Question About Mammal-like-Reptiles

In the history of life, when did the Mammal-like-Reptiles live? _If_ I remember correctly, cladist represent them as ancestor to mammals, yet they are found in strata that is older dinosaurs.

What are the facts concerning these animals?

H. Robert Dean

Cladists don't call them 'mammal-like reptiles', since they don't belong to Clade _Reptilia_. A more accurate term would be 'non-mammalian synapsids'.

The first synapsid existed well before the first dinosaur, during the Pennsylvanian Age. One synapsid clade, _Therapsida_, replaced other synapsids during the Permian. Most types of therapsid died out by the end of the Triassic

The mammal-like reptiles first appeared in the permain, along with the thecodonts (dinosaur ancestors). Dinosaurs and Mammals more or less

The term 'thecodont' as meaning any non-dinosaurian, non-pterosaurian, non-crocodyliform archosauromorph is falling into disuse. Instead of just being 'dinosaur ancestors', this is a highly diverse group of archosauromorphs that really doesn't have any uniting features, and hence is not a particularly useful group.

'Thecodont' is still used to describe a dental condition, though.

I realize that 'dinosaur ancestors' doesn't nearly do the thecodonts justice, but I was trying to keep thing simple.

Incidently, I was under the impression that crocodylians were considered a type of thecodont, rather than as a separate group (like Dinosaurs and Pterosaurs). E.g., not a group derrived from thecodonts, but a group that retains (almost?) every characteristic of their thecodont ancestors with little-to-no derrived characteristics...

Were the synodonts marsupial type mammals?

Bob Dean

Crocodylians are actually fairly derived in some ways. They have a complex heart structure, although it's currently impossible to know how far down the line that trait extends.

But, as I said, the taxon Thecodontia is a rather outdated concept, and falling from use. There is no trait which unites them

sallan wrote:

Were the synodonts marsupial type mammals?

Bob Dean

That is a difficult question to answer as soft tissue doesn't commonly fossilze. However, let me correct a few things:

1. Cynodonts are a group of "non-mammalian synapsids," and not TRUE mammals. (Synapsids is term that usually refers to mammals, yet, as in this case, refers to Cynodonts as well.)

2. These animals were more of a transition between the Reptilia and Mammalia Classes.


Basically, there is NO evidence of marsupial characteristics among this group.

Hope this helps...though I hope I didn't get TOO technical...