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A Triassic crown squamate

Citation

Whiteside, David I.; Chambi-Trowell, Sophie A. V.; Benton, Michael J. (2022), A Triassic crown squamate, Dryad, Dataset, https://doi.org/10.5061/dryad.hhmgqnkkr

Abstract

Mammals, birds, and squamates (lizards, snakes, and relatives) are key living vertebrates, and thus understanding their evolution underpins important questions in biodiversity science. Whereas the origins of mammals and birds are relatively well understood, the roots of squamates have been obscure. Here, we report a modern-type lizard from the Late Triassic of England [202 million years (Ma)], comprising a partial skeleton, skull, and mandibles. It displays at least 15 unique squamate traits and further shares unidentatan and anguimorph apomorphies. The new discovery fixes the origin of crown Squamata as much older than had been thought, and the revised dating shows substantial diversification of modern-type squamates following the Carnian Pluvial Episode, 232 Ma ago.

Methods

The specimen is NHMUK PV R36822. We performed two scans, using the X-Tek XT H 225 ST micro-focus CT scanner (Nikon Metrology), with a micro-focus 225 kV source, which is fitted with a tungsten reflection target, and Perkin Elmer detector (XRD 1620 detector), belonging to the Palaeobiology Research Group, and located in the Life Science Building, University of Bristol. The scan settings for the left mandible and associated anterior portion of the articulated skeleton of NHMUK PV R36822 used 222 kVp, 153 μA, 500 ms exposure, and 3141 projections which were acquired by a full rotation of 360o using 4 frames per projection. A 17.5-μm reconstructed voxel resolution resulted from a source-to-object distance of 103 mm and source to detector distance of 1176 mm. There was no filtration of these scans other than the beryllium window. The scan settings for the associated skull of NHMUK PV R36822 used 200 kVp, 200 μA, 708 ms exposure, and 6,284 projections which were acquired by a full rotation of 360° using 1 frame per projection. A 43.2-μm reconstructed voxel resolution resulted from a source-to-object distance of 255 mm and source to detector distance of 1180 mm. For these scans, we used 0.5-mm Cu filtration and a beryllium window. The scan settings for the braincase NHMUK PV R37377 used 130 kVP, 48 μA, 1000 ms exposure, and 3,141 projections, which were acquired by a full rotation of 360° using 1 frame per projection. A 6.3-μm reconstructed voxel resolution resulted from a source-to-object distance of 37 mm and source to detector distance of 1175 mm. There was no filtration of these scans other than the beryllium window. In all cases, the specimens were mounted with phenolic foam to prevent movement during rotation.

Scans were reconstructed using CT Pro 3D (Nikon Metrology). The two batches of CT scans were then processed using Avizo 8.0 (FEI Visualization Sciences Group) 3D-visualization software. Automated segmentation proved impossible as the density of the limestone and fossil bone were very similar, resulting in poorly contrasted x-ray attenuation properties on the CT scans. Therefore, segmentation was performed manually. Bones were individually segmented from the matrix, then identified and assigned labels within Avizo, before a 3D surface model was generated. In general, although the contrast was low, it was possible to confidently visualize all fossil bones within the scanned regions of NHMUK PV R36822. The only exception was the two lacrimal bones, which could only be partially discerned in the CT scans but were both clearly visible under the microscope.

We also provide the NEXUS command file for the MrBayes run, comprising the data matrix plus commands.

Usage Notes

All 3D files are saved in .stl format, which can be viewed through many standard graphics programs, including Preview on Mac computers.

Funding

Natural Environment Research Council, Award: NE/I027630/1

European Research Council, Award: 788203