|Skeleton in Moscow Paleontological Museum|
Ichthyostega (Greek: "fish roof") is an early genus of tetrapodomorphs that lived at the end of the Upper Devonian period. It was one of the first four-limbed vertebrates in the fossil record. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps. Although Ichthyostega is often labelled a "tetrapod" due to the possession of limbs and fingers, it was more basal ("primitive") than true crown-tetrapods, and could more accurately be referred to as a stegocephalian or stem tetrapod. Likewise, while undoubtedly of amphibian build and habit, it is not considered a true member of the group in the narrow sense, as the first modern amphibians (members of the group Lissamphibia) appeared in the Triassic period. Until finds of other early stegocephalians and closely related fishes in the late 20th century, Ichthyostega stood alone as a transitional fossil between fish and tetrapods, combining fish- and tetrapod-like features. Newer research has shown that it had an unusual anatomy, functioning more akin to a seal than a salamander, as previously assumed.
Ichthyostega was a fairly large animal, broadly built and about 1.5 m (4.9 ft) long. The skull was flat with dorsally placed eyes and armed with large labyrinthodont teeth. The posterior margin of the skull formed an operculum covering the gills. The spiracle was situated in an otic notch behind each eye.
The limbs were large compared to contemporary relatives, and it had seven digits on each hind limb. The exact number of digits on the forelimb is not yet known, since fossils of the manus (hand) have not been found. It had a fin containing fin rays on its tail.
History and classification
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In 1932 Gunnar Säve-Söderbergh 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 related to Acanthostega gunnari, also from East Greenland. Ichthyostega's 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 ribs and stronger vertebrae with more developed zygapophyses. Whether these traits were independently evolved in Ichthyostega is debated. It does however show that Ichthyostega may have ventured onto land on occasions, unlike contemporaneous limbed vertebrates such as Elginerpeton and Obruchevichthys.
Traditionally, Ichthyostega was considered part of an order named for it, the "Ichthyostegalia". however, this group represents a paraphyletic grade of primitive stem-tetrapods and is not used by many modern researchers. Phylogenetic analysis has shown Ichthyostega is intermediate between other primitive stegocephalian stem-tetrapods. The evolutionary tree of early stegocephalians below follows the results of one such analysis performed by Swartz in 2012.
Adaptations for terrestrial life
Early limbed vertebrates like Ichthyostega and Acanthostega differed from earlier tetrapodomorphs such as Eusthenopteron or Panderichthys in their increased adaptations for life on land. Though tetrapodomorphs possessed lungs, they used gills as their primary means of discharging carbon dioxide. Tetrapodomorphs used their bodies and tails for locomotion and their fins for steering and braking; Ichthyostega may have used its forelimbs for locomotion on land and its tail for swimming.
Its massive ribcage was made up of overlapping ribs and the animal possessed a stronger skeletal structure, a largely fishlike spine, and forelimbs apparently powerful enough to pull the body from the water. These anatomical modifications may have evolved to handle the lack of buoyancy experienced on land. The hindlimbs were smaller than the forelimbs and unlikely to have borne full weight in an adult, while the broad, overlapping ribs would have inhibited side-to-side movements. The forelimbs had the required range of movement to push the body up and forward, probably allowing the animal to drag itself across flat land by synchronous (rather than alternate) "crutching" movements, much like that of a mudskipper or a seal. It was incapable of typical quadrupedal gaits as the forelimbs lacked the necessary rotary motion range.
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- Haaramo, Mikko. "Taxonomic history of the genus †Ichthyostega Säve-Söderbergh, 1932". Mikko's Phylogeny Archive. Blom, 2005. Retrieved 24 October 2015.
- "Ichthyostega". Paleofile. Retrieved 24 October 2015.
- Stephanie E. Pierce; Jennifer A. Clack; John R. Hutchinson (2012). "Three-dimensional limb joint mobility in the early tetrapod Ichthyostega". Nature. 486: 524&ndash, 527. doi:10.1038/nature11124. PMID 22722854.
- Evolutionary developmental biology, by Brian Keith Hall, 1998, ISBN 0-412-78580-3, p. 262
- Swartz, B. (2012). "A marine stem-tetrapod from the Devonian of Western North America". PLoS ONE. 7 (3): e33683. doi:10.1371/journal.pone.0033683. PMC 3308997. PMID 22448265.
- Devonian Times - Tetrapods Answer
- Williams, James J. (May 24, 2012). "Ichthyostega, one of the first creatures to step on land, could not have walked on four legs, say scientists". BelleNews. Retrieved 4 November 2013.
- Mosher, Dave (May 23, 2012). "Evolutionary Flop: Early 4-Footed Land Animal Was No Walker?". National Geographic News. Retrieved 4 November 2013.
- Blom, H (2005). "Taxonomic Revision Of The Late Devonian Tetrapod Ichthyostega from East Greenland". Palaeontology. 48 (1): 111–134. doi:10.1111/j.1475-4983.2004.00435.x.
- Westenberg, K (1999). "— From Fins to Feet". National Geographic. 195 (5): 114–127.
- Clack, J.A. (2012). Gaining ground: the origin and evolution of tetrapods (2nd ed.). Bloomington, Indiana, USA.: Indiana University Press.
- Tree of Life Site on early tetrapods
- Getting a Leg Up on Land Scientific American Nov. 21, 2005, article by Jennifer A. Clack.
- BBC News: Ancient walking mystery deepens
- 3D computer model, forelimb maximal joint range, and hindlimb maximal joint range of Icthyostega on YouTube, videos by Stephanie E. Pierce, Jennifer A. Clack, & John R. Hutchinson