Small, enigmatic alligatoroid from the Middle Eocene Clarno Formation, John Day Fossil Beds, Oregon

Alligatoroidea is the crocodylian superfamily that includes extant alligators, caimans and many related extinct taxa. Palaeogene North America offers a rich record of alligatoroid diversity representing more than a dozen forms. The Eocene-aged Hancock Mammal Quarry (HMQ, Clarno Formation, John Day Fossil Beds National Monument, central Oregon, USA) has yielded two crocodylian fossils, a right humerus and the anterior portion of a left lower jaw from an unknown alligatoroid.

Though fragmentary, discrete morphological traits of the HMQ alligatoroid are shared with several contemporaneous alligatoroids, but the suite of character states is unique. Though its phylogenetic placement is uncertain, the presence of an alligatoroid in north-western North America, along with the biostratigraphic record of other Palaeogene alligatoroids and Asian floral and faunal associations of the HMQ suggest that an Asian origin for the taxon is plausible.