The ornithopod that lived fast and died young: Gasparinisaura

A life model of Gasparinisaura cincosaltensis at the Museo Cinco Saltos.

 
In previous posts, I have used a lot of superlatives to describe the quality of various specimens. Of those covered so far, Anabisetia and Macrogryphosaurus were the most spectacular among the ornithopods, and Megaraptor was awe-inspiring. I will be sure to utilise somewhat different superlatives to describe the dinosaur I worked on during my week in Trelew, and for the one I will be studying in my week in Río Gallegos. That said, I really should reserve some for the smallest dinosaur I have worked on so far, for it is one of historical significance in Argentinean palaeontology, and is represented by some absolutely wonderful specimens.
 
The year was 1996. I was finishing high school. Non-hadrosaurid ornithopods from South America were effectively unknown. And then, with one publication in Journal of Vertebrate Paleontology, Rodolfo Coria and Leonardo Salgado changed that. Although they provided scant details about how and when the fossils came to be found, or who collected the material, the specimens they described were amazing. The crown jewel was an almost complete skull, associated with an incomplete skeleton, of a dinosaur that was little bigger than a chicken. Coria and Salgado elected to name the new ornithopod Gasparinisaura cincosaltensis, alluding to the place in which it was found (Cinco Saltos) with the species name, and honouring Zulma Gasparini, a world-renowned Argentinean palaeontologist, with the genus name. I have no doubt that she would have been absolutely chuffed to have such a magnificent specimen named after her, even though she mainly works on crocodiles and their kin, rather than dinosaurs

The holotype skull of Gasparinisaura cincosaltensis at the Proyecto Dino Palaeontological Centre, Lago Barreales. Simply magnificent.

 
Having now observed it for myself, I can say that the type skull of Gasparinisaura is absolutely exquisite. To be able to see the ridges and wear facets on the teeth and the holes that facilitated the passage of blood vessels into the jaw bones was simply phenomenal. I also realised that the palpebral — the bone above the eye socket that might have made these small ornithopods look perpetually angry — was broken. This was not evident from the published illustrations. The bones preserved with the skull were similarly spectacular, including the partial shoulder and pelvic girdles and even some complete limb bones.
 
In their initial publication, Coria and Salgado mentioned only two specimens of Gasparinisaura, and almost all of the illustrations they provided (none of which were photographs) were based on the skeleton with the skull. The other specimen they reported, a partial tail, was only briefly summarised. However, the following year, Salgado, Coria and Susana Heredia reported the discovery of several new specimens of Gasparinisaura. They revealed that the locality from which the material was collected was situated 2.5 km southeast of the Cinco Saltos Cemetery — a juxtaposition of two graveyards of markedly disparate ages. In total, they reported twelve new Gasparinisaura specimens of varying levels of completeness: some comprised substantial portions of skeletons, others were only represented by one or two incomplete bones. Taken together, these specimens spanned quite a size range: from sub-chicken-sized individuals that were almost certainly juveniles to substantially larger exemplars. However, even the largest were still quite small by dinosaur standards! On one of the thigh bones of a particularly large individual, I observed a puncture mark that was clearly made prior to the bone being fossilised. This specimen was not illustrated by Salgado and colleagues, but they did briefly allude to it. Their conclusion as to the origin of the puncture seems perfectly reasonable to me: they interpreted it as a crocodylomorph tooth mark. This implied that this and other Gasparinisaura individuals might have fallen victim to an aquatic predator attack as they came down to drink at a waterhole. Whatever the crocs did not eat was eventually buried in sediment at the bottom of the water body and, ultimately, fossilised. Alternatively, the Gasparinisaura individuals might have been clustered around a drying waterhole, and later scavenged by opportunistic crocs who scattered their carcasses.
 
With these new specimens available, the overall anatomy and appearance of Gasparinisaura became rather well understood. Some of the new specimens included portions of the tail, bones from the upper arm and forearm, partial pelves, articulated ankles, and even in two instances a fairly complete foot. However, many of the long bones (especially those of larger individuals) were incomplete: their top and/or bottom ends were often preserved, but their shafts were almost always missing. Unfortunately, this meant that gauging the relative proportions of the hind limbs was difficult.
 
Amazingly enough, Gasparinisaura is now known from more than the 14 specimens described in the mid-1990s. In 2008, Ignacio Cerda announced the discovery of two new partial skeletons of Gasparinisaura. However, his interest in these specimens was not anatomical, even though they were quite complete (one of them includes a skull that has still never been described); instead, he reported the presence, in both new specimens and in the type specimen, of gastroliths: stomach stones! To find them associated with three specimens of the same species, in the same deposit, when no other rocks were found in the associated sediment, meant that their interpretation was unambiguous. Nevertheless, it seems strange that Gasparinisaura would need rocks to aid in digestion, because its teeth were clearly adapted for mastication. Perhaps, as a small dinosaur, it needed to be able to extract as much nutriment from its food as possible, and in order to achieve this it needed to pulverise whatever it was eating well beyond what its teeth were capable of. Cerda made a strong case for the interpretation of these rock clusters as gastroliths, even going so far as to demonstrate that the ratio between the mass of the stomach stones in a Gasparinisaura individual and its approximate mass was comparable to that found in modern birds.

A partial skeleton of Gasparinisaura cincosaltensis with the forelimbs towards the left, the pelvis to the top right and the hind limbs to the right. Note the gastroliths (stomach stones) in front of the pelvis.

The most recent publication on Gasparinisaura was published in 2012 by Ignacio Cerda and Anusuya Chinsamy. In this work, they focused on the histology of Gasparinisaura: its internal bone structure. What they found, from their substantial sampling, was that Gasparinisaura grew and lived fast and — if the specimens known to date are anything to go by — often died young. Based on their observations, Gasparinisaura grew relatively rapidly until it reached ~60% of the size of the largest individual preserved. This growth did not always take place at a continuous rate, with some individuals showing evidence of brief periods of stasis. However, only after Gasparinisaura exceeded this size did growth slow down more dramatically: evidence of slower, periodically arrested growth was only seen in larger individuals. Nevertheless, it was clear that even the largest preserved individuals were still growing at their time of death. This suggested two possibilities: that Gasparinisaura’s growth was indeterminate — i.e. it never stopped growing until it died — or that no adult Gasparinisaura individuals were known. Cerda and Chinsamy suggested that the latter was more likely, and I agree.
 
Overall, then, we have an amazingly clear picture of what Gasparinisaura looked like, how it grew, and of some aspects of its lifestyle. Having studied, photographed and measured all of the specimens I could lay my hands on, I can only say that it was an absolute delight. Studying the articulated specimen with gastroliths, in particular, was amazing.

Me with the magnificent Gasparinisaura cincosaltensis-with-gastroliths specimen at the Museo Cinco Saltos.

​References
 
Cerda, I.A., 2008. Gastroliths in an ornithopod dinosaur. Acta Palaeontologica Polonica 53, 351–355.
 
Cerda, I.A. & Chinsamy, A., 2012. Biological implications of the bone microstructure of the Late Cretaceous ornithopod dinosaur Gasparinisaura cincosaltensis. Journal of Vertebrate Paleontology 32, 355–368.
 
Coria, R.A. & Salgado, L., 1996. A basal iguanodontian (Ornithischia: Ornithopoda) from the Late Cretaceous of South America. Journal of Vertebrate Paleontology 16, 445–457.
 
Salgado, L., Coria, R.A. & Heredia, S.E., 1997. New materials of Gasparinisaura cincosaltensis (Ornithischia, Ornithopoda) from the Upper Cretaceous of Argentina. Journal of Paleontology 71, 933–940.