A little Linguipolygnathus
Published 12 November 2010
The objects in the figure above represent P-elements of an ozarkodinid conodont. Linguipolygnathus linguiformis is the type species of Linguipolygnathus, one of the genera carved by Bardashev et al. (2002) out of the large older genus Polygnathus. I’ve commented on the taxonomic insanity of Bardashev et al. in a previous post, though the idea of subdividing Polygnathus is not in itself a bad one (and note that if Linguipolygnathus were synonymised with its supposed polyphyletically-ancestral genus Eolinguipolygnathus we’d be left with a single monophyletic genus).
Many discussions of conodonts make reference to their minuteness (I’ve done it myself in the past) and the preserved conodont fossils are certainly minute. However, I must confess to only realising fairly recently that, just because the preserved fossils are minute, doesn’t necessarily mean that the (largely soft-bodied and hence rarely preserved) animals themselves were. Of the two best-preserved body fossils of conodonts available to us, the remains of Promissum are those of an animal about 20 cm long. Even the more modestly sized Clydagnathus, which is apparently more like the usual run of conodonts, would have been about 6 cm long in life: not huge, but still comparable in size to a modern anchovy.
Published 18 June 2012
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.
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.
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. Slavík & J. I. Valenzuela-Ríos. 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.