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 had 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 of bony fish commonly referred to as lobe-finned fish. 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.
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.
Protosphyraena is a fossil genus of swordfish-like marine fish, that thrived worldwide during the Upper Cretaceous Period (Coniacian-Maastrichtian). Though fossil remains of this taxon have been found in both Europe and Asia, it is perhaps best known from the Smoky Hill Member of the Niobrara Chalk Formation of Kansas. Protosphyraena was a large fish, averaging 2–3 metres in length. Protosphyraena shared the Cretaceous oceans with aquatic reptiles, such as mosasaurs and plesiosaurs, as well as with many other species of extinct predatory fish. The name Protosphyraena is a combination of the Greek word protos ("early") plus Sphyraena, the genus name for barracuda, as paleontologists initially mistook Protosphyraena for an ancestral barracuda. Recent research shows that the genus Protosphyraena is not at all related to the true swordfish-family Xiphiidae, but belongs to the extinct family Pachycormidae.
Diplichnites are arthropod trackways with two parallel rows of blunt to elongate, closely spaced tracks oriented approximately perpendicularly to the mid-line of the trackway. The term is more often used for the ichnofossils of this description; however, similar trackways from recent arthropods are sometimes given this name as well.
Undichna is a fish-fin, or fish-swimming fossil trail left as a fossil impression on a substrate, or the opposite impression on an overlying substrate; this type of fossil is an ichnofossil, in this case a specific ichnogenus, Undichna; the term "undichna" is composed of the words: 'und'–'ichna', for "wave-trace".
Undichna britannica is a fish-fin, or fish-swimming fossil trackway left as a fossil impression on a substrate; this type of fossil is an ichnofossil, and in this case an ichnospecies.
Palmichnium is an ichnofossil genus, interpreted as a eurypterid trackway. It has been found by many places around the world, such as Australia, Canada, United States and Wales.
Jefitchia is an extinct genus of prehistoric drum. Species lived from 48.6–33.9 mya . Jefitchia have been uncovered in Texas, Louisiana, and Portugal.
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.
Pteronisculus is an extinct genus of prehistoric ray-finned fish that lived during the Early Triassic and Middle Triassic epochs of the Triassic period worldwide.
Prehistoric Trackways National Monument is a national monument in the Robledo Mountains of Doña Ana County, New Mexico, United States, near the city of Las Cruces. The monument's Paleozoic Era fossils are on 5,255 acres (2,127 ha) of land administered by the Bureau of Land Management. It became the 100th active U.S. national monument when it was designated on March 30, 2009.
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.
Britannica is the Encyclopædia Britannica, a general knowledge English-language encyclopaedia.
The evolution of tetrapods began about 400 million years ago in the Devonian Period with the earliest tetrapods evolved from lobe-finned fishes. Tetrapods are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals. While most species today are terrestrial, little evidence supports the idea that any of the earliest tetrapods could move about on land, as their limbs could not have held their midsections off the ground and the known trackways do not indicate they dragged their bellies around. Presumably, the tracks were made by animals walking along the bottoms of shallow bodies of water. The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear. They are areas of active research and debate among palaeontologists at present.
Cephalaspidida is an extinct order of jawless fish in the subclass Cornuata.
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.
