How well does a death assemblage of marine mammal bones reflect the diversity, species composition, and proportion of bone types in the living fauna? Marine mammal remains were surveyed along the beaches of the Colorado River delta, Baja California, Mexico. Three carcasses and 470 bones were found among 112 localities along 4.0 km of shoreline. The location of each site was recorded and each bone was identified, photographed, and measured and its taphonomic condition was noted. The proportion of bone types found was compared to the proportions known in living marine mammals. The list of species found as bones was compared to the list of species known to live in the northern Gulf of California.

The maximum skeletal ratio of skull:vertebrae:ribs:phalanges:girdles/limbs in a typical Gulf of California marine mammal is 1:74:30:56:16. The 28 skulls and 442 post-cranial bones found provided a skeletal ratio of 1:12:3:1:1. Although vertebrae are the most common bones in the bone assemblage, only 316 were found, not the ∼2,000 predicted by the 28 skulls, indicating that vertebrae are under-represented. Therefore, skulls provide the best estimate of the minimum number of individuals. Smaller bones appear to be more easily destroyed, buried, or transported away. Most vertebrae were in good condition, suggesting that most bones arrived on the beach recently.

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

Differences in population size, habitat use, and behavior among species 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.