''Megazostrodon'' is an extinct genus of basal
mammaliaform
Mammaliaformes ("mammalian forms") is a clade that contains the crown group mammals and their closest extinct relatives; the group radiated from earlier probainognathian cynodonts. It is defined as the clade originating from the most recent com ...
s belonging to the order
Morganucodonta. It is approximately 200 million years old.
[Fur and Fangs: Mammal Origins](_blank)
. 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 Triassic-J ...
of
Lesotho
Lesotho ( ), officially the Kingdom of Lesotho, is a country landlocked country, landlocked as an Enclave and exclave, enclave in South Africa. It is situated in the Maloti Mountains and contains the Thabana Ntlenyana, highest mountains in Sou ...
and
South Africa
South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring countri ...
, 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 ...
of France.
Discovery
The
type species
In 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 species that contains the biological type specimen ...
''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, south ...
of
Lesotho
Lesotho ( ), officially the Kingdom of Lesotho, is a country landlocked country, landlocked as an Enclave and exclave, enclave in South Africa. It is situated in the Maloti Mountains and contains the Thabana Ntlenyana, highest mountains in Sou ...
, southern
Africa
Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
, 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
Saint-Nicolas-de-Port () is a commune in the Meurthe-et-Moselle ''département'' in north-eastern France.
The town's basilica, '' Saint Nicolas'', is a pilgrimage site, supposedly holding relics of Saint Nicholas brought from Italy. It is one of ...
, 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 different fa ...
-like animal between long which probably ate insects and small
reptile
Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...
s. It is thought to have been
nocturnal
Nocturnality is an animal behavior characterized by being active during the night and sleeping during the day. The common adjective is "nocturnal", versus diurnal meaning the opposite.
Nocturnal creatures generally have highly developed sens ...
as it had a larger brain than earlier
cynodont
The cynodonts () (clade Cynodontia) are a clade of eutheriodont therapsids that first appeared in the Late Permian (approximately 260 mya), and extensively diversified after the Permian–Triassic extinction event. Cynodonts had a wide variety ...
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 period, between 243 and 233.23 million years ago (mya), although the exact origin and timing of the evolution of dinosaurs is t ...
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
An interclavicle is a bone which, in most tetrapods, is located between the clavicles. Therian mammals (marsupials and placentals) are the only tetrapods which never have an interclavicle, although some members of other groups also lack one. In t ...
is also present, which is still present in
monotremes
Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials ( Metatheria). Monotremes are typified by structural differences in their br ...
but lost in the line leading to
theria
Theria (; Greek: , wild beast) is a subclass of mammals amongst the Theriiformes. Theria includes the eutherians (including the placental mammals) and the metatherians (including the marsupials) but excludes the egg-laying monotremes.
Ch ...
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
''Dinnetherium'' is an extinct genus of morganucodont mammaliaform that is part of the monotypic order Dinnetheria and is also part of the monotypic family Dinnetheriidae.A. O. Averianov and A. V. Lopatin. 2011. Phylogeny of Triconodonts and Symm ...
'', ''
Wareolestes
''Wareolestes rex'' ("Ware's Brigand king") is a mammaliaform from the Middle Jurassic (Bathonian) rocks of England and Scotland. It was originally known from isolated teeth from England, before a more complete jaw with teeth was found in the ...
'' and ''
Brachyzostrodon''. The megazostrodontids used to be classified as members of a group of mammals called the
triconodont
Eutriconodonta is an order of early mammals. Eutriconodonts existed in Asia, Africa, Europe, North and South America during the Jurassic and the Cretaceous periods. The order was named by Kermack ''et al.'' in 1973 as a replacement name for the p ...
s, which are thought to have evolved from a specific group of cynodonts
[Mammalogy](_blank)
Sam Houston State University. during the late
Triassic
The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
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 Mya. The J ...
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 dentition consisting of four types of 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, wh ...
,
canines
Canine may refer to:
Zoology and anatomy
* a dog-like Canid animal in the subfamily Caninae
** ''Canis'', a genus including dogs, wolves, coyotes, and jackals
** Dog, the domestic dog
* Canine tooth, in mammalian oral anatomy
People with the surn ...
,
premolars
The premolars, also called premolar teeth, or bicuspids, are transitional teeth located between the canine and molar teeth. In humans, there are two premolars per quadrant in the permanent set of teeth, making eight premolars total in the mouth ...
and
molars
The molars or molar teeth are large, flat teeth at the back of the mouth. They are more developed in mammals. They are used primarily to grind food during chewing. The name ''molar'' derives from Latin, ''molaris dens'', meaning "millstone to ...
, as opposed to the uniform (homodont) teeth of most reptiles.
[First Mammals Appear](_blank)
/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
Respiration may refer to:
Biology
* Cellular respiration, the process in which nutrients are converted into useful energy in a cell
** Anaerobic respiration, cellular respiration without oxygen
** Maintenance respiration, the amount of cellul ...
. Their lower jaw comprised a single bone — the dentary
In anatomy, the mandible, lower jaw or jawbone is the largest, strongest and lowest bone in the human facial skeleton. It forms the lower jaw and holds the lower tooth, teeth in place. The mandible sits beneath the maxilla. It is the only movabl ...
(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 ἔνδον ''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 inste ...
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
Placental mammals (infraclass Placentalia ) are one of the three extant subdivisions of the class Mammalia, the other two being Monotremata and Marsupialia. Placentalia contains the vast majority of extant mammals, which are partly distinguishe ...
and possibly ''Erythrotherium
''Erythrotherium'' (meaning "red beast") is an extinct genus of basal mammaliaforms from the Late Triassic to Lower Jurassic. It is related to ''Morganucodon''. Only one species is recorded, ''Erythrotherium parringtoni'', from Red Beds, Stormber ...
'', ''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 placental mammals, who lack them).
They first o ...
bones.[Jason A. Lillegraven, Zofia Kielan-Jaworowska, 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 monotremes
Monotremes () are prototherian mammals of the order Monotremata. They are one of the three groups of living mammals, along with placentals (Eutheria), and marsupials ( Metatheria). Monotremes are typified by structural differences in their br ...
, 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 sub-period of the late Carboniferous period. By the mid-Triassic, there were many synapsid species that looked l ...
* Probainognathus
''Probainognathus'' meaning “progressive jaw” is an extinct genus of cynodonts that lived around 235 to 221.5 million years ago, during the Late Triassic in what is now South America. ''Probainognathus'' is a member of the family Probainogna ...
References
External links
FUR AND FANGS-MAMMAL ORIGINS
First Mammals and Plate Tectonics
Vertebrate paleontology-Michael J. Benton.
{{Taxonbar, from=Q132813
Morganucodonts
Prehistoric cynodont genera
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