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Santa Maria Formation

The Santa Maria Formation is a Late Triassic sedimentary rock unit situated in the Paraná Basin of Rio Grande do Sul state, southern Brazil, renowned for its exceptional record of early dinosaur evolution.^[1] It spans the late Carnian stage, with high-precision U-Pb zircon dating constraining its depositional age to approximately 233.23 ± 0.73 million years ago.^[1] Composed primarily of fine- to medium-grained sandstones, siltstones, and mudstones formed in fluvial environments—including anastomosing channels, floodplains, and braided systems—the formation exhibits sedimentary structures such as mud cracks, caliche concretions, and interstratified illite-smectite clays, reflecting a humid subtropical climate punctuated by seasonal aridity. Stratigraphically, the Santa Maria Formation constitutes the lower portion of the Santa Maria Supersequence within the broader Triassic succession of the Paraná Basin, overlying the Sanga do Cabral Formation and underlying the Caturrita Formation (dated to ~225 Ma).^[1] It is subdivided into members such as the Alemoa Member (characterized by anastomosing fluvial deposits) and the Passo das Tropas Member, with the uppermost levels belonging to the Hyperodapedon Assemblage Zone, which correlates globally with other Carnian tetrapod-bearing units across Pangaea. Geochemical proxies, including chemical index of alteration (CIA) and thorium/uranium ratios, further indicate a shift toward more humid conditions during deposition, potentially linked to the broader Carnian Pluvial Episode—a global climatic perturbation that influenced terrestrial ecosystems.^[2] The formation's paleontological significance stems from its abundant and well-preserved vertebrate fossils, representing one of the most diverse Late Triassic tetrapod assemblages in Gondwana.^[1] Early dinosaurs comprise about 5% of the fauna but include pivotal taxa such as the basal sauropodomorph Saturnalia tupiniquim and the herrerasaurid Staurikosaurus pricei, alongside more recent discoveries like the theropod-like Nhandumirim waldsangae.^[3] Non-dinosaurian components dominate, featuring synapsids (e.g., traversodontid cynodonts), rhynchosaurs, temnospondyl amphibians, and archosauromorph reptiles, which collectively document the competitive dynamics and rapid diversification of archosaurs leading to dinosaur dominance.^[1] These assemblages, primarily from sites near the city of Santa Maria, have yielded over a dozen tetrapod species and provide critical evidence for correlating Triassic biostratigraphy and understanding the environmental drivers of the dinosaur rise.^[4]

Geological Setting

Location and Extent

The Santa Maria Formation is situated within the Paraná Basin in the state of Rio Grande do Sul, southern Brazil, primarily in the central-southern region of the state. It is centered around the city of Santa Maria, located at approximately 29°41′42″S 53°47′42″W.^[5] This positioning places the formation as a key component of the broader Triassic sedimentary record in the basin, which spans across southern South America but with exposures limited to this Brazilian state.^[6] The formation's surface exposures form an east-west trending belt spanning approximately 200–300 km in the central-southern part of Rio Grande do Sul, reflecting the depositional extent of the Santa Maria Supersequence within the basin.^[7] Notable outcrop areas include the regions of Chiniquá and Pinheiros in the Pinheiros-Chiniquá Sequence, as well as Candelária in the Candelária Sequence, where ravines and erosion features reveal the strata.^[6] These exposures are confined to the Rosário do Sul Group, overlying the Early Triassic Sanga do Cabral Formation and representing the basal to middle units of the Santa Maria Supersequence, with upper portions correlating laterally to equivalents of the Ischigualasto Formation in Argentina.^[8]^[9] Modern access to the formation's outcrops is facilitated through protected paleontological sites, including the Geopark Paleorrota, which encompasses multiple localities around Santa Maria and promotes geotourism while preserving the Triassic heritage.^[10] The Paleorruta, a designated fossil route within the geopark, connects key exposures such as those in Candelária and provides guided pathways for researchers and visitors.^[11]

Lithology and Stratigraphy

The Santa Maria Formation is composed primarily of red beds, including mudstones, sandstones, siltstones, and subordinate conglomerates and evaporites, reflecting a continental depositional regime dominated by fluvial and floodplain processes. These sediments exhibit characteristic reddish hues due to iron oxide staining, with mudstones and siltstones forming the bulk of the succession and sandstones occurring as lenticular bodies or interbeds. Minor evaporites, such as gypsum nodules, and pedogenic features like calcretes are sporadically present, indicating periodic arid conditions.^[12] The formation belongs to the Santa Maria Supersequence, which is hierarchically divided into three third-order depositional sequences: the lower Pinheiros-Chiniquá Sequence, the middle Santa Cruz Sequence, and the upper Candelária Sequence, with the Santa Maria Formation encompassing primarily the lower portions of this framework. These sequences are bounded by erosional surfaces and reflect transgressive-regressive cycles within a second-order supersequence framework. The Pinheiros-Chiniquá Sequence consists mainly of coarse-grained fluvial sands and gravels fining upward into overbank fines, while the Santa Cruz and Candelária Sequences feature finer-grained floodplain deposits with increasing lacustrine influences upward.^[12]^[13] The formation is further subdivided into informal members, including the basal Passo das Tropas Member and the overlying Alemoa Member. The Passo das Tropas Member comprises interbedded medium- to fine-grained sandstones and mudstones, with trough cross-stratification, feldspathic pebbles, and mudstone intraclasts indicating channelized fluvial environments; paleosols are developed in the mudstone intervals, signifying periodic subaerial exposure. The Alemoa Member is dominated by massive, dark red mudstones with abundant carbonate concretions and intercalated fine sandstone lenses representing crevasse splay and channel fills. Overall thickness of the Santa Maria Formation varies regionally from 100 to 300 m, with the sequences displaying fining-upward cycles that denote episodic sedimentation pulses linked to tectonic and climatic fluctuations. These cycles typically progress from coarse basal conglomeratic sands to overlying siltstones and mudstones, capped by paleosol horizons. The stratigraphic framework correlates with global Triassic stages through biozonation, such as the Dinodontosaurus Assemblage Zone in the Pinheiros-Chiniquá Sequence.^[14]^[13]

Age and Chronostratigraphy

The Santa Maria Formation is primarily assigned to the Late Triassic Carnian stage, with depositional ages spanning approximately 237 to 228 Ma based on radiometric and biostratigraphic data, though the lower sections may extend into the late Ladinian or even earlier Middle Triassic (potentially Late Anisian) according to some geochemical studies.^[15] This temporal framework is established through a combination of radiometric dating and biostratigraphic correlations, providing a robust chronostratigraphic context for the formation's fossil assemblages. The formation's age aligns with a period of significant global climatic shifts, including the onset of the Carnian Pluvial Episode around 234–232 Ma, which influenced terrestrial ecosystems across Gondwana.^[1] High-precision U-Pb zircon geochronology from tuffaceous layers offers the most direct constraints on depositional ages. A key date of 233.23 ± 0.73 Ma (2σ uncertainty) was obtained from ash-fall zircons in the Alemoa Member, approximately 5 m below its top, representing a maximum depositional age for this mid-formation unit and confirming its placement in the late Carnian (Tuvalian substage).^[16] Detrital zircon maximum ages of around 237 Ma have been reported for lower sequences such as the Santa Cruz Sequence, supporting the inclusion of late Ladinian deposits at the base.^[17] The upper boundary remains constrained below the ~225 Ma age of the overlying Caturrita Formation (early Norian), with the Santa Maria Formation thus primarily within the Carnian, though further radiometric work is needed to precisely resolve internal boundaries.^[16] Biochronology refines the formation's internal subdivisions through tetrapod assemblage zones (AZ), which correlate with global Triassic stages. The lower Dinodontosaurus AZ, spanning the Ladinian-Carnian boundary (~237 Ma), features dicynodont-dominated faunas indicative of pre-Carnian continental conditions.^[18] The overlying Hyperodapedon AZ, dated to the early to late Carnian (~233–230 Ma), is characterized by rhynchosaur-cynodont assemblages and marks the diversification of early dinosaurs.^[16] These zones facilitate precise correlations with contemporaneous units, such as the Ischigualasto Formation in Argentina, where the Hyperodapedon AZ aligns with the late Carnian Scaphonyx-Exaeretodon-Herrerasaurus biozone, linking both to the Carnian Pluvial Episode's environmental perturbations.^[1]

Research History

Discovery and Naming

The Santa Maria Formation was first recognized in the early 20th century through informal references to "Santa Maria beds" by pioneer stratigraphers studying the Paraná Basin, including I. C. White in 1908 and A. I. Oliveira in 1918, who designated these layers for their rich paleotetrapod content as part of the broader Triassic sequence.^[19] The formal naming of the Santa Maria Formation occurred in 1947, proposed by M. Gordon Jr. to describe the continental sedimentary unit overlying the Sanga do Cabral Formation and underlying the Caturrita Formation.^[19] Initial fossil collections from the formation began in the 1920s, when German paleontologists led by Friedrich von Huene conducted expeditions in the Chiniquá region of Rio Grande do Sul, recovering dicynodont specimens such as those later named Stahleckeria potens in 1935.^[20] These efforts marked the first systematic vertebrate paleontological work in the area, highlighting the Triassic deposits' potential for synapsid remains. In the 1930s, Brazilian paleontologist Llewellyn I. Price, working for the National Department of Mineral Production, discovered early dinosaur fossils, including the holotype of Staurikosaurus pricei in 1936 from outcrops near Santa Maria. Further expeditions organized by Price in the 1940s and 1950s expanded fossil recovery from key sites like Sanga da Alemoa, yielding additional therapsids, archosaurs, and the first evidence of early dinosaurs in Brazil, which informed preliminary stratigraphic correlations within the formation.^[21] Early stratigraphic mapping during this period, building on Gordon's framework, was advanced through Price's fieldwork and subsequent analyses by researchers like M. C. Barberena in the late 1950s and 1960s, establishing the basic lithological and biostratigraphic divisions of the formation's mudstone-dominated sequences.^[22]

Key Expeditions and Studies

During the 1970s and 1980s, collaborative expeditions between Argentine and Brazilian paleontologists, notably led by José F. Bonaparte and Mário C. Barberena, focused on the Santa Maria Formation in Rio Grande do Sul, Brazil, resulting in the recovery of important pseudosuchian and lepidosauromorph fossils, including additional specimens of the rauisuchian Prestosuchus chiniquensis and rhynchocephalian taxa such as Sphenodon-like forms.^[23] These efforts built on earlier finds by integrating cross-border expertise to excavate and describe the diverse Late Triassic vertebrate assemblages, enhancing understanding of Gondwanan archosaur evolution.^[24] In the 1990s and 2000s, projects coordinated by the Universidade Federal do Rio Grande do Sul (UFRGS) and Universidade de São Paulo (USP) advanced the biostratigraphy of the formation through systematic fieldwork and taxonomic revisions, particularly refining the vertebrate assemblage zones such as the Dinodontosaurus and Hyperodapedon zones. Max C. Langer's research during this period, including the description of the early sauropodomorph Saturnalia tupiniquim from the upper Santa Maria Formation, provided key evidence for early dinosaur diversification and zonal correlations across southern Gondwana.^[25] These initiatives emphasized multidisciplinary approaches, combining excavation with phylogenetic analyses to delineate temporal boundaries within the Carnian stage. Geochemical and sedimentological investigations complemented these field efforts, with studies like Horn et al. (2002) [using] elemental analysis to sediments from the Santa Maria Formation, revealing insights into depositional environments characterized by fluvial and lacustrine systems under semi-arid conditions.^[26] Such analyses helped reconstruct paleoenvironmental shifts, including fluctuations in humidity and provenance of sediments from the Paraná Basin. The establishment of the Geopark Paleorrota in 2000 marked a pivotal advancement in conservation and public engagement, encompassing over 50 municipalities in Rio Grande do Sul and integrating paleontological sites from the Santa Maria Formation to promote sustainable tourism and protect fossil resources.^[27] This initiative facilitated ongoing research access while educating communities on the geological heritage of the region. In the 2010s, bio- and magnetostratigraphic studies led by César L. Schultz and colleagues integrated fossil distributions with magnetic polarity data from the Santa Maria Formation, confirming its predominantly Carnian age (approximately 233–225 Ma) and supporting correlations with global Triassic chronostratigraphy. These works refined the temporal framework for the formation's assemblage zones, highlighting faunal turnovers linked to the Carnian Pluvial Episode.

Recent Developments (2020–2025)

In 2020, a geochemical analysis of vertebrate assemblage zones within the Santa Maria Supersequence utilized rare earth elements to infer sediment provenance and depositional conditions, revealing distinct signatures that differentiate the Dinodontosaurus and Santacruzodon zones from overlying units.^[4] The year 2023 marked the description of Amanasaurus nesbitti, a novel silesaurid dinosauromorph from the mid-to-late Carnian beds of the formation, based on disarticulated postcranial remains that bridge gaps in the early radiation of dinosaur relatives.^[28] This discovery, from the Pivetta site (lower Candelária Sequence) in the Hyperodapedon Assemblage Zone, underscores the formation's role in avian-line archosaur evolution. Proterochampsid research advanced in 2024 with the identification of Retymaijychampsa beckerorum, a new archosauriform known primarily from well-preserved hindlimb elements recovered from the Pinheiros-Chiniquá Sequence.^[29] These fossils, dated to the Middle-Late Triassic, provide insights into pseudosuchian locomotor adaptations during a transitional climatic phase. In 2024, the description of Gondwanax paraisensis from the Dinodontosaurus Assemblage Zone introduced a new dinosauromorph taxon at the Ladinian-Carnian boundary, enhancing understanding of early Gondwanan archosauromorph diversity. Ongoing efforts at the Universidade Federal de Santa Maria (UFSM) include digitization initiatives for fossil specimens from the formation, alongside 2024 stable isotope analyses (δ¹³C and δ¹⁸O) of teeth and bones from the Dinodontosaurus and Hyperodapedon zones, which indicate elevated values linked to the Carnian Pluvial Episode and refine paleoenvironmental reconstructions of increased humidity.^[30] Post-2020 radiometric dating and biostratigraphic correlations have integrated the formation's assemblages into global chronostratigraphic frameworks, confirming ties to the Carnian Pluvial Episode around 232 Ma and updating entries in paleontological databases like the Paleobiology Database.^[31]

Paleoenvironment

Depositional Environment

The Santa Maria Formation consists of continental red beds deposited within the intracratonic Paraná Basin of southern Brazil, influenced by rift-related tectonics associated with the initial breakup of Pangea during the Middle to Late Triassic. This setting reflects a foreland-like basin dynamics driven by the Gondwanide orogeny to the south, where uplift of the Sul-Rio-Grandense and Uruguayan shields provided sediment provenance, leading to polycyclic sedimentation episodes in a predominantly fluvial-lacustrine system.^[32] Sedimentary facies indicate an alluvial plain environment with fluvial channels of varying sinuosity, including meandering rivers in the Alemoa Member that formed point bars and crevasse splays, alongside low-sinuosity channels in broader floodplain settings. Ephemeral lacustrine phases occurred in overbank areas, evidenced by thin mudstone cycles representing shallow, seasonal water bodies. Paleosols, including calcretes in the Alemoa Member, signify periodic subaerial exposure and soil formation under semi-arid conditions, with root traces and pedogenic features highlighting stability between flood events.^[32] Key sedimentary structures support these interpretations: trough and planar cross-bedding in medium- to fine-grained sandstones records channel migration and bar accretion, while ripple marks and lamination in siltstones indicate low-energy overbank flow. Desiccation cracks and mud curls in mudstones further attest to episodic drying of floodplains and lake margins. The Carnian Pluvial Episode, around 234 Ma, exerted a significant influence by increasing regional humidity, which enhanced fluvial discharge, promoted sheetflood deltaic deposition, and elevated sedimentation rates, shifting the system toward more hydromorphic paleosols and reduced calcrete development in upper strata.^[14]^[6]

Paleoclimate and Non-Vertebrate Biota

The paleoclimate of the Santa Maria Formation during the Late Triassic is interpreted as semi-arid to subhumid, characterized by seasonal precipitation and high evaporation rates. This reconstruction is supported by the prevalence of red beds indicative of oxidizing conditions in well-drained soils, the presence of minor evaporite minerals such as gypsum nodules, and widespread calcretes formed through pedogenic processes in paleosols. Calcretes, including root casts and concentric concretions, developed via the per ascensum model, where capillary rise of groundwater led to carbonate precipitation under fluctuating phreatic levels, consistent with semi-arid conditions that limited vegetation cover while allowing periodic moisture influx associated with fluvial systems. Recent stable isotope analyses of carbon (δ¹³C) and oxygen (δ¹⁸O) from fossil bones and teeth reveal elevated δ¹⁸O values (16.6–27.4‰) in the upper units, signaling warmer temperatures during the Carnian Pluvial Episode, with mean paleotemperatures estimated at 25–30°C based on comparisons to modern analogs; δ¹³C enrichments suggest intermittent arid pulses amid overall increased humidity. Intra-tooth δ¹⁸O variation up to 6.8‰ indicates a more variable environment compared to lower units.^[33] Non-vertebrate biota in the Santa Maria Formation reflects post-Permian-Triassic recovery, with sparse plant fossils dominated by a recovering Gondwanan flora. Seed ferns of the Dicroidium-type, including species such as Dicroidium odontopteroides and Zuberia zuberi, represent the primary component, alongside fragmentary remains of horsetails (Neocalamites sp.) and rare conifer shoots, indicating a low-diversity woodland adapted to seasonal aridity. These plants, preserved in fine-grained mudstones from lacustrine margins, underscore limited biomass production in a recovering ecosystem following the end-Permian extinction.^[34] Invertebrate evidence is scarce but includes coprolites and trace fossils. Coprolites, often ovoid and composed primarily of calcite, are abundant in the Dinodontosaurus Assemblage Zone and contain inclusions like plant fragments and rare nematode eggs (Paleoxyuris cockburni), linking them to herbivorous cynodont diets through reanalysis of shape, size, and content.^[35] Insect traces, such as borings in synapsid bones, and rare bivalve shells in mudstone layers further attest to a modest invertebrate presence. Ichnofossils provide additional biotic signals, with lacertoid tracks (Rhynchosauroides retroversipes) preserved in concave epirelief on sandy substrates and rhizoliths in calcic paleosols evidencing rooting structures of herbaceous vegetation under a high water table.^[36]

Vertebrate Paleofauna

Temnospondyls

The temnospondyl record of the Santa Maria Formation is limited to a single taxon, Compsocerops sp., a chigutisaurid belonging to the brachyopoid lineage of temnospondyl amphibians, recovered from the Candelária Sequence. This specimen, consisting of two partial skull fragments (preserving the orbital and postorbital regions), a mandibular fragment, a clavicular blade, and a humerus, was collected from mudstone deposits near São João do Polêsine in Rio Grande do Sul, Brazil, within the Hyperodapedon Assemblage Zone of Carnian age.^[37] Anatomical features of the skull include thin dermal bones (1–2 mm thick), laterally positioned orbits, a long and pointed tabular horn oriented parallel to the skull midline, and an alar process of the jugal along the ventral margin of the orbit; the jugal does not extend anteriorly beyond the orbit, and the tabular fails to contact the parietal. The mandibular fragment measures approximately 30 mm in length, while the humerus exhibits an expanded proximal epiphysis and a slender diaphysis, and the clavicle features a deep groove for articulation with the cleithrum. These traits indicate a semi-aquatic lifestyle, with the limb elements supporting propulsion in floodplain lakes and shallow river systems.^[37] The specimen represents a juvenile individual, preserved without significant dorsoventral compression in fine-grained mudstones, a taphonomic mode that reflects low-energy depositional conditions in aquatic habitats and underscores the rarity of temnospondyl fossils in the formation. It co-occurs with actinopterygian fish and dipnoans in these lacustrine settings.^[37] Compsocerops sp. holds paleobiogeographic importance as one of the few documented Gondwanan temnospondyls from the Carnian stage, marking the earliest known occurrence of chigutisaurids in South America and predating Norian records from Argentina and India; this extends the group's distribution across western Gondwana and highlights evolutionary continuity from Permian-Triassic temnospondyl ancestors.^[37]

Fish

The fish assemblage of the Santa Maria Formation is characterized by a low diversity of osteichthyan remains, primarily from disarticulated elements such as teeth and scales, with no complete skeletons reported.^[38] Actinopterygian fossils include fragmentary remains preliminarily attributed to palaeonisciforms and perleidiforms, recovered from lacustrine facies associated with conchostracans and the flora Dicroidium, indicating small-bodied ray-finned fishes adapted to shallow aquatic settings.^[38] Sarcopterygian remains are represented by a single lungfish tooth plate identified as Ptychoceratodus cf. philippsi, collected from red beds in the uppermost part of the formation near São João do Polêsine, Rio Grande do Sul.^[39] This specimen marks the oldest Mesozoic record of lungfish in South America and suggests an early Triassic radiation of the genus from eastern Gondwanan regions, with migration into the Paraná Basin during more humid climatic phases.^[39] Overall diversity is limited to approximately three to five taxa, reflecting the ephemeral nature of the depositional environments. These fishes inhabited freshwater to brackish systems, such as seasonal lakes and fluvial channels, where they likely preyed on invertebrates and small aquatic organisms. The co-occurrence of fish remains with temnospondyl amphibians points to shared exploitation of wetter intervals within the formation's semi-arid paleoclimate.

Dicynodonts

Dicynodonts represent the dominant component of the vertebrate paleofauna in the Dinodontosaurus Assemblage Zone of the Santa Maria Formation, comprising the most abundant tetrapods and forming the majority of fossils recovered from multiple quarries such as Chiniquá and Pinheiros.^[40]^[41] These Middle Triassic (late Ladinian–early Carnian) therapsids were primarily herbivorous, characterized by keratin-covered beaked jaws adapted for cropping vegetation, short tails, and robust bodies suited to a terrestrial lifestyle.^[40] Body sizes ranged from 1–3 meters in length for medium-sized forms to over 3 meters for larger taxa, with evidence suggesting possible burrowing behaviors in some individuals based on limb morphology and trace fossils from contemporaneous Gondwanan sites.^[42] The key dicynodont taxa include Dinodontosaurus brevirostris, a medium-to-large kannemeyeriiform with tusks, a broad skull (up to ~40 cm long), and features like an elongate curved suborbital process of the postorbital process, known from the type locality at Pinheiros in the Pinheiros-Chiniquá Sequence.^[40] Another prominent form is the large, tusked Stahleckeria potens, a stahleckeriid reaching 3–4 meters in length and up to 400 kg, identified by diagnostic femoral features such as a dorsally projected head and concave lateral surface; abundant specimens, including juveniles, have been recovered from Chiniquá outcrops in reddish sandstones.^[43]^[44] These taxa highlight the diversity within the assemblage, where dicynodonts often exceed 50% of the recovered fossils, underscoring their ecological dominance as primary herbivores.^[40]^[41] This assemblage captures the last major radiation of dicynodonts in Gondwana during the Middle Triassic, following their survival of the Permo-Triassic extinction and preceding their global decline by the Late Triassic, when they were largely replaced by other herbivorous groups.^[42]^[40] The dicynodonts coexisted with early cynodonts in this environment, contributing to a diverse synapsid-dominated ecosystem.^[41]

Cynognathians

The cynognathians of the Santa Maria Formation represent basal eucynodonts, primarily traversodontids, that occupied key ecological roles in Middle to Late Triassic ecosystems of southern Brazil. These taxa, part of the non-probainognathian eucynodont lineage, exhibit robust builds and adaptations suited to terrestrial environments, serving as important components of the post-Permian-Triassic extinction recovery fauna.^[45] The known taxa include Exaeretodon riograndensis and E. major, both traversodontids known from skulls, lower jaws, and fragmentary postcrania such as humeri and vertebrae. These species are distinguished by differences in cranial proportions, such as the shape of the prefrontal bone and prootic crests in E. riograndensis, and share features like reduced postcanine counts in larger individuals. No confirmed records of Cynognathus cf. crateronotus occur in the formation, though the assemblage reflects broader Gondwanan patterns of cynognathian distribution.^[46] Anatomically, these cynognathians feature sectorial canines indicative of carnivorous capabilities, complemented by multi-cusped, leaf-shaped postcanines adapted for herbivory or omnivory, with occlusal basins for grinding plant material. Skulls reach preorbital lengths of up to 12 cm, corresponding to body lengths of approximately 1-1.5 m, making them among the larger synapsids in the assemblage. Postcranial elements, including robust humeri and vertebrae, suggest a low-slung, quadrupedal posture suited to navigating varied terrains. Evidence from ontogenetic series indicates growth-related changes in dentition, with fewer postcanines in adults.^[46]^[47]^[48] Specimens are distributed across the Santa Cruz and Candelária sequences of the Santa Maria Supersequence, with concentrations in the Hyperodapedon Assemblage Zone of the upper formation. Approximately 40-50 individuals of Exaeretodon have been documented across sites like Santa Cruz do Sul and Várzea do Agudo, including bonebeds with multiple ontogenetic stages, suggesting gregarious behavior possibly akin to herd dynamics rather than pack hunting.^[45]^[47]^[49] Paleoecologically, these traversodontids functioned as dominant medium-to-large herbivores in fluvial and floodplain settings, filling a critical niche left vacant after the Permian-Triassic mass extinction by processing vegetation in a landscape dominated by dicynodonts and early archosaurs. Bonebed accumulations, often with signs of post-mortem insect activity, indicate seasonal flooding events concentrated remains in riverine environments. As basal forms, they represent a stem group ancestral to more derived probainognathians that gave rise to mammaliaforms.^[47]^[45]

Probainognathians

Probainognathians represent a diverse clade of advanced non-mammaliaform cynodonts within the Santa Maria Formation, characterized by specialized cranial and dental features that foreshadow mammalian adaptations, such as improved occlusion and jaw mechanics.^[50] These taxa evolved from earlier cynodont lineages, including those documented among cynognathians, and are prominent in the Late Carnian Hyperodapedon Assemblage Zone.^[51] Their fossils contribute significantly to understanding the radiation of mammal-like reptiles during the Late Triassic in southern Brazil.^[52] Key taxa include Prozostrodon brasiliensis, represented by a well-preserved holotype consisting of a partial cranium, mandible, and complete dentition, recovered from outcrops near Santa Maria city.^[51] This species exhibits multicuspate postcanine teeth with precise occlusion, a slender lower jaw, and an estimated skull length of 6.7 cm, suggesting a body size around 30 cm.^[51] Its dental morphology indicates an insectivorous diet, with shearing capabilities suited for processing small arthropods.^[53] A second specimen, including additional cranial elements, further reveals details of the endocranial anatomy, such as expanded cerebral hemispheres and well-developed olfactory bulbs, highlighting early neural enhancements.^[54] Brasilodon quadrangularis is known from partial dentition and associated postcranial elements from the Linha São Luiz site in the upper portion of the formation, providing insights into the earliest stages of mammaliaform evolution.^[55] Its teeth feature multiple cusps arranged in a quadrangular pattern, enabling complex occlusion typical of early mammaliamorphs, and the overall small size (approximately 12-20 cm in length) supports an insectivorous lifestyle.^[56] Postcranial remains indicate a more upright posture in the hindlimbs compared to basal cynodonts, reflecting locomotor advancements.^[55] The discovery of Alemoatherium huebneri in 2017, based on a partial skull and dentition from the Hyperodapedon Assemblage Zone, further enriches the probainognathian record by demonstrating greater taxonomic diversity among Late Triassic forms near the origin of mammals.^[50] This species shares triconodont-like postcanines and a reduced canine with other prozostrodontians, underscoring experimental dental specializations in the clade.^[52] Collectively, these probainognathians are among the earliest known mammaliamorphs, bridging non-mammalian cynodonts to true mammals through traits like diphyodonty and enhanced bite efficiency.^[57] The delicate skeletal elements of these small-bodied taxa are exceptionally preserved in the fine-grained, red laminated mudstones of the Alemoa Member, which represent low-energy lacustrine depositional environments conducive to the fossilization of fragile bones without significant distortion.^[58] This lithology facilitated the recovery of detailed cranial and dental material, essential for reconstructing their anatomy and evolutionary significance.^[59]

Pseudosuchians

Pseudosuchians, the crocodylomorph-line archosaurs, were prominent predators in the Santa Maria Formation, occupying apex roles in the Late Triassic ecosystems of southern Brazil.^[60] These archosaurs, including members of Loricata such as rauisuchids, dominated the carnivorous niches during the Carnian stage, with fossils primarily from the Dinodontosaurus Assemblage Zone.^[61] Key taxa include Prestosuchus chiniquensis, a large loricatan known from multiple well-preserved specimens, including nearly complete skeletons measuring up to 7 meters in length. Decuriasuchus quartacolonia, another loricatan, is represented by fossils from a monospecific bonebed preserving at least ten individuals, suggesting gregarious behavior among these mid-sized predators. In 2025, the proterochampsid Retymaijychampsa beckerorum was added to the assemblage based on an almost complete hindlimb from the formation, enhancing understanding of early pseudosuchian diversity.^[62] A November 2025 discovery, Tainrakuasuchus bellator, a poposauroid pseudosuchian from the Dinodontosaurus Assemblage Zone, is known from a partial skeleton including lower jaw, vertebrae, and pelvic girdle elements; this armored predator, approximately 3-4 meters long with ziphodont teeth, represents an early member of the group and highlights predatory diversity before widespread dinosaur radiation.^[63] Anatomically, these pseudosuchians exhibited facultative bipedalism or quadrupedality, with robust limbs adapted for terrestrial locomotion and ziphodont teeth—laterally compressed and serrated—for slicing flesh, marking them as specialized carnivores and apex predators capable of tackling large prey.^[64] Over ten individuals of Prestosuchus and related forms have been recovered from the Chiniquá outcrops, highlighting the abundance of these archosaurs in localized deposits.^[65] Ecologically, pseudosuchians like Prestosuchus and Decuriasuchus likely functioned as ambush hunters in the riverine habitats of the Santa Maria Formation, exploiting fluvial environments for predation on herbivores and smaller vertebrates.^[66] Their dominance as top predators may have influenced competition with contemporaneous early dinosaurs, shaping trophic dynamics in the assemblage.^[60]

Avemetatarsalians

Avemetatarsalians, the clade encompassing dinosaurs and their close relatives, are represented in the Santa Maria Formation by some of the earliest unequivocal records of these bird-line archosaurs from the Late Triassic. These fossils, primarily from the Carnian stage (~233 Ma), highlight the initial diversification of avemetatarsalians in Gondwana, coexisting briefly with pseudosuchians in a dynamic terrestrial ecosystem.^[28] Key taxa include Staurikosaurus pricei, a herrerasaurid theropod known from a partial skeleton discovered in the upper levels of the formation. This bipedal, gracile dinosaur, estimated at about 2 meters in length, features a slender build adapted for agility, with elongated hindlimbs and a long tail for balance. As the oldest named Brazilian dinosaur, it provides critical evidence for the early radiation of saurischian dinosaurs.^[67] Another significant find is Saturnalia tupiniquim, a basal sauropodomorph from the Alemoa Member, represented by multiple partial skeletons including cranial and postcranial elements. This small, bipedal animal, roughly 1.5 meters long, exhibited a gracile anatomy with reduced skull size and leaf-shaped teeth bearing denticles, suggesting an omnivorous or partially faunivorous diet that included both plant matter and small prey. Its morphology bridges early theropods and later herbivorous sauropodomorphs, underscoring dietary transitions in the clade.^[68] The 2023 discovery of Amanasaurus nesbitti, a silesaurid dinosauriform from the lower Candelária Sequence, further enriches the avemetatarsalian record with femoral elements indicating a body size of around 1 meter. This taxon, characterized by a clefted anterior trochanter on the femur, supports ornithischian affinities among silesaurs and fills stratigraphic gaps in the evolution of avian-line archosaurs during the mid-to-late Carnian.^[28] In 2025, Itaguyra occulta, a new silesaurid from the Santa Cruz Sequence (early Carnian), was described based on isolated pelvic bones, representing one of the earliest ornithischians or close relatives and confirming continuous dinosauromorph presence through the Middle to Late Triassic in South America.^[69] These avemetatarsalians are rare in the Santa Maria Formation, comprising a minor portion of the vertebrate assemblage dominated by synapsids, yet they hold iconic status for documenting the dawn of dinosaurian dominance. The Alemoa Member stands out as a primary locality, yielding articulated specimens that illuminate the clade's anatomical and ecological innovations around 233 million years ago.^[68]^[67]^[28]

Other Reptiles

The Santa Maria Formation preserves a diverse array of non-archosauromorph reptiles, including rhynchocephalians, procolophonids, and fragmentary remains attributable to basal lepidosauromorphs, contributing to the overall vertebrate assemblage diversity of this Late Triassic ecosystem.^[70]^[71] Rhynchocephalians, represented by taxa such as Clevosaurus brasiliensis, Clevosaurus hadroprodon, Microsphenodon bonapartei, and Lanceirosphenodon ferigoloi, are small, lizard-like reptiles typically measuring 20–30 cm in length with skulls 19–31 mm long.^[70]^[72] These forms exhibit acrodont dentition, featuring robust, triangular teeth with a single enlarged tusk-like anterior tooth and multiple palatal tooth rows, suited for an insectivorous diet focused on arthropods.^[72]^[73] Their endocasts reveal a relatively small brain with a reptilian encephalization quotient of 0.27–0.45, indicating modest cognitive capabilities consistent with understory insectivores in fluvial-deltaic environments.^[73] While some species like C. brasiliensis are relatively abundant locally, rhynchocephalians overall comprise less than 5% of the recovered fauna and occur scattered across Carnian–Norian sequences.^[70]^[36] Procolophonids, exemplified by Cornualbus primus from the Passo das Tropas Member, are another minor component of the reptile fauna, known from partial skeletons of these small (under 30 cm), robust-bodied parareptiles with triangular skulls and acrodont marginal teeth.^[71] Phylogenetic analyses place C. primus within Procolophoninae, highlighting its role as the first tetrapod described from this stratigraphic unit and bridging gaps in South American procolophonid evolution.^[71] These lizard-like forms likely occupied understory niches as herbivores or omnivores, feeding on vegetation or mixed diets in the formation's diverse paleoenvironments.^[36] Like rhynchocephalians, procolophonids are rare, representing fewer than 5% of specimens and primarily known from scattered occurrences in mudstone deposits.^[36] Fragmentary basal lepidosauromorph remains, including jaw fragments and possible osteoderm scales, further attest to the presence of early squamate relatives, though these are too incomplete for precise taxonomic assignment.^[74] Such elements suggest additional understory insectivores or herbivores integrated into the ecosystem, but their scarcity underscores the dominance of larger vertebrates in the preserved assemblages.^[36]

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