Athena Review Image Archive ™
Hadrocodium wui skull
Skull of Hadrocodium wui (afte Wui 1985)
Hadrocodium wui was a small, Early Jurassic Mammaliaform dating from about 195 mya, found in Yunnan, China. The genus name derives from
Hadro- "fullness," and -codium "head," for its large brain
capacity, and the species name wui after the discoverer, Dr. X.-C. Wui.
The type specimen, a nearly complete skull (IVPP 8275, now at the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing) was found by Wui in 1985 in Sinemurian (Early Jurassic) deposits in the Lower Lufang Formation in Yunnan Province. It was originally classified as Morganucodon sp.
The skull, 12 mm long and 8 mm wide, shows several adult features. The estimated body weight of 2 grams makes it among the smallest known mammals, and the smallest mammal yet discovered in the Mesozoic. This contrast with contemporary mammaliaforms of the Late Triassic and Early Jurassic such as Sinconodon, whose body size ranged from 13 to 517 grams, based on skull length from 22 to 62 mm, and Morganucodon, with a size from 27 to 89 grams.
The skull of Hadrocodium shows an early mammalian correlation of both the separation of the middle ear bones from the mandible, and the expanded brain vault. This indicates that several key mammalian evolutionary innovations in the ear region, including the temporo-mandibular joint, evolved incrementally along with increasing size of the brain vault. Its cranial vault is wider and more expanded than those of all other nonmammalian mammaliaforms and all other known Jurassic mammals.
Hadrocodium had triconodont-like teeth for
an insectivorous diet. Based on analysis of 90 cranial and dental
characters, Hadrocodium is the sister taxon to the clade of triconodontids
and extant Mammalia. It is more closely related to living mammals than
Adelobasileus, Sinoconodon, morganucodontids, and
Haldanodon.
References:
Wui 1985
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