The earliest known ankylosaurs possessed a unique and elaborate body armor, including a sharp tail ornament, according to an investigation published in Nature. These extreme features of the armor are unlike those of any other vertebrate, including other ankylosaurs later in their evolutionary history. Analyses of Middle Jurassic fossils from Morocco, dating to about 165 million years ago, offer new insights into the evolution of tail weapons and armor in ankylosaurs.
Ankylosaurs are a group of armored dinosaurs known mainly from the Cretaceous (143–66 million years ago) of North America and Asia. Their initial evolution, in the Middle Jurassic (about 175–162 million years ago), is unclear due to the scarcity of fossil records from that period.
A fossil from the Middle Jurassic of Morocco was reported as being the oldest known ankylosaur, named Spicomellus afer, but the specimen was only a partial rib, which meant it was difficult to determine the body plan of the early ankylosaurs.
A new partial skeleton of Spicomellus, found in the same rocks of the Atlas Mountains in Morocco as the original specimen, is described by Susannah Maidment and colleagues. The new specimen confirms affinities with ankylosaurs and reveals an elaborate dermal armor.
The skeleton includes six ribs with spines attached to their upper surface, a bony collar with plates and two pairs of spines (one spine almost complete measures 87 cm in length) and a pelvic shield with long and short spines. Distinct bones in the tail are indicative of a tail weapon.
Maidment and her colleagues suggest that the features observed in Spicomellus may have served both display and defense. Later ankylosaurs had a simpler and less extravagant body armor, which may suggest a shift toward a primarily defensive function, the authors propose.
Finally, the evidence of a tail weapon in this specimen indicates that adaptations for tail-armament evolved about 30 million years earlier than previously thought, the authors conclude.
