Meseta de Somuncura (La Colonia mammal) (Cretaceous to of Argentina)

Where: Chubut, Argentina (43.0° S, 67.7° W: paleocoordinates 45.8° S, 53.8° W)

• coordinate estimated from map

• outcrop-level geographic resolution

When: La Colonia Formation, Campanian to Campanian (83.6 - 66.0 Ma)

• in the second facies association , vertebrates apparently come from four separate levels spanning approx. 10m. The youngest verts are approx. 10m below the contact with overlying basalts.

Environment/lithology: marginal marine; carbonaceous claystone and gypsiferous, brown, sandy siltstone

• see above description
• The first and lowest facies association is composed of a single facies, and is a coarse-grained sandy-conglomeratic and crossed-bedded granule conglomeratic deposit. It reaches a maximum thickness of 16m, shaping prominent re-entrants in the cliffs along local valleys, as along Arroyo Mirasol Chico. This facies is attributed to a non-marine setting, drained by a channeled fluvial system featuring moderate to low sinuosity implanted on the sandy substratum of the Cerro Barcino Formation. The overall lithology, its yellowish brown color, and the remarkable grain-size change at the boundary, contrast with the Cerro Barcino Formation, and reveal that deposition was interrupted by a modification of the paleoenvironmental conditions...The second facies association crops out approximately in the middle part of the sequence, and is composed of three facies: laminated mudstones, pelite-fine sandstones, and banded siltstones and claystones. Laminated mudstones are yellowish brown in color, sometimes interstratified with psammitic nodules. Pelites are mostly claystones and siltstones, greenish-brown to grayish-brown in color, and featuring parallel lamination and lenticular stratification. Usually, it is interstratified not only with reddish fine-grained sandstones but also with black pelites and gypsum; the sandstone beds are tabular or lenticular with planar and trough cross-bedding, most of them about 0.30 m thick, and occasionally up to 1 m thick, bearing frequent but fragmentary remains of fresh water fish. The black pelites are about 0.10 m thick, occasionally reaching a thickness of 0.60 m, bearing frequent plant remains. The most common evaporitic component is gypsum, which is light brown in color. It is arranged in tabular beds internally featuring cone-in-cone structures and dessication cracks. Finally, the banded siltstones and claystones are yellowish brown in color...Internally, the beds show parallel lamination or lenticular stratification. This second facies association is interpreted as having been deposited in an estuary, tidal flat or coastal plain environment, influenced both by occasional high gresh water streamflow from the continent and tidal currents from the sea. This environment occurred under a seasonal climate (cite refs) alternating periods of humidity and aridity. The humid periods are represented by some laminated pelites with intense bioturbation, frequent remains of either aquatic animals (e.g. fresh-water fish, such as ceratodontid dipnoans, turtles, crocodiles and marine plesiosaurs) or land plants and tetrapods (lizards, snakes, dinosaurs and mammals). The arid periods are represented by saline mudflats with dessication cracks, evaporitic deposits and some red beds. The only macrofossils recorded in these sedimants are dental plates of a ceratodontid dipnoan fish...The third facies association represents the upper section of the sequence. Like the first one, it is composed by only one facies, so called laminated pelites. This is a distinct set of yellow, yellowish brown or greenish brown laminated silty claystones, without sandy beds...This facies is interpreted as the same transitional paleoenvironment as the preceding, but corresponding to the upper part of an intertidal flat.

Size class: mesofossils

Primary reference: R. Pascual, F. J. Goin, P. Gonzalez, A. Ardolino, and P. F. Puerta. 2000. A highly derived docodont from the Patagoinan Late Cretaceous: evolutionary implications for Gondwanan mammals. Geodiversitas 23(3):395-414 [J. Alroy/E. Leckey/R. Benson]more details

Purpose of describing collection: taxonomic analysis

PaleoDB collection 36447: authorized by John Alroy, entered by Erin Leckey on 29.01.2004

Creative Commons license: CC BY (attribution)

Taxonomic list

 Meridiolestida - Reigitheriidae
Reigitherium bunodontum Bonaparte 1990 mammal