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 ...
which lived during 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, ...
period
Period may refer to:
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* Era, a length or span of time
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, around 251 to 260
million years ago
The abbreviation Myr, "million years", is a unit of a quantity of (i.e. ) years, or 31.556926 teraseconds.
Usage
Myr (million years) is in common use in fields such as Earth science and cosmology. Myr is also used with Mya (million years ago). ...
. Remains of ''Saharastega'', discovered by paleontologist Christian A. Sidor at the
Moradi Formation
The Moradi Formation is a geological formation in Niger. It is of Late Permian age. It is informally divided into three subunits. The lower portion of the formation consists of red mudstone, with muddy calcareous sandstone and quartz-granlule co ...
in
Niger
)
, official_languages =
, languages_type = National languages The description is based on a skull lacking the lower jaws.
Description
Skull roof
The skull is somewhat wide and gradually narrows towards the snout, with the portion of the skull in front of the
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 ...
(eye sockets) longer than that behind them. It was 36.8 centimeters (14.5 inches) in length and 30 centimeters (11.8 inches) in width at its widest point. The cranial bones are somewhat eroded, but preserved portions are finely textured with pits and ridges as in most adult temnospondyls, but lacking the unornamented areas adjacent to the midline which characterize
edopoids
Edopoidea is a clade of primitive temnospondyl amphibians including the genus ''Edops'' and the family Cochleosauridae. Edopoids are known from the Late Carboniferous and Early Permian of North America and Europe, and the Late Permian of Afric ...
. The skull lacks
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 ...
grooves, which may indicate terrestriality, but the overall size and shape is akin to aquatic temnospondyls. The numerous teeth were broken, but their bases were all the same size and elliptical in cross-section.
The nares (nostril holes) are narrow and angled almost perpendicular to the long axis of the skull, a unique feature which is dissimilar to the nares of other temnospondyls, which were either circular or angled parallel to the long axis of the skull. The orbits are small, rounded, and widely separated. The
pineal foramen
A parietal eye, also known as a third eye or pineal eye, is a part of the epithalamus present in some vertebrates. The eye is located at the top of the head, is photoreceptive and is associated with the pineal gland, regulating circadian rhyth ...
is absent. The suspensorium (jaw joint) is shifted forwards, which has the effect of twisting the
quadrate bone
The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids (reptiles, birds), and early synapsids.
In most tetrapods, the quadrate bone connects to the quadratojugal and squamosal bones in the skull, and forms upper ...
s forwards and closing up the squamosal embayments (large emarginations along the rear of the skull also known as "
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 ...
es").
The arrangement of the cranial bones is generally similar to that of other early temnospondyls. The nasal bones (on the upper surface of the snout) are long, while the
frontal
Front may refer to:
Arts, entertainment, and media Films
* ''The Front'' (1943 film), a 1943 Soviet drama film
* ''The Front'', 1976 film
Music
* The Front (band), an American rock band signed to Columbia Records and active in the 1980s and e ...
and parietal bones (at the rear of the skull) are wide, which are traits related to the skull proportions. 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 ...
s (on the side of the snout) are isolated, contacting neither the nares nor the orbits. These traits are akin to the cochleosaurid '' Chenoprosopus'', but ''Saharastega'' lacks the characteristic
postparietal
Postparietals are cranial bones present in fish and many tetrapods. Although initially a pair of bones, many lineages possess postparietals which were fused into a single bone. The postparietals were dermal bones situated along the midline of the ...
shape of cochleosaurids. As with other early temnospondyls (but not later members of the group), ''Saharastega'' retains an
intertemporal bone
The Intertemporal bone is a paired Skull, cranial bone present in Osteichthyes, bony fish and certain extinct amphibian-Evolutionary grade, grade tetrapods. It lies in the rear part of the skull, behind the eyes.
Many lineages of four-limbed verte ...
behind the eyes. Likewise, 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 ...
forms a portion of the rear edge of the skull, rather than being excluded by the
tabular
Table may refer to:
* Table (furniture), a piece of furniture with a flat surface and one or more legs
* Table (landform), a flat area of land
* Table (information), a data arrangement with rows and columns
* Table (database), how the table data ...
and
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 ancestral co ...
bones which were present in that area. The tabulars themselves were uniquely shaped. They were thick, bluntly-tipped bones which twisted outwards and downwards along the rear edge of the skull. This is in contrast to other temnospondyls, which generally had smaller, triangular tabulars directed straight back.
The braincase was complex and strongly built, though it was rather shallow owing to the short height of the skull. Unusually among temnospondyls, the skull possessed a large plate of bone above the
foramen magnum
The foramen magnum ( la, great hole) is a large, oval-shaped opening in the occipital bone of the skull. It is one of the several oval or circular openings (foramina) in the base of the skull. The spinal cord, an extension of the medulla oblon ...
, which underlaid a system of ridges forming an inverted T shape. This plate of bone may have been a single large
supraoccipital
The occipital bone () is a cranial dermal bone and the main bone of the occiput (back and lower part of the skull). It is trapezoidal in shape and curved on itself like a shallow dish. The occipital bone overlies the occipital lobes of the cereb ...
, which was rare among Paleozoic amphibians, but was more likely formed by a contribution of the supraoccipital and postparietal bones as in baphetoids. This portion of the skull was somewhat similar to that of the
dvinosauria
Dvinosaurs are one of several new clades of Temnospondyl amphibians named in the phylogenetic review of the group by Yates and Warren 2000. They represent a group of primitive semi-aquatic to completely aquatic amphibians, and are known from t ...
n ''
Isodectes
''Isodectes'' is an extinct genus of dvinosaurian temnospondyl within the family Eobrachyopidae. The genus ''Saurerpeton'', named in 1909, is considered to be a junior synonym
The Botanical and Zoological Codes of nomenclature treat the concep ...
''. The exoccipital bones on either side of the foramen magnum stretch downwards to underscore the braincase, a rare feature also seen in some "advanced" temnospondyls.
Palate
The palate (roof of the mouth) is similar to that of ''Chenoprosopus'' and other early temnospondyls. The bones of the palate are covered with small, dome-like structures known as denticles. The interpterygoid vacuities (holes between the pterygoid bones which were characteristic of temnospondyls) were relatively small and semicircular, located more than halfway towards the rear of the skull. Their small size and rearward location means that their outer edge was completely formed by the pterygoid bones, without any contribution from other palatal bones at the edge of the skull. There was also a small, circular pair of holes ("anterior palatal vacuities") at the tip of the snout and elongated, rectangular
choana
The choanae (singular choana), posterior nasal apertures or internal nostrils are two openings found at the back of the nasal passage between the nasal cavity and the throat in tetrapods, including humans and other mammals (as well as crocodilia ...
e along the edge of the snout. The premaxilla bones at the tip of the snout had a complex
tongue-and-groove
Tongue and groove is a method of fitting similar objects together, edge to edge, used mainly with wood, in flooring, parquetry, panelling, and similar constructions. Tongue and groove joints allow two flat pieces to be joined strongly together t ...
connection to the maxilla bones directly behind them. This configuration, which is only visible from the palate, is seemingly unique to ''Saharastega'' among temnospondyls. The pterygoid bones had a narrow, bulbous contact with the parasphenoid bone (which forms the base of the braincase). While this contact is by no means flexible, it does contrast with the larger and more robust connection present in later temnospondyls.
On the other hand, the palate also has several features in line with more "advanced" temnospondyls. A small depression known as a "fossa subrostralis media" was present where the premaxillae contact each other along the midpoint of the snout tip. This depression is also known is several stereospondylomorphs. Each of the large, plate-like vomer bones lie between the choanae and possess a single diagonally-oriented row of teeth (known as a "transvomerine tooth row") as well as a large fang. Vomer fangs (and palatal fangs in general) are abundant among early tetrapods, but a transvomerine tooth row is characteristic of "advanced" temnospondyls rather than early taxa such as edopoids.
Classification
The first papers to discuss or describe ''Saharastega'' considered it to be part of the group Edopoidea, an early branch of temnospondyl amphibians. Most edopoids lived in the
Carboniferous
The Carboniferous ( ) is a geologic period and system of the Paleozoic that spans 60 million years from the end of the Devonian Period million years ago ( Mya), to the beginning of the Permian Period, million years ago. The name ''Carbonifero ...
, but ''Saharastega'' (and its equally unusual contemporary ''Nigerpeton'') survived until the late Permian as a relict of the early temnospondyl radiation. ''Saharastega'' shares some seemingly unique features with edopoids (particularly the shape of the lacrimal bone), and particularly resembles the cochelosaurids, but also is dissimilar from edopoids in many ways. In the original papers, Saharastega was generally placed at the very base of Edopoidea. Regardless, it was clear that ''Saharastega'' was very basal ("primitive") compared to most temnospondyls, due to the retention of intertemporals, absence of lateral lines, small interpterygoid vacuities, and a narrow contact between the palate and braincase.
Since these studies, the position of ''Saharastega'' has been controversial. Pawley (2006) placed it deeper within the edopoids, as the sister taxon to Cochleosauridae.Chapter 6: "Walking with early tetrapods: evolution of the postcranial skeleton and the phylogenetic affinities of the Temnospondyli (Vertebrata: Tetrapoda)." ''In'': Kat Pawley (2006). " The postcranial skeleton of temnospondyls (Tetrapoda: temnospondyli)." PhD Thesis. La Trobe University, Melbourne. An informal suggestion by Australian paleontologist
Adam Yates
Adam Richard Yates (born 7 August 1992) is a British road and track racing cyclist who currently rides for . He placed fourth overall at the 2016 Tour de France and became the first British rider to win the young rider classification, one year ...
even proposed that it was not a temnospondyl at all, but rather a
seymouriamorph
Seymouriamorpha were a small but widespread group of limbed vertebrates (tetrapods). They have long been considered reptiliomorphs, and most paleontologists may still accept this point of view, but some analyses suggest that seymouriamorphs are s ...
based on general similarities to the skull of ''
Seymouria
''Seymouria'' is an extinct genus of seymouriamorph from the Early Permian of North America and Europe. Although they were amphibians (in a biological sense), ''Seymouria'' were well-adapted to life on land, with many reptilian features—so ...
''. McHugh (2012) tentatively placed it crownward within Temnospondyli (i.e. deeper within the temnospondyl family tree) by placing it as an eryopoid related to
Zatracheidae
Zatracheidae (sometimes mistakenly spelled Zatrachydidae or Zatrachysdidae) is a family of Late Carboniferous and Early Permian temnospondyli, temnospondyl amphibians known from North America and Europe. Zatracheidids are distinguished by lateral ...
. A comprehensive 2013 study of temnospondyl relations did not include ''Saharastega'' due to it being an unstable taxon.
Marjanovic & Laurin (2019) utilized ''Saharastega'' as part of their many reanalyses of Ruta & Coates (2007)'s tetrapod study. The original 2007 study did not include ''Saharastega'', so it was added along with ''Nigerpeton'' and many other taxa in some of the reanalyses. Although Marjanovic & Laurin (2019) did not place ''Saharastega'' close to cochleosaurids or other edopoids (at least in their parsimony analyses), the study consistently placed it as the sister taxon to ''Nigerpeton''. The clade containing these two unusual temnospondyls had an inconsistent placement, with a connection to ''Eryops'', stereospondylomorphs, or the very base of Temnospondyli each considered equally likely. The main purpose of Marjanovic & Laurin (2019)'s study was to analyze different origins for modern
lissamphibia
The Lissamphibia is a group of tetrapods that includes all modern amphibians. Lissamphibians consist of three living groups: the Salientia (frogs, toads, and their extinct relatives), the Caudata (salamanders, newts, and their extinct relatives), ...
ns. When the study restrained its results so that some or all modern lissamphibians were members of Temnospondyli, the results shifted the internal structure of the temnospondyl family tree. In these restrained reanalyses, ''Saharastega'' and ''Nigerpeton'' could additionally be within Stereospondylomorpha or close to Dvinosauria, meaning that there were five equally likely positions under certain parsimony methodologies, which relied on the principal that the simplest family tree is the most likely.
On the other hand, the single bootstrap analysis of Marjanovic & Laurin (2019) which included ''Saharastega'' and ''Nigerpeton'' concluded that they were cochleosaurids. The connection between the ''Saharastega'' and ''Nigerpeton'' was relatively robust, found by 50% of bootstrap trees. Their connection to cochleosaurids was much more tentative, found by only 4% of bootstrap trees, although this was still the most consistent position under the bootstrap methodology. The
bayesian
Thomas Bayes (/beɪz/; c. 1701 – 1761) was an English statistician, philosopher, and Presbyterian minister.
Bayesian () refers either to a range of concepts and approaches that relate to statistical methods based on Bayes' theorem, or a followe ...
analysis had a similar (but less unsteady) result, placing ''Saharastega'' and ''Nigerpeton'' (connected by 99% of bayesian trees) next to the cochleosaurid ''Chenoprosopus'' (connected by 33% of bayesian trees).