A new stegosaurian genus and species from the Middle Jurassic epoch has been identified from the postcranial remains found in the Middle Atlas Mountains south of Fès, Morocco.
Thyreosaurus atlasicus lived in northern Africa during the Middle Jurassic epoch, approximately 165 million years ago.
The species is closely related to Dacentrurus, a member of the clade Dacentrurinae within the thyreophoran dinosaur family Stegosauridae.
“Thyreophora is a diverse clade of herbivorous ornithischian dinosaurs composed of stegosaurs, ankylosaurs, and basal forms,” said Dr. Mostafa Oukassou from the Hassan II University of Casablanca and colleagues.
“Thyreophorans have an extensive fossil record, spanning from the Lower Jurassic to the Upper Cretaceous, which is best known from well-preserved specimens in Laurasia.”
“In contrast, fossil remains are poorly known and mostly partial in Gondwana,” they added.
“In Africa, thyreophorans could have been present since the Early-Middle Jurassic.”
“Stegosaurian body fossils are documented in the Middle Jurassic of Morocco, the Upper Jurassic of Tanzania and the Lower Cretaceous of South Africa.”
“African ankylosaurs are represented only by Spicomellus afer from the Middle Jurassic of Morocco.”
The disarticulated partial skeleton of Thyreosaurus atlasicus was found in the gray marls of the El Mers III Formation in the northern Boulahfa locality, near Boulemane, Middle Atlas of Morocco.
“In 2021, during a geological mapping mission south of Boulemane, we discovered a new site rich in dinosaur remains in the El Mers III Formation,” the paleontologists said.
“This site, named Boulahfa North, is located north of the Boulahfa quarry.”
“The material was collected over several consecutive field trips, and consists of a partial postcranial skeleton of a stegosaur that includes dorsal vertebrae and ribs, a limb bone and dermal armor.”
According to the team, Thyreosaurus atlasicus had an asymetric bizarre dermal armor, unique among stegosaurs.
“The dermal armor of Thyreosaurus atlasicus consists of thick — up to 4 cm (1.6 inches) — subovate to subrectangular-shaped osteoderms,” the researchers said.
“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 stegosaurian 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.”
Thyreosaurus atlasicus had an estimated body length of over 6 m (19.7 feet).
“Axial characters indicate that the specimen belongs to a medium to large-sized stegosaur,” the scientists said.
“The holotype corresponds to an adult individual who did not reach its maximum body size.”
The discovery of Thyreosaurus atlasicus is important in understanding the evolutionary history of stegosaurs and other thyreophoran dinosaurs.
“The recent discoveries of Adratiklit and Thyreosaurus atlasicus provide insight into the early evolution of stegosaurs in the Middle Jurassic of Africa,” the authors concluded.
The findings appear in the journal Gondwana Research.
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Omar Zafaty et al. 2024. A new stegosaurian dinosaur (Ornithischia: Thyreophora) with a remarkable dermal armour from the Middle Jurassic of North Africa. Gondwana Research 131: 344-362; doi: 10.1016/j.gr.2024.03.009