Hynerpeton bassetti

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 gills.

This genus is known from few remains discovered at the Red Hill fossil site in Hyner, Pennsylvania. The most notable fossil is a large endochondral shoulder girdle consisting of the cleithrum, scapula, and coracoid (but not the interclavicle and clavicles), all connected into one shoulder bone. The inner surface of this shoulder bone possesses an array of depressions believed to have been attachment points for a unique set of powerful muscles around the chest. This may have given Hynerpeton improved mobility and weight-bearing abilities compared to other Devonian limbed vertebrates such as Ichthyostega and Acanthostega. The cleithrum (upper blade of the shoulder) is fused to the scapulocoracoid (lower plate of the shoulder, in front of the shoulder socket), unlike in most tetrapods, but the shoulder girdle is independent of the skull, unlike in most fish.

Early four-limbed vertebrates are sometimes referred to as tetrapods (using a trait-based definition of the term), although animals like Hynerpeton, Ichthyostega, and Acanthostega are placed outside the crown group Tetrapoda by the vast majority of paleontologists. From a cladistic (relations-based) point of view, a more accurate term would be "stem-tetrapod" or "stegocephalian", indicating that they were part of the lineage of animals that would lead to true tetrapods such as modern amphibians (Lissamphibia), reptiles, mammals, and birds.

In 1993, paleontologists Edward "Ted" Daeschler and Neil Shubin found the first Hynerpeton fossil at the Red Hill fossil site near Hyner, Pennsylvania, USA. They were surveying the Devonian rocks of Pennsylvania in search of fossil evidence for the origin of limbed vertebrates. This initial find was a robust left endochondral shoulder girdle, belonging to an animal which had powerful appendages. This fossil, designated ANSP 20053, is now considered the holotype specimen of Hynerpeton, which Daeschler and his colleagues formally named in an article published by Science Magazine in 1994. At the time of its discovery, Hynerpeton was the oldest four-limbed vertebrate known from the United States, and its presence in a complex ecosystem such as that preserved at Red Hill helped to answer some of Daeschler and Shubin's questions on the origin and lifestyle of limbed vertebrates. The generic name Hynerpeton is in reference to Hyner and herpeton ("creeping animal"), a greek word which is commonly used as a suffix for newly named ancient amphibians. The specific name, bassetti, is named in honor of Edward M. Bassett, an American city planner and Daeschler's grandfather.

The most fossiliferous layer of the Red Hill site, the "Hynerpeton lens", was named after the genus. It is believed to have been deposited during the Middle to Upper Famennian stage, about 365 to 363 million years ago. Since 1993, more stegocephalian remains have been found in the Hynerpeton lens (also known as the Farwell paleosols). These include shoulder bones, jaw bones, skull fragments, gastralia (belly scutes), a femur, and a large and unusually-shaped humerus. In 2000, a pair of jaw bones were assigned to a second genus, Densignathus, and other studies have argued that several additional unnamed taxa were present at the site, including possibly the oldest known whatcheeriid.

Hynerpeton individuals were presumably similar to other early limbed vertebrates such as Ichthyostega or Acanthostega. Although a lack of sufficient fossil material makes it unwise to come to specific conclusions about the anatomy of Hynerpeton, the structure of the preserved endochondral shoulder girdle offers some information on its classification. The endochondral shoulder girdle is the portion of the shoulder girdle containing the scapula, coracoid, and cleithrum, but not the clavicles and interclavicle. As a whole, the endochondral shoulder girdle is massive and cleaver-shaped. The upward-pointing "shaft" is formed by the cleithrum, a blade-like bone of the shoulder girdle lost by most amniotes. The rear-pointing "blade" is formed by the scapulocoracoid, a plate-like bone which also possesses the glenoid fossa (shoulder socket) along its rear edge and in later tetrapods would separate into the scapula and coracoid. In lobe-finned fish ancestral to tetrapods, such as Eusthenopteron, the endochondral shoulder girdle was attached to the skull. In true tetrapods, the endochondral shoulder girdle is divided into two separate bones: the cleithrum and scapulocoracoid. Hynerpeton is intermediate between these two states, as the endochondral shoulder girdle is separated from the skull but not yet divided into two separate bones. In this way Hynerpeton is comparable to Devonian stem-tetrapods rather than true tetrapods, which did not appear in the fossil record until the Carboniferous. Based on the size of the bone, the Hynerpeton individual to which it belonged had an estimated length of 0.7 meters (2.3 feet).

The cleithrum portion is smooth, unlike the rough-textured cleithrum of tetrapodomorph fish. In addition, the upper portion of the cleithrum is expanded and slightly tilted forwards, a derived character similar to Tulerpetonand true tetrapods.

The original 1994 description of Hynerpeton tentatively placed it within the order Ichthyostegalia of the superclass Tetrapoda. At the time, "Tetrapoda" referred to any four-limbed vertebrate and "Ichthyostegalia" referred to "primitive", Ichthyostega-like Devonian members of the category. However, the arrival and popularity of cladistics has altered both of these terms. The cladistic definition of "tetrapod" now popular among biologists and paleontologists refers to a clade (relations-based grouping) containing only descendants of the last common ancestor of living tetrapods. While Hynerpeton is a tetrapod in the sense that it is a four-limbed vertebrate, it is not a member of the clade Tetrapoda because its lineage went extinct long before the lineages of modern tetrapods evolved. Likewise, "Ichthyostegalia" has been abandoned in the age of cladistics due to being an evolutionary grade leading to true tetrapods, rather than a relations-based clade. The traditional, non-cladistic definition of Tetrapoda, which begins at the earliest limbed vertebrates, corresponds closely to a clade named Stegocephalia, which is defined as all animals more closely related to temnospondyls than to Pandericthys.

Hynerpeton has not been included in many phylogenetic analyses, but those that have included it usually place it as a transitional form on a series of stem-tetrapods leading to Tetrapoda. The shape of the cleithrum and the loss of the postbranchial lamina allow it to be placed higher than Acanthostega (and usually Ichthyostega as well), but the retention of a single-piece endochondral shoulder girdle usually means that it is not placed higher than Tulerpeton. The following is a simplified cladogram based on Ruta, Jeffery, & Coates (2003):

Hynerpeton was found at the Red Hill site of Pennsylvania. This roadcut preserved fossils from the Duncannon member of the Catskill Formation, which was laid down in an ancient coastal floodplain. During the late Devonian, the floodplain was close to the equator, so the climate was warm and humid, with a dry and wet season. It formed along the coast of a shallow sea which bisected the continent Euramerica, and was dominated by several small, slow rivers flowing down from the Acadian mountains in the eastern part of the continent. These rivers were prone to changing their course dramatically, creating oxbow lakes and ponds adjacent to the main river channels. The most abundant plants were forests of ancient broadleaf trees (Archaeopteris) supplemented with marshes filled with fern-like plants (Rhacophyton). Wildfires were common during the dry season, as evidenced by the large amount of charred Rhacophyton material. Other plants include lycopsids such as Lepidodendropsis and Otzinachsonia, as well as difficult-to-place herbs and shrubs such as Barinophyton and Gillespiea.

The animal life of Red Hill was also quite diverse. Early arachnids (Gigantocharinus), millipedes (Orsadesmus), and undescribed scorpions were among the few fully terrestrial members of the Red Hill fauna. The waterways were inhabited by a large variety of fish. Benthic placoderms such as the rare Phyllolepis, the common Groenlandaspis, and the abundant Turriaspis were a large part of the fish assemblage. The early ray-finned fish Limnomis was also abundant, likely forming large schools. Early sharks were present, including the minuscule Ageleodus and the spine-finned Ctenacanthus. Various lobe-finned fish populated the floodplain, as well as the large acanthodian Gyracanthus. The apex predator of the assemblage was Hyneria, a 3 meter (10 feet) long tetrapodomorph fish.