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''Hadrocodium wui'' is an extinct
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 comm ...
that lived during the
Sinemurian In the geologic timescale, the Sinemurian is an age and stage in the Early or Lower Jurassic Epoch or Series. It spans the time between 199.3 ± 2 Ma and 190.8 ± 1.5 Ma (million years ago). The Sinemurian is preceded by the Hettangian and i ...
stage of the
Early Jurassic The Early Jurassic Epoch (in chronostratigraphy corresponding to the Lower Jurassic Series) is the earliest of three epochs of the Jurassic Period. The Early Jurassic starts immediately after the Triassic-Jurassic extinction event, 201.3 Ma&nb ...
approximately in the Lufeng Formation of the Lufeng Basin in what is now the
Yunnan Yunnan , () is a landlocked province in the southwest of the People's Republic of China. The province spans approximately and has a population of 48.3 million (as of 2018). The capital of the province is Kunming. The province borders the ...
province in south-western China (, paleocoordinates ). It is considered as the closest relative of the class
Mammalia Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur o ...
. The fossil of this mouse-like, paper-clip sized animal was discovered in 1985 but was then interpreted as a juvenile
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 represe ...
tid. ''Hadrocodium'' remained undescribed until 2001; since then its large brain and advanced ear structure have greatly influenced the interpretation of the earliest stages 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 ...
, as these mammalian characters could previously be traced only to some . ''Hadrocodium'' is known only from a skull long, and the body mass is estimated to have been , making it one of the smallest Mesozoic mammaliaforms (in contrast, the related ancestral
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
would have been bigger, about ). The name ''Hadrocodium'' alludes to its large cranial cavity, deriving from the Greek word ( 'large, heavy, fullness') and the Latin word , from Greek ( 'head f a plant). The species name, ''wui'', is the Latinized version of discoverer Xiao-Chun Wu's name.


Features

''Hadrocodium'' might have been the first animal to have a nearly fully mammalian middle ear. It is the earliest known example of several features possessed only by mammals, including the middle-ear structure characteristic of modern mammals and a relatively large brain cavity. These features had been considered limited to the crown group mammals, which emerged in the
Middle Jurassic The Middle Jurassic is the second epoch of the Jurassic Period. It lasted from about 174.1 to 163.5 million years ago. Fossils of land-dwelling animals, such as dinosaurs, from the Middle Jurassic are relatively rare, but geological formations c ...
; the discovery of ''Hadrocodium'' suggests that these attributes appeared 45 million years earlier than previously thought. Whether ''Hadrocodium'' was
warm-blooded Warm-blooded is an informal term referring to animal species which can maintain a body temperature higher than their environment. In particular, homeothermic species maintain a stable body temperature by regulating metabolic processes. The onl ...
or cold-blooded has not been settled, although its apparent
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 ...
features would seem to place it in the former group. Hadrocodium were capable of laying eggs like today's platypus and echidna.


Family Tree

;Phylogeny


See also

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Smallest organisms The smallest organisms found on Earth can be determined according to various aspects of organism size, including volume, mass, height, length, or genome size. Given the incomplete nature of scientific knowledge, it is possible that the smalle ...
*
Evolution of mammalian auditory ossicles The evolution of mammalian auditory ossicles was an evolutionary event that resulted in the formation of the bones of the mammalian middle ear. These bones, or ossicles, are a defining characteristic of all mammals. The event is well-documented an ...


References


Bibliography

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