Using glacial erratics to study nautiloids in eastern New York

Nautiloid cephalopods from the Rickard Hill facies of the Saugerties Member of the Schoharie Formation, eastern New York, USA (late Emsian, Devonian): A case study in taphonomy 

Martin A. Becker, Harry M. Maisch IV, Rebecca A. Chamberlain, John A. Chamberlain Jr., Christi G. Kline, and Clint F. Mautz

Summarized by Leighanne Haverlin. Leighanne Haverlin is a geology major at the University of South Florida. She will graduate in December of 2019 and plans to enter the workforce in the field of environmental consulting, coastal geology, or geophysics. After some time working, she hopes to further her education and earn a master’s of science in one of the fields previously mentioned. In her free time, she enjoys running, kayaking, and listening to music. 

What data were used? Glacial fragments containing Nautiloid cephalopods found in the Rickard Hill facies (RHf; facies refer to a certain group of rocks that share the same characteristics) of Lower New York and northern New Jersey were compared to outcrops from the Helderberg Mountains near Clarksville, New York. 

Methods: This study used petrographic thin sections (microscope analysis of the rock) along with hand samples to look at chemical and physical erosion to determine depositional environment (the environments in which rocks were formed in) and lifestyle of nautiloids, animals closely related to the octopus today. 129 specimens in glacial fragments from the RHf were examined. 

Results: By studying the nautiloid assemblages, the study determined that the depositional environment was an inner shelf reef. The glacial erratics (rock that is different than the surrounding rock where it lies) that were found in the Lower Hudson Valley of New York were similar to the rock found in the Helderberg Mountains of New York which are approximately 200 km to the north. The erratics contain nautiloid fossils with coiled and orthoconic (long and narrow) shapes. They were physically and chemically weathered when the Laurentide Ice Sheet, the major ice sheet that covered much of northern North American approximately 20,00 years ago) moved. The nautiloid fossils that were found were determined to be assembled after their death in an area that sustained living organisms. This deposition occurred during a sea level regression (a sea level drop). The RHf glacial fragments featured jointed bedding that is also present in the outcrop in Clarksville, NY. For 5,000 years, the nautiloids were eroded during the movement of the ice sheet and experienced dissolution (meaning they and the rock they were contained in began to dissolve over time) which exposed and preserved structures. The study determined that the depositional environment was an inner shelf reef that harvested great biodiversity (a high number of species of animals). To confirm this hypothesis, it was argued that it is uncommon for dead organisms (especially nautiloids) to drift very far distances after death because the shells would sink to the sea floor. It was determined that juvenile nautiloids lived in a different depositional environment than adults, which explains the large size of the nautiloids in the RHf. The nautiloids were continually buried and unburied due to storm events and sea level change. This same hypothesis was used to understand the fossil assemblages of the Wadleigh Formation in Alaska, the Cherry Valley Limestone in NY, and the Trebotov Chotec Limestone in the Czech Republic

Nautiloid casts from RHf glacial erratics. Both orthoconic (long and narrow) and coiled forms are depicted in the image.

Why is this study important? Prior to this study, no research had been conducted that explained the conditions that resulted in the large numbers and sizes of both coiled and long and narrow (orthoconic) nautiloid cephalopods. This study provided evidence that indicates that much of what we thought we understood of cephalopod ecology (where an animal lives and how it interacts with other animals) and preservation needs to be revised. This was also the first study that used glacial erratics and principles of sequence stratigraphy to address the lives of an assemblage of nautiloids which makes this study unique. 

The big picture: This study used a new method of using glacial erratics and principles of stratigraphy to determine the lifestyle of a species. Nautiloid adults and juveniles are not found in the same depositional environment. The glacial erratics that do not belong to the RHf show similar assemblages of nautiloids which can be analyzed using the same method. Overall, the study brought in a new method of studying taphonomy which could be used in future studies.   

Citation: Becker, M.A, Maisch, H.M., Chamberlain, R.A., Chamberlain, J.A., Kline, C.G. and Mautz, C.F.., 2018, Nautiloid cephalopods from the Rickard Hill facies of the Saugerties Member of the Schoharie Formation, eastern New York, USA (late Emsian, Devonian): A case study in taphonomy : Palaeontologia Electronica, 21.3.42.