Maevarano-Formation

The Maevarano Formation is a sequence of continental sedimentary rocks of the Upper Cretaceous of Madagascar and a significant fossil site. This geological formation was fluvial, therefore, deposited in a river system.

Geology

Outcrops of this formation are in the Mahajanga Basin in the province of Mahajanga in the northwest of the island, and in particular within the village Berivotra near the coast. At the time of their formation, there was the formation between 30 ° and 25 ° south latitude, as Madagascar drifted northward, after it has separated from India around 88 million years ago.

The exact geological age of the Maevarano formation is unclear. The adjacent marine Berivotra lineup, which was partially deposited at the same time as the upper portions of Maevarano lineup, shows that at least the upper portions fall into the Maastrichtian. Some authors give the Campanian as their old age, but for which there is no evidence. In the upper layers of Berivotra lineup are hints to a reversal of the geomagnetic field: This reversal is interpreted as the change between Chron 30N and Chron 29R - a polarity event, which took place in front of about 65.8 million years, which is about 300,000 years before the Cretaceous-Tertiary boundary and the associated mass extinction at the end of the Cretaceous. This suggests that the organisms of Maevarano fauna lived until just before the mass extinction.

The Maevarano lineup consists of three laminated limbs. The lowest layer, the Masorobe member is at least 80 meters thick. It mainly consists of reddish, grobsandigem sandstone with variable grain size ( poorly sorted ), some banks also show a fine -grained sandstone. Separated by a erosional follows the middle layer member of the formation, the Anembalemba Members. The lower section of this layer element consists of white to light gray, fine-to coarse sandy show clay-rich sandstones, the cross-bedding. The upper section consists of tonreichem sandstone with variable grain size together, showing a slightly olive-green to gray color. Most vertebrate fossils of the formation are from the Anembalemba Members, especially from the upper half. The third and uppermost layer member, the Miadana Members, consists of clay, silt and sand stone. These rocks show different colorations, with cross-bedding is missing. The Maevarano lineup as a whole is on the Marovoay -Beds; covered will of the Berivotra lineup.

Paleoecology

The Maevarano lineup at the time of deposition is interpreted as a plane river landscape. The deposition was completed with the time by slowly advancing sea, which marks the beginning of the deposition of the over the Maevarano lineup lying, marine Berivotra lineup. Wide and shallow rivers flow from the central highlands to the northwest. Leave evidence of debris flows suggest that the levels of the rivers were subject to strong fluctuations; so there were periods of low water level as well as periods with faster erosion, which sediments were deposited in the riverbeds. Occurring paleosols are reddish and contain root casts. The paleosols exhibit along with other sedimentological conditions on well-drained floodplain out with a rich vegetation, which was adapted to a relatively dry and strongly seasonal and partly semi-arid climate with dry and rainy seasons.

Vertebrate fauna

Fossil vertebrates of the formation include frogs such as the gigantic Beelzebufo, turtles, snakes, lizards and at least seven different species of Crocodylomorpha (including types of Mahajangasuchus, Trematochampsa, Simosuchus and Araripesuchus ) with a. Dinosaurs are the Abelisauriden Majungasaurus, the Noasauriden Masiakasaurus, the Dromaeosauriden Rahonavis and two forms of titanosaurs represented ( Rapetosaurus and a yet unnamed second form ). In addition, several fossil birds have been identified including Vorona. Crocodile relatives were very diverse and often. With six to seven meters in length Majungasaurus was probably the Spitzenprädator this ecosystem.

History of Research

The Maevarano lineup was first explored in 1895 by the French army surgeon Dr. Félix Salètes and his staff officer Landillon which sent fossils and geological material to the paleontologist Charles Depéret. Depéret published a short scientific description of the formation and named two new dinosaurs from the fossils sent to him - Titanosaurus madagascariensis and Megalosaurus crenatissimus (now Majungasaurus ). More often fragmentary fossils were collected during the 20th century - including a partial skull, which became the holotype specimen of the supposed Pachycephalosauriers Majungatholus. Later it turned out that it was a part of the skull ornaments of Majungasaurus in this fossil. A series of seven large-scale expeditions of Stony Brook University and the University of Antananarivo, the Mahajanga Basin Project, launched in 1993. These expeditions contributed significantly to the understanding of the formation and the organisms that lived at the time of the deposits.