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Re: Segregated vs age-mixed sauropod herds



Tom Holtz <tholtz@umd.edu> wrote:


> > Prototheria has AFAIK never been defined, and hasn't been used at all
> > since phylogenetic nomenclature (and, basically, cladistics) got going
> > in the field Mesozoic mammal phylogeny.
>
> Sereno does so in:
> Sereno, P. C. Shoulder girdle and forelimb in a Cretaceous
> multituberculate: Form, functional evolution, and a proposal for basal
> mammalian taxonomy; Chpt. 10, pp. 315-370 in M. T. Carrano, T. J. 
> Gaudin, R. W. Blob, and J. R. Wible (eds.), Amniote Paleobiology: 
> Perspectives on the Evolution of Mammals, Birds, and Reptiles. 
> University of Chicago Press, Chicago. (13)


Yes, here Prototheria is formally revived as a stem-based clade that includes 
the node-based Monotremata, which is effectively restricted to the crown clade. 
 This contradicts the approach of Archer et al. (1993), who used a stem-based 
Monotremata.  Archer &c' approach ensures that fossil critters like 
_Steropodon_ and _Kollikodon_ are still monotremes, even if they lie outside 
the crown clade.  By contrast, under Sereno's definitions, _Steropodon_ and 
_Kollikodon_ would be non-monotreme prototherians.

Similar to his approach to Prototheria/Monotremata, Sereno defines Allotheria 
as the stem-based clade that is more inclusive than the node-based 
Multituberculata (except that unlike Monotremata, is not a crown clade!)


Mike Keesey <keesey@gmail.com> wrote:
 
> Popular old names like "Mammalia" originated in the neontological
> literature and are still more prevalent in use there. Inferences are
> commonly made based on living forms, and these can go awry
> when members of the stem group are referred to by names commonly
> used to refer to the crown. For example, we all know that mammals have
> determinate growth and are diphyodont -- oh, but Sinoconodon had 
> indeterminate growth and was polyphodont. Did it lactate? Was it
> endothermic? Did it have pinnae? Who knows? What does it buy us to
> anchor such a common name to an arbitrarily selected member of the
> stem group (especially when
s the number of things we can confidently
> state about mammals, and thus depreciates the value of the term.


I take your point, and it's a good one.  But I think it's okay to depreciate 
the value of the term 'mammal'.  After all Mammalia is just one of many clades: 
Mammaliamorpha, Mammaliaformes (anchored in _Sinoconodon_), Theriomorpha, 
Theriiformes, Theria...  etc.  Although we cannot be certain when certain 
mammal traits evolved (e.g., determinate growth, lactation, diphyodonty, 
pinnae), it is unlikely that all of these traits are exclusive to crown mammals.

To me, the danger of crown groups is that they have the (completely 
unintentional) tendency of emphasizing typological differences: in this case, 
mammals vs non-mammals.  However, because 'mammal' traits were acquired 
gradually, I think it's a positive that the definition of Mammalia is somewhat 
arbitrary (such as anchoring Mammalia in _Sinoconodon_).  It relieves Mammalia 
of its Linnaean baggage.

To return to dinosaurs, I think there is a similar danger to defining Aves to 
be the crown group.  Birds were not cut from whole cloth, and individual 
'avian' characters first appeared at different stages in evolution.  Attaching 
the name Aves to the clade that includes _Archaeopteryx_ and modern birds 
(rather than to just the crown group) is certainly arbitrary; but I think this 
is a good thing.  It underlines the fact that most of the traits that make 
birds so distinctive in the modern world (feathers, flight, furcula, pygostyle, 
etc) were first acquired by taxa that are long extinct, and these traits were 
merely inherited by modern birds.  



Cheers

Tim