The Ankylosaurs were a group of heavily-armoured, short-limbed, wide-bodied, Ornithischian Dinosaurs, best known from Late Cretaceous deposits in Eurasia and North America. Although the group is known to have existed by the Middle Jurassic, remains from this period are limited to a jawbone and a few teeth from two sites in England, and it has been speculated that earlier members of the group may have lacked the heavy armour of the Late Cretaceous species. This is not unreasonable, the Late Cretaceous saw the emergence of a range of new predators, including novel groups of Theropod Dinosaurs and Crocodilians, and the first large predatory Mammals and Snakes, leading to the evolution of novel defence strategies in many herbivorous groups.
Although well known from the Laurasian Continents, Ankylosaurs were for a long time thought to be absent from Gondwana. However, in 1986 this was changed by the discovery of Antarctopelta oliveroi, from the Late Cretaceous of Antarctica, the first known Dinosaur from that continent. This was followed in 1989 by the discovery of Kunbarrasaurus ieversi, a small Ankylosaur from the Early Cretaceous of Australia. The subsequent discovery of Stegouros elengassen from the Late Cretaceous of southern Chile and Patagopelta cristata from the Late Cretaceous of Argentina confirmed that Ankylosaurs were present, if uncommon, across Gondwana in the Cretaceous, and it was speculated that these Dinosaurs were members of a single clade, the Parankylosauria, which had split from its Laurasian cousins early in the history of the group, and developed a range of distinctive traits, including a distinctive tail weapon formed from five pairs of robust osteoderms, fused together to form a flat, fan-like weapon called a 'macuahuitl', in reference to a traditional weapon from Mesoamerica, made from a series of obsidian blades mounted on a wooden club, giving a sword-like appearance.
Spicomellus afer was first described in 2021 from a single rib-fragment with fused spikes from Morocco. The fossil appeared to have clear Ankylosaurian affinities, and, importantly, came from the Middle Jurassic, apparently confirming the hypothesis of an early-branching clade of Ankylosaurs reaching Gondwana and diversifying there.
In a paper published in the journal Nature on 27 August 2025, Susannah Maidment of the Fossil Reptiles, Amphibians and Birds Section a the Natural History Museum, and the School of Geography, Earth and Environmental Sciences at the University of Birmingham, Driss Ouarhache, Kawtar Ech-charay, Ahmed Oussou, Khadija Boumir, and Abdessalam El Khanchou of the GERA Laboratory at Sidi Mohamed Ben Abdellah University, Alison Park of Emanya Fossil Preparation and Conservation, Luke Meade, also of the School of Geography, Earth and Environmental Sciences at the University of Birmingham, Cary Woodruff of the Phillip and Patricia Frost Museum of Science, and the Museum of the Rockies, Simon Wills and Mike Smith, also of the Fossil Reptiles, Amphibians and Birds Section a the Natural History Museum, Paul Barrett, again of the Fossil Reptiles, Amphibians and Birds Section a the Natural History Museum, and of the Evolutionary Studies Institute at the University of the Witwatersrand, and Richard Butler, once again of the School of Geography, Earth and Environmental Sciences at the University of Birmingham, describe a new partial skeleton of Spicomellus afer, enabling a more complete reconstruction of the species and a re-interpretation of the history of the Ankylosauria.
The new material comprises a left quadrate, two cervical, two dorsal, four sacral and four free caudal vertebrae, two handle vertebrae, six dorsal ribs with spikes fused to their dorsal surfaces, both scapulocoracoids, ilia and pubes, the right ischium, two metatarsals, an elaborate cervical half ring bearing extremely long spikes, a sacral shield bearing small and large spikes, numerous plates, large and small spikes of various morphologies, and several compound osteoderms comprising combinations of small and large spikes. These suggest an unusually well-armoured Animal, even compared to other Ankylosaurs, casting doubt upon the prevailing idea that heavy armour developed in the group in the Late Cretaceous as a result of increased predation pressures.
The deposits from which the skeleton was recovered come from the Bathonian stage, making them between 168.2 and 165.3 million years old, making Spicomellus afer equal in age to the oldest known Ankylosaur fossils, a scattering of isolated from the Bathonian White Limestone of England, and close to the predicted split between the Ankylosauria and the Stegosauria.
A phylogenetic analysis carried out by Maidment et al. was unable to recover the Parankylosauria as a distinct clade of Gondwanan Ankylosaurs. The Chilean Stegouros elengassen was found to be the sister species to all other Anylosaurs, apparently representing an early-diverging, and otherwise lost, lineage, while other members of the proposed Parankylosauria were scattered throughout the Ankylosauria as a whole, with their formerly apparent similarities being a result of convergent evolution.
Based upon this, Maidment et al. conclude that heavy and extensive armour first appeared in Ankylosaurs in the Middle Jurassic, and was apparently a defining feature of the group from the outset. The further speculate that this may have been a result of sexual selection rather than predation pressure, something which could drive the rapid development of an anatomy distinct from that of their closest relatives. Under this scenario, the widespread appearance of heavily armoured Ankylosaurs in the Late Cretaceous might represent not the adaptation of a less well armoured group to a high predation environment, but rather the advantageous pre-adaptation of the group to such an environment, enabling the Ankylosaurs to flourish at the expense of less well-armoured rival groups.
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