Theriosuchus undetermined

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 : 455

IMPORTANCE OF THERIOSUCHUS SP.

The new crocodyliform specimen from Skye represents the oldest definitive specimen of Theriosuchus , and the oldest clearly diagnostic member of Atoposauridae . This provides several important insights into the evolution and distribution of this clade, which would evolve into a speciose group that persisted for around 100 000 000 years and became widely distributed across the globe. First, the Skye specimen indicates that atoposaurids were present by the Bajocian to Bathonian, and had evolved their characteristic small body size and heterodont dentition by this time. Although the morphology of the oldest known specimens does not necessarily indicate the plesiomorphic conditions of a clade, the antiquity of the Skye specimen and its general similarity to later atoposaurids suggests that small size and heterodont dentition were ancestral features of this group. This hypothesis can be tested through character optimization on a comprehensive phylogeny of Atoposauridae , which is currently in development by JPT.

The presence of a diagnostic skeletal fossil of Atoposauridae in the Middle Jurassic lends support to the atoposaurid identification of isolated heterodont teeth from the Bathonian of Madagascar, France, and the UK ( Evans & Milner, 1994; Kriwet et al., 1997; Flynn et al., 2006; Knoll et al., 2013). Previously, their atoposaurid identity was based on similarity to the heterodont dentition of Theriosuchus , but without diagnostic skeletal fossils of atoposaurids from this time it was possible that these teeth belonged to a different group of heterodont crocodylomorphs (or other archosaurs). Although that is still a possibility (as is always the case with isolated teeth), the new specimen from Skye is definitive evidence that atoposaurids were present during the Bathonian, and as such are the primary candidate for the identity of these distinctive teeth. Further examination of associated body fossils, such as those preliminarily identified by Flynn et al. (2006), as well as recently identified small indeterminate neosuchian fossils from the Bathonian Kilmaluag Formation of the Isle of Skye that potentially could belong to atoposaurids or their close relatives ( Wills, Barrett & Walker, 2014), should provide much key insight into the early radiation of atoposaurids.

The occurrence of Theriosuchus sp. provides a new datum point that, in concert with the tooth record, indicates that atoposaurids were fairly geographically widespread during the very earliest part of their recorded history. This may seem at odds with the limited fossil record of this group during this time, but their small size may explain their rarity. This should stimulate more detailed examination of the biogeography and place of origin of Atoposauridae . It is always tempting to hypothesize that a group originated where its earliest fossils were found, but the patchy Middle Jurassic record of atoposaurids renders such speculation unwise at this point. In the future, however, cladistic biogeographical analysis could be applied to a species-level phylogeny of Atoposauridae to better understand their distribution and biogeographical evolution over time.

Here we have identified numerous dentary and dentition characteristics that vary in Theriosuchus , and possibly more broadly amongst atoposaurids. Coupled with their distinct heterodonty, these morphological differences may indicate that dietary specialization, or character displacement relating to prey choice, was a major driver of early atoposaurid evolution. This variation is also likely to be phylogenetically informative, and may help to elucidate the poorly understood evolutionary relationships within Atoposauridae . To our knowledge, the variation in dentition and dentary morphologies described herein has never been included in a comprehensive analysis of atoposaurid relationships. Conducting a phylogenetic analysis is out of the scope of this paper, although it is in progress by J. P. T., along with a systematic reevaluation of the other Theriosuchus species. The variable dentary and dentition characters that we have outlined here will be incorporated into this analysis.