Sauropod dinosaurs gravitated towards a shared body shape: a massive, barrel-like torso with a long, tapered tail and neck extending from either end. But new research indicates that at least some sauropod species also sported tall sails along the back of their necks.
Fossil remains of several so-called “dicraeosaurid” sauropods described over the past century revealed that these animals had strange neck vertebrae, equipped with long, two-pronged spines that extended upwards. It has been suggested that the puzzling structures acted as supports for a thick hump of tissue, a membranous sail or even the bony cores for keratin-based horns like those displayed by goats and antelopes today.
To help decide which interpretation is most likely, a team in Argentina led by at the Carlos Ameghino Provincial Museum in Cipolletti studied the spines in great anatomical detail.
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The researchers examined the vertebrae of Amargasaurus — a sauropod unearthed in Argentina that lived a little under 130 million years ago and was adorned with tall neck spines — and the fragmentary remains of another dicraeosaur of an undetermined species. They described the spines’ external features and, by taking very thin slices of the bone, they analysed their microscopic internal characteristics too.
Rather than being deeply grooved like the bone cores seen in modern horned animals, the sauropod spines had relatively smooth surfaces. Also, as seen in cross-section, the spines had growth signatures that showed they had changed shape as the animal grew. This wouldn’t be possible if the spines had originally formed the core of keratin-coated horns, since a keratin sheath would be made of non-living tissue that couldn’t morph its shape to match changes to the bone underneath.
Based on the direction and spacing of bone fibres in the spines, the researchers think there was a system of tough ligaments connecting the spines to each other. They argue that soft tissue covered each spine from the base to the tip and stretched between the spines down the neck to form a pair of membranous sails — one for each fork of the vertebral spines.
The interpretation of dicraeosaurs like Amargasaurus having horns “has been very popular in the last decades”, says at the National University of Distance Education in Spain, so his first impression upon reading the study’s sail-centered conclusion was that it is “somewhat shocking, since the look of this animal is deeply engraved in public awareness”.
Vidal says the evidence presented by Cerda and his colleagues is solid, refuting the horn hypothesis, and he notes that “all evidence points in the same direction” for the presence of a sail.
“As a sauropod specialist, I’m especially excited to see this rebooted vision of such bizarre anatomy,” says at Macalester College in Minnesota. “It helps to rescue the reputation of sauropods as overgrown reptiles doomed to extinction, and instead paints a new picture of a much more dramatic, wildly successful group of dinosaurs.”
It isn’t yet known what Amargasaurus might have used a sail for, but both Vidal and Curry Rogers agree with the Argentina team’s suggestion of visual display. It is known that modern animals with sails – which are mostly reptiles – may use them to signal mates or intimidate rivals.
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But none of these creatures are nearly as large as Amargasaurus, nor do they have the bifurcated shape or the position of the sail mostly on the neck, Vidal points out. As such, “there might be different functions or uses that are not present in any [living] animal”, he says.
Other closely related dicraeosaurs with long neck spines — like Pilmatueia and Bajadasaurus — may have also had similar sails, although we will have to wait for future research on their fossils to explore the possibility.
Journal of Anatomy
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