''Riograndia'' is an extinct

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of prozostrodontian

cynodont Cynodontia () is a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 Megaannum, mya), and extensively diversified after the Permian–Triassic extinction event. Mammals are cynodonts, as are their extin ...

s from the

Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch a ...

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

type Type may refer to: Science and technology Computing * Typing, producing text via a keyboard, typewriter, etc. * Data type, collection of values used for computations. * File type * TYPE (DOS command), a command to display contents of a file. * ...

and only

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

is ''Riograndia guaibensis'', named after the State of

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 ...

and Guaíba Basin, where it was discovered. Remains have been found in the Caturrita Formation of the

geopark A geopark is a protected area with internationally significant geology within which Sustainability, sustainable development is sought and which includes tourism, conservation, education and research concerning not just geology but other relevant s ...

of Paleorrota. It was a small non-mammalian

, with several advanced features also present in mammals. Several specimens of ''Riograndia guaibensis'' have been found in the towns of Candelária and

Faxinal do Soturno Faxinal do Soturno is a municipality in the state of Rio Grande do Sul, Brazil. See also *List of municipalities in Rio Grande do Sul This is a list of the municipalities in the state of Rio Grande do Sul (RS), located in the South Region of ...

in the Caturrita Formation. The genus defines the ''Riograndia'' Assemblage Zone.

Description

Holotype A holotype (Latin: ''holotypus'') is a single physical example (or illustration) of an organism used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of s ...

(MCN-PV 2264) is an anterior part of a

skull The skull, or cranium, is typically a bony enclosure around the brain of a vertebrate. In some fish, and amphibians, the skull is of cartilage. The skull is at the head end of the vertebrate. In the human, the skull comprises two prominent ...

, from the tip of snout to the fronto-parietal contact, with complete dentition. The unique feature of ''Riograndia'' are lobed postcanine teeth with approximately equal 5-9 sharp cuspules located along the almost semicircular edge of the crowns of the upper postcanine teeth and in the posterodorsal edge of the lower postcanine teeth. Most specimens have skulls of approximately 35 mm long. Study of the postcranial skeleton shows that ''Riograndia'' had a semi-sprawling forelimbs, similar to those of more basal synapsids. The limbs of more advanced cynodonts are positioned more straight. The body of ''Riograndia'' was held above the ground with the help of adductor muscles attached to the forelimbs.

Classification

''Riograndia'' was originally assigned to the monotypic family Riograndidae within Ictidosauria. Later some authors considered it as a non-tritheledontid ictidosaur or as a basal

in the

family Family (from ) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). It forms the basis for social order. Ideally, families offer predictabili ...

Tritheledontidae Tritheledontidae, the tritheledontids or ictidosaurs, is an extinct family of small to medium-sized (about 10 to 20 cm long) cynodonts. They were highly mammal-like, specialized cynodonts, although they still retained a few reptile-like ...

. Other tritheledontids include the related ''

Irajatherium ''Irajatherium'' is an extinct genus of cynodonts, known only of the type species ''Irajatherium hernandezi''.Martinelli et al., 2005 It is named in honor of Irajá Damiani Pinto. Species ''Irajatherium hernandezi'' is a species known only by ...

'', and two clades, a more basal group including ''

Sinoconodon ''Sinoconodon'' is an extinct genus of mammaliamorphs that appears in the fossil record of the Lufeng Formation of China in the Sinemurian stage of the Early Jurassic period, about 193 million years ago. While sharing many plesiomorphic traits w ...

'', '' Brasilitherium'', ''

Brasilodon ''Brasilodon'' ("tooth from Brazil") is an extinct genus of small, mammal-like cynodonts that lived in what is now Brazil during the Norian age of the Late Triassic epoch, about 225.42 million years ago. While no complete skeletons have been foun ...

'', and ''

Morganucodon ''Morganucodon'' ("Glamorgan tooth") is an early mammaliaform genus that lived from the Late Triassic to the Middle Jurassic. It first appeared about 205 million years ago. Unlike many other early mammaliaforms, ''Morganucodon'' is well represent ...

'', and a more derived clade of ''

Chaliminia ''Chaliminia'' is an extinct genus of tritheledontid cynodont from the Triassic Period of what is now Argentina. The type and only species is ''C. musteloides''. See also *List of therapsids This list of therapsids is an attempt to create a com ...

'', ''

Elliotherium ''Elliotherium'' is an extinct genus of cynodonts which existed in South Africa during the upper Triassic period. The type species is ''Elliotherium kersteni'', named after the Elliot Formation The Elliot Formation is a geological formation a ...

'', ''

Pachygenelus ''Pachygenelus'' is a genus of extinct cynodont. Fossils have been found from the Karoo basin in South Africa and date back to the Early Jurassic. The genus was named in 1913 on the basis of a partial lower jaw found from South Africa, with the ...

'', ''

Diarthrognathus ''Diarthrognathus'' ("Two joint jaw") is an extinct genus of tritheledontid cynodonts, known from fossil evidence found in South Africa and first described in 1958 by A.W. Crompton.Rieppel, Olivier. Evolutionary Theory and the Creation Controver ...

'', and ''

Tritheledon ''Tritheledon'' is an extinct genus of cynodonts that lived during the Lower Jurassic. Fossils were found in the Elliot Formation, South Africa. Like all cynodonts, it had many traits shared by mammals. Tritheledonts were probably insectivores ...

''. The below cladogram was found by Martinelli and Rougier in

2007 2007 was designated as the International Heliophysical Year and the International Polar Year. Events January * January 1 **Bulgaria and Romania 2007 enlargement of the European Union, join the European Union, while Slovenia joins the Eur ...

and modified by Soares ''et al.'' in

2011 The year marked the start of a Arab Spring, series of protests and revolutions throughout the Arab world advocating for democracy, reform, and economic recovery, later leading to the depositions of world leaders in Tunisia, Egypt, and Yemen ...

, with Tritheledontidae added after Ruta ''et al.'' (2013): A cladogram after Stefanello ''et al.'' (2023):

Paleoecology

All specimens of ''Riograndia'' come from the

aged Caturrita Formation. The formation dates to about 225.42

million years ago Million years ago, abbreviated as Mya, Myr (megayear) or Ma (megaannum), is a unit of time equal to (i.e. years), or approximately 31.6 teraseconds. Usage Myr is in common use in fields such as Earth science and cosmology. Myr is also used w ...

Dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...

s from the formation only include the

sauropodomorph Sauropodomorpha ( ; from Greek, meaning "lizard-footed forms") is an extinct clade of long-necked, herbivorous, saurischian dinosaurs that includes the sauropods and their ancestral relatives. Sauropods generally grew to very large sizes, had lo ...

s ''

Unaysaurus ''Unaysaurus'' is a genus of unaysauridae, unaysaurid sauropodomorpha, sauropodomorph herbivore dinosaur. Discovered in southern Brazil, in the geopark of Paleorrota, in 1998, and announced in a press conference on Thursday, December 3, 2004, it ...

'' and '' Guaibasaurus''. Non-dinosaurian

animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...

s include the

dinosauriform Dinosauromorpha is a clade of avemetatarsalians (archosaurs closer to birds than to crocodilians) that includes the Dinosauria (dinosaurs) and some of their close relatives. It was originally defined to include dinosauriforms and lagerpetids, ...

Sacisaurus ''Sacisaurus'' (" Saci lizard") is a silesaurid dinosauriform from the Late Triassic (Norian) Caturrita Formation of southern Brazil. The scientific name, ''Sacisaurus agudoensis'', refers to the city where the species was found, Agudo in the ...

''; the

dicynodont Dicynodontia is an extinct clade of anomodonts, an extinct type of non-mammalian therapsid. Dicynodonts were herbivores that typically bore a pair of tusks, hence their name, which means 'two dog tooth'. Members of the group possessed a horny, t ...

'' Jachaleria''; an unnamed

phytosauria Phytosaurs (Φυτόσαυροι in greek language, Greek, meaning 'plant lizard') are an extinct group of large, mostly semiaquatic Late Triassic archosauriform or Basal (phylogenetics), basal archosaurian reptiles. Phytosaurs belong to the ord ...

n; isolated archosaurian teeth; an

amphibia Amphibians are ectothermic, anamniotic, four-limbed vertebrate animals that constitute the class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all tetrapods, but excluding the amniotes (tetrapods with an amniotic ...

n classified in

Stereospondyli The Stereospondyli are a group of extinct temnospondyl amphibians that existed primarily during the Mesozoic period. They are known from all seven continents and were common components of many Triassic ecosystems, likely filling a similar ecologi ...

; and many common

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 ...

smaller than long. Among the tetrapods is the

procolophonid Procolophonidae is an extinct family of small, lizard-like parareptiles known from the Late Permian to Late Triassic that were distributed across Pangaea, having been reported from Europe, North America, China, South Africa, South America, Antarc ...

'' Soturnia''; the sphenodontid ''

Clevosaurus ''Clevosaurus'' (meaning "Gloucester lizard") is an extinct genus of rhynchocephalian reptile from the Late Triassic and the Early Jurassic periods. Species of ''Clevosaurus'' were widespread across Pangaea, and have been found on all continents ...

''; the

lepidosauria The Lepidosauria (, from Greek meaning ''scaled lizards'') is a Order (biology), superorder or Class (biology), subclass of reptiles, containing the orders Squamata and Rhynchocephalia. Squamata also includes Lizard, lizards and Snake, snakes. Sq ...

n '' Cargninia''; the archosaurian ''

Faxinalipterus ''Faxinalipterus'' is a genus of ornithodiran archosaur, originally described as a pterosaur, from the Late Triassic Caturrita Formation of southern Brazil. A study from 2022 reinterpreted the fossil remains and instead suggests them to belong to ...

'', and an assortment of mammaliamorphs including ''Riograndia'', ''Brasilodon'', and ''Irajatherium''.

References

External links

*
''Dinossauros do Rio grande do Sul''
*

{{Portal bar, Brazil, Paleontology Tritheledontidae Norian genera Late Triassic synapsids of South America Triassic Brazil Fossils of Brazil Paraná Basin Fossil taxa described in 2001 Taxa named by José Bonaparte