North slope of Muddy Mountain (Triassic of the United States)

Where: Wyoming (42.7° N, 106.3° W: paleocoordinates 10.3° N, 31.7° W)

• coordinate estimated from map

• small collection-level geographic resolution

When: Alcova Limestone Member (Crow Mountain Formation), Spathian (251.3 - 247.2 Ma)

• Alcova (Limestone) Member, Crow Mountain Formation, Chugwater Group

•The stratigraphic correlation and hence determination of the relative geological age of the Alcova Limestone is rendered difficult by the general paucity of fossils, vertebrates (Corosaurus) and invertebrates alike (for a review see Storrs, 1991). The presence of the nothosauriform reptile Corosaurus has led some workers (Colbert, 1957; Zangerl, 1963) to assign the Alcova to the Upper Triassic. Storrs (1991:101) accepted a late Lower Triassic (Scythian, Spathian) or, perhaps, an early Middle Triassic (Anisian) age for the Alcova Limestone

Environment/lithology: shallow subtidal; lithified, dolomitic, stromatolitic, shelly/skeletal, gray, silty, carbonaceous limestone

• The Alcova is a very hard, dense, resistant, microsparitic limestone in beds approximately 2-25 cm thick...The limestone is microlaminated, slightly fossiliferous, and locally dolomitic. Clastic content is generally low but vriable; quartz silt particles are locally common, increasingly so near pinchout boundaries. This silt is good evidence for natural lateral termination in these areas. Carbonate-pebble conglomerate is occasionally present in the unit. The Alcova limestone is usually grey in color, but can exhibit mottling of pink, red, yellow, and brown as a result of staining by ferric iron (hematite). Algal stromatolites are abundant in the lower portion of the unit...The limestone is ubiquituously petroliferous but is particularly high in organic content in the Alcova area where the rock is dark grey in color... The horizontal-to-wavy algal laminations of the Alcova are the principal bedding structures of the unit.

Size class: macrofossils

Collected by Field Museum field party in 1996

Collection methods: surface (in situ),

Primary reference: O. Rieppel. 1998. Corosaurus alcovensis Case and the phylogenetic interrelationships of Triassic stem-group Sauropterygia. Zoological Journal of the Linnean Society 124:1-41 [S. Peters/D. Lovelace]more details

Purpose of describing collection: taxonomic analysis

PaleoDB collection 138082: authorized by Roger Benson, entered by Roger Benson on 04.01.2013

Creative Commons license: CC BY (attribution)

Taxonomic list

 Sauropterygia -
Corosaurus alcovensis Case 1936 diapsid
FMNH 2018, partially articulated skeleton