Do virtual dinosaurs dream of evolutionary innovations?

Do virtual dinosaurs dream of evolutionary innovations?

Sauropod dinosaurs, the largest terrestrial animals in the fossil record, have always fascinated me. Quadrupeds with long tails and necks, they were traditionally depicted with towering necks feeding from tree tops, like reptilian giraffes. This view was challenged during the early 2000s, when pioneer computerized analyses suggested straight, horizontal vertebral columns with slightly inflexible necks to be the norm for many sauropod species. Furthermore, exciting new fossil finds in the last decades have revealed sauropods likely explored many different feeding strategies, whose capabilities were constrained by their body proportions. Sauropod functional morphology was a big interest for me, and a large part of my research has focused on it, using high resolution virtual skeletons. Although this could seem a trivial issue, sauropod functional anatomy is one of the keys to understand what mechanisms and selective pressures may have driven their evolution. 

Figure 1
The holotypic specimen of Spinophorosaurus nigerensis during its excavation in 2007, in an area close to Azenak at the Irhazer plain (Agadez, Niger).

Spinophorosaurus nigerensis, an exceptionally complete and well-preserved early branching sauropod from the Jurassic period of Niger, seemed to be a prime candidate as the first step in understanding how a sauropod individual might have functioned. Given the quick advances of 3D digitization, the complete skeleton was digitized in high resolution and assembled in a computer. This virtual environment, where the skeleton could be mounted with just two bones visible at once, enabled a reconstruction based on minimizing preconceived notions.

Figure 2
Original skeletal reconstruction of Spinophorosaurus crafted in 2009 (A) and the new virtual skeletal reconstruction assembled from virtual fossils in 2019 (B).

Surprisingly, the resulting virtual skeleton turned out to be radically different from all previous reconstructions. Not only in the proportions of some bones, but in the overall disposition of its body plan. Whereas earlier reconstructions depicted Spinophorosaurus with a horizontal vertebral column, our reconstruction showed clearly an elevated neck and taller shoulders, while the tail remained horizontal. Despite its elevation, the neck articulated almost completely straight with the anterior dorsal vertebrae, like previous studies had concluded. This meant that there had to be a keystoning point elsewhere in the vertebral column and we found it in the sacrum, the vertebrae ankylosed to the pelvis. This was shocking to us, as most paleontologists were expecting signs of an elevated neck in sauropods at the cervico-dorsal transition, but it turned out to be in the sacrum. The sacrum is wedged, making the torso and neck to be more elevated. Since Spinophorosaurus also had a neck with mobility comparable to that of a giraffe, it likely could have had the same feeding strategies, including high browsing. It is one of the earlier sauropods to have such capabilities. 

Figure 3
High browsing osteological adaptations in Spinophorosaurus nigerensis.

However, not all sauropods had long forelimbs or very flexible necks, therefore not all had the same high browsing capabilities of Spinophorosaurus. We focused on studying the sacra of other sauropods, expecting to confirm that other sauropods with high browsing capabilities had also wedged sacra, while those without high browsing capabilities had more rectangular sacra. Our surprise came when we found the large majority of sauropods, all within the highly diverse Eusauropoda clade, had wedged sacra: the higher the wedging angle, the longer the forelimbs. We quickly understood this meant wedged sacra were not a convergence in high browsers, but all eusauropods had inherited it from a common ancestor.

But, what happened with taxa with shorter forelimbs and necks? Surprisingly, we found additional modifications in the middle dorsal vertebrae that make the torso and neck  curve down in these sauropods. A secondary modification to counteract the wedging of the sacrum which never returned to the basal rectangular condition. Once the eusauropod lineage evolved a wedged sacrum, the character never reversed to a rectangular morphology in any eusauropod. Given the big impact sacrum morphology had in the evolutionary history of sauropods, it appears sacrum wedging was likely an evolutionary innovation.

Figure 4
Simplified evolutionary history of sauropod dinosaurs, showing how despite having tremendous changes in their food obtention strategies and body proportions, the sacrum always remained wedged. Drawings by Diego Cobo.

Finally, although now we know sacra has a larger role in understanding sauropod body plan evolution and their feeding capabilities than we previously thought, we still have plenty of unanswered questions. When did this feature evolve? Did it really never return to the basal condition? If so, how much constraint did it actually impose to the evolution of sauropod body plan? Although our results would not have been possible without the advent of virtual paleontology, I think new discoveries in the field remain the best way to find an answer to these questions.

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