First Occurrence of a Giant Sea Cow (cf. Hydrodamalis cuestae) from the Pliocene Pico Formation of Santa Clarita, Southern California

Sirenians, such as manatees and dugongs, are herbivorous marine mammals commonly called sea cows with a fossil record extending from middle Eocene deposits to the present (Domning 2001). An extinct, nine-meter-long sirenian species called Hydrodamalis cuestae (Domning 1978) (Fig. 1) once lived along the American Pacific coast. It is closely related to the similarly large Steller’s sea cow, Hydrodamalis gigas (Zimmerman 1780), sharing a common ancestor with H. gigas and H. spissa (Furusawa 2004). Hydrodamalis gigas, also known as the Steller’s sea cow, once inhabited the Bering Strait and was driven to extinction likely by 1768 due to overhunting (Domning and Furusawa 1994, Turvey and Risley 2006, Stejneger 1887). In 2018, we discovered a large sirenian vertebra on private land on the eastern side of U.S. Interstate HWY 5 in the Newhall area, containing exposures of the Pico Formation (see Fig. 2 in Squires 2012). The Pico Formation is a marine deposit which has been dated at 3.6-2.5 million years (Winterer and Durham 1962) in the Santa Clarita area containing marine invertebrate fauna indicative of an inner sublittoral marine environment (Squires 2012). There are abundant invertebrate fossils at the site (e.g., turritellid gastropods, pectinid bivalves, echinoids, etc.) buried with pebbleto cobble-sized plutonic, volcanic, and metamorphic clasts in a sandy matrix. Specifically, we collected and identified the following invertebrate taxa at the site: Tresus nuttallii, Glossaulax reclusiana, Turritella cooperi, Nassarius californianus, and Compsomyax subdiaphana. N. californianus is restricted to the Pliocene (Squires 2012), confirming the deposit as Pliocene. These fossiliferous beds appear to be storm deposits, as has been inferred for other Santa Clarita Pico Formation sites (Squires 2012, Winterer and Durnham 1962). Vertebrate fossils found at this Santa Clarita site include bones from mysticete and odontocete cetaceans, as well as a shark tooth (Carcharodon sp.). All of the vertebrate fossils found at the site so far are isolated and fragmentary. A large vertebra (formerly TMU 0101, now LACM 162832) was found by Matthew McLain on a hill slope at the site as float while leading a geology class field trip. We are confident it came from the Pico Formation and not from an overlying formation because there are no other formations above the fossil site at that location. It was collected with permission from the landowner and was donated to the Los Angeles County Museum of Natural History. Precise locality information is available upon request for qualified researchers. The vertebra is large (dimensions listed in Table 1) and is still partially embedded in sandstone. The centrum is distinctly heart-shaped in anterior and posterior view, which is characteristic for sirenians (see, for example, Hautier et al. 2012). There is a sagittal keel on the ventral surface of the centrum. The left superior costal facet is present, but the right was lost due to fragmentation sometime after the fossil was exposed by weathering. The