Isisfordia molnari sp. nov., a new basal eusuchian from the mid-Cretaceous of Lightning Ridge, Australia.

Abstract

The Australian Mesozoic crocodyliform record is sparse in comparison to other Gondwanan localities. A single formally-named taxon is known from this interval; Isisfordia duncani (Winton Formation, Albian–Turonian, Queensland). We present a previously undescribed crocodyliform braincase from the Griman Creek Formation (Cenomanian), New South Wales, which we assign to Isisfordia molnari sp. nov. Assignment to the genus is based on the possession of a newly-defined autapomorphy of Isisfordia: a broadly exposed prootic within the supratemporal foramen. A second autapomorphy of I. duncani (maximum diameter of the caudal aperture of the cranioquadrate siphonium approximately one-third the mediolateral width of the foramen magnum, with the lateral wall of the caudal aperture formed exclusively by the quadrate) may also be present in I. molnari; however, definitive recognition of this feature is marred by incomplete preservation. The new taxon is differentiated from I. duncani based on the absence of a median ridge on the parietal, and the lack of characteristic ridges on the parietal that form the medial margin of the supratemporal foramina. Reanalysis of a second specimen (the former holotype of the nomen dubium,‘Crocodylus (Bottosaurus) selaslophensis’) allows for its referral to the genus Isisfordia. Crucial to this reappraisal is the reinterpretation of the specimen as a partial maxilla, not the dentary as previously thought. This maxillary fragment possesses specific characteristics shared only with I. duncani; namely an alveolar groove. However, several key features differentiate the maxillary fragment from I. duncani, specifically the presence of continuous alveolar septa, the thickening of the medial alveolar rim, and the alveolar and crown base morphology. These findings constitute the first evidence of Isisfordia outside of the type locality and indicate its widespread occurrence on the freshwater floodplains along the eastern margin of the epeiric Eromanga Sea during the Albian–Cenomanian.