The El Mers Group is a geological group in the Middle Atlas of Morocco. It is subdivided into three formations named the El Mers I, II, and III Formations, respectively. It is a marine deposit primarily consisting of marl, with gypsum present in the upper part of unit 3. It is the lateral equivalent of the terrestrial Guettioua Sandstone. Dinosaur remains are among the fossils that have been recovered from the group, most notably those of sauropods and the unusual thyreophoransAdratiklit, Thyreosaurus, and Spicomellus. This formation records a series of coastal to shallow marine settings of Middle Jurassic (with an age of Bajocian-Bathonian for El Mers 1 and a Bathonian-Callovian? for El Mers II-III), based on detailed stratigraphical biomarkers such as ammonites or Foranimifera.[1]
A Modern analogue for El Mers Group is found in the coastal sequences of Coorong, South Australia.
The El Mers formations are composed primarily of marls with intermittent layers of limestone and sandstone, characterized by abundant organic matter and benthic fauna.[2] It is delineated into five distinct members, each exhibiting diverse lithological characteristics. The three members comprise a combination of layers of marls, sandstones, and limestones, marked by various unique fossils and organic materials. Sedimentary discontinuities reveal deltaic sequences that exhibit varying sedimentary structures and lithological compositions, reflecting dynamic depositional environments ranging from subtidal to continental. The facies associations within the El Mers Group are organized into elementary and mesosequences, delineating the transition from prodelta to deltaic plain environments. These sequences capture the evolution of tidal regimes and depositional dynamics.[3][4]
During the Bajocian-Bathonian transition the local faices paint a picture of a dynamic paleoenvironment, where the interplay between land and sea was pronounced.[1] This setting oscillated between a proximal continental shelf, marked by significant terrestrial input, and episodic marine incursions that suggest fluctuating sea levels.[1] The lower parts of the section hint at shallow, sometimes oxygen-deprived, stagnant waters, much like modern lagoon or restricted bays where circulation is limited. Upwards there are signs of marine influence growing stronger, particularly at the base of "Interval C", where a notable sea-level rise seems to have occurred, akin to what we might see on today's outer continental shelves, like those in the North Sea, where marine life diversity peaks, specially Bivalves and Ammonites. However, this marine dominance is not consistent, reverting back to a more coastal, land-influenced environment towards the top, similar to modern deltaic regions where riverine and marine waters mix, such as the Louisiana coast or the Coorong National Park. The environment would have experienced variable salinity, much like estuaries where fresh and marine waters blend, affecting the types of biota that could thrive.[1]
The shifts in facies and microfauna evolution in the area revelated intriguing patterns, from the establishment of initial supratidal environments characterized by charophytic marls and ostracod limestones followed by a resurgence of marine conditions, evident from the reappearance of coastal facies and the diversification of ostracod faunas, indicating a significant transgressive phase during the Upper Bathonian–Lower Callovian period. While in the Central High Atlas marine influences ceased by the Upper Bajocian–Lower Bathonian giving way to continental red layers, in the Middle Atlas marine influences persisted until the Upper Bathonian–Lower Callovian boundary, indicating a diachronic filling of the Atlas gulfs towards the Tethyan domain from west to east.[5]
A freshwater ostracod of the family Limnocytheridae. Local members of the genus genus would not be confined to limnic environments, but would be euryhaline or brackish, since they disappear when faunal diversity is at its maximum.
Isolated teeth, cranial and postcranial material, both isolated and semi-articulated
A marine crocodylomorph of the family Teleosauridae. The referral to the genus Steneosaurus should be taken with caution, as this genus was overlumped.
Isolated teeth of unidentified or undescribed/studied theropods. Teeth associated with the Teleosaur "Megalosaurus"mersensis have recently been suggesting to come from indeterminate Averostrans. Other remains need revision.[13][14]
Sauropod tracks of uncertain affinity. There are both small and enormous sauropod tracks, including some among the largest known worldwide, with a pes track length of up to 130 cm.[10]
NHMUK PV R37366 (A dorsal vertebra), three cervical vertebrae (NHMUK PV R37367; R37368), a dorsal vertebra (NHMUK PV R37365), and a left humerus (NHMUK PV R37007)
HIIUC-BN00, a partial postcranial skeleton that includes nine dorsal vertebrae and 21 dorsal rib remains, a limb bone (fibula?), and six dermal elements
A dacentrurine stegosaur with unusual recumbent osteoderms
↑ Läng, E. (2008). "Les Cétiosaures (Dinosaura, sauropoda) et les sauropodes du Jurassique moyen: revision systématique, nouvelles découvertes et implications phylogénétiques". (Doctoral dissertation, Paris, Muséum national d'histoire naturelle): 1–223. S2CID160327994.
↑ Zafaty, O.; Oukassou, M.; Riguetti, F.; Company, J.; Bendrioua, S.; Tabuce, R.; Charrière, A.; Pereda-Suberbiola, X. (2024). "A new stegosaurian dinosaur (Ornithischia: Thyreophora) with a remarkable dermal armour from the Middle Jurassic of North Africa". Gondwana Research. 131 (In press): 344–362. Bibcode:2024GondR.131..344Z. doi:10.1016/j.gr.2024.03.009.
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