Ichthyostega
Encyclopedia
Ichthyostega is an early tetrapod
genus
that lived at the end of the Upper Devonian
period (Famennian
age, 374 – 359 million years ago). It was a labyrinthodont
, one of the first fossil record of tetrapods. Ichthyostega possessed lungs and limb
s that helped it navigate through shallow water in swamp
s. Though undoubtedly of amphibian
build and habit, it is not considered a true member of the group in the narrow sense, as the first true amphibians appeared in the Carboniferous
period.
described four Ichthyostega species from the Upper Devonian of East Greenland
and one species belonging to the genus Ichthyostegopsis, I. wimani. These species could be synonymous (in which case only I. stensioei would remain), because their morphological
differences are not very pronounced. The species differ in skull proportions, skull punctuation and skull bone patterns. The comparisons were done on 14 specimens collected in 1931 by the Danish East Greenland Expedition. Additional specimens were collected between 1933 and 1955.
The genus is closely related to Acanthostega gunnari
, also from East Greenland. Ichthyostegas skull seems more fish-like than that of Acanthostega, but its girdle
(shoulder and hip) morphology seems stronger and better adapted to land-life. Ichthyostega also had more supportive rib
s and stronger vertebrae with more developed zygapophyses. The first tetrapods (who probably didn't walk on land) were Elginerpeton
and Obruchevichthys
.
on each hind limb
. The exact number of digits on the forelimb
is not yet known, since fossils with forelimbs have not been found. It had a fin containing fin rays on its tail.
(for instance Eusthenopteron
or Panderichthys
) in their increased adaptations for life on land. Though Crossopterygians possessed lung
s, they used gill
s as their primary means of acquiring oxygen
; Ichthyostega appears to have relied on its lungs as its primary apparatus for breathing. The skin
of early tetrapods, unlike that of Crossopterygians, helped retain bodily fluids and deter desiccation
. Crossopterygians used their body
and tail
for locomotion and their fin
s for balance; Ichthyostega used its limbs for locomotion and its tail for balance.
The size of an adult Ichthyostega (1.5 m) precluded completely terrestrial locomotion. Yet the massive ribcage was made up of overlapping ribs and the animal possessed a stronger skeletal structure, a more rigid spine, and forelimbs apparently powerful enough to pull the body from the water. These anatomical modifications clearly evolved to handle the lack of buoyancy experienced on land. The hindlimbs were smaller than the forelimbs and unlikely to have born full weight in an adult, while the broad, overlapping ribs would have inhibited side-to-side movements. Jennifer A. Clack
suggests that Ichthyostega and its relatives spent time basking in the sun to raise their body temperatures, much as some animals do today: the Marine Iguana
s on the Galapagos Island or the Gharial
. They would have returned to the water to cool themselves, hunt for food and reproduce. A lifestyle that required strong forelimbs to pull at least their anterior part out of the water, and a stronger ribcage and spine to support them while sunbathing on their abdomen like modern crocodile
s. New studies suggests that the juveniles were more aquatic than the adults, and the possibility that Ichthyostega came out of the water only as a fully mature adult.
Water was also still a requirement, because the gel
-like eggs
of the earliest terrestrial tetrapods couldn't survive out of water, so reproduction could not occur without it. Water was also needed for their larvae and external fertilization. Most land-dwelling vertebrates have since developed two methods of internal fertilization; either direct as seen in all amniote
s and a few amphibians, or indirect for many salamander
s by placing a spermatophore
on the ground which then is picked up by the female salamander.
The Ichthyostegalians (Elginerpeton
, Acanthostega
, Ichthyostega, etc.) were succeeded by temnospondyls and anthracosaurs, such as Eryops
, amphibians that truly developed the ability to walk on land. Until 2002, there was a gap of 20 million years between the two groups ( Romer's Gap
). In 2002 a 350 million year old fossil from the lower Mississippian, Pederpes finneyae was described and helped to close the gap: it is the earliest-known tetrapod to show the beginnings of terrestrial locomotion.
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...
genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
that lived at the end of the Upper Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
period (Famennian
Famennian
The Famennian is one of two faunal stages in the Late Devonian epoch. It lasted from 374.5 ± 2.6 million years ago to 359.2 ± 2.5 million years ago. It was preceded by the Frasnian stage and followed by the Tournaisian stage and is named after Famenne, a natural region in southern Belgium.It was...
age, 374 – 359 million years ago). It was a labyrinthodont
Labyrinthodontia
Labyrinthodontia is an older term for any member of the extinct subclass of amphibians, which constituted some of the dominant animals of Late Paleozoic and Early Mesozoic times . The group is ancestral to all extant landliving vertebrates, and as such constitutes an evolutionary grade rather...
, one of the first fossil record of tetrapods. Ichthyostega possessed lungs and limb
Limb (anatomy)
A limb is a jointed, or prehensile , appendage of the human or other animal body....
s that helped it navigate through shallow water in swamp
Swamp
A swamp is a wetland with some flooding of large areas of land by shallow bodies of water. A swamp generally has a large number of hammocks, or dry-land protrusions, covered by aquatic vegetation, or vegetation that tolerates periodical inundation. The two main types of swamp are "true" or swamp...
s. Though undoubtedly of amphibian
Amphibian
Amphibians , are a class of vertebrate animals including animals such as toads, frogs, caecilians, and salamanders. They are characterized as non-amniote ectothermic tetrapods...
build and habit, it is not considered a true member of the group in the narrow sense, as the first true amphibians appeared in the 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"...
period.
History and systematics
In 1932 Gunnar Säve-SöderberghGunnar Säve-Söderbergh
Gunnar Säve-Söderbergh was a Swedish palaeontologist and geologist. Säve-Söderbergh was born at Falun, the son of the neurologist Gotthard Söderbergh and Inga Säve. He passed his G.C.E. at Gothenburg in 1928 and took bachelor's and licentiate's degrees at Uppsala University in 1931 and 1933,...
described four Ichthyostega species from the Upper Devonian of East Greenland
Greenland
Greenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
and one species belonging to the genus Ichthyostegopsis, I. wimani. These species could be synonymous (in which case only I. stensioei would remain), because their morphological
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
differences are not very pronounced. The species differ in skull proportions, skull punctuation and skull bone patterns. The comparisons were done on 14 specimens collected in 1931 by the Danish East Greenland Expedition. Additional specimens were collected between 1933 and 1955.
The genus is closely related to Acanthostega gunnari
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...
, also from East Greenland. Ichthyostegas skull seems more fish-like than that of Acanthostega, but its girdle
Girdle
A girdle is a garment that encircles the lower torso, perhaps extending below the hips, and worn often for support. The word girdle originally meant a belt. In modern English, the term girdle is most commonly used for a form of women's foundation wear that replaced the corset in popularity...
(shoulder and hip) morphology seems stronger and better adapted to land-life. Ichthyostega also had more supportive rib
Rib
In vertebrate anatomy, ribs are the long curved bones which form the rib cage. In most vertebrates, ribs surround the chest, enabling the lungs to expand and thus facilitate breathing by expanding the chest cavity. They serve to protect the lungs, heart, and other internal organs of the thorax...
s and stronger vertebrae with more developed zygapophyses. The first tetrapods (who probably didn't walk on land) were Elginerpeton
Elginerpeton
Elginerpeton is a monotypic genus of early tetrapod, the fossils of which were recovered from Scat Craig, Scotland, in rocks dating to the late Devonian Period...
and Obruchevichthys
Obruchevichthys
Obruchevichthys is an extinct genus of tetrapod from Latvia during the Late Devonian. When the jawbone, the only known fossil of this creature, was uncovered in Latvia, it was mistaken as a lobe-fin fish. However, when it was analyzed, it proved to hold many similarities to Elginerpeton, from...
.
Characteristics
Ichthyostega was about 1.5 m long and had seven digitsPolydactyly in early tetrapods
Polydactyly in early tetrapod aquatic animals 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 digits for a given taxon. The condition appear to have arisen from a limb with a fin...
on each hind limb
Hind limb
A hind limb is a posterior limb on an animal. When referring to quadrupeds, the term hind leg is often instead used....
. The exact number of digits on the forelimb
Forelimb
A forelimb is an anterior limb on an animal's body. When referring to quadrupeds , the term foreleg is often instead used....
is not yet known, since fossils with forelimbs have not been found. It had a fin containing fin rays on its tail.
Adaptations for land life
Early tetrapods like Ichthyostega and Acanthostega differed from animals like CrossopterygiansSarcopterygii
The Sarcopterygii or lobe-finned fishes – sometimes considered synonymous with Crossopterygii constitute a clade of the bony fishes, though a strict classification would include the terrestrial vertebrates...
(for instance Eusthenopteron
Eusthenopteron
Eusthenopteron is a genus of prehistoric lobe-finned fish which has attained an iconic status from its close relationships to tetrapods. Early depictions of this animal show it emerging onto land, however paleontologists now widely agree that it was a strictly aquatic animal...
or Panderichthys
Panderichthys
Panderichthys is a 90–130 cm long fish from the Devonian period 397 million years ago, of Latvia. It is named after the german-baltic palaeontologist Christian Heinrich Pander. It has a large tetrapod-like head...
) in their increased adaptations for life on land. Though Crossopterygians possessed lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...
s, they used gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
s as their primary means of acquiring oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
; Ichthyostega appears to have relied on its lungs as its primary apparatus for breathing. The skin
Skin
-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
of early tetrapods, unlike that of Crossopterygians, helped retain bodily fluids and deter desiccation
Desiccation
Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.-Science:...
. Crossopterygians used their body
Body
With regard to living things, a body is the physical body of an individual. "Body" often is used in connection with appearance, health issues and death...
and tail
Tail
The tail is the section at the rear end of an animal's body; in general, the term refers to a distinct, flexible appendage to the torso. It is the part of the body that corresponds roughly to the sacrum and coccyx in mammals, reptiles, and birds...
for locomotion and their 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...
s for balance; Ichthyostega used its limbs for locomotion and its tail for balance.
Jennifer A. Clack
Jennifer Alice Clack, FRS, is an English paleontologist, an expert in the field of evolutionary biology. She studies the "fish to tetrapod" transition— the origin, evolutionary development and radiation of early tetrapods and their relatives among the lobe-finned fishes...
suggests that Ichthyostega and its relatives spent time basking in the sun to raise their body temperatures, much as some animals do today: the Marine Iguana
Marine iguana
The Marine Iguana is an iguana found only on the Galápagos Islands that has the ability, unique among modern lizards, to live and forage in the sea, making it a marine reptile. The Iguana can dive over 30 ft into the water. It has spread to all the islands in the archipelago, and is...
s on the Galapagos Island or the Gharial
Gharial
The gharial , , also called Indian gavial or gavial, is the only surviving member of the once well-represented family Gavialidae, a long-established group of crocodilians with long, slender snouts...
. They would have returned to the water to cool themselves, hunt for food and reproduce. A lifestyle that required strong forelimbs to pull at least their anterior part out of the water, and a stronger ribcage and spine to support them while sunbathing on their abdomen like modern crocodile
Crocodile
A crocodile is any species belonging to the family Crocodylidae . The term can also be used more loosely to include all extant members of the order Crocodilia: i.e...
s. New studies suggests that the juveniles were more aquatic than the adults, and the possibility that Ichthyostega came out of the water only as a fully mature adult.
Water was also still a requirement, because the gel
Gel
A gel is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute cross-linked system, which exhibits no flow when in the steady-state...
-like eggs
Egg (biology)
An egg is an organic vessel in which an embryo first begins to develop. In most birds, reptiles, insects, molluscs, fish, and monotremes, an egg is the zygote, resulting from fertilization of the ovum, which is expelled from the body and permitted to develop outside the body until the developing...
of the earliest terrestrial tetrapods couldn't survive out of water, so reproduction could not occur without it. Water was also needed for their larvae and external fertilization. Most land-dwelling vertebrates have since developed two methods of internal fertilization; either direct as seen in all 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...
s and a few amphibians, or indirect for many salamander
Salamander
Salamander is a common name of approximately 500 species of amphibians. They are typically characterized by a superficially lizard-like appearance, with their slender bodies, short noses, and long tails. All known fossils and extinct species fall under the order Caudata, while sometimes the extant...
s by placing a spermatophore
Spermatophore
A spermatophore or sperm ampulla is a capsule or mass created by males of various animal species, containing spermatozoa and transferred in entirety to the female's ovipore during copulation...
on the ground which then is picked up by the female salamander.
The Ichthyostegalians (Elginerpeton
Elginerpeton
Elginerpeton is a monotypic genus of early tetrapod, the fossils of which were recovered from Scat Craig, Scotland, in rocks dating to the late Devonian Period...
, 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...
, Ichthyostega, etc.) were succeeded by temnospondyls and anthracosaurs, such as Eryops
Eryops
Eryops meaning "drawn-out face" because most of its skull was in front of its eyes is a genus of extinct, semi-aquatic amphibian found primarily in the Lower Permian-aged Admiral Formation of Archer County, Texas, but fossils are also found in New Mexico and parts of the eastern United...
, amphibians that truly developed the ability to walk on land. Until 2002, there was a gap of 20 million years between the two groups ( 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 2002 a 350 million year old fossil from the lower Mississippian, Pederpes finneyae was described and helped to close the gap: it is the earliest-known tetrapod to show the beginnings of terrestrial locomotion.
See also
- Prehistoric amphibian
- Evolutionary history of lifeEvolutionary history of lifeThe evolutionary history of life on Earth traces the processes by which living and fossil organisms have evolved since life on Earth first originated until the present day. Earth formed about 4.5 Ga and life appeared on its surface within one billion years...
- List of transitional fossils
- HynerpetonHynerpetonHynerpeton was a basal carnivorous tetrapod that lived in the lakes and estuaries of the Late Devonian period around 360 million years ago. Like many primitive tetrapods, it is sometimes referred to as an "amphibian", though it is not a true member of the modern Lissamphibia...
External links
- Excellent site on early tetrapods
- Course site
- Course site
- First Four-Legged Animals Inched Along
- Getting a Leg Up on Land Scientific American Nov. 21, 2005, article by Jennifer A. ClackJennifer A. ClackJennifer Alice Clack, FRS, is an English paleontologist, an expert in the field of evolutionary biology. She studies the "fish to tetrapod" transition— the origin, evolutionary development and radiation of early tetrapods and their relatives among the lobe-finned fishes...
.