Taphonomy of the oldest communal latrines from the Triassic Chañares Formation (La Rioja, Argentina): palaeoecological and paleoclimate considerations

LUCAS E. FIORELLI; ELOISA ARGAÑARAZ; JULIA DESOJO

Mendoza

Congreso; 4th INTERNATIONAL PALAEONTOLOGICAL CONGRESS; 2014

International Palaeontological Association

Defecation spots are extremely rare in the fossil record because dung is frequently consumed  by coprophagous, destroyed by termites and/or decomposed by microorganisms. Naturally, feces require exceptional and particular fossilization conditions to avoid being destroyed by these natural recyclers and their preservation involve special conditions. Moreover, herbivorous coprolites can provide unique information about paleoecological and paleoclimatic conditions of ancient ecosystems. Recently, several megaherbivorous dicynodont communal latrines composed of thousands of in situ coprolites have been reported from the Middle-Upper Triassic Chañares Formation in La Rioja province, northwestern Argentina. This formation was deposited in an alluvial to fluvial-lacustrine environment that received copious amounts of ash. In this volcanic palaeoenvironment, gravity flows of ash-mud buried and entombed the fossil vertebrate fauna in massive levels and volcanogenic concretions that typify the lower member of the unit. Here we recapitulate the discovery of the dicynodont communal latrines and describe in detail the biostratinomy and fossil diagenesis assessing paleoenvironmental and paleoclimatic conditions of Ladinian-Carnian Chañares Formation. The communal latrines (biogenic concentrations) could represent multiple 'fossil censuses' and fit into the Johnson´s model 1 ('census assemblage'), with a 'clump type' geometric accumulation and an 'intrinsic' biogenic concentration which imply gregariousness. The autochthonous coprolites preserve their original positions and show pristine surfaces with low proportion of broken samples: some display weathering in the upper surfaces (desiccation cracks before burial) with smooth lower ones. In general, coprolites lack evidence of bioturbation, physical reworking, corrosion or erosion, implying a rapid burial by catastrophic sedimentation ('in mass deposition'). However, constant and copious volcanic ash rains may have also covered the dung heaps preserving in situ the communal latrines. Nevertheless, the thanatocenosis and time-averaging were virtually zero. A paleogeographic reconstruction of westernmost Gondwana during the middle-late Triassic has located Chañares nearly to 40° south latitude, inside a subtropical dry fringe. Some taphonomic features (like weathering desiccation, differential coprofabric) and sedimentary structures may suggest seasonal conditions in a semi-arid paleoclimate, characterized by long dry periods and short wet seasons with general regionally warm temperatures. Moreover, the variation in coprolite shapes (spheroid to flat-round) and their coprofabrics could indicate diet seasonal changes. The temporal and spatial resolution of the coprolite deposits in the fossil communal latrines can supply information regarding the paleobiology and behaviour of kannemeyeriiform in Chañares Formation and provide more information to the paleoflorists, paleoecology, and paleoclimates of western margin of Gondwana during Triassic times.