Elpistostegalia
Encyclopedia
Elpistostegalia or Panderichthyida is an order
of prehistoric lobe-finned fishes which lived during the Late Devonian period (about 385 to 374 million years ago). They represent the advanced tetrapodomorph
stock, the fishes more closely related to tetrapod
s than the osteolepiform
fishes. The elpistostegalians, combining fishlike and tetrapod-like characters, are sometimes called fishapods, a phrase coined for the advanced elpistostegalian Tiktaalik
.
A rise in global oxygen content allowed for the evolution of large, predatory fish that were able to exploit the shallow tidal areas and swamplands as top predators. Such environments would have had periodic oxygen deficiency. In comparable modern aquatic environments like shallow eutrophic lakes and swampland, modern lungfish
and some genera of catfish
also rely on the more stable, atmospheric source of oxygen.
The elpistostegalids gave rise to the labyrinthodonts in the Eifelian (early middle Devonian
) around 395 million years ago. While the early tetrapods flourished and diversified over the next 30 million years, the elpistostegalians disappear from the fossil record fairly quickly in the early Frasnean around 380 million years ago, leaving the labyrinthodonts the sole survivors of their line.
for Elpistostegalia (and thus Tetrapoda):
has become a synonym for Panderichthyida. In most analysis the group is an evolutionary grade
, the last "fishes" of the tetrapod stem line, though Chang and Yu (1997) treat them as the sister clade
to Tetrapoda.
The elpistostegalians encompasses a number of well known transitional fossil
s such as Tiktaalik
, Panderichthys
and Elginerpeton
.
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
of prehistoric lobe-finned fishes which lived during the Late Devonian period (about 385 to 374 million years ago). They represent the advanced tetrapodomorph
Tetrapodomorpha
Tetrapodomorpha is a clade of vertebrates, consisting of tetrapods and their closest sarcopterygian relatives that are more closely related to living tetrapods than to living lungfish...
stock, the fishes more closely related to tetrapod
Tetrapod
Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...
s than the osteolepiform
Osteolepiformes
Osteolepiformes are a group of prehistoric lobe-finned fishes which appears first time during the Devonian period. The order contains five families: Canowindridae, Elpistostegidae, Megalichthyidae, Osteolepidae and Tristichopteridae...
fishes. The elpistostegalians, combining fishlike and tetrapod-like characters, are sometimes called fishapods, a phrase coined for the advanced elpistostegalian Tiktaalik
Tiktaalik
Tiktaalik is a genus of extinct sarcopterygian from the late Devonian period, with many features akin to those of tetrapods . It is an example from several lines of ancient sarcopterygian "fish" developing adaptations to the oxygen-poor shallow-water habitats of its time, which led to the...
.
Palaeobiology of the elpistostegalians
The elpistostegalids were shallow-water fishes. Many of the basic adaptions that later allowed the tetrapods to become terrestrial animals took place in the group. The most important ones were the shift of main propulsion apparatus from the tailfin to the pectoral and pelvic fins, and a shift to reliance on lungs rather than gills as the main means of obtaining oxygen. Both of these appear to be a direct result of moving to an inland freshwater mode of living.A rise in global oxygen content allowed for the evolution of large, predatory fish that were able to exploit the shallow tidal areas and swamplands as top predators. Such environments would have had periodic oxygen deficiency. In comparable modern aquatic environments like shallow eutrophic lakes and swampland, modern lungfish
Lungfish
Lungfish are freshwater fish belonging to the Subclass Dipnoi. Lungfish are best known for retaining characteristics primitive within the Osteichthyes, including the ability to breathe air, and structures primitive within Sarcopterygii, including the presence of lobed fins with a well-developed...
and some genera of catfish
Catfish
Catfishes are a diverse group of ray-finned fish. Named for their prominent barbels, which resemble a cat's whiskers, catfish range in size and behavior from the heaviest and longest, the Mekong giant catfish from Southeast Asia and the second longest, the wels catfish of Eurasia, to detritivores...
also rely on the more stable, atmospheric source of oxygen.
The elpistostegalids gave rise to the labyrinthodonts in the Eifelian (early middle Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
) around 395 million years ago. While the early tetrapods flourished and diversified over the next 30 million years, the elpistostegalians disappear from the fossil record fairly quickly in the early Frasnean around 380 million years ago, leaving the labyrinthodonts the sole survivors of their line.
Traits
Professor Per Ahlberg has identified the following traits as synapomorphicSynapomorphy
In cladistics, a synapomorphy or synapomorphic character is a trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait. A synapomorphy is thus an apomorphy visible in multiple taxa, where the trait in question originates in...
for Elpistostegalia (and thus Tetrapoda):
- The endocraniumEndocraniumFor internal cast of the cranium, see Endocast.The endocranium in comparative anatomy is a part of the skull base in vertebrates and represent the basal, inner part of the cranium. The term is also applied to the outer layer of the dura mater in human anatomy.-Basic structure:Structurally, the...
is hinged, the hinge forming the profundus nerve foramen. The cranial kinesis is also visible in the skull roofSkull roofThe 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, hence the alternative name dermatocranium...
, between the parietal boneParietal boneThe parietal bones are bones in the human skull which, when joined together, form the sides and roof of the cranium. Each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named from the Latin pariet-, wall....
s and the postparietal bones. - A rather small shoulder girdle
- The anal and posterior dorsal fin supported by a basal plate and three unjointed radials.
- The pectoral fin skeleton composed of bones homologous to the tetrapod humerusHumerusThe humerus is a long bone in the arm or forelimb that runs from the shoulder to the elbow....
, ulnaUlnaThe ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form...
and radiusRadius (bone)The radius is one of the two large bones of the forearm, the other being the ulna. It extends from the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna, which exceeds it in length and size. It is a long bone, prism-shaped and slightly curved longitudinally...
, followed by a host of smaller bones anchroing the fin rays, the pelvic fin skeleton similarly has femurFemurThe femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...
, tibiaTibiaThe tibia , shinbone, or shankbone is the larger and stronger of the two bones in the leg below the knee in vertebrates , and connects the knee with the ankle bones....
and fibula.
Phylogeny
The name, originally coined around the genus ElpistostegeElpistostege
Elpistostege is an extinct genus of tetrapod-like fish that lived in the Late Devonian period . Fossils of skull and a part of the backbone have been found at Escuminac Formation in Quebec, Canada.-External links:* *...
has become a synonym for Panderichthyida. In most analysis the group is an evolutionary grade
Evolutionary grade
In alpha taxonomy, a grade refers to a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit.-Definition:...
, the last "fishes" of the tetrapod stem line, though Chang and Yu (1997) treat them as the sister clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...
to Tetrapoda.
The elpistostegalians encompasses a number of well known transitional fossil
Transitional fossil
A transitional fossil is any fossilized remains of a lifeform that exhibits characteristics of two distinct taxonomic groups. A transitional fossil is the fossil of an organism near the branching point where major individual lineages diverge...
s such as Tiktaalik
Tiktaalik
Tiktaalik is a genus of extinct sarcopterygian from the late Devonian period, with many features akin to those of tetrapods . It is an example from several lines of ancient sarcopterygian "fish" developing adaptations to the oxygen-poor shallow-water habitats of its time, which led to the...
, Panderichthys
Panderichthys
Panderichthys is a 90–130 cm long fish from the Devonian period 397 million years ago, of Latvia. It is named after the german-baltic palaeontologist Christian Heinrich Pander. It has a large tetrapod-like head...
and Elginerpeton
Elginerpeton
Elginerpeton is a monotypic genus of early tetrapod, the fossils of which were recovered from Scat Craig, Scotland, in rocks dating to the late Devonian Period...
.