The oldest known communal latrines provide evidence of gregarism in ...

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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 Formation1 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 sediments2,3. The basin contains the reddish Talampaya and Tarjados formations as its lowermost 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 authors3,4. 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 sections4. 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 systems4. 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 stratification3. 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 climates5,6.

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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.

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There is a silcrete level of uncertain origin upholstering the palaeotopography of the Tarjados Formation3, 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 relief3. 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 formations2,3. The unconformity is associated with an early extensional episode deposited after a second synrift stage4 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 worldwide3 and is characterized by fluvial and alluvial deposits that represent the beginning of a synrift phase of sedimentation4. The sedimentary unit was deposited in an active rift basin that received sediments from surrounding highlands, as well as copious amounts of volcanic ash3,4. 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 units1,3 (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 Formation3. 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

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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 channels7. 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 climate5,8,9. 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 mineral3. 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.

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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.

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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.

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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 authors3. 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 content3,10. 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 eruptions10.

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 deposits in a relatively more humid setting. Fossils are preserved almost exclusively within volcanogenic concretions and articulated, even showing some taphonomic and biogenic associations without weathering. The high density and the extraordinary preservation (see Taphonomy) of the concretioned fossils suggest mass mortality due to ongoing catastrophic events.

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Supplementary Figure 4 | Overview of the Chañares type locality –middle lower member– with concretioned fossil remains. (a) Some broken concretions at the type locality of Chañares Formation. (b) Large concretions at the Brazo del Puma locality. (c - e) Some in situ concretioned fossil cynodont.

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The communal latrines described here (e.g., Supplementary Figure 5) are situated at the top of the Bed 1 of the lower member, between 8–15 metres from the base of the Chañares Formation (at the sedimentary microfabrics of Supplementary Figure 2c). The boundary between both beds is represented by different sedimentary structures (mainly volcanogenic concretions of 1–2 metres in diameter) and taphonomic attributes of the fossil remains. The boundary between the lower and upper members has three variable sandstone bodies of 1 to 2 metres in thickness that correspond to large river systems3. However, in some localities these bodies are absent (e.g., El Torcido locality; profile Supplementary Figure 2) and the boundary between the units can be defined by a different colour, the presence of trace fossils and burrows, and some sedimentary structures3.

Latrine coordinates (Supplementary Figure 1a)

Latrine #1: 29°48'40.75"S - 67°46'23.11"W (Supplementary Figure 5a,b) Latrine #2: 29°48'54.86"S - 67°47'0.64"W (Supplementary Figure 6) Latrine #3: 29°49'S - 67°47'W Latrine #4: 29°49'S - 67°46'W Latrine #5: 29°49'38.65"S - 67°46'45.29"W Latrine #6: 29°49'30.26"S - 67°46'4.12"W Latrine #7: 29°49'28.37"S - 67°47'23.13"W (Supplementary Figure 7a, b) Latrine #8: 29°49'0.28"S - 67°47'51.49"W(Supplementary Figure 7c, d)

Upper member The upper member of the Chañares Formaton has a thickness of ~30 metres. It is represented by very massive and concretioned light-gray sediments bearing mostly siliceous concretions, contrasting with the volcanogenic concretions from the lower member3. The upper member

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has diagenetic features and taphonomic attributes of their fossil content, which is reported here for the first time, different from those of the lower member, bolstering its recognition as a valid unit. The first metres represent amalgamated beds of concretionated massive claystones with abundant bioturbations and subvertical burrows of ~0.5 cm wide (cf. Taenidium)3. In the uppermost levels of the Chañares Formation occur several massive horizontal coarse sandstone beds of 1 metre thick, some of them lenticular, with erosive surfaces and diverse stratifications. These sandy and gravel-rich bodies represent braided to meandering wide rivers systems of low to high sinuosity, and low stability3,10. These coarse sandy channels and a volcanoclastic concretion very close to the top of formation yielded fragmentary remains of an indeterminate actinopterygii fish, therapsids and large carnivorous archosauriform remains. The occurrece of these vertebrate fossil remains are of special importance because the upper member of the Chañares Formation was currently considered fossiliferously sterile. There are two or three bentonites fine layers intercalated between sandy bodies3 that are usually formed from weathering of volcanic ash frequently in the presence of water11.

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Supplementary Figure 5 | Fossil communal latrines and coprolites from the Chañares Formation. (a) Overview of latrine #1; red arrows shows the coprolite level. (b) In situ and non-concretioned coprolites from latrine #1. (c) Some collected coprolites from the latrine #1. (d) In situ non-concretioned coprolites from the latrine #5 –red arrows–. (e) In situ nonconcretioned coprolites from the latrine #6 –red arrows–. (f) Coprolite from the latrine #1.

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Supplementary Figure 6 | Concretioned coprolites. (a – d) Coprolites –red arrows– within concretions at communal latrine #2, El Torcido locality, Chañares Formation.

2. Depositional setting Interpretation and reconstruction of an ancient depositional setting depend of the identification of the sedimentary rocks and their environmental parameters10. The most important criteria for their recognition are physical (e.g., facies associations and sedimentary structures and textures), chemical (major-element composition) and biological properties (kinds of fossils and trace fossils)10. Following these premises and according to Rogers and col.3, the depositional environment of the Chañares Formation is complex and difficult to determine accurately because the unit is characteristically massive and concretionary. However, recent works found that the formation was accumulated in a fluvial-alluvial-tolacustrine setting affected by volcanism within an active rift basin where muddy streams with abundant volcanic detritus were deposited from surrounding highlands3,12. 13

Supplementary Figure 7 | Latrines #7 (a–b) and #8 (c–d). (a) Overview of latrine #7; brown rocks on the surface are all coprolites. (b) Example of random sampling using a grid of one meter. (c–d) Large coprolites (maximum size = 345.4 mm) from latrine #8.

Although the Chañares Formation has different depositional palaeoenvironments (see above the characteristics of both members), the general pyroclastic features and volcanoclastic materials of the massive and concretionary unit implies a more complex history. Regionally explosive volcanic eruptions were probably involved in the depositional setting of the unit because of the presence of unconsolidated and consolidated pyroclastic fragments –ash and tuff–, and involving both pyroclastic gravity currents flow and fallout deposits (based on Orton13). These ash flows and tuffaceous sandstone levels were deposited and collapsed downward under gravity as mass flows (flood events of Lahars type) on fluvial-alluvial 14

surfaces and in lakes that occupied the Ischigualasto-Villa Unión Basin during the syneruptive period (after on Orton13). However, it is not known whether these currents were deposited as primary or as secondary flows3. According to Rogers and col.3, the massive and unsorted sediments from the siltstone section of the lower member and the claystone section of the upper member probably represent devitrified pyroclastic flow deposits and currents3,13,14. The Chañares palaeoenvironment may correspond to a medial depositional system, where secondary pyroclastic flows in valleys developed from runoff, based on evidence derived from the particular facies of the sedimenrary unit –en masse deposition–, and depending of the topography on pyroclastic gravity currents (based on Fisher15). However, it is possible that the Chañares Formation represents part of a proximal depositional system where dominant facies of large volume of pyroclastic flows were more prevalent13,16. In summary, the present observations on the sedimentary characteristics of the Chañares Formation are in agreement with previous interpretations and analyses about their palaeoenvironmental features and regarding to the pyroclastic nature of the sedimentary environment1,3,12. In this regard, the taphonomic attributes exhibited by the vertebrate remains found within concretions at the middle levels of the lower member favor a rapid burial in pyroclastic flow deposits (see taphonomy section in Rogers and col., p472)3. The fossil remains preserved in the lower first 10 metres of the lower member are not concretioned and exhibit different taphonomic attributes related with a different history of death and burial than the concretionated fossils of the Chañares Local Fauna. The former fossil remains can be considered within the typical tapho-model of fluvial systems17,18.

3. Taphonomy The Chañares Local Fauna of the Chañares Formation19 is situated in the middle section –bed #2– of the lower member (Supplementary Figures 1, 5a, and 6), where the typical volcanogenic concretion occur1,3. Previous authors stated that fossils may occasionally occur

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without concretions in the sandy basal levels –bed #1– of the lower member, but they are rare, fragmentary and isolated3. Recent field works have improved the knowledge of the faunal diversity, geology and regional taphonomy of the sandy levels of the lowermost Chañares Formation20. Indeed, tetrapod assemblages within the Chañares Formation were more disparate than previously thought, showing palaeoecological differences between them20. Rogers and col.3 described that vertebrate fossil remains in the basal levels of the Chañares Formation are generally associated with sandy paleochannels (Supplementary Figure 3). These specimens usually represent isolated and partially articulated individuals (depending on the proximity to the main channel), but always are not concretionary. However, different taxa may be associated in a restricted area of a few square meters. Indeed, most communal latrines occur between the 8 and 15 metres from the Tarjado-Chañares boundary outside the concretions

(Supplementary

Figure

5)

and

they

are

generally

associated

with

kannemeyeriiform remains. Some coprolites from some latrines (which are higher in the sequence) are preserved within concretions (Supplementary Figure 6). As it is the case of the non-concretioned fossils, the coprolites preserved outside of the concretions, as usually occurs in the latrines, also show some degree of weathering exposure with cracks and pits, microfracture surfaces, and a few trace fossils (Supplementary Figures 8 and 9). By contrast, coprolites within concretions exhibit no weathering (Supplementary Figure 6), as also occur in concretionary vertebrate remains, implying a rapid burial (and almost null biostratinomic history of the Bed #2). The coprolites lack evidence of corrosion or erosion and it can be discerned upper and lower surfaces in the majority of coprolites preserved outside concretions (Supplementary Figures 7 and 8). The upper surfaces are rough, uneven, and with deep grooves and desiccation cracks, mostly produced by weathering before burial (Supplementary Figures 8 and 9). Conversely, the lower surfaces are smooth, with small pits and holes produced by tiny stones and detritus on the soil surface that contacted the dung immediately after defecation (Supplementary

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Figure 9a-c). Commonly these two surfaces differ in their coloration, with the upper surface being darker. Some coprolites possess internal diagenetic cracks generally filled with microcrystalline (equigranular) CaCO3 drusy cement and the internal matrix diagenized to microspary calcite cement (Supplementary Figures 9g and 10).

Supplementary Figure 8 | Coprolite morphological diversity. Shape and size diversity of some coprolites found at the communal latrines of the Chañares Formation. 17

Supplementary Figure 9 | Taphonomical attributes. (a – c) Coprolite in dorsal (a), ventral (b) and side (c) views showing desiccation grooves –red arrows–and pits –blue arrows–. (d – f) Cut coprolites for thin sections showing homogeneous matrix with large inner cracks –red arrows– and surface pits –blue arrows–. (g) Coprolite thin section showing a recrystallized internal crack with drusy calcite cement –arrow–. (h) Coprolite thin section showing two different coprofabric types –arrows–. 18

The thousands of coprolites lack evidence of transport and the accumulation in communal latrines represent “biogenic concentrations” associated with a catastrophic sedimentation that entomb the assemblages in a mass deposition (Supplementary Figures 5, 6 and 7). It is logical to assume that the thanatocenosis and time-averaging were virtually zero by the rapid mass burial (likely a catastrophic burial) of thousands of associated coprolites at the latrines. As a result, bioturbation and physical reworking were null. The coprolites associated in the communal latrines consist of autochthonous remains preserved in their original positions. According to the concept of “fossil censuses”21, which is like a snapshot of population dynamics, the communal latrines could represent as multiple snapshots of a fossil biota and represent a temporal variation in populations. According to Johnson’s models of fossil assemblage formation, the fossil communal latrines fits well into the model 1 (“census assemblage”) characterized by possessing high proportion of in-life fossil with pristine surfaces and low proportion of broken remains. Although latrines have associated some kannemeyeriiform skeletal remains, the biostratinomic classification exhibits a monotypic taxonomic composition and a matrix supported packing. Each latrine possesses a clump type of geometric accumulation as a result of the same biogenic accumulation process. This biogenic concentration is “intrinsic” which is produced by the gregarious behavior of the organisms. The latter also supports the gregarious social behavior of the kannemeyeriiforms that produced the communal latrines in the lower member of the Chañares Formation. In the thin section, the coprofabric exhibits fragmentary and taxonomically unrecognizable woody plant remains, possibly due to mastication process and digestion (Supplementary Figure 12). However show a high density of triads, microspores, and megaspores and other plant reproductive structures (e.g., parts of sporangia) and silica phytoliths-like structures of horsetails, which are very resistant elements to chemical abrasion of the gastric juices. Also exhibits possible fragmentary remains of ostracods, parasites, seeds, and arthropods. Futures studies on these lines may shed light on the possible dietary residues providing evidence for

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unexpected feeding behavior by dicynodonts and the intestinal contents of microorganisms and flora.

Supplementary Figure 10 | 3D reconstructions and CT-scan slice images of two coprolites from latrine #1. The CT-scan shows different views of two coprolites with internal cracks (black grooves) and some infilling or recrystallized (white grooves in a3, a4, and b4) but no signal of micro-bone fragments. The red, yellow, and green lines indicate the intersection of cut planes corresponding to the images 2, 3 and 4 respectively. See the Supplementary Movie files 1 and 2.

4. Statistics Coprolite size distribution––Maximum measurements of complete coprolites of three latrines were sampled (Latrine 1: 29°48'40.75"S - 67°46'23.11"W; Latrine 7: 29°49'28.37"S 67°47'23.13"W; Latrine 8: 29°49'00.28"S - 67°47'51.49"W) and statistics for each latrine were calculated using the software environment R (Supplementary Table 1 and Supplementary Figure 12). Distribution histograms were plotted and fitted to a theoretical distribution using the package fitdistrplus version 1.0-1 written for R. The coprolites average per square metre in the sampled areas was 66.6 (min = 28 coprolites / m2; max = 94 coprolites / m2). 20

Supplementary Figure 11 | Coprolite thin sections from the Chañares Formation. Some coprolites possess a high density of woody coprofabric and an important diversity of spores of non-spermatopsid embryophites and, in a few occasions, some gymnosperms seeds. These coprofabrics are typical of herbivore coprolites.

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See the supplementary excel tables with the provenance and sizes of each coprolite used for this statistical analyzes. The empirical distributions of coprolite sizes were fitted to Gamma distributions with the following parameters: i) Latrine 1 fitted to a Gamma distribution with a shape of 2.04±0.27 and a rate of 0.028±0.004 (Likelihood = -500.80, AIC = 1005.61), ii) Latrine 7 fitted to a Gamma distribution with a shape of 3.22±0.29 and rate of 0.07±0.01 (Likelihood = -1014.03, AIC = 2032.06), and iii) Latrine 8 fitted to a Gamma distribution with a shape of 1.52±0.29 and rate of 0.020±0.004 (Likelihood = -242.9, AIC = 489.9). The overall shape of the distributions is similar for each latrine, being considerably skewed to the left. These distributions suggest that the dicynodont populations that generated the latrines were possibly similar in age composition, being mainly formed by juvenile to sub-adult individuals and with scarce fully grown animals.

Table S1 | Statistics describing size (in mm.) distribution of coprolites of latrines 1, 7 and 8. Abbreviations: Max, maximum; Min, minimum, N, number of samples; St, standard. Latrine 1 Latrine 7 Latrine 8

N 97 226 46

Min. value 10.5 5.8 7.5

Max. value 220.2 204.5 345.4

Median 60.5 38.2 52.15

Mean 72.66 43.2 75.87

St. Deviation 50.1 26.0 70.1

Supplementary Figure 12 | Histograms and theoretical fitted Gamma distribution of coprolite sizes of latrines 1, 7, and 8. 22

5. Age of the Chañares Formation The age of deposition of the Chañares Formation is a contentious topic because published absolute datings are currently lacking. The Chañares Formation was traditionally considered to be of a rough Middle Triassic age19,22,23. Subsequently, Rogers and col.3 bolstered a biostratigraphical correlation between the Chañares Formation and the Dinodontosaurus Assemblage Zone of southern Brazil, which was considered younger than the OlenekianAnisian Cynognathus Assemblage Zone of South Africa. In addition, Rogers and col.3 highlighted that the base of the Ischigualasto Formation was dated as early Carnian and that the thick Los Rastros Formation intervened between the Chañares and Ischigualasto; and, as a result, it was very unlikely that the Chañares Formation may reach the Late Triassic. However, recent modifications of the Triassic time scale24 lead to reconsider the dating of the base of the Ischigualasto Formation as late Carnian in age25. Accordingly, Desojo and col.26 proposed that the Los Rastros Formation must be considered as early–middle Carnian or even Ladinian in age and, as a result, the Chañares Formation should be considered late Ladinian or earliest Carnian in age (ca. 239–235 Ma27). Recent U-Pb radioisotopic datings of detrital zircons of the Dinodontosaurus Assembalge Zone of southern Brazil recovered a maximum age of deposition of 236 million years ago (earliest Carnian sensu Gradstein and col.27)28. The absolute dating of the supposed coeval Dinodontosaurus Assemblage Zone of southern Brazil is in agreement with the age proposed by Desojo and col.26 for the Chañares Formation. In particular, the communal latrines described here occur in the lowermost levels of the Chañares Formation and, as a result, it is very likely that they are between late Middle Triassic to early Late Triassic age (late Ladinian-early Carnian) (Supplementary Figure 1b).

6. Fossil tetrapods from the Chañares Formation The currently known vertebrate fossil assemblage of the Chañares Formation is represented by taxonomically diverse synapsid and archosauriform groups. The large bulk of these fossils

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were collected by field trips leaded by Alfred S. Romer from the University of Harvard (USA) during the 1960s and, subsequently, José F. Bonaparte from the Instituto Miguel Lillo of Tucumán (Argentina) during the 1970s. The most numerically abundant species in the lower levels of the lower member of the Chañares Formation is the large-sized kannemeyeriid dicynodont Dinodontosaurus pedroanum (= Chanaria platyceps; sensu King29; = Dinodontosaurus platyceps, = Dinodontosaurus brevirostris, = Dinodontosaurus oliverai, = Dinodontosaurus platygnathus, = Jachaleria platygnathus; sensu Lucas and Harris30 and Langer and col.31; but see Domnanovich and Marsicano32 for a more taxonomically diverse dicynodont assemblage in the Chañares Formation), which represents around the 70% of the sampled fossil tetrapods20. Conversely, in the middle levels of the lower member of the Chañares Formation the most numerically abundant species in the small traversodontid cynodont Massetognathus pascuali (= Massetognathus terugii, = Massetognathus major, = Massetognathus oligodens; sensu Abdala and Giannini33), which represents around the 75% of the known individuals of the “Chañares type” locality34. The other known fossil tetrapod species of the Chañares Formation are considerably numerically less abundant34, and include the chiniquodontid cynodont Chiniquodon theotonicus (= Probelesodon lewisi; sensu Abdala and Giannini33), the probainognathid cynodont Probainognathus jenseni22,35–37, a preliminary reported indeterminate rhynchosaurid38, and at least ten valid species of basal archosauriforms (i.e. non-archosaur archosauriforms and early members of the crown-group Archosauria). These basal archosauriforms are represented by the proterochampsids Chanaresuchus bonapartei, Gualosuchus reigi and Tropidosuchus romeri, the doswelliid Tarjadia ruthae, the pseudosuchian archosaurs Gracilisuchus stipanicicorum and Luperosuchus fractus, and multiple basal dinosauromorph species, namely Lagerpeton chanarensis, Marasuchus lilloensis (= Lagosuchus lilloensis; sensu Sereno and Arcucci39), Lewisuchus admixtus and Pseudolagosuchus major (this species has been considered as a possible junior synonym40–42 of Lewisuchus admixtus)37,43–50. In addition, Sereno and Arcucci39 considered the small-sized

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“Lagosuchus talampayensis” as a nomen dubium because it is based on a non-diagnostic basal dinosauromorph partial skeleton. Rhynchosaur remains have been recently reported from the lowermost levels of the lower member of the Chañares Formation and represent the first record of the group in the unit and the oldest from Argentina51. The taxonomic diversity of the fossil tetrapod assemblage of the Chañares Formation is dominated by small-sized cynodonts and archosauriforms3. The numerically poorly abundant carnivorous rauisuchian Luperosuchus fractus (only known by two specimens52) and the very abundant herbivorous dicynodont Dinodontosaurus pedroanum are the only large-sized fossil animals (skull length more than 30 cm and some Brazilian specimens have skull lengths that fairly exceed 50 cm; see below) currently known for the Chañares assemblage. The presence of plant remains inside the coprolites, size (i.e., > 30 cm) and their abundance exclude all known tetrapods from the Chañares assemblage as the probable producer-species with exception of the dicynodont Dinodontosaurus. Accordingly, we consider that the kannemeyeriid species Dinodontosaurus pedroanum (and/or a species from the same genus; see Domnanovich and Marsicano32) was mostly probably the coprolite and communal latrine producer.

7. Dinodontosaurus body-size Dinodontosaurus pedroanum was a bulky and quadruped large-sized kannemeyeriform dicynodont from the Chañares and lower Santa Maria formations of northwestern Argentina and southern Brazil, respectively35,53. Dinodontosaurus was a larger animal than the temporally younger dicynodont species Ischigualastia jenseni, from the Ischigualasto Formation of northwestern Argentina. In particular, the holotype specimen of Ischigualastia has a skull length of 55 cm, femoral length estimated in 43 cm (39.5 cm as preserved) and a minimum femoral shaft circumference of 27.7 cm (Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”, Paleontología de Vertebrados; MACN-Pv 18055). The minimum

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circumference of humeral and femoral shafts proved to be reliable proxies to estimate body mass in extinct amniotes based on equations derived from regressions in extant species (e.g., Anderson and col.54). Extant species with femoral minimum circumferences greater than 20.0 cm are mammal megaherbivores that have a body mass equal to or greater than 1,000 kg (e.g., Loxodonta africana, Hippopotamus amphibius; see table 1 in Anderson and col.54). Accordingly, the holotype specimen of Ischigualastia should have fairly exceeded a body mass of 1,000 kg. Regarding Dinodontosaurus pedroanum, the largest known specimen of the species described so far comes from the lower Santa Maria Formation of Brazil and it has a femoral length of 94 cm55 and an estimated femoral length of 64 cm56. Using the femur of Ischigualastia

as

proxy,

the

minimum

femoral

circumference

of

the

Brazilian

Dinodontosaurus specimen should have been greater than 40.0 cm (it is likely that a larger individual would have more robust long bone proportions). Extant African elephants with femoral circumferences equal or higher than 40.0 cm fairly exceed the 5,500 kg. As a result, it can be very confidently inferred that Dinodontosaurus was a megaherbivore (>1,000 kg animals57,58), probably exceeding weights of 4,000 kg. In the Chañares Formation there are also very large undescribed Dinodontosaurus specimens with sizes closely approaching that of the Brazilian individual. In the Brazo del Puma locality it was found a Dinodontosaurus specimen (29°52'16.40"S - 67°42'58.2"W) with a complete skull length of 50 cm and a maximum canine diameter at its base of 38.9 mm (Supplementary Figure 13b). In another locality of the Chañares Formation we found large Dinodontosaurus canines (29°50'08.2"S 67°47'01.9"W) with a maximum diameter at its base of 47.6 mm (Supplementary Figure 13c). The latter specimen should have belonged to an animal with a skull length of around 61 cm based on extrapolations with the complete skull of Brazo del Puma and should have exceeded a body mass of at least 2,500 kg. As a result, Dinodontosaurus specimens of the Chañares Formation can be also confidently included within the category of megaherbivore.

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Supplementary Figure 13 | Dicynodonts teeth from Chañares Formation. (a) Almost complete tooth with a diameter of 35.4 mm. (b) Cross section of a tooth with a diameter of 38.9 mm. (c) Concretionary remains showing a tooth in section with a diameter of 47.6 mm.

Dinodontosaurus was one of the largest known dicynodonts together with an unnamed large dicynodont from the early Late Triassic of Poland (femoral length of 56 cm)59. Dzik and col.59 stated that the unnamed Polish dicynodont was similar in size to a modern rhino (>3,000 kg), which is considered one of the megaherbivores of extant African ecosystems57 and is in agreement with our estimations for Dinodontosaurus.

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8. Dinodontosaurus as a gregarious megaherbivore Owen-Smith57 considered as extant megaherbivores the elephant, rhinoceros, hippopotamus and giraffe, in which the latter is marginally included within the group. As a result and in a strict sense, it is considered a megaherbivore such herbivorous animals that animals which their body mass exceeds one metric tonelade58. However, in a broader sense, the equids, bovids, cervids, tapirs, antelopes and camelids are considered members of herbivorous megafauna58. Contrasting with rhinoceros, which usually are more solitary animals, the other megaherbivores (hereafter referred in a strict sense, i.e. elephants and hippopotamus) are social and communal animals, coexisting in herds composed of large families57. Almost all of these megaherbivorous mammals, even the more solitary rhinoceros, produce considerably large amounts of feces defecated in communal latrines57,60–64. These communal latrines are the result of complex defecation behaviour of several individuals, including males, females and juveniles57,61,63. The fossil mammals that are considered megaherbivores exceeded a body mass of 1,000 kg, in some cases achieving 10,000 kg, and most of them are considered to be gregarious. Some Cenozoic examples of fossil megaherbivores are memebers of the proboscideans65, indricotheriines66, rhinoceros67 and megatherids68. However, some of the most emblematic examples of gregarious megaherbivores are found among ornitischian and the giant sauropod dinosaurs. These megaherbivore dinosaurs had complex social behaviours and lived in large herds69–76. For example, some of them nested together in large areas of breeding colonies77–79, which shows an important feature of social gregariousness. We add here the dicynodont Dinodontosaurus to the list of fossil megaherbivores (sensu Owen-Smith57). Social behaviour in dicynodonts has been suggested based on fossil footprints found at the first few metres of the early Late Triassic Los Rastros Formation (IschigualastoVilla Unión Basin) of La Rioja Province, NW Argentina80. Dinodontosaurus fossil dungheaps from the Chañares communal latrines reflect a complex social relationship and suggest a

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gregarious behaviour in herds probably similar to that of extant megafaunal species (e.g., South American camelids, proboscideans, cervids, and antelopes). More detailed future research on the coprolite-bearing areas of the Chañares Formation may provide valuable palaeobiological information about the complexity and organization of the dicynodont populations and gregarious habits, feeding and trophic ecological reconstitutions, and migrating behaviour81–84. The coprolites from the Chañares communal latrines can potentially provide information of dietary residues, ancient diets, trophic relationships and digestive efficiency. They can also provide isotopic information of the local vegetation characteristics, type of herbivory and foraging locations, reproductive behaviour and palaeoecology. Beyond of these future lines of research, the abundant coprolite associations described here reveals that this social eschatological behaviour is not unique to mammals and constitutes the oldest evidence of communal predating previous records in 220 million years.

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