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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 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 to be named from the 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 , 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 following behind. The success of these scans suggests that computed tomography scanning could reveal excellent detail of all the Elgin in the future. Supplementary material: Three-dimensional models of the two 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 (Nesbitt 2011) that are known from pertaining to Stagonolepis robertsoni based on the identifi- every continent, save and , and lived cation of osteoderms and caudal vertebrae with those of that from the to 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 ‘-line’ from the Formation. In more detail, archosaurs (Nesbitt 2011; Stubbs et al. 2013), among which the osteoderms are identical in size and shape and the 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 (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 , isolation from other skeletal elements, and this is as true of the is known exclusively from the Late Triassic Stagonolepis as with other genera, such as the North (Nesbitt et al. 2014). The of aetosaurs is also American forms Longosuchus (Sawin 1947), unusual, with features such as highly ornamented osteoderms (Case 1922) and (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 in (Desojo et al. 2013). (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 , who, based solely on phylogenetic signal (Parker 2007). It is important to ensure drawings of the ventral dermal armour of the 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 (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 , 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 and the rauisuchians. This protective function limbs (Walker 1961). (Chen et al. 2014) was enabled by the fact that the

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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- 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 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 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 (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 , 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 , 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 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 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. performed manually in Avizo 8.0 using a Wacom Cintiq 24HD tablet. This enabled us to create clear 3D models of the areas where the distinction between rock and bone was poor, Institutional abbreviation and this allowed a greater degree of detail to be visualized. ELGNM, Elgin Museum, 1 High Street, Elgin, Morayshire Spin cycles showing the models in 3D were also created in IV30 1EQ, UK. Avizo and are available to view here (https://doi.org/10. 6084/m9.figshare.c.4824183). These animated rotating 3D Anatomical abbreviations specimen scans provide a full idea of the shape and spatial relation of the elements discussed. Naming conventions of osteoderms and their positions are based on Parker (2016). c, centrum; ch, chevron; lo, lateral Results osteoderm; na, neural arch; nc, neural canal; ns, neural spine; po, paramedian dorsal osteoderm; pz, prezygapophysis; tp, ELGNM 2018.6.1 transverse process; v, ; vo, paramedian ventral In the 3D model of ELGNM 2018.6.1 (Fig. 1), there are osteoderm. seven paramedian dorsal osteoderms, forming three pairs of osteoderms and a portion of an anterior osteoderm on the left- Materials and methods hand side. Two of these osteoderms (IV and V) are almost complete, whereas the other five are only partially preserved Materials with a reduction in preservation quality anteriorly. The The specimens (Supplementary material, Fig. S1) were osteoderms are arranged in matching, symmetrical pairs, collected by David Longstaff in 2018 from Spynie Quarry showing thin edges where they contact along the midline of Downloaded from http://sjg.lyellcollection.org/ by guest on May 13, 2020

Tail armour of the Triassic aetosaur Stagonolepis 57

Fig. 1. Three-dimensional model of ELGNM 2018.6.1, showing (a) dorsal view of the osteoderm and (b) internal view of the ventral side of the dorsal paramedian osteoderm. Details of two matching paramedian osteoderm, from (c) left and (d) right sides. Osteoderm are numbered (I–VII) from front to back of the individual, with I, III, V and VII on the left side, and II, IV and VI on the right side. Anterior of the is to the right. the back, and thickening laterally to form an antero-posterior removed. The internal faces (Fig. 1b) are smooth, with signs inflection marked by an external ridge that rises to a peak just of bone layering. behind mid-length, and marked by a slightly elevated boss, All osteoderms bear pitting on the external surface that also corresponds to the centre of ossification of the (Fig. 1a, c and d) in patterns that reflect the natural growth osteoderm (Cerda and Desojo 2011). When fitted together, of the osteoderm from a growth centre that corresponds to the these inflections, ridges and bosses line up, marking a off-centre boss (Cerda and Desojo 2011). Growth in these squared box-edge along the side of the animal. All two osteoderms was then more forwards than backwards, osteoderms are elongate, suggesting an assignment to the assuming the juvenile had a more or less equidimensional more posterior end of the tail (Walker 1961). osteoderm around the central boss. Presumably, as the animal The best-preserved osteoderm, from the left-hand side, grew in size, bone was also laid down internally to make the rendered in dark purple (Fig. 1a and c), measures 42 mm osteoderm thicker, as well as round the edges to make it more from anterior bar to posterior surface. It shows a roughly extensive. The external pitting, comprising circular and midline peaked structure that represents a substantial rounded, elongate concavities radiating from the growth thickening of the osteoderm overall. In life, this peak centre of each element, is typical of Stagonolepis robertsoni marks an oblique-angled bend in the external face, and the (Walker 1961). As Huxley (1859, p. 442), described it peaks on successive osteoderms line up with each other along the length of the tail, making an angle of the box-like The face of the is ornamented with a peculiar enclosing armour between the dorsal and dorso-lateral faces. sculpture, consisting of distinct deep pits. The casts of The matching osteoderm from the left-hand side (Fig. 1a and these are of course elevated, and lie like drops upon the d) is less complete, and the dorso-lateral portion has been general surface of the impression … Downloaded from http://sjg.lyellcollection.org/ by guest on May 13, 2020

58 E. Keeble and M. J. Benton

The osteoderms overlap antero-posteriorly: the posterior relation along the antero-posterior length cannot be com- edge of each osteoderm sits on the anterior margin of the mented on from this model. osteoderm behind. The zone of overlap is short, comprising a There are two pairs of articulated ventral paramedian depressed margin on the osteoderm behind, and a thin, flat osteoderms, highlighted in blue and yellow (Fig. 2b; vo.I– edge on the osteoderm in front. The pairs of paramedian VI). A partial third pair of ventral osteoderms is seen in osteoderms meet along a slightly grooved, thickened margin orange (Fig. 2b; vo.V) and it seems these have moved that provides a tight interlock, but without overlap. Along somewhat post-mortem. The fused appearance of these their lateral margins, the osteoderms also show a thickened, osteoderms is likely more due to taphonomic processes that grooved margin (Fig. 1b) for contact with the lateral resulted in some crushing of these elements together, rather osteoderms. than their being fused in life, as this is not seen in other specimens of S. robertsoni (Parker 2018). The largest and presumably most complete osteoderm (Fig. 2b; vo.II) is ELGNM 2018.6.2 36 mm long. The armour of ELGNM 2018.6.2 is not quite as well Lateral osteoderms are seen in both views (Fig. 2a and b, preserved as in ELGNM 2018.6.1 and does not exhibit the lo.I–VI). These match in length the dorsal and ventral excellent detail of the dorsal surface sculpture. The specimen paramedian osteoderms, and they show a distinct curve in preserves 14 osteoderms, comprising two paramedian dorsal these views. Evidence of their original curved shape can be osteoderms, six paramedian ventral osteoderms, and six seen in the transverse section of the tail (Fig. 3), which shows lateral osteoderms. The paramedian dorsal osteoderms in the dorsal, lateral and ventral osteoderms all slightly mauve and white (Fig. 2a, po.I, po.II) are reasonably square displaced, presumably by some post-mortem rotation by in shape and overlap medially, though this degree of overlap transverse movement during compaction of the sandstone. is likely due to crushing, seen in po.I (Fig. 2a) which shows a All osteoderms seem to show an equivalent amount of large diagonal crack through its entire length. As there is only curvature, although the dorsal lateral osteoderms (Fig. 3, lo.I, one pair of dorsal paramedian osteoderms preserved, their VI) appear to show a sharp, nearly right-angled bend,

Fig. 3. Three-dimensional model of ELGNM 2018.6.2, showing (a) Fig. 2. Three-dimensional model of ELGNM 2018.6.2, showing (a) dorsal anterior and (b) posterior views of the block with the overall cross- and (b) ventral views of the block. lo, lateral osteoderm; po, paramedian sectional shape of the tail segment, with vertebrae in the centre and a full dorsal osteoderm; v, vertebra; vo, paramedian ventral osteoderm. Anterior suite of dorsal, lateral and ventral osteoderms around. Abbreviations as of the animal is to the right. Figure 2 except ch, chevron. Downloaded from http://sjg.lyellcollection.org/ by guest on May 13, 2020

Tail armour of the Triassic aetosaur Stagonolepis 59 presumably marking one of the edges of the box-like margin It might have been expected that all caudal vertebrae and of the tail armour box. chevrons would have been present and complete within these At least two vertebrae can be distinguished within the tail segments. Based on the shape of the centrum visible on enclosing armour and a single, well-preserved and differ- the vertebra shown in red (Figs 3b, 4a and b), there may have entiated chevron can be seen lying beneath these (Figs 3 been some dorsoventral deformation during fossilization of and 4). The centrum width of vertebrae ranges from 10 to ELGNM 2018.6.1 as the centrum of the more posterior 13 mm. Parts of the vertebrae (Fig. 4a and b) seem to be caudal vertebrae of S. robertsoni are taller dorsoventrally lacking, especially the neural spines and the full extent of the than they are wide (Walker 1961), whereas the centrum of the transverse processes, which are fairly extensive towards the vertebra (Figs 3b, 4a and b) is more nearly circular. Further, anterior end of the tail (Desojo et al. 2013), though become the whole vertebra is not exactly symmetrical, but appears to greatly reduced in size moving posteriorly down the caudal have been twisted. This either reveals that it is more anterior vertebral series. Anterior caudal vertebrae of Stagonolepis than it seems, and extensive transverse processes have been robertsoni (Walker 1961, fig. 10) show down-turned lost to damage, or there has been dorsoventral crushing, transverse processes and a tall neural spine with a dorsal which would also explain the apparent misalignment of the spine table to support the dorsal paramedian osteoderm. The more anterior osteoderms. caudals become smaller and simpler distally, and these Attempts were made to fit the two blocks together, but differences in shape confirm that our examples are from the this could not be managed. They could therefore come anterior half of the tail. from one or more individuals. Comparison of the two The single chevron visible in the scan is not preserved in blocks suggests that ELGNM 2018.6.2 is the more anterior life position and has been dislodged laterally. The chevron block. In this specimen, the lack of a space between the shows remarkable detail (Fig. 4c and d), with a pair of dorsal dorsal and ventral lateral osteoderms suggests it comes processes that show rounded facets for articulation with from a medial or posterior section of the tail (Walker 1961). ventral facets on the centra of the caudal vertebrae. These Rough estimates of osteoderm shape resolve the broken branching processes combine in the ventral keel, which is narrow in anterior and posterior views, but forms a deep- sided blade in lateral view, as shown by Walker (1961, fig. 11) from other material of S. robertsoni. Their shape again confirms a position in the anterior portion of the tail.

Fig. 5. Comparison between osteoderms of the Elgin archosaurs. (a, b) Stagonolepis robertsoni caudal dorsal paramedian osteoderms Fig. 4. Three-dimensional model of elements of ELGNM 2018.6.2 ELGNM 2018.6.1. (c) Ornithosuchus caudal osteoderm BMNH R 3561, showing (a) anterior and (b) posterior views of an anterior caudal adapted from Walker (1964).(d) Stagonolepis robertsoni dorsal vertebra, and also (c) anterior and (d) posterior views of the chevron. c, paramedian osteoderm ELGNM 24. (e) pre-sacral centrum; na, neural arch; nc, neural canal; ns, neural spine; pz, osteoderm BMNH R3139, adapted from Benton and Walker (2002). prezygapophysis; tp, transverse process. (f, g) Stagonolepis robertsoni caudal osteoderms ELGNM 3R. All scale bars are 20 mm. Downloaded from http://sjg.lyellcollection.org/ by guest on May 13, 2020

60 E. Keeble and M. J. Benton osteoderm into a more laterally lengthened plate than those The final Elgin taxon with which the new osteoderms in ELGNM 2018.6.1, which comes from further back in the might be compared is ‘Dasygnathoides’, a form with a tail (Walker 1961). chequered history. Walker (1964) assigned it to Ornithosuchus, but more recently it has been considered as unassignable further than because the remains Discussion are scant, and it differs in size from other Ornithosuchus It is difficult to position the two blocks relative to each other, material (Baczko and Ezcurra 2016). It lacks all postcranial first because they do not fit together and, second, because material, including any osteoderms. typically in aetosaurs ventral osteoderms are shorter than the Finally, our caudal osteoderms (Fig. 5a and b) compare equivalent dorsal elements (Parker 2018). Therefore, we closely with previously discovered caudal osteoderms of cannot assume that the osteoderms become longer or shorter Stagonolepis, such as ELGNM 3R (Fig. 5f and g). in a particular direction along the tail. It is also possible that In terms of the preserved caudal vertebrae, the current the two blocks represent material from two individuals that specimens are entirely consistent with anterior caudals of differ in size, although their closeness in the quarry and Stagonolepis (Walker 1961, fig. 10), as noted. Caudal similarity in lithology, preservation and anatomy suggest vertebrae are not known in Erpetosuchus, nor are they well they may pertain to a single individual. represented in other erpetosuchids; those of Parringtonia are, As noted earlier, we identify these specimens as of course, much smaller than our specimens, and have Stagonolepis robertsoni because they are standard aetosaur- relatively much narrower centra (Nesbitt and Butler 2013, ian osteoderms and S. robertsoni is the only aetosaur from fig. 5r). Anterior caudals of Ornithosuchus are narrower and Elgin. In more detail, the pattern of pitting on the dorsal have taller neural spines (Walker 1964, fig. 8l, m) than the paramedian osteoderm of ELGNM 2018.6.1 is identical to specimens here. that described in other specimens of Stagonolepis robertsoni, Huxley (1877, p. 13) said, ‘in transverse section, the fossil with an asymmetrical, anastomosing ‘droplet’ pattern [Stagonolepis robertsoni] would have the contour of a high (Taborda et al. 2015) radiating outwards from a raised arched window, the ventral scutes representing the sill, and centre of ossification (Walker 1961; Parker 2018). This the dorsal, the arch of the window’. And this, with the aid of pattern is seen in other aetosaurs around the world (e.g. CT scanning, is indeed what we see. The ventral scutes do ) but affinity with these can be discounted not possess as much of a curvature as the laterals or dorsals based on temporal and geographical differences. and so almost produce a ‘sill’. That being said, there is not so While the imbricating overlap of osteoderms in these new much ‘arch’ as Huxley suggested in the CT renderings, but Elgin specimens is typical of many crurotarsans, including rather the transverse section looks more of an oblong, being erpetosuchids, the osteoderms we describe here exhibit somewhat dorsoventrally flattened. This may have been lateral thickening to an antero-posterior inflection, something because Huxley observed dorsal paramedian osteoderms that that is not seen in erpetosuchids. When compared to other had been displaced by the underlying neural spines during Lossiemouth Sandstone Formation archosaurs (Fig. 5), the preservation (Walker 1961; Parker 2018). However, as identity seems clear. previously noted, there appears to have been some The most striking difference between ELGNM 2018.6.1 dorsoventral crushing of ELGNM 2018.6.2, which might (Fig. 5a and b) and Ornithosuchus (Fig. 5c) is that the latter, have affected the observed shape of this caudal section. A like other ornithosuchids, had only a paired series of dorsal similar transverse section shape has also been noted in other osteoderms (Walker 1964, pp. 96–99). Further, the caudal aetosaurs (e.g. Typothorax coccinarum and Stegomus osteoderms of Ornithosuchus are virtually oval in shape, arcuatus), making it possibly a typical feature of the when viewed from above with a defined central ridge divided bauplan of desmatosuchine aetosaurs (Jepsen 1948; into two portions (Walker 1964, fig. 14). The pattern of Heckert et al. 2010). ornamentation also differs, with Ornithosuchus exhibiting a The spaces observed between each row of osteoderms, pattern of primarily linear striations radiating from the especially clearly in the render of ELGNM 2018.6.1 (Fig. 1) longitudinal ridge (Fig. 5c). The same is true of other and the animated rotation of this, most likely allowed for soft ornithosuchids for which the armour is preserved (Baczko and connective tissue that interlinked the osteoderms, and the and Ezcurra 2014). keratinous sheath of the dermis itself, as is seen today in Direct comparison between this specimen and modern crocodilians (Chen et al. 2014). A CT scan study of a Erpetosuchus cannot be made because caudal material is goniopholid crocodilian (Puértolas-Pascual and not known from Erpetosuchus. However, comparison of Mateus 2019) showed an analogous space, which was dorsal osteoderms of Stagonolepis (Fig. 5d) and interpreted as space for a bracing system between the Erpetosuchus (Fig. 5e) shows that those of the latter are osteoderms and the vertebrae. Note, however, that the much smaller, measuring typically 5 × 8 mm, and the goniopholid crocodilian, as with modern relatives, has only sculpture consists of a finely pitted radiating pattern dorsal osteoderms and the tail is not entirely surrounded by (Benton and Walker 2002, fig. 3). Further, there is no an armour sheath. evidence Erpetosuchus had ventral osteoderms. Evidence In comparison with other aetosaurs, Stagonolepis robert- from more complete erpetosuchids with caudal material soni retains a fairly typical caudal anatomy, with the addition (e.g. Dyoplax) suggests that they lacked ventral osteoderm of ventral osteoderms that are not seen in all species (Maisch et al. 2013), thus excluding them as a possible (e.g. Desmatosuchus spurensis; Parker 2008). To give a source of the current material on grounds of size and clearer view of the relation of internal anatomy to the external limitations of the armour cover. osteoderms, it is possible that some attempt at Downloaded from http://sjg.lyellcollection.org/ by guest on May 13, 2020

Tail armour of the Triassic aetosaur Stagonolepis 61 retrodeformation (Arbour and Currie 2012) could be Chen, I.H., Yang, W. and Meyers, M.A. 2014. osteoderms: mechanical behaviour and hierarchical structure. Materials Science and Engineering, C, attempted, but without further CT data as a comparison 35, 441–448, https://doi.org/10.1016/j.msec.2013.11.024 this could produce incorrect results. Clarac, F., Goussard, F., de Buffrenil, V. and Sansalone, V. 2019. The function(s) of bone ornamentation in the crocodylomorph osteoderm: a biomechanical model based on finite element analysis. Paleobiology, 45, 182–200, https:// Conclusion doi.org/10.1017/pab.2018.48 Desojo, J.B. and Vizcaino, S.F. 2009. Jaw biomechanics in the South American The CT scans of the tail region of Stagonolepis robertsoni aetosaur Neoaetosaurides engaeus. Paläontologische Zeitschrift, 83, 499, https://doi.org/10.1007/s12542-009-0032-6 confirm previous accounts of the caudal osteoderm anatomy, Desojo, J.B., Heckert, A.B., Martz, J.W., Parker, W.G., Schoch, R.R., Small, B.J. and increase our understanding of the arrangement of the and Sulej, T. 2013. Aetosauria: a clade of armoured pseudosuchian archosaurs dorsal and ventral (or paramedian and lateral) osteoderms in from the Upper Triassic continental beds. Geological Society, London, Special Publications, 379, 203–239, https://doi.org/10.1144/SP379.17 desmatosuchine aetosaurs, in addition to those described Ezcurra, M.D., Fiorelli, L.E. et al. 2017. Deep faunistic turnovers preceded the already in Typothorax coccinarum and Stegomus arcuatus. rise of the in southwestern Pangea. Nature Ecology and Evolution, 1, 1477–1483, https://doi.org/10.1038/s41559-017-0305-5 The new work also sheds light on how imaging can be used Heckert, A.B. and Lucas, S.G. 2000. , phylogeny, , in research on dermal armour. The next steps are to increase biochronology, paleobiogeography and evolution of the Late Triassic our library of CT data for aetosaurs. With good quality Aetosauria (Archosauria: ). Zentralblatt für Geologie und Paläontologie, Teil 1, 11–12, 1539–1587. models of the armour plates and their attachments and Heckert, A.B., Hunt, A.P. and Lucas, S.G. 1996. Redescription of imbrications, functional experiments could be carried out to reseri, a Late Triassic aetosaur (Reptilia: Archosauria) from (USA), and the biochronology and phylogeny of aetosaurs. test the strength not only of individual osteoderms, but of the Geobios, 29, 619–632, https://doi.org/10.1016/S0016-6995(96)80028-3 whole armour when multiple osteoderms are locked and Heckert, A.B., Lucas, S.G., Hunt, A.P. and Spielmann, J.A. 2007. Late Triassic imbricated together, as in life. aetosaur biochronology revisited. New Mexico Museum of Natural History and Science Bulletin, 41,49–50. Heckert, A.B., Lucas, S.G., Rinehart, L.F., Celeskey, M.D., Spielmann, J.A. and Hunt, A.P. 2010. Articulated skeletons of the aetosaur Typothorax coccinarum We thank David Longstaff for making the original Acknowledgements Cope (Archosauria: Stagonolepidae) from the Upper Triassic Bull Canyon find, in bringing the specimens to the ELGNM, and for photographs, and Janet formation (Revueltian: early-mid ), eastern New Mexico, USA. Trythall and Alison Wright for their curatorial work and for delivering and Journal of , 30, 619–642, https://doi.org/10.1080/ collecting the specimen. We thank Dr Tom Davies for endless help in making the 02724631003763524 CT scans and in advice over the segmentation and image processing. Finally, we Hoffman, D.K., Heckert, A.B. and Zanno, L.E. 2018. Under the armour: X-ray are very grateful to reviewers Bill Parker and Davide Foffa for such helpful computer tomographic reconstruction of the internal skeleton of comments that materially improved the paper. Coahomasuchus chathamensis (Archosauria: Aetosauria) from the Upper Triassic of , USA and a phylogenetic analysis of Aetosauria. PeerJ, 6, e4368, https://doi.org/10.7717/peerj.4368 Funding This research received no specific grant from any funding agency in Hoffman, D.K., Heckert, A.B. and Zanno, L.E. 2019. Disparate growth strategies the public, commercial, or not-for-profit sectors. within Aetosauria: novel histological data from the aetosaur Coahomasuchus chathamensis. The Anatomical Record, 302, 1504–1515, https://doi.org/10. 1002/ar.24019 – Author contributions EK: investigation (lead), writing original draft Huxley, T.H. 1859. On the Stagonolepis robertsoni (Agassiz) of the Elgin – (equal); MJB: conceptualization (lead), investigation (supporting), writing Sandtones; and on the recently discovered footmarks in the sandstones of original draft (equal). Cummingstone. Quarterly Journal of the Geological Society, 15, 440–460, https://doi.org/10.1144/GSL.JGS.1859.015.01-02.54 Scientific editing by Tom Challands Huxley, T.H. 1877. The crocodilian remains found in the Elgin sandstones, with remarks on the ichnites of Cummingstone. Memoirs of the Geological Survey, UK, Monograph, 3,1–52. References Jepsen, G.L. 1948. A Triassic armored reptile from New Jersey. State of New Agassiz, L. 1844. Monographie des Poissons Fossiles du Vieux Gres̀ Rouge ou Jersey Department of Conservation Miscellaneous Geologic Paper,1–20. Systemè Dévonien () des Isles Britanniques et de Russie. Maisch, M.W., Matzke, A.T. and Rathgeber, T. 2013. Re-evaluation of the Soleure, Neuchâtel. enigmatic Dyoplax arenaceus O. Fraas 1867 from the Arbour, V.M. and Currie, P.J. 2012. Analyzing taphonomic deformation of Schilfsandstein ( Formation, lower Carnian, Upper Triassic) of ankylosaur using retrodeformation and finite element analysis. PLOS Stuttgart, . Neues Jahrbuch für Geologie und Paläontologie, One, 7, e39323, https://doi.org/10.1371/journal.pone.0039323 Abhandlungen, 267, 353–362, https://doi.org/10.1127/0077-7749/2013/0317 Baczko, M.B.v. and Ezcurra, M.D. 2014. : a group of Triassic Nesbitt, S. 2011. The early evolution of the archosaurs: relationships and the archosaurs with a unique ankle . Geological Society, London, Special origin of major . Bulletin of the American Museum of Natural History, Publications, 379, 187–202, https://doi.org/10.1144/SP379.4 352,1–292, https://doi.org/10.1206/352.1 Baczko, M.B.v. and Ezcurra, M.D. 2016. Taxonomy of the archosaur Nesbitt, S.J. and Butler, R.J. 2013. Redescription of the archosaur Parringtonia Ornithosuchus: reassessing Ornithosuchus woodwardi Newton, 1894 and gracilis from the Manda Beds of Tanzania, and the antiquity Dasygnathoides longidens (Huxley 1877). Earth and Environmental Science of Erpetosuchidae. Geological Magazine, 150, 225–238, https://doi.org/10. Transactions of the Royal Society of Edinburgh, 106, 199–205, https://doi.org/ 1017/S0016756812000362 10.1017/S1755691016000104 Nesbitt, S., Sidor, C.A., Angielczyk, K.D., Smith, R.M.H. and Tsuji, L.A. 2014. Baczko, M.B.v., Taborda, J.R.A. and Desojo, J.B. 2018. Paleoneuroanatomy of A new archosaur from the Manda beds (, Middle Triasssic) of southern the aetosaur Neoaetosauroides engaeus (Archosauria: Pseudosuchia) and its Tanzania and its implications for character state optimizations at Archosauria paleobiological implications among archosauriforms. PeerJ, 6, e5456, https:// and Pseudosuchia. Journal of , 34, 1357–1382, doi.org/10.7717/peerj.5456 https://doi.org/10.1080/02724634.2014.859622 Benton, M.J. and Walker, A.D. 1981. The use of flexible synthetic rubbers for Parker, W.G. 2007. Reassessment of the aetosaur ‘Desmatosuchus’ chamaensis casts of complex fossils from natural moulds. Geological Magazine, 118, with a reanalysis of the phylogeny of Aetosauria (Archosauria: Pseudosuchia). 551–556, https://doi.org/10.1017/S0016756800032921 Journal of Systematic Palaeontology, 5,41–68, https://doi.org/10.1017/ Benton, M.J. and Walker, A.D. 1985. Palaeoecology, taphonomy, and dating of S1477201906001994 Permo-Triassic reptiles from Elgin, north-east . Palaeontology, 28, Parker, W.G. 2008. Description of new material of the aetosaur Desmatosuchus 207–234. spurensis (Archosauria: ) from the of and a Benton, M.J. and Walker, A.D. 2002. Erpetosuchus, a crocodile-like basal revision of the Desmatosuchus. PaleoBios, 28, 281–240. archosaur from the Late Triassic of Elgin, Scotland. Zoological Journal of Parker, W.G. 2016. Osteology of the Late Triassic aetosaur Scutarx deltatylus the Linnean Society, 136,25–47, https://doi.org/10.1046/j.1096-3642.2002. (Archosauria: Pseudosuchia). PeerJ, 4, e2411, https://doi.org/10.7717/peerj. 00024.x 2411 Case, E.C. 1922. New reptiles and stegocephalians from the Upper Triassic of Parker, W.G. 2018. Anatomical notes and discussion of the first described western . Carnegie Institution of Washington, 321,7–84. aetosaur Stagonolepis robertsoni (Archosauria: Suchia) from the Upper Cerda, I.A. and Desojo, J.B. 2011. Dermal armour histology of aetosaurs Triassic of , and the use of plesiomorphies in aetosaur biochronology. (Archosauria: Pseudosuchia), from the Upper Triassic of and PeerJ, 6, e5455, https://doi.org/10.7717/peerj.5455 . Lethaia, 44, 417–428, https://doi.org/10.1111/j.1502-3931.2010. Parker, W.G. and Martz, J.W. 2010. Using positional homology in aetosaur 00252.x (Archosauria: Pseudosuchia) osteoderms to evaluate the taxonomic status of Downloaded from http://sjg.lyellcollection.org/ by guest on May 13, 2020

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Lucasuchus hunti. Journal of Vertebrate Palaeontology, 30, 1100–1108, end-Triassic . Proceedings of the Royal Society B, 280, 20131940, https://doi.org/10.1080/02724634.2010.483536 https://doi.org/10.1098/rspb.2013.1940 Puértolas-Pascual, E. and Mateus, O. 2019. A three-dimensional skeleton of Taborda, J.R.A., Heckert, A.B. and Desojo, J.B. 2015. Intraspecific variation in from the of Portugal: implications for the Aetosauroides scagliai Casimiquela (Archosauria: Aetosauria) from the bracing system. Zoological Journal of the Linnean Society, Upper Triassic of Argentina and Brazil: an example of ? https://doi.org/10.1093/zoolinnean/zlz102 Ameghiniana, 52, 173–187, https://doi.org/10.5710/AMGH.05.01.2015.2824 Sawin, H.J. 1947. The pseudosuchian reptile Typothorax meadei. Journal of Walker, A.D. 1961. Triassic reptiles of the Elgin area: Stagonolepis, Palaeontology, 21, 201–238, https://www.jstor.org/stable/1299332 and their allies. Philosophical Transactions of the Royal Society, Series B, 244, Seidel, M.R. 1979. The osteoderms of the and their functional 103–204, https://doi.org/10.1098/rstb.1961.0007 significance. Herpetologica, 35, 375–380. Walker, A.D. 1964. Triassic reptiles from the Elgin area: Ornithosuchus and the Stubbs, T.L., Pierce, S.E., Rayfield, E.J. and Anderson, P.L. 2013. Morphological origin of carnosaurs. Philosophical Transactions of the Royal Society, Series and biomechanical disparity in crocodile line archosaurs following the B, 248,53–134, https://doi.org/10.1098/rstb.1964.0009