Silesaurs as primitive ornithischians

[DD1991]

A new paper is out online that you'll find shocking:

 

Müller RT, Garcia MS. 2020. A paraphyletic ‘Silesauridae’ as an alternative hypothesis for the initial radiation of ornithischian dinosaurs. Biol. Lett.16:20200417. http://dx.doi.org/10.1098/rsbl.2020.0417

 

I remember that several Late Triassic animals erected upon teeth (Crosbysaurus, Galtonia, Krzyzanowskisaurus, Lucianosaurus, Pekinosaurus, Protecovasaurus, Revueltosaurus, and Tecovasaurus) were once classified within Ornithischia because the type teeth of those taxa are similar to those of ornithischians (some early sauropodomorphs have leaf-shaped teeth, too), but Revueltosaurus was later reclassified as an extinct relative of alligators and crocodiles based on complete material, and Galtonia and Pekinosaurus were also recognized as suchians closely related to Revueltosaurus, meaning that other putative Triassic ornithischian taxa from the American Southwest were placed in Archosauriformes incertae sedis by Parker et al. (2005) and Irmis et al. (2007) due to the presence of triangular leaf-shaped teeth being convergent among ornithischian dinosaurs and some non-dinosaurian clades. The hyper-sparse record of Triassic Ornithischia left people scratching their heads to explain the paucity of Triassic ornithischian fossils, with some speculating that ornithischians did not diversify until the Early Jurassic; the fact that Pisanosaurus combines some craniodental traits of Ornithischia more advanced than those of Lesothosaurus and the postcranial traits of basal dinosauriforms complicated matters further, because this mosaic of morphological features threatened the status of Pisanosaurus as the oldest ornithischian. Agnolin and Rozadilla (2018) tidied up matters by concluding that Pisanosaurus is a silesaurid that evolves craniodental features convergent with those of advanced ornithischians.

 

The new paper by Muller and Garcia (2020) has a pretty novel hypothesis to explain the virtually non-existence of Ornithischia in Triassic deposits. It shockingly recovers Silesauridae as paraphyletic with respect to traditional Ornithischia (containing Genasauria and Eocursor), suggesting that the earliest relatives of Ornithischia evolved a purely faunivorous diet, given that Asilisaurus from Tanzania has sub-triangular crowns and a constricted root and dentaries with a beak-like anterior tip. For instance, Lewisuchus is recovered a more primitive than Soumyasaurus and Asilisaurus. Since Pisanosaurus has a mosaic of ornithischian-like features and postcranial traits usually seen in non-dinosaurian dinosauromorphs, it may be surmised that the earliest dinosaurs had some postcranial features similar to those of lagerpetids and Lagosuchus. Of interesting note is the fact that Technosaurus (named for Texas Tech University where the holotype is stored) was once considered a 'fabrosaurid' ornitischian before it was classified as a silesaurid, so the placement of all silesaurs as closely related to Ornithischia and recovery of Pisanosaurus as sister to Ornithischia means that the peculiar morphology of Pisanosaurus renders silesaurs more closely related to ornithischians.

 

Randall B. Irmis, William G. Parker, Sterling J. Nesbitt & Jun Liu (2007) Early ornithischian dinosaurs: the Triassic record. Historical Biology, 19:1, 3-22, DOI: 10.1080/08912960600719988

 

Parker, W. G., Irmis, R. B., Nesbitt, S. J., Martz, J. W., & Browne, L. S. (2005). The Late Triassic pseudosuchian Revueltosaurus callenderi and its implications for the diversity of early ornithischian dinosaurs. Proceedings. Biological sciences, 272(1566): 963–969. https://doi.org/10.1098/rspb.2004.3047