Conodont evidence for a latest Emsian to early Eifelian (Devonian) age for the phacopid trilobite Plagiolaria kitabi from Uzbekistan

GILBERT KLAPPER, CATHERINE CRÔNIER & STANISLAVA VODRÁŽKOVÁ

THE DEVONIAN phacopid trilobite Plagiolaria kitabi Crônier & Tsmeyrek (2010) was described from the Akbasai Formation of Uzbekistan, from a part of the formation that has been considered to be late Famennian on the basis of a few conodont specimens attributed to Hindeodella and Polygnathus (collected and determined by local worker M. V. Erina). The conodont species misidentified as Polygnathus inornatus E.R. Branson (1934) by C. Randon (in Crônier & Tsmeyrek 2010, p. 44, figs. 2G-J) occurs in the same beds as Plagiolaria kitabi. A major problem with the identification of this conodont species and the dating of the trilobite-bearing beds as late Famennian is that the genus Plagiolaria is known only from the uppermost Emsian to lower Eifelian elsewhere in the world (Crônier & Tsmeyrek 2010, p. 49). Thus, the authors of Plagiolaria kitabi raised doubt about the late Famennian age of the beds in question.

This problem is resolved by identification of the illustrated conodont specimen as Polygnathus serotinus Telford (1975), a species that ranges from the upper Emsian to the lower Eifelian (serotinus to costatus zones) in Devonian tropical areas (Klapper & Johnson 1980, tables 6, 7). This is in full agreement with the known range of Plagiolaria.

Polygnathus inornatus was originally illustrated by E.R. Branson (1934, pl. 25, figs 8, 26) and E.B. Branson & Mehl (1934, pl. 24, figs 5-7) and has been reviewed in the Catalogue of Conodonts (Klapper 1975, p. 293-297) with a comprehensive synonymy and new photographs of the lectotype and paralectotype in both upper and lower views. Polygnathus inornatus is restricted in range to the lower

Mississippian (Tournaisian), although there are reported occurrences in the latest Famennian, which may represent a related but possibly homeomorphic species (the latter has primarily been indicated in lists, rather than illustrated in the conodont literature). The most important distinction between Polygnathus inornatus and P. serotinus involves the character of the basal pit. In Polygnathus inornatus the pit is symmetrical, whereas it is highly asymmetrical in P. serotinus. Furthermore, the carina of P. inornatus is almost straight to slightly sinuous, whereas it is strongly incurved in Polygnathus serotinus. Also, the outer and inner sides of the platform of Polygnathus inornatus are more or less symmetrical, whereas the posterior outer margin of P. serotinus is sharply incurved near the posterior end making the platform outline strongly asymmetrical. Outer and inner margins of Polygnathus inornatus are equal in height, whereas the posterior outer margin of P. serotinus is distinctly higher than the inner margin.

Two additional specimens of Polygnathus serotinus from the Plagiolaria kitabi beds in Uzbekistan are illustrated here in Figure 1, showing both the upper and lower views. These specimens represent an early ontogenetic stage of Polygnathus serotinus in which there is a flat, asymmetrically curved area to the outer side of the growth centre of the pit (Fig. 1B, D). In later ontogenetic stages this flattened area develops into a small, subcircular, shelf-like protuberance on the outer side of the pit. Such a protuberance is completely lacking in Polygnathus inornatus. A number of junior synonyms have been proposed for the early ontogenetic stage

KLAPPER, G., CRÔNIER, C. & VODRÁŽKOVÁ, S., 2012:04:26. Conodont evidence for a latest Emsian to early Eifelian (Devonian) age for the phacopid trilobite Plagiolaria kitabi from Uzbekistan. Memoirs of the Association of Australasian Palaeontologists 44, 25-26. ISSN 0810-8889.

The Devonian phacopid trilobite Plagiolaria kitabi Crônier & Tsmeyrek was described from a part of the Akbasai Formation in Uzbekistan that had been considered tentatively to be of late Famennian age based on the misidentification of Polygnathus inornatus Branson, a late Famennian? – Tournaisian conodont species. The revised identification herein of the associated conodont as Polygnathus serotinus Telford, in the unit containing Plagiolaria kitabi, supports the uppermost Emsian to lower Eifelian range of the trilobite genus Plagiolaria. Two new conodont specimens represent an early ontogenetic stage of Polygnathus serotinus in which there is a flat, asymmetrically curved area to the outer side of the growth center of the pit.

G. Klapper (g-klapper@northwestern.edu), Earth and Planetary Sciences, Northwestern University, Evanston, IL 60208, USA; C. Crônier (catherine.cronier@univ-lille1.fr), Université Lille I, UFR Sciences de la Terre, UMR 8217 Géosystèmes, 59655 Villeneuve d’Ascq Cedex, France; S. Vodrážková (stana.vodrazkova@seznam.cz), GeoZentrum Nordbayern, Fachgruppe Paläoumwelt, Friedrich-Alexander Universität Erlangen Nürnberg Loewenichstr. 28, Erlangen Germany and Czech Geological Survey, P.O.B. 85, 118 21 Prague 1, Czech Republic. Received 21 February 2012.

Keywords: Devonian, trilobite, conodont, Plagiolaria, Uzbekistan.

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of Polygnathus serotinus and these are reviewed in a paper by Klapper & Vodrážková (in preparation) on the ontogeny and intraspecific variation of Polygnathus serotinus and P. bultyncki Weddige, 1977.

Thus, the uppermost Emsian to lower Eifelian range of the trilobite genus Plagiolaria is fully supported by the identification of Polygnathus serotinus in the Plagiolaria kitabi-beds in Uzbekistan.

ACKNOWLEDGEMENTSThis is a contribution to IGCP 596 ‘Climate change and biodiversity patterns in the Mid-Palaeozoic (Early Devonian to Early Carboniferous)’.

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CrôniEr, C. & TsMEyrEk, h.s., 2010. First record of the Devonian phacopid trilobite Plagiolaria from Uzbekistan. Memoirs of the Association of Australasian Palaeontologists 39, 43-50.

klappEr, G., 1975. Polygnathus inornatus. 293-297 in Ziegler, W. (ed.), Catalogue of Conodonts, Volume II. E. Schweizerbart’sche Verlagsbuchhandlung, Stuttgart.

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Figure 1. Polygnathus serotinus Telford (1975), upper Emsian to lower Eifelian (serotinus to costatus zones), from same beds as Plagiolaria kitabi Crônier & Tsmeyrek, 2010, Lower-Middle Devonian, Kitab State Geological Reserve, southern Uzbekistan. A-B, KSGR VI-670b/334, upper and lower views of the platform. C-D, KSGR VI-670c/334, upper and lower views of the platform. Described and figured specimens are deposited in the museum of the Kitab State Geological Reserve (KSGR).