Hadrocodium

Hadrocodium Temporal range: Early Jurassic, 195 Ma PreЄ Є O S D C P T J K Pg N ↓
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Synapsida
(unranked):	Mammaliaformes
Genus:	Hadrocodium Luo, Crompton & Sun 2001
Species
H. wui Luo, Crompton & Sun 2001

Hadrocodium wui (hadro from Greek ἁδρός/hadros, "large, heavy, fullness";^[1] Latin: codium, from Greek κώδεια/kodeia, "head [of a plant]" ^[2] (alluding to its enlarged cranial cavity);^[3] and wui, the Latinized version of discoverer Xiao-Chun Wu's name^[4]) is an extinct mammaliaform that lived during the Sinemurian stage of the Early Jurassic approximately 195 million years ago^[5] in the Lufeng basin in what is now the Yunnan province in south-western China^[4] (25°12′N 102°06′E / 25.2°N 102.1°E, paleocoordinates 34°18′N 104°54′E / 34.3°N 104.9°E).^[6]

The fossil of this mouse-like, paper-clip sized animal was discovered in 1985 but was then interpreted as a juvenile morganucodontid.^[1] Hadrocodium remained undescribed until 2001; since then its large brain and advanced ear structure^[7] 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.^[8] 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 mammals ever.

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.^[7] These features had been considered limited to the crown group mammals, who emerged in the Middle Jurassic; the discovery of Hadrocodium suggests that these attributes appeared earlier (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 endotherm group.

Family Tree

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

• Evolution of mammalian auditory ossicles

References

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

Bibliography

• "Hadrocodium wui". Carnegie Museum of Natural History. 2001. Retrieved April 2013. Check date values in: |access-date= (help)
• "Tiny creature may be ancestor of all mammals". CNN. May 24, 2001. Retrieved April 2013. Check date values in: |access-date= (help)
• Liddell, Henry George; Scott, Robert (1940). "A Greek–English Lexicon: κώδεια". Perseus Digital Library. Retrieved April 2013. Check date values in: |access-date= (help)
• Luo, Zhe-Xi; Crompton, Alfred W.; Sun, Ai-Lin (2001). "A New Mammaliaform from the Early Jurassic and Evolution of Mammalian Characteristics" (PDF). Science. 292 (5521): 1535–1540. Bibcode:2001Sci...292.1535L. doi:10.1126/science.1058476. PMID 11375489.
• Parsell, D.L. (May 24, 2001). "Tiny Fossil From Early Jurassic Fills New Niche in Mammal Evolution". National Geographic News. Retrieved April 2013. Check date values in: |access-date= (help)
• Rowe, Timothy; Macrini, Thomas E; Luo, Zhe-Xi (May 2011). "Fossil Evidence on Origin of the Mammalian Brain". Science. 332 (955): 955–7. Bibcode:2011Sci...332..955R. doi:10.1126/science.1203117. PMID 21596988. (Supporting online material)

External links

• Rowe, Timothy; Macrini, Thomas E; Luo, Zhe-Xi (2006). "Hadrocodium wui (On-line)". Digital Morphology at the University of Texas. Retrieved April 2013. Check date values in: |access-date= (help) — 3D models from CT scans of the original fossil
• Palaeocritti - a guide to prehistoric animals