''Megazostrodon'' is an extinct genus of basal mammaliaforms belonging to the order

Morganucodonta Morganucodonta (" Glamorgan teeth") is an extinct 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 America, India 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 Triassic-J ...

Lesotho Lesotho ( ), officially the Kingdom of Lesotho, is a country landlocked as an enclave in South Africa. It is situated in the Maloti Mountains and contains the highest mountains in Southern Africa. It has an area of over and has a population ...

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 coun ...

, and ''M. chenali'' from the

Late Triassic The Late Triassic is the third and final epoch of the Triassic Period in the geologic time scale, spanning the time between Ma and Ma (million years ago). It is preceded by the Middle Triassic Epoch and followed by the Early Jurassic 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 specim ...

''M. rudnerae'' was first discovered in 1966 in the Elliot Formation of

, 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 ''Cingula'' is a genus of minute sea snails, marine gastropod mollusks or micromollusks in the family Rissoidae Rissoidae is a large family of very small and minute sea snails with an operculum, marine gastropod mollusks in the superfamily ...

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 differ ...

-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 rhynchocephalia ...

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 varie ...

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 is also present, which is still present in monotremes 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. ...

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 Ki ...

'' 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 ( ) 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 per ...

and early

Jurassic The Jurassic ( ) is a geologic period and 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 Jurassic constitutes the middle period of ...

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. In vertebrates, heterodont pertains to animals where teeth are differentiated into different forms. For exampl ...

dentition consisting of four types of teeth: incisors, canines, premolars and molars, 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 (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 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'', ''Megazostrodon'' is unique among mammaliaforms in lacking epipubic 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, laid eggs.

References

External links