Tremarctos floridanus
Tremarctos floridanus is an extinct species of short-faced bear from North America. Its fossils have been largely found throughout the Southeastern United States and Mexico from the Late Pleistocene, and from earlier epochs at some sites in western North America. T. floridanus became extinct at the end of the Pleistocene.
Taxonomy
Tremarctos floridanus is called the Florida spectacled bear, Florida cave bear, or rarely Florida short-faced bear. Originally, James W. Gidley named this animal Arctodus floridanus in 1928, until affinities with the spectacled bear were realised by Chester Stock when specimens from San Josecito Cave were recovered in 1950. Described as Tremarctos mexicanus, it was recombined as T. floridanus by Björn Kurtén, Lundelius, and Kurtén and Anderson. Despite at least 35 individuals being recovered by 1967, almost all specimens consist of isolated teeth & skull fragments, save for the type specimen. Despite one such common name, T. floridanus is not considered a close relative of the Eurasian cave bear, which belonged to a different genus and subfamily.Diagnostics
T. floridanus is morphologically very similar to its sister species T. ornatus. However, not only was T. floridanus around twice as large as T. ornatus, the proportions of the humerus, femur and neck in particular are longer than T. ornatus, with T. floridanus being described as a relatively long-limbed species. Additionally, while the forelimbs are longer than the hindlimbs in T. ornatus, they are of equal length in T. floridanus. Additionally, T. floridanus has also been described as possessing much more robust limb bones. However, the paws of T. floridanus are proportionally shorter and smaller than T. ornatus.Notable differences also occur in the craniums of T. floridanus and T. ornatus, with the rostrum of T. floridanus being relatively narrow compared with T. ornatus. Additionally, unlike T. ornatus, T. floridanus possesses a signature "glabella" on the frontal bone of the cranium. Both species share practically identical dentitions, though the dentition of T. floridanus was larger, often with a reduced number of premolars and relatively longer molars. Tremarctos floridanus has mandibular condyles raised well above the plane of the teeth, while T. ornatus does not. The lower jaws of T. floridanus are larger; while the ramus of the mandible is taller in T. floridanus, the relative height of the mandible's coronoid process is the same in both species. Kurtén compared the differences between the Tremarctos species as the differences between brown bears and Eurasian cave bears.
Other bears
The American black bear and T. floridanus are regularly co-specific; the M2 molar is often used to differentiate between the species. Despite their similar sizes, the M2 of T. floridanus has a subrectangular form with truncated ends, a flat crown profile, a straight line on the lingual side, a high anterior region, and lacks a cingulum ridge. Meanwhile, the M2 of Arctodus is broader and more robust than Tremarctos.In addition to general cranial and dental differences between tremarctine and ursine bears, T. floridanus can also be distinguished from American black bears via their postcranial skeletons. The forelimbs of T. floridanus evidenced an entepicondylar foramen on the humerus, with the proximal end of the radius in T. floridanus being oval-shaped. The cervical vertebrae are divergent in the orientation, form and proportions, while the number of ribs is also different. The ischial bar and bone ridge of the acetabulum in the pelvis of T. floridanus are more narrow, while the greater trochanter is taller in the femur of T. floridanus.
Evolution
The closest living relative of the T. floridanus is the spectacled bear of South America; they are classified together with other short-faced bears in the subfamily Tremarctinae. Tremarctine bears first appear as Plionarctos during the late Miocene epoch of North America. An investigation into the mitochondrial DNA of bear species indicates that short-faced bears diverged from the Ursinae subfamily approximately 5.7 million years ago. Around the Miocene-Pliocene boundary, tremarctine bears, along with other ursids, experienced an explosive radiation in diversity, as C4 vegetation and open habitats dominated. The world experienced a major temperature drop and increased seasonality, and a faunal turnover which extinguished 70–80% of North American genera.Correspondingly, the genetic divergence date for Arctodus is between 5.5 million and 4.8 million years ago, and between Arctotherium and Tremarctos at 4.1 million years ago. Researchers believe that Arctotherium was a sister lineage to Tremarctos, or even emerged from the Tremarctos genus. All three genera evolved from Plionarctos in the Blancan faunal stage of North America, and are first recorded as the medium-sized Arctodus pristinus, ''Tremarctos floridanus and Arctotherium sp. from the Late Blancan of North America circa 2.6 million years ago. These first appearances near the Plio-Pleistocene boundary coincide with the start of the Quaternary Glaciation, the formation of the Panama Land Bridge, and the second phase of the Great American Biotic Interchange, with the first records of the main South American faunal wave into the United States. A Plionarctos harroldum specimen from Taunton appears evolutionarily intermediate between Plionarctos harroldum and Tremarctos floridanus, affirming that Plionarctos harroldum is the likely ancestor of Tremarctos.
The Intermontane Plateaus preserve the oldest possible remains of T. floridanus, being from Palm Spring Formation, Grand View fauna, and San Simon, although the Grand View specimen may instead represent Plionarctos. Additionally, though originally described as Arctodus sp., researchers suggest that indeterminate ursid from the mid-Blancan Buckhorn fauna may represent either Tremarctos sp. or Protarctos abtrusus.
Unlike its Neotropical sister species, T. floridanus was a temperate species that has almost entirely been recovered from Nearctic sites. The fossil record of T. ornatus is unknown, as T. ornatus remains do not appear in the South American record until the Holocene, which may indicate that T. ornatus emerged from T. floridanus in the Holocene. However, as the montane niche was otherwise open, T. ornatus may have instead evolved in the Pleistocene.
Genetic research on the mitochondrial DNA of tremarctine bears indicates Tremarctos was more closely related to Arctotherium than Arctodus. However, a preliminary investigation of tremarctine bear's nuclear DNA suggests an extensive history of hybridization between Tremarctos and Arctodus in North America, although hybridization with Arctotherium as the Tremarctos genus migrated southwards into South America is also possible. Evidence of gene flow between Tremarctos and an ursine bear was also uncovered, most likely due to the extensive overlap between Tremarctos'' and the ancestors of the American black bear in Pleistocene North America.
Description
Skull
The canalis semicircularis lateral suggests that T. floridanus had a head posture of 38°, which is more oblique than its sister species T. ornatus ; as T. ornatus inhabits densely vegetated areas, the more oblique head posture in T. floridanus could infer a greater capacity for long distance vision. Tremarctos floridanus has mandibular condyles raised well above the plane of the teeth, while T. ornatus does not, suggesting T. ornatus potentially possesses a larger gape. As in other tremarctine bears, tooth size could be variable, as exemplified by an average sized female from Tennessee possessing the largest known teeth in T. floridanus, with some even comparable to Arctodus.Postcranial
T. floridanus is thought to have been almost twice the size of T. ornatus, averaging around the size of a large black bear, with one specimen calculated to 219 kg. Like other tremarctine bears, T. floridanus was highly sexually dimorphic, with males being 25% larger than females. Though noticeably larger than its South American relative, T. floridanus was still much smaller than its contemporary Tremarctine relative Arctodus. Like T. ornatus and A. simus, T. floridanus possessed a false thumb.Range
Blancan and Irvingtonian
The distribution of Tremarctos floridanus changed across its natural history. In the Blancan and Irvingtonian faunal stages, T. floridanus remains are sparse, and solely recovered from western North America. The Late Blancan records Tremarctos sp. from San Simon, Arizona, and T. floridanus possibly from the Grand View fauna of Idaho, while Irvingtonian remains have been recovered from El Golfo in Sonora, with the Anza-Borrego Desert in California recording both Late Blancan and Irvingtonian remains. T. floridanus was recorded from the Blancan Hagerman Fossil Beds, but these have been reassigned to Protarctos abstrusus. Irvingtonian remains were reported from Cumberland Bone Cave in Maryland, but subsequent research establishes Arctodus pristinus was the only tremarctine bear present at the locality.Rancholabrean
However, in the proceeding Rancholabrean epoch, T. floridanus was widely distributed along the eastern Atlantic Plain and the broader Gulf Coast. Fossils have been recovered from the US states of Florida, Georgia, Mississippi, South Carolina, Tennessee, and Texas, and the Mexican states of Michoacán, and Nuevo León.T. floridanus is suggested to have expanded its range into higher altitudes during warmer interstadials and interglacials, which would explain its presence in Inner Space Cavern on Edwards Plateau, Texas. The Grassy Cove Saltpeter Cave specimen, from Cumberland Plateau, Tennessee, has been radiocarbon dated to within the climatically unstable MIS 3.
T. floridanus has been described from Belize, however the recent reassignment of a T. floridanus specimen to the morphologically similar Arctotherium wingei from the Hoyo Negro, and other positive identifications of A. wingei from Belize have put under question current T. floridanus identifications in Yucatán Peninsula. Current scholarly analysis asserts that A. wingei may have restricted the range of T. floridanus outside of Central & South America until the extinction of A. wingei, where subsequently Tremarctos begins to be found in the South America.
Additionally, T. floridanus specimens assigned by Kurtén in New Mexico have been reassigned to Arctodus simus - no western specimens of T. floridanus have been found in the Rancholabrean.