San Diego Natural History Museum--Your Nature Connection[BRCC San Diego Natural History Museum: Paleontology]

Skull of Giant Pliocene Sea Cow
Discovered at Otay Ranch, May 2000

On 23 May 2000 Pat Sena, a field paleontologist with the San Diego Natural History Museum, discovered a complete skull of the giant Pliocene sirenian (sea cow) Hydrodamalis cuestae at a construction site at Otay Ranch in Chula Vista. This extinct species of marine mammal lived in the North Pacific region during latest Miocene through late Pliocene time, approximately 6 to 2 million years ago. It is estimated that H. cuestae reached a maximum body length of about 9 meters (30 feet) and probably weighed close to 10 metric tons. Hydrodamalis cuestae is a fossil species considered to be ancestral to Steller's Sea Cow (Hydrodamalis gigas). A population of approximately 2000 individuals of H. gigas was discovered in 1741 living around Bering Island and Copper Island in the Aleutian Archipelago. Less than 30 years later this species was extinct, the victim of overhunting by Russian sailors. Hydrodamalis gigas was almost as large as its fossil ancestor and was a marine herbivore, feeding primarily on brown and red algae. The extinct species of Hydrodamalis were cold adapted sirenians related to the still living Dugong of the tropical Indo-Pacific region.

Figure 1. View of newly discovered sea cow skull encased in plaster and burlap field jacket. Note that the skull is still partially entombed by sandstone and cobble matrix. The large central opening in the skull is the nasal passage. Scale is in cm.
Figure 2. View of partially uncovered skull after 4 days of preparation. View is from the right front side and shows the large narial opening and rectangular brain case. The skull measures 79 cm (31 in.) in length.

The new skull is the most complete and best preserved specimen of Hydrodamalis cuestae known and was discovered in Pliocene-age marine sandstones of the San Diego Formation as exposed at Otay Ranch. Otay Ranch is being developed by the Otay Ranch Company as a planned community. The fossil was discovered as graders were constructing the Village 1 West phase at Otay Ranch. Recovery of the specimen took about four hours and was conducted by Museum staff (Richard Cerutti, Scott Musick, and Pat Sena) under contract to the Otay Ranch Company. Recovery did not interfere with the construction schedule. The skull is now being prepared in the fossil preparation lab at the Museum.

Text and photographs: Dr. Tom Deméré