Re: LATE SURVIVING CYNODONT (lenghty and boring)

[Pieter.Depuydt@rug.ac.be]

on 6 June, Ron Dass wrote:

>         First of all, I confess that I am at least as taken with ancient
> mammals as 
> with dinosaurs, so I followed this thread with interest. Here's my question.
> I invite
> your educated guesses and wild speculation.
>         If presented with a *living* specimen of an advanced cynodont, could
> one differentiate it from a "real" mammal without dissecting it? If memory
> serves me, the advanced "mammal-like reptiles" (sorry cladists!) were so
> mammal like that a more or less arbitrary skeletal feature was selected as a
> kind of borderline that separated 
> the fossil cynodonts from the mammals. A two part jawbone or something,
> where the 
> posterior part in true mammals migrated into the middle ear. Sorry, I don't
> know too
> many Latin bone names. 

The evolution of the cynodont therapsid lineage towards "mammalness" 
went via the progressive accumulation of anatomical mammal-like 
novelties, with lots of intermediate forms, convergences and mosaics 
of more primitive traits together with more derived features. As new 
intermediate taxa are discovered or re-studied, the definition of 
what is a mammal constantly needs revision. For example: before the 
study of Sinoconodon, a distinct mammal apomorphy was considered the 
presence of postcanine teeth differentiated in premolars (which are 
replaced once in a lifetime) and molars (which are not changed) (so 
called diphyodonty). Sinoconodon's postcanine teeth are not 
differentiated in premolars and molars and were probably replaced 
multiple times in the animal's life. However in the structure of the 
inner ear and the jaw articulation, Sinoconodon is clearly a 
mammal...
Another example: the transition from the primitive articular-quadrate jaw hinge 
to the 
mammalian dentary-squamosal articulation went via a series of 
intermediate forms who possessed a double hinge (Probainognathus, 
Pachygelenus,...)
Convergence also blurs things: for example: a jaw movement in which 
the lower teeth pass medial to the upper teeth during closure, 
leading to a triangular lower jaw movement curve (only one side of 
the lower jaw "bites" at a time) is considered as a mammal apomorphy 
(in non-mammalian cynodonts such as the gomphodonts, the lower teeth 
close behind the upper teeth; the lower jaw has a slight 
anteroposterior movement); but closer examination of the wear facets 
of the postcanines of the trithelodonts (a certain clade of 
non-mammalian cynodonts, considered by some as the sister group to 
mammals) reveals they had also a medial rather than posterior 
movement of the lower jaw, whereas they were more primitive in other 
skull features.
The decision of what is a mammalian and what is a non-mammalian cynodont 
is a subjective one, it depends on what node on the cladogram you 
define as the first (hypothetical) mammal.

In all of this writing, I assume of course cynodonts are the 
therapsid line leading to mammals; this is accepted by most of the 
scientific community, however some paleontologists argue mammals 
arose from derived therocephalians (the small 'scaloposaurs' or 
'bauriids')

> Is it widely accepted that Cynodonts were fur
> covered? How far 
> back did this likely go? (furry Dimetrodon? What a concept!)

There is no direct evidence of fur covering in non-mammalian 
cynodonts or other (non-mamm...) therapsids. The oldest record of fur 
is from Paleogene excreta (see in some 1997 Nature issue).
The "classic" evidence cited in the literature is the presence of 
small pits in the snout region of some well preserved cynodont skulls 
(Thrinaxodon) as well as some therocephalians (Moschowhaitsia), which 
are interpreted as the holes through which vessels and nerves passed 
to whiskers (vibrissae). However some extant lizards also show such 
pits in their skull and moreover, the presence of whiskers does not 
imply the presence of a fur covering of the whole body.
More evidence of (non m...) cynodont endothermy (and indirectly of 
cynodont fur) is found in the bone histology (fibrolamellar) bone and 
the presence of nasal turbinates; but as everyone on the list already 
knows, there is quite some controversy about the value of these 
findings...
The only direct evidence of therapsid skin comes from fossilized skin 
of estemmenosuchids, which appears to be smooth, withour scales, scutes or 
hair, 
and had probably a lot of glands.
The presence of fur on Dimetrodon seems very unlikely, given its
primitive "reptilian" body form and the presence of the elongated 
neural spines, which offered a very useful tool for thermoregulatory purposes.
Dimetrodon was probably to some degree an inertial homeoterm 
(especially the later, larger species e.g. D. grandis), which could 
also warm up more quickly by the aid of its sail.

 I also remember an 
> undergraduate zoology instructor saying that if monotremes had been found in
> the 
> fossil record before the living animals were seen they might well have been
> classified
> as reptiles. this class was taken eons ago in the precladist era.
>         Incidently, on this very subject I once expained to tenth grade
> science class
> that if reptiles are in New York and mammals are in San Francisco, and I-80
> is the 
> evolutionary path between, the platypus took a left in Chicago and wound up
> in Baton Rouge ;)

Monotremes retain some primitive features reminiscent of 
morganucodontids and the cynodont ancestors especially in the 
slightly sprawling position of the front limb. The middle ear 
ossicles are also somewhat different from those of the Therian 
(Placental and Marsupial mammals). However Monotremata probably 
branched off quite some time after the appearance of the first true 
mammal, well up in the "mammal" tree.
The problem in establishing their relationships is the very poor fossil
record of monotremes and the high  specialisation of the extant platypus
and echidnas.

Pieter Depuydt