The Linnean and the Phylogenetic (cladistic) arrangements

To give an example: the linnean system distinguishes separate classes for reptiles, birds and mammals.  Reptiles are cold-blooded and scaley and crawl (or slither in the case of snakes) and lay eggs which they then abandon (the only exception being the crocodylia which guard their nest), and grow new teeth their whole lives.  Birds are warm-blooded, feathered and fly (or with flightless birds descend from flying ancestors), lay eggs and care for their young, and have erect stance and a toothless beak.   Mammals are warm-blooded, furry, have erect stance, give birth to live young and care for them, and replace their teeth only once.  So there are clear morphological differences.

But when you trace back the evolutionary tree you find that mammals merge into mammal-like reptiles (cynodont therapsids) and birds into bird-like reptiles (theropod dinosaurs).  The cladistic classification has the ancestral amniote (egg-laying) stock giving rise to two lines, Sauropsids (reptiles, dinosaurs and birds) and Synapsids (mammal-like reptiles and mammals).  Both Sauropsids and Synapsids start as conventional scaly reptiles, but one develops into birds and the other into mammals.
 

Linnean system - morphology
Cladistic system - ancestor-descendent
Class Reptilia
(cold-blooded, scaley, lay eggs)
mammal-like reptile
reptile
dinosaur
Division Sauropsida
(common ancestor)
 reptile
dinosaur
bird
Class Aves (Birds)
(warm-blooded, feathered, lay eggs)
bird
Class Mammalia
(warm-blooded, furry,  live young)
 mammal
Division Synapsida
(common ancestor)
 mammal-like reptile
mammal

internal linkThe difficulty of reconciling the Linnean and the cladistic systems
 
 
to Vertebrate page
Systematics page


 
 

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Page History

page uploaded 13 December 1998
revised 9 & 14 January 2000