''Antarctanax'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

of basal archosauriform that during the

Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...

lived in

Antarctica Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest cont ...

. 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 specime ...

is ''Antarctanax shackletoni''. It was a reptile around the size of an

iguana ''Iguana'' (, ) is a genus of herbivorous lizards that are native to tropical areas of Mexico, Central America, South America, and the Caribbean. The genus was first described in 1768 by Austrian naturalist Josephus Nicolaus Laurenti in his ...

Discovery and naming

During a paleontological expedition to the

Transantarctic Mountains The Transantarctic Mountains (abbreviated TAM) comprise a mountain range of uplifted (primarily sedimentary) rock in Antarctica which extend, with some interruptions, across the continent from Cape Adare in northern Victoria Land to Coats Land. ...

in 2010 and 2011, a small skeleton was discovered at the

Graphite Peak Graphite Peak () is a peak, high, standing at the northeast end of a ridge running northeast from Mount Clarke, just south of the head of Falkenhof Glacier in Antarctica. It was so named by the New Zealand Geological Survey Antarctic Expedition ...

. In 2019, the type species ''Antarctanax shackletoni'' was named and described by Brandon Robert Peecook, Roger Malcolm Harris Smith and Christian Alfred Sidor. The generic name combines a reference to Antarctica — itself derived from Greek ''anti'', "opposite of", and ''arktikos'', "of the Bear" — with a Greek ''anax'', "ruler", a reference to the Archosauria, the "Ruling Reptiles". The specific name honours

Ernest Shackleton Sir Ernest Henry Shackleton (15 February 1874 – 5 January 1922) was an Anglo-Irish Antarctic explorer who led three British expeditions to the Antarctic. He was one of the principal figures of the period known as the Heroic Age of ...

, the polar explorer who named the Beardmore Glacier which runs along the Graphite Peak. 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 sever ...

, UWBM 95531, was found in a layer of the lower

Fremouw Formation The Fremouw Formation is a Triassic-age rock formation in the Transantarctic Mountains of Antarctica. It contains the oldest known fossils of tetrapods from Antarctica, including synapsids, reptiles and amphibians. Fossilized trees have also been ...

, which dates to the Early Triassic. It consists of a partial skeleton lacking the skull. It contains eight vertebrae of the neck or back, ribs, the left humerus, all five metatarsals of the left foot, phalanges, among them the claws, of the left foot, and the almost complete right foot. Apart from the feet, the skeleton is largely disarticulated. It represents an adult individual.

Description

''Antarctanax'' was about the size of an iguana, long. The describing authors indicated a unique combination of traits that in themselves are not unique. These traits prove that ''Antarctanax'' is a distinct

taxon In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular n ...

. The cervicodorsal vertebrae, those spanning the rear neck and front back, have per side a ''lamina centrodiapophysealis anterior'', a ridge running on the front underside of the side process towards the vertebral body. The cervicodorsal vertebrae have per side a ''lamina prezygodiapophysealis'', a ridge running from the side process to the front articulation process. The cervicodorsal vertebrae have per side a ''lamina postzygodiapophysealis'', a ridge running from the side process to the rear articulation process. The cervicodorsal vertebrae lack a ''lamina centrodiapophyseal posterior'', a ridge running on the rear underside of the side process towards the vertebral body. The neck vertebrae have no keels on the underside. The neck vertebrae have deep pits to the side of the base of the

neural spine 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 ...

. The back vertebrae have deep pits to the side of the base of the neural spine. The fourth

metatarsal The metatarsal bones, or metatarsus, are a group of five long bones in the foot, located between the tarsal bones of the hind- and mid-foot and the phalanges of the toes. Lacking individual names, the metatarsal bones are numbered from the me ...

is more than 30% longer than the third metatarsal.

Phylogeny

''Antarctanax'' was, within the larger Archosauromorpha, placed in the Archosauriformes. 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 showed that it had a more derived position than the

Proterosuchidae Proterosuchidae is an early family of basal archosauriforms whose fossils are known from the Latest Permian and the Early Triassic of Europe, Asia, Africa, Australia and possibly South America. The name comes from Greek πρότερο- ("first" ...

. It was in a

polytomy An internal node of a phylogenetic tree is described as a polytomy or multifurcation if (i) it is in a rooted tree and is linked to three or more child subtrees or (ii) it is in an unrooted tree and is attached to four or more branches. A tr ...

with '' Fugusuchus'' and ''

Sarmatosuchus ''Sarmatosuchus'' is an extinct genus of archosauriform reptile found in sediments of Middle Triassic age and known from the single species ''Sarmatosuchus otschevi''. It is one of the earliest stem-archosaur species known. The holotype and only ...

'', all three forms being more basal than ''

Cuyosuchus ''Cuyosuchus'' is an extinct genus of archosauriform reptile. Its fossils have been found in Late Triassic-aged rocks of the Cacheuta Formation, Cuyo Basin, Mendoza, Argentina. ''Cuyosuchus'' is based on MCNAM 2669, a partial postcranial skel ...

'', the

Erythrosuchidae Erythrosuchidae (meaning "red crocodiles" in Greek) are a family of large basal archosauriform carnivores that lived from the later Early Triassic (Olenekian) to the early Middle Triassic (Anisian). Naming The family Erythrosuchidae was named by ...

and the

Eucrocopoda Archosauriformes (Greek for 'ruling lizards', and Latin for 'form') is a clade of diapsid reptiles that developed from archosauromorph ancestors some time in the Latest Permian (roughly 252 million years ago). It was defined by Jacques Gauthier ...

. The derived position of ''Antarctanax'' indicates that several more basal archosauromorph groups must have

ghost lineage A ghost lineage is a hypothesized ancestor in a species lineage that has left no fossil evidence yet can be inferred to exist because of gaps in the fossil record or genomic evidence. The process of determining a ghost lineage relies on fossilized ...

s dating back to the

Late Permian Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia ** Local-authority trading enterprise, a New Zealand business law ** Local average treatment effect, ...

References

{{Taxonbar, from=Q61498546 Prehistoric archosauriforms Early Triassic reptiles Prehistoric reptiles of Antarctica Triassic Antarctica Fossils of Antarctica Fossil taxa described in 2019 Prehistoric reptile genera