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The evolution of tetrapods began about 400 million years ago in the Devonian Period with the earliest tetrapods evolved from lobe-finned fishes. [1] Tetrapods (under the apomorphy-based definition used on this page) are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals.
The universal tetrapod characteristics of front limbs that bend backward at the elbow and hind limbs that bend forward at the knee can plausibly be traced to early tetrapods living in shallow water. [18] Panderichthys is a 90–130 cm (35–50 in) long fish from the Late Devonian period (380 Mya). It has a large tetrapod-like head.
The transition from fins to limbs occurred once an endoskeleton entered the base of the fin, as seen in today's lungfish. [21] This is thought to have originated in the group Sarcopterygians, including osteolipiforms like Eusthenopteron, due to the homology of the tetrapod forelimb and the osteolepiform fin endoskeleton. [21]
The first tetrapods appeared in the fossil record over a period, the beginning and end of which are marked with extinction events. This lasted until the end of the Devonian 359 mya. The ancestors of all tetrapods began adapting to walking on land, their strong pectoral and pelvic fins gradually evolved into legs (see Tiktaalik). [38]
Tiktaalik, a lobe-finned fish with some anatomical features similar to early tetrapods. It has been suggested to be a transitional species between fish and tetrapods. [81] 365 Ma Acanthostega is one of the earliest vertebrates capable of walking. [82] 363 Ma By the start of the Carboniferous Period
However, most tetrapod species today are amniotes, most of which are terrestrial tetrapods whose branch evolved from earlier tetrapods early in the Late Carboniferous. The key innovation in amniotes over amphibians is the amnion , which enables the eggs to retain their aqueous contents on land, rather than needing to stay in water.
Synapsida [a] is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant reptiles and birds).
An informal term with a broader meaning is stem tetrapod, a stem group consisting of all species more closely related to modern tetrapods than to lungfish, but excluding the crown group. This group includes both traditional "labyrinthodonts" as well as more basal tetrapodomorph fish, though its total content is a matter of some uncertainty, as ...