Theriosuchus, OWEN, 1878

Young, Mark T., Tennant, Jonathan P., Brusatte, Stephen L., Challands, Thomas J., Fraser, Nicholas C., Clark, Neil D. L. & Ross, Dugald A., 2016, The first definitive Middle Jurassic atoposaurid (Crocodylomorpha, Neosuchia), and a discussion on the genus Theriosuchus, Zoological Journal of the Linnean Society 176 (2), pp. 443-462 : 456-457

STATUS OF THERIOSUCHUS

Alongside the description of Theriosuchus sp. here, the genus Theriosuchus contains five different species: T. guimarotae from the Kimmeridgian of Portugal ( Schwarz & Salisbury, 2005); T. pusillus from the Kimmeridgian to Berriasian of the UK ( Owen, 1878); T. ibericus from the Barremian of Spain ( Brinkmann, 1989, 1992); T. grandinaris from the Berriasian to early Barremian of Thailand ( Lauprasert et al., 2011); and T. sympiestodon from the Maastrichtian of Romania ( Martin et al., 2010, 2014b). The status of these species is in some flux, with T. ibericus being of questionable validity ( Schwarz & Salisbury, 2005; Lauprasert et al., 2011).

Alongside these remains, a multitude of material from the Jurassic and the Cretaceous has been attributed to Theriosuchus , or described as having a Theriosuchus - like morphology, with differing degrees of confidence ( Figs 4 and 5 ). These include: atoposaurid teeth similar to those of Theriosuchus from the early Bathonian of southern France ( Kriwet et al., 1997); a fragmentary atoposaurid tooth from the middle Bathonian of southern France ( Knoll et al., 2013); indeterminate teeth and possible but not clearly diagnostic cranial and osteoderm remains from the Bathonian of Madagascar ( Flynn et al., 2006); an indeterminate but Theriosuchus -like set of teeth from the Oxfordian of north-west China, assigned to Mesoeucrocodylia indet. ( Wings et al., 2010); Theriosuchus -like teeth assigned to a dwarf mesosuchian from the Kimmeridgian of northern Germany ( Thies & Broschinski, 2001); another specimen comprising the anterior part of crushed skeleton from the Kimmeridgian of northern Germany tentatively referred to Theriosuchus pusillus by Karl et al., 2006); and cf. Theriosuchus sp. from north-western Germany ( Thies et al., 1997). Moreover, isolated teeth from the Tithonian of northern France have been referred to cf. Theriosuchus sp. ( Cuny et al., 1991), whereas isolat- ed teeth from the Tithonian of western France have been referred to Theriosuchus cf. pusillus ( Vullo et al., 2014) . Other Cretaceous specimens include: Theriosuchus sp. , tentatively referred from the Early Cretaceous of Utah ( USA; Fiorillo, 1999); Theriosuchus sp. based on a tooth from the Berriasian of south-west France ( Pouech, Mazin & Billon-Bruyat, 2006; Pouech et al., 2014); Theriosuchus sp. from the Berriasian of Sweden and Denmark ( Schwarz-Wings, Rees & Lindgren, 2009); a skull fragment described as Theriosuchus sp. from the Wealden of the Isle of Wight (Berriasian–Barremian; Buffetaut, 1983); a range of material ascribed to cf. Theriosuchus sp. , Theriosuchus sp. , and Theriosuchus cf. pusillus from the Early Cretaceous of Thailand ( Cuny et al., 2010) and cf. Theriosuchus sp. from Inner Mongolia, China ( Wu et al., 1996). In the Late Cretaceous, there are Theriosuchus -like teeth from the Cenomanian of south-western France, ascribed to Atoposauridae ( Vullo & Néraudeau, 2008) ; Theriosuchus - like teeth from the Santonian of western Hungary, conservatively referred to Mesoeucrocodylia indet. ( Ösi et al., 2012); and a Theriosuchus -like tooth from the Campanian–Maastrichtian of Portugal ( Galton, 1996), although this tooth may be more comparable to Bernissartia ( Lauprasert et al., 2011) .

These specimens, if all from the same lineage, would undoubtedly make Theriosuchus one of the longestlived and most successful crocodylomorphs of the Mesozoic. However, most of these taxonomic identifications are based on teeth, which makes precise referrals to Theriosuchus based on apomorphies difficult, as well as creating issues in constructing species-level diagnoses and deciphering the relationships of Theriosuchus . At the very least, most authors agree that Theriosuchus (and the various specimens assigned to the lineage) comprise a subgroup of Atoposauridae , and recent phylogenetic analyses have demonstrated the atoposaurid affinities of Theriosuchus with strong support (e.g. Martin et al., 2010). Some authors take a different view, however, with Buffetaut (1982, 1983) and Kälin (1955) considering Theriosuchus to be distinct enough to form a separate family from Atoposauridae , although this idea has not gained much traction in recent years. However, the monophyly of all known Theriosuchus species has yet to be tested.

The heterodont dentition of Theriosuchus has been considered diagnostic for the genus since first described by Owen (1878), with four distinct morphotypes present: (1) pseudocaniniform tooth crowns, with lingual apicobasally aligned enamel ridges, typically found in the anterior-most regions of the premaxillae and dentaries; (2) lanceolate-shaped tooth crown crowns with lingual striae, which are present in the middle to posterior sections of the jaws and have a constriction at the crown–root junction; (3) a low-crowned morphotype present only in T. pusillus , T. ibericus , and T. sympiestodon ; and (4) broad and labiolingually compressed teeth with striations on the labial and lingual faces, found in T. ibericus and T. pusillus , and inferred for the dentary of the Skye specimen based on the shape of the alveoli ( Owen, 1878; Brinkmann, 1992; Thies et al., 1997; Salisbury, 2002; Schwarz & Salisbury, 2005; Schwarz-Wings et al., 2009). Theriosuchus sp. would have had at least two of these morphotypes, as inferred from alveolus shape (the pseudocaniniform morphotype in anterior alveoli, and either the lanceolate or labiolingually compressed morphotype in more posterior dentary alveoli), representing a transitional heterodont morphology that strongly unites the specimen with other Theriosuchus specimens. We conclude here, based on our comparative study of all Theriosuchus material ( Table 1), that this form of heterodonty is a diagnostic feature of Theriosuchus , and that isolated teeth fitting one or more of these morphotypes are likely to be referable to Theriosuchus .

The new mandibulodental characteristics described here indicate that a unique dental morphology is present in each previously recognized species of Theriosuchus ( Table 1), reaffirming their validity. We therefore tentatively accept the presence of five distinct known Theriosuchus species. Combinations of these dental characteristics may identify more inclusive subclades within Theriosuchus , or show that some species of Theriosuchus may be more closely related to other atoposaurid genera (in which case, this would suggest that the genus Theriosuchus should be restricted to the type species and those other taxa most closely related). However, testing of this and the taxonomic validity of these species will require a detailed phylogenetic analysis and comparative anatomical study amongst all currently known species.

Figures

Figure 4. Chronostratigraphical chart of the Theriosuchus species, cf. Theriosuchus specimens, and Theriosuchus-like specimens. See the Discussion for the relevant references.

Figure 5. Map of Theriosuchus and Theriosuchus-like specimens from the (A) Middle Jurassic, (B) Upper Jurassic, (C) Lower Cretaceous, and (D) Upper Cretaceous. See the Discussion for the relevant references. Palaeomaps are modified version of high-resolution versions kindly provided by Ron Blakey (http://cpgeosystems.com/).