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Re: Cetartiodactyla (was Re: Cladistic taxonomy (was Dietary factors))



Tim,
That sounds very interesting. I recognize a separate Order Mesonychiformes, so no matter how it and artiodactyls and cetaceans are interrelated, I will be able to reflect it in my classifications without having to change the contents of any of these three orders.
Therefore retaining the traditional Artiodactyliformes (without cetaceans) gives me more flexibility. Whether it is paraphyletic (gave rise to cetaceans) or holophyletic, I still have the same stable groups (Orders) and can reflect all these controversial interrelationships with an easily modified coding sequence. Thus a cladisto-eclectic approach is informative and flexible without the instability of a purely cladistic approach.
I rest my case,
Ken
********************************************************
From: "Tim Williams" <twilliams_alpha@hotmail.com>
Reply-To: twilliams_alpha@hotmail.com
To: dinosaur@usc.edu
Subject: Cetartiodactyla (was Re: Cladistic taxonomy (was Dietary factors))
Date: Wed, 09 May 2001 13:43:12 -0500



Jaime A. Headden wrote:

  I'm afraid that Cetartiodactyla and Artiodactyla do not have
the same content ... not explicitly. The new name was chosen to
reflect a paradigm.

The monophyly of the Artiodactyla does not change if cetaceans (as their
descendents) are included in the clade. It remains a valid taxon. A strict
cladist might therefore say that replacing "Artiodactyla" with
"Cetardiodactyla" is unnecessary. It's as redundant as changing the name of
the Dinosauria to Avedinosauria in light of the fact that birds (Aves) are
now almost universally recognized as dinosaur descendents.


But as Jaime said, the change of name does reflect a paradigm shift. The
(Cet)artiodactyla now not only includes the majority of modern ungulates but
the majority of modern marine mammals as well. The term "Cetartiodactyla"
also gives notice to the reader that cetaceans are being included with the
Artiodactyla, and becomes a shorthand way of saying "Artiodactyla, including
Cetacea".


To date, almost all the support for the Cetartiodactyla (Cetacea evolving
from *within* the Artiodactyla, close to the hippopotamids) comes from
molecular data (sequence alignments from 12S rRNA, cytochrome b,
retinoid-binding protein genes, non-coding SINE elements, etc). By
contrast, analyses based on anatomical (skeletal and dental) characters from
fossil and extant mammals tend to support a close relationship between
Cetacea and Artiodactyla, but with both descending independently from
ancestors close to (or within) the Mesonychia. (The mesonychians
[Hapalodectidae + Mesonychidae] are a group of extinct terrestrial
carnivores formerly included in the Condylarthra).


However, there is a paper soon to be published which proposes a reason for
this apparent conflict between morphological and molecular data in the
question of Cetacea / Artiodactyla relationships.  The proposed explanation
ties in with Ralph Chapman's point about exercising greater discretion in
choosing the anatomical characters used in cladistic analyses.  I can't say
much, so I'll *bite* my tongue for now...


Tim


------------------------------------------------------------

Timothy J. Williams

USDA/ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163

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