Hynerpeton

Hynerpeton (/haɪˈnɜːrpətɒn/ lit. 'creeping animal from Hyner') 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.

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 often referred to as tetrapods (using a common trait-based definition of the term), although animals like Hynerpeton, Ichthyostega, and Acanthostega are placed outside the crown group Tetrapoda by paleontologists. From a cladistic (relations-based) point of view, alternative terms include "stem-tetrapod" or "stegocephalian", indicating that they were part of the lineage of animals that would lead to crown-tetrapods such as modern amphibians (Lissamphibia), reptiles, mammals, and birds.

Hynerpeton hails from the Red Hill fossil site, which, during the Late Devonian, was a warm floodplain inhabited by a diverse ecosystem of aquatic fish and terrestrial invertebrates. Hynerpeton was one of several genera of four-limbed vertebrates known from the site, although it was the first to be discovered. It has been theorized that animals like Hynerpeton were able to use their amphibious lifestyle to find shallow pools where they could spawn, isolated from predatory fish which inhabited the deeper rivers.