Thyreosaurus

MEANING: Shield lizard

PERIOD: Middle Jurassic

CONTINENT: Africa

Thyreosaurus is a stegosaurian dinosaur characterized by a unique armor resembling those of nodosaurid ankylosaurs more than those of its fellow stegosaurs. The bony growths appear to have laid flat on the animal's back, rather than standing erect. However, due to the disarticulated nature of the fossil specimen, their exact position remains unknown.

Abstract from paper: In recent years the Middle Atlas of Morocco has become an area of interest for the study of dinosaurs in northern Africa. The Boulahfa locality, near Boulemane, has produced a diverse dinosaur assemblage from the Middle Jurassic of the El Mers Group. Fossil remains of sauropods and thyreophorans, such as ankylosaurs (Spicomellus) and stegosaurs (Adratiklit), have been reported thus far in this region. Here, we describe a new partial thyreophoran skeleton found in the gray marls of the El Mers III Formation (Bathonian-? Callovian), which mainly consists of disarticulated dorsal vertebrae and ribs, and associated dermal armour elements. Axial characters (e.g., elongated pedicels of the dorsal neural arches; upturned transverse processes and dorsal ribs with straight axes suggesting a narrow ribcage) indicate that the specimen belongs to a medium to large-sized stegosaur. The dorsal vertebrae show differences with those of Adratiklit, whose material has been found at the same stratigraphic levels. Thyreosaurus atlasicus gen. et sp. nov. is characterized by a remarkable dermal armour, which consists of thick (up to 4 cm) subovate to subrectangular-shaped osteoderms. The asymmetrical texture of their sides, one roughly ornamented with small pits and fiber bundles, the other with a well-marked cross-hatched pattern, is clearly different from that observed to date in other stegosaurs (and ankylosaurs). The bone histology of these osteoderms is reminiscent of that of stegosaur tail spines. It is interpreted that these osteoderms were arranged in a recumbent position over the body of the animal, instead of an erect position. The holotype corresponds to an adult individual who did not reach its maximum body size (estimated body length 6 m). The phylogenetic analysis suggests that Thyreosaurus is closely related to Dacentrurus within Dacentrurinae. The recent discoveries of Adratiklit and Thyreosaurus provide insight into the early evolution of stegosaurs in the Middle Jurassic of Africa.

Thyreosaurus is from the Middle Jurassic. The Middle Jurassic, spanning from approximately 174 to 163 million years ago, was a period of increasing tectonic activity and evolutionary innovation. By this time, the supercontinent Pangaea had begun to split more significantly, with Laurasia in the north and Gondwana in the south drifting apart. This movement created new coastlines, shallow seas, and rift valleys that fostered diverse ecosystems. The climate remained generally warm and humid, promoting the spread of lush vegetation, including ferns, cycads, and conifers, which blanketed much of the land and supported a wide variety of herbivorous dinosaurs.

Though less well known than the Late Jurassic, the Middle Jurassic was an important evolutionary chapter. Several major dinosaur groups began to diversify, including the stegosaurs and more derived long-necked sauropods that would later dominate the landscape. Theropods also continued to evolve, giving rise to new lineages like the megalosaurids and the early ancestors of more derived carnivores. Fossil evidence from this interval is relatively scarce compared to later stages, but what we do have paints a picture of an increasingly complex world, setting the stage for the iconic ecosystems of the Late Jurassic.

Thyreosaurus is a stegosaur. Stegosauria is a group of herbivorous ornithischian dinosaurs best known for their distinctive back plates and tail spikes. They first appeared in the Middle Jurassic and were most diverse during the Late Jurassic, although a few persisted into the Early Cretaceous. Stegosaurian fossils have primarily been discovered in the Northern Hemisphere, particularly in North America, Europe, and Asia, with fewer remains found in the Southern Hemisphere. Their distribution suggests they were successful in a range of environments, from floodplains to more arid regions, though they eventually declined in diversity as other herbivorous groups rose to prominence.

Stegosaurians were part of a larger group called Thyreophora, which also includes the heavily armored ankylosaurs. Early in their evolutionary history, stegosaurs developed simple rows of bony osteoderms for defense. Over time, these structures became more elaborate, evolving into the tall, broad plates and formidable tail spikes that characterize the group. These plates typically ran in two rows along the back and may have served multiple purposes including defense, thermoregulation, and display, though their exact function is still debated. At the end of the tail, most stegosaurs bore a set of long paired spikes which could have been used as a powerful weapon against predators.