Maevarano formation
The Maevarano Formation is a sequence of continental sedimentary rocks of the Upper Cretaceous in Madagascar and an important fossil site. This geological formation was deposited fluviatil , i.e. 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 in the vicinity of the village of Berivotra near the coast. At the time of its formation, the formation was between 30 ° and 25 ° south latitude when Madagascar drifted northward after separating from India about 88 million years ago .
The exact geological age of the Maevarano Formation is unclear. The adjacent marine Berivotra Formation , which was partially deposited at the same time as the upper sections of the Maevarano Formation, shows that at least the upper sections fall into the Maastrichtian . Some authors give the Campanium as the age, but there is no evidence for this. In the upper layers of the Berivotra Formation there are indications of a polarity reversal of the earth's magnetic field : This polarity reversal is interpreted as the change between Chron 30N and Chron 29R - a polarity event that took place about 65.8 million years ago, i.e. about 300,000 years before the Cretaceous-Tertiary border and the associated mass extinction at the end of the Cretaceous. This suggests that the organisms of the Maevarano fauna lived until just before mass extinction.
The Maevarano formation consists of three layers. The lowest layer, the Masorobe member, is at least 80 meters thick. It consists mainly of reddish, coarse sandstone with variable grain size ( poorly sorted ), with some banks also showing fine-grained sandstone. Separated by an erosion discordance , the middle layer of the formation follows, the anembalemba member. The lower section of this stratum consists of whitish to slightly gray, fine to coarse sand, clay-rich sandstones that show oblique stratification. The upper section is composed of clay-rich sandstone with variable grain size, which is slightly olive-green to gray in color. Most of the formation's vertebrate fossils come from the Anembalemba member, especially the upper half. The third and topmost layer member, the Miadana member, consists of clay , silt and sandstone. These rocks show different colors, but there is no oblique stratification. The Maevarano formation as a whole lies on the Marovoay beds; it is covered by the Berivotra Formation.
Paleoecology
The Maevarano formation at the time of the deposition is interpreted as a flat river landscape. The deposition was terminated over time by the slowly advancing sea , which marks the beginning of the deposition of the marine Berivotra Formation above the Maevarano Formation. Broad and shallow rivers flowed from the central plateaus to the northwest. Evidence of debris flows suggests that the levels of the rivers were subject to strong fluctuations; there were periods of low water level as well as periods of rapid erosion, as a result of which sediments were deposited in the river beds. Occurring paleo soils are reddish and contain root prints. The paleo-soils, together with other sedimentological conditions, indicate well-drained flood plains, with rich vegetation that was adapted to a relatively dry, highly seasonal and partly semi-arid climate with dry and rainy seasons.
Vertebrate fauna
The formation's fossil vertebrates include frogs such as the gigantic Beelzebufo , turtles, snakes, lizards, and at least seven different species of crocodylomorpha (including species of Mahajangasuchus , Trematochampsa , Simosuchus, and Araripesuchus ). Dinosaurs are represented with the Abelisauriden Majungasaurus , the Noasauriden Masiakasaurus , the Dromaeosauriden Rahonavis and two forms of titanosaurs ( Rapetosaurus and an as yet unnamed second form). Several fossil birds were also discovered, including Vorona . Crocodile relatives were very diverse and frequent. At six to seven meters in length, Majungasaurus was probably the top predator of this ecosystem.
Research history
The Maevarano formation was first discovered in 1895 by the French military doctor Dr. Félix Salètes and his colleague, Officer Landillon, are researching which fossils and geological material sent to the paleontologist Charles Depéret . Depéret published a brief 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 - among them is a partial skull, which the holotype -Exemplar the alleged Pachycephalosauriers Majungatholus was. Later it turned out that this fossil was part of the skull ornaments of Majungasaurus . A series of seven large-scale expeditions from Stony Brook University and Antananarivo University, the Mahajanga Basin Project, started in 1993. These expeditions contributed considerably to knowledge of the formation and the organisms that lived at the time of the deposits.
Individual evidence
- ^ A b c d e Raymond R. Rogers, Krause, David W .; Curry Rogers, Kristina; Rasoamiaramanana, Armand H .; & Rahantarisoa, Lydia .: Paleoenvironment and Paleoecology of Majungasaurus crenatissimus (Theropoda: Abelisauridae) from the Late Cretaceous of Madagascar . In: Sampson, Scott D .; & Krause, David W. (Ed.): Majungasaurus crenatissimus (Theropoda: Abelisauridae) from the Late Cretaceous of Madagascar (= Society of Vertebrate Paleontology Memoir 8) 2007, pp. 21-31.
- ↑ David B. Weishampel, Barrett, Paul M .; Coria, Rodolfo A .; Le Loueff, Jean; Xu Xing; Zhao Xijin; Sahni, Ashok; Gomani, Elizabeth MP; and Noto, Christopher N .: Dinosaur distribution . In: Weishampel, David B .; Dodson, Peter; and Osmólska, Halszka (Ed.): The Dinosauria , 2nd. Edition, University of California Press, Berkeley 2004, ISBN 0-520-24209-2 , p. 604.
- ^ Susan E. Evans, Marc EH Jones, David W. Krause: A giant frog with South American affinities from the Late Cretaceous of Madagascar . Proc. Natl. Acad. Sci. USA, 10.1073 / pnas.0707599105 Abstract
- ^ A b David W. Krause, Sampson, Scott D .; Carrano, Matthew T .; & O'Connor, Patrick M .: Overview of the history of discovery, taxonomy, phylogeny, and biogeography of Majungasaurus crenatissimus (Theropoda: Abelisauridae) from the Late Cretaceous of Madagascar . In: Sampson, Scott D .; & Krause, David W. (Ed.): Majungasaurus crenatissimus (Theropoda: Abelisauridae) from the Late Cretaceous of Madagascar (= Society of Vertebrate Paleontology Memoir 8) 2007, pp. 1-20.
- ↑ Charles. Depéret: Note on the Dinosauriens Sauropodes et Théropodes du Crétacé supérieur de Madagascar . In: Bulletin de la Société Géologique de France . 21, 1896, pp. 176-194.
- ^ Hans-Dieter Sues, & Taquet, Phillipe .: A pachycephalosaurid dinosaur from Madagascar and a Laurasia − Gondwanaland connection in the Cretaceous . In: Nature . 279, No. 5714, 1979, pp. 633-635. doi : 10.1038 / 279633a0 .