RE: Turtles and Crocodylians are not Reptiles - no? What are they?

[Tim Williams <twilliams_alpha@hotmail.com>]

Erik Boehm wrote:


> Its impossible to form a monophyletic definition, there is no way to 
> reconcile the exclusion of birds, or the
> inclusion of synapsids but not mammals.


True.  The issues are summarized here....

Modesto, S., and Anderson, J. (2004).  The phylogenetic definition of Reptilia. 
 Systematic Biology 53: 815-821.

They arrive at a stem-based definition for Reptilia (the most inclusive clade 
containing _Lacerta agilis_ and _Crocodylus niloticus_, but not _Homo 
sapiens_).  However, this revised Reptilia consciously excludes turtles from 
the definition, because of their uncertain phylogenetic position in amniote 
phylogeny.  Personally, I think the definition of Gauthier et al. (1988) is 
better, because it uses turtles, as well as squamates and crocs, as reference 
taxa.  So turtles, crocs and squamates would always be reptiles, and birds 
would fall inside Reptilia, whereas mammals (and all other synapsids) would 
fall outside.  


> But a paraphyletic definition is pretty easy.


Paraphyletic definitions are not standard practice in phylogenetic taxonomy, 
and I think they're probably a bad idea on the whole.  


> Wouldn't this be a good start for a paraphyletic definition:
> All aminotes excluding those closer to birds than to crocodiles, and 
> excluding all those descended from the 
> cynodonts


>From a historical/typological perspective, birds fall more comfortably inside 
>Reptilia than mammals given that they are descended from scaly ancestors (and 
>retain scales on the feet, etc).  On the other hand, mammals do not have true 
>scales, and never did.  Our skin has always been glandular, more like that of 
>amphibians than the reptiles around today.  As thrashed out in a recent 
>thread, the term "mammal-like reptile" is a misnomer.

Every phylogeny I have seen recovers the Synapsida (including mammals) as 
diverging from the lineage that gave rise to all other extant amniotes (crocs, 
birds, turtles, lizards, snakes, tuatara) and a host of extinct reptilian 
groups (plesiosaurs, ichthyosaurs, and the like).  Accordingly, new 
(phylogenetic) definitions of Reptilia include extant reptiles and birds, but 
not mammals.  

BTW, as a clade descended from cynodonts, mammals are themselves cynodonts.   
In other words, Mammalia is a clade inside Cynodontia.


Cheers

Tim

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