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There are some things that you find yourself returning to like an itching scab. Yes, it's time for me to once again wade into the unsettling world of polygnathid conodont taxonomy.
Previous comments on the subject can be found here and here. To briefly recap: Bardashev et al. (2002) divided the Devonian conodonts of the Polygnathidae, most of them previously assigned to a single genus Polygnathus, between a number of families and genera. However, they explicitly represented a number of both families and genera as extensively polyphyletic. Later, Weddige (2005) responded to criticisms of the divided taxonomy by claiming that it represented a form taxonomy only. In my first post on the subject, I expressed confusion at what exactly Weddige meant by that claim.
On closer examination, I'm somewhat more inclined to take Weddige's claim at face value. Despite proposing detailed phylogenetic relationships between the species studied, Bardashev et al.'s (2002) taxonomy is supposed to prioritise identification above all. The primary division, Polygnathidae vs 'Eopolygnathidae', is based on a single character: the development of the basal cavity on the underside of the Pa element of the polygnathid apparatus. Presence of a basal cavity is the ancestral condition; within the 'eopolygnathids', the margins of the basal cavity become progressively closed in a number of lineages until, in the 'polygnathids', there is only a small basal pit remaining. So, for instance, the genera Eolinguipolygnathus and Linguipolygnathus are placed in separate families, despite the facts that (a) they differ in no other characters (the diagnoses provided for the two genera by Bardashev et al. are effectively identical), (b) 'Linguipolygnathus' is proposed to have arisen no less than five separate times from 'Eolinguipolygnathus' ancestors (and is not directly connected phylogenetically to other genera in its own family), and (c) relative to the other polygnathids examined, the two 'genera' supposedly share a clear and (more significantly) phylogenetically coherent character in the formation of the posterior part of the Pa element into a transversely ridged tongue.
Bardashev et al. argued that this division was necessary because the restricted basal cavity was the original character used to establish the Polygnathidae, so the inclusion of taxa with an open basal cavity violated the original diagnosis of the family. The possibility of revising the family diagnosis is not raised, despite their own research apparently showing that it does not diagnose a coherent group. The underlying motivation for this prioritisation of diagnostic characters over phylogeny seems to be the use of conodonts as markers in biostratigraphy. For instance, the type of Eolinguipolygnathus, Polygnathus dehiscens, has been proposed as the marker for the beginning of the section of the Devonian known as the Emsian. But does this emphasis on diagnostic features truly serve even biostratigraphy? Carls et al. (2008), for instance, claim that emphasis on characters of the ventral side of the conodont Pa element has lead to a number of distinct taxa being confused under the name 'Polygnathus dehiscens', leading to misdiagnosis of the Emsian boundary.
Just as I have stated before that a key should not be a taxonomy, a taxonomy should not be a key. Both are important, but both have their own roles to play.
REFERENCES
Bardashev, I. A., K. Weddige & W. Ziegler. 2002. The phylomorphogenesis of some Early Devonian platform conodonts. Senckenbergiana Lethaea 82 (2): 375-451.
Carls, P., L. SlavÃk & J. I. Valenzuela-RÃos. 2008. Comments on the GSSP for the basal Emsian stage boundary: the need for its redefinition. Bulletin of Geosciences 83 (4): 383–390.
Weddige, K. 2005. Contra Ruth Mawson’s critizising Bardashev, Weddige & Ziegler 2002, e.g. in SDS Newsletters 20 (2004). Subcommission on Devonian Stratigraphy Newsletter 21: 51-52.
If the polygnathid condition has arisen independently many times, surely it must be basically useless for stratigraphy? One can't very well assume all acquisitions were simultaneous.
ReplyDeleteIt's always funny when people get crazy notions about what rules nomenclature has to follow. Like Hutchinson and Chiappe (1998) claiming Parvicursorinae wasn't a senior synonym of Mononykinae because its authors didn't think Mononykus belonged in it. Or the increasingly disasterous idea of Wilson and Upchurch (2003) that family-level taxa need to be based on eponymous holotypes diagnostic at species level, which is causing all sorts of unnecessary petitions for neotypes (Allosaurus, Anchisaurus, Stegosaurus).
ReplyDeleteIf the polygnathid condition has arisen independently many times, surely it must be basically useless for stratigraphy? One can't very well assume all acquisitions were simultaneous.
ReplyDeleteThe zonation diagrams presented in Bardashev et al. (2002) would seem to indicate that the polygnathid condition did arise more or less simultaneously, at least within the larger lineages. I'll admit, it looks iffy to me, but not being a worker in the field myself I have to start on the presumption that Barsashev et al. know their subject better than me. If I correctly understand Talent & Mawson's (2003) criticism of Bardashev et al. (cited in one of the earlier posts on this subject), one of their complaints was that Bardashev et al. didn't provide stratigraphic diagrams, so not providing the physical data on which their zonation was based.