Armored dinosaurs form the suborder thyreophores, which means “shield bearers” in Greek. The thyreophores are classified into two large groups, the ankylosaurs, with the body covered by an armor of bony plates, such as Borealopelta, which we have already discussed in Fossil Zoo; and stegosaurs, with a double row of plates or spikes along their backs, which have also had their episode in this podcast. But we know of a few more primitive thyreophores that don’t fall into either group, what paleontologists call “basal members.”
Thyreophores evolved from small running bipedal herbivorous dinosaurs. They appeared on the northern continent of Laurasia, which later gave rise to Eurasia and North America. The most basal thyreophore that we know of is Scutellosaurus, which lived in Arizona during the Lower Jurassic, around 196 million years ago. Scutellosaurus, whose name means “lizard with a small shield”, was still a small and slender bipedal herbivore, measuring 1.30 meters in length, 50 centimeters high at the hips and weighing between three and ten kilos. The head is small, nine centimeters long. The front teeth are wide, and the lateral ones, fluted and leaf-shaped, are adapted to chewing plant matter. The arms are long, with large hands and small, pointed claws. Scutellosaurus is covered with several hundred small oval bony plates distributed in up to ten parallel rows along the back, from neck to tail. Some of these osteoderms are flat, while others are rough and covered by a longitudinal ridge. The tail represents two thirds of the total length of the animal, and served to maintain balance when running on the two hind legs, long and slender.
Emausaurus, which lived in northern Germany fifteen million years later, represents the transition from bipedia to quadrupedia. Its name is formed by the acronym for Ernst-Moritz-Arndt Universität, the University of Greifswald, in northeastern Germany, and by the Greek term “sauros”, lizard. The remains of Emausaurus are rather sparse and incomplete, but at least we know that it was a quadrupedal herbivore, perhaps partly bipedal, and that it fed on low, hard vegetation, such as the large leaves of cycads and benetitals. It was between three and four meters long, and could weigh up to 240 kilos. Not enough osteoderms have been found to determine the extent or arrangement of their armor.
Best known is Scelidosaurus, which lived 190 million years ago in what is now the British Isles. In fact, it is one of the oldest complete dinosaurs known. Scelidosaurus, four meters long and weighing just under three hundred kilos, was already mainly quadrupedal, also feeding on low vegetation. Although some tracks discovered in 1991 in the mountains of Santa Cruz, in southern Poland, and attributed to Scelidosaurus or a close relative, show an animal capable of walking on its hind legs.
The name of this dinosaur comes from a mistake. Its earliest remains appeared in a quarry in the cliffs of Black Ven on the south coast of Dorset in south-west England. These fossils came into the hands of the collector Henry Norris, who sent some fragments of bones from the hind legs to Richard Owen, superintendent of the Natural History department of the British Museum, who in 1959, with the intention of calling the animal “the lizard of the hind leg”, he named them Scelidosaurus. But Owen confused the Greek terms “skélos” (σκέλος), “leg” and “sjélis” (σχελίς), “beef rib”, so the dinosaur, although described from leg bones, is actually called “rib lizard”.
The head of Scelidosaurus is small, about eight inches long, and viewed from above it is triangular in shape, longer than it is wide. The snout is flat on top, and is equipped with a small horny beak. The teeth are longer and more triangular than in more advanced armored dinosaurs; they do not get to crush the food, but they crumble it by the lateral rubbing between the upper and lower teeth. Above the eyes is a small crest of bone, and parts of the skin on the skull and lower jaw are ossified. The neck is moderately long; the trunk is thick and elongated, but taller than it is wide. The tail is relatively short, not longer than the rest of the body. The spine in the area of the hips and the base of the tail has a large number of ossified tendons that give it rigidity. The hind legs are longer than the front legs, as in most dinosaurs. The hands, recently discovered, have not yet been described. The feet, quite large and broad, are turned outwards. The first finger is small, and the other four are robust. The hoof-like claws curve inward.
From the neck to the tail, along the animal’s back, several parallel rows of osteoderms extend. On the neck there are two rows on each side; at the base of the neck they are flat and very large. The first osteoderms, on the back of the head, form a pair of scutes with three points each, pointing backwards. On each side of the body are three rows of large oval osteoderms, with a triangular ridge, which is most conical on the most lateral row. Between these rows are one or two rows of smaller, flattened osteoderms. There are also osteoderms on the outside of the legs. In the tail there are four rows, one at the top, one at the bottom, and one on each side. In some specimens, somewhat different osteoderms appear, with a spine instead of a crest; these specimens also have small horns on the back of the head.
Fossilized skin impressions have also been found. Thus we know that between the scutes, the skin of Scelidosaurus is covered by rounded scales that do not overlap, and there are also small flat granules of bone a few millimeters in diameter. And in 2000 a fossil made up of a series of eight tail vertebrae came to light in which part of the skin has been preserved; it can be seen that the osteoderms were covered by a hard layer of keratinous skin.
In recent years, the fossil remains of a new basal thyreophore have been discovered and excavated in central Argentina: Jakapil, which in the Puelche language of the Pampas and Patagonia means “shield bearer”. Jakapil is much more modern than the other basal thyreophores; more modern even than many more advanced armored dinosaurs; It lived about 95 million years ago, in the Upper Cretaceous. Being so recent, its classification as a basal thyreophore has been much discussed: It has been proposed that it may be a basal ceratopsian, a relative of the famous Triceratops, or that it belongs to a new group of dinosaurs hitherto unknown.
Jakapil is a small bipedal dinosaur. The only known specimen, which had not reached maturity, measured less than five feet and weighed about five kilos. Its skull is short and stout, with a narrow snout and only eleven pairs of leaf-shaped teeth in the lower jaw. The body is covered by osteoderms of at least five different types: on the sides of the neck they are large and flat, D-shaped; on the upper part of the neck they are long and pointed, like thorns; in the throat they are thick and flat; on the back, smaller; and in the rest of the body, oval. The arms are so short that they seem useless, but they are well muscled; its function is ignored. He lived in a desert region dotted with oases.
Throughout the Jurassic and Cretaceous, the thyreophores increased in size and their osteoderms evolved in two directions. For one, they enlarged to cover the entire body and gave rise to ankylosaurs. On the other, they were concentrated along the back in a double row of plates or spikes in stegosaurs. Both groups were distributed throughout the world; Stegosaurs went extinct about 125 million years ago, in the Early Cretaceous, while ankylosaurs survived until the Cretaceous-Paleogene mass extinction, about 66 million years ago.
(Germán Fernández, 01/26/2023)
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