''Epachthosaurus'' (meaning "heavy lizard") was a

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 ...

titanosaur Titanosaurs (or titanosaurians; members of the group Titanosauria) were a diverse group of sauropod dinosaurs, including genera from all seven continents. The titanosaurs were the last surviving group of long-necked sauropods, with taxa still thr ...

ian

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 b ...

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 ...

from the Late

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

. It was a basal lithostrotian titanosaur. Its fossils have been found in Central and Northern

Patagonia Patagonia () is a geographical region that includes parts of Argentina and Chile at the southern end of South America. The region includes the southern section of the Andes mountain chain with lakes, fjords, temperate rainforests, and glaciers ...

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a considerably smaller portion in the Northern Hemisphere. It can also be described as the southern Subregion#Americas, subregion o ...

Discovery and naming

The

type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...

, ''E. sciuttoi'', was described by Powell in 1990. The bones assigned to it by Powell in 1990 were, originally, assigned to '' Antarctosaurus sp.'', and then to '' Argyrosaurus superbus?'', before being named as a new taxon. The

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 ...

specimen is MACN-CH 1317, which consists of an incomplete posterior

dorsal vertebra In vertebrates, thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. In humans, there are twelve thoracic vertebrae of intermediate size between the cervical and lumbar ve ...

. Another specimen, the

paratype In zoology and botany, a paratype is a specimen of an organism that helps define what the scientific name of a species and other taxon actually represents, but it is not the holotype (and in botany is also neither an isotype (biology), isotype ...

MACN-CH 18689, consists of a cast of six articulated caudal vertebrae, the partial

sacrum The sacrum (: sacra or sacrums), in human anatomy, is a triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1S5) between ages 18 and 30. The sacrum situates at the upper, back part of the pelvic cavity, ...

, and a fragmentary pubic peduncle from the right ilium. A nearly complete specimen referred to ''Epachthosaurus'', UNPSJB-PV 920, was recovered during field research conducted as part of the project ''Los vertebrados de la Formación Bajo Barreal, Provincia de Chubut, Patagonia, Argentina'' by researchers from the Laboratorio de Paleontologia de Vertebrados of the Universidad Nacional de la Patagonia. The specimen, which is one of the most complete titanosaurian skeletons known, consists of a complete, well-preserved, and articulated skeleton only missing the skull, neck, four or five cranial dorsal vertebrae, and several distal caudals.

Description

The autapomorphies that distinguish ''Epachthosaurus'' from other genera are: middle and caudal dorsal vertebrae with unique articular processes extending ventrolaterally from the hyposphene; a strongly developed intraprezygapophyseal lamina, and processes projecting laterally from the dorsal portion of the spinodiapophyseal lamina; hyposphene-hypantrum articulations in caudals 1–14; and a pedal phalangeal formula of 2-2-3-2-0. The genus shares the following apomorphies with various titanosaurians: caudal vertebrae with ventrally expanded posterior centrodiapophyseal laminae; six sacral vertebrae; an ossified ligament or tendon above the sacral neural spines; procoelous proximal, middle, and distal caudal centra with well-developed distal articular condyles; semilunar sternal plates with cranioventral ridges; humeri with squared proximolateral margins and proximolateral processes; unossified carpals; greatly reduced manual phalanges; nearly horizontal, craniolaterally expanded iliac preacetabular processes; pubes proximodistally longer than ischia; and transversely expanded ischia.

Classification

''Epachthosaurus'' is considered to be the most basal titanosaurian known with procoelous caudal vertebrae. Below is a phylogenetical cladogram showing the position of ''Epachthosaurus'' within

Titanosauria Titanosaurs (or titanosaurians; members of the group Titanosauria) were a diverse group of Sauropoda, sauropod dinosaurs, including genera from all seven continents. The titanosaurs were the last surviving group of long-necked sauropods, with tax ...

: The cladogram below follows Mocho ''et al.'' (2019), placing ''Epachthosaurus'' within Lithostrotia, instead of a basal titanosaur.

Paleoecology

''Epachthosaurus'' is known from the early Late Cretaceous of the

Bajo Barreal Formation The Bajo Barreal Formation is a geological Formation (geology), formation in the Golfo San Jorge Basin of Chubut Province, Chubut and Santa Cruz Province, Argentina, Santa Cruz, Argentina whose strata date back to the Cenomanian, Middle Cenomanian ...

. Other fauna from the formation include the basal

chelid Chelidae is one of three living families of the turtle suborder Pleurodira, and are commonly called Austro-South American side-neck turtles. The family is distributed in Australia, New Guinea, parts of Indonesia, and throughout most of South Amer ...

turtles '' Bonapartemys'' and ''

Prochelidella ''Prochelidella'' is an extinct genus of Early Cretaceous, Early to Late Cretaceous chelidae, chelid turtles from the Bajo Barreal Formation, Bajo Barreal, Candeleros Formation, Candeleros, Cerro Barcino Formation, Cerro Barcino and Portezuelo Fo ...

'', the

abelisaurid Abelisauridae (meaning "Abel's lizards") is a family (or clade) of ceratosaurian theropod dinosaurs. Abelisaurids thrived during the Cretaceous period, on the ancient southern supercontinent of Gondwana, and today their fossil remains are foun ...

'' Xenotarsosaurus'', and an unidentified carnotaurine

. The Bajo Barreal Formation dates back to the late

Cenomanian The Cenomanian is, in the International Commission on Stratigraphy's (ICS) geological timescale, the oldest or earliest age (geology), age of the Late Cretaceous epoch (geology), Epoch or the lowest stage (stratigraphy), stage of the Upper Cretace ...

and early

Turonian The Turonian is, in the International Commission on Stratigraphy, ICS' geologic timescale, the second age (geology), age in the Late Cretaceous epoch (geology), Epoch, or a stage (stratigraphy), stage in the Upper Cretaceous series (stratigraphy), ...

of the

. Other genera that lived alongside ''Epachthosaurus'' are ''

Secernosaurus ''Secernosaurus'' (meaning "severed lizard") is a genus of herbivorous dinosaur. ''Secernosaurus'' was a hadrosaur, a "duck-billed" dinosaur which lived during the Late Cretaceous. ''Secernosaurus'' and its close relatives lived in South America, ...

'', '' Notohypsilophodon'', '' Drusilasaura'', ''

Campylodoniscus ''Campylodoniscus'' is a genus of titanosaur sauropod dinosaur from the Late Cretaceous Period of what is now Argentina. The type species was first named and described by Friedrich von Huene in 1929 as ''Campylodon ameghinoi'', the genus name me ...

'', ''

Aniksosaurus ''Aniksosaurus'' (meaning "spring lizard", from Modern Greek Άνοιξη, "Spring", referring to the fact it was found on 21 September 1995, the onset of Spring on the Southern Hemisphere) is a genus of avetheropod dinosaur from what is now Chub ...

'', and ''

Sarmientosaurus ''Sarmientosaurus'' is a genus of titanosaurian sauropod dinosaur belonging to the Titanosauria. It lived in what is now South America, specifically Argentina, during the Upper Cretaceous Period (geology), Period about 95 million years ago. The ...

''.

References

{{Taxonbar, from=Q134607 Titanosauria Dinosaur genera Turonian dinosaurs Bajo Barreal Formation Taxa named by Jaime Powell Fossil taxa described in 1990 Dinosaurs of Argentina