A paleontological deposit near San Clemente de Térapa represents one of the very few Rancholabrean North American Land Mammal Age sites within Sonora, Mexico. During that time, grasslands were common, and the climate included cooler and drier summers and wetter winters than currently experienced in northern Mexico. Here, we demonstrate restructuring in the mammalian community associated with environmental change over the past 40,000 years at Térapa. The fossil community has a similar number of carnivores and herbivores whereas the modern community consists mostly of carnivores. There was also a 97% decrease in mean body size (from 289 kg to 9 kg) because of the loss of megafauna. We further provide an updated review of ungulates and carnivores, recognizing two distinct morphotypes of Equus, including E. scotti and a slighter species; as well as Platygonus compressus; Camelops hesternus; Canis dirus; and Lynx rufus; and the first regional records of Palaeolama mirifica, Procyon lotor, and Smilodon cf. S. fatalis. The Térapa mammals presented here provide a more comprehensive understanding of the faunal community restructuring that occurred in northern Mexico from the late Pleistocene to present day, indicating further potential biodiversity loss with continued warming and drying of the region.

Glyptosaurinae is an extinct subfamily of lizards of the family Anguidae. Glyptosaurine lizards are known exclusively from the Paleogene of North America and Eurasia, reaching their peak of diversity and distribution in the Eocene. In North America these lizards are largely restricted to the intermontane basins along the Rocky Mountain range, with only sparse, indeterminately-identified skeletal elements known from outside of this region. Glyptosaurine lizards are split into two tribes: the monophyletic Glyptosaurini and paraphyletic “Melanosaurini”. Within Glyptosaurini, the most common and widespread genus is Glyptosaurus. In this study I describe a new specimen assignable to G. sylvestris, notable for being recovered from the late Uintan of the Santiago Formation in southern California, significantly outside the typical known geographic range of well-preserved glyptosaurine fossils. The presence of Glyptosaurus in southern California at a time of widespread tectonic and climatic change and increasing regional endemism in mammalian faunas, when also considering the results other studies of Eocene lizards, indicates a pattern of evolution for lizards of the time that is quite different from the dramatic turnovers and regional restrictions observed in Eocene mammals. The specimen described here also shows features consistent with ontogenetic variation and may help to provide insight into the life history of glyptosaurine lizards.

The Rancholabrean Land Mammal Age (RLB) occurred in the late Pleistocene, incorporating at least the Wisconsin Glaciation and possibly the preceding interglacial ('Sangamon'). The RLB is defined based on the occurrence of Bison. The first occurrence (arrival time) of Bison is not well established and is still debated. A conservative approach toward the RLB includes: (1) restricting the RLB to faunas south of 55°. N latitude in North America, (2) defining the RLB by the earliest arrival of Bison south of this latitude, (3) placing the best-corroborated arrival time of Bison (therefore the beginning of the RLB) between approximately 210 and 160. ka, and (4) the end of the RLB is marked by the extinction of mainly large mammals as an event or process culminating at approximately 11 radiocarbon ka BP. RLB faunas contain a wealth of information including amphibians, reptiles, birds, and mammals, but clearly the most is known about the latter. Glacial climate regimes forced species into communities that do not occur today (nonanalog or disharmonious scenarios). The debate about the terminal RLB extinctions continues with the cause either being (1) overkill due to the arrival of the First Americans, or (2) the climatic and environmental defragmentation of biotic communities.

The Rancholabrean Land Mammal Age (RLB) occurred in the late Pleistocene, incorporating at least the Wisconsin Glaciation and possibly the preceding interglacial ('Sangamon'). The RLB is defined based on the occurrence of Bison. The first occurrence (arrival time) of Bison is not well established and is still debated. A conservative approach toward the RLB includes: (1) restricting the RLB to faunas south of 55°. N latitude in North America, (2) defining the RLB by the earliest arrival of Bison south of this latitude, (3) placing the best-corroborated arrival time of Bison (therefore the beginning of the RLB) between approximately 210 and 160. ka, and (4) the end of the RLB is marked by the extinction of mainly large mammals as an event or process culminating at approximately 11 radiocarbon ka BP. RLB faunas contain a wealth of information including amphibians, reptiles, birds, and mammals, but clearly the most is known about the latter. Glacial climate regimes forced species into communities that do not occur today (nonanalog or disharmonious scenarios). The debate about the terminal RLB extinctions continues with the cause either being (1) overkill due to the arrival of the First Americans, or (2) the climatic and environmental defragmentation of biotic communities.

Fossils of the giant short-faced bear, Arctodus simus (Cope, 1879), have been recovered from over 100 localities in North America, extending from Mexico to Alaska and California to Virginia. Despite this large range, the species has never been recorded from the southeastern United States. The lesser short-faced bear, Arctodus pristinus Leidy, 1854 is well represented from this region, particularly Florida, but all known occurrences are late Pliocene middle Pleistocene in age (about 2.5 to 0.3 Ma). Differentiating A. simus from A. pristinus can be difficult because large individuals of A. pristinus overlap in size with small individuals of A. simus, and there are few morphological differences. However, these two taxa can be clearly separated based on the relative proportions of their molars and premolars. Two Pleistocene records of A. simus representing a minimum of three individuals from the Withlacoochee River drainage of central Florida are reported here, substantially extending the distribution of this massive bear into southeastern North America. A late Pleistocene age for these occurrences is corroborated by an associated Rancholabrean fauna and rare earth elemental analyses. One of the reported individuals is quite large, supporting the hypothesis of extreme sexual dimorphism in A. simus and rejecting a hypothesis of two subspecies.