Polydactyly in early tetrapods
Encyclopedia
Polydactyly in early tetrapod
aquatic animal
s is not to be confused with polydactyly
in the medical sense, i.e., it was not an anomaly
in the sense it was not a condition
of having more than the typical number of digit
s for a given taxon
. The condition appear to have arisen from a limb with a fin
rather than digits, as found in lobe-finned fishes. From this condition a new pattern of limb formation evolved, where the development axis of the limb rotated to sprout secondary axes along the lower margin, giving rise to a variable number of very stout skeletal supports for a paddle-like foot. The condition is thought to have arisen from the loss of the fin ray-forming proteins actinodin 1 and actinodin 2.
Early groups like Acanthostega
had eight digits, while the more derived Ichthyostega
had seven digits, the yet-more derived Tulerpeton
had six toes.. Crassigyrinus
from the fossil-poor Romer's gap
in early Carboniferous
is usually thought to have had five digits to each foot. The amphibian Anthracosaurs
had a five-toe pattern still found in their Amniote
descendants, while further reduction had taken place on other Labyrinthodont lines, leaving the forefoot with four toes and the hind foot with five, a pattern still found in modern amphibians
. The increasing knowledge of Labyrinthodonts from Romer's gap has led to the challenging of the hypothesis that pentadactyly, as displayed by most modern tetrapods, is plesiomorphic. The number of digits may have reduced in amphibians and reptiles independently. However, even the early Ichthyostegalians like Acantostega and Ichthyostega had the forward bony toes combined in a single stout digit, making them effectively five-toed.
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
aquatic animal
Aquatic animal
An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...
s is not to be confused with polydactyly
Polydactyly
Polydactyly or polydactylism , also known as hyperdactyly, is a congenital physical anomaly in humans, dogs, and cats having supernumerary fingers or toes....
in the medical sense, i.e., it was not an anomaly
Anomaly
Anomaly may refer to:-Astronomy and celestial mechanics :* In astronomy, an anomaly is a quantity measured with respect to an apsis, usually the periapsis...
in the sense it was not a condition
Congenital disorder
A congenital disorder, or congenital disease, is a condition existing at birth and often before birth, or that develops during the first month of life , regardless of causation...
of having more than the typical number of digit
Digit (anatomy)
A digit is one of several most distal parts of a limb, such as fingers or toes, present in many vertebrates.- Names:Some languages have different names for hand and foot digits ....
s for a given taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
. The condition appear to have arisen from a limb with a fin
Fin
A fin is a surface used for stability and/or to produce lift and thrust or to steer while traveling in water, air, or other fluid media, . The first use of the word was for the limbs of fish, but has been extended to include other animal limbs and man-made devices...
rather than digits, as found in lobe-finned fishes. From this condition a new pattern of limb formation evolved, where the development axis of the limb rotated to sprout secondary axes along the lower margin, giving rise to a variable number of very stout skeletal supports for a paddle-like foot. The condition is thought to have arisen from the loss of the fin ray-forming proteins actinodin 1 and actinodin 2.
Early groups like Acanthostega
Acanthostega
Acanthostega is an extinct labyrinthodont genus, among the first vertebrate animals to have recognizable limbs. It appeared in the Upper Devonian about 365 million years ago, and was anatomically intermediate between lobe-finned fishes and the first tetrapods fully capable of coming onto...
had eight digits, while the more derived Ichthyostega
Ichthyostega
Ichthyostega is an early tetrapod genus that lived at the end of the Upper Devonian period . It was a labyrinthodont, one of the first fossil record of tetrapods. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps...
had seven digits, the yet-more derived Tulerpeton
Tulerpeton
Tulerpeton is a fossil of an extinct genus of Devonian labyrinthodont that was found in the Tula Region of Russia at a site named Andreyevka...
had six toes.. Crassigyrinus
Crassigyrinus
Crassigyrinus is an extinct genus of carnivorous stem tetrapod from the Early Carboniferous of Scotland and possibly Greer, West Virginia. The type specimen was originally described as Macromerium scoticum and lacked a complete skull...
from the fossil-poor Romer's gap
Romer's gap
Romer's Gap is an example of a gap in the fossil record used in the study of evolution. Such gaps represent a period from which excavators have found no or very few fossils. Romer's gap is named after paleontologist Dr...
in early Carboniferous
Carboniferous
The Carboniferous is a geologic period and system that extends from the end of the Devonian Period, about 359.2 ± 2.5 Mya , to the beginning of the Permian Period, about 299.0 ± 0.8 Mya . The name is derived from the Latin word for coal, carbo. Carboniferous means "coal-bearing"...
is usually thought to have had five digits to each foot. The amphibian Anthracosaurs
Anthracosauria
Anthracosauria is an order of extinct reptile-like amphibians that flourished during the Carboniferous and early Permian periods, although precisely which species are included depends on one's definition of the taxon.-Various definitions:...
had a five-toe pattern still found in their Amniote
Amniote
The amniotes are a group of tetrapods that have a terrestrially adapted egg. They include synapsids and sauropsids , as well as their fossil ancestors. Amniote embryos, whether laid as eggs or carried by the female, are protected and aided by several extensive membranes...
descendants, while further reduction had taken place on other Labyrinthodont lines, leaving the forefoot with four toes and the hind foot with five, a pattern still found in modern amphibians
Lissamphibia
The subclass Lissamphibia includes all recent amphibians and means smooth amphibia.Extant amphibians fall into one of three orders — the Anura , the Caudata or Urodela , and the Gymnophiona or Apoda .Although the ancestry of each group is still unclear, all share certain common characteristics,...
. The increasing knowledge of Labyrinthodonts from Romer's gap has led to the challenging of the hypothesis that pentadactyly, as displayed by most modern tetrapods, is plesiomorphic. The number of digits may have reduced in amphibians and reptiles independently. However, even the early Ichthyostegalians like Acantostega and Ichthyostega had the forward bony toes combined in a single stout digit, making them effectively five-toed.