Counillonia and Repelinosaurus Type Locality (Triassic of Laos)

Also known as Luang Prabang Basin

Where: Laos (19.9° N, 102.1° E: paleocoordinates 0.6° S, 94.4° E)

• coordinate stated in text

• outcrop-level geographic resolution

When: Purple Claystone Formation, Early/Lower Triassic (252.2 - 247.2 Ma)

• "Three samples from the Purple Claystone Formation, including one collected at the dicynodont site, were dated using U-Pb geochronology on detrital zircon (Rossignol et al., 2016). The sample collected at the dicynodont fossil site yielded a maximum depositional age of 252.0 ± 2.6 Ma, whereas the other volcaniclastic samples collected in the same formation yielded maximum depositional ages of 251.0 ± 1.4 and 300.5 ± 3.7 Ma. The various volcaniclastic textures, their roundness, and the relatively low volcaniclast content (below 20%) implying an important and protracted mixing with other detrital particles, as well as the fact that some of the volcaniclasts underwent at least two sedimentary cycles (Bercovici et al., 2012; Blanchard et al., 2013), suggest that these dates, obtained from zircon grains interpreted as being detrital in origin, represent maximum depositional ages. The actual age of deposition of the Purple Claystone Formation is therefore likely to be younger. Both youngest maximum depositional ages (i.e., 252.0 ± 2.6 and 251.0 ± 1.4 Ma) encompass the P-Tr boundary (251.902 ± 0.024 Ma; Burgess et al., 2014) within uncertainties. The consideration of a late Permian age, potentially plausible, would nonetheless imply that the reworking of the zircon grains took place within an unlikely brief time span. Given the age of the overlying formation (224.9 ± 1.0 Ma; Blanchard et al., 2013), an age up to the Carnian could be proposed as the theoretical upper age limit for the Purple Claystone Formation. However, the occurrence of a regional Middle Triassic unconformity (e.g., Racey, 2009), probably superimposed onto the reverse fault separating the Purple Claystone Formation from other sedimentary units to the southeast, reduces the likely time span for the deposition of the Purple Claystone Formation. Consequently, an Early Triassic age for the Purple Claystone Formation and its enclosed fossils is considered to be the most likely." (Olivier et al. 2019)

• group of beds-level stratigraphic resolution

Environment/lithology: "floodplain"; silty claystone

• "The sedimentary facies association indicates braided river depositional environments, evolving vertically to alluvial plain environments, probably including ponds (Bercovici et al., 2012)." (Olivier et al. 2019)
• "The Limestone and Sandstone Formation is overlain by the Purple Claystone Formation, from which various fossil remains have been excavated (dicynodonts and a chroniosuchian; Steyer, 2009; Arbez et al., 2018). The Purple Claystone Formation is mainly composed of homogeneous silty claystones, silts, and more rarely clays (Bercovici et al., 2012). The formation also comprises volcaniclastic siltstones and sandstones, with millimeter- to centimeter-sized rounded and highly weathered volcaniclasts (up to about 20 vol.%). These volcaniclasts exhibit a variety of volcanic textures (microlithic, trachytic, porphyritic) and are sometimes embedded within lithic fragments, attesting to multiple reworking events for these volcaniclasts (Bercovici et al., 2012; Blanchard et al., 2013). The Purple Claystone Formation also contains subordinate amounts of coarser deposits, including sandstone and conglomeratic facies with three-dimensional megaripples typical of braided river deposits. The conglomeratic levels consist of rounded pebbles of highly fossiliferous limestones (foraminifers, corals, bryozoans), subangular to rounded pebbles of volcanic rocks, black cherts, red quartzites, red sandstones, and siltstones. Paleosols, sometimes exhibiting vertical root traces, are developed within this formation (Bercovici et al., 2012)." (Olivier et al. 2019)

Size class: macrofossils

Collected by P. Taquet and colleagues in 1993 - 2003

• LPB - Luang Prabang Basin specimens stored in the Savannakhet Dinosaur Museum, Savannakhet, Laos

Primary reference: C. Olivier, B. Battail, S. Bourquin, C. Rossignol, J.-S. Steyer and N.-E. Jalil. 2019. New dicynodonts (Therapsida, Anomodontia) from near the Permo-Triassic boundary of Laos: implications for dicynodont survivorship across the Permo-Triassic mass extinction and the paleobiogeography of Southeast Asian blocks. Journal of Vertebrate Paleontology [A. Dunhill/B. Allen]more details

Purpose of describing collection: taxonomic analysis

PaleoDB collection 201493: authorized by Alex Dunhill, entered by Bethany Allen on 10.05.2019

Creative Commons license: CC BY (attribution)

Taxonomic list

Show authors, comments, and common names
Osteichthyes
 Dicynodontia -
Counillonia superoculis n. gen. n. sp., Repelinosaurus robustus n. gen. n. sp.
Counillonia superoculis n. gen. n. sp. Olivier et al. 2019 dicynodont
LPB 1993-3 (holotype), partial skull without mandible
Repelinosaurus robustus n. gen. n. sp. Olivier et al. 2019 dicynodont
LPB 1993-2 (holotype), partial skull without mandible; LPB 1995-9, partial skull without mandible