Acanthostega

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Acanthostega
Temporal range:
Ma
↓
Skeletal reconstruction
Scientific classification Edit this 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 (meaning "spiny roof") is an extinct genus of

lobe-finned fishes and those that were fully capable of coming onto land.[1][2]

Discovery

Zoological Museum of Copenhagen

The

Jennifer A. Clack in East Greenland in 1987, though fragments of the skull had been discovered in 1933 by Gunnar Säve-Söderbergh and Erik Jarvik
.

Description

Life restoration

The 60 cm (24 in) Acanthostega

weight 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 the pectoral girdle to the pelvic girdle.[4]

There are many morphological changes that allowed the pelvic girdle of Acanthostega to become a weight-bearing structure. In more ancestral states the two sides of the girdle were not attached. In Acanthostega 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]

Classification

An illustration of the Devonian speciation of lobe-finned fish

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 earlier Tiktaalik, which like Ichthyostega showed signs of greater abilities to move around on land, but is thought to have been primarily aquatic.[2]

In

stem-tetrapods in weed-filled swamps, such as Acanthostega, which had eight-digited feet; and Ichthyostega, with full limbs. Their descendants also included pelagic lobe-finned fish such as coelacanth
species.

Paleobiology

Hunting strategy

Musee De L'Histoire Naturelle Brussels

It has been inferred that Acanthostega 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 that Acanthostega was primarily an aquatic animal descended from fish that never left the sea, and that the specializations of the tetrapod lineage were exaptations: 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 larger fish to swim in; Clack remarks on how the lower jaw of Acanthostega 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]

Acanthostega AMNH model - head detail

Research based on analysis of the

terrestrial animals. Their results indicate that Acanthostega 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]

Lifestyle

Model reconstruction at the State Museum of Natural History in Stuttgart, Germany

While normally considered more basal than Ichthyostega, it is possible that Acanthostega was actually more derived. Since Acanthostega resembles juvenile Ichthyostega and shows a lot less differences from juveniles to adults than the latter, it has been suggested that Acanthostega might be descended from a neotenic 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 in Ichthyostega 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 of Acanthostega were similar to those of crown-tetrapods, suggesting that it evolved from an ancestor that had more terrestrial adaptations than itself.[7]

Development

A

bimodal distribution which could confirm the latter hypothesis.[8]

See also

Sources

  1. ^ Benton, Michael J. (2019). "Acanthostega". Vertebrate Palaeontology (Kindle ed.). Wiley. p. 90.
  2. ^ a b c d e [1] Archived February 25, 2007, at the Wayback Machine
  3. ^ "Acanthostega gunneri". Devonian Times. Retrieved 2013-11-04.
  4. ^
    S2CID 4412211
    .
  5. PMID 17438285
    .
  6. ^ Fossils suggest earlier land-water transition of tetrapod
  7. S2CID 28461841
    .
  8. PMID 27602519
    .

Sources

External links
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