Do more sacrals mean a bipedal stance?
Short answer: yes and no. Sacral vertebrae connect the backbone to the pelvis. Occasionally other vertebrae lie between the ilia without connecting to them. These are not real sacrals. Occasionally other vertebrae do not lie between the ilia, yet converge on them. These are real sacrals. Sometimes there is a relationship between ilial length and number of sacrals. Sometimes there isn’t.
Generally in basal tetrapods (pre-reptiles/amphibians), there is only one sacral. However, these basal tetrapod/pre-reptiles appear to have two sacrals: Batropetes, Microrater and Cacops. Let me know if this is wrong.
Reptiles – The New Lepidosauromorpha
Most reptiles have two sacrals or more, but two taxa in this study revert to one sacral: Milleretta and Eunotosaurus. Others in the Milleretta clade, like Acleistorhinus and the bolosaurids, may also have had one sacral, but they are known only from skulls.
Saurosternon reverses this pattern back to two sacrals. It was much smaller than its predecessors and likely arboreal.
Jesairosaurus, Hypuronector and drepanosaurs have more than two sacrals. The fenestrasaurs continue this pattern. Cosesaurus has four. Sharovipteryx has nine. Longisquama has five. Pterosaurs have more than four. I have argued (Peters 2000) that the fenestrasaurs were increasingly bipedal with occasionally bipedal Rotodactylus tracks matching Cosesaurus feet. Beachcombing pterosaurs reverted to quadrupedalism, especially while feeding. There is no doubt that Sharovipteryx was a biped, although all that Google shows on this subject for the first few pages have been originated by me! No one else wants to concur? This is particularly strange and bears the mark of widespread bias or blindness. The drepanosaurs were likely arboreal lizards, some of which probably adopted a tripedal stance assisted by a prehensile tail. Jesairosaurus is a puzzle in this department.
Snakes from both branches revert to one sacral or none.
Reptiles – The New Archosauromorpha
Sphenacodont pelycosaurs have more than two sacrals as do therapsids. No bipeds here. This has been attributed to increasing size and increasing leg length raising the body further off the substrate (if only slightly). The initiation of elongated dorsal spines may have contributed to this support between the pecs and pelvis.
Ornithosuchids have more than two sacrals. So do a sprinkling of rauisuchians, including Vjushkovia, Smok and Postosuchus. I’m guessing on the number of sacrals in Vjushkovia based on a lateral view (which can be misleading) and would appreciate any better data. The other three look to be tentative bipeds.
Among archosaurs on the basal croc line, Pseudhesperosuchus has three or four sacrals. So do Scleromochlus + Saltopus. So do Terrestrisuchus + Saltoposuchus. All were likely bipeds. The number of sacrals decreased to two in derived quadrupedal crocs, but the quadrupedal Protosuchus may have retained three (covered by scutes).
Only two sacrals were reported for the theropod dinosaur Tawa, but three vertebrae were pressed between the ilia. Coelophysis had five or more sacrals highly pressed between the ilia. Herrerasaurus had only two sacrals, but they were greatly expanded as they attached to the broad ilia. Two other verts, one fore and one aft, had tiny sacrals between the ilia. These theropods were bipeds.
The phytodinosaurs (poposaurs + sauropodomorphs) all had three or four sacrals. Basal forms were bipeds. A few, like Lotosaurus and Shuvosaurus, reverted to quadrupedal locomotion. The other phytos – the Ornithischia had five or more and these likewise began as bipeds but most reverted to quadrupedalism, some of them quite early on.
Now, how do they stack up with regard to bipedalism?
As you can see from the above list, having more than two sacrals is a convergent trait among many clades, some of them completely aquatic. Some of this increase in sacral number was due to nothing more than increasing mass. Even so, an increased number of sacrals did develop among tentative and facultative bipeds at the fulcrum of the lever developing at the acetabulum, as a response to the stresses developing there when the forelimbs are raised. In later dinosaurs and pterosaurs the reversion to quadrupedal locomotion did not diminish the number of sacrals as members of both clades occasionally rose to hind limbs for feeding, fighting or takeoff and landing.
As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.