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

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...



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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