The skull of Sclerocormus reinterpreted.

Figure 1. Large Sclerocormus and its much smaller sister, Cartorhynchus. These nest with basal sauropterygians, not ichthyosauriforms.

Figure 1. Large Sclerocormus and its much smaller sister, Cartorhynchus. These nest with basal sauropterygians, not ichthyosauriforms. The odd thing about this genus is really the short neck, not the small head.

Yesterday we looked at the new basal sauropterygian with a tiny head, Sclerocormus (Figs. 1, 2). Originally Jiang et al. 2016 considered Sclerocormus ‘a large aberrant stem ichthyosauriform,’ but their cladogram did not have the stem ichthyosauriforms recovered by the 684-taxa reptile tree, Wumengosaurus, Thaisaurus and Xinminosaurus.

Basal sauropterygians often have a tiny skull. 
Check out these examples: Pachypleurosaurus, Keichousaurus, Plesiosaurus, Albertonectes. Given this pattern, the odd thing about Sclerocormus is its short neck, not its tiny skull. The outgroup, Qianxisaurus has a skull about equal to the cervical series.

As noted previously
the terms ‘aberrant’ or ‘engimatic’ usually translate into “somewhere along the way we made a huge mistake, but don’t know what to do about it.” For the same reason, pterosaurs are widely considered ‘aberrant’ archosaurs, Vancleavea is an ‘aberrant’ archosauriform, Daemonosaurus and Chilesaurus are aberrant theropods and caseasaurs are ‘aberrant’ synapsids. All of these taxa also nest elsewhere in the large reptile tree.

Moreover
several of the Jiang et al interpretations of the skull could not by confirmed by DGS tracings (Fig. 2). Others were just fine.

Figure 2. Sclerocormus skull as originally interpreted and reinterpreted here.

Figure 2. Sclerocormus skull as originally interpreted and reinterpreted here.

Reinterpretations

  1. Jiang et al. nasals  >  nasals + premaxillae
  2. Jiang et al. premaxilla (lower portion)   >  anterior maxilla
  3. Jiang et al. premaxilla (upper portion)  >   left dentary
  4. Jiang et al. missed the right dentary and all teeth
  5. Jiang et al. missed the occipitals (postparietals, tabulars, supra occipital)
  6. Jiang et al. maxilla   >   lacrimal
  7. Jiange et al. scapula    >  coracoid + scapula
  8. Jiang et al. mandible elements? are confirmed as actual mandible elements
  9. Jiang et al. left postfrontal   >   postorbital
  10. Jiang et al. left squamosal and postfrontal   >  left posterior mandible elements

Phylogenetically
here are the stem ichthyosaurs and a sampling if ichthyosaurs (Fig. 3). Note where hupehsuchids nest, as derived utatsusaurs and shastasaurs. Cartorhynchus and Sclerocormus (Fig. 1) do not nest here.

Figure 2. Subset of the large reptile tree focusing on ichthyosaurs. Note most of the more derived ichthyosaurs from Marek et al. 2015, are not listed here. So we're not comparing apples to apples here.

Figure 2. Subset of the large reptile tree focusing on ichthyosaurs. Note most of the more derived ichthyosaurs from Marek et al. 2015, are not listed here. So we’re not comparing apples to apples here.

References
Jiang D-Y, Motani R, Huang J-D, Tintori A, Hu Y-C, Rieppel O, Fraser NC, Ji C, Kelley NP, Fu W-L and Zhang R 2016. A large aberrant stem ichthyosauriform indicating early rise and demise of ichthyosauromorphs in the wake of the end-Permian extinction. Nature Scientific Reports online here.

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