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During the late Miocene and early Pliocene about 5.7 to 4.75 million years ago, a distinctive suite of four species of extinct horses (Family Equidae) were widespread in North America. This includes Nannippus aztecus, Neohipparion eurystyle, Astrohippus stocki, and Dinohippus mexicanus. In Florida, two additional horse species, Pseudhipparion simpsoni and Cormohipparion emsliei, are also typically found. Here we compare horses from four Florida fossil sites of this age, including three from the Bone Valley mines and a fourth from the recently discovered Montbrook site. Two of these sites have all six predicted species, one has five species, and one has only four species present. To explain these differences, we used species occurrences from research databases to better understand the relative abundances, species richness, and occurrences of these horses from these four sites. One site (Palmetto Mine Agrico), with five equid species, appears to lack the sixth species owing to ecological reasons. This is different from Montbrook, the site with only four of the six species. Results indicate that Montbrook is likely missing the two horse species for a couple of reasons: sampling bias and biological/ecological causes. Our results demonstrate that fossil sampling biases can account for observed horse species occurrences when the overall abundance of certain species is low. Nevertheless, other factors, including ecology and with sufficient resolution, perhaps also time, may also explain the distribution and occurrences of individual horse species at these and other fossil sites. During the late Miocene and early Pliocene (latest Hemphillian, Hh4 interval, 5.7 to 4.75 Ma) a distinctive suite of four species of extinct horses (Family Equidae) were widespread in North America. This includes Nannippus aztecus, Neohipparion eurystyle, Astrohippus stocki, and Dinohippus mexicanus. In Florida, two additional equid species, Pseudhipparion simpsoni and Cormohipparion emsliei, are also typically found at Hh4 localities. Here we compare horses from four Hh4 Florida fossil sites, including three from the Bone Valley mines and a fourth from the recently discovered Montbrook site. Two of these sites have all six expected species, one has five species, and one has only four species present. To explain these differences, we used species counts from research databases and rarefaction simulation to clarify the relative abundances, species richness, and occurrences of these horses from these four sites. The Palmetto Mine (Agrico) site, with five equid species, appears to lack the sixth species owing to ecological reasons. This is different from Montbrook, the site with only four of the six species. Results indicate that Montbrook is likely lacking two missing equid species for a couple of reasons: sampling bias and biological/ecological causes. Our results demonstrate that sampling biases can account for observed equid species richness when the overall abundance of certain equid species is low. Nevertheless, other factors, including ecology and with sufficient resolution, perhaps also time, may also explain the distribution and occurrences of individual species at these and other fossil sites. In a broader perspective, analyses such as this example provide an opportunity to address a persistent challenge in paleontology, that is, how do we explain absences of extinct taxa from the fossil record?

Fossils of the ursid Indarctos from Withlacoochee River 4A of Florida (late early Hemphillian North American Land Mammal Age, Hh2, ~ 7.5–6.5 Ma) represent the best sample of this genus in North America, including both craniodental and postcranial specimens, yet only the skull has been described. In this study, we describe the other material of this bear from the same locality and review the records of Indarctos in North America. Indarctos from Withlacoochee River 4A has dental characters in accordance with those of typical Indarctos oregonensis , but has distinctly more slender postcranial bones. Indarctos from other localities of North America contains two morphs with regards to postcranial robustness, but lacks a clear geographic pattern. Indarctos from Withlacoochee River 4A shares traits with the Old World Indarctos zdanskyi and may be its descendant in the New World. The machairodont from Withlacoochee River 4A shows typical characters (e.g., presence of distinct P4 preparastyle and mandibular flange) that allow referral to Amphimachairodus rather than to Nimravides . Its morphology also shows a stronger affinity of this population to the Old World Amphimachairodus horribilis rather than to Hh3 Amphimachairodus coloradensis . Previously, most North American records of Amphimachairodus were late Hemphillian (Hh3–4, ~ 6.5–4.5 Ma), when it co-occurred with a different ursid, “ Agriotherium. ” The Withlacoochee River 4A specimens provide evidence that Amphimachairodus dispersed from Asia during the Hh2. The special morphology of Indarctos and the presence of Amphimachairodus in a Hh2 fauna suggest that the environment had begun to change before the significant fauna turnover between the early and late Hemphillian.

The fossil record of the Pacific 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 identified 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. 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 fieldwork and through reidentification of previously collected fossils. Newly identified taxa include the borophagine canid Borophagus secundus (Matthew and Cook, 1909), the camelids MegatylopusMatthew and Cook, 1909 and PleiolamaWebb and Meachen, 2004, a dromomerycid, and the equids CormohipparionSkinner and MacFadden, 1977 and PseudhipparionAmeghino, 1904. Specimens previously assigned to NeohipparionGidley, 1903 and Hipparionde 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 refined 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 significant Miocene locality.

New material of Agriotherium from the late Hemphillian (~6 Ma) Quiburis Formation in Arizona, North America is reported. These specimens represent a new species, Agriotherium hendeyi , sp. nov., of small size and with a bucco-lingually narrow lower dentition distinguishing it from North American “ Agriotherium ” schneideri as well as the genotype and other Old World species of Agriotherium . Strikingly, the m1 metaconid-entoconid complex of the new species exhibits a morphologically transitional state between the pattern observed in Indarctos (three cusps) and the typical Agriotherium pattern (two cusps). Together with a review of the variation in A. africanum , a geometric morphometric analysis permits identification and discussion of a proposed transformation pathway from the Indarctos pattern to the Agriotherium pattern. It is shown that the two cusps in the metaconid-entoconid complex in Agriotherium correspond to the two entoconids in Indarctos , whereas the ancestral metaconid is reduced or lost in Agriotherium . From a developmental perspective, the metaconid fused to the anterior entoconid as a result of the shortening of the talonid, rather than via replacement of the metaconid by a posterior shift of the entoconid, presumably under selective pressure towards a more typically hypercarnivorous tooth morphology and carnivorous diet.

In March 2017, the partial right mandible (FHSM VP-19466) of the giant Miocene-Pliocene bear, Agriotherium schneideri (Ursidae; Agriotheriinae), was recovered from the sandy sediments of an exposed paleo stream channel in northern Sedgwick County, Kansas. The mandible includes the right canine, fourth premolar, and m1-m3 molars. The vacant p1 alveolus is visible immediately behind the canine. The overall size of the mandible, measurements of the teeth, presence of a ‘chin,’ and a large premasseteric fossa confirm the specimen to be Agriotherium schneideriSellards 1916. The worn condition of the teeth suggests that the bear was an older adult at the time of death. The relatively complete and un-abraded condition of the mandible would preclude long distance stream transport. All of the previously documented North American specimens of Agriotherium are dated from late Miocene to earliest Pliocene (∼ 10-4 Ma). Fragmentary remains attributable to Agriotherium have been previously identified from several Miocene localities in Kansas.

A new genus and species of megalonychid sloth, Zacatzontli tecolotlanensis n. gen. n. sp., is described from the late Hemphillian of Jalisco, Mexico. Comparison and analysis of the type specimen, a mandible, with other megalonychid sloths shows a closer relationship to South American taxa than those from North America or the Caribbean. This suggests that during the early stages of the Great American Biotic Interchange there were two separate dispersal events of megalonychid sloths—an earlier one represented by Pliometanastes and the later one by Zacatzontli n. gen. While the morphology of the spout of Zacatzontli more closely resembles that of Megalonyx, based on the current record, Zacatzontli does does not enter North America until after the evolution of Megalonyx from Pliometanastes. The role of the northern neotropics in South America as a staging area for South American taxa that entered North America is discussed.

Two peccary species, Mylohyus elmorei and Prosthennops serus are described from the medium-bodied fauna of the Gray Fossil Site (GFS) of northeastern Tennessee. This site, recognized as an oak-hickory forest, is latest Hemphillian or earliest Blancan based on mammalian biochronology, with an estimated age of 4.9–4.5 Ma. The GFS represents the only site outside the Palmetto Fauna of Florida with M. elmorei , greatly expanding the species range north over 920 km, well into the Appalachian region. This is also the first Appalachian occurrence of the relatively widespread P. serus . Our understanding of intraspecific variation for both M . elmorei and P . serus is expanded due to morphological and proportional differences found in cranial and dental material from the GFS, Tyner Farm locality, Palmetto Fauna, and within the literature. The GFS M. elmorei material represents the most complete mandible and second cranium for the species, and preserve intraspecific variation in the length of the diastema, dental proportions, and the complexity of the cuspules of the hypoconulid complex. Similarly, mandibular material from the GFS for P. serus exhibited larger dentitions and a greater degree of robustness than currently recognized for the species.

Thousands of vertebrate fossils have been recovered from the Gray Fossil Site, Tennessee, dating to the Miocene-Pliocene boundary. Among these are but eight specimens of bats representing two different taxa referable to the family Vespertilionidae. Comparison of the fossils with Neogene and Quaternary bats reveals that seven of the eight specimens pertain to a species of Eptesicus that cannot be distinguished from recent North American Eptesicus fuscus . The remaining specimen, a horizontal ramus with m3, is from a smaller vespertilionid bat that cannot confidently be assigned to a genus. Although many vespertilionid genera can be excluded through comparisons, and many extinct named taxa cannot be compared due to nonequivalence of preserved skeletal elements, the second taxon shows morphological similarities to small-bodied taxa with three lower premolar alveoli, three distinct m3 talonid cusps, and m3 postcristid showing the myotodont condition. It resembles especially Nycticeius humeralis and small species of Eptesicus . Eptesicus cf. E. fuscus potentially inhabited eastern North America continuously since the late Hemphillian land mammal age, when other evidence from the Gray Fossil Site indicates the presence in the southern Appalachian Mountains of a warm, subtropical, oak-hickory-conifer forest having autochthonous North American as well as allochthonous biogeographical ties to eastern Asia and tropical-subtropical Middle America.

Trachemys (Testudines: Emydidae) represents one of the most well-known turtle genera today. The evolution of Trachemys , while being heavily documented with fossil representatives, is not well understood. Numerous fossils from the late Hemphillian Gray Fossil Site (GFS) in northeastern Tennessee help to elucidate its evolution. The fossil Trachemys at the GFS represent a new species. The new taxon, Trachemys haugrudi , is described, and currently represents the most thoroughly described fossil emydid species known. A phylogenetic analysis, including 31 species, focusing on the subfamily Deirochelyinae is performed that includes the new fossil species, along with numerous other modern and fossil deirochelyine species, representing the first phylogenetic analysis published that includes several fossil deirochelyines. The phylogenetic analysis, utilizing morphological evidence, provides monophyletic clades of all modern deirochelyines, including Chrysemys , Deirochelys , Pseudemys , Malaclemys , Graptemys , and Trachemys . A strict consensus tree finds the recently described fossil species Graptemys kerneri to be part of a clade of Graptemys + Malaclemys . Three fossil taxa, including one previously referred to Pseudemys ( Pseudemys caelata ) and two to Deirochelys ( Deirochelys carri and Deirochelys floridana ) are found to form a clade with modern Deirochelys reticularia reticularia , with D. floridana sister to the other members of the clade. Chrysemys is found to be part of a basal polytomy with Deirochelys in relation to other deirochelyine taxa. Two fossil taxa previously referred to Chrysemys ( Chrysemys timida and Chrysemys williamsi ) form a paraphyly with the modern Chrysemys picta picta and Deirochelys , and may be referable to distinct genera. Additionally, fossil taxa previously attributed to Trachemys ( Trachemys hillii , Trachemys idahoensis , Trachemys inflata , and Trachemys platymarginata ) and T. haugrudi are found to form a clade separate from clades of northern and southern Trachemys species, potentially suggesting a distinct lineage of Trachemys with no modern survivors. Hypotheses of phylogenetic relationships mostly agree between the present study and previous ones, although the inclusion of fossil taxa provides further clues to the evolution of parts of the Deirochelyinae. The inclusion of more fossil taxa and characters may help resolve the placement of some taxa, and further elucidate the evolution of these New World turtles.

Dozens of well-preserved fossil burrow systems in upper Miocene sedimentary rocks of the Ogallala Group at a site in east-central Nebraska record rodent behavior and the subsurface ecology of grasslands just as the modern Great Plains was developing. These burrow systems include one to four entrance or exit tunnels, large underground nesting chambers at depths of several decimeters below ancient land surfaces, and incisor grooves on the walls. Tunnels average 89.8 mm in diameter, a value similar to the burrow diameters of multiple living North American rodents. Chambers vary in shape and typically exceed 500 mm in length; some attain 1000 mm in length. Living marmotine ground squirrels (tribe Marmotini) construct burrow systems of varying degrees of complexity, but they do not engage in shallow subsurface foraging. Extinct members of this group were the most likely excavators of the fossil burrows. In contrast, extant pocket gophers (family Geomyidae) and, presumably their fossil relatives, are obligate subterranean animals that produce linked deep and shallow burrow subsystems, the latter representing their chief foraging strategy. Our results raise issues regarding the relationships between the architecture of fossil rodent burrow systems and aspects of rodent behavior and life history, such as litter size, developmental rates, seasonal torpor, hibernation, and sociality in grasslands. An improved understanding of the burrowing behaviors of ancient rodents will highly complement the growing body of knowledge about the development of grasslands on Earth over time, but truly ichnological analyses of the burrows and burrowing behaviors of extant rodents are much needed.