Problems in the reevaluation of Caseosaurus

Baron and Williams 2018
bring us a new evaluation of a Late Triassic ilium considered to be an enigmatic (because nothing more than an illiim is known) dinosauriform, Caseosaurus crosbyensis.

From their abstract
“Historically, Caseosaurus crosbyensis has been considered to represent an early saurischian dinosaur, and often a herrerasaur. More recent work on Triassic dinosaurs has cast doubt over its supposed dinosaurian affinities and uncertainty about particular features in the holotype and only known specimen has led to the species being regarded as a dinosauriform of indeterminate position. Here, we present a new diagnosis for Caseosaurus crosbyensis and refer additional material to the taxon—a partial right ilium from Snyder Quarry. Our comparisons and phylogenetic analyses suggest that Caseosaurus crosbyensis belongs in a clade with herrerasaurs and that this clade is the sister taxon of Dinosauria, rather than positioned within it.”

Unfortunately
in order to determine which taxa are in and out of the Dinosauria and the Archosauria, you have to include basal bipedal crocodylomorphs, the outgroup for the Dinosauria in the large reptile tree (LRT, 1176 taxa). Baron and Williams omit this vital clade and so come up short in their evaluation. Baron recently made waves when he united Ornithischia with Theropoda to the exclusion of Sauropodomorpha. That error was also due to taxon exclusion, covered earlier here and here.

Worse yet, the authors report, 
“In addition, our analysis recovers the enigmatic European taxon Saltopus elginensis among herrerasaurs for the first time.”

Hopefully that will be the first and only time.
In the LRT, Saltopus nests with Scleromochlus and Gracilisuchus as basal crocodylomorphs.” Again, this is due to taxon exclusion.

Worst yet, the authors report,
Dimorphodon macronyx was included as an additional outgroup taxon, following its use in the study by Baron et al. (2017a).”

Hopefully this will be the last time
pterosaurs are used in dinosaur phylogenetic analysis. They are not related to each other.

Building on these fails, the authors continue:
“If this hypothesis is correct then this clade of herrerasaurs also represents the first clade of non-dinosaurian dinosauromorphs known to contain large-bodied carnivorous species.”

This hypothesis is not correct.
In the LRT, Herrerasaurus and kin nest as the last common ancestor of all dinosaurs, and so, by definition, Herrerasaurus and kin are dinosaurs, basal carnivorous dinosaurs. Based on the proximity of basal bipedal crocs to dinosaurs, there are no known non-dinosaur dinosauromorphs at this time. The Dinosauromorpha is a junior synonym for Archosauria and should be dropped from usage.

And now, the big reveal:
We only know the ilium from Caseosaurus, the holotype UMMP 8870 and the referred material NMMNH P-35995. Are they conspecific? No (Fig 1). Are they congeneric? No.

Figure 1. GIF animation comparing the holotype pelvis of Caseosaurus (big and red) to the referred material (small and blue). Perhaps you are asking yourself, what were these authors thinking? The two pelves are not even congeneric.

Figure 1. GIF animation comparing the holotype pelvis of Caseosaurus (big and red) to the referred material (small and blue). Perhaps you are asking yourself, what were these authors thinking? The two pelves are not even congeneric.

Presently in the LRT
the PVL 4597 specimen (Fig. 2, wrongly attributed to Gracilisuchus) nests as the last common ancestor of dinos and crocs. That’s what the ilium looks like compared to the basalmost dino, Herrerasaurus (Fig. 2).

Figure 1. The PVL 4597 specimen attributed to Gracilisuchus by Lecuona et al. 2017, but nesting at the base of the Dinosauria in the LRT.

Figure 1. The PVL 4597 specimen attributed to Gracilisuchus by Lecuona et al. 2017, but nesting at the base of the Dinosauria in the LRT.

According to Wikipedia
Langer (2004) examined the ilium and reassigned it to the genus Chindesaurus, which lived during the same period and geological region.

References
Baron MG and Williams ME 2018. A re-evaluation of the enigmatic dinosauriform Caseosaurus crosbyensis from the Late Triassic of Texas, USA and its implications for early dinosaur evolution. Acta Palaeontologica Polonica 63(1): 129–145.
Langer M 2004. Basal Saurischia. In Weishampel, Dodson and Osmolska. The Dinosauria Second Edition. University of California Press. 861 pp.

https://en.wikipedia.org/wiki/Caseosaurus

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