Defining taxon: first appearance of the hemicyonine bear Plithocyon (Tedford et al., 2004)

Basis of name: Wood et al. (1941) based the name on the Barstow Formation in the Mojave Desert of southern California. This formation takes its name from the nearby city of Barstow in San Bernardino County. The Barstow Formation is over 1000 meters thick and contains several superposed vertebrate fossil faunas as well as datable ash beds. Woodburne et al. (1990) demonstrated that the lower portion of the Barstow Formation produces Hemingfordian vertebrate fossils, while higher strata contain early and late Barstovian fossils.

Barstovian faunas are also found elsewhere in southern California, including the well-known Shark Tooth Hill locality near Bakersfield . Outside of California, Barstovian sites and faunas are known from Oregon, Nevada, Idaho, Montana, and New Mexico from the western US, from southern Texas, eastern Colorado, Nebraska, and South Dakota from the central US, and from Florida, Georgia, and Maryland from the eastern US. Of these, the localities from Nebraska and Colorado are the richest, and contain superposed Clarendonian faunas stratigraphically above Barstovian localities(Tedford et al., 1987). In southern Texas, there is also a stratigraphic sequence of Miocene vertebrate faunas, of which the Burkeville and Cold Spring represent the early and late Barstovian, respectively (Tedford et al., 1987; 2004). Because of their relatively close proximity, these faunas from the Texas Coastal Plain are frequently very similar to those of contemporaneous faunas from Florida. Barstovian fossils are also known from five faunas in Mexico, from as far south as Chiapas (Montellano-Ballesteros and Jiménez-Hildalgo, 2006), and from southern Canada (Tedford et al., 2004).

The Barstovian is divided into two subintervals: the Ba1 from 15.9 to 14.8 million years ago; and the Ba2 from 14.8 to 12.5 million years ago (Tedford et al., 2004). All of the Barstovian NALMA falls within the middle Miocene Epoch of the standard geologic time scale.

Index species for Ba1 in Florida: Perognathus minutus, Bouromeryx americanus, Acritohippus isonesus, Merychippus brevidontus, “Merychippus” goorisi

Significant Ba1 localities and faunas in Florida: Alum Bluff; Willacoochee Fauna

Index species for Ba2 in Florida: Leptarctus progressus, Calippus proplacidus, Calippus circulus, Cormohipparion quinni, Protohippus perditus, Aphelops megalodus, Teleoceras meridanum

Significant Ba2 localities and faunas in Florida: Bradley Fauna (including the Red Zone sites in the Phosphoria Mine and multiple sites in the Kingsford and Nichols mines); Ashville

Characteristic taxa for the Barstovian in Florida: Carcharocles megalodon, Carcharodon hastalis, Hemipristis serra, Galeocerdo aduncus, Batrachosauroides dissimulans, Thecachampsa americana, Peratherium sp., Protospermophilus sp., Copemys sp., Proheteromys sp., Amblycastor fluminis, Paratoceras sp., Procranioceras sp., Metaxytherium cratagense, Metaxytherium floridanum, Gomphotherium sp., Zygolophodon tapiroides, Hypohippus chico.

Major extinctions: Significant but not particularly extensive extinctions occurred in both the Ba1 and Ba2 (Tedford et al., 2004). Among taxa known from Florida, Parahippus (sensu lato) and Osbornodon both became extinct during the Ba1. The last known occurrences of Amphicyon, Archaeohippus, Anchitherium, Amblycastor, Prosynthetoceras, Moropus, and Bouromeryx are during or at the end of the Ba2. The genera Proheteromys and Harrymys, the most common small rodents in Florida Hemingfordian and Arikareean faunas, last occur in the Barstovian.

Comments: The Barstovian is the poorest known of the Miocene NALMAs in Florida. There is no equivalent Barstovian “megasite” like Thomas Farm from the Hemingfordian or the Love Site from the Clarendonian. Both Ba1 and Ba2 species in Florida tend to be represented by a limited number of specimens, by relatively incomplete specimens, or both, making species-level identifications impossible in many cases. This is one of the reasons that many species and even entire faunas remain unstudied despite being collected decades ago. Barstovian mammals are better known in Florida than are birds, reptiles, and amphibians, for which there are very few described species.

The Barstovian is the oldest NALMA when proboscideans became widespread across the North American continent. The mastodon family apparently dispersed first, in the late Hemingfordian, but is only known from a few western faunas of that age. The first mastodon in North America is the genus Zygolophodon. Gomphotheres followed soon after and are found across North America by the Ba2. Both of these proboscidean families originated in the Old World. The oldest records of both families in Florida is the Barstovian.

Major turnover within the horse family Equidae occurred during the Barstovian. Ba1 horses consisted of a high diversity of moderately high-crowned species (“mesodont”) that were traditionally all assigned to the genus Merychippus, along with rarer taxa with more brachydont teeth, such as Archaeohippus, Anchitherium, Hypohippus, and Parahippus (Hulbert and MacFadden, 1991; Hulbert, 1993). Some of the species traditionally in Merychippus are now placed in new genera such as Acritohippus, Parapliohippus, and Scaphohippus. By the Ba2, Parahippus and most of the merychippine-grade genera were extinct, replaced by more hypsodont species assigned to genera such as Cormohipparion, Pseudhipparion, Nannippus, Calippus, Protohippus, and Pliohippus. This assemblage would be the dominant group of North American horses through the Clarendonian and on into the early Hemphillian (Webb et al, 1995).

Among the carnivores, the most diverse taxa during the Barstovian were the borophagine canids, the mustelids, and, to a lesser extent, the procyonids. True cats (Felidae) were present in North America during this time, although not known as yet from Florida, but were not diverse. The diversity of amphicyonids or bear-dogs was much lower than in preceding NALMAs.

The defining taxon for the start of the Barstovian is the appearance of the hemicyonine bear Plithocyon in North America (Tedford et al., 2004). But it is only known from three early Barstovian localities, all in the southwestern US (Hunt, 1998). While it is known to have reached the Great Plains by the late Barstovian, it is still rare and not present at most Barstovian fossil localities. In practice, most Barstovian faunas are distinguished from those of the preceding Hemingfordian and succeeding Clarendonian by species-level identifications of canids, horses, rhinos, camels, protoceratids, and dromomerycids. If present and identifiable, rodents are also good biochronologic indicators for early to middle Miocene faunas, particularly beavers, cricetids, heteromyids, and geomyoids.



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