Riojasauridae is an extinct

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 ...

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 ...

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

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 ...

Period (late

Carnian The Carnian (less commonly, Karnian) is the lowermost stage (stratigraphy), stage of the Upper Triassic series (stratigraphy), Series (or earliest age (geology), age of the Late Triassic Epoch (reference date), Epoch). It lasted from 237 to 227.3 ...

Norian The Norian is a division of the Triassic geological period, Period. It has the rank of an age (geology), age (geochronology) or stage (stratigraphy), stage (chronostratigraphy). It lasted from ~227.3 to Mya (unit), million years ago. It was prec ...

Ages). It contains the

genera Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial s ...

Riojasaurus ''Riojasaurus'' (meaning "lizard from La Rioja") was a herbivorous sauropodomorph dinosaur named after La Rioja Province in Argentina where it was found in the Los Colorados Formation in the Ischigualasto-Villa Unión Basin by José Bonaparte. ...

'' and '' Eucnemesaurus''. The Riojasauridae is considered a stem taxon, and is defined as "the most inclusive clade containing ''Riojasaurus incertus'' but not '' Plateosaurus engelhardti'', '' Massospondylus carinatus'', or '' Anchisaurus polyzelus''".

Geologic 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 exp ...

s containing riojasaurid fossils include the Lower

Elliot Formation The Elliot Formation is a geological formation and forms part of the Stormberg Group, the uppermost geological group that comprises the greater Karoo Supergroup. Outcrops of the Elliot Formation have been found in the northern Eastern Cape, so ...

Orange Free State The Orange Free State ( ; ) was an independent Boer-ruled sovereign republic under British suzerainty in Southern Africa during the second half of the 19th century, which ceased to exist after it was defeated and surrendered to the British Em ...

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...

(where fossils of ''Eucnemesaurus'' have been found), and the

Los Colorados Formation The Los Colorados Formation is a sedimentary geological formation, rock formation of the Ischigualasto-Villa Unión Basin, found in the provinces of San Juan Province, Argentina, San Juan and La Rioja Province (Argentina), La Rioja in Argentina. ...

, in La Rioja Province,

Argentina Argentina, officially the Argentine Republic, is a country in the southern half of South America. It covers an area of , making it the List of South American countries by area, second-largest country in South America after Brazil, the fourt ...

(where fossils of ''Riojasaurus'' have been recovered).

Evolutionary relationships

The Riojasauridae are considered to be sauropodomorphs, but not sauropods themselves. This means that they were generally much smaller than the Sauropods of the

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

and

Cretaceous Period The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 77.1 million years, it is the ninth and longest geologi ...

s, and members of this family may not have been

obligate {{wiktionary, obligate As an adjective, obligate means "by necessity" (antonym '' facultative'') and is used mainly in biology in phrases such as: * Obligate aerobe, an organism that cannot survive without oxygen * Obligate anaerobe, an organism ...

quadruped Quadrupedalism is a form of locomotion in which animals have four legs that are used to bear weight and move around. An animal or machine that usually maintains a four-legged posture and moves using all four legs is said to be a quadruped (fr ...

s, the way more derived Sauropods were. The relationships of this family within other sauropodomorphs has been considered by studies in 2007 and 2020 with slightly different results. The

cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...

below shows basal sauropodomorph relationships simplified after Yates, 2007. In this analysis, Riojasauridae was recovered as the earliest-diverging lineage of the clade

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

, and represents a

sister group In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...

to the node formed by

Massospondylidae Massospondylidae is a family (biology), family of early massopod dinosaurs that existed in Asia, Africa, North America, South America and AntarcticaHellert, Spencer M. "A New Basal Sauropodomorph from The Early Jurassic Hanson Formation of Antarc ...

, the genus '' Jingshanosaurus'' and the Anchisauria. The following cladogram is from a phylogenetic analysis by Peyre de Fabrègues ''et al.'', 2020. Here, Riojasauridae was placed in a more derived position, lying closer to

Sauropoda Sauropoda (), whose members are known as sauropods (; from ''wikt:sauro-, sauro-'' + ''wikt:-pod, -pod'', 'lizard-footed'), is a clade of saurischian ('lizard-hipped') dinosaurs. Sauropods had very long necks, long tails, small heads (relative t ...

than to

, and appearing as the out group to the clade formed by the genus ''Seitaad'' and the Sauropodiformes. The Massospondylidae and the taxa considered to be related to the family by Yates, are shown to be represent more primitive lineages.

References

{{Taxonbar, from=Q3234495 Massopoda Dinosaur families Norian dinosaurs