New Fossil Finds Track When Armored Dinosaurs Spread Around the World

In popular depictions of paleontology, armored dinosaurs are often treated like icing on the Mesozoic cake. No Jurassic floodplain feels quite complete without a dinosaur like the spike-tailed Stegosaurus trundling around in the background somewhere. The same goes for Late Cretaceous forests, where the heavily-armored Ankylosaurus held a club-tipped tail capable of busting T. rex shins, if need be. We’ve known about such creatures for about as long as the word “dinosaur” has existed, but it’s only now that paleontologists are beginning to unravel the evolutionary epic of these armored marvels. In the past year, especially, paleontologists have described multiple new finds that outline how these armor-encased dinosaurs spread around the world in the early chapters of their history.

Both the spiky stegosaurs and the heavily-armored ankylosaurs belonged to a particular dinosaur group called thyreophorans. For decades, it seemed that stegosaurs were the banner bearers for armored dinosaurs only to go extinct in the early days of the Cretaceous—about 115 million years ago—and be replaced by the extensively-ornamented ankylosaurs. But discoveries in the past 30 years have dramatically altered that clean and neat story. In the Morrison Formation of the American west, for example, paleontologists have found that the famed Stegosaurus lived alongside an ankylosaur named Mymoorapelta. Not only did stegosaurs and ankylosaurs overlap in time, but paleontologists are discovering that these “living tanks” spread around the world much faster than anyone anticipated. Far from being dinosaurian sideshows, armored dinosaurs were evolutionary success stories.

In broad strokes, stegosaurs and ankylosaurs can often told apart by their armor arrangements. Stegosaurs often had combinations of large, flat plates along the midlines of their backs and long spikes on their tails. Ankylosaurs, by contrast, had heavier coats of armor that could take the form of pebble-like dots to massive spikes and even club tails.

Experts are still unsure as to why these closely-related dinosaurs evolved such different styles, but it might have to do with the fact that these dinosaurs often coexisted in the Jurassic and evolved different strategies to fend off predators. Stegosaurs, says Indiana University of Pennsylvania paleontologist Shundong Bi, used their spike-tipped tails for “active defense” when carnivores came close—an idea supported by injured bones of the carnivore Allosaurus that seem to have been punctured by swings from Stegosaurus tails. Ankylosaurs were more extensively covered in armor and may have evolved their bony coverings as “passive defense,” Bi says, which would have made it very difficult for tyrannosaurs or other carnivores to bite without breaking their teeth in the process. “They adapted to different niches,” Bi says, “so these two groups coexisted for so long.”

But the earliest thyreopharns looked quite different from their famous successors. Scutellosaurus, found in the roughly 200-million-year-old rock of the Navajo Nation, was not a big, four-on-the-floor pincushion like some of its later cousins. This little creature, one of the earliest armored dinosaurs yet known, was about the size of a Labrador retriever and walked on two legs, its neck, back, sides and tail dotted with small pieces of bone armor called osteoderms. These ornaments probably would have been of little use in defense, says Natural History Museum London paleontologist Thomas Raven, meaning that it’s unclear why such osteoderms evolved in the first place. But the origins of an anatomical feature and its later modifications are two different things, and so a body covering of osteoderms could later be adapted into the plates, spikes and other body coverings of thyreophorans.

Paleontologists had uncovered a few dinosaurs that show the modifications that popped up between Scutellosaurus and later rock stars like Stegosaurus. The dinosaur Scelidosaurus, from the 191-million-year-old rock of England, walked on all fours and had a more ankylosaur-like look even though it probably was not technically an ankylosaur yet itself. But there had to be more species out there. Experts have only just begun to find them.

The critical fossils didn’t come from the deserts of the American west or the rock quarries of western Europe. Instead, paleontologists have started to turn up new armored dinosaurs from Africa and Asia. The finds are doing more than filling in the timeline—they’re indicating that dinosaurs decked out in bony armor spread around the planet quickly after they stepped onto the scene around 200 million years ago.

Late last year paleontologists named an early ankylosaur, Spicomellus afer, from the roughly 165-million-year-old rock of Morocco. Little is known about this dinosaur, but the creature had pointed spikes jutting from its ribs in a way not seen in any other known species. And paleontologists have found the oldest stegosaur in Africa, named Adratiklit boulahfa, in the same deposits, indicating that both stegosaurs and ankylosaurs were living in the same places long before the days of Stegosaurus.

Another new dinosaur has helped add another facet to the story. Paleontologists have suspected that there were early thyreophorans in China for decades, but all the evidence was too sparse to be conclusive. But earlier this year Bi and colleagues named Yuxisaurus kopchicki from even older rock layers, between 174 and 192 million years old, in China. The dinosaur wasn’t an ankylosaur or a stegosaur, but might have been close to the point where both groups split from each other—offering researchers a way to put a timer on when these dinosaur lineages started to get their distinctive styles and behaviors.

Yuxisaurus wasn’t the only dinosaur around at the time. The armored dinosaurs Scelidosaurus and Emausaurus lived in Jurassic Europe around the same time as Yuxisaurus lived in ancient Asia. These armored dinosaurs can’t conclusively be identified as either stegosaurs or ankylosaurs, suggesting that they were precursors to these later lineages. The discovery of contemporary armored dinosaurs in places that would have been thousands of miles apart, Bi notes, indicates that these dinosaurs spread around the Northern Hemisphere before stegosaurs and ankylosaurs began to evolve their trademark specializations.

Paleontologists are just beginning to connect the dinosaurian dots and reveal the story of the thyreophorans in more detail. For dinosaurs that are often thought of as sturdy and covered in armor, it’s actually very difficult to find complete fossils from early thyreophorans. “The early armored dinosaurs are often quite fragmentary,” Raven says, meaning that most are known only from a few parts of the skeleton or little more than pieces of their bony armor. Experts hope to find more to better understand these animals and how they relate to each other.

Raven and colleagues are starting to look to previously-overlooked parts of the world to fill in the gaps. “The continents that were once Gondwana,” or a massive southern continent made up of Africa, Australia and South America, “are definitely the places to look for clues,” he says. Recent finds in Morocco hint that there’s far more yet to be uncovered in the rocks of the Global South, perhaps going back to the earliest members of the armored dinosaur family. By tracing the origins of the armor-studded group, paleontologists may be able to learn why these dinosaurs evolved spikes and plates, the beginnings of the shields, spikes, and clubs that continue to inspire our fascination with these armor-studded dinosaurs.

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.