right p3; collected by Richard Harington and team from the Canadian Museum of Nature in 1973. CMN 38053, isolated left m1, collected by William Irving and Brenda Beebe of the University of Toronto in 1977 (Irving and Beebe, 1984). Locality—Old Crown Basin CRH (C. Richard Harington) Locality 11A, Yukon Territory, Canada. The locality is a point bar along the lower Old Crow River (67.819° N, 139.904° W). The area is very remote, only accessible by plane to the village of Old Crow, and from there either on the river or by helicopter. Age—Approximately Irvingtonian to Rancholabrean NALMA, or from about 1.4 Ma to 0.012 Ma (Harington, 2011). We interpret the most likely age of the Old Crow Chasmaporthetes to be no younger than the end of Irvington I (approximately 0.85 Ma; see discussion), likely closer to 1.4 Ma.

Description CMN 24958, right p3: The main cusp is slightly worn, and has distinct, smooth ridges running down the mesial and distal faces. A well-developed mesial accessory cusp (MAC) is separated from the main cusp by a pronounced notch. No mesial cingulum is visible. A well-developed distal accessory cusp (DAC) is similar in size to the MAC, with a similarly notched boundary with the main cusp. Bulging cingula are developed on the buccal and labial sides of the DAC. The distal face of the DAC forms a depression on the distal edge of the tooth, separating the lingual and labial cingula. Vertical surface furrows are visible throughout the tooth, being most pronounced on the surface of the main cusp. CMN 38053, left m1: A moderate shear facet is developed on the carnassial blade, exposing dentine within a rim of enamel. The protoconid is taller than the paraconid; both cusps are only slightly worn at the apices. There is no metaconid. A low cingulum is present around the base of the crown. The talonid has a single centrally situated cuspid, the hypoconid, with a raised ridge connecting its apex to the base of the protoconid forming a notch at their contact point. There are three small accessory cuspids formed on the lingual to distal rim of the talonid basin. The labial edge of the talonid is broken, so the possible presence of additional cuspids cannot be determined. For additional details see Electronic Supplemental Material (ESM). CMN 24958 has an overall black sheen. Small regions of milky yellow to white preservation allows Hunter-Schreger bands to be observed under a dissection microscope at 30–40× magnification. The specimen is broken off at the base of the crown. Two roots are visible, with the distal being larger than the mesial one. Preservation of CMN 38053 is similar to CMN 24958. There are two broken roots at the base of the crown, with the mesial being the larger. Both CMN 38053 and CMN 24958 exhibit transitional, acute-undulating Hunter-Schreger Band enamel microstructure, indicating moderate capability to resist abrasive and/or hard foods (Tseng, 2011).

Comparison with other Chasmaporthetes The nearly symmetrical arrangement of p3 MAC and DAC around the main cusp, the linear alignment of the three cusps on p3 in occlusal view, and the centrally situated and trenchant hypoconid in the talonid basin are all characteristic of Chasmaporthetes, and are present on the referred specimens from Old Crow. Compared to specimens of C. ossifragus and other species of Chasmaporthetes, the p3 from Old Crow is relatively wide for its length (Figure 2A; Table S1). Lower first molar (m1) dimensions of the Old Crow specimen fall in line with the proportions observed in the global Chasmaporthetes sample (Figure 2B; Table S1). However, the m1 trigonid of the Old Crow specimen is relatively broad and short compared to the majority of the Chasmaporthetes measured (Figure 2C; Table S1). The MAC on CMN 24958 is relatively larger than the MAC on PPM (Panhandle-Plains Historical Museum) 2343 (C. ossifragus, Cita Canyon, Texas) and UF (Florida Museum of Natural History, University of Florida) 18088 (C. ossifragus, Inglis 1A, Florida), whereas the m1 trigonid and talonid proportions are similar among all three specimens, as well as to the isolated m1 from Inglis 1A (UF 18089). Because there are only two teeth from the Old Crow Basin in this report, other characteristics such as degree of curvature of the dentition and p4 to m1 length ratios cannot be used to assign the specimens to an existing species, nor are there enough characters to warrant naming a new species. The main dental feature that has been described as characteristic of C. ossifragus (Berta, 1981) is consistent with our observations of the Old Crow specimen: a well-developed MAC is present in the Old Crow p3 (CMN 24957), similar to C. ossifragus specimens from Florida, and relatively larger in size compared to p3 MAC in other Chasmaporthetes species, especially Old World species such as C. lunensis. Following previous reviews of dental (Kurtén and Werdelin, 1988), and lacking more specific evidence to unambiguously assign the Old Crow fossils to any one of existing species of Chasmaporthetes other than from geographic delineation, we reserve judgment on species designation and instead identify the new fossils as Chasmaporthetes cf. C. ossifragus.

Data Accessibility Statement All data used in the study are included in the publication.