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RE: Non-Diagnostic Typification



> Nonetheless, the study shows us that,
> even though the type specimen appears to lack _autapomorphies_, it is
probably
> _apomorphic_ and may bear an _apomorphic suite_ which distinguishes it
from
> other hadrosaurids, and allows its placement within Hadrosaurinae.
There is
> thus no question of its value as a useful taxon, and one should not
regard it
> as a _nomen dubium_ because, as it may be, the cranial material is so
scarce.

It is exactly because of such potential debate about whether a taxon
represents a _nomen dubium_ or not that "_nomen dubium_" is a working
concept only, without any official nomenclatural status.

>  At what exact level does a taxon become useless for a type? 

The 'legal' answer for this, of course, is "no level" - any taxon can
potentially be used as type of a higher taxon. The practical answer is
that we can't really answer that beyond 'the level that it can still be
demonstrated to possess the character(s) of the taxon it typifies'.
This, of course, will vary from taxon to taxon. Also, even a quite
detailed description can be insufficient if it is missing the crucial
characters. In many invertebrate groups, for instance, even different
families can sometimes only be distinguished by examination of the
genitalia. I remember talking to someone who worked on jumping spiders,
and complaining about the illustrations of many species in the original
19th Century descriptions, which were beautifully detailed, artistic,
life-like - and completely useless, because for all their detail they
didn't show the crucial features for identification.
    I take particular exception to one sentence in Prieto-Marquez et al.
(2006) when they touch on this question: "It is unclear what the meaning
of "well-known" is, but if it refers to complete knowledge of the
anatomy of a taxon, the genus _Hadrosaurus_ would not follow this
recommendation" [that only well-known taxa be used as types]. This may
be just sloppy sentence composition, but I feel compelled to reply that
complete knowledge of a taxon is _never_ possible, and even what was
once _sufficient_ knowledge may become no longer so as our knowledge of
related taxa increases (compare, for instance, the recent debate on this
list about how _Troodon_ is only a diagnostic taxon so long as only one
species with that form of teeth is known to have inhabited the type
locality and time).

> Are we required
> to have autapomorphies when diagnosing taxa or names based on them? Or
are
> apomorphic suites sufficient and thus indicative of a diagnostic
taxon? Are
> apomorphic suites useful for distinguishing species?

I'm not exactly sure what an 'apomorphic suite' is, I'm sorry, but I'm
not a fan of the idea that species must be defined by 'autapomorphies'.
Firstly, if the characters defining species have been derived multiple
times, then species may differ in the combination of characters, without
a phylogenetic analysis being able to distinguish which are apomorphies
and which plesiomorphies for each particular species. Second, if one
species is ancestral to another, then the former (by definition) does
not possess any autapomorphies relative to its descendant.

One question which came into my mind - if _Hadrosaurus_ turns out not to
be a member of the clade usually called Hadrosaurinae, what would the
correct name for this clade be (under the ICZN, at least)? I seem to
recall seeing a few tribes named within Hadrosaurinae ('Kritosaurini'
seems to ring a bell). Would one of these take priority?

    Cheers,

        Christopher Taylor

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