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 thyreophorans Adratiklit, 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.
Paleoenvironment
The El Mers formations are composed primarily of marls with intermittent layers of limestone and sandstone, characterized by abundant organic matter and benthic fauna. 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.
During the Bajocian-Bathonian transition the local faices paint a picture of a dynamic paleoenvironment, where the interplay between land and sea was pronounced. This setting oscillated between a proximal continental shelf, marked by significant terrestrial input, and episodic marine incursions that suggest fluctuating sea levels. 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.
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.
Fossils
Foraminifera GenusSpeciesStratigraphic positionMaterialNotesImagesBullopora B. tuberculata SW part of the Skoura synclineTests/ShellsA benthic foraminiferan of the family PolymorphinidaePfenderella P. cf. arabica SW part of the Skoura synclineTests/ShellsA benthic foraminiferan of the family PfenderininaePraekurnubia P. crusei SW part of the Skoura synclineTests/ShellsA benthic foraminiferan of the family Pfenderinidae; a genus found in the Tethys between Bajocian and Oxfordian-aged layersPseudocyclammina P. maynci SW part of the Skoura synclineTests/ShellsA benthic foraminiferan of the family Hauraniidae
Dinoflagellates GenusSpeciesStratigraphic positionMaterialNotesImagesAldorfia A. aldorfensis FD section, Skoura SynclineCystsA Dinoflagellate of the family GonyaulacaceaeCribroperidinium C. crispum C. venustum FD section, Skoura SynclineCystsA Dinoflagellate of the family GonyaulacaceaeCtenidodinium C. combazii C. continuum C. cornigerum FD section, Skoura SynclineCystsA Dinoflagellate, incertae sedis Dichadogonyaulax D. sellwoodii FD section, Skoura SynclineCystsA Dinoflagellate, incertae sedis Korystocysta K. gochtii K. pachyderma FD section, Skoura SynclineCystsA Dinoflagellate of the family GonyaulacaceaeMeiourogonyaulax M. caytonensis M. reticulata M. valensii FD section, Skoura SynclineCystsA Dinoflagellate of the family GonyaulacaceaeNannoceratopsis N. deflandrei FD section, Skoura SynclineCystsA Dinoflagellate of the family NannoceratopsiaceaePareodinia P. ceratophora FD section, Skoura SynclineCystsA Dinoflagellate of the family PareodiniaceaeRhynchodiniopsis R. cladophora FD section, Skoura SynclineCystsA Dinoflagellate of the family GonyaulacaceaeTrichodinium T. scarburghense FD section, Skoura SynclineCystsA Dinoflagellate of the family GonyaulacaceaeTubotuberella T. dangeardii FD section, Skoura SynclineCystsA Dinoflagellate of the family GonyaulacaceaeWanaea W. acollaris W. indotata W. verrucosa FD section, Skoura SynclineCystsA Dinoflagellate of the family GonyaulacaceaeWilleidinum W. spp. FD section, Skoura SynclineCystsA Dinoflagellate, incertae sedis
Brachiopoda GenusSpeciesStratigraphic positionMaterialNotesImagesCymatorhynchia C. quadriplicata El MersIsolated shellsA branchipod of the family Tetrarhynchidae Daghanirhynchia D. termierae El MersIsolated shellsA branchipod of the family Tetrarhynchidae Formosarhynchia F. (Rhynchonella?) dumortieri El MersIsolated shellsA branchipod of the family RhynchonellidaeRugitela R. praebullata El MersIsolated shellsA branchipod of the family ZeilleriaceaeStiphrothyris S. mouterdei El MersIsolated shellsA branchipod of the family LobothyrididaeTubithyris T. chouberti T. cf. wrighti El MersIsolated shellsA branchipod of the family LobothyrididaeZeilleria Z. subbucculenta El MersIsolated shellsA branchipod of the family Zeilleriaceae
Ostracoda GenusSpeciesStratigraphic positionMaterialNotesImagesCytherella C. index Ait HammouValvesA freshwater/brackish ostracod of the family CytherellidaeFastigatocythere F. aff. brentonensis Ait HammouValvesA freshwater/brackish ostracod of the family ProgonocytheridaeKinkelinella K. aff. triangula Ait HammouValvesA freshwater ostracod of the family ProgonocytheridaeMetacypris M. spp. Ait HammouValvesA freshwater ostracod of the family Limnocytheridae. Local members of the genus would not be confined to limnic environments, but would be euryhaline or brackish, since they disappear when faunal diversity is at its maximum.Procytheridea P. ihopyensis Ait HammouValvesA freshwater ostracod of the family Schulerideidae
Xiphosura GenusSpeciesStratigraphic positionMaterialNotesImagesSelenichnites S. tesiltus S. isp. El Mers Tassmante O'MocheIsolated crescent-shaped trace fossilsTraces of xiphosurans, representing the second occurrence of this ichnogenus in Morocco
Mollusca GenusSpeciesStratigraphic positionMaterialNotesImagesAstarte A. cf. rayensis El Mers Skoura synclineIsolated ShellsA marine clam of the family AstartidaeCadomites C. bremeri C. cf. bremeri East Skoura synclineIsolated shellsAn ammonite of the family Stephanoceratidae and indicator of the Bathonian ageCercomya C. undulata El MersIsolated shellsA marine clam of the family LaternulidaeChoffatia C. cf. urinacensis East Skoura synclineIsolated shellsAn ammonite of the family Perisphinctidae and indicator of the Callovian ageCorbula C. pectinata C. lyrata Aït KermoussIsolated shellsA marine clam of the family Corbulidae Clydoniceras C. discus Tamguert n'Tarit Tizi n'JuillerhIsolated shellsAn ammonite of the family Clydoniceratidae Eomiodon E. angulatus E. gardeti E. algeriensis E. indicus El Mers Skoura synclineIsolated ShellsA marine clam of the family Neomiodontidae Isognomon I. (Mythiloperna) murchisoni El MersIsolated shellsA marine oyster of the family MalleidaeLima L. (Plagiostoma) subcardiformis Aït KermoussIsolated shellsA marine file clam of the family LimidaeLiostrea L. sandalina El Mers Skoura synclineIsolated ShellsA marine oyster of the family OstreidaeLopha L. costata Aït KermoussIsolated shellsA marine oyster of the family OstreidaeModiolus M. imbricatus M. cf. bipartitus Aït Kermouss El Mers Tamguert n'TaritIsolated shellsA marine mussel of the family MytilidaeMorphoceras M. macrescens East Skoura synclineIsolated shellsAn ammonite of the family Morphoceratidae and indicator of the Bathonian ageMytilus M. (Falcimytilus) tumidus Aït KermoussIsolated shellsA marine mussel of the family MytilidaeNeomiodon N. skouraensis El Mers Skoura synclineIsolated ShellsA marine clam of the family NeomiodontidaeNucula N. (Paleonucula) waltoni El MersIsolated shellsA marine nut clam of the family NuculanidaeOraniceras O. sp. East Skoura synclineIsolated shellsAn ammonite of the family Parkinsoniidae and indicator of the Bathonian ageOrionoides O. cf. pseudorion East Skoura synclineIsolated shellsAn ammonite of the family PerisphinctidaeOstrea O. sandalina O. (Liostrea) hebridica Aït Kermouss El Mers Tamguert n'TaritIsolated shellsA marine oyster of the family OstreidaeOxyeurax O. fimbriata Aït Kermouss El Mers Tamguert n'TaritIsolated shellsA marine clam of the family AstartidaeParacenoceras P. sp. Aït KermoussIsolated shellsA Nautilidan of the family ParacenoceratidaeParkinsonia P. sp. Skoura synclineIsolated shellsAn ammonite of the family Parkinsoniidae and indicator of the Bajocian age for the lower portions of the GroupProtocardia P. tikechkachensis El Mers Tamguert n'TaritIsolated shellsA marine cockle of the family CardiidaeTrigonia T. kheraensis El Mers Skoura synclineIsolated ShellsA marine clam of the family Trigoniidae
Echinoderms GenusSpeciesStratigraphic positionMaterialNotesImagesAcrosalenia A. spinosa El Mers Ich TimellalineIsolated IndividualsAn Echinoidean of the family AcrosaleniidaeHemicidaris H. jauberti H. cf. icaunensis Ich TimellalineIsolated IndividualsAn Echinoidean of the family Hemicidaridae
Fishes GenusSpeciesStratigraphic positionMaterialNotesImagesScheenstia S. cf. mantelli S ("Lepidotes") ssp. Ait Hammou Djmila Oued El Mers Tizi n'JuillerhPartial specimen wirth cranial material; Referred cranial material, isolated teeth and scalesA marine/brackish bony fish of the family Lepidotidae
Testudinata GenusSpeciesStratigraphic positionMaterialNotesImagesTestudinata indet. Indeterminate Tamguert n'Tarit Tizi n'JuillerhVery numerous debris: escutcheons, plates, vertebrae, ribs, long bonesTurtle remains of uncertain affinity
Crocodylomorpha GenusSpeciesStratigraphic positionMaterialNotesImagesHatcherichnus H. ispp. Inzar O'FounassFootprintsIndeterminate Crocodrylomorph Tracks"Megalosaurus" "M." mersensis Botane Tizi n'JuillerhIsolated teeth and vertebraA marine crocodylomorph of the family Teleosauridae. Mistaken as Theropod remains in the past.Steneosaurus? S.? spp. Bou Iferaoun Botane Darak Djmila Oued Tamemecht Tamguert r'Tane Tirardine TissenfeltIsolated teeth, cranial and postcranial material, both isolated and semi-articulatedA marine crocodylomorph of the family Teleosauridae. The referral to the genus Steneosaurus should be taken with caution, as this genus was overlumped.
Dinosauria Theropoda