Spicomellus

Spicomellus Temporal range: Middle Jurassic (Bathonian–Callovian), ~
Illustration of the holotype.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	†Ornithischia
Clade:	†Thyreophora
Clade:	†Ankylosauria
Genus:	†Spicomellus Maidment et al., 2021
Species:	†S. afer
Binomial name
†Spicomellus afer Maidment et al., 2021

Spicomellus is an extinct genus of early ankylosaurian dinosaur from the El Mers III Formation (Bathonian-Callovian) of Morocco. The genus contains a single species, S. afer, known from a single rib with fused osteoderms. Spicomellus represents the oldest named ankylosaur.

Discovery and naming

The Spicomellus holotype specimen, NHMUK PV R37412, 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-ossified spines. The holotype was CT scanned 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 generic name, Spicomellus, combines the Latin words "spicus", meaning "spike" and "mellus", which refers to a collar of spikes. The specific name, afer, is a Latin word referring to something inhabiting Africa.

Abundant diverse eurypodan dinosaurs have been found in Jurassic Laurasian sediments, but their remains are rarer in Gondwanan 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 pseudosuchian), have superficially similar modified ribs. In all other known ankylosaurs, the osteoderms 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 ankylosaurian. 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 orthodentine—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. An unnamed thyreophoran from the Bajocian-aged Bearreraig Sandstone Formation of the Isle of Skye, Scotland, could be older than Spicomellus, but it is unclear if these fragmentary remains belonged to a stegosaur or an ankylosaur.

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. Predators of the ecosystem consisted of indeterminate theropods (possible megalosaurids). The sauropod Atlasaurus is also known from the contemporaneous terrestrial Guettioua Formation.

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, evaporitic and carbonate sediments.