A new partial skeleton including dermal armor of a thyreophoran (Ornithischia) dinosaur from Middle Jurassic of North Africa (Morocco).

ZAFATY, OMAR; OUKASSOU, MOSTAFA; PEREDA-SUBERBIOLA, XABIER; RIGUETTI, FACUNDO J.; COMPANY, JULIO; BENDRIOUA, SAAD; TABUCE, RODOLPHE; CHARRIÈRE, ANDRÉ

Simposio; 14th Symposium on Mesozoic Terrestrial Ecosystems and Biota; 2023

In the last decades, the Middle Atlas region has becomean important area for the study of Moroccan´s Mesozoicvertebrates, especially the Boulemane-El Mers region,rich in diverse dinosaur bones and tracks.The first dinosaur skeletal fossils have been found inMiddle Jurassic (El Mers I Formation) sites of this area(du Dresnay, 1963; Lapparent, 1955; Termier et al., 1940)were attributed to sauropods: i.e., ‘Cetiosaurus’ moghrebiensis (Lapparent, 1955; Monbaron et al., 1999).In the Bathonian-?Callovian continental deposits(Charrière & Haddoumi, 2016) of this area (El MersGroup), new genera and species of thyreophoran dinosaurs have been described recently, include the stegosaur Adratiklit boulahfa (Maidment et al., 2020), thefirst eurypodan thyreophoran from North Africa, andthe ankylosaur Spicomellus afer (Maidment et al.,2021), a bizarre armored dinosaur with spiked osteoderms fused to dorsal surface of the one preserved Tshaped rib. Additionally, an isolated well-preservedstegosaur footprint has also been described in thebasal sandstone beds of the ?Upper Jurassic-?LowerCretaceous Oued El Atchane Formation (Oukassouet al., 2023).The El Mers III Formation, in which our new site(BN-HIIUC) was discovered, is represented by a seriesof many tens of meters of fine friable marls and greenpinkish siltstones interbedded with relatively hard calcareous marls. At the base of the series, siltstones areorganized in irregular pinkish and greenish laminaewhere sauropod footprints (a manus-pes set) have beenidentified. This bioturbated level laterally changes togreenish grey marls rich in fossil wood, dinosaurbones, and pyrite concretions, indicating a continentalsedimentary environment with reducing conditions. Atthe top of the series, the marls are locally interbeddedwith small lenses of poorly-graded sandstones, andcovered unconformably by sandstone paleochannels ofthe Oued El Atchane Formation, and by pink siltstones with root traces indicating the development of apaleosol.Herein, we describe a new specimen discovered in 2021and collected over several consecutive field trips. The preserved material is composed by a partial, disarticulatedskeleton found in the grey siltstones of the El Mers IIIFormation. The specimen consists of postcranial remains:dorsal vertebrae and ribs, osteoderms, and other incomplete elements. All the fossils were found associated in asmall area, so it is likely that they belong to a single individual. The new thyreophoran remains shows stegosaurian features, such as moderately elongated pedicels of thedorsal neural arches, and osteoderms are reminiscent ofthat of ankylosaurs.Osteoderms show a quite asymmetrical structure. Theinternal surface shows a homogeneously ornamentedsurface, with a cross-hatched pattern of longitudinalridges at angles between 90 and 110 representingcollagen fibers. Two different osteoderms have beensampled for histological analyses. Both osteoderms arevery thick and show a massive and compact structure.The microstructure of these osteoderms consists oftwo outer coarse cortices composed of compact, primary woven bone, with simple vascular canals, enclosing a thick trabecular core composed of coarsecancellous secondary bone, resulting from a heavilyremodelation of the primary tissues by secondaryosteons. There is no evidence of the interwovenarranged structural fiber bundles and large resorptioncavities that constitutes the general pattern observedin ankylosaurs dermal ossifications (Burns et al., 2014;Scheyer & Sander, 2004).In general, the morphological features of the vertebraeare very similar to those of the stegosaurian dinosaurs,notably Adratiklit boulahfa. The osteoderms are very particular and their structure resembles at first glance thesacropelvic armor described in certain nodosaurid ankylosaurs (e.g., Polacanthus).Therefore, preliminary observations of the new material argue for the occurrence in the Middle Jurassic ofNorth Africa of a stegosaur with osteoderms that lookexternally like those of ‘polacanthid’ ankylosaurs.Additional work is needed for clarifying the phylogenetic position of this new specimen. These newdata represent an extremely important point to understand the diversity of thyreophoran (i.e., armored)dinosaurs, their paleobiogeography, and evolutionaryrelationships.