A new concentration lagerstätte for marine vertebrates of Tithonian (Uppermost Jurassic) age in southern Coahuila, Mexico
Final Report Abstract
The Gomez Farias coquina deposit consists of ammonites, aptychi, and bivalves, in addition to marine vertebrates. The faunal elements present in this unit were assigned to the uppermost Kimmeridgian Beckeri Zone and are therefore slightly older than originally expected. The unit accumulated in a hemipelagic mud bottom environment under low oxygen conditions. A combination of upwelling, winnowing, offshore winds, circulatory nutrient traps, low oxygenated bottom waters, transgressional regime with reduced net sedimentation, marine phosphate generation and phosphorus migration was crucial for the high concentration and excellent preservation of marine reptilians. Condensed intervals are repeatedly present in the lower La Caja Formation in southern Coahuila and Lagerstatten-like concentration of fossils may therefore be a widespread phenomenon in the area. Microfossil and invertebrate assemblages identified in both the La Caja and La Casita formations provide evidence for partial isolation of the Gulf of Mexico from both the Paleopacific and the western and central Tethys. The intermittent presence of lithostratigraphic units with explicit high latitude Boreal (e.g. Cylindroteuthis, Buchia) and Austral proveniences (Anopaea, Protarctis) reflects short periods of cold-water ingression into the Gulf of Mexico, caused either by geotectonic changes associated with the opening of the Hispanic corridor, or by periods of short-term cooling. Our findings also provide evidence for an intermittent latest Jurassic faunal pathway between the Boreal, the Western Gulf of Mexico, and the Pacific Realm. This situation only changed during the earliest Berriasian, when Mediterranean taxa (e.g. abundant calpionellids, Mesohibolitid Rhaphibelus aciculiformis) arrive in northeastern Mexico and the area is permanently connected with the Tethys.
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