page_title: Hynerpeton
section_title: Description
hierarchical_section_title: Hynerpeton / Description
caption_attribution_description: Hynerpeton bassetti, a basal tetrapoda from the Late Devonian of North America, pencil drawing
context_page_description: 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, 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.
context_section_description: 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 Tulerpeton and true tetrapods. The scapulocoracoid region is large when seen from the side but very thin when seen from below. The glenoid fossa (shoulder socket) is positioned on the posterolateral (outer and rear) edge of the scapulocoracoid, a position significantly more lateral than other Devonian tetrapods (apart from Tulerpeton). Above the glenoid fossa is a raised area known as the supraglenoid buttress.
  Hynerpeton also has several autapomorphies, unique features which no other known stem-tetrapod possesses. The inner face of the scapulocoracoid has a large, deep depression known as a subscapular fossa. The upper rim of this depression is very roughly textured due to being covered with muscle scars. The rear edge of the subscapular fossa, on the other hand, is formed by a massive raised area, known as an infraglenoid buttress. A second depression known as the infraglenoid fossa, which is continuous with the glenoid fossa, wraps around the rear of the bone to bisect the infraglenoid buttress.
  These characteristics combined seem to support the idea that Hynerpeton had very powerful muscles attached to the scapulocoracoid portion of the endochondral shoulder girdle. The infraglenoid fossa is particularly well-developed in this genus, and may have been an origin point for retractor muscles. A similar groove on the front edge of the bone may have helped with elevating or protracting the limb. The rim of the subscapular fossa also evidently provided a site for muscle attachment. As these features are unknown in other stem- and crown-tetrapods, it is likely that Hynerpeton's musculature was used for some unique, experimental form of movement that did not survive the Devonian. The original describers suggested that strong muscles could have been equally viable for walking or swimming.
  Unusually, Hynerpeton seems to lack a postbranchial lamina. This blade of bone, preserved in many fish and some stem-tetrapods (Acanthostega, for example) extends lengthwise along the inner edge of the cleithrum. It typically forms the rear wall of the branchial chamber (gill cavity), and may have helped ensure that water flows in a single direction through the gills. Its absence in Hynerpeton may indicate that it lacked gills, a
caption: Highly speculative life restoration by Nobu Tamura
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