* * *


The Therapsids
Previous Page

Therapsid Ancestors  —  the Pelycosaurs
We humans can trace our lineage back 300 million years
to the very successful pelycosaurs. Well-adapted to lush equatorial lowlands,  these early synapsids*  flourished. The very first complex ecosystem on land  —  including large plant-eaters and big meat-eating predators  —  was woven from the many lines of our pelycosaur ancestors.

But, about 270 million years ago  —  in the middle of the Permian — the climate changed drastically. The constant warm, wet lushness at the equator gave way to the first real seasons since the start of the Permian period.  Cooler seasonal temperatures and drier habitats created an alien world.  Unable to survive this new environment, most of the pelycosaurs became extinct.

Fortunately for us, this was not the end of the synapsid story  —  pelycosaurs left  heirs apparent to their empire.

A Mid-Permian Pelycosaur Predator  —  Dimetrodon limbatus  —  above left
The famous fin-backed pelycosaur,  Dimetrodon  ( 'two-types of teeth' - 3m long ) needs little introduction.  Despite its sprawling stance and thick-limbs, the toothy Dimetrodon and its kin were the top, unrivalled land predators of their time.

So, what did pelycosaurs use that big fin for?  Some say temperature control. If so, it is very odd that only some pelycosaurs were endowed with this dorsal radiator.

Perhaps it depended upon environment.  Dimetrodon and Edaphosaurus,  the sail- backed plant-eater, may have lived in open, sunny areas while their finless equivalents,  Sphenacodon and Casea, dwelt in heavily forested regions. Under the treed canopy, a large fin would offer no advantage in controlling body temperature and the tall spines supporting the fin would make manouvring beneath low branches very awkward.

* In phylogeny (the tracing of the origins and evolution of the different groups of living things), the great Class Synapsida includes both the pelycosaurian and therapsid Orders. To phylogenists, you, me and all other therian mammals are both synapsids and living therapsids.

The Therapsids
Previous Page

* * *

Earliest Therapsids
Next Page