Leonerasaurus and the path to sauropodhood
In the background, between the flurry of new ceratopsian papers and the normal constant rain of theropod research, there has been a marked uptick in the number of basal sauropodomorph (‘prosauropod’ if you want) papers over the last couple of years. Simply in terms of new taxa we’ve had Panphagia, Massospondylus kaalae, Aardonyx, Seitaad, Sarahsaurus, Chromogisaurus, Ignavusaurus, Xixiposaurus, Chuxiongsaurus as well as the admission of an old favourite, Eoraptor, to the ranks. And now we have one more; Leonerasaurus traquetrensis.
Its an interesting little beast, with some quite advanced sauropod-like characteristics packaged in quite a lithe, small body. One stand-out feature is the number of vertebrae joining the pelvis to the spine (sacral vertebrae). Most basal sauropodomorphs had three such vertebrae, whereas the large sauropods have at least four, usually five or or more. Leonerasaurus is like a sauropod with four vertebrae joining the pelvis. The extra vertebra has been recruited from the base of the tail. One might think that this extra bracing was related to weight support for increasing size as these sauropodomorphs edged closer to gigantism and ‘sauropodhood’. But not so, Leonerasaurus is rather small (bone histology confirms that the skeleton was of an adult), and a reconstruction of ancestral body sizes based on the authors’ own cladistic analysis shows that the aquisition of the extra sacral vertebra happened before the big spike in body size on the lineage leading to sauropods. In other words one more feature characteristic of later gigantic sauropods has been shown to have evolved long before they were giants.
Another aspect that is becoming a theme in early sauropodomorph evolution is rampant homoplasy (messy evolution that involves either convergent evolution of characteristics or reversal back to the ancestral state). Pol et al. find that Leonerasaurus is really quite derived actually slotting in between Aardonyx and Melanorosaurus on their cladogram. That makes sense in the light of the increased sacral count and a few other very ‘sauropody’ features, such as the spoon-shaped front teeth, but much less sense when you look at the primitive low neural spines of the dorsals and the slender metatarsal of the first toe of the foot. Hence the massive sauropod-like foot of Aardonyx (that bore the bulk of the weight on the inner digits) was not present in this guy. Did it reverse in Leonerasaurus or is it convergent in Aardonyx and sauropods? Another option is that Leonerasaurus is simply in the wrong place. Indeed it does share some features with a more basal early sauropodomorph familiar to many: Anchisaurus. These include a long, spine-like process sticking out of the front of the ilium in the pelvis (it is short and triangular in other basal sauropodmorphs) and quite slender sacral ribs. So is Leonerasaurus a sister taxon of Anchisaurus that has picked up some sauropod-like characteristics convergently? That would have been my guess but its an hypothesis that the authors directly tested and no, trees that enforce such a relationship are significantly longer (=significantly worse explanation of the data) than those that place it up as the sister to what I call the quadrupedal clade (Melanorosaurus + Sauropoda). Of course who knows what future observations may bring?
I’m writing this before the embargo breaks so I cannot provide a link to the paper. However if you want to see it just head on over to PloS ONE and you’ll find it.