A tetrapod is any four-limbed vertebrate animal of the superclass Tetrapoda. Tetrapods include all extant and extinct amphibians and amniotes, with the latter in turn evolving into two major clades, the sauropsids and synapsids. Some tetrapods such as snakes, legless lizards, and caecilians have evolved to become limbless via mutations of the Hox gene, although some do still have a pair of vestigial spurs that are remnants of the hindlimbs.
Sarcopterygii — sometimes considered synonymous with Crossopterygii — is a clade including both a group of bony fish commonly referred to as lobe-finned fish, and tetrapods. They are characterised by prominent muscular limb buds (lobes) within their fins, which are supported by articulated appendicular skeletons. This is in contrast to the other clade of bony fish, the Actinopterygii, which have only skin-covered bony spines supporting the fins.
A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. This is especially important where the descendant group is sharply differentiated by gross anatomy and mode of living from the ancestral group. These fossils serve as a reminder that taxonomic divisions are human constructs that have been imposed in hindsight on a continuum of variation. Because of the incompleteness of the fossil record, there is usually no way to know exactly how close a transitional fossil is to the point of divergence. Therefore, it cannot be assumed that transitional fossils are direct ancestors of more recent groups, though they are frequently used as models for such ancestors.
Eusthenopteron is a genus of prehistoric sarcopterygian fishes known from several species that lived during the Late Devonian period, about 385 million years ago. It has attained an iconic status from its close relationship to tetrapods. Early depictions of animals of this genus show them emerging onto land, but paleontologists now widely agree that eusthenopteron species were strictly aquatic animals.
Panderichthys is a genus of extinct sarcopterygian from the late Devonian period, about 380 Mya. Panderichthys, which was recovered from Frasnian deposits in Latvia, is represented by two species. P. stolbovi is known only from some snout fragments and an incomplete lower jaw. P. rhombolepis is known from several more complete specimens. Although it probably belongs to a sister group of the earliest tetrapods, Panderichthys exhibits a range of features transitional between tristichopterid lobe-fin fishes and early tetrapods. It is named after the German-Baltic paleontologist Christian Heinrich Pander. Possible tetrapod tracks dating back to before the appearance of Panderichthys in the fossil record were reported in 2010, which suggests that Panderichthys is not a direct ancestor of tetrapods, but nonetheless shows the traits that evolved during the fish-tetrapod evolution
Hynerpeton is an extinct genus of early four-limbed vertebrate that lived in the rivers and ponds of Pennsylvania during the Late Devonian period, around 365 to 363 million years ago. The only known species of Hynerpeton is H. bassetti, named after the describer's grandfather, city planner Edward Bassett. Hynerpeton is known for being the first Devonian four-limbed vertebrate discovered in the United States, as well as possibly being one of the first to have lost internal (fish-like) gills.
Tiktaalik is a monospecific genus of extinct sarcopterygian from the Late Devonian Period, about 375 Mya, having many features akin to those of tetrapods. Tiktaalik is estimated to have had a total length of 1.25–2.75 metres (4.1–9.0 ft) based on various specimens.
The Fram Formation is an Upper Devonian (Frasnian) sequence of rock strata on Ellesmere Island that came into prominence in 2006 with the discovery in its rocks of examples of the transitional fossil, Tiktaalik, a sarcopterygian or lobe-finned fish showing many tetrapod characteristics. Fossils of Laccognathus embryi, a porolepiform lobe-finned fish, and Qikiqtania, a close relative of Tiktaalik, were also found in the formation. The Fram Formation is a Middle to Upper Devonian clastic wedge forming an extensive continental facies consisting of sediments derived from deposits laid down in braided stream systems that formed some 375 million years ago, at a time when the North American craton ("Laurentia") was straddling the equator.
A walking fish, or ambulatory fish, is a fish that is able to travel over land for extended periods of time. Some other modes of non-standard fish locomotion include "walking" along the sea floor, for example, in handfish or frogfish.
Edward B. 'Ted' Daeschler is an American vertebrate paleontologist and Associate Curator and Chair of Vertebrate Biology at the Academy of Natural Sciences in Philadelphia. He is a specialist in fish paleontology, especially in the Late Devonian, and in the development of the first limbed vertebrates. He is the discoverer of the transitional fossil tetrapod Hynerpeton bassetti, and a Devonian fish-like specimen of Sauripterus taylori with fingerlike appendages, and was also part of a team of researchers that discovered the transitional fossil Tiktaalik.
Elpistostegalia is a clade containing Panderichthys and all more derived tetrapodomorph taxa. The earliest elpistostegalians, combining fishlike and tetrapod-like characters, such as Tiktaalik, are sometimes called fishapods. Although historically Elpistostegalia was considered an order of prehistoric lobe-finned fishes, it was cladistically redefined to include tetrapods.
The skull roof or the roofing bones of the skull are a set of bones covering the brain, eyes and nostrils in bony fishes and all land-living vertebrates. The bones are derived from dermal bone and are part of the dermatocranium.
Ichthyostegalia is an order of extinct amphibians, representing the earliest landliving vertebrates. The group is thus an evolutionary grade rather than a clade. While the group are recognized as having feet rather than fins, most, if not all, had internal gills in adulthood and lived primarily as shallow water fish and spent minimal time on land.
Laccognathus embryi is an extinct species of porolepiform lobe-finned fish recovered from Ellesmere Island, Canada. It existed during the Frasnian age of the Late Devonian epoch.
Why Evolution is True is a popular science book by American biologist Jerry Coyne. It was published in 2009, dubbed "Darwin Year" as it marked the bicentennial of Charles Darwin and the hundred and fiftieth anniversary of the publication of his On the Origin of Species By Means of Natural Selection. Coyne examines the evidence for evolution, some of which was known to Darwin (biogeography) and some of which has emerged in recent years. The book was a New York Times bestseller, and reviewers praised the logic of Coyne's arguments and the clarity of his prose. It was reprinted as part of the Oxford Landmark Science series.
Farish Alston Jenkins was a professor at Harvard University who studied and taught paleontology. His discoveries included a transitional creature with characteristics of both fish and land animals — Tiktaalik roseae —and one of the earliest known frogs, Prosalirus bitis.
Innovations conventionally associated with terrestrially first appeared in aquatic elpistostegalians such as Panderichthys rhombolepis, Elpistostege watsoni, and Tiktaalik roseae. Phylogenetic analyses distribute the features that developed along the tetrapod stem and display a stepwise process of character acquisition, rather than abrupt. The complete transition occurred over a period of 30 million years beginning with the tetrapodomorph diversification in the Middle Devonian.
Qikiqtania is an extinct genus of elpistostegalian tetrapodomorph from the Late Devonian Fram Formation of Nunavut, Canada. The genus contains a single species, Q. wakei, known from a partial skeleton. Analysis of the fin bones suggests that Qikiqtania was well-suited to swimming, and likely incapable of walking or supporting itself out of the water, as has been suggested for the closely related Tiktaalik.
The Zachelmie trackways are a series of Middle Devonian-age trace fossils in Poland, purportedly the oldest evidence of terrestrial vertebrates (tetrapods) in the fossil record. These trackways were discovered in the Wojciechowice Formation, an Eifelian-age carbonate unit exposed in the Zachełmie Quarry of the Świętokrzyskie Mountains (Holy Cross Mountains]. The discovery of these tracks has complicated the study of tetrapod evolution. Morphological studies suggest that four-limbed vertebrates are descended from a specialized type of tetrapodomorph fish, the epistostegalians. This hypothesis was supported further by the discovery and 2006 description of Tiktaalik, a well-preserved epistostegalian from the Frasnian of Nunavut. Crucial to this idea is the assumption that tetrapods originated in the Late Devonian, after elpistostegalians appear in the fossil record near the start of the Frasnian. The Zachelmie trackways, however, appear to demonstrate that tetrapods were present prior to the Late Devonian. The implications of this find has led to several different perspectives on the sequence of events involved in tetrapod evolution.
The evolution of fishes took place over a timeline which spans the Cambrian to the Cenozoic, including during that time in particular the Devonian, which has been dubbed the "age of fishes" for the many changes during that period.
