Hynerpeton

Hynerpeton (/haɪˈnɜːrpətɒn/; from Hyner, Pennsylvania and Ancient Greek ἑρπετόν herpetón, 'creeping animal,' meaning '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 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. 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. 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. Some of this material has been assigned to Hynerpeton, but in many cases, these assignments were reverted. For example, paleontologist Jenny Clack referred several addition fossils to the genus in her 1997 review of Devonian trackways. These fossils, which had not been previously noted in the scientific literature, included a jugal (cheek bone), belly scutes, and a portion of the mandible (lower jaw). In 2000, Daeschler described the mandible (ANSP 20901) in more depth, and compared and contrasted it with the remains of Densignathus. A more comprehensive review of Red Hill 'tetrapod' fossils was undertaken by Daeschler, Clack, and Shubin in 2009. They noted that most fossils were assigned to Hynerpeton based on their close proximity to the point where the original endochondral shoulder girdle was discovered. However, they argued that, since there were other unique animals (i.e. Densignathus, the owner of the unusual humerus, and whatcheerids) close to this point, proximity was not a sufficient reason to consider these referrals valid. Therefore, they did not consider the remains described by Clack (1997) and Daeschler (2000) to be guaranteed examples of Hynerpeton material. Nevertheless, they did retain a referred left cleithrum, ANSP 20054, within the genus due to its structure being practically identical to that of the holotype. 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).