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 groupTetrapoda, 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.
History
In 1993, paleontologistsEdward "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.[1][2] 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 nameHynerpeton 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.[1][3]
The most fossiliferous layer of the Red Hill site, the "Hynerpetonlens", 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.[1] 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.[4] In 2000, a pair of jaw bones were assigned to a second genus, Densignathus,[5] and other studies have argued that several additional unnamed taxa were present at the site, including possibly the oldest known whatcheeriid.[4]
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).[6] In 2000, Daeschler described the mandible (ANSP 20901) in more depth, and compared and contrasted it with the remains of Densignathus.[5] 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.[4]
Description
Highly speculative life restoration by Nobu Tamura
Hynerpeton individuals were presumably similar to other early limbed vertebrates ("tetrapods") 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 endochondralshoulder 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.[1] 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).[7]
Line drawing of the holotype left shoulder girdle in medial (internal) view
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.[1]
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.[1]
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.[1] 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.[8] The original describers suggested that strong muscles could have been equally viable for walking or swimming.[1]
Unlike most of its contemporaries, 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, and that the Hynerpeton lineage may have been among the first vertebrates to evolve this adaptation.[1] However, this interpretation is not without controversy. Janis & Farmer (1999) noted that postbranchial laminae were absent in some eusthenopterid fish (which retained gill bones) and present in Whatcheeria (which had no evidence for gills despite well-preserved remains).[9] Shoch & Witzmann (2011) note that it is not always clear when or how postbranchial laminae are 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 nor possess postbranchial laminae.[10] Daeschler et al. (1994) considered the loss of the postbranchial lamina to be a derived feature indicating that Hynerpeton was more "advanced" than Acanthostega.[1] On the contrary, Schoch & Witzmann (2011) found evidence for a postbranchial lamina in crown-tetrapods such as the temnospondylsTrematolestes and Plagiosuchus. Thus, Hynerpeton's loss of a postbranchial lamina (and possibly internal gills) likely evolved independently of crown-tetrapods.[10]
Classification
The original 1994 description of Hynerpeton tentatively placed it within the orderIchthyostegalia 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.[1] However, the arrival and popularity of cladistics has altered both of these terms. Many paleontologists continue to use the traditional definition of "tetrapod"; a few instead opt for a cladistic definition which restricts the term to the crown group, a clade (relations-based grouping) containing only descendants of the last common ancestor of living tetrapods.[11] While Hynerpeton is a tetrapod in the sense that it is a four-limbed vertebrate, it is not a member of the tetrapod crown group, as its lineage went extinct long before the lineages of modern tetrapods evolved.[12] 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.[13] The traditional, non-cladistic definition of Tetrapoda, which begins at the earliest limbed vertebrates, is roughly equivalent to a clade named Stegocephali, which is defined as all animals more closely related to temnospondyls than to Pandericthys.[14]
Hynerpeton has not been included in many phylogenetic analyses due to its limited amount of fossil material. Those analyses that have included it usually place it as a transitional form on a series of stem-tetrapods leading to crown-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 endochondral shoulder girdle usually means that it is not placed higher than Tulerpeton. The following is a simplified cladogram based on Ruta, Jeffery, & Coates (2003):[12]
The modern day Murray River of Australia, a similar environment to the Catskill Formation during the Devonian
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 Gillespiea.[15]
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. Benthicplacoderms such as the rare Phyllolepis, the common Groenlandaspis, and the abundant Turrisaspis 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 acanthodianGyracanthus. The apex predator of the assemblage was Hyneria, a 3-meter (10 feet) long tetrapodomorph fish.[15]
Hynerpeton was not the only purported tetrapod at the site. A somewhat larger genus, Densignathus, coexisted alongside it.[5][4] In addition, an unusual humerus incompatible with the endochondral shoulder girdle of Hynerpeton may show that a third genus lived in the floodplain.[16] Skull fragments similar to those of whatcheeriids such as Pederpes and Whatcheeria may indicate that a fourth genus was also present,[4] though their referral to whatcheeriids has been questioned.[17] The depositional environment and fauna of the Red Hill site offered new hypotheses for the questions on why and how 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.[15] A modern equivalent would probably be the Murray River of Australia. In this subtropical modern environment experiencing wet and dry seasons, spawninggolden 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.[18]
A tetrapod is any four-limbed vertebrate animal of the superclass Tetrapoda. Tetrapods include all extant and extinct amphibians and amniotes, with the latter in turn evolving into two major clades, the sauropsids and synapsids. Hox gene mutations have resulted in some tetrapods becoming limbless or two-limbed. Nevertheless, these limbless groups still qualify as tetrapods through their ancestry, and some retain a pair of vestigial spurs that are remnants of the hindlimbs.
Sarcopterygii — sometimes considered synonymous with Crossopterygii — is a clade of vertebrate animals which includes a group of bony fish commonly referred to as lobe-finned fish. These vertebrates are characterised by prominent muscular limb buds (lobes) within their fins, which are supported by articulated appendicular skeletons. This is in contrast to the other clade of bony fish, the Actinopterygii, which have only skin-covered bony spines supporting the fins.
Ichthyostega is an extinct genus of limbed tetrapodomorphs from the Late Devonian of what is now Greenland. It was among the earliest four-limbed vertebrates ever in the fossil record and was one of the first with weight-bearing adaptations for terrestrial locomotion. Ichthyostega possessed lungs and limbs that helped it navigate through shallow water in swamps. Although Ichthyostega is often labelled a 'tetrapod' because of its limbs and fingers, it evolved long before true crown group tetrapods and could more accurately be referred to as a stegocephalian or stem tetrapod. Likewise, while undoubtedly of amphibian build and habit, it is not a true member of the group in the narrow sense, as the first modern amphibians appeared in the Triassic Period. Until finds of other early stegocephalians and closely related fishes in the late 20th century, Ichthyostega stood alone as a transitional fossil between fish and tetrapods, combining fish and tetrapod features. Newer research has shown that it had an unusual anatomy, functioning more akin to a seal than a salamander, as previously assumed.
Acanthostega is an extinct genus of stem-tetrapod, among the first vertebrate animals to have recognizable limbs. It appeared in the late Devonian period about 365 million years ago, and was anatomically intermediate between lobe-finned fishes and those that were fully capable of coming onto land.
Eusthenopteron is a genus of prehistoric sarcopterygian fish known from several species that lived during the Late Devonian period, about 385 million years ago. It has attained an iconic status from its close relationship to tetrapods. Early depictions of animals of this genus show them emerging onto land, but paleontologists now think that eusthenopteron species were strictly aquatic animals, though this is not completely known.
Hyneria is a genus of large prehistoric predatory lobe-finned fish which lived in fresh water during the Famennian stage of the Devonian period.
Tiktaalik is a monospecific genus of extinct sarcopterygian from the Late Devonian Period, about 375 Mya, having many features akin to those of tetrapods. Tiktaalik is estimated to have had a total length of 1.25–2.75 metres (4.1–9.0 ft) on the basis of various specimens.
Ventastega is an extinct genus of stem tetrapod that lived during the Upper Fammenian of the Late Devonian, approximately 372.2 to 358.9 million years ago. Only one species is known that belongs in the genus, Ventastega curonica, which was described in 1996 after fossils were discovered in 1933 and mistakenly associated with a fish called Polyplocodus wenjukovi. ‘Curonica’ in the species name refers to Curonia, the Latin name for Kurzeme, a region in western Latvia. Ventastega curonica was discovered in two localities in Latvia, and was the first stem tetrapod described in Latvia along with being only the 4th Devonian tetrapodomorph known at the time of description. Based on the morphology of both cranial and post-cranial elements discovered, Ventastega is more primitive than other Devonian tetrapodomorphs including Acanthostega and Ichthyostega, and helps further understanding of the fish-tetrapod transition.
Tulerpeton is an extinct genus of Devonian four-limbed vertebrate, known from a fossil that was found in the Tula Region of Russia at a site named Andreyevka. This genus and the closely related Acanthostega and Ichthyostega represent the earliest tetrapods.
Whatcheeriidae is an extinct family of stem-tetrapods which lived in the Mississippian sub-period, a subdivision of the Carboniferous period. It contains the genera Pederpes, Whatcheeria, and possibly Ossinodus. Fossils of a possible whatcheeriid have been found from the Red Hill locality of Pennsylvania. If these remains are from a whatcheeriid, they extend the range of the family into the Late Devonian and suggest that advanced tetrapods may have lived alongside primitive tetrapod ancestors like Hynerpeton and Densignathus. They also imply that a very long ghost lineage of whatcheeriids lived through Romer's gap, a period during the Early Carboniferous conspicuously lacking in tetrapod remains.
Edward B. 'Ted' Daeschler is an American vertebrate paleontologist and Associate Curator and Chair of Vertebrate Biology at the Academy of Natural Sciences in Philadelphia. He is a specialist in fish paleontology, especially in the Late Devonian, and in the development of the first limbed vertebrates. He is the discoverer of the transitional fossil tetrapod Hynerpeton bassetti, and a Devonian fish-like specimen of Sauripterus taylori with fingerlike appendages, and was also part of a team of researchers that discovered the transitional fossil Tiktaalik.
Stegocephali is a clade of vertebrate animals containing all fully limbed tetrapodomorphs. It is equivalent to a broad definition of the superclass Tetrapoda: under this broad definition, the term "tetrapod" applies to any animal descended from the first vertebrate with four limbs each with digits in the extremity (pentadactyly), rather than fins of their sarcopterygian relatives.
Elpistostegalia is a clade containing Panderichthys and all more derived tetrapodomorph taxa. The earliest elpistostegalians, combining fishlike and tetrapod-like characters, such as Tiktaalik, are sometimes called fishapods. Although historically Elpistostegalia was considered an order of prehistoric lobe-finned fishes, it was cladistically redefined to include tetrapods.
The skull roof or the roofing bones of the skull are a set of bones covering the brain, eyes and nostrils in bony fishes and all land-living vertebrates. The bones are derived from dermal bone and are part of the dermatocranium.
Ichthyostegalia is an order of extinct amphibians, representing the earliest landliving vertebrates. The group is thus an evolutionary grade rather than a clade. While the group are recognized as having feet rather than fins, most, if not all, had internal gills in adulthood and lived primarily as shallow water fish and spent minimal time on land.
Antlerpeton is an extinct genus of early tetrapod from the Early Carboniferous of Nevada. It is known from a single poorly preserved skeleton from the Diamond Peak Formation of Eureka County. A mix of features in its compound vertebrae suggest that Antlerpeton is a primitive stem tetrapod that has affinities with later, more advanced forms. Its robust pelvis and hind limbs allowed for effective locomotion on land, but the animal was likely still tied to a semiaquatic lifestyle near the coast.
Ymeria is an extinct genus of early stem tetrapod from the Devonian of Greenland. Of the two other genera of stem tetrapods from Greenland, Acanthostega and Ichthyostega, Ymeria is most closely related to Ichthyostega, though the single known specimen is smaller, the skull about 10 cm in length. A single interclavicle resembles that of Ichthyostega, an indication Ymeria may have resembled this genus in the post-cranial skeleton.
Tinirau is an extinct genus of sarcopterygian fish from the Middle Devonian of Nevada. Although it spent its entire life in the ocean, Tinirau is a stem tetrapod close to the ancestry of land-living vertebrates in the crown group Tetrapoda. Relative to more well-known stem tetrapods, Tinirau is more closely related to Tetrapoda than is Eusthenopteron, but farther from Tetrapoda than is Panderichthys. The type and only species of Tinirau is T. clackae, named in 2012.
Innovations conventionally associated with terrestrially first appeared in aquatic elpistostegalians such as Panderichthys rhombolepis, Elpistostege watsoni, and Tiktaalik roseae. Phylogenetic analyses distribute the features that developed along the tetrapod stem and display a stepwise process of character acquisition, rather than abrupt. The complete transition occurred over a period of 30 million years beginning with the tetrapodomorph diversification in the Middle Devonian.
The vertebrate land invasion refers to the transition of vertebrate animals from being aquatic/semiaquatic to predominantly terrestrial during the Late Devonian period. This transition allowed some vertebrates to escape competitive pressure from other aquatic animals and explore niches on land, which eventually established the vertebrates as the dominant terrestrial phylum. Fossils from this period have allowed scientists to identify some of the species that existed during this transition, such as Tiktaalik and Acanthostega. Many of these species were also the first to develop adaptations suited to terrestrial over aquatic life, such as neck mobility, more robust lungs and hindlimb locomotion.
1 2 3 4 5 Daeschler, Edward B.; Clack, Jennifer A.; Shubin, Neil H. (19 May 2009). "Late Devonian tetrapod remains from Red Hill, Pennsylvania, USA: how much diversity?". Acta Zoologica. 90: 306–317. doi:10.1111/j.1463-6395.2008.00361.x. ISSN0001-7272.
1 2 3 Daeschler, Edward B. (March 2000). "Early tetrapod jaws from the Late Devonian of Pennsylvania, USA". Journal of Paleontology. 74 (2): 301–308. doi:10.1017/S0022336000031504. ISSN0022-3360.
↑ Janis, Christine M.; Farmer, Colleen (May 1999). "Proposed habitats of early tetrapods: gills, kidneys, and the water–land transition". Zoological Journal of the Linnean Society. 126 (1): 117–126. doi:10.1006/zjls.1998.0169. ISSN0024-4082.
↑ Laurin, M (1998). "The importance of global parsimony and historical bias in understanding tetrapod evolution. Part I. Systematics, middle ear evolution, and jaw suspension". Annales des Sciences Naturelles, Zoologie, Paris. 13e Série. 19: 1–42. doi:10.1016/S0003-4339(98)80132-9.
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