Re: Dinosaurs versus mammals

[Pieter.Depuydt@rug.ac.be]

> At 10:25 AM 5/23/97 -0400, Jeff Poling wrote:
> >   According to the recent AMNH book about their collection, it's the 
> >other way around:  true mammals evolved before true dinosaurs.  Is 
> >there any consensus on this?
> 
> Odd, I have never heard this.
> 
> The earliest "true" mammals I know of are the Haramiyids, Morganucodontids
> and Kuehneotheriids of the Late Triassic, specifically the Late Norian (or
> Rhaetian, if one accepts that time division). This is substantially alter
> than the earliest known dinosaur, which are the early forms from the
> ?Carnian of South America.  Mammals certainly do not become really diverse
> and widespread until the earliest Jurassic.

New haramiyid material from the Norian of Greenland reveals these 
early mammals have a  highly specialized dentitition 
(puncture-crushing occlusion) radically differing from the teeth of 
other Late Triassic/Early Jurassic mammals (morganucondontids and 
kuehneotheriids) which have a grinding or shearing dentition
(See Jenkins et al: Nature 20 Febr 1997).
So mammals had by end-Triassic times already radiated substantially, 
which led the authors to suggest a much earlier origin of true 
mammals, perhaps as far bach as Middle Triassic (this last dating is 
speculative of course).

> If one allows that the first mammals to predate the earliest records, one
> must likewise allow that the first dinosaurs preceded *their* first
> records, so that is a wash.  One is probably talking an early to mid-Norian
> origin, at the earliest, for mammals, and probably a mid-Triassic origin
> for dinosaurs.

> Now, the real weakness of this exclusionary model is that the later
> cynodonts were very similar to mammals, especially the Traversodonts,
> Tritylodontids, and Thrinaxodontids.  And these occur rather earlier than
> the mid-Norian - in fact Traversodonts are known from earlier than
> dinosaurs, dating from the end of the Early Triassic.

Cynodont evolution indeed shows a gradual evolution towards more 
"mammalness" in tooth morphology, gait, jaw and middle ear 
structure,... The most derived non-mammalian  cynodonts
 ('chiniquodontids' (I'm not sure this is still considered as a monophyletic 
taxon..), 
'traversodontids' (a grade, not a clade), Probainognathus, Tritylodontidae and 
Trithelodontidae) are very mammal-like in their general anatomy. 
The recent interest in microvertebrate remains has revealed the 
coexistence of very small carnivorous and insectivorous non-mammalian 
cynodonts together with the first true mammals (at least in the 
Middle European locality of Lorraine(See Fraser and Sues: "In the 
Shadow of the Dinosaurs"). Why did the mammal survive and 
did their near relatives not  make it?....
Note:  Thrinaxodon is now considered as the sister group to all Middle 
Triassic and later cynodonts (Eucynodontia Kemp is defined as all 
cynodonts closer to mammals than to Thrinaxodon)(pardon me if my 
terminology/vocabulaire isn't exact; I am a layman who tries 
understanding cladistics).

> Whatever went on, it looks like it was fairly subtle and complex.

You are completely right.

Last note: the last chapters of the forementioned "In the Shadow of 
the Dinosaurs" and its 'predecessor' Padian et al: "The Beginning of 
the Age of Dinosaurs" give a nice synopsis of the current held views 
on dinosaur and mammalian origins.
> --------------
> May the peace of God be with you.         sarima@ix.netcom.com
>                                           sfriesen@netlock.com
> 
> 
>