Late Eocene (Chadronian) fossil mammals from the Palm Park Formation, Caballo Mountains, Sierra County, New Mexico
— Spencer G. Lucas and Justin A. Spielmann

The Palm Park Formation in the southern Caballo Mountains of Sierra County, New Mexico, is a volcaniclastic deposit overlain by the ~36 Ma Bell Top Formation. The vertebrate fauna of this formation includes: turtles (aff. Stylemys sp. and an emydid) and four identifiable mammal taxa (Hyaenodon horridus, Hyracodon sp., Mesohippus cf. M. texanus and Merycoidodon presidioensis). Much of the turtle material consists of vertebral elements and isolated shell fragments. Hyaenodon horridus is recognized from a maxillary fragment. Dentary fragments with damaged teeth indicate the presence of Hyracodon sp., and a dentary with cheek teeth documents the presence of Mesohippus cf. M. texanus. The most complete specimen from the Palm Park Formation is a partial skull of the oreodont Merycoidodon presidioensis that includes both maxillae. The fossil mammals from the Palm Park Formation indicate an early Chadronian age of ~36-37 Ma. They can be correlated to the Chadronian mammals from the Rubio Peak Formation in the Black Range of Sierra County. The total age range of the Palm Park Formation is likely ~ 36-38 Ma.