Downloaded from http://sjg.lyellcollection.org/ by guest on May 13, 2020 Research article Scottish Journal of Geology Published online February 14, 2020 https://doi.org/10.1144/sjg2019-026 | Vol. 56 | 2020 | pp. 55–62 Three-dimensional tomographic study of dermal armour from the tail of the Triassic aetosaur Stagonolepis robertsoni Emily Keeble and Michael J. Benton* School of Earth Sciences, University of Bristol, Life Sciences Building, Tyndall Avenue, Bristol BS8 1TQ, UK MJB, 0000-0002-4323-1824 * Correspondence: [email protected] Abstract: The aetosaur Stagonolepis robertsoni was the first reptile to be named from the Late Triassic Lossiemouth Sandstone Formation of Morayshire. Its characteristic rectangular armour plates have been reported in isolation and in association with skeletal remains. Here we present for the first time a three-dimensional reconstruction of the armour plates around the tail in association with caudal vertebrae and a chevron, to give direct evidence of the body outline. The caudal vertebral column was surrounded by eight bony osteoderms, paired paramedian dorsal and ventral plates, and a pair of lateral osteoderms on right and left. The tail shape was subcircular, broader than high. The osteoderms overlap like roofing tiles, the posterior margin of each overlapping the osteoderm following behind. The success of these scans suggests that computed tomography scanning could reveal excellent detail of all the Elgin reptiles in the future. Supplementary material: Three-dimensional models of the two fossil specimens are available at: https://doi.org/10.6084/m9. figshare.c.4824183 Received 23 September 2019; revised 24 November 2019; accepted 20 January 2020 Aetosaurs were unusual quadrupedal, highly armoured We identify the current specimens unequivocally as suchian archosaurs (Nesbitt 2011) that are known from pertaining to Stagonolepis robertsoni based on the identifi- every continent, save Australia and Antarctica, and lived cation of osteoderms and caudal vertebrae with those of that from the Carnian to Rhaetian in the Late Triassic (Heckert taxon. First, the shape and pitting of the osteoderms are and Lucas 2000; Nesbitt 2011). This was a time of great unique to Aetosauria, and Stagonolepis is the only aetosaur diversity and disparity of crurotarsans, the ‘crocodile-line’ from the Lossiemouth Sandstone Formation. In more detail, archosaurs (Nesbitt 2011; Stubbs et al. 2013), among which the osteoderms are identical in size and shape and the aetosaurs became specialized as one of the only groups of droplet-like external pitting to all previously described Triassic archosaurs to be herbivorous, though some species Stagonolepis scutes (Agassiz 1844; Walker 1961). Further, may also have been partially or fully insectivorous and better as discussed below, the caudal vertebrae in our specimens are suited to feeding on soft grubs (Desojo and Vizcaino 2009). identical to those of S. robertsoni (Walker 1961). Though phylogenetic analysis makes it clear that Aetosauria Aetosaur armour plates may occur commonly, even in must have diverged from other suchians in the Early Triassic, isolation from other skeletal elements, and this is as true of the clade is known exclusively from the Late Triassic Stagonolepis as with other genera, such as the North (Nesbitt et al. 2014). The anatomy of aetosaurs is also American forms Longosuchus (Sawin 1947), unusual, with features such as highly ornamented osteoderms Desmatosuchus (Case 1922) and Typothorax (Case 1922), and an upturned shovel-shaped rostrum that may have been whose plates may bear spines. Where they are abundant, used for digging or foraging for roots, tubers or insects aetosaur osteoderms have been used as index fossils in (Desojo et al. 2013). biochronology (Heckert et al. 1996, 2007). The ornamenta- The first aetosaur to be named anywhere in the world was tion and pitting on the dermal surface of plates may be Stagonolepis robertsoni from the Lossiemouth Sandstone diagnostic of genera or species of aetosaurs, though care Formation of Lossiemouth East Quarry, near Elgin, should be taken if using only the paramedian osteoderms, Morayshire (Walker 1961; Benton and Walker 1985). It because the lateral osteoderms may provide a stronger was described by Louis Agassiz, who, based solely on phylogenetic signal (Parker 2007). It is important to ensure drawings of the ventral dermal armour of the holotype that homologous osteoderms are used for species identifica- (ELGNM 27R), wrongly assumed it to be a fish (Agassiz tion, as their morphology changes within individuals and 1844). This was later corrected by Huxley, who reclassified species according to their position on the body, whether Stagonolepis as a reptile among the Crocodilia (Huxley dorsal, lateral or ventral, but also along the length of the body 1859, 1877). Today, Stagonolepis sits firmly within (Parker and Martz 2010; Desojo et al. 2013). Aetosauria and is considered a typical, if basal, member of Stagonolepis and other aetosaurs were encased in plates the group (Desojo et al. 2013). Stagonolepis has ventral presumably for defence against large predators such as armour and small rounded armour plates over most of the Ornithosuchus and the rauisuchians. This protective function limbs (Walker 1961). (Chen et al. 2014) was enabled by the fact that the © 2020 The Author(s). Published by The Geological Society of London for EGS and GSG. All rights reserved. For permissions: http://www.geolsoc.org.uk/ permissions. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sjg.lyellcollection.org/ by guest on May 13, 2020 56 E. Keeble and M. J. Benton osteoderms overlapped in imbricating patterns, like slates on after blasting of the rock had taken place during small-scale a roof (Clarac et al. 2019), as is also seen in other quarrying. This very recent find shows that, despite the pseudosuchians, for example, erpetosuchids (Benton and historical nature of most Elgin specimens, the site is still Walker 2002; Ezcurra et al. 2017). However, experimental productive and it is possible that new material could be studies show that aetosaur osteoderms were weaker than uncovered in the future. There are two blocks (ELGNM those of at least some contemporaneous and modern 2018.6.1 and 2) that might come from one individual, as they crocodilomorphs and crocodilians when stresses similar to were found close together, and both represent portions of tail a predatory attack were applied to them (Clarac et al. 2019). and osteoderms from a similarly sized individual. However, All aetosaurs possessed dorsal paramedian and lateral the two sharp-edged blocks cannot be fitted together. The osteoderms, many also had ventral osteoderms, and some matrix is the typical fine-grained, well-sorted, light buff- even had appendicular osteoderms, marking them as some of coloured sandstone of the Lossiemouth Sandstone the most heavily armoured animals in the fossil record Formation. (Desojo et al. 2013; Clarac et al. 2019). ELGNM 2018.6.1 (Fig. S1A) is the larger of the two The aetosaurian dermal armour might also have played a blocks, and contains a more posterior section of the tail, role in thermoregulation (Seidel 1979; Chen et al. 2014). though it shows only the dorsal caudal paramedian plates. This is suggested by the broad surface of the osteoderm, with The internal surfaces of the osteoderm are exposed. ELGNM high vascularity, as suggested by the ornamental pattern on 2018.6.2 (Fig. S1B) likewise contains an example of the the osteoderm, which provided ample channels for cutaneous dorsal paramedian osteoderm, but also includes the corre- blood vessels to allow rapid heat transfer (Seidel 1979). sponding caudal lateral and ventral plates as well as at least Previously, the best way to study the remains of the Elgin two largely complete vertebrae and a chevron, though these reptiles was to destroy the bones, which are usually crumbly are not visible externally. and contained in a tough sandstone matrix, leaving only the mould of the fossil in the rock, from which a replica can then be CT scanning cast (Walker 1961; Benton and Walker 1981, 1985). Earlier efforts to remove the sandstone using chisels resulted in Both blocks were scanned using a Nikon XT H 225 CT damage to the soft bone, and no chemical treatment has been scanner with rotating target at the University of Bristol. found to remove the rock at all, least of all to remove the rock ELGNM 2018.6.1 was scanned at 223 kV and 399 uA using neatly and leave the bone undamaged. This all makes it difficult a 2 mm Cu filter. The gain was 24 with a 0.5 s exposure, to study the Elgin reptiles, and especially surface details such as providing 3141 projections and 4× frame averaging, resulting the unique external ornamentation of aetosaur osteoderms. in a voxel size of 0.065 mm. ELGNM 2018.6.2 was scanned Here we use computed tomography (CT) scanning to overcome at 213 kV and 179 uA with no filter. The gain was 18 and a this problem. Only two other studies have successfully 0.5 s exposure, also with 3141 projections and 4× frame attempted CT scanning of an aetosaur: Baczko et al. (2018) averaging, resulting in a voxel size of 0.0380 mm. Both scans show CT scans of the braincase of Neoaetosauroides and were run for c. 1 h 40 min each. The use of a micro-CT Hoffman et al. (2018, 2019) provide scans of scattered scanner allowed for higher resolution than a standard postcranial elements of Coahomasuchus. Parker (2018) also scanner, thus enabling a clear image, despite the difficulty attempted to CT scan the skull of S. robertsoni,buttheX-rays of scanning such a dense matrix. failed to penetrate the sandstone matrix of the specimen. Although the separation between many bones and matrix Here, we explore the imbrication of multiple armour plates was good and allowed some degree of automatic volume of an aetosaur and their relation to the underlying bones, using rendering, additional sectioning of CT scans had to be CT scanning to produce three-dimensional digital models.