Researcher Olivier Strauss with assembled postcranium of Purussaurus mirandai laid out. Image credit: Torsten Scheyer (CC BY 4.0)
All living crocodiles, alligators, caimans and gharials – collectively called crocodylians – have a similar body plan that suits their lifestyles as aquatic ambush predators. This similarity extends throughout their bodies, including the skeleton. Their backbones, for example, always have nine vertebrae in the neck, 15 in the trunk, and two in the pelvis. Closely related extinct crocodylians also organize their spines in the same way.
Scheyer et al., however, now report that one extinct caiman called Purussaurus mirandai had a spine that was structured unlike any other known crocodylian. Weighing almost three tons (~2,600 kg), the 10-meter-long Purussaurus was more than twice as heavy as the largest living crocodylian, the saltwater crocodile. When Scheyer et al. examined fossilized remains from Venezuela that are estimated to be between 7–5 million years old, they found an extra vertebra in the creature's pelvic area and one less in its trunk. Scheyer et al. speculate that this unusual arrangement may have helped the extinct creature to support its massive weight and compensate for the strain imposed on its skeleton.
Within the animal kingdom, so-called homeobox genes dictate how different body structures, including the spine, develop in embryos. Shifts in where these genes are active in the embryo can result in an extra pelvic vertebra in humans and other animals. Scheyer et al. conclude that changes in the boundaries of the activity of homeobox genes may also explain the extra pelvic vertebra in this ancient caiman.
It is not yet clear if other extinct crocodylians had extra pelvic vertebrae as well. But these new findings are likely to lead to more research on related giant crocodylian fossils to find out. Such research could help scientists to better understand the biomechanics of crocodylians and may lead to new insights on caimans, which have thrived in the tropics of northern South America for the past seven million years. Further research in this area may also help explain how these reptiles have adapted to their environment and the role they play in their ecosystems, which is currently threatened by human activity.
