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Isisfordia molnari
Hart, Bell, Smith & Salisbury. 2019
Reconstruction by José Vitor Silva. |
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.
Systematic Palaeontology
CROCODYLIFORMES Hay, 1930
MESOEUCROCODYLIA Whetstone and Whybrow, 1983
NEOSUCHIA Clark, 1988
EUSUCHIA Huxley, 1875
Genus ISISFORDIA Salisbury, Molnar, Frey and Willis, 2006
Isisfordia duncani Salisbury, Molnar, Frey and Willis, 2006
Holotype: QM F36211 (near complete skeleton, missing the rostral part of the skull).
Referred material: QM F44320 (skull).
Locality, horizon and age: Winton Formation, Albian–Turonian, Queensland.
Diagnosis: Species of Isisfordia with a median ridge on parietal (a); ridges on the parietal forming the medial margin of the supratemporal foramina (a); caudal maxillary tooth crown bases and alveoli ovate (a).
Isisfordia molnari sp. nov.
Holotype: AM F125553 (braincase).
Referred material: AM F15818 (maxillary fragment).
Locality, horizon and age: Both AM F125553 and AM F15818 were recovered from underground opal mines in the Lightning Ridge district, in or around the years 2000 and 1914, respectively. Precise locality data is not available; however, AM F15818 probably derives from an older field near the township, possibly the ‘Three Mile’ field. AM F125553 is likely to be from the one of the Coocoran fields, 30–40 km to the west of Lightning Ridge. Fossil-bearing rocks across the region pertain to the Wallangulla Sandstone Member of the Griman Creek Formation (part of the Surat Basin), which crops out in northern New South Wales and southeastern Queensland (Fig. 1). The Griman Creek Formation at Lightning Ridge was previously considered Albian in age (e.g., Dettman et al., 1992; Smith, 1999; Smith & Kear, 2013), but recent radiometric dating indicates a Cenomanian (96.6–100.2 Ma; Bell et al., 2019) age, slightly younger than the lower Winton Formation at Isisford (see Tucker et al., 2013). The Griman Creek Formation has produced a diverse assemblage of vertebrate fossils (see Bell et al., 2019 for a comprehensive overview) apart from crocodyliforms, including aspidorhynchids (Bell et al., 2019), lamniforms (Bell et al., 2019), dipnoans (Kemp, 1997a; Kemp, 1997b), meiolaniform and chelid testudines (Smith, 2009; Smith, 2010; Smith & Kear, 2013), leptocleidid plesiosaurs (Kear, 2006), anhanguerian pterosaurs (Brougham, Smith & Bell, 2017), ankylosaurians (Bell, Burns & Smith, 2017), small and large bodied ornithopods (Molnar & Galton, 1986; Molnar, 1996; Bell et al., 2018), titanosauriform sauropods (Molnar & Salisbury, 2005), megaraptorid theropods (Bell et al., 2015; Brougham, Smith & Bell, 2019), enantiornithes (Bell et al., 2019) and australosphenid mammals (Archer et al., 1985; Flannery et al., 1995; Clemens, Wilson & Molnar, 2003; Pian et al., 2016; T. Rich in Poropat et al., 2018).
Diagnosis: Species of Isisfordia with a flat dorsal surface of the parietal (a); parietal contribution to medial margin of supratemporal fenestrae flat (does not form raised rim) (a); caudal maxillary alveoli circular and separated by interalveolar septa along entire caudal portion of the maxillary alveolar groove(a).
Etymology: After palaeontologist Ralph Molnar, who has made significant contributions to Australian vertebrate palaeontology, including work on the crocodylomorphs from the Griman Creek Formation.
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Figure 6: Reconstruction of Isisfordia molnari sp. nov. In life mode, swimming next to a wading sauropod.
Reconstruction by José Vitor Silva. |
Conclusions:
Isisfordia molnari sp. nov., from the Griman Creek Formation, is described based on a partial braincase (AM F125553). This specimen is assigned to Isisfordia based on the presence of one unambiguous (and newly-defined) autapomorphy: a broadly exposed prootic within the supratemporal foramen, rostral to the rostral aperture of the posttemporal canal. Differences in the morphology of the parietals and the apparent inclusion of the supraoccipital in the dorsal margin of the foramen magnum of AM F125553 warrant the exclusion of AM F125553 from I. duncani. A maxillary fragment, AM F15818 (formerly the holotype of ‘Crocodylus (Bottosaurus) selaslophensis’), is also referred to this new species. This is based on unique features of the dentition and the presence of an alveolar groove, shared only with Isisfordia duncani from the Lower Cretaceous portion of the Winton Formation. We justify specific separation based on differences in alveolar and tooth crown base morphology, and the presence of interalveolar septa in the caudal part of the maxillary toothrow and a thickening of the medial alveolar wall of AM F15818.
Isisfordia represents the first multispecific Australian Mesozoic archosaur, underscoring Australia’s poor Mesozoic terrestrial vertebrate fossil record. In light of these findings, a reappraisal of all previously-described crocodyliform material from the Griman Creek Formation is now pertinent, although future work will continue to be hampered by the isolated and often fragmentary specimens that are the result of the unique mining setting in which these fossils are found. Nevertheless, we agree with Molnar (1980) that Griman Creek Formation crocodyliform material likely derives from a single taxon (identified here as Isisfordia molnari), providing a working hypothesis on which to base future work.
Lachlan J. Hart, Phil R. Bell, Elizabeth T. Smith and Steven W. Salisbury. 2019.
Isisfordia molnari sp. nov., A New Basal Eusuchian from the mid-Cretaceous of Lightning Ridge, Australia.
PeerJ. 7:e7166. DOI:
10.7717/peerj.7166
