Evaluation of the photosensory characteristics of the lateral and pineal eyes of Plioplatecarpus (Squamata, Mosasauridae) based on an exceptionally preserved specimen from the Bearpaw Shale (Campanian, Upper Cretaceous) of southern Alberta

ABSTRACT

The skull of Plioplatecarpus possesses large orbits and a large parietal foramen. Although the large orbits suggest that Plioplatecarpus possessed large eyes with enhanced visual performance, none of the previously described skulls include a sclerotic ring, and so it has been impossible in the past to reconstruct their size or optical properties. A recently collected skull with a sclerotic ring preserved in situ provides for the first time the data to do so. The dimensions of the sclerotic ring demonstrate that, although the f-number of the eyeball was not low enough to support the hypothesis that Plioplatecarpus was adapted to low light conditions, the eyeball was nevertheless large and possessed increased visual acuity and sensitivity that enabled the mosasaur to operate efficiently in both light-rich and light-poor conditions and possibly other conditions of reduced visibility. Proportions of this skull also suggest that Plioplatecarpus possessed a relatively wide binocular field of vision, providing stereoscopic vision, and further increasing both acuity and sensitivity, enhancing its ability to effectively detect and track quickly moving objects under a wide range of light conditions. A greatly enlarged parietal foramen implies the presence of a large parietal eye, the significance of which is uncertain, but it may have enhanced the ability to regulate migratory behavior.

ACKNOWLEDGMENTS

I am indebted to D. Brinkman for drawing my attention to the specimen discussed in this paper and providing images and measurements of the postcranial skeleton, M. Laframboise and M. Michell for expert preparation as well as discussions and observations regarding this material, and B. Strilisky for arranging for a loan of the skull of TMP 2022.043.0001. Also, thanks to M. Everhart, T. Konishi, A. Murray, and S. Walsh, for discussion of sensory systems of vertebrates and reading earlier versions of this paper and providing valuable criticism and helpful suggestions, and Shyong En Pan for providing photography expertise. We also owe a debt to Korite International for alerting the Tyrrell Museum of Paleontology to the discovery of the skeleton, and allowing a museum team access to the quarry to collect it. Finally, thank you to JVP editors M. D’Emic and D. Madzia for helping me navigate the revision process.

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the author(s).

DATA AVAILABILITY STATEMENT

The author confirms that all data supporting the findings of this study are available within the body of the article and Tables 1–3 contained therein.