SUPPLEMENTARY INFORMATION The oldest known communal latrines provide evidence of gregarism in Triassic megaherbivores Lucas E. Fiorelli*, Martín D. Ezcurra, E. Martín Hechenleitner, Eloisa Argañaraz, Jeremías R. A. Taborda, M. Jimena Trotteyn, M. Belén von Baczko & Julia B. Desojo *To whom correspondence should be addressed. E-mail: [email protected] 1. Provenance, authenticity, geological setting and stratigraphy of the communal latrines of the Chañares Formation 2. Depositional setting 3. Taphonomy 4. Statistics 5. Age of the Chañares Formation 6. Fossil tetrapods from the Chañares Formation 7. Dinodontosaurus body size 8. Dinodontosaurus as a gregarious megaherbivore 9. References 1. Provenance, authenticity, geological setting and stratigraphy of the communal latrines of the Chañares Formation. Several communal latrines were found in successive palaeontological field works conducted in 2011 and 2012 in outcrops of the Chañares Formation situated in the Talampaya National Park, La Rioja Province, northwestern Argentina (Supplementary Figure 1a). The Chañares Formation 1 crops out as part of the Ischigualasto-Villa Unión Basin, which represents a succession of continental deposits composed of 4,000 metres of alluvial, fluvial and lacustrine sediments 2,3 . The basin contains the reddish Talampaya and Tarjados formations as its lower- most units and corresponds to the Synrift 1 tectonic phase. The Talampaya Formation is dated as Induan/Olenekian (Early Triassic) and the Tarjados Formation as Anisian (early Middle Triassic) according to some authors 3,4 . The lower section of the Talampaya Formation is represented by alluvian fan deposits followed by fluvial and playa lake deposits in the middle and upper sections 4. The Tarjados Formation has aerealy extensive outcrops in the Talampaya National Park but at the moment no significant fossil vertebrate remains were reported. The Tarjados Formation is represented by a succession of fluvial-playa lake-fluvial depositional systems 4. The upper levels of the Tarjados Formation are characterized by thick and massive red sandstones and sporadic lents interspersed by coarse sandy and conglomeratic river channels, some of them showing cross-bedding stratification 3. Some pebbles, bioturbations (invertebrate burrows) and numerous carbonate nodules may occur together with abundant thick silicified rhizoliths. This rhizoliths possess ramifications with an important horizontal development, indicating a dense packing between lenticular bodies of ephemeral river channels. The latter strongly suggests the presence of an edaphic organization and pedogenetic alterations in the upper levels of the Tarjados Formation, which indicate the development of incipient palaeosoils in palaeoenvironments with relatively humid climates 5,6 . 2 Supplementary Figure 1 | Geological setting of the communal latrines and geochronology of the Chañares Formation. (a) Geologic map of the Chañares and El Torcido localities showing the communal latrine locations. ( b) Chronostratigraphical position of the Chañares Formation according to Desojo and col.26 . The Geologic Time Scale based on the GSA. Maps drawn in Corel Draw Graphics Suite X5 based on Google Earth images and personal field observations. 3 There is a silcrete level of uncertain origin upholstering the palaeotopography of the Tarjados Formation 3, showing some thick vertical veins irrumpting the upper levels of the sedimentary unit. The contact between the Tarjados and Chañares formations is a regional unconformity that locally exhibits up to 2 metres of relief 3. This unconformity delimits the base of the Agua de la Peña Group, which is represented by four very fossiliferous formations: the Chañares, Los Rastros, Ischigualasto, and Los Colorados formations 2,3 . The unconformity is associated with an early extensional episode deposited after a second synrift stage 4 that is characteristic of the Chañares-Los Rastros sequence. The Chañares Formation is one of the most fossiliferous Middle Triassic-early Late Triassic tetrapod-bearing assemblages worldwide 3 and is characterized by fluvial and alluvial deposits that represent the beginning of a synrift phase of sedimentation 4. The sedimentary unit was deposited in an active rift basin that received sediments from surrounding highlands, as well as copious amounts of volcanic ash 3,4 . The best sampled locality of the formation is the classic “Los Chañares” locality (Supplementary Figure 1a), which yielded hundreds of fairly complete and articulated tetrapod specimens (see Rogers et al. [2001] for a detail geological and sedimentological description of the Chañares Formation) 3. The Chañares Formation has two clearly distinct lithological units 1,3 (Supplementary Figure 1b) and, as a result, we suggest a more formal subdivision of the formation into a lower and an upper member. The lower member corresponds to the lower lithological unit that bears the volcanogenic concretions that typify the formation and contains most of the vertebrate fossil remains historically collected in the Chañares Formation 3. The two members of the Chañares Formation are described as follows. Lower member The lower member represents a rather homogeneous unit in the studied outcrops of the Talampaya National Park and reaches a thickness of 35 metres. The first metres of the 4 member are massive sandstones with some thin lenticular coarse sandy structures that correspond to sporadic anastomosed river channels (Supplementary Figure 2). These levels yielded non-concretioned vertebrate fossil remains, some of them partially articulated, with a different preservation than those yielded within the concretions (Supplementary Figure 3 and 4). In these levels, silicified and carbonate root traces –or rhizoconcretions– (Supplementary Figure 3a-b) are common and with a strong horizontal development across the fluvial system along together with some meandering horizontal burrows. The latters are immature paleosoils with poor vertical development, representing a typical poorly developed paleosoil near fluvial channels 7. The rhizolith morphology is not plant specific but is ecophenotypic (i.e., root patterns produced by specific environmental conditions) and suggests the development of incipient pedogenic horizons that correspond to a fossil rhizosphere in a palaeoenvironment with a relatively humid climate 5,8,9 . The lower five metres of the sedimentary unit are represented by fine and massive, slightly friable (Supplementary Figure 2), and yellowish-gray sandstones with a calcareous diagenetic matrix of microcrystalline silica and montmorillonite clay mineral 3. The grains generally have a size that ranges from 0.1 to 0.25 mm and are subangular with moderate sphericity and selection (Supplementary Figure 2d-e). Nevertheless, a sediment mixture is present, in which mono- and polycrystalline quartz grains are dominant and with a relatively high amount of sedimentary and volcanic lithic fragments and a lower concentration of feldspars forming a graywacke matrix-rich rock. The rest of the member is consolidated by a very fine and high relief calcite matrix (carbonate-mud matrix) (Supplementary Figure 3c), deposited together with grains, representing a quartz wacke sandstone and a grain-supported rock – packstone –. This rock has a poor grain selection and classification, with subrounded grains of 0.5 to 1.5 mm dominated by monocrystalline quartz. There are some microcrystalline quartz derived from the overlain Tarjados Formation and sedimentary and volcanic lithic fragments. 5 Supplementary Figure 2 | Sedimentary structures and rock thin sections from the lower member of the Chañares Formation. (a) Stratigraphic profile of the Top Ten locality. ( b) Thin section and microfabric of a typical volcanogenic concretion. ( c) Thin section and microfabric of the massive sedimentary level at the communal latrines. ( d and e) Typical microfabrics of the sandstone lenticular bodies at the first metres of the lower member. 6 Supplementary Figure 3 | Structures and fossils from the lower member of the Chañares Formation. (a and b) In situ rizoliths at the first metres of the lower member. ( c - e) Nonconcretioned vertebrate pinkish fossil remains from the first metres of the lower member. 7 Volcanogenic concretions (Supplementary Figures 2b and 4a,b) are common in the lower member and are particularly very large (~1–2 metre in diameter) in its upper half. The concretions in the upper half of the lower member yield the concretioned vertebrate fossils (Supplementary Figure 4c-d) described originally by Romer and collaborators1 and subsequent authors 3. The sedimentary mass-transport processes in the lower member suggest a gravity flow of mud, with a plastic mechanical behaviour and a sheared transport distributed through sediment mass, in which the strength principally from cohesion is due to clay content 3,10 . Subaerial mud flows occur commonly in semiarid regions after heavy rainfalls, but are also common in volcanic regions where volcanic debris are water saturated with deposits of muddy streams during heavy rains that accompanying eruptions 10 . We define two different beds within the lower member of the Chañares Formation (Supplementary Figure 1b): Bed #1–– Lower fluvial/paleosoil levels with abundant fossil remains associated to river channels in a semi-arid region. Fossil remains are generally not concretioned in a taphonomic association, but rarely articulated and present some weathering. Bed #2–– Middle/upper levels that correspond to floodplain