Maevarano Formation

The Maevarano Formation is an Upper Cretaceous sedimentary rock formation found in the Mahajanga Province of northwestern Madagascar. It is most likely Maastrichtian in age, and records a seasonal, semiarid environment with rivers that had greatly varying discharges. Notable animal fossils recovered include the theropod dinosaur Majungasaurus, the early bird Vorona, the flying dromaeosaur Rahonavis, the titanosaurian sauropod Rapetosaurus, and the giant frog Beelzebufo.

Description

The Maevarano Formation outcrops in the Mahajanga Province of Madagascar, particularly within 50 kilometers (31 mi) southeast of the provincial capital, Mahajanga (marked with a red dot on the map).

The Maevarano Formation is well-exposed in the Mahajanga Basin, in particular near the village of Berivotra near the northwestern coast of the island where its outcrops have been heavily dissected by erosion. At the time it was being deposited, its latitude was between 30°S and 25°S as Madagascar drifted northward after splitting from India about 88 million years ago. It is composed of three smaller units or members. The lowest is the Masorobe Member, which is usually reddish and is at least 80 meters thick (262 ft). Its rocks are mostly poorly sorted coarse-grained sandstones with some finer-grained beds. It is separated by an erosional disconformity from the next member, the Anembalemba Member. The lower portion of the Anembalemba Member is fine to coarse clay-rich sandstone, whitish or light grey in color, with cross-bedding. The upper portion of this member is made of poorly sorted clay-rich sandstone, light olive-grey in color, that lacks cross-bedding. Most vertebrate fossils come from the Anembalemba Member, especially from the upper portion. The Miadana Member, the third and uppermost member, is not always present, and is up to 25 meters thick (82 ft); in some places. Elsewhere, it is replaced by the marine Berivotra Formation. The Miadana Member is made up of claystone, siltstone, and sandstone, lacks cross-bedding, and has several colors of rock. The Maevarano Formation as a whole is underlain by the Marovoay beds and capped by the Berivotra Formation.^[1]

The age of the Maevarano Formation has been debated; the Berivotra Formation, which is partially contemporaneous with the upper portions of the formation, shows that at least the upper part of the Maevarano is Maastrichtian in age. There is no evidence that it is Campanian,^[1] despite previous reports to that effect.^[2] The Berivotra Formation appears to include near its top a magnetic reversal, interpreted as the shift from Chron 30N to Chron 29R, which occurred approximately 65.8 million years ago (about 300 000 years before the Cretaceous–Paleogene boundary and associated Cretaceous–Paleogene extinction event. This suggests that Maevarano organisms also lived shortly before (geologically speaking) the extinction event.^[1]

History of exploration

The Maevarano Formation was first explored by French military physician Dr. Félix Salètes and his staff officer Landillon in 1895, and fossils and geologic data were sent to paleontologist Charles Depéret.^[3] He briefly described the formation and named two dinosaurs from the remains (Titanosaurus madagascariensis and Megalosaurus crenatissimus, now Majungasaurus).^[4] Similar collections were made throughout the 20th century, yielding mostly fragmentary fossils;^[3] one such specimen, a rough partial skull roof, became the holotype of supposed pachycephalosaur (bonehead dinosaur) Majungatholus in 1979.^[5] (This specimen was later shown to be part of the skull ornamentation of a Majungasaurus.) Large-scale expeditions (seven to date), under the banner of the Mahajanga Basin Project, began in 1993. These expeditions, conducted jointly by Stony Brook University and the University of Antananarivo, have greatly expanded knowledge of this formation and the organisms that lived while it was being deposited.^[3]

Paleoenvironment

The Maevarano Formation is interpreted as a low-relief alluvial plain that over time was covered by a marine transgression. Broad, shallow rivers flowed to the northwest from central highlands; evidence for debris flows suggests that the discharges of the rivers varied greatly, with periods of dilute water flow, and periods of rapid erosion dumping sediment into the channels. Paleosols are reddish and include root casts. The paleosols and other sedimentologic evidence indicate well-drained floodplains with abundant vegetation adapted to a relatively dry climate, strongly seasonal (rainy and dry seasons) and at times semiarid (not unlike the present climate of the area).^[1]

Vertebrate paleofauna

Majungasaurus crenatissimus, top land predator of the Maevarano Formation.

Animals found in the formation include frogs (including Beelzebufo ampinga),^[6] turtles, snakes, lizards, at least seven species of crocodyliforms (including species of Mahajangasuchus and Trematochampsa), abelisaurid theropods Majungasaurus, noasaurid Masiakasaurus, two types of titanosaurian sauropods (Rapetosaurus and an unnamed second form), and at least five species of birds or very bird-like dinosaurs, including Rahonavis. The 6 to 7 meter long (20 to 23 ft) Majungasaurus was likely the apex predator in the terrestrial environment. Crocodyliforms were very diverse and abundant.^[1]

Amphibians

Amphibians of the Maevarano Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Beelzebufo	Beelzebufo ampinga				A large frog.	Beelzebufo

Non-Avian Dinosaurs

Indeterminate Lithostrotia remains formerly attributed to the titanosauridae. Undescribed Lithostrotia form. Indeterminate Enantiornithes remains. Possible indeterminate spinosaurid remains.

Non-Avian Dinosaurs reported from the Maevarano Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Majungasaurus[7]	M. crenatissimus[7]			"scattered remains leading to nearly most of the animal."[8]	An abelisaur.	Majungasaurus Masiakasaurus Rahonavis Rapetosaurus
Masiakasaurus[7]	M. knopfleri[7]			"Disarticulated remains of at least 6 individuals," as well as other isolated fossils.[9]	A noasaurid abelisaur.
Rahonavis[7]	R. ostromi[7]			"Partial postcranial skeleton."[10]	A dromaeosaurid.
Rapetosaurus[7]	Rapetosaurus krausei[7]			"[Three] skulls, at least [one] postcranial skeleton."[11]	A titanosaur.
Stegosaurus[7]	S. madagascariensis[7]			"Teeth."[12]	Possible indeterminate ornithischian remains.[7]
Titanosaurus[7]	T. madagascariensis[7]
Vahiny[13]	Vahiny depereti[13]			"Partial braincase."[13]	A titanosaur.

Birds

Crocodylomorphs

Crocodylomorphs of the Maevarano Formation
Genus	Species	Location	Stratigraphic position	Material	Notes	Images
Mahajangasuchus	M. insignis				A peirosaurid.	Mahajangasuchus Simosuchus
Miadanasuchus	M. oblita				Formerly known as Trematochampsa oblita.
Simosuchus	S. clarki				A chimaerasuchid.
Araripesuchus	A. tsangatsangana				A notosuchian.

Snakes

Mammals

Mammals known from the Maevarano Formation include the gondwanathere Lavanify miolaka, known from two teeth, two more teeth that may represent a second gondwanathere, the broken tooth UA 8699, which as been interpreted both as metatherian and as eutherian, a multituberculate tooth fragment, and a yet undescribed mammal known from an articulated skeleton.^[15]

Footnotes

References

• Weishampel, David B.; Dodson, Peter; and Osmólska, Halszka (eds.): The Dinosauria, 2nd, Berkeley: University of California Press. 861 pp. ISBN 0-520-24209-2.