Aistopoda (Greek for " avingnot-visible feet") is an order of highly specialised snake-like

stegocephalians Stegocephali (often spelled Stegocephalia) is a group containing all four-limbed vertebrates. It is equivalent to a broad definition of Tetrapoda: under this broad definition, the term "tetrapod" applies to any animal descended from the first ve ...

known from 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 ...

and

Early Permian 01 or '01 may refer to: * The year 2001, or any year ending with 01 * The month of January * 1 (number) Music * '01 (Richard Müller album), 01'' (Richard Müller album), 2001 * 01 (Son of Dave album), ''01'' (Son of Dave album), 2000 * 01 (Urban ...

of Europe and North America, ranging from tiny forms only , to nearly in length. They first appear in the fossil record in the Mississippian period and continue through to the

. The

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 ...

is small but very specialised, with large

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 ...

, and large

fenestra A fenestra (fenestration; plural fenestrae or fenestrations) is any small opening or pore, commonly used as a term in the biological sciences. It is the Latin word for "window", and is used in various fields to describe a pore in an anatomical st ...

e. The primitive form ''

Ophiderpeton ''Ophiderpeton'' (from el, ὄφῐς , 'snake' and el, ἑρπετόν 'creeper') is an extinct genus of aistopod tetrapodomorphs from the early Carboniferous to the early Permian. Remains of this genus are widespread and were found in Ohio ...

'' has a pattern of

dermal bone A dermal bone or investing bone or membrane bone is a bony structure derived from intramembranous ossification forming components of the vertebrate skeleton including much of the skull, jaws, gill covers, shoulder girdle and fin spines rays (lepi ...

s in the skull similar in respects to the

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 ...

s. But in the advanced genus ''

Phlegethontia ''Phlegethontia'' is an extinct genus of aïstopod stegocephalians from the Carboniferous and Permian periods of Europe and North America. It was about long, and possessed a lightly built skull with many openings, unlike some earlier relative ...

'' the skull is very light and open, reduced to a series of struts supporting the braincase against the lower jaw, just as in

snakes Snakes are elongated, limbless, carnivorous reptiles of the suborder Serpentes . Like all other squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales. Many species of snakes have skulls with several more joi ...

, and it is possible that the aistopods filled the same

ecological niche In ecology, a niche is the match of a species to a specific environmental condition. Three variants of ecological niche are described by It describes how an organism or population responds to the distribution of resources and competitors (for ...

s in the Paleozoic that snakes do today. Pseudophlegethontia turnbullorum

They had an extremely elongated body, with up to 230

vertebra 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 ...

e. The vertebrae were holospondylous, having only a single ossification per segment. They lacked intercentra, even in the tail, and had are no free

haemal arch A haemal arch also known as a chevron, is a bony arch on the ventral side of a tail vertebra of a vertebrate. The canal formed by the space between the arch and the vertebral body is the haemal canal. A spinous ventral process emerging from the hae ...

es. The

neural arch 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 i ...

was low and fused to the

centrum (Latin for ''center'') may refer to: Places In Greenland * Nuuk Centrum, a district of Nuuk, Greenland * Centrum Lake, Greenland In the Netherlands * Amsterdam-Centrum, the inner-most borough of Amsterdam, Netherlands * Rotterdam Centrum, a borou ...

. All of these features are very similar to those of the

Nectridea Nectridea is the name of an extinct order of lepospondyl tetrapods from the Carboniferous and Permian periods, including animals such as ''Diplocaulus''. In appearance, they would have resembled modern newts or aquatic salamanders, although they ...

, both representing the typical lepospondylous condition. A recent paper described the internal organization of the aistopod head, finding that aïstopods retained many fish-like features of the skull and brain, including a persistent extension of the

notochord In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consis ...

into the head and an open canal between the

pituitary gland In vertebrate anatomy, the pituitary gland, or hypophysis, is an endocrine gland, about the size of a chickpea and weighing, on average, in humans. It is a protrusion off the bottom of the hypothalamus at the base of the brain. The ...

and the mouth. Because these features are lost early in tetrapod evolution, this may be evidence that aistopods diverged from other tetrapods soon after the origin of digits. The ribs were slender, either single or double-headed, with the head shaped like a letter "K". There is no trace of limbs or even limb girdles in any known fossil, and the tail was short and primitive.

Relationships

Evolutionary relationships with other early

tetrapod Tetrapods (; ) are four-limbed vertebrate animals constituting the superclass Tetrapoda (). It includes extant and extinct amphibians, sauropsids ( reptiles, including dinosaurs and therefore birds) and synapsids (pelycosaurs, extinct theraps ...

s remain controversial, as even the earliest aistopod, the

Viséan The Visean, Viséan or Visian is an age in the ICS geologic timescale or a stage in the stratigraphic column. It is the second stage of the Mississippian, the lower subsystem of the Carboniferous. The Visean lasted from to Ma. It follows the ...

species '' Lethiscus stocki'', was already highly specialised. Aïstopods have been variously grouped with other lepospondyls, or placed at or prior to the batrachomorph-

reptiliomorph Reptiliomorpha (meaning reptile-shaped; in PhyloCode known as ''Pan-Amniota'') is a clade containing the amniotes and those tetrapods that share a more recent common ancestor with amniotes than with living amphibians ( lissamphibians). It was de ...

divide. However, a cladistic analysis by Pardo ''et al.'' (2017) recovered Aistopoda at the base of Tetrapoda.Jason D. Pardo, Matt Szostakiwskyj, Per E. Ahlberg & Jason S. Anderson (2017) Hidden morphological diversity among early tetrapods. Nature (advance online publication) doi:10.1038/nature22966> The group was quite diverse during the

Late Carboniferous 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, ...

, with a few forms continuing through to 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 ...

. 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 Anderson ''et al.'' (2003) showing the

phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

relationships of aistopods:

References

* Benton, M. J. (2000), ''Vertebrate Paleontology'', 2nd ed. Blackwell Science Ltd * Carroll, RL (1988), ''Vertebrate Paleontology and Evolution'', WH Freeman & Co. pp. 176–7 * Reisz, Rober
Biology 356 - Major Features of Vertebrate Evolution
(online) * von Zittel, K.A (1932), ''Textbook of Paleontology'', CR Eastman (transl. and ed), 2nd edition, vol.2, p. 221-2, Macmillan & Co.

External links

{{Taxonbar, from=Q1876138 Prehistoric animal orders Aistopods