Sixmile Canyon, near Gallup, NMMNH L-5700 (Triassic of the United States)

Where: McKinley County, New Mexico (35.5° N, 108.6° W: paleocoordinates 11.0° N, 47.2° W)

• coordinate estimated from map

• small collection-level geographic resolution

When: Bluewater Creek Member (Chinle Formation), Norian (228.0 - 208.5 Ma)

• "Lower Bluewater Creek Formation, Chinle Group (Upper Triassic: Adamanian) [= Bluewater Creek Member of Chinle Formation of other authors]. The fossils were found weathering from a purple mudstone unit immediately below the first prominent red band of Bluewater Creek strata in Sixmile Canyon, in the middle of unit 4 of the Sixmile Canyon I measured section of Heckert and Lucas (2002a, 2003; Fig. 1C). In 2002, we acquired samples of a tuffaceous sandstone at the base of the Blue Mesa Member locally and had the detrital zircons analysed by U-Pb geochronology, but were unable to report results until Heckert et al. (2009). Other workers have also obtained age estimates for this horizon, including Irmis and Mundil (2008) and Irmis et al. (2011) Despite using three different techniques, each at different laboratories, all have estimated an age of approximately 220 Ma (Table 1). As a detrital zircon estimate, this is by definition a maximum age, and the dated horizon is approximately 43 m above the Doswellia occurrence, so it is possible that the site is as much as several million years older than 220 Ma. However, given the rarity of Late Triassic age constraints associated with biostratigraphically useful tetrapods, c. 220 Ma is a reasonable approximation for the numerical age of the D. sixmilensis type locality, although it is several million years younger than the age estimates of homotaxial strata recently published by Ramezani et al. (2011)."

•Although Heckert et al (2012) argue that an age of 220 Ma places this locality in the Carnian, most recent work and recent geological timescales use an age of approximately 228 Ma for the Carnian/Norian boundary (e.g. International Committee on Stratigraphy, 2012) and so the locality is entered here as Norian.

• bed-level stratigraphic resolution

Environment/lithology: terrestrial; mudstone

• purple mudstone unit

Size class: macrofossils

Reposited in the NMMNH

Collection methods: surface (in situ),

• NMMNH, New Mexico Museum of Natural History

Primary reference: A. B. Heckert, S. G. Lucas, and J. A. Spielmann. 2012. A new species of the enigmatic archosauromorph Doswellia from the Upper Triassic Bluewater Creek Formation, New Mexico, USA. Palaeontology 55(6):1333-1348 [R. Benson/R. Benson]more details

Purpose of describing collection: taxonomic analysis

PaleoDB collection 136057: authorized by Roger Benson, entered by Roger Benson on 15.11.2012

Creative Commons license: CC BY (attribution)

Taxonomic list

Reptilia
 Eosuchia - Doswelliidae
Doswellia sixmilensis n. sp. Heckert et al. 2012 archosauromorph
Holotype: NMMNH P-61909, an incomplete skeleton consisting of portions of the skull and lower jaws, several centra, and osteoderms, as well as two possible limb bone fragments