News on the Origin of Dinosaurs: Megapnosaurus and Zupaysaurus

Coelophysis bauri and the former Syntarsus rhodesiensis
(= Coelophysis rhodesiensis, Coelophysis kayentakatae, Megapnosaurus kayentakatae)
 have been considered congeneric. Unfortunately, the large reptile tree (subset Fig. 1) did not recover that relationship based on data below.

Figure 6. Proto-dinosaurs including Zupaysaurus and Megapnosaurus.

Figure 1. Proto-dinosaurs including Zupaysaurus and Megapnosaurus. They do not nest with the theropod dinosaur Coelophysis.

According to Wikipedia
on the topic of its name, “It was formerly called Syntarsus, but that name was already taken by a beetle, and was subsequently given the name Megapnosaurus by Ivie, Ślipiński & Węgrzynowicz, in 2001, though many subsequent studies have classified it in the genus Coelophysis.”

According to Wikipedia
on the topic of Coelophysis rhodesiensis, “Syntarsus” rhodesiensis was first described by Raath (1969) and assigned to Podokesauridae.[6] The taxon “Podokesauridae”, was abandoned since its type specimen was destroyed in a fire and can no longer be compared to new finds. Over the years paleontologists assigned this genus to Ceratosauridae (Welles, 1984), Procompsognathidae (Parrish and Carpenter, 1986) and Ceratosauria (Gauthier, 1986). Most recently, is has been assigned to Coelophysidae by Tykoski and Rowe (2004), Ezcurra and Novas (2007) and Ezcurra (2007), which is the current scientific consensus.”

Figure 1. The palates of Coelophyis and Megapnosaurus (illustrated by Cope 1989) together with the palates of Lewisuchus and Pseudhesperosuchus in phylogenetic order based on the large reptile tree. Note the gradual evolution of the elements here and the certainty that Megapnosaurus is not congeneric with Coelophysis. The palates of Lewisuchus and Pseudhesperosuchus are evidently only partially preserved. These line drawings are the only data currently available here. 

Figure 2. The palates of Coelophyis and Megapnosaurus (illustrated by Cope 1989) together with the palates of Lewisuchus and Pseudhesperosuchus in phylogenetic order based on the large reptile tree. Note the gradual evolution of the elements here and the certainty that Megapnosaurus is not congeneric with Coelophysis. The palates of Lewisuchus and Pseudhesperosuchus are evidently only partially preserved. These line drawings are the only data currently available here.

Colbert (1989) illustrated the two palates together
of Coelophysis and Megapnosaurus (Fig. 2) and they sure do not look alike. I wondered about this discrepancy in two supposedly close sister taxa. I finally found a solution when I added Megapnosaurus and Zupaysaurus to the large reptile tree. They both nested between the basal proto-dinosaur Lewisuchus and the clade of basal pro to-dinosaurs that includes Pseudhesperosuchus.

Note in the palates gradual evolution in
the coming together and fusion of the vomers, the development of the anterior and posterior embayments of the palatine, the enlargement of the cheek articulation of the ectopterygoid and the anterior angle of the transverse process of the pterygoid, among other evolutionary advances here, along with individual variations.

Megapnosaurus_skull588

Figure 3. Megapnosaurus kayentakatae does not nest with Coelophysis, but with Zupaysaurus, between Lewisuchus and the proto-dinosaurs in the Pseudhesperosuchus clade.

Megapnosaurus rhodesiensis (kayentakatae) (Raath 1969, renamed by Ivie, Ślipiński & Węgrzynowicz, in 2001, Fig. 3, early Jurassic, 3m length) 30 individuals were found in a fossil bed. According to Wikipedia, “Over the years paleontologists assigned this genus to Ceratosauridae (Welles, 1984), Procompsognathidae (Parrish and Carpenter, 1986) and Ceratosauria (Gauthier, 1986). Most recently, is has been assigned to Coelophysidae by Tykoski and Rowe (2004), Ezcurra and Novas (2007) and Ezcurra (2007), which is the current scientific consensus.”

This is what happens with taxon exclusion.

Figure 3. Zupaysaurus nests with Megapsnosaurus as a proto-dinosaur.

Figure 4 Zupaysaurus nests with Megapsnosaurus as a proto-dinosaur.

Zupaysaurus rougirez (Arcucci and Coria 2003, Latest Triassic to Earliest Jurassic,  PULR-076, up to 4m long, 45cm skull length, Fig. 4) nests as a late-surviving pre-dinosaur (dinosauromorph) with Megapnosaurus between Lewisuchus and the Pseudhesperosuchus clade. Everyone else considers it a theropod, but my guess is they have not tested against these candidate taxa. Traditional paleontologists are still stuck on Lagerpeton. Here are the verified dinosaur precursors to scale (Fig. 5).

Figure 2. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

Figure 5. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Trialestes, Herrerasaurus, Tawa and Eoraptor.

And here’s the skull of Coelophysis for comparison (Fig. 6).

Figure 4. Coelophysis skull for comparison.

Figure 6. Coelophysis skull for comparison.

Just found this, so this was added to the post less than 24 hours later.
Down 2000 compared Coelophysis bauri with Syntarsus rhodesiensis and concluded that both were “remarkably similar morphologically.” He reported, “Although I have not been able to personally study fossils of Syntarsus rhodesiensis, I am confidant that, except for possible mis-articulation of the skull roof, the drawings prepared for Raath’s dissertation accurately represent this taxon. Raath’s illustrations, in almost every case, depict the Ghost Ranch theropod Coelophysis bauri more exactly than do the drawings and descriptions in Colbert’s (1989) monograph (which I glean data from). C. bauri and S. rhodesiensis…differ only in minor details such as neck length, proximal and distal hind limb proportions and proximal caudal anatomy. These differences do not justify a generic separation. The genus Coelophysis has priority over the genus Syntarsus.”

Take a look for yourself
at figures 3 and 6. They appear to be distinct to my eye. What do you think? Note that Downs does not mention the distinct palate. I’d like to see more postcranial data on Megapnosaurus. It’s out there. If you have it, send it. A shorter neck than in Coelophysis is to be expected if Megapnosaurus is a sister to Lewisuchus. There is no indication that Downs compared his taxa to Lewisuchus or the Pseudhesperosuchus clade, nor is there any indication of phylogenetic analysis.

References
Arcucci AB and Rodolfo AC 2003. A new Triassic carnivorous dinosaur from Argentina. Ameghiniana 40(2):217-228.
Cope ED 1889. On a new genus of Triassic Dinosauria. American Naturalist 23: 626
Late Triassic Norian.
Colbert E 1989. The Triassic Dinosaur Coelophysis. Museum of Northern Arizona Bulletin 57: 160.
Downs A 2000. Coelophysis bauri and Syntarsus rhodesiensis compared, with comments on the preparation and preservation of fossils from the Ghost Ranch Coelophysis quarry. In:Lucas, S.G.; Heckert, A.B. (eds.). “Dinosaurs of New Mexico”. New Mexico Museum of Natural History Bulletin 17: 33–37.
Raath  MA 1969. A new Coelurosaurian dinosaur from the Forest Sandstone of Rhodesia. Arnoldia Rhodesia. 4 (28): 1-25.
Raath MA 1977. The Anatomy of the Triassic Theropod Syntarsus rhodesiensis (Saurischia: Podokesauridae) and a Consideration of Its Biology. Department of Zoology and Entomology, Rhodes University, Salisbury, Rhodesia 1-233.
Rowe T 1989. A new species of the theropod dinosaur Syntarsus from the Early Jurassic Kayenta Formation of Arizona. Journal of Vertebrate Paleontology. 9, 125-136.

wiki/Zupaysaurus
wiki/Coelophysis_rhodesiensis

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2 thoughts on “News on the Origin of Dinosaurs: Megapnosaurus and Zupaysaurus

  1. I see you’ve published on this problem, Rob. Can you provide the correct lateral view for Colbert’s
    ‘Syntarsus’ palate? Several specimens appear to be intermixed here and I don’t want to create a chimaera of data.

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