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I think the reason stability of definition won out is three-fold.
- First, given a topology, a definition is easy and uncontroversial to apply. Content can change in so many ways though, each bringing a new subjective question to 'answer' before knowing what to do with the clade. Sure your phytosaur example might seem
obvious, but what if ornithosuchids move a node stemward of Euparkeria? Do we exclude ornithosuchids from Archosauria or bring Euparkeria into the fold? Or what if turtles are archosaurs? That's a huge change in content, especially for living species.
At that point someone concerned with content might be excused for dumping Archosauria and using Archelosauria for the clade instead. One end result of the latter is placental mammal nomenclature from molecular studies circa 2000. Cetartiodactyla because
whales weren't originally artiodactyls, Eulipotyphla and Euarchonta because some taxa were removed from Lipotyphla and Archonta, etc..
- Second, stability of definition makes it so that saying a taxon is a member of clade X actually tells you something specific about that taxon's relationships. For content-based groups, it's more of a fuzzy, subjective "similar enough to at least the majority
of members to be classified here."
- This subjectivity in both points gets at the deeper reason stability of definition won- it's based on the real spatiotemporal phenomenon of cladogenesis. There are an actual finite number of branching points to be discovered, and basing our nomenclature
on these means we're naming objective concepts. There's actually some foreseeable final design that we're approaching as we learn more about the tree of life. But what would a final, objective, idealized version of content-based definitions even be? A list
of all taxa that must belong to a group and a list of those which can never belong to it? Or maybe you can only add a certain number of taxa from a list of non-members before you have to abandon the group name? Obviously any such rules around content-based
groups would be just the subjective whim of each author, not representing an existing structure in nature. And thus like any subject unmoored from reality, there could never be any real progress.
Mickey Mortimer
From: dinosaur-l-request@mymaillists.usc.edu <dinosaur-l-request@mymaillists.usc.edu> on behalf of dawidmazurek@wp.pl <dawidmazurek@wp.pl>
Sent: Saturday, June 13, 2020 12:14 AM
To: dinosaur-l@usc.edu <dinosaur-l@usc.edu>
Subject: Odp: [dinosaur] Party like it's 1758!
Consider recent ideas on phytosaur phylogenetic position. If
you prefer the stability of definitions, phytosaurs fall outside Archosauria. But there is a clade for phytosaurs + archosaurs, that have basically the same content as was though for Archosauria before. So, for the 'other' stability, why not broaden Archosauria,
with phytosaurs considered just the first offshot? While the name flips to another node, the stability of content is saved, and 'new' archosaurs, for practical purposes, remain the same group as 'old' archosaurs.
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