| Acanthostega | |
|---|---|
 | |
| Skeletal reconstruction | |
Scientific classification | |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Sarcopterygii |
| Clade: | Tetrapodomorpha |
| Clade: | Stegocephali |
| Family: | †Acanthostegidae Jarvik, 1952 |
| Genus: | †Acanthostega Jarvik, 1952 |
| Species: | †A. gunnari |
| Binomial name | |
| †Acanthostega gunnari Jarvik, 1952 | |
Acanthostega, fromAncient Greek ἄκανθα (ákantha), meaning "spine", and στέγη (stégē), meaning "roof", is an extinctgenus ofstem-tetrapod, among the firstvertebrate animals to have recognizablelimbs. It appeared in the lateDevonian period (Famennian age) about 365 million years ago, and was anatomically intermediate betweenlobe-finned fishes and those that were fully capable of coming onto land.[1][2]
Thefossilized remains are generally well preserved, with the famousfossil by which the significance of this species was discovered being found byJennifer A. Clack in EastGreenland in 1987, though fragments of the skull had been discovered in 1933 byGunnar Säve-Söderbergh andErik Jarvik.
The 60 cm (24 in)Acanthostegahad eight digits on each hand (the number of digits on the feet is unclear) linked by webbing, it lacked wrists, and was generally poorly adapted for walking on land. It also had a remarkably fish-like shoulder and forelimb.[3] The front limbs ofAcanthostega could not bend forward at the elbow, and therefore could not be brought into aweight bearing position, appearing to be more suitable for paddling or for holding on to aquatic plants.Acanthostega is the earliest stem-tetrapod to show the shift in locomotory dominance from thepectoral girdle to the pelvic girdle.[4]
There are many morphological changes that allowed the pelvic girdle ofAcanthostega to become a weight-bearing structure. In more ancestral states the two sides of the girdle were not attached. InAcanthostega there is contact between the two sides and fusion of the girdle with the sacral rib of the vertebral column. These fusions would have made the pelvic region more powerful and equipped to counter the force of gravity when not supported by the buoyancy of an aquatic environment.[4] It had internal gills that were covered like those of fish. It also had lungs, but its ribs were too short to support its chest cavity out of water.[2]
Acanthostega is seen as part of widespread evolutionary radiation in the late Devonian period, starting with purely aquatic finned tetrapodomorphs, with their successors showing increased air-breathing capability and related adaptions to the jaws and gills, as well as more muscular neck allowing freer movement of the head than fish have, and use of the fins to raise the body of the fish.[2] These features are displayed by the earlierTiktaalik, which likeIchthyostega showed signs of greater abilities to move around on land, but is thought to have been primarily aquatic.[2]
InLate Devonianvertebrate speciation, descendants ofpelagiclobe-finned fish –likeEusthenopteron– exhibited a sequence of adaptations:Panderichthys, suited to muddy shallows;Tiktaalik with limb-like fins that could take it onto land;stem-tetrapods in weed-filled swamps, such asAcanthostega, which had eight-digited feet; andIchthyostega, with full limbs. Their descendants also included pelagic lobe-finned fish such ascoelacanth species.
It has been inferred thatAcanthostega probably lived in shallow, weed-choked swamps, its legs apparently being adapted for these specific ecosystems. Apart from the presence of limbs, it was not adapted in any way for walking on land.Jennifer A. Clack interprets this as showing thatAcanthostega was primarily an aquatic animal descended from fish that never left the sea, and that the specializations of the tetrapod lineage wereexaptations: features which would later be useful for terrestrial life, even if they originated for a different purpose. At that period,deciduous plants were flourishing and annually shedding leaves into the water, attracting small prey into warm oxygen-poor shallows that were difficult for largerfish to swim in; Clack remarks on how the lower jaw ofAcanthostega shows a change from those of fish that have two rows of teeth, with a large number of small teeth in the outer row, and two large fangs and some smaller teeth in the inner row. This difference likely corresponds to a shift in stem-tetrapods from feeding exclusively in the water to feeding with the head above water or on land.[2]
Research based on analysis of thesuturemorphology in the skull ofAcanthostega indicates that the species was able to bite prey at or near the water's edge. Markey and Marshall compared the skull with the skulls of fish, which use suction feeding as the primary method of prey capture, and creatures known to have used the direct biting on prey typical ofterrestrial animals. Their results indicate thatAcanthostega was adapted for what they call terrestrial-style feeding, strongly supporting the hypothesis that the terrestrial mode of feeding first emerged in aquatic animals. If correct, this shows an animal specialized for hunting and living in shallow waters in the line between land and water.[5]
While normally considered morebasal thanIchthyostega, it is possible thatAcanthostega was actually more derived. SinceAcanthostega resembles juvenileIchthyostega and shows a lot less differences from juveniles to adults than the latter, it has been suggested thatAcanthostega might be descended from aneotenic lineage. Although it appears to have spent its whole life in water, its humerus also exhibits traits that resemble those of later, fully terrestrial stem-tetrapods (the humerus inIchthyostega being somewhat derived from, and homologous with the pectoral and pelvic fin bones of earlier fishes). This could indicate that vertebrates evolved terrestrial traits earlier than previously assumed, and numerous times independently from another.[6] Muscle scars on the forelimbs ofAcanthostega were similar to those of crown-tetrapods, suggesting that it evolved from an ancestor that had more terrestrial adaptations than itself.[7]
Ahistological study ofAcanthostega humeri, assisted bysynchrotron scans, indicates that the animal matured slowly. Some individuals reached sexual maturity (based on a fully ossified humerus) at more than six years of age, and adult fossils are much rarer than juveniles. Late ossification of the humerus supports a fully aquatic lifestyle forAcanthostega. There is barely any correlation between humerus size and maturity, suggesting that there was significant size variation among individuals of the same age. This may be due to competitive pressures, differing adaptive strategies, or evensexual dimorphism. However, the small sample size prevents recognition of abimodal distribution which could confirm the latter hypothesis.[8]
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