Basal tetrapods evolved from lobe-finned fish during the Devonian Period, and had features unique to tetrapods. Tiktaalik was an intermediate form between fish and amphibians, while Ichthyostega was one of the first true tetrapods with breakthroughs for terrestrial life.
Basal tetrapods refer to the tetrapods (four-legged animals) at the root of the tetrapod evolutionary tree, which contains all land vertebrates and also some marine animals such as cetaceans: whales and relatives. Basal tetrapods evolved from sarcopterygians (lobe-finned fish), represented today by living fossils the coelacanth and lungfish.
380 million years ago, during the Devonian Period, the earth lacked the lush forests that would characterize all subsequent eras. True trees were just beginning to evolve, but primitive vascular plants would have been everywhere, creating short shrub-like forests. These were populated by arthropods such as mites, scorpions, and myriapods, as well as some simple molluscs that didn’t fossilize well. The bizarre fungal monolith Prototaxites, up to 9 m (30 ft) tall, was the tallest thing around. The Earth was warm, causing continental shelves to flood and lowlands to be covered in nutrient-rich marshes. The environment was perfect for encouraging a small family of fish to develop stronger bottom fins used to propel themselves through the vegetation of shallow marshes.
Early lobe-finned fishes such as Eusthenopteron, which lived about 380 million years ago, are known to possess many features that are now known to be unique to tetrapods: internal nostrils, a two-part skull with an intercranial joint, and, specifically, a radius, ulna, and humerus in the anterior fin and fibula, tibia, and femur in the pelvic fin. The latter feature is now known to be common to all sarcopterygians.
Around the same time lived Panderichthys, a shallow-water fish with even more basal tetrapod-like features, most notably a broad tetrapod-like head. Panderichthys is a crucial transitional fossil, showing a midpoint in the evolution of the pelvic girdle that made basal tetrapod legs possible. It even had a spiracle, or oxygen breathing tube, on its head, which later evolved into the stapes bone in tetrapod ears. This allowed him to breathe adequately in muddy oxygen-poor water.
Lived about 475 million years ago was the famous Tiktaalik, whose reconstructions and analyzes most clearly show an intermediate form between a fish and an amphibian. Whether Tiktaalik was a true tetrapod, venturing onto land, is unknown, although it seems probable. Studies of Tiktaalik’s skeleton have shown that it could have supported its own weight within reach. Its fins had a wrist structure not seen in any previous fish. Its head had sizable spiracles that probably channeled oxygen into primitive lungs. This sarcopterygian also had a broad crocodile-like head that could turn left and right, independent of its body, an adaptation that was ideal for hunting on land and unlike any fish before it. Tiktaalik is the best preserved sarcopterygian to be classified in the subclass Tetrapodomorpha. Tiktaalik and earlier members of Tetrapodmorpha, such as Kenichthys, are considered so similar to basal tetrapods that they have been called “fishapods,” referring to their combination of fish and basal tetrapod characteristics.
One of the first true tetrapods, which probably didn’t yet walk predominantly on land, but used its primitive legs to navigate through swamps, was Ichthyostega, which lived 367-362.5 million years ago. Ichthyostega is one of the few basal tetrapods, a tetrapod that is neither an amphibian, nor a synapsid (mammal and relatives), nor a sauropsid (reptile). These animals had a number of further evolutionary breakthroughs for terrestrial life. For one thing, Ichthyostega primarily used its lungs to breathe, rather than its gills. It had a special skin coating that helped prevent it from drying out. It had a skeletal structure (especially the spine and rib cage) which clearly shows that this animal spent a lot of time standing. Ichthyostega and other ichthyostegoids (Acanthostega, Elginerpeton, Ichthyostega and others) were eventually replaced by temnospondyls and anthracosaurs, including the famous Eryops. Unfortunately, there is a 20-30 million year gap in the fossil record between both groups, filled by only a few specimens.
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