COLLAGEN ISOTOPE ANALYSIS OF EXTREMELY WELL-PRESERVED FOSSILS FROM A BLUE HOLE SITE IN THE BAHAMAS

Stable isotopes from collagen have been used to reconstruct diet, trophic levels, and migration patterns in modern animals, early human civilizations, and mammal-dominated Pleistocene continental fossil sites. However, island and reptile-dominated fossils have not been explored for pre-human ecosystem structure. Nitrogen has been analyzed in extinct non-mammalian taxa (including non-avian dinosaurs and plesiosaurs) but these Mesozoic bones have less reliable nitrogen values due to the effects of fossilization and diagenesis. Phenomenally well-preserved sub-fossils from the Sawmill Sink blue hole on the island of Abaco in the Bahamas have in many cases near pristine bone collagen preservation. This allows an opportunity to assess the now extinct fauna of Abaco for trophic levels and test the terrestriality of the larger organisms (crocodiles and tortoises) which lived there prior to human occupation but have since died out in the Bahamas. We sampled 22 crocodile and tortoise bones and were able to obtain sufficient bone collagen from 13 for isotopic sampling. The crocodiles and tortoises (with one exception) yielded δ¹⁵N values below 7.5‰ and δ¹³C values more negative than -16‰. These results indicate the large reptiles dominated the terrestrial fauna, with the typically aquatic crocodiles (Crocodylus rhombifer) obtaining a large part of their diet from a terrestrial ecosystem. Likewise the dominant terrestrial herbivore was an extinct tortoise (Chelonoidis alburyorum). Numerous crocodile bite marks on tortoise shells corroborate these findings. In addition, we sampled three ancient human bones (which date after the occurrence of tortoise and crocodile bones) which all yielded δ¹⁵N values between 9.6 and 10.7‰ and δ¹³C values between -14.8 and -16.0‰. These data indicate a large part of the diet of these early Bahamians was likely fish. This application is the first use of collagen isotope sampling to learn of the structure of a pre-human island ecosystem. This method could be readily applied to other Holocene island fossil sites to gain new insight into the structure and diets of now extinct and extirpated taxa.