Panthera atrox Leidy 1853

– Panthera atrox Leidy, 1853 Referred material. El Barrio locality: UAHMP-4221, left lower canine; UAHMP-4222, left fifth metacarpal. Distribution and age. The American lion was widely distributed across North America from Alaska (Whitmore and Foster, 1967) to southern Mexico (Montellano-Ballesteros and Carbot-Chanona, 2009; Carbot-Chanona and Gómez-PØrez, 2014); some records from Alaska and western Canada maybe represent members that are more closely related to the cave lion (P. spelaea) (Barnett et al., 2009). It is a taxon limited to the Rancholabrean NALMA (Kurten and Anderson, 1980; Lange, 2002). 4.1 Description The canine UAHMP-4221 is large and robust (Table 1). A thin layer of enamel (<1 mm thick) covers the first third of the tooth. The tip has broken anterolaterally after death (given that it does not show wear) and is subacute in shape. The specimen is moderately curved, and in cross section it is oval to oval-elongate toward to the base. The lateral surface of the tooth is slightly convex, whereas the medial surface is flattened (Fig. 2). The metacarpal UAHMP-4222 is slender and long, showing a relatively gracile appearance (Table 2, Fig. 3). The shaft is oval in cross section and without sharply defined borders between the dorsal and palmar surfaces. At the proximal end, the articular surface for the unciform is narrow. The medial face presents the articular surface for metacarpal IV, formed by a large projection (“ear-shaped projection” of Merriam and Stock, 1932) and a narrow notch placed on the palmar side. The lateral surface presents a convex tuberosity. At the distal end, the lateral tuberosity is more prominent than the medial tuberosity. The distal articulation surface is situated at a 10 ◦ angle from the axis of the shaft and a prominent palmar keel is apparent. 4.2 Taxonomic assessment The deciduous lower canine of felids is distinguished by having an accessory cusp situated on the anterolingual side of the tooth (Salles, 1992). The specimen UAHMP-4221 does not show this condition, indicating that it is a permanent tooth; furthermore, it resembles the lower canine of pantherines in the absence of a lingual cavity (a widespread condition among felids), and differs from an upper canine in the absence of a lingual ridge (Salles, 1992) and in being more robust. By contrast, the lower canine of Smilodon is smaller and slender, strongly curved, and shows a median lateral ridge (Merriam and Stock, 1932). The size of the tooth is comparable to that observed for lower canines belonging to Panthera atrox, including USNM 23619 (right lower jaw with c, p3–m1) from Lost Chicken Creek, Rancholabrean of Fairbanks, Alaska; it falls in the upper limit of the observed range in specimens from Rancho La Brea, California, United States; and it is nearly 25 % larger than the specimen IHNFG-2678 (isolated lower canine) from Chiapas, southern Mexico (Table 1). The difference in size between the specimens from Hidalgo and Chiapas could be explained by intraspecific variation (related to age and/or sex), considering that UAHMP-4221 shows dimensions comparable to the larger lower canines from Rancho La Brea, whereas the dimensions of IHNFG-2678 correspond to those of the smaller ones (Table 1). It should be stated that the specimen UAHMP-4221 is significantly larger (ca. 33 %) than lower canines of Panthera leo and P. tigris; however, the size of this tooth between those extant species is similar (Table 1). Among felids, the morphology of the limb elements is somewhat homogeneous, including the metacarpals. In general, the metacarpal bones of felids are characterized by being short and robust with a broad and curved diaphysis as well as broad proximal and narrow distal ends (Morales-Mejía and Arroyo-Cabrales, 2012). All these features are observed in the specimen UAHMP-4222, indicating its felid condition. In particular, the fifth metacarpal from Hidalgo resembles those of Panthera atrox in the following characters: (1) well-developed projection on the palmar side at the proximal end; (2) the articulating surface for the unciform is narrow; (3) the diaphysis is relatively slender; (4) the notch on the articulating surface for the fourth metacarpal is narrow; and (5) the diaphysis at the middle is oval in cross section (Merriam and Stock, 1932). Furthermore, the size of UAHMP-4222 (greatest length = 105.9 mm) is within the observed range of fifth metacarpals of P. atrox from Rancho La Brea, California, United States (Table 2); however, it is about 25 % larger than the fifth metacarpal of OCMP-077 belonging to P. tigris (greatest length = 80.3 mm). It should be noted that the fifth metacarpal of Smilodon is distinguished in having a poorly developed projection on the palmar side at the proximal end, a broad articulating surface for the unciform, and a relatively stout diaphysis (Merriam and Stock, 1932). The comparative study indicates that the size and morphology of dental and postcranial remains from Hidalgo are closely comparable to those observed in specimens belonging to Panthera atrox. Hence, the studied sample is formally assigned to that large-sized cat species. 5 Paleobiological significance 5.1 Paleoecology The American lion was one of the largest cats that inhabited North America during the late Pleistocene (KurtØn and Anderson, 1980; Lange, 2002). Body size estimations indicate that this felid had a mean body mass of 300 kg, ranging from 200 to 400 kg (Van Valkenburgh et al., 2016). It has been observed that prey body size tends to increase with the predator size (Sinclair et al., 2003). Therefore, it should be expected that large mammalian herbivores were common prey of Panthera atrox. Van Valkenburgh et al. (2016: fig. 2, p. 865) predicted a typical prey size ranging from 60 to 900 kg for the American lion, and the maximum prey size could have been about 1000 kg. Given the above and considering the taxonomic mammalian composition known at the El Barrio locality, it seems that potential prey for Panthera atrox in that site could include adult individuals of llamas (Hemiauchenia gracilis and Camelops sp.), deer (Odocoileus cf. virginianus), horses (Equus conversidens), and even bison (Bison sp.); the body mass among these herbivores is estimated to have been 200 to 900 kg (Fig. 4). Hunting in groups increases the upper range of available prey size; consequently, it has been suggested that large-sized Pleistocene cats (such as Panthera atrox) were able to kill prey with a body mass of about 6000 kg (Van Valkenburgh et al., 2016). Assuming a hunting group behavior and potential presence of other individuals belonging to Panthera atrox at southeastern Hidalgo during the late Pleistocene, it is also probable that adult individuals of glyptodonts (Glyptotherium floridanum) and ground sloths (Paramylodon cf. harlani), as well as young and/or subadult individuals of proboscideans (Cuvieronius sp. and Mammuthus sp.), could represent other, perhaps occasional prey. The American lion was the second largest carnivore in the late Pleistocene ecosystems of North America, exceeded by only the short-faced bear Arctodus simus (KurtØn and Anderson, 1980). The large size and restricted dietary behavior of Panthera atrox (a carnivore that feeds mostly on meat, i.e., a hypercarnivore) suggest that this extinct cat occupied the top of the trophic chain, considering that both conditions are typical of extant top predators (Ritchie and Johnson, 2009). At the El Barrio locality, it is probable that the individual of P. atrox described here had the ecological role of the top predator, displacing other carnivores of small to medium size, such as the dire wolf (Canis dirus) to the mesopredator guild. 5.2 Geographic distribution By the late Pleistocene, the American lion was widely distributed from Alaska to southern Mexico. The earliest known occurrences are from the Sangamonian interglacial stage, including localities in the western of the United States, as well as northern and central Mexico (KurtØn and Anderson, 1980; Van Devender et al., 1985), indicating that this large-sized cat reached regions of southern temperate North America in a relatively short time. Subsequently, the American lion spread its distribution to the Great Plains, the Great Basin, the California Coast, the Gulf Coast, and Mexico during the Wisconsinan (KurtØn and Anderson, 1980; Lange, 2002). Based on the known geographic distribution of Panthera atrox, it seems that it was a common inhabitant of temperate areas of central-western North America, although it was able to reach tropical areas that now are part of southern Mexico (Fig. 5). Previous to this study, the American lion Panthera atrox has been reported from nine Mexican localities in northern (La Brisca (Sonora); Arroyo-Cabrales et al., 2005), central (San Josecito (Nuevo León), El Cedral (San Luis Potosí), El Cedazo (Aguascalientes), Chapala–Zacoalco (Jalisco), Tequixquiac (State of Mexico); Freudenberg, 1910; Mooser and Dalquest, 1975; Lorenzo and Mirambell, 1981; Arroyo-Cabrales and Polaco, 2003; Arroyo-Cabrales et al., 2005; Lucas, 2008), and southern (La Simpatía, La Tejería y Villa Corzo (Chiapas); Aviaea, 1969; Montellano-Ballesteros and Carbot-Chanona, 2009; Carbot-Chanona and Gómez-PØrez, 2014) areas of the country. The record reported here supplements its presence in central Mexico, specifically in areas that now are part of southeastern Hidalgo. It is noted that most of the localities where Panthera atrox has been reported are located in central Mexico, between 19 and 24 ◦ N at an altitude from 1500 to 2250 m a.s.l. (including El Barrio locality at 2184 m a.s.l., Hidalgo (present study)), whereas the occurrences in northern (La Brisca, Sonora) and southern (Chiapasan localities) Mexico are located at a mean altitude of 750 m a.s.l. (Fig. 6a). According to the major biogeographic corridors of Ceballos et al. (2010), the records of Panthera atrox from northwestern and central Mexico (inclu

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