DESCRIPTION OF A SIRENIAN FOSSIL SPECIMEN FROM THE LATRANIA FORMATION, SOUTHERN CALIFORNIA

Sirenians (dugongs, manatees, and their extinct relatives) are marine mammals with a fossil record extending from the lower Eocene to the present. Although sirenians are known from southern California in Pliocene deposits, their discovered remains from the Latrania Formation of the Anza-Borrego region are poor (two rib fragments). Additionally, vertebrate fossils in the Latrania Formation, a marine sandstone, are almost always isolated remains. In 2017, an associated sirenian ribcage was discovered by Malakai Damata near Ocotillo, California in the lower Latrania Formation of the Imperial Group, which locally lies immediately above pre-Mesozoic basement rock in the Coyote Mountains north of Ocotillo, California. The only bones observed were ribs of a dugongid (cf. Hydrodamalis cuestae) laying horizontally in near life position. The fossils were exposed in several blocks that had fallen off of a cliff face, and the proximal heads of the ribs remain in place in the cliff. The associated ribs in life position suggest the presence of an articulated individual specimen, but we cannot confirm this hypothesis because of the hardness of the rock and the inability to excavate the cliff further. It is possible that the vertebrae and other bones are still embedded within the cliff. Surrounding the sirenian are fossils of bivalves, gastropods, and barnacles, and beds containing abundant worm burrows occur lower than the unit containing the sirenian. The taphonomy of this specimen is unique for the Latrania Formation and demands an explanation. As of yet, we do not know why this animal remained articulated and associated while other vertebrates did not, but actualistic studies of decay and disarticulation of large marine animals suggest that the creature must have been buried relatively rapidly (less than a year). Further analysis of the site should yield a better understanding of the preservation of this surprising sirenian.