How pneumatic were the presacral vertebrae of dicraeosaurid (Sauropoda: Diplodocoidea) dinosaurs?

Abstract

Skeletal pneumaticity implies bone invasion via air sacs that are diverticula of the respiratory system. Among extantvertebrates, this feature is found only in birds, and in extinct taxa it occurs in saurischian dinosaurs and pterosaurs.The sauropod axial skeleton is characterized by having a complex architecture of laminae and fossae that haveusually been related to some degree of pneumaticity. We examined the external anatomy of the presacral vertebrae oftwo dicraeosaurid sauropods holotype specimens, Amargasaurus cazaui and Brachytrachelopan mesai, and obtainedcomputed tomography scan images from mid- and posterior cervical vertebrae of both specimens and an anteriordorsal vertebra of Brachytrachelopan. In all cases, we recognized a ‘procamerate’ internal pneumatization pattern,confirming previous hypotheses that dicraeosaurid vertebral pneumaticity is reduced relative to other eusauropodtaxa. Thus, pneumatic diverticula were present in Amargasaurus, Brachytrachelopan, Dicraeosaurus, Pilmatueia and,possibly, other dicraeosaurid sauropods, but these diverticula did not invade their presacral vertebrae extensively.Furthermore, we found that the more pneumatic dicraeosaurid taxa, with some exceptions, occupy a basal positionwithin Dicraeosauridae. There is some variability in pneumaticity among dicraeosaurids from Gondwana, withPilmatueia achieving the highest degree of pneumatization.