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; <i>Isisfordia duncani</i> (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 <i>Isisfordia molnari</i> sp. nov. Assignment to the genus is based on the possession of a newly-defined autapomorphy of <i>Isisfordia</i>: a broadly exposed prootic within the supratemporal foramen. A second autapomorphy of <i>I. duncani</i> (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>I. molnari</i>; however, definitive recognition of this feature is marred by incomplete preservation. The new taxon is differentiated from <i>I. duncani</i> 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,‘<i>Crocodylus (Bottosaurus) selaslophensis</i>’) 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>I. duncani</i>; namely an alveolar groove. However, several key features differentiate the maxillary fragment from <i>I. duncani</i>, 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 <i>Isisfordia</i> 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.