Cheek Tooth Morphology and Ancient Mitochondrial DNA of Late Pleistocene Horses from the Western Interior of North America: Implications for the Taxonomy of North American Late Pleistocene Equus

Horses were a dominant component of North American Pleistocene land mammal communities and their remains are well represented in the fossil record. Despite the abundant material available for study, there is still considerable disagreement over the number of species of Equus that inhabited the different regions of the continent and on their taxonomic nomenclature. In this study, we investigated cheek tooth morphology and ancient mtDNA of late Pleistocene Equus specimens from the Western Interior of North America, with the objective of clarifying the species that lived in this region prior to the end-Pleistocene extinction. Based on the morphological and molecular data analyzed, a caballine (Equus ferus) and a non-caballine (E. conversidens) species were identified from different localities across most of the Western Interior. A second non-caballine species (E. cedralensis) was recognized from southern localities based exclusively on the morphological analyses of the cheek teeth. Notably the separation into caballine and non-caballine species was observed in the Bayesian phylogenetic analysis of ancient mtDNA as well as in the geometric morphometric analyses of the upper and lower premolars. Teeth morphologically identified as E. conversidens that yielded ancient mtDNA fall within the New World stilt-legged clade recognized in previous studies and this is the name we apply to this group. Geographic variation in morphology in the caballine species is indicated by statistically different occlusal enamel patterns in the specimens from Bluefish Caves, Yukon Territory, relative to the specimens from the other geographic regions. Whether this represents ecomorphological variation and/or a certain degree of geographic and genetic isolation of these Arctic populations requires further study

Fast spread followed by anagenetic evolution in Eurasian and North American Amphimachairodus

The charismatic sabretooth cat Amphimachairodus has numerous but largely fragmentary records across late Miocene deposits of Africa, Eurasia and North America. The genus has a complex taxonomic history, and the majority of Amphimachairodus materials come from isolated localities, often studied without stratigraphic context. Here, we analyse the long, continuous records from the classic Chinese Baode strata, which produce Amphimachairodus throughout the section, and demonstrate that an A. palanderi-horribilis chronospecies succession represents a continuum of in situ anagenetic evolution of increasing size. We then synthesise chronological occurrences of Amphimachairodus from all Holarctic records and reframe their evolution as a case of chronospecies succession. Two parallel anagenetic lineages are evident: a Eurasian A. giganteus-palanderi-horribilis chronospecies succession and second, a North American A. coloradensis-alvarezi chronospecies succession following an immigration event in the early Hemphillian. In addition to greater hypercarnivory evidenced by dental specialisation, the Eurasian lineage shows a trend towards a large body size, whereas the North American lineage decreases in size. We take this opportunity to describe materials of Amphimachairodus alvarezi from Yepómera (latest Hemphillian) in the state of Chihuahua, Mexico, and previously undescribed materials from San Miguel de Allende Basin. We review taxonomic status of Chinese A. horribilis and related taxa

Notes on the origin of extensive endorheic regions in central and northern Mexico, and some implications for paleozoogeography

The recent discovery of a fossil of Enhydritherium terraenovae in upper Miocene fluvial deposits in Juchipila (Mexico), nearly 200 km away from the nearest coast, together with other known occurrences of the same species in Florida and California, made possible to envision an alternative to the Panamanian and Polar routes of migration through fluvial systems in Mexico. In order to cross from one ocean to the other, individuals of E. terraenovae must have passed the continental divide, which is a physiographic feature that separates surface waters that flow into the Atlantic and Pacific versants. Two vast endorheic regions, which together span more than 400,000 km2 in area, currently dominate drainage systems in northern and central Mexico. The endorheic regions are broadly bounded by two mountain ranges and coincide with the arid and semi-arid regions of the Chihuahuan desert. These closed basins are an additional obstacle for migration. However, drainage systems are constantly varying and adjusting to changing conditions imposed by climate, tectonic activity, volcanism, and pronounced asymmetries in topography and rainfall distribution. The migration route across Mexico for Enhydritherium terraenovae in the late Miocene (≥6 Ma) could have been facilitated by one or more river captures that inverted the flow direction near the headwaters of a drainage system that debouched either into the Gulf of Mexico or the Pacific coast. Biologists studying fresh water fish faunas in the southern part of the United States and in northern and central Mexico have documented several living species that occur in both the Rio Grande and in the Mezquital rivers, two drainages that are not presently connected, drain in opposite directions (i.e. towards the Gulf of Mexico and the Gulf of California, respectively) and are separated by the endorheic regions. Furthermore, systematic studies of fresh water fish faunas in the region has numerous examples of endemicity and allopatric speciations that are interpreted as evidence of several events of drainage system rearrangement either by stream capture, beheading, or river diversion or by fragmentation caused by formation of closed basins by faulting, volcanic activity or isostatic rebound in the footwall block of normal faults. The western boundaries of the extensive endorheic regions in Mexico are controlled mainly by a highland produced by intense felsic volcanism during the Sierra Madre Occidental activity (Eocene – Miocene), and the eastern hydrological divide coincides with mountains associated with the fold and thrust belt produced by the Hidalgoan orogeny in early Cenozoic time. Normal faulting associated with Basin and Range extension formed additional closed basins. Another major factor in the formation of the endorheic regions is the terminal Miocene climate change, which caused runoff reduction and lowered the rivers capacity of erosion. Opening of the Gulf of California changed the overall slope of rivers draining into the Pacific in northwestern Mexico triggering the capture of rivers formerly confined to the closed basins or draining to the Gulf of Mexico. Quaternary normal faults have been documented a few tens of kilometers from the hydrological divides of the endorheic regions, suggesting that in some regions there is an active tectonic component in their formation. The location of the crossing point for the otters is uncertain as the fossil record is scant and investigation on the evolution of drainage systems through time in Mexico is in the early stages. However, it is argued that there is a fair chance that the crossing occurred via the Mezquital river capture near Durango city. Isotopic age of the sediments where the Enhydritherium terraenovae fossil was collected, imply that the capture must have occurred in the late Miocene

A New Pliocene Capybara (Rodentia, Caviidae) from Northern South America (Guajira, Colombia), and its Implications for the Great American Biotic Interchange

One of the most striking components of the modern assemblage of South American mammals is the semiaquatic capybara (Caviidae, Hydrochoerinae), the biggest rodent in the world. The large hydrochoerines are recorded from the middle Miocene to the present, mainly in high latitudes of South America. Although less known, they are also recorded in low latitudes of South America, and in Central and North America. We report the first record of capybaras from the late Pliocene of Colombia, found in deposits of the Ware Formation, Guajira Peninsula in northeastern Colombia. We analyze the phylogenetic position within Caviidae, the possible environmental changes in the Guajira Peninsula, and the implications of this finding for the understanding of the Great American Biotic Interchange. The morphological and phylogenetic analyses indicate that the hydrochoerine of the Guajira Peninsula is a new species, ?Hydrochoeropsis wayuu, and this genus is most closely related to Phugatherium. According to the latest phylogenetic results, this clade is the sister group of the lineage of the recent capybaras (Neochoerus and Hydrochoerus). ?Hydrochoeropsis wayuu is the northernmost South American Pliocene hydrochoerine record and the nearest to the Panamanian bridge. The presence of this hydrochoerine, together with the fluvio-deltaic environment of the Ware Formation, suggests that during the late Pliocene, the environment that dominated the Guajira Peninsula was more humid and with permanent water bodies, in contrast with its modern desert habitats