The Spicomellusholotype 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-ossified spines. The holotype was CT scanned and histologically sectioned to confirm that it was an ankylosaurian.[1]
In 2021, Maidment et al. describedSpicomellus afer as a new genus and species of ankylosaurian thyreophoran based on these fossil remains. The generic name, 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.[1]
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.[1][2]
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.[1][3]
Classification
Preliminary assessments of the holotype led researchers to consider stegosaurian relationships for the species.[3] 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.[1]
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.[1] 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.[4]
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.[1]
↑ Clark, N. D. L. (2001). "A thyreophoran dinosaur from the Early Bajocian (Middle Jurassic) of the Isle of Skye, Scotland". Scottish Journal of Geology. 37 (1): 19–26. Bibcode:2001ScJG...37...19C. doi:10.1144/sjg37010019.
↑ 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
↑ 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.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
