Sauropod Update: Abdarainurus

There’s a new titanosaur in town: Abdarainurus, described by Averianov and Lopatin. The holotype of Abdarainurus hails from the Alagteeg Formation of Mongolia, which was formerly considered to be the lower layers of the Djadochta Formation. It was collected way back in 1970, but only now has been studied.

Abdarainurus is only known from nine partial caudal vertebrae and some chevrons, which is not a particularly inspiring fossil, even by titanosaur standards. However, these happen to be some extremely weird caudal vertebrae, with several traits either rare in other sauropods or entirely unique to the new species. However, Darren Naish noted on Twitter that one of these reportedly unique traits—ridges inside the neural canal—is actually widespread in sauropods:

In their description of the new Mongolian #sauropod Abdarainurus, Averianov & Lopatin provide excellent figures of the exact same structures: they suggest that they might be NCRs, unique to Abdarainurus, but didn't know that they've been reported in diverse sauropod taxa. pic.twitter.com/s45D3pujrD

— Darren Naish (@TetZoo) February 11, 2020

Other notable traits include the postzygapophyses being offset onto processes (a trait shared with the South American rinconsaurs and a handful of other species) and a pair of prong-like projections, which they dub the postprezygapophyseal process, in between the prezygapophyses.

The most important of these unusual traits is probably that the vertebrae are opisthocoelous—that is, the articulations between the vertebrae are convex on the front end and concave on the back end. The exact functional purpose of this configuration is unknown, but it suggests that the tail was adapted for bearing unusual forces. This configuration is extremely rare—in almost every other sauropod, either the back end of each tail vertebra is convex or neither end is. The only other sauropods with a convex front end in the anterior caudal vertebrae are Sonidosaurus and Opisthocoelicaudia, both of which are also from the Late Cretaceous of the Gobi.

Does this mean there was a group of titanosaurs with unusual tail specializations in Asia during the Late Cretaceous? Oddly, apparently not: in the phylogenetic analyses Averianov and Lopatin conducted, Opisthocoelicaudia and Abdarainurus wound up in very different places (Sonidosaurus is too poorly known to be included). I have to wonder if that result will be borne out by future studies, though—it would be a rather peculiar coincidence for the same unusual character to have evolved repeatedly in only one place in the world.