Megazostrodon
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''Megazostrodon'' is an extinct genus of basal
mammaliaform 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 a ...
s belonging to the order
Morganucodonta Morganucodonta ("Glamorgan teeth") is an extinct Order (biology), order of basal Mammaliaformes, a group including crown-group mammals (Mammalia) and their close relatives. Their remains have been found in Southern Africa, Western Europe, North A ...
. It is approximately 200 million years old.Fur and Fangs: Mammal Origins
. Palaeobiology and Biodiversity Research Group, University of Bristol.
Two species are known: ''M. rudnerae'' from the
Early Jurassic The Early Jurassic Epoch (geology), Epoch (in chronostratigraphy corresponding to the Lower Jurassic series (stratigraphy), Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassicā ...
of Lesotho and South Africa, and ''M. chenali'' from the
Late Triassic The Late Triassic is the third and final epoch (geology), epoch of the Triassic geologic time scale, Period in the geologic time scale, spanning the time between annum, Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch a ...
of France.


Discovery

The
type species In International_Code_of_Zoological_Nomenclature, zoological nomenclature, a type species (''species typica'') is the species name with which the name of a genus or subgenus is considered to be permanently taxonomically associated, i.e., the spe ...
''M. rudnerae'' was first discovered in 1966 in the
Elliot Formation The Elliot Formation is a geological formation and forms part of the Stormberg Group, the uppermost geological group that comprises the greater Karoo Supergroup. Outcrops of the Elliot Formation have been found in the northern Eastern Cape, so ...
of
Lesotho Lesotho, formally the Kingdom of Lesotho and formerly known as Basutoland, is a landlocked country in Southern Africa. Entirely surrounded by South Africa, it is the largest of only three sovereign enclave and exclave, enclaves in the world, t ...
, southern
Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...
, by palaeontologist and archaeologist Ione Rudner. It was first described by A. W. Crompton and F. A. Jenkins Jr. in 1968. The generic name ''Megazostrodon'' means, literally, 'large girdle tooth' (from the Greek ''mega'' -large, ''zostros'' -girdle and ''don'' -tooth—referring to the large external cingula of the upper molars). The specific name honours Rudner for her discovery. A second species, ''M. chenali'', was named in 2015 based on remains found in Saint-Nicolas-de-Port, France. It is named after the French palaeontologist Emmanuel Chenal.


Characteristics

''Megazostrodon'' was a small,
shrew Shrews ( family Soricidae) are small mole-like mammals classified in the order Eulipotyphla. True shrews are not to be confused with treeshrews, otter shrews, elephant shrews, West Indies shrews, or marsupial shrews, which belong to dif ...
-like animal between long which probably ate insects and small
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. It is thought to have been
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 ...
as it had a larger brain than earlier
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 the enlarged areas of its brain were found to be those that process sounds and smells. This was probably in order to avoid being in competition with the reptiles or becoming prey to the
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. Although considered a close relative of mammals, it did have some non-mammalian characteristics inherited from its predecessors: the first two vertebrae (atlas and axis) were still unfused as in earlier cynodonts, and it only had three sacral vertebrae instead of the usual mammalian five. An interclavicle is also present, which is still present in
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 lost in the line leading to
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 ...
n mammals.


Evolution

''Megazostrodon'' is the best-known genus of the family Megazostrodontidae, part of the larger group Morganucodonta. The other members of this family that are currently known are '' Indozostrodon'', '' Dinnetherium'', '' Wareolestes'' and '' Brachyzostrodon''. The megazostrodontids used to be classified as members of a group of mammals called the triconodonts, which are thought to have evolved from a specific group of cynodontsMammalogy
Sam Houston State University.
during the late
Triassic The Triassic ( ; sometimes symbolized šŸˆ) 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 ...
and early
Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...
periods. However, recent classifications consider the megazostrodontids to be mammaliaforms outside of the stricter grouping of Mammalia proper, while the triconodonts remain within the mammalian crown group. These early mammaliaforms possessed many traits which made them well suited for an active lifestyle. They had a
heterodont In anatomy, a heterodont (from Greek, meaning 'different teeth') is an animal which possesses more than a single tooth morphology. Human dentition is heterodont and diphyodont as an example. In vertebrates, heterodont pertains to animals wher ...
dentition consisting of four types of teeth:
incisor 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 ...
s, canines,
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 and
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, '' ...
, as opposed to the uniform (homodont) teeth of most reptiles.First Mammals Appear
/ref> This enabled them to chew and therefore process their food more thoroughly than their reptilian cousins. There is evidence that the movement of the mandible allowed a shearing action to chew food. Their skeletons changed so that their limbs were more mobile, being less laterally splayed, and allowing faster forward motion. They had a short ribcage and large lungs, which allowed efficient respiration. Their lower jaw comprised a single bone—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 ...
(as opposed to the multiple bones in the jaws of their ancestors, or seven different bones found in reptilian lower jaws). The other bones which once made up the jaw had reduced, and in later mammals would become incorporated into the middle ear, enhancing their hearing. Probably the most important change in the evolution of the first mammals was that their ancestors, the cynodonts, had become
endotherm An endotherm (from Greek ἔνΓον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions inst ...
ic. This meant that they generated their own body heat, relying on the food they ate to help sustain their body temperature rather than depending on their surrounding environment. This permitted higher, more sustained activity levels during the day than reptiles (reptiles must frequently perform temperature regulation activities such as sun basking and seeking shade). It was probably the key to becoming nocturnal—a major advantage in a world where most predators were active during the day. ;Phylogeny


Reproduction

Like placentals and possibly ''
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 ...
'', ''Megazostrodon'' is unique among mammaliaforms in 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.Jason A. Lillegraven,
Zofia Kielan-Jaworowska Zofia Emilia Kielan-Jaworowska (25 April 1925 – 13 March 2015) was a Polish paleobiologist. In the mid-1960s, she led a series of Polish-Mongolian paleontological expeditions to the Gobi Desert. She was the first woman to serve on the executi ...
, William A. Clemens, ''Mesozoic Mammals: The First Two-Thirds of Mammalian History'', University of California Press, 17/12/1979 – 321
It is likely that ''Megazostrodon'', like the modern
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 ...
s, laid eggs.


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 ...
* Probainognathus


References


External links


FUR AND FANGS-MAMMAL ORIGINSFirst Mammals and Plate TectonicsVertebrate paleontology-Michael J. Benton.
{{Taxonbar, from=Q132813 Morganucodonta Jurassic mammals of Africa Rhaetian life Hettangian life Late Triassic synapsids Jurassic synapsids Triassic France Fossils of France Jurassic South Africa Fossils of South Africa Fossil taxa described in 1986 Taxa named by Alfred W. Crompton Taxa named by Farish Jenkins