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.