BONES ON THE BEACH: MARINE MAMMAL TAPHONOMY OF THE COLORADO RIVER DELTA, MEXICO

How well does a death assemblage of marine mammal bones reflect the diversity, species composition, and proportion of bone types in the living fauna? We surveyed marine mammal remains on the Colorado River Delta, Baja California, Mexico. We found 470 bones and 3 carcasses among 112 sites along a total of 4.0 km of shoreline.

The maximum skeletal ratio of skull: vertebrae: ribs: phalanges: girdle/limbs in a marine mammal is 1:74:30:68:16. We found 28 skulls and 442 post-cranial bones, and a skeletal ratio of 1:12:3:1:1. Although vertebrae are the most common bones in the bone assemblage, all post-cranial skeletal elements are under-represented. Skulls provide the best estimate of the minimum number of individuals. Smaller bones may be more easily destroyed, buried or transported. The variation in taphonomic condition of the bones suggests accumulation over some period of time, perhaps less than 50 years.

We found remains of 8 of the 18 species recorded in the northern Gulf: Zalophus californianus (California Sea Lion, 8 skulls), Delphinus delphis (Common Dolphin, 7 skulls), Tursiops truncatus (Bottlenose Dolphin, 6 skulls), Phocoena sinus (Vaquita, five skulls), one skull each of Pseudorca crassidens (False Killer Whale), Kogia breviceps (Pygmy Sperm Whale), and a possible Mesoplodon sp. (Beaked Whale). One Physeter macrocephalus (Sperm Whale) was identified by its large vertebrae.

Differences in population size, habitat use and behavior may affect species composition and abundance within the bone assemblage. Migrants and rare species are not as abundant as residents in the bone assemblage. Coastal species are more common than oceanic ones.

Marine mammal remains are common within the 3% of Colorado Delta shoreline surveyed, and provide a remarkably good sample of the living fauna. Surveys of mammal remains may be a valuable and cost-effective supplement to aerial and nautical surveys of the live fauna.