Integrated taphonomy of an avian death assemblage in marine sediments from the late Pliocene of Florida

Abstract

We integrate taphonomic data on vertebrate and invertebrate paleontology, micropaleontology and palynology to explain the formation of a late Pliocene death assemblage of marine birds and fish in the “Pinecrest Sand”, Gulf Coastal Florida. Stereonet plots of orientation data on over 1500 cormorant (Phalacrocoracidae: Phalacrocorax) bones indicate that this fossil assemblage formed first from gradual accumulation of bone, shell and sediments on a barrier island beach, and second by rapid sedimentation in a quiet, back-beach setting associated with multiple episodes of breaching of the barrier. This latter event resulted in the preservation of 137 partial and complete cormorant skeletons and thousands of isolated bones that show a high angle of dip and a preferred orientation to the northeast. Invertebrate fossils exhibit taphonomic signatures characteristic of high-energy reworking with a large percentage of abraded shell fragments similar to beach deposits. Moreover, these data indicate that more than a single depositional episode caused the formation of the deposits referred to as the bird layers. Palynological evidence supports this conclusion. The pollen recovered from the deposits is highly abraded and broken and does not represent an in situ vegetational environment as compared to other deposits of this age in Florida and Georgia. The large number of cormorant and other seabird and fish remains in the bird layers appears to have been caused by a series of toxic red tides that occurred on the Gulf Coast of Florida. These events today cause die offs of large flocks of cormorants and bottom-dwelling fish similar to those recovered from the site. Palynological analysis of the sediments revealed abundant cysts of a dinoflagellate species known to produce toxic red tides. The most likely cause of the death of the cormorants and other vertebrates at this site is a toxic bloom of a variety of Pyrodinium bahamense, the thecate form of the dinoflagellate cyst Polysphaeridium zoharyi, which has a stratigraphic range from the lower Eocene to the Holocene.