Wednesday, May 3, 2023

[Paleontology • 2023] Computed Tomography and Three-dimensional Reconstruction of the Skull of the Stem Tetrapod Crassigyrinus scoticus

Crassigyrinus scoticus Watson, 1929

in Porro, Rayfield & Clack, 2023.
Image by Bob Nicholls 

The early tetrapod Crassigyrinus scoticus was a large aquatic predator known from the lower- to mid-Carboniferous (upper Tournasian to upper Visean/lower Serpukovian, approximately 350–330 Ma) of Scotland and Canada. Crassigyrinus is enigmatic in terms of its phylogenetic position due to its unusual morphology, which features a mixture of primitive and derived characters. Previous reconstructions, based on five incomplete and deformed specimens, have suggested a dorsoventrally tall skull with a short and broad snout, large orbits and external nares, and an extended postorbital region. In this study, we scanned four specimens using computed tomography and segmented imaging data to separate bone from matrix and individual bones from each other. Based on these data, we present a revised description of the upper and lower jaws, including sutural morphology and abundant new anatomical information. Damage was repaired and the skull retrodeformed to create a hypothetical three-dimensional reconstruction of the skull of Crassigyrinus that is dorsoventrally flatter than earlier reconstructions, yet still morphologically unique amongst early tetrapods. Overall skull shape, the size and distribution of the teeth, sutural morphology, and the specialized anatomy of the jaw joint and mandibular symphysis all suggest that Crassigyrinus was a powerful aquatic predator capable of hunting and subduing large prey.

The results presented here supplement and amend previous descriptions of C. scoticus, including new information on skull anatomy, sutural morphology, and a detailed description of the jaw joint and mandibular symphysis. Computed tomography and 3D visualization permitted a new 3D reconstruction of the upper and lower jaws of C. scoticus. The reconstructed cranium is dorsoventrally flatter than previous attempts and features a vaulted palate and enormous subtemporal fossae; the new lower jaw is less curved than earlier reconstructions, and the jaw joint is anteromedially tilted rather than transversely oriented. Evidence from skull anatomy, sensory systems, tooth size, shape and distribution, sutural morphology, and the shape and orientation of the jaw joint and mandibular symphysis are used to predict the feeding mechanism and ecology in this unusual stem tetrapod, hypotheses that will be tested further using biomechanical modeling.

Laura B. Porro, Emily J. Rayfield and Jennifer A. Clack. 2023. Computed Tomography and Three-dimensional Reconstruction of the Skull of the Stem Tetrapod Crassigyrinus scoticus Watson, 1929. Journal of Vertebrate Paleontology. e2183134. DOI: 10.1080/02724634.2023.2183134