DOI: 10.5061/dryad.hhmgqnkkr
title: A Triassic crown squamate

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