Vertebrate taphonomy and paleoecology of a Cretaceous-Paleogene marine bonebed

Zachary M. Boles

doi:10.17918/etd-7587

In this dissertation, I characterize the vertebrate taphonomy and paleoecology of the Cretaceous-Paleogene Main Fossiliferous Layer (MFL) of the Hornerstown Formation, exposed at the Rowan Fossil Quarry in Mantua Township, New Jersey. The MFL has been proposed to represent a: 1) reworked deposit; 2) time-averaged deposit; or 3) a mass-death assemblage, possibly related to the K/Pg impact event. If the mass-death scenario is correct, the MFL would represent the only known vertebrate bonebed located stratigraphically at the K/Pg Boundary and would offer unique insight into the immediate aftermath of the bolide impact event. In spite of the potential importance of this site, no systematic taphonomic studies had been conducted prior to my research. Beginning in 2012, the MFL was systematically excavated and all contextual information was recorded. Several associated and articulated partial skeletons were recovered from an area of ~ 150 m2 in addition to hundreds of isolated bones. A "bloat and float" scenario can explain the abundance of isolated skeletal elements. Preservation of skeletal elements varies, although abrasion is typically minor or absent. Bioerosion is relatively common and may account for much of the damage to the bones. My taphonomic data, when viewed in light of other previous studies, favors the hypothesis that the MFL formed over a relatively short period of time due to a sudden influx of vertebrate carcasses. I also report the first recovery of soft-tissue-like structures from marine vertebrate fossils with known handling history and negative controls. Osteocyte-like structures were isolated from turtle shell elements of various taxa as well as from an indeterminate turtle humerus. Osteocyte- and vessel-like structures were also isolated from the femur of a marine crocodile, Thoracosaurus neocesariensis. However, not all of the sampled bone samples yielded soft-tissue-like structures. At least within the MFL, preservation of these microstructures is independent of taxon, skeletal element, or the macroscopic preservation of the bone. Instead, the best indicator of both the preservation of microstructures and bone microstructure is the fossil color. In the sampled bone fragments, tan colored samples yielded no or poorly preserved osteocyte-like structures whereas darker colored samples provided abundant and/or well-preserved cell-like structures. This study indicates that soft-tissue-like structures can be preserved in a marine setting and that they can be recovered from sediments completely waterlogged over geologic time. My third chapter focuses on the preservation and identification of vertebrate coprolites in the MFL. At least 178 coprolites were recovered from the MFL and at least seven morphotypes are recognized based on size, morphology, and the presence of inclusions. Heteropolar spiral coprolites are abundant and were likely produced by chondrichthyans with a spiral valve. Pellets, small cylindrical coprolites and large cylindrical coprolites may have been produced by bony fish, sea turtles, crocodilians, and/or mosasaurs. None of these morphotypes, except for the single large heteropolar spiral coprolite and a few indeterminate coprolite fragments, possess identifiable inclusions. Instead, most visible inclusions are small brown or black structures that may represent highly digested bone fragments. The phosphatic nature of the coprolites and the presence of some bone material suggest that carnivores likely produced many of them. However, the specific producer(s) cannot be identified. Complete coprolites are uncommon with most exhibiting some degree of pre-fossilization damage including breaks and bioerosion (e.g. coprophagy). For my final chapter, I described the shell bone histology of six of the eight turtle taxa known from the MFL. Based on the histology, overall architecture, and compactness of the bones, I was able to infer the habitat preference of the turtles. Agomphus pectoralis and Adocus beatus likely inhabited freshwater habitats near the shore, whereas Bothremys barberi and Taphrosphys sulcatus were shallow marine sea turtles. The bone histology of Peritresius ornatus suggests a shallow marine environment whereas the compactness values and overall shell morphology suggest a more pelagic lifestyle. Catapleura repanda is suggested to be a pelagic turtle based on bone histology, bone compactness values, and overall shell morphology.

Vertebrate taphonomy and paleoecology of a Cretaceous-Paleogene marine bonebed

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Vertebrate taphonomy and paleoecology of a Cretaceous-Paleogene marine bonebed

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