Hadrocodium

Hadrocodium wui is an extinct mammaliaform that lived during the Sinemurian stage of the Early Jurassic approximately 195 million years ago[1] in the Lufeng Formation of the Lufeng Basin in what is now the Yunnan province in south-western China[2] (25.2°N 102.1°E / 25.2; 102.1, paleocoordinates 34.3°N 104.9°E / 34.3; 104.9).[3] It is considered as the closest relative of the class Mammalia.

Hadrocodium
Temporal range: Sinemurian
~
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Synapsida
Clade: Therapsida
Clade: Cynodontia
Clade: Mammaliaformes
Genus: †Hadrocodium
Luo, Crompton, & Sun, 2001
Species

The fossil of this mouse-like, paper-clip sized animal was discovered in 1985 but was then interpreted as a juvenile morganucodontid.[4] Hadrocodium remained undescribed until 2001; since then its large brain and advanced ear structure[5] have greatly influenced the interpretation of the earliest stages of mammalian evolution, as these mammalian characters could previously be traced only to some 150 million years ago.[6] Hadrocodium is known only from a skull, but the body is estimated to have been a mere 3.2 cm (1.3 in) in length and about 2 g (0.071 oz) in mass, making it one of the smallest mammaliaform ever (in contrast, the related ancestral mammal would have been bigger, about 10 cm or 4 inches).

The name Hadrocodium alludes to its large cranial cavity,[7] deriving from the Greek word hadrós (ἁδρός 'large, heavy, fullness')[4] and the Latin word codium, from Greek kṓdeia (κώδεια 'head [of a plant]').[8] The species name, wui, is the Latinized version of discoverer Xiao-Chun Wu's name.[2]

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,[9] including the middle-ear structure characteristic of modern mammals and a relatively large brain cavity.[5] These features had been considered limited to the crown group mammals, which emerged in the Middle Jurassic; the discovery of Hadrocodium suggests that these attributes appeared 45 million years earlier than previously thought.

Whether Hadrocodium was warm-blooded or cold-blooded has not been settled, although its apparent nocturnal features would seem to place it in the former group.

Hadrocodium are capable of laying eggs like today's platypus and echidna.

Family Tree
Main article: Evolution of mammals
Cynodontia
 
 

Dvinia

 
 

Procynosuchidae

 
 
Epicynodontia
 

Thrinaxodon

 
Eucynodontia
 
 

Cynognathus

 
 
 

Tritylodontidae

 
 

Traversodontidae

 
 
 
Probainognathia
 
 

Tritheledontidae

 
 

Chiniquodontidae

 
 
 
 

Prozostrodon

 
Mammaliaformes
 

Morganucodontidae

 
 
 

Docodonta

 
 
 

Hadrocodium

 
 
 

Kuehneotheriidae

 
 

crown group Mammals

 
 
 
 
 
 
 
 
 
 
Phylogeny [10]
Mammaliaformes 
 

 Adelobasileus

 
 
 

 Sinoconodon

 
 
 
 

 Morganucodon

 
 

 Megazostrodon

 
 
 
 

 Haramiyida

 
 
 
 

 Haldanodon

 
 

 Castorocauda

 
 
 
 

 Hadrocodium

 
 

 Mammalia

 
 
 
 
 
 
 

See also

References
  1. Luo, Crompton & Sun 2001, Abstract
  2. Parsell 2001
  3. Hei Koa Peng, Lufeng (CUP, IVPP) (Jurassic of China) in the Paleobiology Database. Retrieved April 2013.
  4. Luo, Crompton & Sun 2001, Note 1
  5. CNN 2001
  6. CMNH 2001
  7. Luo, Z.-X. (2001). "A New Mammaliaform from the Early Jurassic and Evolution of Mammalian Characteristics". Science. 292 (5521): 1535–40. Bibcode:2001Sci...292.1535L. doi:10.1126/science.1058476. PMID 11375489. S2CID 8738213.
  8. Liddell & Scott 1940
  9. Symmetrodonta - Palaeos
  10. Close, Roger A.; Friedman, Matt; Lloyd, Graeme T.; Benson, Roger B.J. (2015). "Evidence for a mid-Jurassic adaptive radiation in mammals". Current Biology. 25 (16): 2137–2142. doi:10.1016/j.cub.2015.06.047. PMID 26190074.

Bibliography


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