Computed tomography and three-dimensional reconstruction of the skull of the stem tetrapodCrassigyrinus scoticusWatson, 1929

Abstract

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.

Descriptive anatomy and three-dimensional reconstruction of the skull of the tetrapod Eoherpeton watsoni Panchen, 1975 from the Carboniferous of Scotland

“…Some limits to the data set used in the description and 3D reconstruction should be noted. Although scan resolution was comparable to those in previous descriptions of Acanthostega and Crassigyrinus (Porro et al 2015b(Porro et al , 2023, contrast between individual bones was sometimes too poor to clearly discern suture morphology, although contacts were usually clear (with the exception of the contacts between the squamosal-quadrate-quadratojugal and the dentary-coronoid series; see details below). We clearly acknowledge any uncertainties in the nature of contacts between elements in the following description; future synchrotron scanning of E. watsoni could potentially resolve these ambiguities.…”

“…Most 3D reconstructions of early tetrapod skulls are based on well-preserved individual specimens (Porro et al 2015a;Lautenschlager et al 2016;Fortuny et al 2017;Arbez et al 2022), with rarer attempts made to reconstruct skulls exhibiting more pronounced deformation (Porro et al 2015b(Porro et al , 2023Rawson G.1950.86.1, although the left vomer and its teeth were used because of the apparent absence of this element on the right, and a portion of the left pterygoid preserving the basal articulation was merged with the right pterygoid. With the exception of single median elements (basisphenoid, parasphenoid), all bones were reflected across midline to create the left side of the skull.…”

“…These issues were corrected by moderate dorsal vaulting of the palate, resulting in an angle of approximately 102°between the horizontal laminae of the left and right pterygoids, and subsequent shifting of other cranial bones. Although early tetrapods have traditionally been reconstructed with horizontal palates, the presence of a dorsally vaulted palate in E. watsoni as well as Acanthostega (Porro et al 2015b), Ichthyostega (Rosen et al 1981), Crassigyrinus (Porro et al 2023) and Whatcheeria (Bolt & Lombard 2018;Rawson et al 2021) suggests that this assumption needs serious reconsideration. As a result of some degree of plastic deformation in the dentary and prearticular bones of the right lower jaw, these elements were segmented in several pieces to remove deformation.…”

“…Medical imaging methods, particularly computed tomography (CT) and micro-computed tomography (μCT) scanning, have revolutionised the study of fossil organisms (Conroy & Vannier 1984;Cunningham et al 2014;Rahman & Smith 2014). Over the past decades, increasingly sophisticated techniques have been developed to digitally repair damage and deformation to create virtual three-dimensional (3D) reconstructions of fossil skulls (Zollikofer et al 1995;Motani 1997;Gunz et al 2009;Lautenschlager 2016), including early tetrapod skulls (Porro et al 2015a(Porro et al , 2015b(Porro et al , 2023Pardo et al 2017;Lemberg et al 2021;Rawson et al 2021;Arbez et al 2022). In this study, we use μCT and visualisation software to digitally prepare the type specimen of E. watsoni, revealing new anatomical details that supplements previous descriptions (Watson 1929;Panchen 1975;Smithson 1985).…”

Using salamanders as model taxa to understand vertebrate feeding constraints during the late Devonian water-to-land transition

“…The only factor that might restrict dimensionally complex movements of the mandible appears to be the articular surface of the jaw joint [ 160 ] or mandibular suspension. We, therefore, encourage the study of early tetrapodomorph jaw articulations and suspensions—morphological details that besides very few considerations [ 196 – 198 ] seem to have largely been ignored. Comparative analyses of fossilised and extant articular surfaces across larval tetrapods might answer whether they used complex chewing mechanics during ontogeny.…”