Ichthyostega is a genus of extinct animals closely related to tetrapods (four-legged terrestrial vertebrates). It is found as a fossil in rocks in eastern Greenland from the late Devonian period about 370 million years ago. Although often referred to as “quadrupeds” due to the presence of limbs and fingers, Ichthyosteg was a more basal “primitive” variety than true crown tetrapods and might more accurately be called a stegocephalic or stem quadruped.
View origin and description
Ichthyostega (from the Greek for “roof fish”) is an early genus of the tetrapodomorph clade (tetrapodomorphs) that lived in the late Devonian period. It was one of the first four-limbed vertebrates found in the fossil record. Ichthyostega had lungs and limbs that helped her move through shallow water in swamps. By structure and habits, it is not considered a true member of the group, since the first modern amphibians (members of the Lissamphibia group) appeared in the Triassic period.
Fun fact: Four species were originally described, and a second genus, Ichthyostegopsis, was described. But further research has shown the existence of three reliable species, based on the proportions of the skull and associated with three different formations.
Until the finds of other early stegocephalians and closely related fishes at the end of the 20th century, Ichthyostega remained the only transitional fish-tetrapod fossil found, combining features of fish and tetrapods at the same time. A newer study showed that she had an unusual anatomy.
Traditionally, Ichthyostega represents a paraphyletic class of the most primitive stem tetrapods, therefore it is not classified by many modern researchers as an ancestor of modern species. Phylogenetic analysis has demonstrated that Ichthyostega is an intermediate link between other primitive stegocephalic stem tetrapods. In 2012, Schwartz compiled an evolutionary tree of early stegocephalians.
Appearance and features
Ichthyostega was about one and a half meters long and had a small dorsal fin along the edge of the tail. The tail itself possessed a series of bony supports typical of the tail supports found in fish. Other features retained in earlier aquatic vertebrates include a relatively short snout area, the presence of a preopercular bone in the cheek region that serves as part of the gills, and many small scales on the body. Advanced tetrapod features include a series of strong bones supporting fleshy limbs, lack of gills, and strong ribs.
Fun fact: Ichthyostega and its relatives represent forms slightly more advanced than the aquatic Eusthenopteron, and appear to be close to the evolutionary line leading to the first tetrapods on land.
The most notable feature of the ichthyostegus axial skeleton is the degree to which the ribs overlap. One pectoral rib may overlap three or four more posterior ribs, forming a barrel-shaped “corset” around the body. This suggests that the animal could not flex its torso laterally while walking or swimming. The vertebrae were not chordates, but the neural arches had more prominent zygapophyses.
It can be assumed that the animal moved more as a result of dorsoventral flexion than during normal lateral walking. The massive forelimbs may have been used to pull the animal forward and then flex the presacral region to pull the hindquarters up. The hind limbs consisted of a short, thick femur with a large flange and an adductor with a deep intercondylar fossa.
The large, almost quadrangular tibia and shorter fibula were flattened. The large intermediate and fibula included most of the ankle bones. A well-preserved specimen collected in 1987 shows a complete set of seven fingers, three small ones on the leading edge and four full ones on the back.
Where does the Ichthyostega live?
Ichthyosteg remains have been found in Greenland. Although the exact habitat of the species is unknown, it can be assumed that ichthyostegi were inhabitants of the northern hemisphere. And they inhabited the current waters of the Atlantic and Arctic Oceans. The Devonian period is characterized by a relatively warm climate and probably the absence of glaciers. The difference in temperature from the equator to the poles was not as great as it is today. The weather was also very dry, mostly along the equator where the weather was driest.
Fun Fact: Tropical sea surface temperature reconstructions suggest an average of 25°C in the early Devonian. Carbon dioxide levels dropped sharply during the Devonian as the burial of newly formed forests pulled carbon from the atmosphere into the sediments. This is reflected in the mid-Devonian period by cooling down to 5 °C. The late Devonian is characterized by an increase in temperature to a level equivalent to the early Devonian.
At that time, there is no corresponding increase in CO² concentrations, and continental weathering is increasing (as shown by higher temperatures). In addition, a number of pieces of evidence, such as the distribution of plants, point to Late Devonian warming. It is in this period that the fossils found are dated. It is possible that the Ichthyostegs survived into the next Carboniferous period. Their further disappearance may be due to a decrease in temperature in their habitats.
During this period, the climate affected the dominant organisms in reefs, microbes were the main reef-building organisms during warm periods, and corals and stromatoporoid sponges played a dominant role in colder times. The warming at the end of the Devonian may even have contributed to the disappearance of stromatoporoids.
Now you know where the ichthyostega was found. Let's see what she ate.
What did Ichthyostega eat?
The fingers of the ichthyostega were poorly bent, and the muscular system was weak, but the animal, in addition to the aquatic environment, could already move through swampy areas of land. If we consider the pastime of ichthyostegi in percentage terms, then 70-80% of the time she conquered the water element, and the remaining time she tried to master the land. Its main food sources were the inhabitants of the seas of that time, fish, marine plankton, and possibly marine plants. Sea levels were generally high in the Devonian.
The marine fauna continued to be dominated by:
- various and abundant brachiopods;
- mysterious hederellids;
- microconchids ;
- crinoid lily-like animals, despite their resemblance to flowers, were abundant;
- trilobites were still quite common.
It is possible that Ichthyostega fed on some of these species. Previously, scientists have associated ichthyostega with the appearance of tetrapods on land. However, most likely, she went to land for a very short time, and returned back to the water. Which of the ancient vertebrates became the real discoverer of land remains to be seen.
By the Devonian period, life was in full swing in the process of colonization of the earth. The moss forests and bacterial mats of the Silurian at the beginning of the period included primitive root plants that created the original stable soils and arthropods such as mites, scorpions, trigonotharbids, and centipedes. Although arthropods appeared on earth earlier than the early Devonian, and the existence of fossils such as Climactichnites suggests that terrestrial arthropods may have appeared as early as the Cambrian.
The first possible fossils of insects appeared in the early Devonian. Evidence for the earliest tetrapods is presented as trace fossils in shallow lagoons of a marine carbonate platform/shelf during the Middle Devonian, although these traces have been questioned and scientists have theorized fish feeding traces. All this rapidly developing flora and fauna was a potential source of food for Ichthyosteg.
Character and lifestyle features
The age of the animal was set at 370 million years and dated to the Devonian period. Ichthyostega — one of the oldest known tetrapods. Because of its features, which include characteristics of both fish and amphibians, Ichthyostega has served as an important springboard and morphological evidence for the theory of evolution.
Fun Fact: One of the coolest facts about Ichthyosteg is not that she has webbed feet, but that she was able to breathe air — at least for short periods of time. However, even with this amazing ability, she probably didn't spend much time on land. This was because she was rather heavy and his legs were not strong enough to move his sturdy body.
In Ichthyostega, the forelimbs seem to have been heavy, and the forearm could not be fully extended. The proportions of the elephant seal are the closest anatomical equivalent among living animals. It is possible that Ichthyostega climbed rocky beaches, moving its forelimbs in parallel and dragging its hind limbs behind it.
The animal was incapable of typical quadrupedal gaits because the forelimbs did not have the necessary range of rotational motion. However, the exact lifestyle of Ichthyostega is not yet clear due to its unusual features.
Social Structure and Reproduction
It is assumed that Ichthyostega and her relatives spent their time basking in the sun to raise their body temperature. They also returned to the water to cool off, hunt for food, and breed. Their way of life called for strong forelimbs to pull at least the front part out of the water, and a stronger chest and spine to support them, sunbathing on their stomachs like modern crocodiles.
Interesting fact: Ichthyostegs became the progenitors of two main branches of amphibians, differing in the structure of the skull and limbs. Labyrinthodonts arose in the Late Devonian. Outwardly, they looked like crocodiles or salamanders. Today, hundreds of species of labyrinthodonts have become known that lived in swampy forests and rivers.
Water was a requirement for the Ichthyostega, since the eggs of the earliest land tetrapods could not survive out of water, so reproduction could not occur without an aquatic environment. Water was also essential for their larvae and external fertilization. Since then, most terrestrial vertebrates have developed two methods of internal fertilization. Either direct, as seen in all amniotes and a few amphibians, or indirect for many salamanders, placing a spermatophore on the ground, which is then lifted up by the female.
Ichthyostega natural enemies
Although the forelimbs have not been reconstructed because they are not found among the known fossils of the animal, it is believed that these appendages were larger than the hind limbs of the animal. Scientists believe that in this way the ichthyostega moved its body from water to land.
It seems that locomotion, which is a function of the instinctive movements of the musculoskeletal system of the body, represented only a minimal variability of movements underwater using a combination of tail movements and legs. In this case, the legs were specially used to pass the muscles through the flooded undergrowth of aquatic plants.
Interesting fact: Although land movements were possible, Ichthyostega was more developed for life in the water, especially in the adult stage of its life. It rarely moved on land, and perhaps the smaller size of juveniles, allowing them to move more easily on land, served not to search for food outside the water element, but as a way to avoid other large predators until they grew large enough to avoid becoming their prey.
The scientists argue that moving to land provided animals with greater safety from predators, less competition for prey, and certain environmental benefits not found in water, such as oxygen concentration, and temperature control — implies that developing limbs also adapt to spending some of their time out of the water.
However, studies have shown that sarcopterygians have evolved quadrupedal-like limbs suitable for walking well before heading to land. This suggests that they adapted to walk on land underwater before moving to land.
Population and species status
Ichthyostega is a species that has been extinct for a very long time. Therefore, today it is difficult to judge how widespread Ichthyostega populations were on Earth. But since fossils were found only within Greenland, the number of individuals was probably negligible. These animals lived in a very difficult period. A major extinction occurred at the beginning of the last phase of the Devonian, the fauna of the Famenzian deposits shows that about 372.2 million years ago, when all the fossil agnatan fishes, with the exception of heterostracan psammosteiids, suddenly disappeared.
The Late Devonian extinction event was one of the five major extinction events in the history of Earth's life, and was more radical than the similar extinction event that closed the Cretaceous. The Devonian extinction crisis primarily affected the marine community and selectively affected shallow water organisms in warm water. The most important group that suffered from this extinction event were the builders of the great reef systems.
Among the marine groups heavily affected were:
- jawless fish;
- all placoderms.
Land plants, as well as freshwater species such as our tetrapod ancestors, were relatively unaffected by the Late Devonian extinction event. The reasons for the extinction of species in the Late Devonian are still unknown and all explanations remain speculative. Under these conditions, Ichthyostega survived and multiplied. Asteroid impacts changed the surface of the Earth and affected its inhabitants.