The re-interpretation of existing fossil specimens is a recurring theme in the study of early tetrapods. Typically, this involved re-assigning a specimen —often a lower jaw— that once believed to be that of a lobe-fin to that of a tetrapod. In contrast, Ichthyostega had been regarded at a tetrapod since its discovery in the 1930s. Indeed, it was essentially the Late Devonian tetrapod for much of the 20th Century. However, it too has been subjected to several significant re-interpretations.
Numerous specimens of Ichthyostega have been recovered from Upper Famennian deposits of eastern Greenland since 1929 and in 1932 Gunnar Säve-Söderberg concluded that there were at least three species of Ichthyostega and a fourth species belonging to Ichthyostegopsis were represented in the collections. Since then, these taxa have variously consolidated into either three species (Ichthyostega eigili, I. stensioei and I. watsoni) or just one species (I. stensioei). A recent study conducted by Henning Blom resolves many of the stratigraphic uncertainties of the different collecting localities and concludes that three valid species (Ichthyostega eigili, I. stensioei and I. watsoni) are represented in the collections. Most of the morphological work done on this genus was conducted on Ichthyostega stensioei.
Jarvik first reconstruction of Ichthyostega was published in 1955. His final effort, with modest differences, accompanied his 1996 monograph. He presented an essentially terrestrial tetrapod with vestiges of its piscine ancestry, most notably its finned tail. It had an extensively ossified and undifferentiated spine, robust limbs and girdles, and a rather conventional pes (foot) containing five digits; the manus (hand) is unknown, but it too was depicted conventionally. The most remarkable features presented by Jarvik was the already mentioned finned tail and the bizarre ribs. These ribs, which are narrow both proximally and distally, broadened dramatically in the middle and extensively overlap adjacent ribs. Presumably, this unique anatomy helped protect the lungs while the animal's trunk was not supported by buoyant water. It would also appear to restrict the side-to-side flexing crucial to conventional locomotion. (See top illustration.)
During the 1990s, studies by Jennifer Clack, Michael Coates, Per Ahlberg, and H.C. Bjerring substantially revised Jarvik's reconstruction. Ichthyostega was now considered to be a primarily aquatic tetrapod, although one that may have hauled itself onto the shore. The hind limbs were smaller than previously depicted and may have served as paddles rather than legs. Moreover, instead of five toes, its pes clearly had seven. (See middle illustration.)
More recently, the aquisition of new specimens and improved access to existing collections has prompted Ahlberg, Clack and others to offer a new —and radical— interpretation of Ichthyostega. The head is somewhat smaller and the tail shorter in relation to the shoulder and pelvis, but the most dramatic differences occur with the ribs and spine. (See bottom illustration.)
Ichthyostega is still distinguished by broadly flanged and overlapping ribs but these are fewer in number and more restricted longitudinally than in Jarvik's reconstruction. The thoracic ribs still formed "barrel-shape corset" that presumably protected the lungs, but those in the neck and lumbar region were greatly reduced and probably did not have inhibited lateral flexion. The neural arches (dorsal elements of the spine) exhibit dramatic changes along the length of the spine. Those in the thorax conventionally orient posteriorly, but there is a rapid transition in the lumbar region to larger, anteriorly-oriented, and heavily muscularized neural arches. Those behind the pelvis rapidly transition to the more conventional size and orientation; information of the neural spines of the neck and proximal tail is meagre.
This reconstruction by Ahlberg et. al. is extraordinary (and not without its critics). They noted that the dramatic logitudinal differentiation of the spine they found in Ichthyostega was not expected in any early tetrapod. Other that this, significant differentation of the spine appears in archeosaurs, theraspids and some other reptilian lineages during the Permian and Triassic. Moreover, they suspect that the lumbar spine was capable of some dorsal-ventral flexion; a character otherwise essentially limited to mammals.
The remarkable morphology resulting from this reconstruction has prompted speculation by Ahlberg and Clack that Ichthyostega may have engaged in a unique form of terrestrial (or shallow water) locomotion somewhat analogous with that employed by modern seals. (However, seals don't have the unusual spinal morphology found in this early tetrapod.) Ichthyostega may have extended the lumbar region to help advance the front half of the body and dorsally flexed it to pull the pelvis and tail forward. In other words, it may have moved like an inchworm.
Ichthyostega was a relatively large (1.5 m or 4 ft) early tetrapod with a stout body. Although it was originally considered to be the transitional form between fishes and Carboniferous amphibians, its skull possess several primitive, fish-like features, whereas those of other early tetrapods (i.e., Acanthostega and Ventastega) were generally more derived. Recent investigations on the otic chamber suggest that Ichthyostega had a uniquely specialized ear that may have use air chambers as underwater sound transducers.
The shoulder girdle of Ichthyostega was generally similar to that of Acanthostga, and considerably less well developed that those of Tulerpeton and Hynerpeton. No evidence for post-branchial lamina has been found, which suggest that it may have lacked internal gills. However, the recent discovery of branchial arches with deep groves suggest otherwise.
The forelimbs of Ichthyostega are generally more robust than those of Acanthostega and may have been able to bear some of its weight. The joint at the shoulder was relatively mobile but the elbow afforded little forward-and-back movement. The manus (hand) is unknown, so we don't know whether the animal had wrists.
The pelvis was large and was apparently attached to the spine. The hind limbs were considerably smaller than the forelimbs and probably functioned more like paddles than legs. The knee was relatively flexible but there are no evidence of an ankle joint. There are seven toes.
The three species of Ichthyostega were collected from three localities consisting of two Famennian formations (Aina Dal and Britta Dal) stratigraphically separated by a 100-150 m nonfossiliferous layer (the Wimans Bjerg Formation). What little sedimentological information that exists suggest that it lived in streams; the tectonics of the region also indicate that the localities were inter-montane. Fishes frequently found in association with it include Remigolepis (a placoderm) and Holoptychius (a lobe-fin). Ichthyostega and Acanthostega have been recovered from within the same deposits, but apparently not from within the same horizons. The temporal and ecological separations between these two early tetrapods is unclear.
Skeletal differences between Acanthostega and Ichthyostega suggest they had rather different life habits. It's been speculated that Acanthostega was probably exclusively aquatic, while Ichthyostega may have hauled itself onto the shore.
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