''Arachana'' is an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

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 ...

of rhinesuchid-like

temnospondyl Temnospondyli (from Greek language, Greek τέμνειν, ''temnein'' 'to cut' and σπόνδυλος, ''spondylos'' 'vertebra') is a diverse order (biology), order of small to giant tetrapods—often considered Labyrinthodontia, primitive amphi ...

known from the

Early Triassic The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which is a un ...

Buena Vista Formation The Buena Vista Formation is a Late Permian/Early Triassic geologic formation of the Cerro Largo Department in northeastern Uruguay. The fluvial sandstones and mudstones preserve temnospondyl and archosaur fossils. Description The Buena Vi ...

of northeastern

Uruguay Uruguay (; ), officially the Oriental Republic of Uruguay ( es, República Oriental del Uruguay), is a country in South America. It shares borders with Argentina to its west and southwest and Brazil to its north and northeast; while bordering ...

. ''Arachana'' was first named by Graciela Piñeiro, Alejandro Ramos and Claudia Marsicano in

2012 File:2012 Events Collage V3.png, From left, clockwise: The passenger cruise ship Costa Concordia lies capsized after the Costa Concordia disaster; Damage to Casino Pier in Seaside Heights, New Jersey as a result of Hurricane Sandy; People gather ...

and 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 ...

is ''A. nigra''. It shares characteristics with both rhinesuchids and lydekkerinids, making it a transitional form between basal and more advanced

stereospondyl The Stereospondyli are a group of extinct temnospondyl amphibians that existed primarily during the Mesozoic period. They are known from all seven continents and were common components of many Triassic ecosystems, likely filling a similar ecologi ...

Description

''Arachana'' is known from 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 ...

FC-DPV 1369, a three-dimensionally preserved nearly complete

skull The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...

that has not been distorted. It was collected in the Colonia Orozco locality from the

of the Norte Basin. The formation spans the boundary between the

Permian The Permian ( ) is a geologic period and stratigraphic system which spans 47 million years from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.9 Mya. It is the last period of the Paleoz ...

and the

Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...

, and it is uncertain whether FC-DPV 1369 was found in rocks above or below the boundary. ''Arachana'' shares several characters with rhinesuchids. For example, it has a large head with

orbits In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as a p ...

or eye sockets placed slightly behind the mid-length of the skull. The surface of the skull is deeply pitted and the margins of the cheek regions are smooth and convex. The

basioccipital The basilar part of the occipital bone (also basioccipital) extends forward and upward from the foramen magnum, and presents in front an area more or less quadrilateral in outline. In the young skull this area is rough and uneven, and is joined t ...

bone at the back of the skull is small. On the

palate The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly sepa ...

, the

pterygoid bone The pterygoid is a paired bone forming part of the palate of many vertebrates, behind the palatine bone In anatomy, the palatine bones () are two irregular bones of the facial skeleton in many animal species, located above the uvula in the th ...

is connected to the

vomer The vomer (; lat, vomer, lit=ploughshare) is one of the unpaired facial bones of the skull. It is located in the midsagittal line, and articulates with the sphenoid, the ethmoid, the left and right palatine bones, and the left and right maxill ...

by a projection called the palatal ramus. The ramus separates the interpterygoid vacuities, two large holes in the palate. In many temnospondyls, the

palatine A palatine or palatinus (in Latin; plural ''palatini''; cf. derivative spellings below) is a high-level official attached to imperial or royal courts in Europe since Roman times.

and ectopterygoid bones also touch the vacuities, but in ''Arachana'' and rhinesuchids, the palatine ramus excludes these bones from the vacuity margins. ''Arachana'' also has characteristics of a more

derived Derive may refer to: * Derive (computer algebra system), a commercial system made by Texas Instruments * ''Dérive'' (magazine), an Austrian science magazine on urbanism *Dérive, a psychogeographical concept See also * *Derivation (disambiguatio ...

group of stereospondyls called lydekkerinids. In both ''Arachana'' and lydekkerinids, the

otic notch Otic notches are invaginations in the posterior margin of the skull roof, one behind each orbit. Otic notches are one of the features lost in the evolution of amniotes from their tetrapod ancestors. The notches have been interpreted as part of an ...

in the back of the skull is shallow and is not touched by the

supratemporal bone The supratemporal bone is a paired cranial bone present in many tetrapods and tetrapodomorph fish. It is part of the temporal region (the portion of the skull roof behind the eyes), usually lying medial (inwards) relative to the squamosal and latera ...

as is otherwise the case in temnospondyls. Sensory

sulci Sulci or Sulki (in Greek , Steph. B., Ptol.; , Strabo; , Paus.), was one of the most considerable cities of ancient Sardinia, situated in the southwest corner of the island, on a small island, now called Isola di Sant'Antioco, which is, howev ...

, grooves in the skull that held a

lateral line The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...

system in life, run across the

lacrimal bone The lacrimal bone is a small and fragile bone of the facial skeleton; it is roughly the size of the little fingernail. It is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders. Several bony landmarks of ...

but are not angled sharply as in other temnospondyls. On the palates of both ''Arachana'' and lydekkerinids, rows of palatine teeth are relatively small and the main body of the pterygoid bone is covered in small projections. None of these features are present in rhinesuchids, suggesting that ''Arachana'' was a

transitional form A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. This is especially important where the descendant group is sharply differentiated by gross a ...

between basal stereospondyls like rhinesuchids and more advanced forms like lydekkerinids. The transitional features of ''Arachana'' place it as part of an entire transitional fauna that existed around the Permo-Triassic boundary. While it is uncertain whether ''Arachana'' lived before or after the Permian-Triassic extinction event, its transitional position shows that some temnospondyls were diversifying rather than diminishing across the boundary.

Phylogeny

Below is a

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

from the analysis of Piñeiro ''et al.'' (2012):

References

{{Taxonbar, from=Q21445996 Triassic temnospondyls of South America Triassic Uruguay Fossils of Uruguay Paraná Basin Fossil taxa described in 2011