Temporal range: Famennian (late Devonian)
|Life restoration of Ichthyostega after Ahlberg, 2005.|
|Class:||"Amphibia" (wide sense)|
Ichthyostega (Greek: "fish roof") 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 tetrapods in the fossil record. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps. 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. Until finds of other early tetrapods and closely related fishes in the late 20th century, Ichthyostega stood alone as the transitional fossil between fish and tetrapods, combining a fishlike tail and gills with an amphibian skull and limbs.
History and systematics
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 closely 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 clearly ventured onto land on occasions, unlike the very first tetrapods like Elginerpeton and Obruchevichthys.
Ichthyostega was a fairly large animal, broadly built and about 1.5 m 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.
Adaptations for terrestrial life
Early tetrapods like Ichthyostega and Acanthostega differed from animals like Crossopterygians (for instance Eusthenopteron or Panderichthys) in their increased adaptations for life on land. Though Crossopterygians possessed lungs, they used gills 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 these animals retain bodily fluids and deterred desiccation. Crossopterygians used their body and tail for locomotion and their fins for balance; Ichthyostega used its fore limbs for locomotion and its tail for balance.
The size of an adult Ichthyostega (1.5 m) would have made terrestrial locomotion difficult. 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 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.
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 Iguanas 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 crocodiles. New studies suggest 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 amniotes and a few amphibians, or indirect for many salamanders 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.
|Wikimedia Commons has media related to Ichthyostega.|
- Evolutionary developmental biology, by Brian Keith Hall, 1998, ISBN 0-412-78580-3, p. 262
- Devonian Times - Tetrapods Answer
- Stephanie E. Pierce, Jennifer A. Clack, & John R. Hutchinson (2012). "Three-dimensional limb joint mobility in the early tetrapod Ichthyostega". Nature 486: 524–527. doi:10.1038/nature11124.
- 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.
- News Staff (April 18, 2009). "Was Ichthyostega The Earliest Land-Water Transition Of Tetrapod". Science 2.0. ION Publications LLC. Retrieved 4 November 2013.
- Blom, H. (2005) — Taxonomic Revision Of The Late Devonian Tetrapod Ichthyostega from East Greenland. Palaeontology, 48, Part 1:111–134
- Westenberg, K. (1999) — From Fins to Feet. National Geographic, 195, 5:114–127
- 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