An association of Carodnia feruglioi (Mammalia: Xenungulata) specimens and the relocation of the Carodnia faunal Zone

GELFO, J.; ACOSTA HOSPITALECHE, C.; BAUZÁ, N.; STILES, E.; STRÖMBERG

Congreso; XII Congreso de la Asociación Paleontológica Argentina; 2021

In the present contribution we discuss several aspects related to the appearance of at least two individuals assigned to Carodnia feruglioi, in the basal levels of the Rio Chico Group in the Bajo de La Palangana locality, Chubut province, Argentina. The specimens correspond to two partially preserved skulls and isolated dental elements, associated with few indeterminate Crocodylia bones. All the elements were recovered in a flat area of approximately 1000 m2 at a height of 34 meters above sea level (measured with Garmin Oregon 550 GPS). The matrix consists of gray to greenish clays, particularly on the surface, with ochre, red and chestnut mottling. These levels, originally identified by Simpson as the Carodnia Faunal Zone and corresponding to the lower levels of the Rio Chico Formation, were in later interpretations considered as part of the Peñas Coloradas Formation (base of the Río Chico Group, early Paleocene) and recently, tentatively interpreted as the more basal section of the Las Flores Formation. The set of remains recovered in situ makes it possible to identify with certainty, the level considered by Simpson in their type locality, and enable detailed studies to better define the lithostratigraphic unit. Sediment samples taken from the same levels as the fossils do not preserve plant silica (phytoliths), but phytoliths extracted from higher or lower in the Peñas Coloradas Formation and the Las Flores Formation provide some paleovegetational context. Each of the three samples show a strong dominance of forest indicators, with extremely rare, or no evidence for grasses. However, whereas the lowermost sample, from the base of the Peñas Coloradas Formation, consists largely of palm phytoliths, the samples from higher in the same formation and from the top of Las Flores Formation contains a highly diverse set of phytoliths from primarily woody angiosperms, with only relatively rare palm morphotypes. This suggests either a shift from more palm-dominated habitats to mixed angiosperm-dominated forests, or spatial heterogeneity in this region during the Paleocene?early Eocene. The cranial elements were found mainly within the sediment, the outcropping portions presented a high degree of erosion at the bone level, and several exposed teeth were broken or with fresh fractures. Preliminary results from the prepared specimens provide, for the first time, new details of the dental morphology Carodnia feruglioi. The right M3 has a long, thick, and very low mesial cingulum close to the neck of the tooth. The protoloph is the highest structure, slightly concave distally, and with a clear facet of mesial wear along its entire length. It corresponds to the longest labiolingual section of the tooth and joins together the para- and protocone. The protocone is the most voluminous cusp and occupy the entire lingual side of the tooth. There is no true lingual cingulum, but a very faint crenulation is present at the base of the protocone, joining the precingulum and the postcingulum. The paracone is smaller than the protocone but larger than the metacone, which is located more labially than the paracone, generating the distal transverse shortening of last upper molars in mammals. A short lophid projects from the apex of the metacone to a weak metaconule (?) and from there, the lophid continues at an angle of 90° towards the low and short postcingulum, which runs from the distolingual face of the protocone to the distolabial side of the metacone. The qualitative differences with the M3 of larger Carodnia vierai from São José de Itaboraí, Brazil are greater than in the case of other, more conservative loci, suggesting the presence valuable phylogenetic characters.