Gravisauria is a

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

sauropod Sauropoda (), whose members are known as sauropods (; from '' sauro-'' + '' -pod'', 'lizard-footed'), is a clade of saurischian ('lizard-hipped') dinosaurs. Sauropods had very long necks, long tails, small heads (relative to the rest of their bo ...

dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...

s consisting of some genera,

Vulcanodontidae The Early Jurassic sauropod dinosaurs ''Zizhongosaurus'', ''Barapasaurus'', ''Tazoudasaurus'', and ''Vulcanodon ''Vulcanodon'' (meaning "volcano tooth") is an extinct genus of sauropod dinosaur from the Early Jurassic of southern Africa. The ...

and

Eusauropoda Eusauropoda (meaning "true sauropods") is a derived clade of sauropod dinosaurs. Eusauropods represent the node-based group that includes all descendant sauropods starting with the basal eusauropods of ''Shunosaurus'', and possibly ''Barapasaur ...

.Allain, R. and Aquesbi, N. (2008). "Anatomy and phylogenetic relationships of ''Tazoudasaurus naimi'' (Dinosauria, Sauropoda) from the late Early Jurassic of Morocco." ''Geodiversitas'', 30(2): 345-424.

Classification

The

Gravisauria was appointed by the French

paleontologist Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (roughly 11,700 years before present). It includes the study of fossi ...

Ronan Allain and Moroccan paleontologist Najat Aquesbi in 2008 when a

cladistic Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups (" clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived char ...

analysis of the dinosaur found by Allain, ''

Tazoudasaurus ''Tazoudasaurus'' is a genus of vulcanodontid sauropod dinosaurs hailing from the Early Jurassic, located in the High Atlas Mountains of Morocco in North Africa. Along with ''Ohmdenosaurus'' is one of the two formally described sauropods from th ...

'', as the outcome was that the

family Family (from la, familia) is a Social group, group of people related either by consanguinity (by recognized birth) or Affinity (law), affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its ...

. The group includes ''Tazoudasaurus'' and ''Vulcanodon'', and the sister taxon Eusauropoda, but also certain

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate s ...

such as ''

Antetonitrus ''Antetonitrus'' is a genus of sauropod dinosaur found in the Early Jurassic Elliot Formation of South Africa. The only species is ''Antetonitrus ingenipes''. As one of the oldest known sauropods, it is crucial for the understanding of the origi ...

'', ''

Gongxianosaurus ''Gongxianosaurus'' is a genus of basal sauropod dinosaur from the early Jurassic Period (Toarcian stage). The only species is ''Gongxianosaurus shibeiensis''. Based on four fragmentary to complete specimens found in China (Sichuan Province), ...

'' and ''

Isanosaurus ''Isanosaurus'' means "North-eastern thailand lizard" was a sauropod dinosaur from Thailand. It was originally dated to approximately 210 million years ago during the Late Triassic (late Norian to Rhaetian stages), which would make it on ...

'' that do not belong in Vulcanodontidae but to an even more basic position occupied in

Sauropoda Sauropoda (), whose members are known as sauropods (; from '' sauro-'' + '' -pod'', 'lizard-footed'), is a clade of saurischian ('lizard-hipped') dinosaurs. Sauropods had very long necks, long tails, small heads (relative to the rest of their bo ...

. It made sense to have Sauropoda compared to this, more derived group that included Vulcanodontidae and Eusauropoda in a definition: Gravisauria (heavy lizards), defined as the group formed by the last common ancestor of ''Tazoudasaurus'' and ''

Saltasaurus ''Saltasaurus'' (which means "lizard from Salta") is a genus of saltasaurid dinosaur of the Late Cretaceous period of Argentina. Small among sauropods, though still heavy by the standards of modern creatures, ''Saltasaurus'' was characterized by ...

'' (Bonaparte and Powell, 1980) and all its descendants. Below is a cladogram found by Nair and Salisbury in 2012 showing the relationships of Gravisauria:

Synapomorphies

Aquesbi mentioned two synapomorphies, shared derived characteristics of Gravisauria: the

vertebrae The spinal column, a defining synapomorphy shared by nearly all vertebrates,Hagfish are believed to have secondarily lost their spinal column is a moderately flexible series of vertebrae (singular vertebra), each constituting a characteristic i ...

are wider side to side than front to rear and possession of asymmetrical condyles femoris at the bottom of the

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates with ...

. Those were previously not thought to be Eusauropoda synapomorphies but Allian found these properties also on ''

''.

Involvement in extinction

Gravisauria split off in the

Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-J ...

, around the

Pliensbachian The Pliensbachian is an age of the geologic timescale and stage in the stratigraphic column. It is part of the Early or Lower Jurassic Epoch or Series and spans the time between 190.8 ± 1.5 Ma and 182.7 ± 1.5 Ma (million years ago). The Plien ...

and

Toarcian The Toarcian is, in the ICS' geologic timescale, an age and stage in the Early or Lower Jurassic. It spans the time between 182.7 Ma (million years ago) and 174.1 Ma. It follows the Pliensbachian and is followed by the Aalenian. The Toarcian ...

, 183 million years ago, and Aquesbi thought that this was part of a much larger revolution in the

fauna Fauna is all of the animal life present in a particular region or time. The corresponding term for plants is ''flora'', and for fungi, it is '' funga''. Flora, fauna, funga and other forms of life are collectively referred to as '' biota''. Zoo ...

, which includes the disappearance of

Prosauropoda Plateosauria is a clade of sauropodomorph dinosaurs which lived during the Late Triassic to the Late Cretaceous. The name Plateosauria was first coined by Gustav Tornier in 1913. The name afterwards fell out of use until the 1980s. Classificati ...

Coelophysoidea Coelophysoidea were common dinosaurs of the Late Triassic and Early Jurassic periods. They were widespread geographically, probably living on all continents. Coelophysoids were all slender, carnivorous forms with a superficial similarity to the ...

and basal

Thyreophora Thyreophora ("shield bearers", often known simply as "armored dinosaurs") is a group of armored ornithischian dinosaurs that lived from the Early Jurassic until the end of the Cretaceous. Thyreophorans are characterized by the presence of body ...

, which they attributed to a worldwide

mass extinction An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms. It ...

References

*Najat Aquesbi, 2008, ''Les Sites de Dinosaures du Lias du Haut Atlas (Maroc), Problèmes de Phylogénie et de Paleogeographie'' Sauropods {{sauropodomorph-stub