Hydrochoerus hesperotiganites

*Hydrochoerus hesperotiganites* is an extinct species of caviid rodent that lived in California during the Late Pleistocene.

## History

*Hydrochoerus hesperotiganites* was named in 2022 by White et al., with the specific epithet meaning "western pancake", based on the holotype being discovered from the western world and the quality of preservation. The holotype is SDSNH 50000, a crushed but nearly complete skull (lacking mandible) with diagnostic features, which were compared to modern *Hydrochoerus*. It was discovered by Bradford O. Riney during mitigation monitoring in the Town Center North shopping center, Oceanside. It was found from 2 lower stratigraphic units consisting of mud, silt and sandstones that likely date to the Rancholabrean (based on a fauna including *Bison latifrons* from the same layer and older layers that may correlate).

## Description

The authors established a midline between the nasals and occipital to see how the holotype was crushed. It is identified as *Hydrochoerus* due to an upper incisor with anterior grooves, faint striations in the lateral groove of the M3 with a bi-lobed first prism anf followed by 12 independent prisms. Defining features are a larger sizes, a wider skull roof and more robust zygomatic processes of the maxilla and descending lacrimal. The anteromedial petrosal reaches and overlaps the alisphenoid in the otic region.

## Classification

Besides New World porcupines, capybaras are the only caviomorphs that colonized temperate North America during the Pliocene and the Pleistocene of the the Great American Interchange. Only 3 genera had colonized here before: *Neochoerus*, *Phugatherium* and *H. hesperotiganites*, amking it the only *Hydrochoerus* to inhabit North America. However, there is no indication for their presence between San Diego County, their northernmost current range and the only Central American species are *Neochoerus*, which makes their route tracing difficult, proposing 2 paths: travelling along the Pacific coastline of Sinaloa and Sonora to find either further up the coast and up the Colorado River or up other rivers that drained into the Gulf of California to disperse further north and eventually southwards. The first would have made finding permanent water sources overland difficult, and thus less likely. They also suggest a similar route along the Rio Yaqui dispersed capybara genera throughout Mexico. White et al. suggest that increasing locality sampling in Mexico may resolve this issue.

## Paleoecology

*Hydrochoerus hesperotiganites* is known from San Diego County, California, where it lived alongside frogs, gastropods, pelecypods, bony fish, turtles, squamates, a diverse assemblage of birds, rabbits, gophers, deer mouse, meadow voles, foxes, tapirs, horses, ground soths, mastodons, deer-like artiodactyls, sycamores and oak trees. This environment would have been a freshwater pond or oxbrow lake formed atop eroded Eocene strata (which were probably ancestral to San Luis Rey River sediments that deposited over floodplains). Chang silt and sandstone layers suggest seasonal sediment input. Upper layers indicate the river was travelling southwards, forming another oxbrow lake while the floodplains were aggrading. Isolated mammalian remains indicate that their corpses washed ashore to rot and fossilize, with others flowing down river. An abundant waterfowl fauna suggests and supports the contemporaneous capybaras, who are known to congregate around water.

## References

https://journals.library.ualberta.ca/vamp/index.php/VAMP/article/view/29379/21433