Author's note: these pages were written some years ago. I am not planning to update them. For a more current coverage, see the Palaeos website (to which many links on these pages point to anyway. More info here

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)		Division Sauropsida (common ancestor)
Class Aves (Birds) (warm-blooded, feathered, lay eggs)
Class Mammalia (warm-blooded, furry,  live young)		Division Synapsida (common ancestor)

The difficulty of reconciling the Linnean and the cladistic systems
 
 

Systematics page

Page History