Geological setting

The Ladinian–earliest Carnian25,26 Chañares Formation crops out in southwestern La Rioja Province, NW Argentina (Fig. 1a), as part of the Ischigualasto-Villa Unión Basin (see Supplementary Figure 1). This basin represents a Triassic continental succession of around 4,000 metres of alluvial, fluvial and lacustrine sediments. The Chañares Formation is one of the most fossiliferous Middle-Late Triassic continental tetrapod-bearing assemblages worldwide27 (see Supplementary Information). Its best-sampled locality is “Los Chañares”25, which historically yielded hundreds of fairly complete and articulated tetrapod specimens27 (Fig. 1 and Supp. Fig. 1).

The Chañares Formation was deposited in an alluvial to fluvial-lacustrine environment within an active rift basin that received sediments from surrounding highlands, as well as copious amounts of volcanic ash27,28. Following previous suggestions that the Chañares Formation comprises two clearly distinct lithological units25,27 (Fig. 1b), the formation is divided here into a lower and an upper member (see Supplementary Information). The lower member reaches up to 35 metres of thickness and represents the lower lithological unit that bears the volcanogenic concretions that characterize the formation and historically yielded the vast majority of vertebrate fossil remains27. Two beds with clear distinguishable lithology are recognized within this member (see Supplementary Information). The communal latrines are situated in the upper section of the lower bed, between 8 to 15 metres from the base of the stratigraphic unit (Suppl. Figs. 1 and 2). The upper member represents 30 metres of very massive and concretioned light-gray sediments bearing mostly siliceous concretions and some massive horizontal coarse sandstone beds at the uppermost levels27 (see Supplementary Information).

The Chañares Formation was traditionally considered Ladinian (late Middle Triassic) in age25,27, but more recent authors considered a Late Ladinian-earliest Carnian age based on vertebrate biostratigraphy and maximum age constrain from radioisotopic datings from overlaying formations26 (see Supplementary Information). However, the communal latrines occur in the lower levels of the Chañares Formation and, therefore, are probably in the Ladinian-Carnian boundary (Suppl. Fig. 1b).

Triassic communal latrines

A fossil communal latrine is defined here as a massive, relatively small (i.e. smaller than the expected home range of the producer herd or population) coprolite-bearing fossil field with evidence of defecation of multiple individuals.

The coprolites were found in the lower member of the Chañares Formation (Fig. 2) and they represent massive autochthonous biogenic accumulations buried in a short-term deposition event (see Supplementary Information). The latter indicates that the high coprolite densities are not taphonomical artefacts or caused by allochthonous accumulations after reworking. The areal density of coprolites is extremely high, averaging 66.6 coprolites/square metre but reaching maximum densities of 94 coprolites/square metre and an estimate of ~30,000 coprolites in the most abundant areas (see Statistics in the Supplementary Information). The autochthony is supported by the nature of the sediments (e.g., matrix supported packing), the bounded and localized coprolites accumulations, the monotypic taxonomic composition and the clump type geometric accumulation of each latrine, as well as their “intrinsic” concentrations, the pristine coprolite surfaces and the low proportion of broken coprolites (see Supplementary Information). As a result, multiple individuals should have generated these massive biogenetic depositions. Although coprolites vary in size and shape in each area, there is no substantial morphological variation among different latrines (Fig. 2c). The coprolite fields have variable areal extensions between 400 to 900 square metres and are separated around 1.5 kilometres from each other, but the latter would be biased by taphonomical artefacts and irregularity of the outcrops. However, the massive27 (see Supplementary Information) condition of the coprolite-bearing sediments prevents inferring synchronicity between these eight coprolite fields (Suppl. Fig. 1a). Coprolite accumulations are frequently associated with juvenile to adult, partially articulated dicynodonts. The areal extension of the coprolite fields is substantially smaller than the inferred distribution of the dicynodont herds (composed of adult animals with a body mass that exceeded 3,000 kg) expected for the Chañares Formation. As an outcome, the massive coprolite deposits should have not been simply the result of gregarism, but of a more complex behaviour of defecation in a punctual relatively small area. Accordingly, the coprolite accumulations of the Chañares Formation are interpreted as fossil communal latrines based on the three lines of evidence outlined above, namely their high density, relatively small areal size and autochthonous condition.

Figure 2 Coprolites from Chañares Formation. (a) In-situ not concretioned coprolites exposed at latrine #1 (see Supplementary Information). (b) Coprolite within concretion at latrine #2. (c) Diversity of coprolite shapes and sizes from several communal latrines (CRILAR-Pv 464). Full size image

Coprolites

Coprolites collected in the Chañares Formation were found inside and outside concretions (Fig. 2a, b; Suppl. Figs. 5 and 6), but the latter condition is the most common in denser latrines (i.e., ~30,000 coprolites). These coprolites vary from whitish grey to dark grey and 0.5 to 35 cm in diameter (Fig. 2c and Suppl. Figs. 7c and 8). By contrast, coprolites preserved within volcanogenic concretions were mostly observed in low-density latrines. They show little variation in size and shape and are dark brown-violet, resembling the colour of the concretions (Fig. 2b and Suppl. Figs. 6). Coprolite morphology is variable, but all have pristine surfaces, clear-cut edges and most of them are ovoid to spheroidal in shape, ranging from 0.5 to 10 cm in diameter. Less abundant coprolites are sausage-like with segmented surfaces, wrapped and oblate, with ragged edges, ranging from 10 to 25 cm in diameter (Fig. 2; see Suppl. Fig. 8). Coprolites with a loop/spiral, coiled or sausage-like shape (Fig. 3g–i), but lumpy and/or with cracks are less abundant. Very large coprolites (i.e., 20–35 cm in diameter; Fig. 3a–c and Suppl. Figs. 7c) are scarce and have generally a cow-dung-shape (flat round piles) with a rough surface. Upper and lower surfaces are discerned in several coprolites (see Fig. 3), in which the upper surface is rough and with deep grooves and desiccation cracks, mostly produced by weathering before burial. Conversely, the lower surface is smooth and possesses small holes produced by tiny stones and detritus on the soil surface that contacted the dung immediately after defecation (see Fig. 3).

Figure 3 Coprolite external features and taphonomical attributes. (a–c) Coprolite in dorsal (a), ventral (b) and side (c) views showing desiccation grooves only on the dorsal surface. (d–f) Coprolite in dorsal (d), ventral (e) and side (f) views showing grooves and some pits generated by desiccation and soil detritus on the ventral surface. (g–i) Coprolite in dorsal (g), ventral (h) and side (i) views showing ventral smooth surface, but with very cracked –by desiccation– dorsal and side surfaces. (j–k) Coprolite in dorsal (j) and ventral (k) views showing smooth surfaces, but with lithoclasts on the ventral surface –black arrows in (k)–. Full size image

Most of the thin sections (Fig. 4) of the sampled coprolites have considerable diagenetic alteration represented by calcite (microspary) replacement. CT scans (see Suppl. Fig. 10) revealed that the coprolites are massive, but have some internal desiccation microfractures infilled by a diagenetic drusy equigranular cement (Fig. 4c and Suppl. Fig. 9g). Some coprolites possess internal microvesicles that are interpreted as gas microbubbles traces. All sampled coprolites lack internal micro-bone remains and, conversely, the coprofabrics bear abundant carbonaceous organic debris, microscopic woody plant remains and micro- and megaspores, as well as possible freshwater ostracods (Fig. 4 and Suppl. Fig. 11).