Tetrapods Roald Smeets

English: Illustration of Tiktaalik roseae, a S...

English: Illustration of Tiktaalik roseae, a Sarcopterygiin fish found at Ellesmere Island, Canada. Famous as one of the best candidates to the title as a missing link between lobe-finned fishes and tetrapods. (Photo credit: Wikipedia)

The tetrapods (Greek τετραπόδηs tetrӑpódēs, “four-footed”) are the amphibians, reptiles, birds and mammals; these four groups are united by the fact that most animals in each group have four limbs. Some animals with only two limbs such as pygopod lizards and some with no limbs at all like snakes and caecilians are considered tetrapods because they are classified among the four groups. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian. They are now a dominant part of the terrestrial fauna, representing all known larger land animals. Some groups have even returned to an aquatic existence, including the largest animal known, the blue whale.

The evolution of the first tetrapods marked the moment when the two basic forms of vertebrates, fishes and tetrapods, diverged. This transition, from a body plan for breathing and navigating in water and a body plan enabling the animal to move on land, involved a series of changes taking place throughout most of the 56.8 million years that make up the Devonian period. While it is one of the most profound evolutionary changes known, it is also one of the best understood, largely thanks to a number of amazing fossil finds in the late 20th century combined with improved phylogenetic analysis.

The Devonian period is traditionally known as the “Age of Fishes”, marking the diversification of numerous extinct and modern major fish groups. Among them were the early bony fishes, who diversified and spread in freshwater and brackish environments at the beginning of the period. The early types resembled their cartilaginous forefathers in many aspects of their anatomy, including a shark-like tailfin, spiral gut, large pectoral fins stiffened in front by skeletal elements and a largely unossified axial skeleton.

They did however have certain traits separating them from cartilaginous fishes, traits that would become pivotal in the evolution of terrestrial forms: With the exception of a pair of spiracles, the gills did not open singly to exterior like in sharks, rather they were hidden behind a bony operculum. The gill chamber was bound posteriorly by a stout cleithrum bone, also functioning as anchoring for the pectoral fins. As part of the overall armour of rhomboid cosmin scales, the skull had a full cover of dermal bone, constituting a skull roof over the otherwise shark-like cartilaginous inner cranium. Importantly, they also had a swim bladder/lung, a feature lacking in all other fishes.