Obscure Dinosaur of the Week: Nqwebasaurus thwazi

Name: Nqwebasaurus thwazi
Etymology: From the Xhosa 'Nqweba' (the name of the Kirkwood region in which it was discovered), and the Greek 'sauros' (lizard); and from the Xhosa 'thwazi' (fast running)
Distribution: Early Cretaceous (Berriasian-Valanginian) of South Africa
Type Specimen: Fragmentary skull and nearly complete skeleton
Estimated size: 0.9 m in length (juvenile)
First described by: de Klerk et al., 2000

Image from de Klerk et al., (2000)

Interesting fact: Gastronomic gastroliths

Smooth stones were found in the abdominal region of this specimen of this ground dwelling carnivorous coelurosaur, Nqwebasaurus thwazi, stones that are not common to the geological formation in which the fossil was preserved. So how did they get there? de Klerk et al. (2000) thought these stones were most likely gastroliths: swallowed by animals to aid in breaking down food in the stomach. 

Modern day herbivorous avian dinosaurs (birds) use gastroliths to help in the digestion of plant matter, and aquatic carnivorous animals such as crocodiles, alligators, seals, and sea lions use them  for ballast or stability. However, some doubt still surrounds these theories for gastrolith use, especially as to the true function of gastroliths in non-avian dinosaurs (see the UCMP Berkeley page on gastroliths for an overall summary). Wings (2007, pp 1) provides an even more detailed review of gastrolith use, and lists the following possible reasons for swallowing rocks:

... mineral supply and storage, stomach cleaning, maintenance of a beneficial microbial gut flora, destruction of parasites and alleviation of hunger. Accidental ingestion of sediment, either by being mistaken for prey, by being attached to it, during playing or due to pathological behaviour, is considered to be common...

If the rounded stones found in and around N. thwazi really are gastroliths, they are a rare find, having only been documented in two other theropod dinosaurs: Baryonyx (Charig et al., 1986) and Caudipteryx (Ji et al., 1998).

Photograph and diagram of the N. thwazi holotype fossil. The dark oval objects seen in the diagram are gastroliths. Image from de Klerk et al., (2000)

This little guy is also import for two other reasons. First, the occurence of N. thwazi in Early Cretaceous strata pre-dates the known North American taxa by approximately 50 million years (de Klerk et al., 2000). And second, N. thwazi is the oldest Gondwanan coelurosaurian (de Klerk et al., 2000). The authors cite paucity of fossil material in southern Africa as the reason for misinterpretations concerning faunal radiations: whether this clade originated in Laurasia or Gondwana. Lets hope that more discoveries like this help clear things up!

Simplified cladogram of all non-avian dinosaurs. N. thwazi was a coelurosaur (possibly a compsognathid), within Theropoda

References
Charig, A. J., Milner, A. C. 1986. Baryonyx, a remarkable new theropod dinosaur. Nature 324: 359-361.

de Klerk, W. J., Forster, C. A., Sampson, S. D., Chinsamy, A., Ross, C. F. 2000. A new coelurosaurian dinosaur from the Early Cretaceous of South Africa. Journal of Vertebrate Paleontology 20 (2): 324-332

Ji, Q., Currie, P.J., Norell, M.A., Ji, S. (1998). Two feathered dinosaurs from northeastern China. Nature 393 (6687): 753–761

Wings, O. 2007. A review of gastrolith function with implications for fossil vertebrates and a revised classification. Acta Palaeontologica Polonica 52 (1): 1–16