The relationships and paleoecology of Pachystropheus rhaeticus, an enigmatic latest Triassic marine reptile (Diapsida: Thalattosauria)

By: Jacob G. Quinn, Evangelos R. Matheau-Raven, David I. Whiteside, John E. A. Marshall, Deborah J. Hutchinson & Michael J. Benton 

Summarized by: Kodi Biscardi, who is a transfer student at Binghamton University studying geology and hopes to one day study geophysics.

What data were used? Researchers present two major types of data in this study: fossil evidence of Pachystropheus rhaeticus, a marine reptile, and the fossilized pollen/spores found in the same rocks that contained Pachystropheus rhaeticus fossils.Researchers revisited all previously discovered and named  fossil specimens of Pachystropheus rhaeticus throughout Britain (particularly from rocks of the Westbury Formation, which dates back to the end of the Triassic ~205 million years ago. Namely, the studied fossils includes a “holotype” of the Pachystropheus rhaeticus, an incompletely preserved fossil of ~35 vertebrae and other bones. A holotype is a singular fossil that is used to define the features of a species, so that researchers can reference it when discovering new fossils. Researchers also present a new, similarly-aged specimen, suspected to belong to the same species of Pachystropheus, found in the same location as the holotype. This new discovery has 62 well-preserved backbones–almost double the amount of the holotype–as well as other preserved animal fossils and, fossilized pollens preserved within the rocks of the Westbury Formation.

What was the question or point of the paper? Researchers set out to establish a comprehensive history of Pachystropheus rhaeticus, a late Triassic marine reptile. This fossil is often misidentified because the preservation is often poor, which means that there can be conflicting information and interpretations about it. Scientists here focus on untangling some of the confusion by re-studying previously named fossils of this species, figuring out who it is closely related to, examining its ecological roles as a predator, and better defining the time that this species went extinct by using pollen fossils from the same layers as the reptiles.

Methods:  Researchers described consistent morphologic characteristics using the Pachystropheus rhaeticus holotype. Using bones from the holotype, the researchers conducted a comparative anatomy study of all previously and newly discovered Pachystropheus rhaeticus specimens, reidentifying many bones as coelacanth fish, instead of reptiles. Focusing on their newer fossil assemblage, having removed the non-reptilian bones from the study group, researchers utilized scanning techniques to produce 3D models that could be reconstructed to resemble what bones previously looked like during life. These scans gave researchers much easier ways to study harder-to-reach fossils and find common patterns in the bone structures, which is called the species’ morphology. By analyzing the morphology of specimens, researchers can infer the relationships between this species and possible ancestors/descendants, causing a shift in the Pachystropheus rhaeticus’ placement in evolutionary trees called a phylogenetic analysis. Some of the latest discovered specimens were believed to be from the early Jurassic (201.3 mya). However, preserved pollen and spores found within the rocks containing the specimen were focused on by researchers in this study. By running various tests on fossilized pollen/spores, researchers were able to precisely date the ages of the rocks these fossils were found in and, therefore, determine the age of the Pachystropheus rhaeticus fossils within it. Studying fossil pollen and spores is called palynology. 

Results: Scientists concluded that none of the named specimens of reptile had preserved teeth, jaws, or skulls; those specimens that had them actually belonged to the coelacanth fish. This means that the reptile’s ecology is speculative. Teeth are highly important in paleontology, as by studying them, paleontologists can guess a species’ diet and thus establish their role in the ecosystem. For instance, predators have sharper teeth designed to tear meat, while herbivores have teeth better designed for grinding down plants. Based upon their previously limited morphological information, the Pachystropheus rhaeticus was originally believed to be the oldest and first species of Choristodere (an order of ancient Jurassic reptiles). However, the results of their analysis of the fossilized pollen dated the youngest fossils to be around late Triassic (237 mya) in age, which means that the species did not survive a major extinction event at the end of the Triassic. Fossilized Pachystropheus rhaeticus ribs and upper leg bones showed specific features that are associated with nearshore aquatic lifestyles, but associated with the limited ability to move on land. These characteristics were highly similar to those of another marine reptile studied in other articles, Endennasaurus acutirostris; with this and the results of their phylogenies, researchers place the Pachystropheus rhaeticus in the same order as the Endennasaurus, which is called Thalattosauria. 

Why is this study important? Validity and reliability are important in any field of science. Previously, Pachystropheus rhaeticus specimens were believed to be much more abundant, leading to consistent misidentification. This study aimed and achieved to narrow down the standards of the species ecology, morphological identity, and chronological placements. This was exceedingly hard due to limited information from the fossil record. This study also closes a long-standing mystery of the evolutionary placement of Pachystropheus rhaeticus, labeling the species one of the last surviving members ofThalattosauria, not the earliest of the Choristodere.

Broader Implications beyond this study: This study has shifted many early marine reptile Choristodere and Thalattosauria evolution models, as well as set a standard for reevaluating old specimens for the possibility of misidentification and chronological mistakes. The accuracy of evolutionary trees is highly important for scientists as they allow us to infer more about how past ecosystems and lifestyles changed over time, especially during mass extinctions. Mass extinctions are especially periods of great interest to paleontologists, as understanding how animals responded to mass extinctions in the past can help us apply that information to modern day climate change science. 

Citation: Quinn, J. G., Matheau-Raven, E. R., Whiteside, D. I., Marshall, J. E. A., Hutchinson, D. J., & Benton, M. J. (2023). The relationships and paleoecology of Pachystropheus rhaeticus, an enigmatic latest Triassic marine reptile (Diapsida: Thalattosauria). Journal of Vertebrate Paleontology, 43(6). https://doi.org/10.1080/02724634.2024.2350408