| Hynerpeton | |
|---|---|
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| The holotype fossil of Hynerpeton, a left shoulder girdle in lateral (external) view | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Clade: | Sarcopterygii |
| Clade: | Tetrapodomorpha |
| Clade: | Stegocephali |
| Genus: | †Hynerpeton |
| Species: | †H. bassetti
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| Binomial name | |
| †Hynerpeton bassetti Daeschler et al., 1994
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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. Conversely, the shoulder girdle is independent of the skull, unlike in most fish.
Early four-limbed vertebrates are often referred to as tetrapods, according to a trait-based definition of the term as commonly used by most paleontologists. Devonian animals like Hynerpeton, Ichthyostega, and Acanthostega are excluded from the crown group Tetrapoda, since they evolved prior to the common ancestor of modern amphibians (Lissamphibia), reptiles, mammals, and birds. For specialists who prefer a neontological definition of tetrapods (i.e., only in the context of modern life), alternative terms for non-crown tetrapods include "stem-tetrapod" or "stegocephalian".
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. Some paleontologists have proposed that animals like Hynerpeton made use their amphibious lifestyle to find shallow pools where they could spawn, isolated from predatory fish which inhabited the deeper rivers.