Sails!
Yes, I'm talking about the tall vertebral fins running across the backs of, most famously, Spinosaurus aegypticus and Dimetrodon ssp. These weird structures have driven paleobiologists crazy for years: what were they for? How did they work? What benefit did they impart to the creatures that used them?
Two interpretations of Spinosaurus. From the excellent blog Mesozoic Archives.
The picture is complicated by the fact that Dimetrodon and Spinosaurus could not have been further apart, taxonomically speaking: Dimetrodon was a synapsid - like mammals - while Spinosaurus and other dinosaurs belonged to the archosaur supergroup, like crocodiles. Their last common ancestor was some kind of primordial lizardy thing, and since then they had nothing to do with each other. They were also separated by 207 million years, which is three times as long as the span separating dinosaurs and us.
Source: Wikimedia
Taking an even broader view, things get weirder. Just after Dimetrodon (geologically speaking) and his ilk were wiped out in the apocalyptic Permian Extinction Event, croc-relatives called Poposaurs popped up in the Triassic; at least three or four of them had sails, too. Going back before Dimetrodon, there were sailed amphibians such as Platyhistrix. Even Dimetrodon's contemporaries were sporting fabulous frillage: herbivorous Edaphosaurus, burly Ctenospondylus, and tiny Ianthasaurus, among others. And then of course there's the rare but certainly weird sailfins of today: the plumed basilisk and the sailfin dragon, two lizards. Sailfins keep popping up in the amphibian-reptile lineage.
Sailfin dragon
Now, various theories have been floated as to what these sails were used for. The most common is that they were used for thermal regulation: turn them toward the sun, you warm up; toward the wind, you cool down. Another idea is display: the fins make the animal look more impressive, or can be used as billboards to express emotions like anger, fear, or lust. It has even been posited that the spines did not support sails, but humps. This last hypothesis is the most dubious, as the spines vary wildly in strength and support: Spinosaurus' spines were very thin, geared toward weight reduction. As to the display aspect, this seems likely for herbivores, but predators would have an extremely tough time of it: these neural spines couldn't be lowered or raised (they were fixed to the vertebra), and would have been a liability for an active predator. Especially since Spinosaurus was, by any measure, the largest predator ever to walk the earth. Predators need stealth. You can't be stealthy when you're the reptilian equivalent of a schoolbus with a billboard stuck on top.
The only real way to get on top of this mystery is to find out what these creatures all had in common. After a little research, one odd denominator begins to pop out: water. Nearly all these creatures were found along the coasts of ancient seas; where they were found inland, I'd posit a large lake or estuary (does anyone know the paleoecology of Vytshegdosuchus? From the map in the linked article, one could posit a large estuary or delta leading out into the bay...)
Let's start with Dimetrodon. In 2014, superstar paleontologist Robert Bakker noticed something odd about the famous finback's fossils: they were nearly always associated with ancient shark teeth (link to IFL article). He went on to note that land-based prey was scarce, both for carnivores and herbivores. These animals were making their living at sea. The skull of Dimetrodon implies that this was not just a desperate switch to fish-eating: like many crocodilians, its upper jaws had a significant notch at the front, an adaptation for snatching onto slimy, wriggly piscines. In addition, its skull was oddly canted, implying a "snout-down" position; add this to the fact that its shoulders were much taller than its hips, and you have an animal built for walking around in shallow water with its head down, ready to snag a passing shark.
by Robert T. Bakker
I'm wondering about evidence for electrical receptors on the snout, which would make it easier for Dimetrodon to snag a fish without having to stare into the water and possibly be fooled by refraction.
Spinosaurus had all of these adaptations, plus several more. Let's count them up...notch in the jaws: check. Canted skull: check. High nostrils: check. Electric receptors: check. Enormous front claws, especially on the first digit: double check. Its fossils are also found in association with giant sawfishes called Onchopristis, usually in shredded pieces - direct evidence that Spinosaurus was ripping the fish apart prior to feeding.
Recent finds of Spinosaurus have shown not only that this beast was a fish-killing machine, it was also one of the weirdest theropods (two-legged lizard-hipped dinos) ever to walk the earth. It was quadrupedal, or nearly so: its back legs were show to be quite short and puny, compared to a T. rex or even its Suchomimid relatives like Baryonyx. Add this to its front-heaviness and powerful arms, and you get a creature that may have spent much of its time on all fours - like a bear (although similarly-proportioned creatures, like prosauropods, are thought to have been bipedal - shows you what a difference a balancing tail can make). It had a long neck, too, and a long and powerful tail similar to a crocodile; it's not hard to imagine Spinosaurus swimming along the coastline, neck arched up like a terrifying swan, powerful sweeping tail and limbs propelling it after its prey.
Not only that, but both of these unrelated creatures had analog herbivores living alongside them. Dimetrodon had Edaphosaurus, an oddly tiny-headed creature with a backward-sweeping sail; its teeth were postlike, perhaps due to a diet of water plants. Spinosaurus' gardens were feasted upon by Ouranosaurus, a duckbilled herbivore. Granted, Ouranosaurus' spines are more reminiscent of a bison, with flanged tips, so perhaps it really did have a hump instead of a sail; and the diet of duckbills, ironically, seemed to have been mostly coarse upland plants than water species. However, the fact of its heightened vertebral spines, combined with its delta habitat, do reinforce the sail-water connection.
We now come to the lesser-known finbacks: the Poposauroids. These Triassic archosaurs were very odd, being a land-living, distant relative of crocodiles with no modern descendants; they walked with their limbs tucked under their bodies, exactly like dinosaurs, and many were even bipedal. If not for the mass extinction at the end of the Triassic, these upright pseudo-crocodilians might have evolved into what we think of as "dinosaurs" themselves. Whatever the case, several of them sported sail-fins, including Arizonasaurus and Xilousuchus. All three were found along ancient shorelines. Arizonasaurus was quite obviously a fish-eater, with a tapered, notched snout and nostrils situated closer to its eyes. Xilousuchus had a more generic, doglike head, but probably hunted and scavenged along the shoreline, much like Skeleton Coast lions do today. From Wikipedia:
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| Arizonasaurus - not a dinosaur. |
To wrap up, here's various assorted sailbacks: Acrocanthosaurus, a huge allosauroid hunting along the coast of the ancient American interior seaway; Platyhistrix, an amphibian living alongside Edaphosaurus and Dimetrodon; the plumed basilisk, a lizard; and the sailback water-dragon. I'm probably missing quite a few others. All are associated with water. (I'm not mentioning the sauropod Amargasaurus here because, despite its association with braided rivers, I can't conclude it lived in a delta or floodplain. However, I'm hoping it fits my hypothesis).
Platyhystrix, an ancient amphibian. From Wikipedia.
So this begs the question: if sails are so often associated with water, what was their function?
I think it's a mixture of thermoregulation and display that would be most useful in an aquatic environment. During the Permian, Triassic, and Cretaceous periods - when the synapsids, Poposaurs, and Spinosaurus lived respectively - the world was a much drier and hotter place, and depending on where you lived there was fiercer competition for fewer resources. You therefore needed a very efficient body that could do a lot with very little. Now, I might get in trouble here; but I'd like to posit that most of the sailbacks, even the dinosaurs, were some form of ectotherm. This is not unprecedented: the roadrunner of the American Southwest, a bird, uses a special patch of skin on its rump to bask in the morning sun, thus saving energy in warming itself up. An animal that spends a significant amount of time in water - especially cold water - would need a long sunning session after an icy dip - see marine iguanas (which incidentally have tall scales on their backs - not quite a sail, but they only just recently adapted to a fully marine diet; give 'em a few million years).
Now, with all that acreage for sunning, the animal also gains a convenient billboard. Think about it: the last thing a swimming Spinosaurus would want is another Spinosaurus surfing along the same stretch of ocean. If Spinosaurus A is dipping its head to snag a fish, how is Spinosaurus B to know it's there? Hence, the sail. With that huge fin projecting above the water, Spinosaurus is now as obvious as a Disney cruiseliner, no matter what it's up to. A potential trespasser would be able to spot him from a mile away, and know to steer clear. If the animal is able to change the color of the sail, or have eye-spots or bright flashes on it, the more the better.
In conclusion, I'm only confirming the leading theories about a sail's function: thermoregulation and display combined. The difference is the environmental context: arid shorelines and cold water. Aquatic-feeding sailfins use their ornamentation both to warm up after hunting, and to advertise their presence while under or near the water's surface. For inland creatures, especially running predators, a huge and flimsy sail doesn't make any sense.
Disagree? Let me know in the comments; I'd love to know if there's some other factors that I'm not considering. And granted, there were many, many fish-eaters and aquatic feeders without sailfins. However, I think I've made a solid hypothesis for the purpose of large, fragile sailfins in at least some of these creatures.
Rick Out.
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