Is the key to #sauropod neck position and high browsing capabilities in their butts (actually, their sacra)? Today, we publish @SciReports how sacra may be key in sauropod evolution. #Paleoart: #Spinophorosaurus and indet. #theropod by Diego Cobo. #thread #science #paleo 1/n 
First, I must say this was the first paper I wrote out of my last 3, and it was the first submitted as well. So I am very glad to see it seeing light after almost 2 years since I wrote the first draft. Here's the link rdcu.be/b3FpJ 2/n
Everything started when I assembled the virtual neck of the Spinophorosaurus holotype in 2015-16. Since it is one of the few known complete and well-preserved sauropod necks, it was a good specimen to test whether previous analyses held true 3/n
And they did hold up. Like previous studies on Plateosaurus (by @H_Mallison ) or #Diplodocus and #Apatosaurus (by Stevens and Parrish), the neck of Spinophorosaurus articulates pretty straight in ONP, though rather a little dorsally sloping like #Plateosaurus 4/n
When I turned visible all the bones (skeleton mounting followed @H_Mallison protocol: only two bones visible at once, to minimize preconceived notions): Surprise! Virtual Spinophorosaurus (B) looked nothing like previous reconstructions (A): its neck was pretty elevated. 5/n
The tail, like that of most sauropods, is subhorizontal, while the presacral vertebrae (dorsals and cervicals) slope dorsally strongly. And this is because of the sacrum and posterior dorsals, which are wedged (particularly, the sacrum) 6/n
This is coupled with a relatively long humerus, which is in proportion longer than that of Camarasaurus (femur/humerus ratio 1.37 vs 1.21 in Spinophorosaurus; humerus/scapula ratio 0.74 vs. 0.86 in Spinophorosaurus). In short, Spino's shoulder was taller than #Camarasaurus 7/n
Regarding the missing forearm/hand and their proportions or the position of the scapulocoracoids, there is more detail on that on the extensive Supplementary. Thread follows the actual paper, but I will refer to Supplementary now and then. static-content.springer.com/esm/art%3A10.1… 8/n
So, the virtual #Spinophorosaurus, the first virtual #sauropod from a fossil individual this complete, has the same feeding capabilities of a giraffe, as is briefly outlined on this paper and more in depth on our previous PLOS ONE paper . LINK: 9/n
To understand how much more range of motion #Spinophorosaurus had in its neck per joint, here's a comparison between Spino (27º), #Haplocanthosaurus (20º) and #Camarasaurus (17º) at the DV1-CV12 joint (from the supplementary). One key factor is prezygapophysis elongation 10/n
All in all, Spinophorosaurus is congruent with some previous results by Stevens and Parrish (presacral series is almost straight), but not others (neck is more flexible than their analyses suggested, and the keystone is not on the cervico-dorsal transition, but the sacrum) 11/n
Now, back to the sacrum: it has been claimed sacral wedging caused pelvic retroversion in some sauropods: that is not true. A sacrum can be wedged and the pelvis not retroverted (#Diplodocus) and retroverted (#Camarasaurus). This is discussed in the supplementary 12/n
And here, the implications for other sauropods other than Spinophorosaurus begin!
Given the key role sacral wedging has on the geometry of the axial skeleton of Spino, we thought brachiosaurs, with their extremely long arms must have had extremely wedged sacra, while titanosaurs or dicraeosaurs with short forelimbs must have had a more rectangular sacra 13/n
Turns out, we were right on the former, but wrong on the latter: a vast majority of sauropods checked had wedged sacra, at least more than 10º. Only non eusauropods had rectangular sacra, more similar to those of "prosauropods". 14/n
Sauropods such as Kotasaurus or Barapasaurus (the basalmost sauropods preserving complete sacra) have relatively slender, likely elongated humeri (unfortunately, no articulated individuals exist for either) and rectangular sacra 15/n
But all eusauropods surveyed have wedged sacra. And, more surprisingly, the amount of sacral wedging appears to correlate with the relative length of the forelimb! The longer the humerus relative to the femur, the more sacral wedging, unlike in non eusauropod sauropodomorphs 16/n
This has two important implications: first, the sacrum became wedged ONCE, and does not appear to have ever reverted to the ancestral rectangular condition, so that makes it a likely SYNAPOMORPHY of Eusauropoda (Sacrum: wedged more than 10º). 17/n
Second, it has important functional implications. The correlation with relative humerus length suggests a possible functional module, in which sacrum and forelimb (at least) could have mutually influenced each other throughout sauropod evolution. How? Let's see: 18/n
Let's say #Brachiosauridae. The only known complete sacra (#Brachiosaurus and #Giraffatitan) are the most extremely wedged (close to 30º), and their arms, the longest. Sacra and forearms evolved toward more verticality, allowing these animals to browse the highest 19/n
In sauropods with shorter arms and necks (potential low/ground browsers), the sacrum is less wedged, with angles closer to 10º. However, this is not enough to get the head to the ground… Why didn’t the sacrum revert to the basal condition (rectangular)? 20/n
We don’t know, but looks it became fixated in sauropod body plan (becoming a constraint). The solution found by different lineages which evolved lower head positions was the same… in convergent fashion: they modified dorsal vertebrae and forelimbs to cope with the wedging 21/n
The middle dorsal vertebrae of some dicraeosaurs and titanosaurs are wedged too, but making an obtuse instead of an acute angle (like the sacrum). By modifying these vertebrae and shortening the forelimbs, the torso is curved, counteracting the keystoning of the sacrum 22/n
Regarding #Dicraeosaurus remember seeing this presacral sequence articulated by Kent Stevens, and something looked weird. Considering the wedging in the sacrum, it looks like this. @amylcampbell any clues? 23/n
Finally, #Diplodocus. Its sacrum is wedged 16º. Gilmore noticed it on the @NMNH specimen, whose back actually curved down as in #Dicraeosaurus. However, the Senckenberg museum mount has a straight back and very steep neck. So, the condition for Dippy remains a doubt for now 24/n
Given the role the sacrum has in the overall body proportions and orientation of eusauropod skeletons and being synapomorphic rather than homoplasic, it appears to be an evolutionary innovation. I would like to engage sauropod fellow researchers in discussing the hypothesis 25/n
Note that many sauropod mounts, even those crafted less than 20 years ago still have disarticulated bones posed in preconceived positions, and that many sauropod reconstructions are therefore subject to change in the close future 26/n
And, FINALLY (sorry, this thread was longer than necessary), I would like to thank my coauthors, @mupe_elche for all the assistance in research and curation, Diego Cobo for his killer art, @SciReports for making the paper open access, and the paper reviewers (great input!). 27/27
EXTRA: I’m going to add to this thread blog posts addressing the paper, so it’s all together and accesible. I’ll start with a really nice entry on @skeletaldrawing
This thread can be more or less summarized in this #behindthepaper post.
