X‐Ray Computed Tomography Reveals Hidden Morphology in an Extinct Mammalian Relative

Abstract

High Resolution X‐ray computed tomography is commonly used to study internal structures of fossil skulls. CT allows inference about the structure of brain surface morphology, the inner ear, and internal bony elements such as turbinates. Computed tomography data is frequently published in figures as isosurface renderings of either the segmented endocranial volume or of the surface of the specimen. Here I demonstrate how CT improves upon superficial anatomical understanding of fossils in a Late Triassic early mammal relative from the Ischigualasto Formation in Argentina. When the specimen, PVSJ 882, was first discovered and initially described, it was assumed to be closely related to mammals on account of the perceived absence of two circumorbital bones, the prefrontal and postorbital. It was not until the specimen could be viewed non‐destructively in cross section that the prefrontal and postorbital were identified to be present. However, other derived early mammalian features grew apparent, including a fusion of the prootic and opisthotic to form the petrosal, as well as cochlea housed within a bony promontorium. The discovery of primitive orbital elements in a specimen with such a derived ear is new to mammal paleontology. The superficial morphology of other derived mammal relatives is encouraged to be studied with CT. Furthermore, I demonstrate how adjusted volume renderings can more accurately communicate the morphology of the specimen in question, while isosurfaces can be misleading for the same reasons that photos of the specimen in rock matrix can be misleading. While isosurface renderings have advantages in figures depicting CT data, they should not be used to the exclusion of volume renderings when volume renderings can depict subtle sutures, unprepared foramina, and deep, delicate bone buried under rock matrix.Support or Funding InformationThe Jackson School of Geosciences at the University of Texas at Austin