Guest post: dishing the dirt on dwarfing in dinosaurs

A couple of weeks ago a new paper came out of sauropod bone microstructure in sauropods, the latest in a series from the Bonn lab of Martin Sander. The lead author, Koen Stein is yet another ex-Bristolian who is now pursuing a PhD and here takes time out of the traditional student panic to write a bit about the growth of the dwarf dinosaur Magyrosaurus.

The first bones of Magyarosaurus were found by Nopcsa’s sister Ilona in 1895 on their estate in Romania, then part of the Austro-hungarian empire. It was Franz Baron Nopcsa (note the spelling: c before s!) himself to suggets that Magyarosaurus was an island dwarf in Nopcsa 1914. This 1914 report (in German) gives a summary of Nopcsa’s presentation (probably not powerpoint) at the meeting of the Zoological-Botanical Association in Vienna in 1912. Nopcsa contrasted the small size of (then still called) Titanosaurus dacus with the large sauropods from North America (Morisson fauna) and East Africa (Tendaguru material), and compared the sauropod size difference with the small size of the Pleistocene elephants described by Dorothea Bate (Bate 1905). Because these elephants lived on islands in the Mediterranean, Nopcsa concluded that Titanosaurus dacus also must have lived on an island.

In one of his latest publications, Nopcsa used dinosaur bone histology in a revolutionary way to address paleobiological issues, namely establishing ontogenetic stage based on bone microstructure (Nopcsa and Hiedseck, 1933). Although mostly overlooked by subsequent authors, this contribution shows how (as underlined by the author himself) “perhaps for the first time in palaeontological research – histology can be brought to settle a problem where other methods fail.”. By comparing (rib) bone histology of the newly described hadrosaurid Tetragonosaurus from the Late Cretaceous of North America, to that of juvenile and adult specimens of a wide array of fossil and Recent reptiles, he was able to show that it actually is a juvenile of larger “ornamented Trachodontids” (lambeosaurine hadrosaurids). His early death kept him from fully utilizing this powerful paleobiological method to firmly establish the dwarf status of the Hateg dinosaurs he studied by showing that their small size is not age-related, juvenile feature. More info on Nopcsa can be found in (Weishampel and Kerscher, in press).

Von Huene renamed Magyarosaurus in his 1932 monograph on the Saurischia (Huene 1932). He even went so far to name three species (M. dacus, M. hungaricus and M. transylvanicus) based on morphological and size differences. No one has actually reviewed the material to confirm or falsify Von Huene’s hypotheses. Jean Leloeuff suggested therefore that the large bones ascribed to “M.” hungaricus in the collections of the NHM and in the Geological Survey of Hungary, Budapest are actually the adult M. dacus and the large number of small bones being found is a sampling or taphonomical bias (Leloeuff, 2005). However, morphological work by Zoltan Csiki and coauthors is underway, suggesting that at least two species can be distinguished. We were able to confirm this with our histological study.

The Hatzeg Basin fauna is now known for a wide array of Late Cretaceous vertebrate remains. Other dwarfs (Telmatosaurus, Zalmoxes shqiperorum, Struthiosaurus) and non dwarf (Zalmoxes robustus) ornithischian dinosaurs, many theropod remains, eggs, mammals, amphibians, and pterosaurs (some big ones) have been found in Transylvania over the decennia. Mike Benton’s review of the Hatzeg Basin fauna covers many aspects of the fauna including diversity and dwarfing (Benton et al. 2010). I should also mention that it was Mike Benton who suggested studying Magyarosaurus bone histology (thanks again Mike!).

I think that one of the most important issues we adress in the paper, is a general question that should be asked when doing paleontology and comparative anatomy. Some morphological features can change during ontogeny. Therefore it is necessary to compare individuals of the same ontogenetic age (preferably adult) in phylogenetic studies, and to ask oneself, what ontogenetic status does the specimen I am studying have? In many cases it is possible to determine this by looking at sutures in cranial or vertebral bones for example. But if only long bones are available, it may become difficult to discriminate juveniles from adults, especially when dealing with sauropod long bones. This is where bone histology may prove useful.

Magyarosaurus bone microstrucutre. Courtesy Koen Stein.

By core drilling several specimens and cross cutting one to check for local variation in the histology, we were able to distinguish M. dacus from “M.” hungaricus (Stein et al. 2010). The degree of remodelling is much higher in the smaller specimens than in the single large one we sampled (we did not get access to the NHM material). Because bone remodelling is a time dependent process, that usually starts late in ontogeny, this suggests that the small individuals were of a later histologic ontogenetic stage than the large ones. The 5 to 6 ton heavy “M.” hungaricus therefore follows a different ontogenetic path than the 600 to 700 kg M. dacus. A lot of this is based on comparison with a large sample of different taxa and histologic ontogenetic stages of other neosauropods (Sander, 2000; Klein and Sander, 2008). A possible flaw in our argument is the absence of an external fundamental system, a series of closely spaced LAGs (lines of arrested growth) indicating a sudden slowing down of growth. It could not been present, or have been destroyed by mechanical preparation or by the remodelling. However, the bone remodelling in M. dacus is of the same intense degree as in the largest adult neosauropods, which probably suggests it was not going to increase much in size.

In a more speculative note, the remodelling we observed in these tiny sauropods, is a feature that seems to be increasingly present in titanosaurs, and could be the result of peramorphic processes (Woodward and Lehmann, 2009; Klein et al 2009). More speculation regards the presence of large sauropods and large ornithopods among the generally dwarfed fauna.

Bate, D. 1905. Further note on the remains of Elephas cypriotes from a cave-deposit in Cyprus. Phil. Trans. R. Soc. Lond. B 197:347-360.

Benton, M. J., Z. Csiki, D. Grigorescu, R. Redelstorff, P. M. Sander, K. Stein, and D. B. Weishampel. 2010. Dinosaurs and the island rule: The dwarfed dinosaurs from Hatzeg Island. Palaeogeography, Palaeoclimatology, Palaeoecology doi:10.1016/j.palaeo.2010.01.026.

Klein, N., and M. Sander. 2008. Ontogenetic stages in the long bone histology of sauropod dinosaurs. Paleobiology 34:248-264.

Klein, N., M. Sander, and V. Suteethorn. 2009. Bone histology and its implications for the life history and growth of the Early Cretaceous titanosaur Phuwiangosaurus sirindhornae. Geol. Soc. Lond. Spec. Pub. 315:217-228.

Le Loeuff, J. 2005. Romanian late cretaceous dinosaurs: big dwarfs or small giants? Historical Biology 17:15-17.

Nopcsa, F. 1914. Über das Vorkommen der Dinosaurier in Siebenbürgen. Verhandlungen der Zoologisch-Botanischen Gesellschaft 54:12-14.

Nopcsa, F. and Heidsieck, E. 1933 Proc. Zool. Soc. Lond., 1, 221

Sander, P. M. 2000. Long bone histology of the Tendaguru sauropods: Implications for growth and biology. Paleobiology 26:466-488.

Stein, K., Z. Csiki, K. Curry Rogers, D. B. Weishampel, R. Redelstorff, J. L. Carballido, and M. Sander. 2010. Small body size and extreme cortical bone remodeling indicate phyletic dwarfism in Magyarosaurus dacus (Sauropoda: Titanosauria). PNAS doi:10.1073/pnas.1000781107.

Weishampel, D. B. and O. Kerscher. In press Franz Baron Nopcsa. [English translation of Tasnádi Kubacska, A. (1945) Verlag Ungar. Naturwiss. Mus., Budapest]. Hatkeniana.

Woodward, H., and T. Lehman. 2009. Bone Histology and Microanatomy of Alamosaurus sanjuanensis (Sauropoda: Titanosauria) from the Maastrichtian of Big Bend National Park, Texas. Journal of Vertebrate Paleontology 29:807-821.