The Rio do Rasto Formation is a

Late Permian Late or LATE may refer to: Everyday usage * Tardy, or late, not being on time * Late (or the late) may refer to a person who is dead Music * ''Late'' (The 77s album), 2000 * Late (Alvin Batiste album), 1993 * Late!, a pseudonym used by Dave Groh ...

sedimentary Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or deposited at Earth's surface. Sedime ...

geological formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics (lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock expo ...

in the South Region of

Brazil Brazil, officially the Federative Republic of Brazil, is the largest country in South America. It is the world's List of countries and dependencies by area, fifth-largest country by area and the List of countries and dependencies by population ...

. The official name is Rio do Rasto, although in some publications it appears as ''Rio do Rastro''. The strata were deposited between the

Wordian In the geologic timescale, the Wordian is an age or stage of the Permian. It is the middle of three subdivisions of the Guadalupian Epoch or Series. The Wordian lasted between and million years ago (Ma). It was preceded by the Roadian and fo ...

and the

Wuchiapingian In the geologic timescale, the Wuchiapingian or Wujiapingian (from in the Liangshan area of Hanzhong, Shaanxi Province ) is an age or stage of the Permian. It is also the lower or earlier of two subdivisions of the Lopingian Epoch or S ...

, from about 266 to 254 million years ago. The geology, alongside its paleobiota, indicate that the locality was a

freshwater Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mi ...

environment. Some of the animals discovered in the formation include ''

Tiarajudens ''Tiarajudens'' () (" Tiaraju tooth") is an extinct genus of saber-toothed herbivorous anomodonts which lived during the Middle Permian period (Capitanian stage) in what is now Rio Grande do Sul, Brazil. It is known from the holotype UFRGS ...

'', '' Parapytanga'' and '' Pampaphoneus''.

Description

Geology

The Rio do Rasto Formation is found mainly in the Brazilian states of Santa Catarina, Paraná, and

Rio Grande do Sul Rio Grande do Sul (, ; ; "Great River of the South") is a Federative units of Brazil, state in the South Region, Brazil, southern region of Brazil. It is the Federative units of Brazil#List, fifth-most populous state and the List of Brazilian s ...

. It was formed during the

and is divided into two members: the lower Serrinha Member and the upper Morro Pelado Member, spanning from the

stages of the

Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...

. The deposits of the Formation are mainly continental, having lacustrine, fluvial and aeolian sediment deposition. Although there have been many interpretations of the Serrinha Member, most agree on it being a shallow lacustrine environment. Hypotheses about the Member's formation range from deposits in

lacustrine A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from t ...

and deltaic environments with large lakes influenced by storm waves to it being a deposit of shallow lakes occasionally influenced by fluvial incursions and storm waves. The member is thick and it is composed chiefly of mudstone and fine-grained sandstone. The Morro Pelado Member was deposited in smaller lakes, alluvial plains, fluvial meanders, delta systems and temporary bodies of water. The rock deposits of Morro Pelado were covered by aeolian stones, such as the dunes of the Pirambóia Formation. It is thick and mainly composed of fine to medium-grained reddish sandstones, usually stratified, with fossils being found on its pelitic intercalations. Tetrapod fossils only occur in this Member. The transition between the two Members occurs in the center-east portion of the

Paraná Basin The Paraná Basin (, ) is a large cratonic sedimentary basin situated in the central-eastern part of South America. About 75% of its areal distribution occurs in Brazil, from Mato Grosso to Rio Grande do Sul states. The remainder area is distribu ...

Age

Estimating the age of the Rio do Rasto formation has long relied solely on the basis of

biostratigraphic Biostratigraphy is the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them.Hine, Robert. "Biostratigraphy." ''Oxford Reference: Dictionary of Biology ...

correlations. Since 2018,

radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to Chronological dating, date materials such as Rock (geology), rocks or carbon, in which trace radioactive impurity, impurities were selectively incorporat ...

has made it possible to specify the age of part of the formation. Based on conchostracan and bivalve faunas, Holz and colleagues have suggested that the sedimentary succession of this formation extended from the

(middle

Guadalupian The Guadalupian is the second and middle Series (stratigraphy), series/Epoch (geology), epoch of the Permian. The Guadalupian was preceded by the Cisuralian and followed by the Lopingian. It is named after the Guadalupe Mountains of New Mexico an ...

) to the

(base of the

Lopingian The Lopingian is the uppermost series/last epoch of the Permian. It is the last epoch of the Paleozoic. The Lopingian was preceded by the Guadalupian and followed by the Early Triassic. The Lopingian is often synonymous with the informal te ...

). Among the tetrapods, the presence of

dinocephalians Dinocephalians (terrible heads) are a clade of large-bodied early therapsids that flourished in the Early Permian, Early and Middle Permian between 279.5 and 260 million years ago (Ma), but became extinct during the Capitanian mass extinct ...

in Fagundes and Boqueirão sites (State of Rio Grande do Sul) as well as in the region of Serra do Cadeado (State of Paraná) indicates a Guadalupian age for these localities, given that this synapsid group is restricted to this age in the rest of the world. In the Fagundes farm site, dinocephalians coexist with the pareiasaur ''Provelosaurus''. The latter is present in the Aceguà area about 2 m above a 30 cm thick layer of

bentonite Bentonite ( ) is an Absorption (chemistry), absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelli ...

radiometrically dated at 266 ± 5.4 million years. The broad error margin of this radiometric dating places the Brazilian dinocephalian sites in the Lower

Roadian In the geologic timescale, the Roadian is an age or stage of the Permian. It is the earliest or lower of three subdivisions of the Guadalupian Epoch or Series. The Roadian lasted between and million years ago (Ma). It was preceded by the Kun ...

– Middle

Capitanian In the geologic timescale, the Capitanian is an age or stage of the Permian. It is also the uppermost or latest of three subdivisions of the Guadalupian Epoch or Series. The Capitanian lasted between and million years ago. It was preceded by t ...

time interval, confirming the Guadalupian age of this part of the Morro Pelado Member. The other

tetrapods A tetrapod (; from Ancient Greek τετρα- ''(tetra-)'' 'four' and πούς ''(poús)'' 'foot') is any four- limbed vertebrate animal of the clade Tetrapoda (). Tetrapods include all extant and extinct amphibians and amniotes, with the lat ...

of the Morro Pelado Member in Rio Grande do Sul suggest an age not exceeding Wordian: Tapinocephalids are only known from Wordian and Capitanian rocks, the basal anomodont ''

'' is closely related to the South African genus ''

Anomocephalus ''Anomocephalus'' is an extinct genus of primitive anomodonts and belongs to the clade Anomocephaloidea. The name is said to be derived from the Greek word ''anomos'' meaning lawless and ''cephalos'' meaning head. The proper word for head in Gree ...

'' of the ''Tapinocephalus'' Assemblage Zone whose age extends from the late Wordian to the late Capitanian, and the temnospondyl '' Konzhukovia'', present at the Boqueirão farm site in São Gabriel, is also known in eastern

European Russia European Russia is the western and most populated part of the Russia, Russian Federation. It is geographically situated in Europe, as opposed to the country's sparsely populated and vastly larger eastern part, Siberia, which is situated in Asia ...

in strata limited to the Wordian-Capitanian interval. The probable Wordian age of the Morro Pelado Member is also reinforced by a radiometric age of the Serrinha Member (lower part of the Rio do Rasto formation) obtained from a

volcanic ash Volcanic ash consists of fragments of rock, mineral crystals, and volcanic glass, produced during volcanic eruptions and measuring less than 2 mm (0.079 inches) in diameter. The term volcanic ash is also often loosely used to r ...

layer which gave an age of 270.61 + 1.76/− 3.27 Ma corresponding to the Roadian (early Guadalupian).

Paleoecology

The rock deposits of the Serrinha and Morro Pelado Members, as well as the abundant conchostracan fossils, indicate a

environment, with Serrinha being composed of shallow lakes with occasional storms and Morro Pelado being composed of small lakes, alluvial plains,

meander A meander is one of a series of regular sinuous curves in the Channel (geography), channel of a river or other watercourse. It is produced as a watercourse erosion, erodes the sediments of an outer, concave bank (cut bank, cut bank or river cl ...

s and temporary bodies of water, which were later covered up by dunes. The increased occurrence of aeolian sandstone in Morro Pelado indicate a drier climate with greater aridity. Fossils of the Rio do Rasto Formation, and neighboring Permian formations, have revealed a flora composed of plants like glossopterids, sphenophytes and

pteridophyte A pteridophyte is a vascular plant (with xylem and phloem) that reproduces by means of spores. Because pteridophytes produce neither flowers nor seeds, they are sometimes referred to as " cryptogams", meaning that their means of reproduction is ...

s. The fauna is composed of Invertebrates, such as

bivalves Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed by a calcified exoskeleton consis ...

gastropods Gastropods (), commonly known as slugs and snails, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and from the land. Ther ...

and conchostracans, and vertebrates like palaeonisciform and

elasmobranch Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including modern sharks ( division Selachii), and batomorphs (division Batomorphi, including rays, skates, and sawfish). Members of this subclass are characterised by h ...

fish such as ''

Xenacanthus ''Xenacanthus'' (from Ancient Greek wikt:ξένος, ξένος, xénos, 'foreign, alien' + wikt:ἄκανθος, ἄκανθος, akanthos, 'spine') is an extinct genus of Xenacanthida, xenacanth cartilaginous fish. It lived in freshwater environ ...

'' and '' Sphenacanthus''. Tetrapods are only found in the upper Morro Pelado Member, and are represented by terrestrial herbivores like the dicynodonts ''

Endothiodon ''Endothiodon'' (/ɛndoʊθiːoʊdɔːn/ "inner tooth" from Greek endothi (ἔνδοθῐ), "within", and odon (ὀδών), "tooth", most likely named for the characteristic of the teeth being placed internally to the maxilla) is an extinct genus ...

'' and '' Rastodon'', the pareiasaur ''

Provelosaurus ''Provelosaurus'' is an extinct Pareiasaur genus of the Late Permian found on the road between Aceguá and Bagé in the Paleorrota, Rio Grande do Sul, Brazil. Found in the Rio do Rasto Formation, aged about 260 million years.http://www.ufrgs.br ...

'', a tapinocephalid and the saber-toothed ''

''. Therapsid predators such as '' Pampaphoneus'' were also present, as well as another unidentified anteosaur, and a titanosuchid. Temnospondyls such as ''

Australerpeton ''Australerpeton'' is an extinct genus of stereospondylomorph temnospondyl currently believed to belong to the family Rhinesuchidae. When first named in 1998, the genus was placed within the new family Australerpetontidae. However, studies publ ...

'', '' Bageherpeton'' and ''

Rastosuchus ''Rastosuchus'' is an extinct genus of stereospondyl temnospondyl within the family Rhinesuchidae. It contains one species, ''Rastosuchus hammeri'', found in the Permian Rio do Rasto Formation of Brazil. History of study The name ''Rastosuchu ...

'' lurked in the abundant bodies of water. These giant amphibians filled the niche of aquatic predators, much like the modern day

crocodile Crocodiles (family (biology), family Crocodylidae) or true crocodiles are large, semiaquatic reptiles that live throughout the tropics in Africa, Asia, the Americas and Australia. The term "crocodile" is sometimes used more loosely to include ...

Coprolite A coprolite (also known as a coprolith) is fossilized feces. Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour (in this case, diet) rather than morphology. The name ...

remains have also been found, some of which likely belong to palaeonisciform and

fish, and some of the coprolites contained pieces of bones and scales, demonstrating clear evidence of predation. Some of the scales preserved peg-and-socket structures, associated with palaeonisciform fish. In at least one coprolite it was possible to recognise a fish jaw with teeth.

Stromatolite Stromatolites ( ) or stromatoliths () are layered Sedimentary rock, sedimentary formation of rocks, formations (microbialite) that are created mainly by Photosynthesis, photosynthetic microorganisms such as cyanobacteria, sulfate-reducing micr ...

s have also been found in the formation, interpreted as having developed in waters that were shallow, clear and warm and in habitats that were inhospitable for competitors.

Fossil content

Flora

Invertebrates

Bivalves

Conchostraca

Gastropods

Vertebrates

Fish

Amphibians

Parareptiles

Synapsids

References

{{Reflist, 30em Geologic formations of Brazil Permian System of South America Permian Brazil Changhsingian Shale formations Sandstone formations Fluvial deposits Formations Permian southern paleotemperate deposits Fossiliferous stratigraphic units of South America Paleontology in Brazil Formations Formations Formations Formations Formations Formations