Meet The 370 Million-Year-Old Four-Legged Fish That Walked On Land

During the Devonian period, creatures of the sea, like Ichthyostega, evolved to move and live on land. This period is also famous as the age of fish. Fish ventured out of the water and evolved limbs for movement on land. These animals belong to the first set of tetrapods on earth.

Tetrapods are vertebrate animals that have four limbs. These animals include reptiles, mammals, birds, and amphibians. Tetrapods can include vertebrate land animals that have evolved to live in water. 

Tetrapods the first tetrapods evolved from vertebrates called tetrapodomorph fishes. These fishes had thick fins that evolved and changed into limbs. This article will discuss what the four-legged fish is, when it existed and how it went extinct.

Description & Size

Ichthyostega is a limbed fish that lived during the Devonian period. This fish was a tetrapod that evolved limbs for movement out of water. They had gills and fins fused with the skull and limbs of an amphibian. Ichthyostega had spiracles in their heads. The limbed fishes of the period could breathe air, and this was possible through their spiracles.

Ichthyostega means ‘fish roof’ in greek. They existed between 370 to 362 million years ago. This period was the Devonian period, the fourth period of the Paleozoic era. Their amphibious body structure was a bridge between amphibians and fish. They had limbs and lungs that aided swimming. Some facts about the Ichthyostega include:

• Weight – 50 lb
• Length – 1.5 meters
• Skin type – Scales
• Attributes – Four limbs, frog/fish-like head, tail fins, and gills

This animal belongs to and has four known species, and they are; Ichthyostega stensioei, Ichthyostega watsoni, Ichthyostega eigili, and Ichthyostega kochi. The species were identified by Gunnar Save-Soderbergh in 1932. The differences in the species were more notable in the patterns of the skull bones.

Ichthyostega moved using their limbs while they balanced their bodies using their tail. The younger ones could have moved on land better because of their smaller size and weight. They would also increase body temperature through exposure to the sun. It is likely one of the reasons why they needed to adapt to the land movement.

Ichthyostega has smaller hind limbs. These limbs likely couldn’t carry them in their adult size. They had skins that decreased evaporation and reduced the loss of water on the body’s surface. The more powerful forelimbs help drag themselves out of the water. 

Diet – What Did Ichthyostega Eat?

Ichthyostega is described as a piscivore. A piscivore is a carnivore that mainly eats fish. They had labyrinthodont teeth that made chewing on meat easy. As a carnivore, they also fed on small reptiles they could hunt on land. These prey were mostly small lizards.

Ichthyostega is known to rely on its gills more than its lungs. They dragged themselves when moving on land, which made it slow, unlike their swift movement in the water. This fact shows that it lived most of its life in water and probably survived on mainly fish. 

Habitat – When and Where Ichthyostega Lived

Ichthyostega lived as far back as 370 million years ago during the early evolution of tetrapods. This period was the third period during the Paleozoic era, the Devonian period. This animal evolved out of the waters to live a measured amphibious life.

Ichthyostega lived in swampy and shallow waters in the now Greenland region. Their limbs weren’t capable of supporting movement on land, which suggests that they lived mainly in water. Their ability to live on land was aided by the spiracles they possessed.

Ichthyostega probably could not survive without a water habitat. They litter 1 to 2 eggs. These eggs were gel-like and needed water to survive. Thus, procreation could only be done in the water.

Threats and Predators to the Ichthyostega

Ichtyostegas were solitary animals. They lived most of their life alone, and this suggests they had no herd movement behavior. Their primary defense mechanism was their labyrinthodont teeth. The Ichthyostega was likely prey to large sea animals of the time.

They were also subject to the inevitability of extinction which all species face. These extinctions were mainly caused by natural disasters. Evidence suggests that the formation of glaciers threatened and affected their habitat.

Discoveries and Fossils – Where Ichthyostega Fossils Were Found

The first fossils of the Ichthyostega were found in the 1920s. These fossils were later identified and described by Gunnar Säve-Söderberg. Gunnar also unearthed four fossils in Devonian Greenland. 

Subsequent fossil discoveries were made in 1929 and 1932. These fossils have described four species of the Ichthyostega. Ichthyostega fossils have been discovered in East Greenland. These fossils were found in deposits of the upper Famennian division. This division is the latter of the Devonian epoch. 

Extinction – When Did It Die Out?

Ichthyostega lived between 370 to 362 million years ago. Their extinction was due to a large-scale fish extinction about 360 million years ago. This extinction event caused a reset to the evolution of life on earth.

The exact cause of this mass extinction is not known yet. Evidence of glacier formation has been found during the late Devonian period. These glaciers were substantial enough to have reduced sea levels. As Ichthyostega was mainly an aquatic animal, this greatly affected them and caused them to become extinct.

Similar Animals to the Ichthyostega

The Ichthyostega belongs to an animal group called tetrapods. Tetrapods are not limited to amphibious or marine animal types. They evolved in a period when marine animals developed limbs to move on land.

Tetrapods are vertebrates with limbs, and this covers a long list of animals. Tetrapods are classified into Amphibia, Aves, Mammalia, Reptilia, and the ancient tetrapods from about 397 million years ago.

During the Devonian period, Ichthyostegas lived with other tetrapods that also started the limbed fish evolution. They also have close relatives today that have similar looks and evolutionary traits.

Similar animals to the Ichthyostega include:

• Acanthostega – Acanthostega means spiky roof in greek. This animal existed about 360 million years ago during the Late Devonian period. They weigh between 5 to 10 lb and can measure up to 2 feet long. Acanthostega is one of the well-known tetrapods that evolved and lived with Ichthyostegas. They also lived in swamps and rivers.
• Lungfish – Lungfish is a freshwater fish that can breathe in water and on land. They evolved about 400 million years ago and have retained some ancient features from evolution. They have an internal lung to eject waste and inhale oxygen. This lung helps give buoyancy when they’re in the water. 
• Tiktaalik – Tiktaalik is an animal from the late Devonian period, about 375 million years ago. They are often called ‘fishapods’ due to their similar features to both fish and tetrapods. They had four legs and a reptilian flat head. It had fish features like scales, fins, and gills with a tetrapod’s wide neck and ribs. Tiktaalik also lived in shallow waters, rivers, and swamps. They also had some human features like wrists, elbows, and shoulders.
• Dolphin – Dolphins are a species of whales. They can measure about 32 feet and weigh about 13,000 lb. They can live long lives to about 60 years of age. Dolphins live in oceans and seas all over the world. They are carnivores, and they hunt other fishes and marine animals. They have strong teeth for tearing the skin of their prey.
• Frog – Frogs are amphibious animals that are related to early tetrapods. They have four limbs, two long hindlimbs to leap, and two small forelimbs. They can live both on land and in water. Frogs are carnivores that prey on insects and small invertebrates. Their leap is a form of defense mechanism from prey. 

Conclusion

The Ichthyostega is an early tetrapod that started the evolution of animals from water to land. This evolution paved the way for some of today’s amphibious animals and some finishes. Their limbs did not provide a seamless moving experience when on land. 

Evidence shows that this animal mainly lived its life underwater. Their biological system supported long-duration underwater as they had gills for breathing. They also had fins and were fast swimmers. They look like some limbed reptiles of today but are different in biological composition.

The photo featured at the top of this post is © iStock.com/AlessandroZocc