Reinterpreting the Skull of Effigia Using DGS

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.

“Extreme Convergence”
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 pedes of Alligator and Effigia

Figure 1. The pedes of Alligator (left) and Effigia (right) demonstrating the convergence of the structure of the ankle bones (astragalus and calcaneum). In basal archosaurs the ankle is a simple hinge, but in Alligator the hinge takes a sharp turn between the astragalus and calcaneum. The astragalus and calcaneum of Effigia articulate in a crocodile-normal configuration and their morphology is similar to Alligator. Note the peg of the astragalus inserts into and rotates on the calcaneum. There are also axes of rotation at the distal tibia/fibula and at the (evidently missing) distal tarsals. 

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.

Figure 2. Comparing the DGS method of skull reconstruction to traditional methods. Above: The skull of Shuvosaurus, a sister taxon. Next: Effigia traced from a photo published in Norell and Nesbitt 2006. Note the surangular extends over the mandibular fenestra, as in other dinosaurs and a predentary is present as in other poposaurids. Next: Original tracing of Effigia skull. Bottom: Original restoration of Effigia skull from Norell and Nesbitt (2004). Note the anterior extent of the surangular.

Figure 2. Comparing the DGS method of skull reconstruction to traditional methods. Above: The skull of Shuvosaurus, a sister taxon. Next: Effigia traced from a photo published in Norell and Nesbitt 2006. Note the surangular extends over the mandibular fenestra, as in other dinosaurs and a predentary is present as in other poposaurids. Next: Original tracing of Effigia skull. Bottom: Original restoration of Effigia skull from Norell and Nesbitt (2004). Note the anterior extent of the surangular.

DGS
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.

Surangular/Dentar/Premaxilla
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.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
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

AMNH Effigia webpage
wiki/Effigia

Advertisements

2 thoughts on “Reinterpreting the Skull of Effigia Using DGS

  1. Pingback: Dgs | TagHall

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s