Spicomellus is an extinct genus of early dinosaur from the El Mers III Formation (Bathonian-Callovian) of Morocco. The genus contains a Monotypic taxon, S. afer, known from a single rib with fused . Spicomellus represents the oldest named ankylosaur.
Discovery and naming
The
Spicomellus holotype specimen,
NHMUK PV R 37412, was discovered in layers of the El Mers III Formation near
Boulahfa in
Boulemane, Fès-Meknès region, Morocco. It was later acquired by London's Natural History Museum from a commercial fossil dealer. The specimen consists of a single
rib with four co-
Ossification spines. The holotype was
and histologically sectioned to confirm that it was an ankylosaurian.
In 2021, Maidment et al. described Spicomellus afer as a new genus and species of ankylosaurian thyreophoran based on these fossil remains. The Genus, Spicomellus, combines the Latin words spica, meaning "spike" and mellum, which refers to a collar of spikes. The specific name, afer, is a Latin word referring to something inhabiting Africa.
Abundant diverse dinosaurs have been found in Jurassic sediments, but their remains are rarer in deposits. Spicomellus is the second described eurypodan taxon from North Africa, after Adratiklit.
Description
The preserved dermal spikes of the holotype were fused directly to the bone, a trait unique to
Spicomellus and not known from any other
vertebrate. Some prehistoric animals, including
Protuberum (a
cynodont) and
Euscolosuchus (a
), have superficially similar modified ribs. In all other known ankylosaurs, the
are embedded into the muscle tissue, rather than fused to underlying bone.
Classification
Preliminary assessments of the holotype led researchers to consider stegosaurian relationships for the species.
[ In their 2021 description of Spicomellus, Maidment et al. discussed several lines of evidence supporting the placement of Spicomellus a basal . They had considered the possibility that the rib was actually part of the jaw of an osteichthyan fish, since some members have teeth fused to their jaws. However, since there is no evidence of ortho—an important component of fish teeth—they considered this classification unsupported. Based on the T-shaped cross section of the rib, Spicomellus can reasonably be assigned to the Eurypoda. Furthermore, the structural fibers of the osteoderms are interwoven, with a plywood-like arrangement, which is seen in ankylosaurs but not other thyreophorans.]
Spicomellus is the oldest known ankylosaur that has currently been named from anywhere in the world. Few other ankylosaurs are known from a similar time. Sarcolestes, known from a partial lower jaw, was found in England's Oxford Clay Formation, which dates to the Callovian age.
Paleoecology
Spicomellus is known from the El Mers III Formation (El Mers Group) of Morocco. This would have been part of the supercontinent Gondwana during the Middle Jurassic. It coexisted with the sauropod " Cetiosaurus" mogrebiensis and the stegosaurs Adratiklit and Thyreosaurus.[J. Jenny, A. Le Marrec, and M. Monbaron. (1981). Les empreintes de pas de dinosauriens dans le Jurassique moyen du Haut Atlas central (Maroc): nouveaux gisements et precisions stratigraphiques. Geobos. 14(3):427-431] The sauropod Atlasaurus is also known from the contemporaneous terrestrial Guettioua Formation.[M. Monbaron, D. A. Russell, and P. Taquet. (1999). Atlasaurus imelakei n.g., n.sp., a brachiosaurid-like sauropod from the Middle Jurassic of Morocco. Comptes Rendus de l'Académie des Sciences à Paris, Sciences de la Terre et des Planètes 329:519-526.]
The discovery of Spicomellus also shows that the two major thyreophoran groups (Ankylosauria and Stegosauria) coexisted for over 20 million years, and implies that the putative extinction of the stegosaurs in the Early Cretaceous may have happened for reasons other than an increased diversity of anyklosaurs at that time.
Studies by the describing authors of the sedimentology and stratigraphy of the locality suggested a shallow marine depositional environment with continental mixed Clastic rock, evaporitic and Carbonate rock.
