Liebig, P. 2004.  Death on the delta: Taphonomic evidence for mass stranding of Pseudorca crassidens. Gulf of California Conference, Tucson, AZ, June, 2004.

I found remains of several Pseudorca crassidens (False Killer Whale) within a small area on the Colorado River delta, approximately 55km north of San Felipe. The site is located high in the tidal range and the remains are disarticulated and scattered over ~5,000 m² of mudflats and shelly beaches. The bones found on the mudflats consist mostly of isolated teeth and skulls, many of which are partially buried, and a few remains of ribs and vertebrae. The bones on the beach consist of skulls with intact and splintered teeth and badly weathered vertebrae and ribs. I found 15 skulls of adult Pseudorca crassidens.

I interpret this site as a mass stranding because of the large number of one species’ skulls in a small area. Post-mortem transport and concentration of dispersed individuals of one species is highly unlikely. The similar taphonomic condition of the bones, suggests that the remains arrived on the beach at the same time. Based on bone weathering studies in Africa, I estimate that these bones are ~30 years old. Pseudorca crassidens are migrants to the northern Gulf of California and are well known for their mass stranding behavior. Although still unclear, scientists hypothesize mass stranding in P. crassidens typically occurs because of parasitic infections, magnetic irregularities or erroneous decisions in searching for food.