jawed fish
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Gnathostomata are the
jaw The jaw is any opposable articulated structure at the entrance of the mouth In animal anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an ...

jaw
ed
vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic ma ...
s. The term derives from
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
: (') "jaw" + (') "mouth". Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates. In addition to opposing jaws, living gnathostomes have teeth, paired appendages, and a horizontal
semicircular canal The semicircular canals or semicircular ducts are three semicircular, interconnected tubes located in the innermost part of each ear, the inner ear. The three canals are the horizontal, superior and posterior semicircular canals. Structure The sem ...
of the inner ear, along with physiological and cellular anatomical characters such as the
myelin Myelin is a lipid-rich (fatty) substance that surrounds nerve cell axons (the nervous system's "wires") to Insulator (electricity), insulate them and increase the rate at which electrical impulses (called action potentials) are passed along the ax ...
sheaths of
neuron A neuron or nerve cell is an membrane potential#Cell excitability, electrically excitable cell (biology), cell that communicates with other cells via specialized connections called synapses. It is the main component of nervous tissue in all Anima ...

neuron
s. Another is an
adaptive immune system The adaptive immune system, also referred as the acquired immune system, is a subsystem of the immune system The immune system is a network of biological processes that protects an organism In biology, an organism (from Ancient Greek, G ...
that uses
V(D)J recombination V(D)J recombination is the mechanism of somatic recombination that occurs only in developing lymphocyte A lymphocyte is a type of white blood cell in the immune system The immune system is a network of biological processes that protects an ...
to create antigen recognition sites, rather than using genetic recombination in the
variable lymphocyte receptorVariable lymphocyte receptors (VLRs) belong to the Leucine-rich repeat (LRR) family and mediate adaptive immune responses in the jawless vertebrates, lamprey Lampreys (sometimes inaccurately called lamprey eels) are an ancient extant lineag ...
gene. It is now assumed that Gnathostomata evolved from ancestors that already possessed a pair of both pectoral and pelvic fins.New study showing pelvic girdles arose before the origin of movable jaws
/ref> Until recently these ancestors, known as antiarchs, were thought to have lacked pectoral or pelvic fins. In addition to this, some placoderms were shown to have a third pair of paired appendages, that had been modified to claspers in males and basal plates in females—a pattern not seen in any other vertebrate group. The
Osteostraci The class (biology), class Osteostraci ("Bony Shells") is an extinct taxon of bony-armored jawless fish, termed "ostracoderms", that lived in what is now North America, Europe and Russia from the Wenlock epoch, Middle Silurian to Late Devonian. A ...

Osteostraci
are generally considered the
sister taxon In phylogenetics In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiolo ...
of Gnathostomata. Jaw development in vertebrates is likely a product of the supporting gill arches. This development would help push water into the mouth by the movement of the jaw, so that it would pass over the gills for gas exchange. The repetitive use of the newly formed jaw bones would eventually lead to the ability to bite in some gnathostomes. Newer research suggests that a branch of Placoderms was most likely the ancestor of present-day gnathostomes. A 419-million-year-old fossil of a placoderm named ''
Entelognathus ''Entelognathus primordialis'' (“primordial complete jaw”) is a placoderm from the late Silurian The Silurian ( ) is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at million years ago ( ...

Entelognathus
'' had a bony skeleton and anatomical details associated with cartilaginous and bony fish, demonstrating that the absence of a bony skeleton in Chondrichthyes is a derived trait. The fossil findings of primitive bony fishes such as ''
Guiyu oneiros ''Guiyu oneiros'' is the earliest articulated bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse Taxonomy (biology), taxonomic group of fish that have skeletons primarily composed of bone tissue. They can be contr ...

Guiyu oneiros
'' and ''
Psarolepis ''Psarolepis'' (; ''psārolepis'', from Greek ψαρός 'speckled' and λεπίς 'scale') is a genus of extinct lobe-finned fish which lived around 397 to 418 million years ago (Pridoli epoch, Pridoli to Lochkovian faunal stage, stages). Fossil ...
'', which lived contemporaneously with ''Entelognathus'' and had pelvic girdles more in common with placoderms than with other bony fish, show that it was a relative rather than a direct ancestor of the extant gnathostomes. It also indicates that spiny sharks and Chondrichthyes represent a single sister group to the bony fishes. Fossil findings of juvenile placoderms, which had true teeth that grew on the surface of the jawbone and had no roots, making them impossible to replace or regrow as they broke or wore down as they grew older, proves the common ancestor of all gnathostomes had teeth and place the origin of teeth along with, or soon after, the evolution of jaws. Late
Ordovician The Ordovician ( ) is a geologic period A geological period is one of the several subdivisions of geologic time enabling cross-referencing of rocks and geologic events from place to place. These periods form elements of a hierarchy of divisions i ...

Ordovician
-aged microfossils of what have been identified as scales of either
acanthodian Acanthodii or acanthodians is an extinct class of Gnathostomata, gnathostomes (jawed fish, fishes), typically considered a paraphyletic group. They are currently considered to represent a Evolutionary grade, grade of various fish lineages leading ...
s or "shark-like fishes", may mark Gnathostomata's first appearance in the fossil record. Undeniably unambiguous gnathostome fossils, mostly of primitive acanthodians, begin appearing by the Llandovery epoch, early Silurian, and become abundant by the start of the Devonian.


Classification

The group, Gnathostomata, is traditionally a phylum (biology), infraphylum, broken into three top-level groupings: Chondrichthyes, or the cartilaginous fish; Placodermi, an extinct clade of armored fish; and Teleostomi, which includes the familiar classes of bony fish, birds, mammals, reptiles, and amphibians. Some classification systems have used the term Amphirhina. It is a sister group of the jawless craniates Agnatha.


References


External links


Tree of Life discussion of Gnathostomata


{{Vertebrate-stub Gnathostomata, Vertebrate taxonomy Darriwilian first appearances Extant Ordovician first appearances Infraphyla