Updated March 12, 2015 with a revised mandible for Shuvosaurus.
The Extreme Strangeness of Effigia
Effigia okeeffeae (Nesbitt and Norell, 2006) Carnian, Late Triassic, ~210 mya, ~ 2 m in length, was originally considered an early theropod dinosaur by Colbert, who collected the specimen in the late 1940s but never removed it from its jacket.
A recent reassessment by Nesbitt and Norell (2006) and Nesbitt (2007) nested Effigia among the poposaurid rauisuchians based largely on the ankle, but they noted “extreme convergence in the body plans” with ornithomimid dinosaurs. They reported that the ankle of Effigia articulated in a crocodile-normal configuration, with a morphology similar to Alligator (Figure 1). The broken and missing calcaneal “heel”would have turned proximally, like that in a sister taxon, Shuvosaurus.
The Calcaneal Tuber and its Distribution
Most paleontologists assert that the calcaneal “heel” is found only in rauisuchians + crocodylians (= pseudosuchians) not dinosaurs and their kin. Without the present expanded inclusion list, prior workers were not aware of the new clade, the Paraornithischia, that nested Effigia as a sister taxon to the phytodinosauria based on more parsimoniously shared traits from head to toe. The “extreme convergence” with restricted to the ankle.
I have never seen the skull of Effigia, only published photos. Even so, it appears that the original reconstruction by Nesbitt (2007) contains certain errors and oversimplifications that I repaired and reidentified. The DGS (Digital Graphic Segregation) method using Adobe Photoshop enabled a test of the original reconstruction and not all the original results could be verified.
Chief among the problems in the Nesbitt and Norell (2006) reconstruction is the identification of the long bone over the mandibular fenestra as the surangular. This arose from the identification of the mandibular tip as the maxilla. Perhaps the authors did not realize that in sister taxa the anterior bone is a predentary or a pseudopredentary (if not homologous with the predentary in ornithischians). The temporal muscles for closing the jaw were also attached to the dentary in all tetrapods, which cannot happen when the dentary is restricted to the jaw tip. Here the toothless dentary is in its standard place and a predentary precedes it at the jaw tip.
A Descending Cranium
In Effigia the posterior skull descends, but that was “fixed” in the reconstruction of Nesbitt and Norell (2006). Here (Fig. 2) what you see is what you get.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
Nesbitt SJ and Norell MA 2006. Extreme convergence in the body plans of an early suchian (Archosauria) and ornithomimid dinosaurs (Theropoda). Proceedings of the Royal Society B 273:1045–1048. online
Nesbitt S 2007. The anatomy of Effigia okeeffeae (Archosauria, Suchia), theropod-like convergence, and the distribution of related taxa. Bulletin of the American Museum of Natural History, 302: 84 pp. online pdf