Ichthyostega

Ichthyostega (gr. "roofed fish") is an extinct genus of tetrapod that lived from the late Upper Devonian during the Famennian age., approximately 365 to 360 million years ago; It was the first basal tetrapod to be discovered.

Ichthyostega already had legs with seven fingers, instead of fleshy fins, unlike Tiktaalik; but it probably did not use them to move on land, as was initially believed, but it is possible that with them it made a movement like that of amphibians and aquatic reptiles to move underwater, using its limbs to propel itself in the swamps, which indicates that he surely did not live on land.

History and systematics

In 1932 Gunnar Säve-Söderbergh described four species of Ichthyostega from the Upper Devonian, from Greenland and one species belonging to the genus Ichthyostegopsis, I. wimani. These species could be more modern synonyms (in which case only I. stensioei would be valid), given that their morphological differences are not very marked. Species differ in skull proportions, skull fenestrae, and cranial bone patterns. Comparisons were made with 14 specimens collected in 1931 by the Danish East Greenland Expedition. Additional specimens were found between 1933 and 1955.

This genus is closely related to Acanthostega gunnari, also found in eastern Greenland. The skull of Ichthyostega appears to be more similar to that of fish than that of Acanthostega, but the morphology of its shoulder and pelvic girdles indicates that they were stronger and better adapted to remain on land. Ichthyostega also has more supporting ribs, and stronger vertebrae with better developed zygapophyses. Whether these characteristics evolved independently in Ichthyostega is a matter of debate. In any case it clearly shows that Ichthyostega ventured onto dry land on occasion, unlike early tetrapods such as Elginerpeton and Obruchevichthys.

Description

Ichthyostega was quite large, measuring around 1.5 meters long. The skull was flat with the eyes located dorsally, and was armed with large teeth. On the posterior margin of the skull there was an operculum covering the gills, and the spiracle was located in an otic notch behind each eye.

The limbs were large compared to those of its contemporary relatives, and it also had seven fingers on each hind limb (polydactyly). The exact number of fingers on the forelimb is not yet known, as no fossils of the hand have been found. It had a caudal fin that had bony rays.

Adaptations for terrestrial life

Early tetrapods such as Ichthyostega and Acanthostega differed in several ways from their ancestors, the sarcopterygian fishes (lobe-finned fish), such as Eusthenopteron or Panderichthys). Although sarcopterygians had lungs, the gills were their main means of acquiring oxygen; Instead, Ichthyostega appears to have used its lungs as its main respiratory system. The skin of the first tetrapods, unlike that of sarcopterygians, helped them conserve body fluids by preventing desiccation. Sarcopterygians used their body and tail for locomotion and their fins to maintain balance; Ichthyostega would use its front limbs for locomotion and its tail for balance.

The specialization of Upper Devonian vertebrates resulted in lobe-finned fish like Panderichthys having descendants like Eusthenopteron that could breathe air in swampy pools; such as Tiktaalik whose paw-like fins could carry it to land, preceding the first amphibian tetrapods such as Acanthostega, whose feet had eight toes, and Ichthyostega with seven. Other sarcopterygians evolved into forms like the coelacanths that still exist today.

The size of an adult Ichthyostega was 1.5 m, which would have made terrestrial locomotion difficult. The massive ribcage is made up of overlapping ribs and the animal possessed a stronger skeletal structure and a very rigid spine; The forelimbs were apparently powerful enough to swim in water. The hind limbs were smaller than the forelimbs and are unlikely to have supported the weight of the animal, while the broad, overlapping ribs had side-to-side movements. The forelimbs had the range of motion required to push the body upwards. and forward, which allowed the animal to crawl on flat land by synchrony (rather than alternating movement), very similar to that of a Boleophthalmus dussumieri or that of modern seals.

Jennifer A. Clack suggested that Ichthyostega spent time sunbathing to raise its body temperature, as do some modern-day animals such as Galapagos marine iguanas, or crocodiles, and would have returned to the water to cool off, search for food and reproduce. A lifestyle in which it was necessary for the front limbs to be strong to go out of the water. And it had to have a strong ribcage and spine to support it while sunbathing, like modern crocodiles. New studies suggest that juveniles were more aquatic than adults, and that Ichthyostega emerged from the water only when it was a fully mature adult.

Water was still a requirement, as the eggs of the first terrestrial tetrapods could not survive outside of water, so reproduction could not occur without it. Water was also needed for their larvae and for fertilization. Most terrestrial vertebrates have developed methods of internal fertilization, either direct as in all amniotes and a few amphibians, or indirect as in many salamanders, by placing a spermatophore on the ground which is then picked up by the female.

Members of the order Ichthyostegalia (Elginerpeton, Acanthostega, Ichthyostega, etc.) were replaced by the temnospondyls and anthracosaurs, such as < i>Eryops, amphibians that actually developed the ability to walk on land. Until 2002, there was a difference of 20 million years between the two groups, this difference is known as the Romer Interval. A 350-million-year-old fossil from the Lower Mississippian, Pederpes, was described in 2002 and helped close the gap; It is the earliest known tetrapod which shows the beginnings of true terrestrial locomotion.