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By Jessie Holmes Chatigny Illustration by Jim Melli
Sweet-faced and absolutely huge, the newest resident in the Fossil Mysteries exhibition was installed in the Museum’s atrium in Fall 2006—our sea cow arrived fashionably late. (The exhibition opened July 1.) At 30 feet long, Hydrodamalis cuestae dominates the northeast corner of the atrium, watching over her calf below. Half “fleshed-out” and half skeletal, the Museum’s modeled body and cast skeleton of this sea cow is currently the only exhibit of this fossil sea cow in the world.
Now extinct, Hydrodamalis cuestae was the largest sirenian to ever live. Sirenians, also popularly known as sea cows, are plant-eating marine mammals distantly related to elephants. Living sirenians (manatees and dugongs) are warm water tropical animals that feed on sea grass and algae—eating for eight hours a day, they consume approximately 15% of their body weight. In contrast, Hydrodamalis cuestae, who may have weighed in at 10 metric tons (22,046 pounds), was a giant cool-water-adapted sea cow that specialized in grazing on marine algae.
In May 2000, Museum Paleontologist Pat Sena discovered fossils including a skull of Hydrodamalis cuestae while working in the field at a construction site in Otay Ranch in Chula Vista. His discovery represents the most complete and best-preserved specimen yet found, and is of great excitement to researchers. This skull is displayed in Fossil Mysteries and a cast of it is mounted with the skeleton .
During the Pliocene Epoch when Hydrodamalis cuestae was living off California, scientists believe that the shoreline of our coast was very different from today. Shallow bays and estuaries provided ideal habitat for the sirenians, unlike today’s mostly rocky, exposed shoreline.
Sirenian evolution in the North Pacific Ocean during the late Miocene and Pliocene involved their gradual adaptation to colder waters and a more northward range extension, accompanied by an increase in body size. Food choice, too, probably changed from marine plants growing in sheltered waters, such as seagrasses, to kelps and other cold-tolerant algae growing in colder, less-protected waters.
Although Hydrodamalis cuestae became extinct toward the end of the Pliocene Epoch approximately two million years ago, its evolutionary descendant Hydrodamalis gigas survived until the mid-18th century. This species, known as Steller’s Sea Cow, lived in the cold coastal waters around the Aleutian Islands in the Bering Sea . Hunted to extinction in the span of 20 years, Hydrodamalis gigas grew up to eight meters (26 feet) in length and may have weighed as much as 8000 kg (17,600 pounds).
Hydrodamalis gigas was observed in 1741 and 1742 by biologist Georg Wilhelm Steller, the only scientist to see it before its extinction around 1768. The ship’s naturalist and physician on board during Vitus Bering’s second expedition to North America, Steller observed the animal while he was shipwrecked on Bering Island and described it in great detail.
Although Steller was unaware of it at the time, the presence of Hydrodamalis gigas off Bering Island was only as an evolutionary relict, a small population confined to a very restricted area of cold waters near the Kamchatkan peninsula.
Because we have historical information about Hydrodamalis gigas, we can infer a good bit about the ecology of Hydrodamalis cuestae. Although we cannot be sure, we can surmise that H. cuestae may have also lived at the ocean surface, in shallow bays, constantly feeding on marine plants in the kelp beds. It was a slow-moving, massive animal that lived in distinct social groups. Since H. gigas demonstrated strong familial bonds, we can assume that H. cuestae may have had similar social leanings.
Like H. gigas, Pliocene H. cuestae may have had claw-like, short front appendages to dig and tear algae off rocks, and propel themselves along in shallow water, in a motion that combined elements of swimming and walking. Steller observed that the sea cows fed on algae near or at the surface, and may not in fact have been able to submerge their entire body, although they were apparently able to keep their heads under water for as long as five minutes.
One of the most intriguing things about sea cows is their incredibly stout skeleton. Their density and heft may have helped counteract some of their buoyancy, acting as ballast to their blubber. An interesting anatomical specialization of both species of Hydrodamalis is the loss of teeth in adults. Instead of teeth, these animals had cornified skin in their mouths. Curiously, they had also lost the terminal phalanges in the digits of their hands.
Fossil remains of Hydrodamalis cuestae have been found in our region at sites in Oceanside, San Diego, Lemon Grove, National City, and Chula Vista. These remains indicate an animal with a total body length estimated at close to 10 meters (33 feet).
The Museum’s mounted specimen is a composite cast skeleton. The original fossil bones from which the casts were made include a complete skull of one individual, the vertebral column of another individual, the right humerus of a third individual, the right ulna and radius of a fourth individual, and four ribs, each from different individuals. The tail and sacral portions of the vertebral column, as well as the right scapula were cast from bones of Hydrodamalis gigas borrowed from the University of California, Berkeley. The lower jaw, sternum, pelvis, right hand, and most of the ribs were sculpted by Don Jeffrey using illustrations and photographs as models.
William Monteleone, a paleo artist in Orange County, sculpted the fleshed-out side of the sea cow and the baby sea cow on the exhibition floor.
SAN DIEGO NATURAL HISTORY: FIELD NOTES, December 2006/January 2007