The Pennsylvanian is, on the ICS geologic timescale, the younger of two subperiods of the Carboniferous Period. It lasted from roughly 323.2 million years ago to 298.9 million years ago. As with most other geochronologic units, the rock beds that define the Pennsylvanian are well identified, but the exact date of the start and end are uncertain by a few hundred thousand years. The Pennsylvanian is named after the U.S. state of Pennsylvania, where the coal-producing beds of this age are widespread.
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.
Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial and semiaquatic vertebrates. Amniotes evolved from amphibian ancestors during the Carboniferous period and further diverged into two groups, namely the sauropsids and synapsids. They are distinguished from the other living tetrapod clade — the non-amniote lissamphibians — by the development of three extraembryonic membranes, thicker and keratinized skin, and costal respiration.
"Labyrinthodontia" is an informal grouping of extinct predatory amphibians which were major components of ecosystems in the late Paleozoic and early Mesozoic eras. Traditionally considered a subclass of the class Amphibia, modern classification systems recognize that labyrinthodonts are not a formal natural group (clade) exclusive of other tetrapods. Instead, they consistute an evolutionary grade, ancestral to living tetrapods such as lissamphibians and amniotes. "Labyrinthodont"-grade vertebrates evolved from lobe-finned fishes in the Devonian, though a formal boundary between fish and amphibian is difficult to define at this point in time.
The Batrachomorpha are a clade containing recent and extinct amphibians that are more closely related to modern amphibians than they are to mammals and reptiles. According to many analyses they include the extinct Temnospondyli; some show that they include the Lepospondyli instead. The name traditionally indicated a more limited group.
Lepospondyli is a diverse taxon of early tetrapods. With the exception of one late-surviving lepospondyl from the Late Permian of Morocco, lepospondyls lived from the Early Carboniferous (Mississippian) to the Early Permian and were geographically restricted to what is now Europe and North America. Five major groups of lepospondyls are known: Adelospondyli; Aïstopoda; Lysorophia; Microsauria; and Nectridea. Lepospondyls have a diverse range of body forms and include species with newt-like, eel- or snake-like, and lizard-like forms. Various species were aquatic, semiaquatic, or terrestrial. None were large, and they are assumed to have lived in specialized ecological niches not taken by the more numerous temnospondyl amphibians that coexisted with them in the Paleozoic. Lepospondyli was named in 1888 by Karl Alfred von Zittel, who coined the name to include some tetrapods from the Paleozoic that shared some specific characteristics in the notochord and teeth. Lepospondyls have sometimes been considered to be either related or ancestral to modern amphibians or to Amniota. It has been suggested that the grouping is polyphyletic, with aïstopods being primitive stem-tetrapods, while recumbirostran microsaurs are primitive reptiles.
Reptiliomorpha is a clade containing the amniotes and those tetrapods that share a more recent common ancestor with amniotes than with living amphibians (lissamphibians). It was defined by Michel Laurin (2001) and Vallin and Laurin (2004) as the largest clade that includes Homo sapiens, but not Ascaphus truei. Laurin and Reisz (2020) defined Pan-Amniota as the largest total clade containing Homo sapiens, but not Pipa pipa, Caecilia tentaculata, and Siren lacertina.
Anthracosauria is an order of extinct reptile-like amphibians that flourished during the Carboniferous and early Permian periods, although precisely which species are included depends on one's definition of the taxon. "Anthracosauria" is sometimes used to refer to all tetrapods more closely related to amniotes such as reptiles, mammals, and birds, than to lissamphibians such as frogs and salamanders. An equivalent term to this definition would be Reptiliomorpha. Anthracosauria has also been used to refer to a smaller group of large, crocodilian-like aquatic tetrapods also known as embolomeres.
Diadectomorpha is a clade of large tetrapods that lived in Euramerica during the Carboniferous and Early Permian periods and in Asia during Late Permian (Wuchiapingian), They have typically been classified as advanced reptiliomorphs positioned close to, but outside of the clade Amniota, though some recent research has recovered them as the sister group to the traditional Synapsida within Amniota, based on inner ear anatomy and cladistic analyses. They include both large carnivorous and even larger herbivorous forms, some semi-aquatic and others fully terrestrial. The diadectomorphs seem to have originated during late Mississippian times, although they only became common after the Carboniferous rainforest collapse and flourished during the Late Pennsylvanian and Early Permian periods.
Seymouria is an extinct genus of seymouriamorph from the Early Permian of North America and Europe. Although they were amphibians, Seymouria were well-adapted to life on land, with many reptilian features—so many, in fact, that Seymouria was first thought to be a primitive reptile. It is primarily known from two species, Seymouria baylorensis and Seymouria sanjuanensis. The type species, S. baylorensis, is more robust and specialized, though its fossils have only been found in Texas. On the other hand, Seymouria sanjuanensis is more abundant and widespread. This smaller species is known from multiple well-preserved fossils, including a block of six skeletons found in the Cutler Formation of New Mexico, and a pair of fully grown skeletons from the Tambach Formation of Germany, which were fossilized lying next to each other.
Westlothiana is a genus of reptile-like tetrapod that lived about 338 million years ago during the latest part of the Viséan age of the Carboniferous. Members of the genus bore a superficial resemblance to modern-day lizards. The genus is known from a single species, Westlothiana lizziae. The type specimen was discovered in the East Kirkton Limestone at the East Kirkton Quarry, West Lothian, Scotland in 1984. This specimen was nicknamed "Lizzie the lizard" by fossil hunter Stan Wood, and this name was quickly adopted by other paleontologists and the press. When the specimen was formally named in 1990, it was given the specific name "lizziae" in homage to this nickname. However, despite its similar body shape, Westlothiana is not considered a true lizard. Westlothiana's anatomy contained a mixture of both "labyrinthodont" and reptilian features, and was originally regarded as the oldest known reptile or amniote. However, updated studies have shown that this identification is not entirely accurate. Instead of being one of the first amniotes, Westlothiana was rather a close relative of Amniota. As a result, most paleontologists since the original description place the genus within the group Reptiliomorpha, among other amniote relatives such as diadectomorphs and seymouriamorphs. Later analyses usually place the genus as the earliest diverging member of Lepospondyli, a collection of unusual tetrapods which may be close to amniotes or lissamphibians, or potentially both at the same time.
Casineria is an extinct genus of tetrapod which lived about 340-334 million years ago in the Mississippian epoch of the Carboniferous period. Its generic name, Casineria, is a latinization of Cheese Bay. The site near Edinburgh, Scotland where the holotype fossil was found. When originally described in 1999, it was identified as a transitional fossil noted for its mix of basal (amphibian-like) and advanced (reptile-like) characteristics, putting it at or very near the origin of the amniotes, the group containing all mammals, birds, modern reptiles, and other descendants of their reptile-like common ancestor. However, the sole known fossil is lacking key elements such as a skull, making exact analysis difficult. As a result, the classification of Casineria has been more controversial in analyses conducted since 1999. Other proposed affinities include a placement among the lepospondyls, seymouriamorphs, "gephyrostegids", or as a synonym of Caerorhachis, another controversial tetrapod which may have been an early temnospondyl.
Embolomeri is an order of tetrapods or stem-tetrapods, possibly members of Reptiliomorpha. Embolomeres first evolved in the Early Carboniferous (Mississippian) Period and were the largest and most successful predatory tetrapods of the Late Carboniferous (Pennsylvanian) Period. They were specialized semiaquatic predators with long bodies for eel-like undulatory swimming. Embolomeres are characterized by their vertebral centra, which are formed by two cylindrical segments, the pleurocentrum at the rear and intercentrum at the front. These segments are equal in size. Most other tetrapods have pleurocentra and intercentra which are drastically different in size and shape.
Branchial arches, or gill arches, are a series of bony "loops" present in fish, which support the gills. As gills are the primitive condition of vertebrates, all vertebrate embryos develop pharyngeal arches, though the eventual fate of these arches varies between taxa. In jawed fish, the first arch develops into the jaws. The second gill arch develops into the hyomandibular complex, which supports the back of the jaw and the front of the gill series. The remaining posterior arches support the gills. In amphibians and reptiles, many elements are lost including the gill arches, resulting in only the oral jaws and a hyoid apparatus remaining. In mammals and birds, the hyoid is simplified further.
Limnoscelis was a genus of large diadectomorph tetrapods from the Late Carboniferous to early Permian of western North America. It includes two species: the type species Limnoscelis paludis from New Mexico, and Limnoscelis dynatis from Colorado, both of which are thought to have lived concurrently. No specimens of Limnoscelis are known from outside of North America. Limnoscelis was carnivorous, and likely semiaquatic, though it may have spent a significant portion of its life on land. Limnoscelis had a combination of derived amphibian and primitive reptilian features, and its placement relative to Amniota has significant implications regarding the origins of the first amniotes.
Gephyrostegus is a genus of extinct gephyrostegid reptiliomorph amphibian. It was a small animal at 22 cm snout-vent length, of generally lizard-like build and presumably habit. It had large eyes and a large number of small, pointed teeth, indicating it was an active insectivorous hunter. The remains have been found in Nýřany, Czech Republic, dating from around 310 million years ago.
Solenodonsaurus is an extinct genus of reptiliomorphs that lived in what is now Czech Republic, during the Westphalian stage.
Colosteidae is a family of stegocephalians that lived in the Carboniferous period. They possessed a variety of characteristics from different tetrapod or stem-tetrapod groups, which made them historically difficult to classify. They are now considered to be part of a lineage intermediate between the earliest Devonian terrestrial vertebrates, and the different groups ancestral to all modern tetrapods, such as temnospondyls and reptiliomorphs.
Brachydectes is an extinct genus of lysorophian tetrapods that lived from the Late Carboniferous to the Early Permian. It had a very small head and long body, B. elongatus had 1–2 centimetres (0.39–0.79 in) long skull and pre-sacral length up to 150 centimetres (59 in), while B. newberryi, which have proportionally larger skull than B. elongatus, for specimen with 7.6 millimetres (0.30 in) skull roof had estimated total length of 11 centimetres (4.3 in), while the largest skull exceeds 3 centimetres (1.2 in).
The Carboniferous rainforest collapse (CRC) was a minor extinction event that occurred around 305 million years ago in the Carboniferous period. The event occurred at the end of the Moscovian and continued into the early Kasimovian stages of the Pennsylvanian.
