Sunday, April 17, 2011

Daemonosaurus chauliodus

Daemonosaurus chauliodus

The Evil Spirit lizard haunts theropod phylogeny

Meet Daemonosaurus chauliodus, a newly discovered theropod dinosaur from the Chinle Formation of the “Coelophysis Quarry” at Ghost Ranch in New Mexico. Its description by Hans-Dieter Sues, Sterling J. Nesbitt, David S.  Berman and Amy C. Henrici, has just been published in the Proceedings of the Royal Society B (Sues et al., 2011).

Fig 1.- A reconstruction of the head of Daemonosaurus. The fur is speculative.

The media has readily dubbed this little critter as the “Bucked-toothed Demon Lizard” but there is actually nothing devilish about the look of the 
animal. The intended meaning of the generic name is in fact a bit different: the authors used the Greek word daimon in its original sense of ‘evil spirit’ and allude to legends about evil spirits haunting Ghost Ranch, the place of discovery. The specific name, derived from the Greek chauliodous and which means ‘with prominent teeth’ is shared with the scientific name of the deep-sea viperfish (Chauliodus sloani). Like the viperfish, Daemonosaurus is indeed characterized by a set of much enlarged fang-like front teeth. Such teeth might be an indication of a specialized diet, but at this point, we don’t have a clue of what it was.

The prominent teeth are not the only things remarkable about this critter. Daemonosaurus is also one of the most primitive theropod which makes him all the more important as the early history of this particular group of dinosaurs is somewhat sketchy. We know that by the Late Triassic Carnian age (216.5-228 MYA) , the three principal lineages of dinosaurs, i.e. the theropods, the sauropodomorphs and the ornithischians were already well differentiated. A sample of the earliest theropods, i.e. Herrerasaurus, Staurikosaurus, Eoraptor and the recently described Eodromaeus (Martinez et al., 2011) and Sanjuansaurus (Alcober et al., 2010) were found in South America in the Ischigualasto Formation of Argentina and the Santa Maria Formation of Brazil. These primitive theropods still retained five digits on both their hands and feet (albeit greatly reduced fifth digits), while the Neotheropoda, a group that contains all the most advanced theropods including Tyrannosaurus, Velociraptor and the birds, had only three digits in their hands (with sometimes a vestigial fourth as in Coelophysis; in T. rex the 3rd digit is also vestigial), and three functional digits per feet (the fifth is vestigial and the first, is reduced to a “dew claw” on the inside or back of the foot).

Until recently, the later Norian and Rhaetian stages of the Late Triassic (199.6 to 216.5 MYA) have only yielded Neotheropods belonging to a single group called Coelophysoidea (some examples include Coelophysis of WWD fame, Segisaurus, Lilensternus and most probably Zupaysaurus), with the sole exception of the enigmatic Chindesaurus from Arizona which is difficult to classify due to the fragmentary nature of its remains (Nesbitt et al., 2007) but which is now generally considered a late survivor of the Herrerasaurus tribe. The evolutionary gap between the early theropods and the neotheropods started to be filled a couple of years ago with the description of a new species, Tawa hallae (Nesbitt et al., 2009), also from the Ghost Ranch locality of New Mexico and that has been shown to be intermediate between the herrerasaurs and the neotheropods. And this year, we have Daemonosaurus.

Fig 2.- Reconstructed skulls of Daemonosaurus chauliodus (top) and Tawa hallae (bottom). Based after Nesbitt et al., 2009 & Sues et al., 2010. Note the shorter snout and proportionally longer front teeth and larger orbital fenestrae of Daemonosaurus.

Daemonosaurus appears to be even more primitive than Tawa. The holotype consists of a crushed skull, vertebrae and ribs, found in the so-called Coelophysis quarry of Rhaetian age (Stewart et al., 1972). The fossil was indeed extracted from a large block containing remains of several Coelophysis bauri, and there is a chance that more postcranial remains of Daemonosaurus will turn up after completion of the preparation of the block. With its short and deep skull equipped with pointy teeth, Daemonosaurus look is surprising, unlike any of the other basal theropods. The structure of the cervical vertebrae indicates that it probably had a long neck like Tawa or Coelophysis. Like most of the other theropods of the Late Triassic, it was pretty small, measuring perhaps no more than 1.5 meters.

It now appears that the theropod fauna of the Late Triassic was far more diverse than originally thought. Due to its unusual skull shape and teeth, Daemonosaurus was however probably not a transitional form to the new theropods but an evolutionary dead-end that left no descendant.

References:

Alcober, O. A.; and Martinez, R. N. 2010. A new herrerasaurid (Dinosauria, Saurischia) from the Upper Triassic Ischigualasto Formation of northwestern Argentina. ZooKeys 63: 55–81.

Martinez, R. N., Sereno, P. C., Alcober, O. A., Colombi, C. E., Renne, P. R., Montan˜ ez, I. P. & Currie, B. S. 2011. A basal dinosaur from the dawn of the dinosaur era in southwestern Pangaea. Science 331, 206–210.


Nesbitt, S. J., Irmis, R. B. & Parker,W. G. 2007 A critical re-evaluation of the Late Triassic dinosaur taxa of North America. J. Syst. Palaeontol. 5, 209–243.


Nesbitt, S. J., Smith, N. D., Irmis, R. B., Turner, A. H., Downs, A. & Norell, M. A. 2009 A complete skeleton of a Late Triassic saurischian and the early evolution of dinosaurs. Science 326, 1530–1533.


Stewart, J. H., Poole, F. G. & Wilson, R. F. 1972 Stratigraphy and origin of the Chinle Formation and related Upper Triassic strata in the Colorado Plateau region. US Geol. Surv. Prof. Pap. 690, 1–336.


Sues, H.-D.; Nesbitt, S.J.; Berman, D.S.; and Henrici, A.C. 2011. A late-surviving basal theropod dinosaur from the latest Triassic of North America. Proceedings of the Royal Society B in press

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