Theriosuchus undetermined
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publication ID |
https://doi.org/ 10.1111/zoj.12315 |
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persistent identifier |
https://treatment.plazi.org/id/8A7087ED-C915-FFA3-8B11-F2B13858C2E3 |
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treatment provided by |
Marcus |
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scientific name |
Theriosuchus undetermined |
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Referred specimen
NMS G. 2014.52.1, an incomplete, three-dimensionally preserved right dentary. It is broken at both the anterior and posterior ends ( Fig. 2 View Figure 2 ).
Referred locality
The specimen was collected ex situ in May 2013 by one of us (T. J. C.) from a landslip below Carraig Mhor (British National Grid NG 88715 88607), Trotternish Peninsula, Isle of Skye , Scotland, UK ( Fig. 1 View Figure 1 ) .
Stratigraphical age and horizon
Valtos Sandstone Formation, Great Estuarine Group. Late Bajocian–Bathonian, Middle Jurassic.
Differential diagnosis
Atoposaurid crocodyliform within Theriosuchus with the following unique combination of characters (putative autapomorphic characters amongst Atoposauridae are indicated by an asterisk *): (1) posterodorsally orientated oblique crenulations (raised ridges) on the medial surface of the D8 and D9 alveoli labial rims (although we cannot entirely preclude these from being taphonomic distortions)*; (2) longitudinally crenulated dorsal dentary surface lingual to the dentary arcade*; (3) distinctive pair of foramina medial to the fourth dentary alveolus, adjacent to the raised D4 alveolar rim and the dorsal margin of the symphysis, respectively* (however, this characteristic varies intraspecifically and requires further study in other Theriosuchus specimens; see description below); (4) parallel symphyseal surface to the dentary arcade and external surface*; (5) inferred transition from a pseudocaniniform to labiolingually compressed dental morphotype from D7 to D8 (based on the shape of the alveoli); (6) D4 and D5 almost twice the size of subsequent posterior alveoli; (7) independent dental alveoli separated by interalveolar septae, with no confluent alveoli chain; and (8) no apparent contribution by the splenial to the symphysis*. (Note we cannot entirely discount the possibility that the anterior-most preserved alveolus is in fact the D3, and not the D4; see below.)
Description
The maximum length of the preserved dentary is 26.1 mm, and in dorsal view it has a maximum width of 9.8 mm at the fourth and fifth alveoli (D4–D5), and a minimum width of 9.1 mm at the D6–D7 interalveolar septum ( Fig. 2 View Figure 2 ). The dorsoventral height of the symphyseal region in medial view is 6.1 mm, and remains constant along what is preserved of the dentary. The external surface decreases in dorsoventral height, from 5.2 mm adjacent to the D4 alveolus, to 3.7 mm lateral to the D8–D9 alveoli. The medial and lateral surfaces are both straight and parallel in dorsal view, unlike some other members of Theriosuchus in which the lateral surface is dorsoventrally convex ( Schwarz & Salisbury, 2005; Martin et al., 2014b; Young et al., 2014b; Table 1).
The anterior and alveolar external surfaces of the dentary are covered in very small, heterogeneously spaced, subcircular pits, similar to other species of Theriosuchus ( Owen, 1878; Brinkmann, 1989, 1992; Schwarz & Salisbury, 2005). However, these pits are not as well developed as those in T. sympiestodon ( Martin et al., 2014b) . In this pitted region, when larger pits are present they are surrounded by a more abundant stippled pattern on the external surface. Between these large pits the external surface of the bone is generally subtly convex. However, posteriorly and ventrally the external surface becomes more rugose in an anteroposterior direction, with no stippled pattern present.
Five complete dentary alveoli are preserved, with a partial sixth at the posterior end of the bone, which we interpret as being the D4 to D9 alveoli (based upon comparisons with other Theriosuchus specimens). We D, dentary alveolus.
are cautious in this interpretation, however, owing to the fragmentary nature of the specimen. In the Skye specimen, the D4 and D5 alveoli are approximately equal size. In T. ibericus , this morphology is unknown, and D4 alveoli are the largest in T. guimarotae , T. pusillus , and T. sympiestodon . In some specimens of T. pusillus (e.g. NHMUK PV OR48244), the D3 alveoli are also enlarged, but not to the same degree as the D4. Therefore, we cannot preclude the possibility that the anterior-most alveolus preserved on the Skye specimen is in fact the D3 alveolus. The preserved alveoli are partially filled with matrix. The maximal anteroposterior lengths of the alveoli are (D4–D8): 3.9, 3.6, 2.7, 2.6, and 3.2 mm. The maximal transverse widths are (D4–D8): 2.6, 2.9, 2.2, 2.1, and 1.8 mm. These show a progressive transformation from a suboval D4–D5, to a subcircular D6–D7, to an anteroposteriorly elongated oval D8, which we interpret as representing a shift from a pseudocaniniform to a labiolingually compressed dental morphology, a feature found only in the heterodont atoposaurid Theriosuchus . A similar alveolar shape transition occurs in the Kimmeridgian species T. guimarotae , in which a shift from pseudocaniniform dental morphology to a lanceolate morphology is observed, but at the D10 to D11 alveoli ( Schwarz & Salisbury, 2005). The transition is also apparent in T. pusillus but beginning at D5 instead of D7. Furthermore, in T. pusillus specimen NHMUK PV OR48244, D10 is occupied by a hypertrophied, lanceolate tooth, with the tooth in D9 representing a transitional morphology between the smaller D5–D8. In T. sympiestodon , D4 is clearly the largest, but it is not clear whether D5–D7 occupy a single confluent groove, or are separated by low septae ( Martin et al., 2014b). There is a clear diastema separating D7 and D 8 in T. sympiestodon , a feature differentiating this taxon from the Skye specimen. As such, the position of this dental morphology transition, as inferred from alveolar morphology, is a feature that differentiates these species of Theriosuchus .
Each tooth in NMS G. 2014.52 occupies an independent alveolus separated by a distinct interalveolar septum. The septum separating the D4 alveolus from the D5 alveolus is thin, whereas all subsequent septa are broader and flatter on the occlusal surface, a condition similar to that of T. pusillus between the D3–D4 alveoli (NHMUK PV OR48244). This is unlike the condition apparent in T. sympiestodon , in which the D4– D7 alveoli appear to be confluent and the only gap is between the D7 and D8 alveoli ( Martin et al., 2014b) – however, this is based on a poorly preserved specimen, and we regard this inference as tentative. It is also different from the morphology in T. pusillus , which has a confluent D4–D7 alveoli chain (NHMUK PV OR48262; Young et al., 2014b), and T. guimarotae , which has confluent third and fourth alveoli, with the interalveolar bar separating the D4 and D5 alveoli being the broadest of all anterior interalveolar septae ( Schwarz & Salisbury, 2005). However, other specimens attributed to this T. pusillus , including NHMUK PV OR48244, show that with the exception of D3–D4, the dentary alveoli are separated by septae. In T. ibericus all teeth occupy a single continuous dental groove (J. P. T., pers. observ.), a feature that is unlikely to be a result of preservation. Although the validity of T. ibericus is questionable ( Schwarz & Salisbury, 2005), this feature is enough to distinguish the Skye specimen from this taxon.
The alveoli in NMS G. 2014.52.1 have raised lingual rims, which are more prominent on the D4–D6 alveoli, and vertically festooned labial rims. Although present on T. guimarotae ( Schwarz & Salisbury, 2005) , the development of these rims is much more pronounced on the Skye specimen. Raised lingual rims are not observed on any specimen of T. pusillus . On the medial surface of the D8 and D9 alveoli labial rims of NMS G. 2014.52.1 there are oblique crenulations (raised ridges), which extend posterodorsally towards the specimen’s posterior break. This feature is not known in any atoposaurid specimen, and therefore we consider it to be diagnostic for the Skye specimen.
Immediately lingual to the dental arcade in NMS G. 2014.52.1 there is a parallel row of foramina extending from the D5 alveolus to at least the D7 alveolus, with at least five foramina present and filled with matrix. This is also seen in T. guimarotae and T. sympiestodon , but in the latter taxon this row is more medially inset and extends at an angle to the obliquely set symphyseal region. In T. pusillus this foramen row either begins adjacent to the D7 alveolus (NHMUK PV OR48262) or adjacent and posteromedial to the D2 alveolus (NHMUK PV OR48244), and runs parallel to the tooth row. The surface that these foramina occupy is flat and longitudinally crenulated in NMS G. 2014.52.1, unlike in T. guimarotae and T. pusillus in which it is smooth. Furthermore, NMS G. 2014.52.1 has a distinctive pair of lingual foramina medial to D4, with one adjacent to the dorsal symphyseal margin and another adjacent to the raised lingual rim of D4. A single foramen is also present in T. pusillus , located more anteromedial to D4 than in NMS G. 2014.52.1. One is also present in T. guimarotae , but immediately medial to a possible diastema separating the D2– D3 alveoli. We note that the presence and distribution of these foramina can vary intraspecifically, but to our knowledge there are no atoposaurid specimens that preserve a foramen directly adjacent to the dorsal margin of the symphysis. Therefore, the position of this pair of foramina could be diagnostic for the Skye specimen, but this determination must await further discovery and examination of atoposaurid specimens preserving the dorsal surface of the dentary.
The symphyseal region is parallel to the dental arcade, and terminates lateral to the D7 alveolus, differing from both T. guimarotae and T. pusillus in which the symphyses are obliquely inclined. Therefore, the parallel orientation of the symphysis is autapomorphic for NMS G. 2014.52.1. Furthermore, the symphysis of the Skye specimen does not appear to receive a contribution from the splenial. However, there is a ‘trough’- shaped surface posterior to the dentary portion of the symphysis, which might be the sutural surface for the missing splenial. If this is the case, the splenial would extend no further anteriorly than the D6 alveolus, and the total length of the symphysis would have a greater posterior extent, in dorsal view. In T. pusillus , the symphysis in both NHMUK PV OR48262 and NHMUK PV OR48244 receives a major contribution from the splenial, extending as far anteriorly as the D3–D4 alveoli in dorsal aspect. In T. guimarotae , the splenial contributes to the symphysis anteriorly as far as the D3 alveoli ( Schwarz & Salisbury, 2005). The symphyseal suture is comprised of ridges radiating from a central point, resulting in a distinctive ‘starburst’ morphology. These ridges reach the dorsal edge of the medial surface, but terminate before reaching the ventral margin. The ventral region of the medial surface lacking the ridges is instead smooth and unsculptured. This is different to T. guimarotae and T. pusillus , in which the symphysis occupies the whole medial surface anterior to the D7 alveoli and the D5–D6 alveoli, respectively, and is slightly offset from the posterior medial dental surface, with this area perforated by a distinctive nutrient foramen ( T. pusillus, NHMUK PV OR 48262).
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