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The angular is a large
bone A bone is a rigid tissue Tissue may refer to: Biology * Tissue (biology), an ensemble of similar cells that together carry out a specific function * ''Triphosa haesitata'', a species of geometer moth found in North America * ''Triphosa dubit ...

bone
in the
lower jaw In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any ind ...

lower jaw
(mandible) of
amphibians Amphibians are ectotherm File:Junonia lemonias DSF by Kadavoor.JPG, ''Junonia lemonias'' is basking under the sun. An ectotherm (from the Ancient Greek, Greek ἐκτός (''ektós'') "outside" and θερμός (''thermós'') "hot") is an ...

amphibians
and
reptiles Reptiles, as most commonly defined, are the animals in the class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or ...

reptiles
(
birds Birds are a group of warm-blooded vertebrate Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With ...

birds
included), which is connected to all other lower jaw bones: the
dentary In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any ind ...

dentary
(which is the entire lower jaw in
mammals Mammals (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the R ...
), the
splenialThe splenial is a small bone A bone is a Stiffness, rigid tissue (anatomy), tissue that constitutes part of the vertebrate skeleton in animals. Bones protect the various organs of the body, produce red blood cell, red and white blood cells, sto ...
, the suprangular, and the
articular The articular bone is part of the lower jaw of most vertebrates Vertebrates () comprise all species of animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Anima ...
. It is homologous to the tympanic bone in mammals, due to the
incorporation of several jaw bones into the mammalian middle ear
incorporation of several jaw bones into the mammalian middle ear
early in mammal evolution. In
therapsids Therapsida is a major group of eupelycosauria Eupelycosauria is a large clade A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic—t ...

therapsids
(mammal ancestors and their kin), the lower jaw is made up of the
dentary In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any ind ...

dentary
(the
mandible In anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science which deals with the structural organization of li ...

mandible
in mammals) and a group of smaller "postdentary" bones near the jaw joint. As the dentary increased in size over million of years, two of these postdentary bones, the articular and angular, became increasingly reduced and the dentary eventually made direct contact with the upper jaw. These postdentary bones, even before their articular function was lost, probably transmitted sound vibrations to the stapes and, in some therapsids, a bent plate that might have supported a membrane capable of detecting vibrations developed on the angular. Eventually, it developed into the
ectotympanic The ectotympanic, or tympanicum, is a bony ring in the tympanic part of the temporal bone The tympanic part of the temporal bone The temporal bones are situated at the sides and base of the skull, and lateral to the temporal lobes of the cerebral ...
ring which supports the
tympanic membrane In the anatomy Anatomy (Greek ''anatomē'', 'dissection') is the branch of concerned with the study of the structure of s and their parts. Anatomy is a branch of natural science which deals with the structural organization of living thing ...

tympanic membrane
in the ears of modern mammals.


References

Vertebrate anatomy {{Musculoskeletal-stub