Popular portrayals of paleontology often treat armored dinosaurs as icing on the Mesozoic cakes. A Jurassic floodplain is incomplete without a Stegosaurus with a spike-tailed tail trundling about in the background. Late Cretaceous forests were the same. The heavily armored Ankylosaurus had a long, club-tipped tail that could bust T. rex's shins if necessary. Although we have known about these creatures for as long as the word dinosaur has been around, paleontologists are only just beginning to understand the evolutionary story of these armored wonders. Paleontologists have made numerous discoveries in the last year that show how armor-encased dinosaurs spread throughout the globe during the early chapters of the animal's history.
The spiky stegosaurs as well as the heavily armored ankylosaurs were both part of a dinosaur group called the 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. However, discoveries made in the last 30 years have changed this story dramatically. Paleontologists discovered that Mymoorapelta, the famous Stegosaurus, lived in the Morrison Formation of America's west. Paleontologists have discovered that stegosaurs were not only overlapping in time but that they spread much faster than expected. Armored dinosaurs are not dinosaurian sideshows. They were evolutionary success stories.
Ankylosaurs and stegosaurs can often be distinguished by their armor patterns. Many stegosaurs had large, flat plates running along their backs with long spikes at their tails. Ankylosaurs had a heavier armor covering that could be pebble-like or large spikes, and even club tails.
Experts remain unsure about the reasons these closely related dinosaurs developed such distinct styles. However, experts believe it may have something to do with the fact these dinosaurs coexisted in Jurassic times and had different strategies for fighting off predators. Shundong Bi from Indiana University of Pennsylvania says that Stegosaurs used their spike-tipped tails to "active defense" against carnivores. This idea is supported by the bones of Allosaurus, which were injured and appear to have been punctured with swings from Stegosaurus tails. Bi suggests that Ankylosaurs had more extensive armor than other carnivores and might have developed their bony covers as "passive defence". This would have made it difficult for tyrannosaurs to bite their teeth without breaking them. Bi states that "they adapted to different niches", which is why the two groups coexisted so long.
The earliest thyreopharns were quite different than their more famous counterparts. Scutellosaurus was found in the Navajo Nation's roughly 200-million year-old rock. It wasn't a large, four-on the-floor pincushion, like its later cousins. The earliest known armored dinosaur was approximately the same size as a Labrador retriever. It walked on two legs and had its neck, back and tail covered with tiny pieces of bone armor called osteoderms. According to Thomas Raven, a Natural History Museum London paleontologist, these ornaments would not have been useful in defense. However, the origins of anatomical features and their later modifications are two distinct things. Therefore, a body cover of osteoderms might later be used to cover plates, spikes, or other body coverings of the thyreophorans.
Paleontologists have discovered a few dinosaurs showing the modifications that were found between Scutellosaurus, and later rock stars such as Stegosaurus. Scelidosaurus, a dinosaur from the 191 million-year-old rock in England, was able to walk on all fours. It had an ankylosaur-like appearance, even though it wasn't technically an ankylosaur. There had to be other species. They are still being discovered by experts.
The crucial fossils weren't found in the American West desert or western Europe's rock quarries. Paleontologists are now discovering armored dinosaurs in Africa and Asia. These finds go beyond filling in the timeline. They indicate that bony armored dinosaurs spread quickly around the planet after their appearance around 200 million years ago.
An early ankylosaur, Spicomellus Afer, was named by paleontologists late last year from the approximately 165-million-year old rock of Morocco. Although little is known about this dinosaur the fact that it had pointed spikes sticking out of its ribs in an unusual way not seen in any other species makes it a unique specimen. Paleontologists also found Adratiklit boulahfa the oldest known stegosaur from Africa in the same deposits. This indicates that both ankylosaurs and stegosaurs lived in the same areas long before Stegosaurus.
The story has been expanded by the discovery of a new dinosaur. For decades, paleontologists suspected there might have been early thyreophorans living in China. However, the evidence was not sufficient to prove this. Bi and his colleagues, Yuxisaurus kopchicki, discovered a layer of rock that was 174 to 192 million years older than expected. Although the dinosaur was not an ankylosaur nor a stegosaur it may have been close enough to split off from one another. Researchers now have a way of putting a timer on when these lineages began to develop their unique styles and behaviors.
Yuxisaurus was not the only dinosaur that lived at the time. Around the same time that Yuxisaurus lived, the armored dinosaurs Scelidosaurus (and Emausaurus) lived in Jurassic Europe. The armored dinosaurs cannot be definitively identified as either ankylosaurs or stegosaurs. This suggests that they were precursors of these later lineages. Bi points out that the discovery of armored dinosaurs in locations that are thousands of miles from each other suggests that these dinosaurs were present before ankylosaurs and stegosaurs developed their signature specializations.
Paleontologists are still learning how to link the dinosaurian dots, and how to tell the story of the Thyreophorans. It's difficult to find complete fossils of early thyreophorans from dinosaurs often thought to be strong and covered with armor. Raven states that early armored dinosaurs can be quite fragmentary, which means that only a small portion of the skeleton and a little bit of their bony armor is known. Experts are hoping to learn more about these animals and their relationships.
Raven and colleagues are starting to look to previously-overlooked parts of the world to fill in the gaps. He says that the continents that once belonged to Gondwana, a large southern continent consisting of South America, Australia, and Africa, are "definitely the best places to search for clues." Recent discoveries in Morocco suggest that there is more to be found in the rocks of South Africa, possibly going back to the early members of the armored dinosaur famiy. Paleontologists might be able to trace the origins for the armor-studded group and discover the reasons these dinosaurs developed spikes and plates. This could also help them to understand the origins of clubs, shields, and spikes that continue to intrigue us about these armor-studded dinosaurs.