Tsoabichi

''Tsoabichi'' is an extinct genus of caimanine crocodylian. Fossils are known from the Green River Formation in Wyoming, and date back to the Ypresian stage of the Eocene (Wasatchian stage of North American age). The genus was named and described in 2010 by paleontologist Christopher A. Brochu, with the type species being ''Tsoabichi greenriverensis''. According to the current understanding of caiman evolutionary relationships, ''Tsoabichi'' is a basal member of Caimaninae and may have evolved after caimans dispersed into North America from northern and central South America, their main center of diversity in the Cenozoic.

Description

Some living caimans such as the Spectacled Caiman have a "spectacle", or a bony ridge between the eyes. ''Tsoabichi'' lacks a spectacle, but it does have three smaller ridges between the orbits, or eye sockets. In ''Tsoabichi'', distinct rims are seen around the supratemporal fenestrae, two holes on the skull table. Much of the supraoccipital bone is also found on the skull table, and forms a V-shape. To either side of the V-shaped supraoccipitals are the parietal bones, which form the posterior margin of the skull table. Along the snout, the nasal bone forms a thin ridge, and narrows as it approaches the external naris where the nostrils are located.

The dorsal osteoderms (bony scutes along the back) are wider than those of other caimans. Some have two keels on their outer surface. ''Tsoabichi'' also has bipartite ventral osteoderms on its underside.

Phylogeny

''Tsoabichi'' was included in a phylogenetic analysis when it was described in 2010. Brochu (2010) found it to be a basal member of Caimaninae closely related to the living genus ''Paleosuchus''. The analysis of Hastings ''et al. ''(2013), which included several more species of caimans, also placed ''Tsoabichi'' in a basal position within Caimaninae as the sister taxon to crown group caimans (the smallest clade that includes all living caimans and their most recent common ancestor). '' Eocaiman'' and '' Culebrasuchus'' were successively more basal than ''Tsoabichi'', as shown in the cladogram below:

Biogeography

''Tsoabichi'' is one of the few caimanines known to have lived north of what is now Mexico; the majority of caimans living and extinct are known from Central and South America. Caimans are thought to have originated in North America in the Late Cretaceous, yet ''Tsoabichi'' most likely originated from a South American population that reentered North America in the Early Eocene. The route by which a population of caimans could have reached North America in the Early Eocene is unknown. A sea route is unlikely given that living caimans have low tolerance for salt water, yet North and South America were separated by a large expanse of ocean. A continuous land route would not appear until the Isthmus of Panama formed several tens of millions of years later in the Pliocene between the two continents occurred in the Late Eocene. An alternative biogeographic explanation is that ''Tsoabichi'' descended from a population of ancestral caimans that never left North America, but this hypothesis is unlikely given that it would necessitate multiple independent dispersals into Central and South America (including one for ''Culebrasuchus'', one for ''Eocaiman'', one for ''Paleosuchus'', and one for derived caimans).

References

{{Taxonbar, from=Q7849815

Alligatoridae
Eocene reptiles of North America
Fossil taxa described in 2010
Eocene crocodylomorphs
Prehistoric pseudosuchian genera