Reichert–Gaupp Theory
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The evolution of mammalian auditory ossicles was an
evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...
ary process that resulted in the formation of the
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
ian
middle ear The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear). The mammalian middle ear contains three ossicles (malleus, incus, and stapes), which transfer the vibrations ...
, where the three middle ear bones or
ossicles The ossicles (also called auditory ossicles) are three irregular bones in the middle ear of humans and other mammals, and are among the smallest bones in the human body. Although the term "ossicle" literally means "tiny bone" (from Latin ''ossi ...
, namely the
incus The ''incus'' (: incudes) or anvil in the ear is one of three small bones (ossicles) in the middle ear. The incus receives vibrations from the malleus, to which it is connected laterally, and transmits these to the stapes medially. The incus i ...
,
malleus The ''malleus'', or hammer, is a hammer-shaped small bone or ossicle of the middle ear. It connects with the incus, and is attached to the inner surface of the eardrum. The word is Latin for 'hammer' or 'mallet'. It transmits the sound vibra ...
and
stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other tetrapods which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the f ...
(a.k.a. "the anvil, hammer, and stirrup"), are a defining characteristic of mammals. The event is well-documented and important academically as a demonstration of transitional forms and
exaptation Exaptation or co-option is a shift in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another. Exaptations are common in both anatomy and be ...
, the re-purposing of existing structures during evolution. The ossicles evolved from skull bones present in most
tetrapod A tetrapod (; from Ancient Greek :wiktionary:τετρα-#Ancient Greek, τετρα- ''(tetra-)'' 'four' and :wiktionary:πούς#Ancient Greek, πούς ''(poús)'' 'foot') is any four-Limb (anatomy), limbed vertebrate animal of the clade Tetr ...
s, including
amphibian Amphibians are ectothermic, anamniote, anamniotic, tetrapod, four-limbed vertebrate animals that constitute the class (biology), class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all Tetrapod, tetrapods, but excl ...
s,
sauropsid Sauropsida ( Greek for "lizard faces") is a clade of amniotes, broadly equivalent to the class Reptilia, though typically used in a broader sense to also include extinct stem-group relatives of modern reptiles and birds (which, as theropod dino ...
s (which include
extant Extant or Least-concern species, least concern is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Exta ...
reptile Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...
s and
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s) and early
synapsid Synapsida is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant rept ...
s (which include ancestors of mammals). The reptilian quadrate,
articular The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals. Anatomy In most vertebrates, the articular bone is connected to two o ...
and
columella Lucius Junius Moderatus Columella (, Arabic: ) was a prominent Roman writer on agriculture in the Roman Empire. His in twelve volumes has been completely preserved and forms an important source on Roman agriculture and ancient Roman cuisin ...
bones are
homolog In biology, homology is similarity in anatomical structures or genes between organisms of different taxa due to shared ancestry, ''regardless'' of current functional differences. Evolutionary biology explains homologous structures as retained her ...
s of the mammalian incus, malleus and stapes, respectively. In reptiles (and early synapsids by association), the
eardrum In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear from the middle ear. Its function is to transmit changes in pres ...
is connected to the
inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
via a single bone, the columella, while the upper and lower jaws contain several bones not found in modern mammals. Over the course of
mammalian evolution The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian (geology), Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synaps ...
, one bone from the upper jaw (the quadrate) and one from the lower jaw (the articular) lost their function in the jaw articulation and migrated to form the middle ear. The shortened columella connected to these bones to form a
kinematic chain In mechanical engineering, a kinematic chain is an assembly of rigid bodies connected by joints to provide constrained motion that is the mathematical model for a mechanical system. Reuleaux, F., 187''The Kinematics of Machinery,''(trans. an ...
of three ossicles, which serve to amplify air-sourced fine vibrations transmitted from the eardrum and facilitate more acute
hearing Hearing, or auditory perception, is the ability to perceive sounds through an organ, such as an ear, by detecting vibrations as periodic changes in the pressure of a surrounding medium. The academic field concerned with hearing is auditory sci ...
in terrestrial environments.


History

Following on the ideas of
Étienne Geoffroy Saint-Hilaire Étienne Geoffroy Saint-Hilaire (; 15 April 177219 June 1844) was a French naturalist who established the principle of "unity of composition". He was a colleague of Jean-Baptiste Lamarck and expanded and defended Lamarck's evolutionary theorie ...
(1818), and studies by
Johann Friedrich Meckel Johann Friedrich Meckel (17 October 1781 – 31 October 1833), often referred to as Johann Friedrich Meckel, the Younger, was a German anatomist born in Halle. He worked as a professor of anatomy, pathology and zoology at the University of Halle, ...
the Younger (1820),
Carl Gustav Carus Carl Gustav Carus (3 January 1789 – 28 July 1869) was a German physiologist and painter, born in Leipzig, who played various roles during the Romanticism, Romantic era. A friend of the writer Johann Wolfgang Goethe, he was a many-sided man: a ...
(1818),
Martin Rathke Martin Heinrich Rathke (25 August 1793, Danzig – 3 September 1860, Königsberg) was a German embryologist and anatomist. Along with Karl Ernst von Baer and Christian Heinrich Pander, he is recognized as one of the founders of modern embryology ...
(1825), and
Karl Ernst von Baer Karl Ernst Ritter von Baer Edler von Huthorn (; – ) was a Baltic German scientist and explorer. Baer was a naturalist, biologist, geologist, meteorologist, geographer, and is considered a, or the, founding father of embryology. He was a m ...
(1828), the relationship between the reptilian jaw bones and mammalian middle-ear bones was first established on the basis of
embryology Embryology (from Ancient Greek, Greek ἔμβρυον, ''embryon'', "the unborn, embryo"; and -λογία, ''-logy, -logia'') is the branch of animal biology that studies the Prenatal development (biology), prenatal development of gametes (sex ...
and
comparative anatomy Comparative anatomy is the study of similarities and differences in the anatomy of different species. It is closely related to evolutionary biology and phylogeny (the evolution of species). The science began in the classical era, continuing in t ...
by
Karl Bogislaus Reichert Karl Bogislaus Reichert (20 December 1811 – 21 December 1883) was a German anatomist, embryologist and histologist. Biography Reichert was born in Rastenburg (Kętrzyn), East Prussia. From 1831, he studied at the University of Konigsberg, ...
(in 1837, before the publication of ''
On the Origin of Species ''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life'')The book's full original title was ''On the Origin of Species by M ...
'' in 1859). These ideas were advanced by
Ernst Gaupp Ernst Wilhelm Theodor Gaupp (13 July 1865 – 23 November 1916) was a German anatomist from Beuthen in Upper Silesia (today Bytom, Poland). He studied natural sciences and medicine in Jena, Königsberg and Breslau, where he received his doctora ...
, and are now known as the ''Reichert–Gaupp theory''. The discovery of the link in homology between the reptilian jaw joint and mammalian malleus and incus is considered an important milestone in the history of comparative anatomy. Novacek references these early works: – – Work on extinct theromorphs by
Owen Owen may refer to: People and fictional characters * Owen (name), including a list of people and fictional characters with the given name or surname Places United States * Owen, Missouri, a ghost town * Owen, Wisconsin * Owen County, Indiana ...
(1845), and continued by
Seeley Seeley may refer to: __NOTOC__ People and fictional characters * Seeley (surname) * Seeley Booth, a fictional character in the American television series ''Bones'' * Seeley G. Mudd (1895-1968), American physician, professor and philanthropist * See ...
,
Broom A broom (also known as a broomstick) is a cleaning tool, consisting of usually stiff fibers (often made of materials such as plastic, hair, or corn husks) attached to, and roughly parallel to, a cylindrical handle, the broomstick. It is thus a ...
, and Watson, was pivotal in discovering the intermediate steps to this change. The transition between the "reptilian" jaw and the "mammalian" middle ear was not bridged in the ''fossil'' record until the 1950s with the elaboration of such fossils as the now-famous ''
Morganucodon ''Morganucodon'' ("Glamorgan tooth") is an early mammaliaform genus that lived from the Late Triassic to the Middle Jurassic. It first appeared about 205 million years ago. Unlike many other early mammaliaforms, ''Morganucodon'' is well represent ...
''. During embryonic development, the incus and malleus arise from the same
first pharyngeal arch The pharyngeal arches, also known as visceral arches'','' are transient structures seen in the embryonic development of humans and other vertebrates, that are recognisable precursors for many structures. In fish, the arches support the gills a ...
as the
mandible In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone i ...
and
maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...
, and are served by mandibular and maxillary division of the
trigeminal nerve In neuroanatomy, the trigeminal nerve (literal translation, lit. ''triplet'' nerve), also known as the fifth cranial nerve, cranial nerve V, or simply CN V, is a cranial nerve responsible for Sense, sensation in the face and motor functions ...
. Recent genetic studies are able to relate the development of the ossicles from the embryonic arch to hypothesized evolutionary history. ''Bapx1'', also known as '' Nkx3.2'' (a member of the NK2 class of
homeobox A homeobox is a Nucleic acid sequence, DNA sequence, around 180 base pairs long, that regulates large-scale anatomical features in the early stages of embryonic development. Mutations in a homeobox may change large-scale anatomical features of ...
genes), is implicated in the change from the jaw bones of non-mammals to the ossicles of mammals. Other implicated genes include the ''Dlx'' genes, ''Prx'' genes, and ''Wnt'' genes. Image:Mammal middle ear.png, 251px, A typical mammalian middle ear: sound makes the tympanum (eardrum) vibrate; 3 small bones, the malleus, incus and stapes, transmit the vibrations to the labyrinth (inner ear), which transforms the vibrations into nerve signals. rect 30 14 118 28
Tensor tympani The tensor tympani is a muscle within the middle ear, located in the bony canal above the bony part of the auditory tube, and connects to the malleus bone. Its role is to dampen loud sounds, such as those produced from chewing, shouting, or thun ...
rect 146 48 181 60
Incus The ''incus'' (: incudes) or anvil in the ear is one of three small bones (ossicles) in the middle ear. The incus receives vibrations from the malleus, to which it is connected laterally, and transmits these to the stapes medially. The incus i ...
rect 169 65 227 77
Stapedius The stapedius is the smallest skeletal muscle in the human body. At just over one millimeter in length, its purpose is to stabilize the smallest bone in the body, the stapes or stirrup bone of the middle ear. Structure The stapedius emerges from ...
rect 177 100 231 113
Labyrinth In Greek mythology, the Labyrinth () is an elaborate, confusing structure designed and built by the legendary artificer Daedalus for King Minos of Crete at Knossos. Its function was to hold the Minotaur, the monster eventually killed by the h ...
rect 164 116 205 130
Stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other tetrapods which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the f ...
rect 154 136 242 151
Tympanic cavity The tympanic cavity is a small cavity surrounding the bones of the middle ear. Within it sit the ossicles, three small bones that transmit vibrations used in the detection of sound. Structure On its lateral surface, it abuts the external audit ...
rect 165 165 226 194
Eustachian Tube The Eustachian tube (), also called the auditory tube or pharyngotympanic tube, is a tube that links the nasopharynx to the middle ear, of which it is also a part. In adult humans, the Eustachian tube is approximately long and in diameter. It ...
rect 18 164 132 194
Ear drum In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear from the middle ear. Its function is to transmit changes in pressur ...
rect 5 138 87 151
Ear canal The ear canal (external acoustic meatus, external auditory meatus, EAM) is a pathway running from the outer ear to the middle ear. The adult human ear canal extends from the auricle to the eardrum and is about in length and in diameter. S ...
rect 28 56 74 71
Malleus The ''malleus'', or hammer, is a hammer-shaped small bone or ossicle of the middle ear. It connects with the incus, and is attached to the inner surface of the eardrum. The word is Latin for 'hammer' or 'mallet'. It transmits the sound vibra ...
# desc top-right


Defining characteristic of mammals

Living mammal species can be identified by the presence in females of
mammary gland A mammary gland is an exocrine gland that produces milk in humans and other mammals. Mammals get their name from the Latin word ''mamma'', "breast". The mammary glands are arranged in organs such as the breasts in primates (for example, human ...
s which produce milk. Other features are required when classifying
fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
, since mammary glands and other soft-tissue features are not visible in fossils.
Paleontologists Paleontology, also spelled as palaeontology or palæontology, is the scientific study of the life of the past, mainly but not exclusively through the study of fossils. Paleontologists use fossils as a means to classify organisms, measure geolo ...
therefore use the ossicles as distinguishing bony features shared by all living mammals (including
monotremes Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...
), but not present in any of the early
Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...
therapsids ("
mammal-like reptiles Synapsida is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant rept ...
"). Early amniotes had a jaw joint composed of the
articular The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals. Anatomy In most vertebrates, the articular bone is connected to two o ...
(a small bone at the back of the lower jaw) and the quadrate (a small bone at the back of the upper jaw). All non-mammalian amniotes use this system including
lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...
s,
crocodilia Crocodilia () is an order of semiaquatic, predatory reptiles that are known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorph pseudosuchia ...
ns,
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
s (and their descendants the
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s) and
therapsids Therapsida is a clade comprising a major group of eupelycosaurian synapsids that includes mammals and their ancestors and close relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including li ...
; so the only
ossicle The ossicles (also called auditory ossicles) are three irregular bones in the middle ear of humans and other mammals, and are among the smallest bones in the human body. Although the term "ossicle" literally means "tiny bone" (from Latin ''ossicu ...
in their
middle ear The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear). The mammalian middle ear contains three ossicles (malleus, incus, and stapes), which transfer the vibrations ...
s is the
stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other tetrapods which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the f ...
. The mammalian jaw joint is composed of different skull bones, including the
dentary In jawed vertebrates, the mandible (from the Latin ''mandibula'', 'for chewing'), lower jaw, or jawbone is a bone that makes up the lowerand typically more mobilecomponent of the mouth (the upper jaw being known as the maxilla). The jawbone ...
(the lower jaw bone which carries the teeth) and the
squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestra ...
(another small skull bone). In mammals, the quadrate and articular bones have evolved into the
incus The ''incus'' (: incudes) or anvil in the ear is one of three small bones (ossicles) in the middle ear. The incus receives vibrations from the malleus, to which it is connected laterally, and transmits these to the stapes medially. The incus i ...
and
malleus The ''malleus'', or hammer, is a hammer-shaped small bone or ossicle of the middle ear. It connects with the incus, and is attached to the inner surface of the eardrum. The word is Latin for 'hammer' or 'mallet'. It transmits the sound vibra ...
bones in the middle ear. The mammalian
middle ear The middle ear is the portion of the ear medial to the eardrum, and distal to the oval window of the cochlea (of the inner ear). The mammalian middle ear contains three ossicles (malleus, incus, and stapes), which transfer the vibrations ...
contains three tiny bones known as the
ossicles The ossicles (also called auditory ossicles) are three irregular bones in the middle ear of humans and other mammals, and are among the smallest bones in the human body. Although the term "ossicle" literally means "tiny bone" (from Latin ''ossi ...
:
malleus The ''malleus'', or hammer, is a hammer-shaped small bone or ossicle of the middle ear. It connects with the incus, and is attached to the inner surface of the eardrum. The word is Latin for 'hammer' or 'mallet'. It transmits the sound vibra ...
,
incus The ''incus'' (: incudes) or anvil in the ear is one of three small bones (ossicles) in the middle ear. The incus receives vibrations from the malleus, to which it is connected laterally, and transmits these to the stapes medially. The incus i ...
, and
stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other tetrapods which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the f ...
. The ossicles are a complex system of
lever A lever is a simple machine consisting of a beam (structure), beam or rigid rod pivoted at a fixed hinge, or '':wikt:fulcrum, fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, l ...
s whose functions include: reducing the
amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...
of the vibrations; increasing the mechanical
force In physics, a force is an influence that can cause an Physical object, object to change its velocity unless counterbalanced by other forces. In mechanics, force makes ideas like 'pushing' or 'pulling' mathematically precise. Because the Magnitu ...
of vibrations; and thus improving the efficient transmission of sound energy from the eardrum to the
inner ear The inner ear (internal ear, auris interna) is the innermost part of the vertebrate ear. In vertebrates, the inner ear is mainly responsible for sound detection and balance. In mammals, it consists of the bony labyrinth, a hollow cavity in the ...
structures. The ossicles act as the mechanical analog of an electrical
transformer In electrical engineering, a transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple Electrical network, circuits. A varying current in any coil of the transformer produces ...
, matching the
mechanical impedance Mechanical impedance is a measure of how much a structure resists motion when subjected to a harmonic force. It relates forces with velocities acting on a mechanical system. The mechanical impedance of a point on a structure is the ratio of the fo ...
of vibrations in air to vibrations in the liquid of the
cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus (cochlea), modiolus. A core component of the cochlea is the organ of Cort ...
. The net effect of this
impedance matching In electrical engineering, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or ...
is to greatly increase the overall sensitivity and upper frequency limits of mammalian hearing, as compared to reptilian hearing. The details of these structures and their effects vary noticeably between different mammal species, even when the species are as closely related as humans and
chimpanzees The chimpanzee (; ''Pan troglodytes''), also simply known as the chimp, is a species of great ape native to the forests and savannahs of tropical Africa. It has four confirmed subspecies and a fifth proposed one. When its close relative the ...
.


Phylogeny

The following simplified
cladogram A cladogram (from Greek language, Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an Phylogenetic tree, evolutionary tree because it does not s ...
displays relationships between
tetrapods A tetrapod (; from Ancient Greek τετρα- ''(tetra-)'' 'four' and πούς ''(poús)'' 'foot') is any four- limbed vertebrate animal of the clade Tetrapoda (). Tetrapods include all extant and extinct amphibians and amniotes, with the lat ...
: The first fully terrestrial
vertebrate Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain. The vertebrates make up the subphylum Vertebra ...
s were
amniotes Amniotes are tetrapod vertebrate animals belonging to the clade Amniota, a large group that comprises the vast majority of living terrestrial and semiaquatic vertebrates. Amniotes evolved from amphibious stem tetrapod ancestors during the ...
, which developed in eggs with internal membranes which allowed the developing
embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...
to breathe but kept water in. The first amniotes arose in the late
Carboniferous The Carboniferous ( ) is a Geologic time scale, geologic period and System (stratigraphy), system of the Paleozoic era (geology), era that spans 60 million years, from the end of the Devonian Period Ma (million years ago) to the beginning of the ...
from the ancestral
reptiliomorph Reptiliomorpha (meaning reptile-shaped; in PhyloCode known as ''Pan-Amniota'') is a clade containing the amniotes and those tetrapods that share a more recent common ancestor with amniotes than with living amphibians (lissamphibians). It was defi ...
s (a group of amphibians whose only living descendants are amniotes). Within a few million years two important amniote lineages became distinct: the
synapsid Synapsida is a diverse group of tetrapod vertebrates that includes all mammals and their extinct relatives. It is one of the two major clades of the group Amniota, the other being the more diverse group Sauropsida (which includes all extant rept ...
ancestors of mammals, and the
sauropsid Sauropsida ( Greek for "lizard faces") is a clade of amniotes, broadly equivalent to the class Reptilia, though typically used in a broader sense to also include extinct stem-group relatives of modern reptiles and birds (which, as theropod dino ...
s ancestors of
lizard Lizard is the common name used for all Squamata, squamate reptiles other than snakes (and to a lesser extent amphisbaenians), encompassing over 7,000 species, ranging across all continents except Antarctica, as well as most Island#Oceanic isla ...
s,
snake Snakes are elongated limbless reptiles of the suborder Serpentes (). Cladistically squamates, snakes are ectothermic, amniote vertebrates covered in overlapping scales much like other members of the group. Many species of snakes have s ...
s,
crocodilian Crocodilia () is an Order (biology), order of semiaquatic, predatory reptiles that are known as crocodilians. They first appeared during the Late Cretaceous and are the closest living relatives of birds. Crocodilians are a type of crocodylomorp ...
s,
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the #Evolutio ...
s and
bird Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s. The evolution of mammalian jaw joints and ears did not occur simultaneously with the evolution of other mammalian features. In other words, jaw joints and ears do not define any except the most recent groups of mammals.


Early tetrapod and amniote ears

In modern amniotes (including mammals), the middle ear collects airborne sounds through an
eardrum In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear from the middle ear. Its function is to transmit changes in pres ...
and transmits vibrations to the inner ear via thin cartilaginous and ossified structures. These structures usually include the
stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other tetrapods which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the f ...
(a
stirrup A stirrup is a light frame or ring that holds the foot of a rider, attached to the saddle by a strap, often called a ''stirrup leather''. Stirrups are usually paired and are used to aid in mounting and as a support while using a riding animal ...
-shaped auditory ossicle). Early
tetrapods A tetrapod (; from Ancient Greek τετρα- ''(tetra-)'' 'four' and πούς ''(poús)'' 'foot') is any four- limbed vertebrate animal of the clade Tetrapoda (). Tetrapods include all extant and extinct amphibians and amniotes, with the lat ...
likely did not possess eardrums. Eardrums appear to have evolved independently three to six times. In basal members of the 3 major
clades In biology, a clade (), also known as a monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach to taxonomy ...
of amniotes (synapsids,
eureptiles Sauropsida (Greek language, Greek for "lizard faces") is a clade of amniotes, broadly equivalent to the Class (biology), class Reptile, Reptilia, though typically used in a broader sense to also include extinct stem-group relatives of modern repti ...
, and
parareptiles Parareptilia ("near-reptiles") is an extinct group of basal sauropsids ("reptiles"), traditionally considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds). Parareptiles first arose near the end ...
) the
stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other tetrapods which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the f ...
bones are relatively massive props that support the
braincase In human anatomy, the neurocranium, also known as the braincase, brainpan, brain-pan, or brainbox, is the upper and back part of the skull, which forms a protective case around the brain. In the human skull, the neurocranium includes the calv ...
, and this function prevents them from being used as part of the hearing system. However, there is increasing evidence that synapsids, eureptiles and parareptiles developed eardrums connected to the inner ear by stapes during the
Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...
.


Early therapsid jaws and ears

The jaws of early synapsids, including the ancestors of mammals, were similar to those of other
tetrapod A tetrapod (; from Ancient Greek :wiktionary:τετρα-#Ancient Greek, τετρα- ''(tetra-)'' 'four' and :wiktionary:πούς#Ancient Greek, πούς ''(poús)'' 'foot') is any four-Limb (anatomy), limbed vertebrate animal of the clade Tetr ...
s of the time, with a lower jaw consisting of a
tooth A tooth (: teeth) is a hard, calcified structure found in the jaws (or mouths) of many vertebrates and used to break down food. Some animals, particularly carnivores and omnivores, also use teeth to help with capturing or wounding prey, tea ...
-bearing dentary bone and several smaller posterior bones. The jaw joint consisted of the
articular The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals. Anatomy In most vertebrates, the articular bone is connected to two o ...
bone in the lower jaw and the quadrate in the upper jaw. The early
pelycosaurs Pelycosaur ( ) is an older term for basal or primitive Late Paleozoic synapsids, excluding the therapsids and their descendants. Previously, the term mammal-like reptile was used, and Pelycosauria was considered an order, but this is now thoug ...
(late Carboniferous and
early Permian 01 or 01 may refer to: * The year 2001, or any year ending with 01 * The month of January * 1 (number) Music * '01 (Richard Müller album), ''01'' (Richard Müller album), 2001 * 01 (Urban Zakapa album), ''01'' (Urban Zakapa album), 2011 * ''01011 ...
) likely did not have
tympanic membrane In the anatomy of humans and various other tetrapods, the eardrum, also called the tympanic membrane or myringa, is a thin, cone-shaped membrane that separates the external ear from the middle ear. Its function is to transmit changes in pressur ...
s (external eardrums). Additionally, their massive stapes bones supported the braincase, with the lower ends resting on the quadrates. Their descendants, the
therapsids Therapsida is a clade comprising a major group of eupelycosaurian synapsids that includes mammals and their ancestors and close relatives. Many of the traits today seen as unique to mammals had their origin within early therapsids, including li ...
(including mammalian ancestors), probably had tympanic membranes in contact with the quadrate bones. The stapes remained in contact with the quadrate bone, but functioned as auditory ossicles rather than supports for the brain case. As a result, the quadrate bones of therapsids likely had a dual function in both the jaw joint and auditory system.


Twin-jointed jaws

During the
Permian The Permian ( ) is a geologic period and System (stratigraphy), stratigraphic system which spans 47 million years, from the end of the Carboniferous Period million years ago (Mya), to the beginning of the Triassic Period 251.902 Mya. It is the s ...
and early
Triassic The Triassic ( ; sometimes symbolized 🝈) is a geologic period and system which spans 50.5 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.4 Mya. The Triassic is t ...
the dentary of therapsids, including the ancestors of mammals, continually enlarged while other jaw bones were reduced. Eventually, the dentary bone evolved to make contact with the
squamosal The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestra ...
, a bone in the upper jaw located
anterior Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position pro ...
to the quadrate, allowing two simultaneous jaw joints: an anterior "
mammal A mammal () is a vertebrate animal of the Class (biology), class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three ...
ian" joint between the dentary and squamosal and a posterior "reptilian" joint between the quadrate and articular. This "twin-jointed jaw" can be seen in late
cynodont Cynodontia () is a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 Megaannum, mya), and extensively diversified after the Permian–Triassic extinction event. Mammals are cynodonts, as are their extin ...
s and early mammaliforms. ''
Morganucodon ''Morganucodon'' ("Glamorgan tooth") is an early mammaliaform genus that lived from the Late Triassic to the Middle Jurassic. It first appeared about 205 million years ago. Unlike many other early mammaliaforms, ''Morganucodon'' is well represent ...
'' is one of the first discovered and most thoroughly studied of the mammaliforms, since an unusually large number of morganucodont fossils have been found. It is an example of a nearly perfect evolutionary intermediate between the mammal-like reptiles and extant mammals.


Early mammals

The earliest mammals were generally small animals, and were likely
nocturnal Nocturnality is a ethology, behavior in some non-human animals characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnality, diurnal meaning the opposite. Nocturnal creatur ...
insectivores A robber fly eating a hoverfly An insectivore is a carnivorous animal or plant which eats insects. An alternative term is entomophage, which can also refer to the human practice of eating insects. The first vertebrate insectivores wer ...
. This suggests a plausible source of evolutionary pressure: with these small bones in the middle ear, a mammal has extended its range of hearing for higher-pitched sounds which would improve the detection of insects in the dark. The evidence that the malleus and incus are homologous to the reptilian articular and quadrate was originally embryological, and since this discovery an abundance of
transitional fossil A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. This is especially important where the descendant group is sharply differentiated by gross ...
s has both supported the conclusion and given a detailed history of the transition. The evolution of the
stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other tetrapods which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the f ...
(from the
columella Lucius Junius Moderatus Columella (, Arabic: ) was a prominent Roman writer on agriculture in the Roman Empire. His in twelve volumes has been completely preserved and forms an important source on Roman agriculture and ancient Roman cuisin ...
) was an earlier and distinct event. The evolution of the mammalian middle ear appears to have occurred in two steps. A partial middle ear formed by the departure of postdentary bones from the dentary, and happened independently in the ancestors of monotremes and
theria Theria ( or ; ) is a scientific classification, subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the Placentalia, placental mammals) and the metatherians (including the marsupials) but excludes the egg-lay ...
ns. The second step was the transition to a definite mammalian middle ear, and evolved independently at least three times in the ancestors of today's monotremes, marsupials and placentals.


Fossil evidence for mammal-like jaws and ears

As the dentary bone of the lower jaw continued to enlarge during the Triassic, the older quadrate-articular joint fell out of use. Some of the bones were lost, but the quadrate, the
articular The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals. Anatomy In most vertebrates, the articular bone is connected to two o ...
, and the angular bones became free-floating and associated with the
stapes The ''stapes'' or stirrup is a bone in the middle ear of humans and other tetrapods which is involved in the conduction of sound vibrations to the inner ear. This bone is connected to the oval window by its annular ligament, which allows the f ...
. This occurred at least twice in the
mammaliformes Mammaliaformes ("mammalian forms") is a clade of synapsid tetrapods that includes the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts during the Late Triassic. It is defined as ...
. The
multituberculates Multituberculata (commonly known as multituberculates, named for the multiple tubercles of their teeth) is an extinct order of rodent-like mammals with a fossil record spanning over 130 million years. They first appeared in the Middle Jurassic, a ...
had jaw joints that consisted of only the dentary and squamosal bones, and the quadrate and articular bones were part of the middle ear. Other features of their teeth, jaws and skulls are significantly different from those of mammals.


''Hadrocodium''

In the lineage most closely related to mammals, the jaws of ''
Hadrocodium ''Hadrocodium wui'' is an extinct mammaliaform that lived during the Sinemurian stage of the Early Jurassic approximately in the Lufeng Formation in what is now the Yunnan province in south-western China (, paleocoordinates ). It is considere ...
'' (about 195M years ago in the very early Jurassic) suggest that it may have been the first to have a nearly fully mammalian middle ear: it lacks the trough at the rear of the lower jaw, over which the eardrum stretched in therapsids and earlier mammaliformes. The absence of this trough suggests that ''Hadrocodium''’s ear was part of the cranium, as it is in mammals, and that the former articular and quadrate had migrated to the middle ear and become the malleus and incus. ''Hadrocodium''’s dentary has a "bay" at the rear which mammals lack, a hint that the dentary bone retained the same shape as if the articular and quadrate had remained part of the jaw joint. However, several studies have cast doubt on whether ''Hadrocodium'' did indeed possess a definitive mammalian middle ear; ''Hadrocodium'' likely had an ossified connection between the middle ear and the jaw, which is not visible in the fossil evidence due to limited preservation. Researchers now hypothesize that the definitive mammalian middle ear did not emerge any earlier than the late Jurassic (~163M years ago).


''Teinolophos''

It has been suggested that a relatively large trough in the jaw bone of the early Cretaceous
monotreme Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...
''
Teinolophos ''Teinolophos'', from Ancient Greek τείνω (''teínō''), meaning "extend", and λόφος (''lóphos''), meaning "crest", is a genus of Teinolophidae, teinolophid monotreme that lived during the late Barremian Age (geology), age of the Ear ...
'' provides evidence of a pre-mammalian jaw joint, because therapsids and many mammaliforms had such troughs in which the articular and angular bones "docked". Thus, ''Teinolophos'' had a pre-mammalian middle ear, indicating that the mammalian middle ear ossicles evolved independently in monotremes and in other mammals. A more recent analysis of ''Teinolophos'' concluded that the trough was a channel for the large vibration and electrical sensory nerves terminating in the bill (a defining feature of the modern platypus). Thus, the trough is not evidence that ''Teinolophos'' had a pre-mammalian jaw joint and a pre-mammalian middle ear.


''Yanoconodon''

A recently discovered intermediate form is the primitive mammal ''
Yanoconodon ''Yanoconodon'' is a monotypic genus of extinct early mammal whose representative species ''Yanoconodon allini'' lived during the Mesozoic in what is now China. The holotype fossil of ''Yanoconodon'' was excavated in the Yan Mountains about 300 k ...
'', which lived approximately 125 million years ago in the
Mesozoic The Mesozoic Era is the Era (geology), era of Earth's Geologic time scale, geological history, lasting from about , comprising the Triassic, Jurassic and Cretaceous Period (geology), Periods. It is characterized by the dominance of archosaurian r ...
era. In ''Yanoconodon'' the ossicles have separated from the jaw and serve the hearing function in the middle ear, yet maintain a slender connection to the jaw via the ossified
Meckel's cartilage In humans, the cartilaginous bar of the mandibular arch is formed by what are known as Meckel's cartilages (right and left; also known as Meckelian cartilages), above which the incus and malleus are located. Meckel's cartilage arises from the fir ...
. Maintaining a connection via the ossified Meckel's cartilage may have been evolutionary advantageous since the auditory ossicles were not connected to the cranium in ''Yanoconodon'' (as they are in
extant Extant or Least-concern species, least concern is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Exta ...
mammals), and required structural support via Meckel's cartilage.


Effects on hearing

The
frequency range Spectral bands are regions of a given spectrum, having a specific range of wavelengths or frequencies. Most often, it refers to electromagnetic bands, regions of the electromagnetic spectrum. More generally, spectral bands may also be means in ...
and sensitivity of the
ear In vertebrates, an ear is the organ that enables hearing and (in mammals) body balance using the vestibular system. In humans, the ear is described as having three parts: the outer ear, the middle ear and the inner ear. The outer ear co ...
is dependent on the shape and arrangement of the middle-ear bones. In the reptilian lineage, hearing depends on the conduction of low-frequency vibrations through the ground or bony structures (such as the
columella Lucius Junius Moderatus Columella (, Arabic: ) was a prominent Roman writer on agriculture in the Roman Empire. His in twelve volumes has been completely preserved and forms an important source on Roman agriculture and ancient Roman cuisin ...
). By modifying the articular bone, quadrate bone, and columella into small ossicles, mammals were able to hear a wider range of high-frequency airborne vibrations. Hearing within mammals is further aided by a tympanum in the outer ear and an elongated lagena (
cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus (cochlea), modiolus. A core component of the cochlea is the organ of Cort ...
) in the inner ear.


See also

*
Evolution of mammals The evolution of mammals has passed through many stages since the first appearance of their synapsid ancestors in the Pennsylvanian (geology), Pennsylvanian sub-period of the late Carboniferous period. By the mid-Triassic, there were many synaps ...
* ''
Yanoconodon ''Yanoconodon'' is a monotypic genus of extinct early mammal whose representative species ''Yanoconodon allini'' lived during the Mesozoic in what is now China. The holotype fossil of ''Yanoconodon'' was excavated in the Yan Mountains about 300 k ...
'' *
quadrate bone The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids ( reptiles, birds), and early synapsids. In most tetrapods, the quadrate bone connects to the quadratojugal and squamosal bones in the skull, and forms up ...
*
articular bone The articular bone is part of the lower jaw of most vertebrates, including most jawed fish, amphibians, birds and various kinds of reptiles, as well as ancestral mammals. Anatomy In most vertebrates, the articular bone is connected to two othe ...
*
columella Lucius Junius Moderatus Columella (, Arabic: ) was a prominent Roman writer on agriculture in the Roman Empire. His in twelve volumes has been completely preserved and forms an important source on Roman agriculture and ancient Roman cuisin ...
*
Vilevolodon ''Vilevolodon'' is an extinct, monotypic genus of volant, arboreal euharamiyids from the Oxfordian age of the Late Jurassic of China. The type species is ''Vilevolodon diplomylos''. The genus name ''Vilevolodon'' references its gliding capabili ...


References


Further reading

* * * * * * * especially pages 85–96 * * * * contains wide bibliography of scientific literature up to 2016 * * * * * *


External links


Your Inner Fish : We Hear With the Bones That Reptiles Eat With (video by Karen Sears and Neil Shubin
{{DEFAULTSORT:Evolution Of Mammalian Auditory Ossicles mammalian auditory ossicles Auditory system Mammal anatomy Mammalian auditory ossicles Otology Audiology