The first Central American chalicothere (Mammalia, Perissodactyla) and the paleobiogeographic implications for small-bodied schizotheriines

SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP


GEOLOGIC, BIOSTRATIGRAPHIC, AND GEOCHRONOLOGICAL CONTEXT
The Panama chalicothere was discovered in the Lirio Norte region of the Gaillard (Culebra) Cut of the Panama Canal ( Fig. 1) preserved within the early Miocene Las Cascadas Formation, a sequence of volcanic agglomerates, lapilli tuffs, and tuffaceous sedimentary rocks (Montes et al., 2012). The upper Las Cascadas Formation provides definitive evidence for the earliest terrestrial paleoenvironments in the Panama Canal Basin, including fluvially deposited lenticular conglomerates and sandstones (Montes et al., 2012;Rincon et al., 2012), subaerially exposed volcaniclastics (Kirby et al., 2008), and in situ root structures. The chalicothere tooth was found as float near the contact of a lapilli tuff and an overlying subaerially exposed red ash with centimeter-scale volcanic lithic fragments.
Recently described species of floridatraguline camels and a bothriodontine anthracothere found in association with protoceratids, equids, and amphicyonids in the upper Las Cascadas Formation indicates the presence of a late Arikareean mammalian assemblage with North American affinities (Rinc on et al., 2012(Rinc on et al., , 2013. Geochronological analyses of the overlying Culebra Formation (Kirby et al., 2008;Montes et al., 2012) and underlying Bas Obispo Formation (Rooney et al., 2010) constrain the age of the Las Cascadas mammalian assemblage to the latest Oligocene to earliest Miocene (~25-19 Ma). The fossiliferous horizons, however, are restricted to the uppermost Las Cascadas Formation, suggesting that the mammalian assemblage likely existed during the younger end of this age range.

DESCRIPTION AND COMPARATIVE MORPHOLOGY
The tooth, UF 280165, is a relatively unworn, partial right lower molar discovered in two pieces as float. The anterolingual portion of the trigonid, including the paraconid and part of the paralophid, is broken and missing as is the apex of the entoconid. No roots are preserved on the specimen. The estimated APL, measured from the anterior-most preserved portion of the anterior cingulid to the posterior-most portion of the complete posterior cingulid, is 22.5 mm, whereas the measured TW is 11.4 mm.
UF 280165 is molariform with a high length-to-width ratio and could potentially represent either a dp4 or any of the permanent lower molar positions. Coombs (1978a) described the dp4 of Moropus elatus as having a trigonid slightly smaller than the talonid. No differences in the relative trigonid and talonid proportions relative to M. elatus were reported in the dp4 of Tylocephalonyx skinneri (Coombs, 1979) or in the Aquitanian species of Moropus from France (Coombs, 1974). The widths of the trigonid and talonid in UF 280165, measured as the TW between the bases of the protoconid/metaconid and entoconid/ metastylid, respectively, are equal at 11.4 mm. The estimated length of the trigonid, measured either by doubling the APL between the protoconid and metaconid (6.3 mm) or measuring *Corresponding author.
Journal of Vertebrate Paleontology e923893 (5 pages) Ó by the Society of Vertebrate Paleontology DOI: 10.1080DOI: 10. /02724634.2014 from the anterior-most edge of the anterior cingulid to the metaconid (7.5 mm), is considerably less than the APL of the talonid (12.7 mm). The m3 of Moropus elatus, M. merriami, and Tylocephalonyx skinneri all exhibit posteriorly displaced hypoconids and entoconids and disproportionately longer metalophids relative to the hypolophids (Coombs, 1978a(Coombs, , 1979(Coombs, , 2004. The metalophid and hypolophid lengths of UF 280165 are comparable at 10.2 and 9.5 mm, respectively. Based on these proportions, UF 280165 most likely represents an m1 or m2. Upon considering UF 280165 as an m1 or m2, it is similar in size to the smallest chalicotheriids from the Oligocene and Miocene (Table 1), more specifically Butleria rusingense, Chalicotherium salinum, Schizotherium avitum, and S. priscum. Coombs et al. (2001) referred isolated chalicothere lower molars from the late Arikareean Toledo Bend Local Fauna (Albright, 1999) and a chalicothere mandible (UF cast 180233) from the St. Mark's River in Florida to Moropus oregonensis. These specimens from the Gulf Coast are also similar in size to UF 280165. Chavasseau et al. (2010) and Liu and Zhang (2012) used lower molar width-to-length ratios to identify chalicothere mandibular fragments without associated upper dentitions or postcrania to the subfamily level. Their use of this metric was based on the observations by previous authors that schizotheriine chalicotheres typically have more elongate lower molars relative to chalicotheriines (e.g., Coombs, 1989Coombs, , 2009Anquetin et al., 2007). Ratios of width to length for the m1 and m2 positions within Chalicotheriinae range from 0.56 to 0.64 and 0.52 to 0.66, respectively, whereas these ratios within Schizotheriinae range from 0.50 to 0.68 and 0.50 to 0.58. The ranges reported here are based on data from Liu and Zhang (2012), available in the online supplemental data. The width-to-length ratio exhibited by UF 280165 (0.51) falls below the range expected for chalicotheriines, suggesting that the specimen from Panama is a schizotheriine chalicothere like all other known North American chalicotheres. This subfamily identification for UF 280165 is highly tentative given the large overlap in ratios for the two subfamilies.
UF 280165 lacks a well-developed metastylid on the posterior surface of the metaconid as is typically seen in the lower molars of schizotheriine chalicotheres. Two small cuspids are present instead, separated from each other and the metaconid by short lingual fissures that would not be apparent after moderate dental wear ( Fig. 2A, E). The metalophid ends lingually between these two small cuspids, whereas the metacristid is confluent with the posterior cuspid (Fig. 2B). The lophids of UF 280165 are slightly curved, similar to the condition exhibited in species of Moropus.
The morphology of the metaconid region and lophids of UF 280165 distinguishes it from other North American species of schizotheriine chalicotheres. Moropus elatus and M. merriami each exhibit strongly developed metastylids on the dp4 and all lower molars (Coombs, 1978a(Coombs, , 2004. Similarly, the metastylid  (Albright, 1999). Although well preserved, the lower dentition of the St. Mark's River M. cf. oregonensis is in a private collection and will not be discussed further. In Tylocephalonyx skinneri, the metastylid of the m1 is lower and barely distinct from the metaconid (Coombs, 1979), but the lophids in T. skinneri are distinctively curved in contrast to the slightly curved lophids of UF 280165. Weak metastylid development or variability in metastylid development on lower molars has been reported in species of Schizotherium (Coombs, 1978b), Butleria rusingese (Butler, 1965), and in Miocene schizotheriines and chalicotheriines from Eurasia and Africa, including Metaschizotherium bavaricum (Coombs, 2009), species of Anisodon (Aquentin et al., 2007;Chavasseau et al., 2010;Fahlke et al., 2013), species of Chalicotherium (Coombs, 1989;Khan et al., 2009;Chavasseau et al., 2010;Fahlke et al., 2013), and Ancylotherium cheboitense (Guerin and Pickford, 2005). The condition of having paired cuspids posterior to the metaconid, however, has not been described in any of these taxa.

DISCUSSION
UF 280165 may represent a new small-bodied genus of schizotheriine endemic to tropical Central America during the late Arikareean. The Panamanian chalicothere lacks a well-developed metastylid and highly curved lophids and consequently cannot be assigned to either of the two known schizotheriine genera (i.e., Moropus and Tylocephalonyx) from North America as they are currently defined. Discovery of chalicothere postcrania in the Las Cascadas Formation will be integral to understanding the taxonomy of this species, because many diagnostic features of both schizotheriine and chalicotheriine taxa are restricted to the postcranial skeleton. Coombs et al. (2001) hypothesized that the small body sizes of Moropus oregonensis from the John Day Formation and the Gulf Coast M. cf. oregonensis reflect the primitive condition exhibited by the first immigrant Moropus from Eurasia. Furthermore, small body size and weakly or variably developed metastylids are characters of Butleria, the basal-most genus within Chalicotheriinae (Anquentin et al., 2007), and Schizotherium, the oldest schizotheriine genus (Coombs, 1978b). These characters in UF 280165 suggest that the Las Cascadas chalicothere represents one of the most primitive chalicotheriids in the western hemisphere, maintaining the lower dental morphology exhibited by the first chalicotheriid immigrants.  hypothesized that first appearances of Eurasian mammals in North America during the late Arikareean were associated with glacioeustatic sea-level changes and opening of Beringian dispersal routes. The earliest, well-documented occurrences of North American chalicotheres (i.e., Moropus oregonensis) are from the Kimberly Member (lithostratigraphic unit M) of the John Day Formation in Oregon (Coombs et al., 2001;Albright et al., 2008). Albright et al. (2008) correlated lithostratigraphic unit M to magnetochrons C7r and C7n, establishing a minimum age of 24.9 Ma for the first chalicotheriid immigrants. The chron C7 correlation of Albright et al. (2008) and the minimum age of chalicotheriid immigration to North America are coincident with the 25.2-24.8 Ma Oi2c d 18 O event (Miller et al., 1998;Pekar et al., 2006) and concurrent drop in global sea levels (~20 m below modern sea level; Miller et al., 2005;Kominz et al., 2008). This evidence suggests that the first appearance of primitive, small-bodied chalicotheriids in North America can be linked to a specific global paleoclimate event (i.e., the Oi2c Antarctic ice sheet expansion/sea-level lowstand) with some confidence.
As noted above, the age of the Las Cascadas Formation is constrained to roughly 25-19 Ma, spanning the Oligocene-Miocene  Boulila et al., 2011: table 1 for the ages of these events). In addition, trace element analyses indicate that the intense volcanic activity preserved in the Las Cascadas Formation may mark the transition from arc magmatism to local extensional magmatism in the Panama Canal Basin during the initial collision with South America (Farris et al., 2011). The combination of intensive volcanic activity, basin extension, and repeated sea-level lowstands (i.e., changes in base level) may explain the repeated volcanic to sedimentary sequences recorded in the Las Cascadas Formation (Montes et al., 2012). The Las Cascadas chalicothere along with the rest of the mammalian fossil assemblage were found in the uppermost volcanosedimentary sequence and are likely synchronous with one of the several Mi1 ice sheet expansions/sea-level lowstands between 23 and 19 Ma. The implication here is that the occurrence of a chalicothere in the late Arikareean New World Tropics could be significantly younger than the first North American immigration of chalicotheriids during the latest Oligocene. In addition, the primitive, small-bodied Las Cascadas chalicothere is likely contemporaneous with the large-bodied Moropus elatus from the Harrison Formation of Nebraska with a minimal age of 22.9 § 0.08 (Coombs, 2004;Tedford et al., 2004).
These observations led to two alternative hypotheses concerning the origin of the Las Cascadas chalicothere. First, primitive, small-bodied chalicotheriids similar to the first North American immigrants persisted in tropical Central America during the late Arikareean as large-bodied taxa became more prevalent at higher latitudes. The taxon represented by the Las Cascadas chalicothere may have been ecologically excluded from these higher latitudes due to changes in vegetation and habitat structure in the earliest Miocene (Str€ omberg, 2002(Str€ omberg, , 2006, a situation similar to that of the late Arikareean Gulf Coast Moropus reported by Frailey (1979), Albright (1999), and Coombs et al. (2001). The second hypothesis is that the Las Cascadas chalicothere represents a second immigration of Eurasian chalicotheriids into North America. Multiple dispersals of mammalian carnivores into North America during the latest Oligocene and earliest Miocene (e.g., ) set a precedent for more than one migration event in this interval, indicating the plausibility of two chalicotheriid dispersals from Eurasia. Additional material of the Las Cascadas chalicothere along with a rigorous phylogenetic analysis of Eurasian and North American chalicotheres will be necessary to test these hypotheses.