New occurrences of mammals from McKay Reservoir (Hemphillian, Oregon)

Non-technical Summary. — The fossil record of the Paci ﬁ c Northwest records millions of years of changing climate and evolving organisms, shedding light not only on life of the past, but also giving us the tools to better predict how future environmental change might play out. A site near Pendleton, Oregon is especially important to understanding ancient ecosystems because it preserves a complete community of vertebrates, from bats and shrews to rhinos and saber-toothed cats. In 2017 and 2021, we returned to this site to collect new fossils. We also visited museum collections that contain fossils from the site to open as clear a window as possible into this 5 – 6 million year old world. We identi ﬁ ed many species new to the site, many of which are also new to the Northwest. These include a bone-crushing dog; camels, both giant and llama-like; an extinct animal that looked like a deer, but that has no modern relatives; and at least two types of ancient horse. We also compiled a complete, up-to-date list of all the mammals ever found at the site. Besides giving us a better idea of what lived in the Pendleton area in the Miocene Epoch, this work will serve as a jumping-off point for later studies focused on how these organisms behaved, functioned, and interacted with each other and with their environments. Abstract. — Encompassing global cooling, the spread of grasslands, and biogeographic interchanges, the Hemphillian North American Land Mammal Age is an important interval for understanding the factors driving ecological and evolutionary change through time. McKay Reservoir near Pendleton, Oregon is a natural laboratory for analyses of these factors. It is remarkable for its small vertebrate fauna including rodents, bats, turtles, and lagomorphs, but also for its larger mammal fossils like camelids, rhinocerotids, canids, and felids. Despite the importance of the site, few revisions to its faunal list have been published since its original description. We expand on this description by identifying taxa not previously known from McKay Reservoir based on specimens collected during ﬁ eldwork and through reidenti ﬁ cation of previously collected fossils. Newly identi ﬁ ed taxa include the borophagine canid Borophagus secundus (Matthew and Cook, 1909), the camelids Megatylopus Matthew and Cook, 1909 and Pleiolama Webb and Meachen, 2004, a dromo-merycid, and the equids Cormohipparion Skinner and MacFadden, 1977 and Pseudhipparion Ameghino, 1904. Specimens previously assigned to Neohipparion Gidley, 1903 and Hipparion de Christol, 1832 lack the features necessary to diagnose these genera, which are therefore removed from the site ’ s faunal list. The presence of Borophagus secundus , Cormohipparion , and Pseudhipparion is especially important, because each occurrence represents a major geographic range extension. This re ﬁ ned understanding of the fauna lays the foundation for future studies of taphonomy, taxonomy, functional morphology, and paleoecology — potentially at the population, community, or ecosystem levels — at this paleobiologically signi ﬁ cant Miocene locality.


Introduction
The Hemphillian North American Land Mammal Age (NALMA; Miocene, 9-4.9 Ma; Bell et al., 2004;Tedford et al., 2004;May et al., 2014) spans an interval of profound ecological change, encompassing climatic cooling, the continued spread of grasslands, and the migration of new taxa from Eurasia and South America (Woodburne, 2004).The McKay Reservoir locality near Pendleton, Oregon (Fig. 1) offers a rare opportunity to study the effects of large-scale change on an ecosystem scale.The site preserves an important Hemphillian-aged vertebrate fauna (Shotwell, 1956).It is especially remarkable for its diversity of small vertebrates, including birds (Brodkorb, 1958), rodents (Martin, 2008), turtles, eulipotyphlans, bats, and lagomorphs (Shotwell, 1956).Larger mammal fossils are also common at the site.The rhinocerotid Teleoceras Hatcher, 1894 is particularly abundant (Shotwell, 1956) and the site has also *Corresponding author.yielded a diversity of carnivoran fossils, including borophagine (Wang et al., 1999) and canine canids (Tedford et al., 2009) and the giant felid Machairodus lahayishupup Orcutt and Calede, 2021 (Orcutt and Calede, 2021).The exceptional completeness of the fossil record from McKay Reservoir has made it a major focus of paleobiological research.Shotwell (1955a) argued that the taphonomy of the McKay Reservoir fossils indicated a pondbank environment and built on this by including the McKay Reservoir fauna in a pioneering analysis of landscape paleoecology in the Columbia River Basin (Shotwell, 1958).The site's continuing importance as a paleobiological study system is underscored by the many subsequent studies incorporating McKay Reservoir fossils to address questions regarding mammalian body-size evolution (Orcutt andHopkins, 2013, 2016), morphological variation in mylagaulid rodents (Calede and Hopkins, 2012), the biogeography of arvicoline rodents (Martin, 2010), the origin of sigmodontine rodents (Ronez et al., 2021), rhinocerotid paleopathology (Stilson et al., 2016), and felid functional morphology and systematics (Orcutt and Calede, 2021).As extensively and intensely as it has been studied, the McKay Reservoir faunal list continues to grow, due to taxonomic revisions, to previously described taxa, and to new collections made by Gonzaga University field crews in 2017 and 2021.Here, we present the occurrence of several mammalian taxa not previously identified from McKay Reservoir and revisit prior identifications by Shotwell (1956), providing a more complete assessment of diversity in this paleoecosystem and laying the foundation for future paleobiological analyses.

Geologic setting
The McKay Formation, from which the specimens described here were collected, is a component of the late Miocene Dalles Group, previously referred to as the 'Shutler Formation' (Farooqui et al., 1981).The fossil-bearing units of the formation outcrop only along a 0.3 km section of shoreline on a peninsula on the northeast shore of McKay Reservoir (Fig. 1).The sediments at the locality range from siltstone to conglomerate (Farooqui et al., 1981), with most fossils preserved in the siltstone and sandstone units (Shotwell, 1956).Most McKay Reservoir fossils are found in float along the lake shore, exposed by wave erosion during periods of high water, making it impossible to assign these specimens to a particular stratigraphic unit.As first noted by Shotwell (1956), McKay Reservoir fossils show minimal evidence of abrasion or any other extensive alteration prior to burial.Although some specimens show signs of weathering and breakage, these appear fresh and are likely the result of changing water levels in the reservoir and a strongly seasonal modern climate.However, a detailed taphonomic analysis of the site has not yet been undertaken.
The fossils uncovered from McKay Reservoir are characteristic of the early late Hemphillian (Hh3) NALMA and allow a biochronologic age estimate of 6.7-5.9Ma (Tedford et al., 2004).Two tephras are interbedded within the fossiliferous layers of the McKay Formation at McKay Reservoir with 40 Ar/ 39 Ar dates of 5.95 ± 0.30 Ma and 5.51 ± 1.22 Ma (Martin et al., 2018).Most fossil sites are between the two tephras but there are rare fossil specimens below the lower ash including possible material of Megatylopus Matthew and Cook, 1909.There are abundant fossils in the unit immediately above the upper tephra (Shotwell, 1956;Martin et al., 2018).However, all of these specimens remain consistent with the Hh3 NALMA (Tedford et al., 2004) and therefore are unlikely to significantly pre-or postdate the tephras.Creek National Wildlife Refuge and collection was carried out under Permits for Paleontological Investigations issued by the US Bureau of Reclamation (CCAO:U17-001 and CCAO:U21-004) and Special Use Permits issued by the US Fish and Wildlife Service (FF01RMCC00-17-021 and FF01RMCC00-21-005).Measurements of specimens were made to the nearest tenth of a millimeter using Mitutoyo Digimatic calipers with an accuracy of ± 0.02 mm.In the text and tables, upper dentition is indicated by uppercase letters, lower dentition indicated by lowercase letters.
Description.-Largesize of teeth and presence of small m2 on UOMNH F-81105 and F-81106 suggest that specimens can be attributed to a borophagine canid (Wang et al., 1999) Remarks.-AlthoughBorophagus secundus has been reported from many Hemphillian-aged sites elsewhere in North America, these specimens represent a northward range extension (Wang et al., 1999).Although some fossils described by Shotwell (1963) from the Drewsey Formation of southeaster Oregon could be attributable to Borophagus secundus, UOMNH F-81105 marks the first definitive occurrence of the species in Oregon and in the Pacific Northwest more broadly.
Figure 3; Table 2 Description.-Largesize (  2010) indicated the presence of the genus at the site without assigning specific specimens to it.Many of the specimens are smaller than those found at sites elsewhere in North America (Table 2; Voorhies and Corner, 1986;Jiménez-Hidalgo and Carranza-Castañeda, 2010) although UOMNH F-4152 is extremely large, indicating a wide range of body sizes (Table 2).Whether this is the result of sexual dimorphism, the presence of multiple species, or simply normal variation within Megatylopus cannot be ascertained based on the sample size currently available.
Tribe Lamini Webb, 1965 Genus Pleiolama Webb andMeachen, 2004 Type species.-Pleiolamamckennai Webb and Meachen, 2004 from the Clarendonian Merritt Dam Member of the Ash Hollow Formation at Kat Quarry, Nebraska, USA, by original designation.
Figure 4; Table 2 Description.-Smallsize of molar is consistent with Pleiolama, particularly with the small species Pleiolama vera (Matthew, 1909); although Pleiolama and Alforjas Harrison, 1979 are similar in size, lower degree of hypsodonty is consistent with Pleiolama (Webb and Meachen, 2004).Elongated, gracile phalanx indicates that UOMNH F-4154 and UOMNH F-81112 are referable to Lamini (Harrison, 1979).Presence of W-shaped suspensory ligament scar that does not extend far down the phalangeal shaft indicates that the specimens can be referred to the lamins Alforjas or Hemiauchenia Gervais and Ameghino, 1880 (sensu Harrison, 1979, equivalent to Journal of Paleontology:1-12  Remarks.-Thesmall camelid described by Shotwell (1956) is referable to Pleiolama.Although lamins, including Pleiolama, are known from regional Hemphillian-aged sites in Oregon's Rattlesnake Formation (Fremd, 2010) and Washington's Ellensburg Formation (Martin and Pagnac, 2009), this is the first identification of the genus from McKay Reservoir.
Family Dromomerycidae Frick, 1937 Dromomerycidae gen.indet.sp.indet.Figure 5; Table 2 Description.-Relativelysmall selenodont and brachydont molar that is consistent with both Cervidae and Dromomerycidae.However, the lack of complex folding on the protoconal and metaconule crests is inconsistent with Bretzia Fry and Gustafson, 1974 and other cervids (Gustafson, 2015).Of the two unequivocally Hemphillian-aged dromomerycid genera (Frick, 1937;Janis and Manning, 1998) (Prothero and Liter, 2008).However, its size falls well below the range reported by Webb (1983) in his description of Yumaceras hamiltoni (Table 2).Pediomeryx hemphillensis is smaller in size than Yumaceras hamiltoni (Webb, 1983;Prothero and Liter, 2008) Remarks.-UOMNHF-4444 is the only specimen in any collection that we could find that was identified in collections as Neohipparion.Shotwell (1955aShotwell ( , 1956Shotwell ( , 1958) ) reported one occurrence of Neohipparion but never identified the specimen.This specimen is broken so that the lingual half is missing, thus removing any of the diagnostic characteristics for any more specific identification.As such, we are  Cormohipparion sp.indet.Figure 6.1-6.5;Table 3 Description.
-Large teeth (Table 3).Protocone small and isolated; complex plications on the pre-and post-fossettes; well-developed pli caballins.More specific identification is difficult with isolated teeth because they lack a majority of diagnostic characters (Famoso and Davis, 2014).Shotwell (1955aShotwell ( , 1956Shotwell ( , 1958) ) are Cormohipparion, because he never identified the specific specimens that he assigned to Hipparion.Additionally, Skinner and MacFadden (1977) named Cormohipparion from material previously identified as Hipparion in North America well after Shotwell's original work.We have reviewed all of the relevant specimens and have not found any that meet the diagnosis for Hipparion and suggest removing it from the faunal list and replacing it with Cormohipparion.
Pseudhipparion sp.indet.Figure 6.6; Table 3 Description.-Smalltooth (Table 3).Protocone large, elliptical, and isolated in early wear stages but appears to connect in later wear stages (estimated from observance on the exterior of the tooth); simple plications on both fossettes; prominent hypoconal lake.More specific identification is difficult with isolated teeth because they lack a majority of diagnostic characters (Famoso and Davis, 2014).Remarks.-UOMNHF-29959 marks the first published occurrence of the genus in Oregon and the Pacific Northwest, as well as west of the Rocky Mountains.When the length and width of the specimen are compared to mean values for previously described species (Table 3), this tooth is not consistent with Pseudhipparion skinneri Webb and Hulbert,  Shotwell (1956Shotwell ( , 1958Shotwell ( , 1967a, b) , b) in his initial work on the locality, by subsequent authors (Lindsay, 1962;Hutchison, 1968;Wagner, 1976;Harrison, 1981;Martin, 1984Martin, , 1998;;Korth, 1999;Prothero, 2005;Orcutt and Calede, 2021), and in this paper are shown.X, the original taxonomic assignment is still valid; O, taxonomy has been updated since its original description; ?, taxonomic assignment uncertain; -, not applicable.

Discussion
Besides adding to the McKay Reservoir faunal list, the specimens described above underscore the paleobiological significance of the site.Most importantly, these occurrences mark major geographic range extensions for three taxa (Borophagus secundus, Pseudhipparion sp.indet., and Cormohipparion sp.indet.),none of which had been definitively identified in Oregon and of which only Cormohipparion had previously been described from any sites in the Pacific Northwest (Rapp, 2006).Hemphill, Lipscomb, downloaded from the Paleobiology Database on 28 November 2023).Having better established the composition and completeness of the mammal fauna, a logical next step along with continuing field work would be reconstructing interactions between the members of that fauna through stable isotope analysis of mammal teeth.Fossils from the site could also inform broader quantitative taxonomic analyses of taxa such as Megatylopus, Cormohipparion, and Pseudhipparion, possibly allowing the identification of existing specimens to the species level.The McKay paleocommunity consists of not only mammals but fish, amphibians, and an abundance of reptiles and birds, most of which have not been a focus of study since the original descriptions by Shotwell (1956) and Brodkorb (1958).Note that, among the comparable regional formations mentioned above, the only description of any nonmammalian taxon is the turtle Clemmys hesperia Gray, 1825 from the Rattlesnake Formation (Hay, 1903).Given the importance of these taxa as environmental indicators, a revision of the ichthyofauna, herpetofauna, and avifauna of McKay Reservoir and other Dalles Group localities will provide a more complete paleoecological context.A clearer understanding of the physical environment of the McKay paleoecosystem is also crucial, particularly because Shotwell's (1955a) interpretation of the site as a pondbank community is not consistent with the coarser-grained lithology of the bluff from which the fossils weather (Martin et al., 2018).Along with sedimentology, stratigraphy, and paleopedology, ichnology could also provide valuable information about the McKay paleoenvironment, because burrows consistent in size with those of fossorial rodents were visible in some of the lower units of the McKay Formation.Establishing as clear as possible an environmental context would make it possible to undertake analyses not feasible in many paleoecosystems, e.g., reconstructing food webs or population-level morphological patterns in well-represented taxa including rhinocerotids, sciurids, and camelids.Additional fieldwork and the review of previously collected material is pivotal to our understanding of the McKay Formation and the Hemphillian of the Pacific Northwest, revealing a more complete picture of the fauna, providing increased sample sizes for paleobiological analyses, and allowing comparisons with coeval localities across North America.
specimens were collected at McKay Reservoir (University of Oregon Locality UO 2222) by field crews from Gonzaga University in July of 2017 and 2021.The reservoir is part of McKay

Figure 1 .
Figure 1.Maps showing location of McKay Reservoir within Oregon (left) and of fossil bearing outcrops along reservoir shoreline (right).Scale bars = 50 km (left); 2 km (right).star, site of fossil bearing outcrops.
Repositories and institutional abbreviations.-Specimensfrom McKay Reservoir are reposited in the following collections: the South Dakota School of Mines and Technology (SDSM; SDSM V for locality numbers), the University of California Museum of Paleontology (UCMP, UCMP V for locality numbers), the University of Oregon Museum of Natural and Cultural History (UOMNH F, UO for locality numbers) and the University of Washington Burke Museum (UWBM VP, UWBM C for locality numbers).Another cited repository is American Museum of Natural History, New York (AMNH).

Table 2
Shotwell (1956)orner, 1986)nd lack of cement in fossettes of cheek teeth are characteristic of Megatylopus (Jiménez-Hidalgo and Carranza-Castañeda, 2010).Phalanges are intermediate in robusticity (Table2), as in TitanotylopusBarbour and Schultz, 1934, Megatylopus,  and Camelops Leidy, 1854; are consistent in size with Megatylopus and Camelops; and have a W-shaped suspensory ligament scar, as in Titanotylopus and Megatylopus(Voorhies and Corner, 1986).Remarks.-Shotwell(1956)indicatedthepresence of a large and small camelid at McKay Reservoir but did not identify these camelids to genus.The larger material described above is referable to Megatylopus, the first definitive identification of this genus at McKay Reservoir, althoughMartin et al. (
, the brachydonty of the McKay Reservoir specimen is consistent with Yumaceras Frick, 1937, but the small size of the tooth might be more consistent withPediomeryx Stirton, 1936 (Table 2; Prothero and Leiter, 2008).
Remarks.-Isolated upper molars such as SDSM 38459 are generally not diagnostic for dromomerycids, in which lower dentition and ossicones are of predominant importance.Pediomeryx is the most common and widespread dromomerycid of the late Hemphillian, but SDSM 38459 bears a stronger resemblance to Yumaceras in its brachydonty (Prothero and Liter, 2008)ed measurements of upper dentition for this species hinders quantitative comparisons.If SDSM 38459 does represent Yumaceras, its occurrence at McKay Reservoir would be a slight range extension; it has previously only been reported in the Pacific Northwest from the earlier Hemphillian-aged Drewsey Formation of southeastern Oregon(Prothero and Liter, 2008).Pediomeryx has not previously been reported from the region.

Table 4 .
Updated faunal list for McKay Reservoir.Taxa first described by Webb and Hulbert, 1986Leidy, 1869), or Pseudhipparion hesseiWebb and Hulbert, 1986.There are no other qualitative or quantitative diagnostic features on this tooth that can be used to differentiate among the three species, therefore we leave the specimen identified to the genus level.
Their presence at McKay Reservoir reflects the remarkable completeness of the paleocommunity and further solidifies the value of the site as a natural laboratory for Hemphillian paleoecology, a point that is illustrated by comparison with other geographically constrained Hemphillian-aged formations in the Pacific Northwest.Following this analysis, 40 mammalian taxa have been reported from McKay Reservoir (Table4).By contrast, the Rattlesnake Formation of the John Day Basin has yielded 29 described mammalian taxa and the