Sauropods always had size on their side. The largest species, like Supersaurus, were so gargantuan that the simple makeup of their muscles, tendons, ligaments, and other tissues would have been quite a task for any predatory to chomp through even with a proffered neck. The fantastic growth rate of these dinosaurs, too, acted as a form of defense. Newly hatched sauropods were Jurassic popcorn, but their growth strategy of rapid bodily inflation offered youngsters the hope of getting to big to easily be eaten.
But that’s hardly all. The whip-like tails of some sauropods have been interpreted as defensive weapons, for example, and dinosaurs like Shunosaurus even had tail clubs. Now, according to a new paper by paleontologist Pablo Gallina and colleagues, sauropods might be able to add spikes to their repertoire. Fossils uncovered in the roughly 139 million-year-old rock of Patagonia have revealed a particularly spiny sauropod.
Paleontologist Pablo Gallina and colleagues have named the dinosaur Bajadasaurus pronuspinax. All else being equal, this dinosaur would have gotten a fair bit of buzz for being a sauropod represented by a skull. Large and sturdy as these dinosaurs might seem, their heads often go missing. (Although, to be honest, I’m not sure if anyone has ever quantified if headless sauropods skeletons are found with any more frequency than other sorts of dinosaurs.) But what made Bajadasaurus briefly blow up paleo-tuned Twitter feeds is the fact this sauropod had curved, forward-pointing spines jutting from its neck vertebrae. Reconstructed, the dinosaur looks like a very angry caterpillar.
That resemblance may not be coincidental. Many caterpillars have spiky, stinging hairs that look as nasty as they feel. It’s a warning to predators. Perhaps, Gallina and colleagues argue, the spikes of Bajadasaurus served a similar function.
Paleontologists have seen structures like this at least once before. The South American sauropod Amargasaurus - a fairly close relative of Bajadasaurus - sported double-rows of backward-pointing spines jutting from its neck. No one knows why. The spikes look too flimsy to be weaponry, so the traditional alternative explanations have been offered - social signaling, sexual selection, and thermoregulation.
Bajadasaurus is a bit different. First, we as yet know little of this dinosaur. The idea that this dinosaur had an especially spiky neck is based on the discovery of a single vertebra with hooked spines and the dinosaur’s relationship to Amargasaurus. The real animal could wind up looking more conservative, or even more extreme. Gallina and coauthors note that the idea the neck spikes of Bajadasaurus were covered in extremely long keratin sheaths is based on previous research of Amargasaurus and other dinosaurs.
So what’s the evidence that Bajadasaurus evolved a “fence” of forward-pointing spikes for defense? For now, the hypothesis is untested.
Paleontologists have grappled with this puzzle for decades, with many “weapons” - like the horns of ceratopsids - turning out to look better as social signals than spears and shields. And there’s rarely ever one single reason for elaborate structures to have the shape they do among vertebrates. "Bizarre structures" often represent compromises. In the various species of modern antelope, for example, horns are often shaped by pressures related to defense, social signaling, sexual competition, and thermoregulation, emphasis in one area creating a different shape than in another. The same would have been true for dinosaurs, so trying to pin down a single reason for an elaborate structure is likely to be off the mark.
All the same, it’s difficult to think of a hungry Early Cretaceous theropod looking at a full-grown Bajadasaurus and wanting to go for the neck. Perhaps the spines didn't need to have any mechanical defense function at all. Perhaps - in addition to carrying social signals to Bajadasaurus themselves - the spines made the dinosaurs look bigger, more intimidating, or otherwise offered an extra deterrent. This is pure speculation, and it's near impossible to test in an extinct animal, but the notion is based upon biological clues in our modern world.
Finding more Bajadasaurus to understand individual variation in spine details, as well as how the spines changed as baby Bajadasaurus grew up, might provide some clues about these structures, as they have for the crests of the shovel-beaked hadrosaurs. A supremely cool new dinosaur has been added to the list. Now our task is to get to know it better.