High browsing skeletal adaptations in Spinophorosaurus reveal an evolutionary innovation in sauropod dinosaurs

Abstract

Sauropods were among the most diverse lineages of dinosaurs, with an ample geographic distribution throughout the Mesozoic. this evolutionary success is largely attributed to neck elongation and its impact on feeding efficiency. However, how neck elongation influenced exactly on feeding strategies is subject of debate. the process of mounting a nearly complete virtual skeleton of Spinophorosaurus nigerensis, from the Middle (?) Jurassic of niger, has revealed several previously unknown osteological adaptations in this taxon. Wedged sacral and posterior dorsal vertebrae cause the presacral column to deflect antero-dorsally. This, together with elongated scapulae and humeri make the anterior region of the skeleton vertically lengthened. Also, elongated prezygapophyseal facets on the cervical vertebrae and a specialized first dorsal vertebra greatly increase the vertical range of motion of the neck. these characters support this early eusauropod as a more capable high browser than more basally branching sauropods. While limb proportions and zygapophyseal facets vary among Eusauropoda, the sacrum retained more than 10° of wedging in all Eusauropoda. This implied a functional constraint for sauropod species which evolved lower browsing feeding strategies: the antero-dorsal sloping caused by the sacrum had to be counteracted with further skeletal modifications, e.g. a ventrally curved mid to anterior presacral spine to hinder the dorsal slope of the whole presacral series caused by the wedged sacrum. this suggests that at least the last common ancestor of eusauropoda developed high browsing capabilities, partially due to the modified wedged sacrum, likely a potential synapomorphy of the clade and key in the evolutionary history of the group. Sauropods were the earliest large phytophagous dinosaurs, with an unparalleled disparity in body size, since their Late Triassic origin until their demise at the end of the Cretaceous 1. Their quadrupedal, long-necked, long-tailed body plan remained fixed during their evolution, although both relatively shorter necks 2,3 and extremely long necks 4-7 appeared on several different clades of sauropods. This characteristic body plan had a direct impact on the feeding efficiency of these animals 1,8 , with some changes on this body plan being likely feeding adaptations, key in their evolutionary history 8. However, whether these adaptations allowed or not high browsing capabilities has been the subject of a lively debate 9-15. Previous studies on sauropod feeding capabilities based on their postcrania have focused mostly on neck posture and range of motion 6,9,11,12,15-17. Evidence from neutral articulation of the bones and computerized analyses suggested straighter, less flexible necks 9,14,16 , with some authors suggesting most sauropods could barely raise the neck above shoulder height 9,16. However, evidence stemming from comparisons with extant relatives and analyses on inter-vertebral stress suggested elevated, more flexible and curved necks 10-13,18. More recent studies have reve...

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