Morganucodonta ("

Glamorgan Glamorgan (), or sometimes Glamorganshire ( or ), was Historic counties of Wales, one of the thirteen counties of Wales that existed from 1536 until their abolishment in 1974. It is located in the South Wales, south of Wales. Originally an ea ...

teeth 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, tear ...

") is an extinct

order Order, ORDER or Orders may refer to: * A socio-political or established or existing order, e.g. World order, Ancien Regime, Pax Britannica * Categorization, the process in which ideas and objects are recognized, differentiated, and understood ...

of basal

Mammaliaformes 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 ...

, a group including

crown-group In phylogenetics, the crown group or crown assemblage is a collection of species composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. ...

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 ...

s (

Mammalia A mammal () is a vertebrate animal of the 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 middle ear bon ...

) and their close relatives. Their remains have been found in

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and

China China, officially the People's Republic of China (PRC), is a country in East Asia. With population of China, a population exceeding 1.4 billion, it is the list of countries by population (United Nations), second-most populous country after ...

. The morganucodontans were probably insectivorous and

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 ...

, though like eutriconodonts some species attained large sizes and were carnivorous. Nocturnality is believed to have evolved in the earliest mammals in the Triassic (called the

nocturnal bottleneck The nocturnal bottleneck hypothesis is an evolutionary biology hypothesis to explain the origin of several mammalian traits. In 1942, Gordon Lynn Walls described this concept which states that placental mammals were mainly or even exclusively ...

) as a specialisation that allowed them to exploit a safer, night-time niche, while most larger predators were likely to have been active during the day (though some dinosaurs, for example, were nocturnal as well).

Anatomy and biology

Morganucodontans had a double

jaw The jaws are a pair of opposable articulated structures at the entrance of the mouth, typically used for grasping and manipulating food. The term ''jaws'' is also broadly applied to the whole of the structures constituting the vault of the mouth ...

articulation made up of 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 ...

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 ...

joint as well as a quadrate-articular one. This implies that they also retained one of their postdentary bones: 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 ...

. There is a trough at the back of the jaw on the inside (lingual) where the postdentary bones sat. The double jaw articulation and retention of the postdentary bones are characteristic of many of the earliest Mammaliaformes, but are absent today in mammals: All crown-group

s have a lower jaw that is composed of a single bone, and the articular has been incorporated into the middle ear, having become the

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 ...

(while the quadrate has become 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 ...

Unlike more basal therapsids such as '' Sinoconodon'', morganucodontan teeth were

diphyodont A diphyodont is any animal with two sets of teeth, initially the ''deciduous'' set and consecutively the '' permanent'' set. Most mammals are diphyodonts—as to chew their food they need a strong, durable and complete set of teeth. Diphyodonts ...

(meaning that they replaced their teeth once, having a 'milk' set and 'adult' set as seen in today's mammals including humans and not

polyphyodont A polyphyodont is any animal whose tooth (animal), teeth are continually replaced. In contrast, diphyodonts are characterized by having only two successive sets of teeth. Polyphyodonts include most toothed fishes (most notably sharks), many repti ...

(meaning that the teeth are constantly replaced, and the animal and its teeth get larger throughout its lifetime, as in reptiles). Evidence of lactation is present in '' Wareolestes'', via tooth replacement patterns. Morganucodontans also had specialised teeth –

incisors Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, wher ...

, canines,

molars The molars or molar teeth are large, flat tooth, teeth at the back of the mouth. They are more developed in mammal, mammals. They are used primarily to comminution, grind food during mastication, chewing. The name ''molar'' derives from Latin, '' ...

and

premolar The premolars, also called premolar Tooth (human), teeth, or bicuspids, are transitional teeth located between the Canine tooth, canine and Molar (tooth), molar teeth. In humans, there are two premolars per dental terminology#Quadrant, quadrant in ...

s – for food processing, rather than having similarly shaped teeth along the tooth row as seen in their predecessors. Morganucodontans are thought to have been insectivorous and carnivorous, their teeth adapted for shearing. Niche partitioning is known among various morganucodontan species, different types specialised for different prey. '' Paceyodon davidi'' was the largest species of morganucodontans. It is known by a right lower molariform in length, which is bigger than molariforms of all other morganucodontans. The teeth of some morganucodonts are structured in such a way that occlusion happens by the lower

cusp A cusp is the most pointed end of a curve. It often refers to cusp (anatomy), a pointed structure on a tooth. Cusp or CUSP may also refer to: Mathematics * Cusp (singularity), a singular point of a curve * Cusp catastrophe, a branch of bifu ...

"a" fitting anteriorly to upper cusp "A", between "A" and "B". This occlusion pattern is by no means universal among morganucodontans, however; '' Dinnetherium'', for example has an occlusion mechanism closer to that of the

eutriconodont Eutriconodonta is an order (biology), order of early mammals. Eutriconodonts existed in Asia (including Insular India, pre-contact India), Africa, Europe, North America, North and South America during the Jurassic and the Cretaceous periods. The ...

s, in which molars basically alternate.

Wear facet Wear is the damaging, gradual removal or deformation of material at Solid, solid surfaces. Causes of wear can be mechanical (e.g., erosion) or Chemistry, chemical (e.g., corrosion). The study of wear and related processes is referred to as trib ...

s are present. The septomaxilla, a primitive feature also found in ''Sinoconodon'', is found in morganucodontans, as well as a fully ossified orbitosphenoid. The anterior lamina is enlarged. The cranial moiety of the

is a narrow bone that is superficially placed to the petrosal and parietal. Unlike its predecessors, the morganucodontans have a larger cerebral capacity and a longer

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 atlas elements are unfused; there is a suture between the dens and

axis An axis (: axes) may refer to: Mathematics *A specific line (often a directed line) that plays an important role in some contexts. In particular: ** Coordinate axis of a coordinate system *** ''x''-axis, ''y''-axis, ''z''-axis, common names ...

. The

cervical rib Cervical ribs are the ribs of the neck in many tetrapods. In most mammals, including humans, cervical ribs are not normally present as separate structures. They can, however, occur as a pathology. In humans, pathological cervical ribs are usually no ...

s are not fused to the centra. The

coracoid A coracoid is a paired bone which is part of the shoulder assembly in all vertebrates except therian mammals (marsupials and placentals). In therian mammals (including humans), a coracoid process is present as part of the scapula, but this is n ...

and procoracoid, which are absent in

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, are present. The head of the

humerus The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius (bone), radius and ulna, and consists of three sections. The humeral upper extrem ...

is spherical as in mammals, but the spiral ulnar condyle is

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 ...

-like. In the pelvic girdle, the pubis, ilium and

ischium The ischium (; : is ...

are unfused. At least ''

Megazostrodon ''Megazostrodon'' is an extinct genus of basal mammaliaforms belonging to the order Morganucodonta. It is approximately 200 million years old.

'' and ''

Erythrotherium ''Erythrotherium'' (meaning "red beast") is an extinct genus of basal mammaliaforms from the Lower Jurassic of South Africa. It is related to ''Morganucodon ''Morganucodon'' ("Glamorgan tooth") is an early mammaliaform genus that lived from t ...

'' are unique among basal mammaliaforms for lacking

epipubic Epipubic bones are a pair of bones projecting forward from the pelvic bones of modern marsupials, monotremes and fossil mammals like multituberculates, and even basal eutherians (the ancestors of placentals, who lack them). They first occur in ...

bones, suggesting that they didn't have the same reproductive constraints. ''Morganucodon'' at least appears to have had a long lifespan and a lower metabolism than modern mammals.

Classification

Because morganucodontans possessed the mammalian dentary-squamosal jaw joint, systematists like G. G. Simpson (1959) considered the morganucodontans to be mammals. Some palaeontologists continue to use this classification. Others use

terminology, which limits

to the descendants of the closest common ancestor of living mammals. By this definition, more basal orders like morganucodontans are not in Mammalia, but are mammaliaformes. This is the most commonly accepted classification system.

References

;Notes ;Bibliography * * *

External links

* {{Taxonbar, from=Q142898 Mammaliaformes Prehistoric tetrapod orders Late Triassic first appearances Early Cretaceous extinctions Taxa named by Kenneth A. Kermack Taxa named by Frances Mussett

Anatomy and biology

Classification

See also

References

External links