''Triassurus'' is an extinct genus of

amphibian Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...

, and the oldest member of

Caudata The Caudata are a group of amphibians containing the extant salamanders (the order Urodela) and all extinct species of amphibians more closely related to salamanders than to frogs. They are typically characterized by a superficially lizard-lik ...

(salamanders and close relalatives). It is known from the

Middle Middle or The Middle may refer to: * Centre (geometry), the point equally distant from the outer limits. Places * Middle (sheading), a subdivision of the Isle of Man * Middle Bay (disambiguation) * Middle Brook (disambiguation) * Middle Creek ...

Upper Triassic The Late Triassic is the third and final epoch of the Triassic Period in the geologic time scale, spanning the time between Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch and followed by the Early Jurassic Epoch. T ...

(

Ladinian The Ladinian is a stage and age in the Middle Triassic series or epoch. It spans the time between Ma and ~237 Ma (million years ago). The Ladinian was preceded by the Anisian and succeeded by the Carnian (part of the Upper or Late Triassic ...

Carnian The Carnian (less commonly, Karnian) is the lowermost stage (stratigraphy), stage of the Upper Triassic series (stratigraphy), Series (or earliest age (geology), age of the Late Triassic Epoch (reference date), Epoch). It lasted from 237 to 227.3 ...

) aged

Madygen Formation The Madygen Formation (Russian language, Russian: Madygen Svita) is a Middle Triassic, Middle–Late Triassic (Ladinian–Carnian) geologic formation and lagerstätte in the Batken Region, Batken and Osh Regions of western Kyrgyzstan, with minor ...

in Kyrgyzstan. The type species is ''T. sixtelae''.

Description

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 a partial and poorly preserved skeleton (PIN-2584/10), including the skull. Holotype specimen had tiny skull just long, but it was probably a larva: the neural arches of the vertebrae were still paired and no vertebral centers show any degree of ossification. The skull closely resembles that of current salamanders, especially as regards the long space in the joint of the maxillary bones and the lack of bone connection between the pterygoid and the maxilla. There were about 20 presacral vertebrae, while the legs were small. In 2020, a more complete adult specimen (FG 596/V/20) with long skull was described.

Classification

''Triassurus'' was first described in 1978, based on a fossil found in the

in Kyrgyzstan. The bizarre planatory reptiles ''

Sharovipteryx ''Sharovipteryx'' ("Sharov's wing", known until 1981 as ''Podopteryx'', "foot wing") is a genus of early gliding reptiles containing the single species ''Sharovipteryx mirabilis''. It is known from a single fossil and is the only glider with a me ...

'' and ''

Longisquama ''Longisquama'' is a genus of extinct reptile. There is only one species, ''Longisquama insignis'', known from a poorly preserved skeleton and several incomplete fossil impressions from the Middle Triassic, Middle to Late Triassic Madygen Formati ...

'' were also found in the same formation. ''Triassurus'' was initially described as the oldest salamander, based mainly on the characteristics of the skull. Some vertebral characteristics, in reality, would lead not only to primitive salamanders such as ''

Hynobius ''Hynobius'' is a genus of salamander (Asian salamanders) in the family Hynobiidae, occurring in Japan, Korea, China, Taiwan and Far East Russia. It contains these species: Species Species included (as of March 2021): *''Hynobius abei'' Sato, ...

'' but also to the larval forms of frogs and to the

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

s. Furthermore, the cheek area is similar to branchiosaurs.R. Estes. 1981. ''Handbuch der Paläoherpetologie/Encyclopedia of Paleoherpetology''. Part 2. Gymnophiona, Caudata. Gustav Fischer Verlag, Stuttgart xv-115 In any case, ''Triassurus'' remains the most salamander-like Triassic amphibian fossil. In 2020 ''Triassurus'' was definitely determined to a caudatan, based on four

apomorphies In phylogenetics, an apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy). A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to hav ...

shared with salamanders: "

parasphenoid The parasphenoid is a bone which can be found in the cranium of many vertebrates. It is an unpaired dermal bone which lies at the midline of the roof of the mouth. In many reptiles (including birds), it fuses to the endochondral (cartilage-derived ...

shape and dorsal surface, with a V-shaped anterior depression, an unpaired posteromedial crest, and a radial arrangement of furrows; parietal not plate-like and rectangular but L-shaped;

squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestra ...

forming a straight transverse strut with slightly expanded lateral end and well-expanded medial end, without squamosal embayment; and straight

scapula The scapula (: scapulae or scapulas), also known as the shoulder blade, is the bone that connects the humerus (upper arm bone) with the clavicle (collar bone). Like their connected bones, the scapulae are paired, with each scapula on either side ...

with expanded ends". In 2022 another phylogenetic analysis, that included members of

Dissorophoidea Dissorophoidea is a clade of medium-sized, temnospondyl amphibians that appeared during the Moscovian in Euramerica, and continued through to the Late Permian and the Early Triassic of Gondwana. They are distinguished by various details of the ...

, caecilians, frogs, as well as both living and fossil salamanders, recovered ''Triassurus'' as a

stem-group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

caudata The Caudata are a group of amphibians containing the extant salamanders (the order Urodela) and all extinct species of amphibians more closely related to salamanders than to frogs. They are typically characterized by a superficially lizard-lik ...

n, outside the

crown group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor ...

of modern salamanders.

References

Fossils of Kyrgyzstan Fossil taxa described in 1978 Prehistoric amphibian genera {{paleo-salamander-stub