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Riograndia Temporal range: Norian ~225.42 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Life restoration of Riograndia guaibensis
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Synapsida
Clade:	Therapsida
Clade:	Cynodontia
Family:	†Tritheledontidae
Genus:	†Riograndia Bonaparte et al., 2001
Species:	†R. guaibensis
Binomial name
†Riograndia guaibensis Bonaparte et al., 2001

Riograndia is an extinct genus of prozostrodontian cynodonts from the Late Triassic of Brazil. The type and only species is Riograndia guaibensis, named after the State of Rio Grande do Sul and Guaíba Basin, where it was discovered. Remains have been found in the Caturrita Formation of the geopark of Paleorrota. It was a small non-mammalian cynodont, 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 in the Caturrita Formation. The genus defines the Riograndia Assemblage Zone.

Description

Holotype (MCN-PV 2264) is an anterior part of a skull, 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 genus in the family Tritheledontidae. Other tritheledontids include the related Irajatherium, and two clades, a more basal group including Sinoconodon, Brasilitherium, Brasilodon, and Morganucodon, and a more derived clade of Chaliminia, Elliotherium, Pachygenelus, Diarthrognathus, and Tritheledon. The below cladogram was found by Martinelli and Rougier in 2007 and modified by Soares et al. in 2011, with Tritheledontidae added after Ruta et al. (2013):

A cladogram after Stefanello et al. (2023):

Paleoecology

All specimens of Riograndia come from the Late Triassic aged Caturrita Formation. The formation dates to about 225.42 million years ago. Dinosaurs from the formation only include the sauropodomorphs Unaysaurus and Guaibasaurus. Non-dinosaurian animals include the dinosauriform Sacisaurus; the dicynodont Jachaleria; an unnamed phytosaurian; isolated archosaurian teeth; an amphibian classified in Stereospondyli; and many common tetrapods smaller than 15 cm (5.9 in) long. Among the tetrapods is the procolophonid Soturnia; the sphenodontid Clevosaurus; the lepidosaurian Cargninia; the archosaurian Faxinalipterus, and an assortment of mammaliamorphs including Riograndia, Brasilodon, and Irajatherium.

References

1. ^ Morgan Lionel Guignard, Agustin G. Martinelli, Marina B Soares (2019). "Postcranial anatomy of Riograndia guaibensis (Cynodontia: Ictidosauria)". Geobios. 53: 9-21. doi:10.1016/j.geobios.2019.02.006.{{cite journal}}: CS1 maint: multiple names: authors list (link)
2. ^ Jose F Bonaparte, Jorge Ferigolo, Ana Maria Ribeiro (2001). "A Primitive Late Triassic 'ictidosaur' from Rio Grande Do Sul, Brazil". Palaeontology. 44 (4): 623-635. doi:10.1111/1475-4983.00194.{{cite journal}}: CS1 maint: multiple names: authors list (link)
3. ^ Soares, M. B.; Schultz, C. L.; Horn, B. L. D. (2011). "New information on Riograndia guaibensis Bonaparte, Ferigolo & Ribeiro, 2001 (Eucynodontia, Tritheledontidae) from the Late Triassic of southern Brazil: anatomical and biostratigraphic implications". Anais da Academia Brasileira de Ciências. 83 (1): 329–354. doi:10.1590/S0001-37652011000100021. ISSN 0001-3765. PMID 21437390.
4. ^ Schultz, C. L.; Martinelli, A. G.; Soares, M. B.; Pinheiro, F. L.; Kerber, L.; Horn, B. L. D.; Pretto, F. A.; Müller, R. T.; Melo, T. P. (2020). "Triassic faunal successions of the Paraná Basin, southern Brazil". Journal of South American Earth Sciences. 104: 102846. doi:10.1016/j.jsames.2020.102846.
5. ^ Ruta, M.; Botha-Brink, J.; Mitchell, S. A.; Benton, M. J. (2013). "The radiation of cynodonts and the ground plan of mammalian morphological diversity". Proceedings of the Royal Society B: Biological Sciences. 280 (1769): 20131865. doi:10.1098/rspb.2013.1865. PMC 3768321. PMID 23986112.
6. Stefanello, M.; Martinelli, A. G.; Müller, R. T.; Dias-da-Silva, S.; Kerber, L. (2023). "A complete skull of a stem mammal from the Late Triassic of Brazil illuminates the early evolution of prozostrodontian cynodonts". Journal of Mammalian Evolution. doi:10.1007/s10914-022-09648-y.

External links

• (in Portuguese) Dinossauros do Rio grande do Sul
• (in Portuguese) Sociedade Brasileira de Paleontologia

v t e Cynodontia
Kingdom: Animalia Phylum: Chordata Clade: Synapsida Clade: Therapsida Clade: Eutheriodontia Synapsida see Synapsida Cynodontia see below↓
Cynodontia Cynodontia †Dvinia †Nanocynodon †Madysaurus †Uralocynodon †Charassognathidae Abdalodon Charassognathus Nshimbodon †Procynosuchidae Procynosuchus Sludica Epicynodontia †Bolotridon †Cynosaurus †Nanictosaurus †Nythosaurus †Platycraniellus †Thrinaxodon †Vetusodon †Galesauridae Galesaurus Progalesaurus Eucynodontia †Kataigidodon †Cynognathia see Cynognathia Probainognathia see below↓
Probainognathia Probainognathia †Agudotherium †Candelariodon †Charruodon †Lumkuia †Protheriodon †Tessellatia †Chiniquodontidae Aleodon Chiniquodon Cromptodon? Riojanodon †Probainognathidae Bonacynodon Probainognathus †Ecteniniidae Diegocanis Ecteninion Trucidocynodon Prozostrodontia †Alemoatherium †Deccanodon †Dromatherium †Inditherium †Microconodon †Polonodon †Pseudotriconodon †Rewaconodon †Santacruzgnathus †Therioherpeton †Tricuspes †Prozostrodontidae Prozostrodon Pseudotherium †Tritheledontidae Chaliminia Diarthrognathus Elliotherium Irajatherium Pachygenelus Riograndia? Tritheledon Mammaliamorpha †Adelobasileus †Botucaraitherium †Brasilodon †Sinoconodon †Tikiodon †Tritylodontidae Bienotherium Bienotheroides Bocatherium Dianzhongia Dinnebitodon Fossiomanus Kayentatherium Lufengia Montirictus Nuurtherium Oligokyphus Polistodon Shartegodon Stereognathus Tritylodon Xenocretosuchus Yuanotherium Yunnanodon Mammaliaformes see below↓
Mammaliaformes Mammaliaformes †Bocaconodon †Brachyzostrodon †Bridetherium †Cherwellia †Cifellilestes †Delsatia †Dianoconodon †Dinnetherium †Dyskritodon †Eozostrodon †Erythrotherium †Gondwanadon †Hadrocodium †Hallautherium †Helvetiodon †Indotherium †Indozostrodon †Megazostrodon †Morganucodon †Paceyodon †Paikasigudodon †Purbeckodon †Rosierodon †Sinoconodon? †Storchodon †Stylidens †Wareolestes †Woutersia †Kuehneotheriidae Fluctuodon Kotatherium Kuehneon Kuehneotherium †Shuotheriidae Feredocodon Itatodon? Paritatodon? Pseudotribos Shuotherium †Docodonta Agilodocodon Borealestes Castorocauda Cyrtlatherium Dobunnodon Docodon Docofossor Dsungarodon Ergetiis Gondtherium? Haldanodon Hutegotherium Itatodon? Khorotherium Krusatodon Microdocodon Paritatodon? Peraiocynodon Sibirotherium Simpsonodon Tashkumyrodon Tegotherium †Haramiyida Avashishta? Hypsiprymnopsis? Haramiyavia Kalaallitkigun Kollikodon? Mojo? Theroteinus Thomasia Euharamiyida Allostaffia? Arboroharamiya Baidabatyr? Butlerodon Cryoharamiya Kermackodon? Maiopatagium Megaconus? Qishou Sharypovoia Shenshou Sineleutherus Vilevolodon Woodeatonia Xianshou Gondwanatheria? Hahnodontidae? Cifelliodon Denisodon Hahnodon Crown-Mammalia see Mammalia
See also Incertae sedis †Cistecynodon †Eoraetia †Gaumia †"Hahnia" †Kunminia †Lepagia †Meurthodon †Mitredon †Panchetocynodon †Redondagnathus †Saurodesmus? Paraphyletic / Polyphyletic groups Dromatheriidae Morganucodonta Symmetrodonta Triconodonta
Category

• 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