Herrerasaurus coracoid: maybe it is not your typical dinosaurian coracoid

Figure 1. Herrerasaurus and Sanjuansaurus scapulocoracoids. The coracoid was largely restored by Sereno 1993, but maybe its not such a disc (in green). It could be more of a transitional strust/disc (in blue), like Pseudhesperosuchus.

Figure 1. Herrerasaurus and Sanjuansaurus scapulocoracoids. The coracoid was largely restored by Sereno 1993, but maybe its not such a disc (in green). It could be more of a transitional strust/disc (in blue), like Pseudhesperosuchus (Fig. 2). In like fashion, Alcober and Martinez 2010 added plaster to their Sanjuansaurus coracoids, both of them. Take away most of the plaster and things become more interesting. Click to enlarge.

As all paleontologists know, crocs have a strut-like coracoid and dinos have a disc-like coracoid (untill it is modified back into a strut in flapping pre-birds). Herrerasaurus and Sanjuansaurus (Fig. 1) are very basal dinosaurs. In 1993 Sereno illustrated what was known of the Herrerasaurus scapulocoracoid. The incomplete coracoid was restored in typical dinosaur fashion as a disc, but maybe it wasn’t quite there yet. Alcober and Martinez 2010 did the same with their Sanjuansaurus, to both of their coracoids.

Maybe all that plaster wasn’t necessary.

Figure 2. Pseudhesperosuchus in various views. Note the scapulocoracoid. This taxon is a proximal outgroup to the Dinosauria and Herrerasaurus (Fig. 1).

Figure 2. Pseudhesperosuchus in various views. Note the scapulocoracoid. This taxon is a proximal outgroup to the Dinosauria and Herrerasaurus (Fig. 1).

One of the proximal outgroups
to dinosaurs includes the clade of Pseudhesperosuchus (Fig. 2) and Junggarsuchus. Both have strut-like coracoids, typical of crocs. So does Trialestes, a taxon even closer to dinos. I haven’t seen that material yet, just going by the Clark and Berman 2000 description.

Put the scapulocoracoid of Pseudhesperosuchus next to those of Herrerasaurus and Sanjuansaurus and you’ll see something on the origin of dinosaurs there. It’s heresy, and I realize that. But it’s worth testing.

Food for thought
Yes, crocs and dinos are distinct from each other (just look at those proximal carpals!), but among the 475 tested taxa, no clade nest closer to dinosaurs than crocs (together, of course, creating the clade Archosauria). Lagerpeton and pterosaurs are not even in the running.

More on the evolution of dinosaurs from basal bipedal crocs coming soon.

References
Clark JM, Sues H-D and Berman DS 2000. A new specimen of Hesperosuchus agilis from the Upper Triassic of New Mexico and the interrelationships of basal crocodylomorph archosaurs. Journal of Vertebrate Paleontology 20(4):683-704.
Novas FE 1994. New information on the systematics and postcranial skeleton of Herrerasaurus ischigualastensis (Theropoda: Herrerasauridae) from the Ischigualasto
Reig OA 1963. La presencia de dinosaurios saurisquios en los “Estratos de Ischigualasto” (Mesotriásico Superior) de las provincias de San Juan y La Rioja (República Argentina). Ameghiniana 3: 3-20.
Sereno PC and Novas FE 1993. The skull and neck of the basal theropod Herrerasaurusischigualastensis. Journal of Vertebrate Paleontology 13: 451-476. doi:10.1080/02724634.1994.10011525.

wiki/Herrerasaurus
wiki/Sanjuansaurus
wiki/Trialestes

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