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

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

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 th ...

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 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 ...

from the Late

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of th ...

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

Patagonia Patagonia () refers to a geographical region that encompasses the southern end of South America, governed by Argentina and Chile. The region comprises the southern section of the Andes Mountains with lakes, fjords, temperate rainforests, and gl ...

South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the southe ...

Discovery and naming

The

type species In 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 species that contains the biological type specimen ...

, ''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 ''Argyrosaurus'' ( ) is a genus of titanosaurian sauropod dinosaur that lived about 70 million years ago, during the Late Cretaceous Period (geology), Period of what is now Argentina. Discovery and naming The type species, ''Argyrosaurus superb ...

?'', before being named as a new taxon. The

holotype A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

specimen is

MACN The Bernardino Rivadavia Natural Sciences Argentine Museum ( es, Museo Argentino de Ciencias Naturales Bernardino Rivadavia) is a public museum located in the Caballito, Buenos Aires, Caballito section of Buenos Aires, Argentina. History and ov ...

-CH 1317, which consists of an incomplete posterior dorsal vertebra. 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 nor a syntype). Of ...

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

sacrum The sacrum (plural: ''sacra'' or ''sacrums''), in human anatomy, is a large, 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 ...

, 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 sauropod dinosaurs, including genera from all seven continents. The titanosaurs were the last surviving group of long-necked sauropods, with taxa still thr ...

: 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 in the Golfo San Jorge Basin of Chubut and Santa Cruz, Argentina whose strata date back to the Middle Cenomanian to Late Turonian. The formation was first described by Teruggi & Rossetto in ...

. Other fauna from the formation include the basal chelid turtles '' Bonapartemys'' and '' Prochelidella'', 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 found ...

Xenotarsosaurus ''Xenotarsosaurus'' is a genus of abelisaurid theropod dinosaur that lived during the Late Cretaceous of Argentina. Discovery In 1980 geologist Juan Carlos Sciutto discovered a rich fossil site six kilometres north of the Ocho Hermanos ranch in ...

'', and an unidentified carnotaurine

. The Bajo Barreal Formation dates back to the late

Cenomanian The Cenomanian is, in the ICS' geological timescale, the oldest or earliest age of the Late Cretaceous Epoch or the lowest stage of the Upper Cretaceous Series. An age is a unit of geochronology; it is a unit of time; the stage is a unit in the s ...

and early

Turonian The Turonian is, in the ICS' geologic timescale, the second age in the Late Cretaceous Epoch, or a stage in the Upper Cretaceous Series. It spans the time between 93.9 ± 0.8 Ma and 89.8 ± 1 Ma (million years ago). The Turonian is preceded by t ...

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 ...

'', '' Aniksosaurus'', 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 about 95 million years ago. The type species is ...

''.

References

{{Taxonbar, from=Q134607 Late Cretaceous dinosaurs of South America Titanosaurs Fossil taxa described in 1990