|The holotype left shoulder (ANSP 20053) in medial (inside) view|
Daeschler et al., 1994
Hynerpeton (//; from Hyner and Ancient Greek ἑρπετόν (herpetón), "creeping animal," meaning "creeping animal from Hyner") is an extinct genus of early four-limbed vertebrates that lived in the rivers and ponds of Pennsylvania during the Late Devonian period, around 365 to 363 million years ago. They are sometimes referred to as tetrapods, although the genus is placed outside the crown group Tetrapoda by the vast majority of paleontologists. A more accurate term would be "stem-tetrapod" or "stegocephalian", indicating that it was part of the lineage of animals that would lead to true tetrapods such as modern amphibians (Lissamphibia), reptiles, mammals, and birds. The only known species of Hynerpeton, Hynerpeton bassetti, is named in honor of Edward M. Bassett, an American city planner and the grandfather of Hynerpeton's discoverer, Ted Daeschler.
This genus is known from few remains, the most notable being a large shoulder girdle. The structure of this shoulder bone indicates that members of this genus had a unique set of powerful muscles around the chest. This may have given Hynerpeton improved mobility and weight-bearing abilities compared to other Devonian stem-tetrapods such as Icthyostega and Acanthostega. Hynerpeton is also 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.
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 shoulder bone, belonging to an animal which had powerful appendages. This fossil, designated ANSP 20053, would come to be known as the holotype specimen of Hynerpeton, which Daeschler and his colleagues formally named in an article published by Science Magazine in 1994.
The most fossiliferous layer of the Red Hill site, the Hynerpeton lens, was named after the genus. 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 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 the new material has been assigned to Hynerpeton, but in many cases these assignments were reverted. For example, Jenny Clack mentioned a jugal (cheek bone), scutes, and a jaw fragment as belonging to Hynerpeton in her 1997 review of Devonian trackways. The jaw fragment, ANSP 20901, was described and listed as a Hynerpeton fossil by Daeschler in 2000.
A more comprehensive review of Red Hill "tetrapod" fossils was undertaken by Daeschler, Clack, & Shubin in 2009. They noted that most fossils were assigned to Hynerpeton based on their close proximity to the point where the original shoulder bone was discovered. However, they argued that, since there were other unique animals close to this point, proximity was not a sufficient reason to consider these referrals valid. 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 their anatomy. Nevertheless, the structure of the preserved shoulder bone does offer some information on its classification. As a whole, the bone is massive and cleaver-shaped, with the upward-pointing "shaft" formed by the cleithrum and the rear-pointing "blade" formed by the scapulocoracoid, which also possesses the shoulder socket. In lobe-finned fish ancestral to tetrapods, such as Eusthenopteron, this bone was attached to the skull. In true tetrapods, the bone is divided into the two separate bones: the cleithrum and scapulocoracoid. Hynerpeton is intermediate between these two states, as the shoulder bone 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 .7 meters (2.3 feet).
The cleithrum portion is smooth, rather than rough-textured as in 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 shoulder socket, 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 shoulder bone. 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 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 members of the genus lacked gills. However, Shoch & Witzmann (2011) note that it is not always clear when or how this structure is preserved due to the divergent morphology of many stegocephalian cleithra. In addition, they point out that aquatic salamanders, which breathe with external gills, do not require or possess postbranchial laminae. Daeschler et al. (1994) considered the loss of the postbranchial lamina to be a derived feature indicating that Hynerpeton was more "advanced" than Acanthostega. On the contrary, Schoch & Witzmann (2011) found evidence for a postbranchial lamina in true tetrapods such as Trematolestes and Plagiosuchus. Thus, Hynerpeton's loss of a postbranchial lamina (and presumably internal gills) likely evolved independently of 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.
Hynerpeton has not been featured 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 shoulder bone 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 Gilleaspiea.
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.
Hynerpeton was not the only purported four-limbed vertebrate at the site. A somewhat larger genus, Densignathus, coexisted alongside it. In addition, an unusual humerus incompatible with the shoulder bone of Hynerpeton may show that a third genus lived in the floodplain. Skull fragments similar to those of whatcheeriids such as Pederpes and Whatcheeria indicate that a potential fourth genus was also present. The Red Hill site is important for finding the answer to the question of why terrestriality evolved in stem-tetrapods. The Catskill floodplain never became dry enough for its waterways to completely dry up, but at certain times of the year shallow ponds became isolated from the main river channels. Terrestrial or semiaquatic animals could have used these ponds as refuge from the larger predatory fish which patrolled the deeper waterways. A modern equivalent would probably be the Murray River of Australia. In this subtropical modern environment experiencing wet and dry seasons, spawning golden perch (Macquaria ambigua) take refuge in oxbow lakes to escape larger, faster murray cod (Maccullochella peeli) in the main river channel. In a Devonian environment, vertebrates with terrestrial capabilities may have had the advantage when navigating between these different environments. The flexibility imparted by such a lifestyle could also have let them take advantage of a larger variety of food sources.
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