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Enrollment and Coaptations in Some Species of the Ordovician Trilobite Genus Placoparia

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Enrollment and Coaptations in Some Species of the Ordovician Trilobite Genus Placoparia Enrollment and coaptations in some sp eeies of the Ordovician trilobite genus Placoparia JEAN-LOUIS HENRY AND EUAN N.K. CLARKSON Henry, J.-L. & Clarkson, E.N.K. 1974 12 15: Enrollment and coaptations in some species of the Ordovician trilobite genus Placoparia. Fossils and Strata, No. 4, pp. 87-95. PIs. 1-3. Oslo ISSN 0300-949l. ISBN 82-00-04963-9. The species Pl. (Placoparia) cambriensis, Pl. (Coplacoparia ) tournemini and Pl. (Coplacoparia ) borni, described by Hammann (1971) from the Spanish Ordovician, are found in Brittany in formations attributed to the Llanvirn and the LIandeilo. The lateral borders of the librigenae of Pl. (Coplacoparia) tournemini and Pl. (Coplacoparia ) borni bear depressions into which the distal ends of the thoracic pleurae and the tips of the first pair of pygidial ribs come and fit during enrollment. These depressions, or coaptative structures sensu Cuenot (1919), are also to be observed in Pl. (Placoparia) zippei and Pl. (Hawleia) grandis, from the Ordovician of Bohemia. However, in the species tournemini and borni, additional depressions appear on the anterior cephalic border, and the coaptations evolve towards an increasing complexity. From Pl. (Placoparia) cambriensis, an ancestrai form with a wide geographic distribution, two distinct populations se em to have individu· alised by a process of allopatric speciation; one of these, probably neotenous, is represented by Pl. (Coplacoparia ) tournemini (Massif Armoricain and Iberian Peninsula), the other by Pl. (Placoparia ) zippei (Bohemia). Geo­ graphic isolation may be indirectly responsible for the appearance of new coaptative devices. jean-Louis Henry, Laboratoire de Paleontologie et de Stratigraphie, Institut de Geologie de l'Universite, B.P. 25 A. F-35031 Rennes Cedex France, et Centre national de la Recherche scientifique (R. C.P. N° 240); Euan N. K. Clarkson, Grant Institute of Geology, University of Edinburgh, West Mains Road, Edinburgh EH 9 3 jW, Scotland 1st August, 1973. The genus Placoparia is common in some Ordovician formations of the Ma:>sifAr moricain, but 50 far no detailed account of the speeies occurring in Brittany has yet been published. The only recent studies were made by Cavet & Pillet (1964, 1968), who described and figured a Placoparia occurring in nodular shales of Llanvirnian age in the synclinorium of Redon-Ancenis, which in 1968 was only provisionally assigned to the speeies zippei. In 1876, De Tromelin & Lebesconte (pp. 636-637) recognized two speeies among the speeimens found in the Palaeozoic synclines south of Rennes: Placoparia tourneminei (Rouault) and Pl. zippei (Boeck) . which differ essentially in that they have respectively 11 and 12 thoracic segments. Later on, in the thesis he devoted to the Ordovician and the Silurian of the Crozon peninsula, Kerforne (1901:53, 178-179) mentioned one form only: Placoparia tourneminei; the author added that he never collected any Pl. zipp ei, "dont l'aire geographique n'est pas assez grande en Bretagne pour qu 'on puisse s'en servir comme fossile caracteristique". Let us add that, what with the lack of good illustrations in the early studies and the loss of part of the material, it is impossible to draw up detailed lists of synonyms. Now that Hammann (1971a, 1971b, 1971c) has provided a firm systematic basis for the subfamily Placopariinae, it seemed to us pertinent to describe the coaptative devices and the enrollment of the Placoparia of Brittany. TERMINOLOGY AND TECHNIQUES USED The terminology used in the present paper is that established by Il upe (1953) . From rear to front the glabellar furrows are numbered Sl, S2, S3, and the corresponding lateral lobes L1, L2, L3. The word coaptation, first introduced by the French biologist Cuenot in 1919, refers to the mechanical adjustment or fitting together of two independent parts of the same animal. The 87 coaptative structures are peculiar morphological devices (cavities, protuberances, notches, furrows etc ....) such as for instance interlock in a fully enrolled trilobite. All the specimens and the latex casts figured here have been slightly whitened with ammo­ nium chloride, and the background in some of the photographs has been partially blocked out with Indian ink. Photographs and drawings are those of the first author. MATERIAL The specimens studied are most often small, hard ly exceeding 3 or 4 cm in length when complete. All of these come from formations assigned to the Llanvirn and to the Llandeilo. Though frequently distorted in lutites, the Placoparia are, however, finely preserved, the ornamentation of the test being perfectly visible on the external moulds. In the species known from the Ordovician of Brittany, the dorsal exoskeleton is compara­ tively thick, being about O.l mm in individuals whose overall length does not exceed 3 cm. Thus the more delicate coaptative structures of the cephalon are very hard to distinguish, or are even invisible, on internal moulds. Precise determination of the species is therefore possible only in latex replicas obtained from external moulds, except of course when the test itself has been preserved. The material comprises exoskeletons which are often incomplete and sometimes enrolled, and isolated pygidia and cranidia. Complete cephala with their fixigenae are rather rare. In some cases the hypostoma is preserved "in situ" . All the specimens are part of the collections ° of the Geology Institute in Rennes (IGR) . Theu are numbered from IGR N 1798 to TGR N° 1966. The letter "a" following a sample number indicates an internal mould, the letter"b" an extemal mould. THE SPECIES OF THE GENUS PLA COPA RIA IN BRITTANY AND THEIR VERTICAL DISTRIBUTION In this paper we have abandoned the stratigraphic nomenclature which students of the Ordo­ vician of the Massif Armoricain have been using for more than a century. This out-of-date nomenclature, which is often misleading, and has become increasingly less useful as research progresses, has been replaced (in all but exceptional cases) by geographical names applying to local lithologically homogeneous units. These units have been known for quite a long time, and besides, the y are those referred to on geological maps. They retain, at least for the time being, a deliberately broad meaning so as to avoid an excessive multiplication of names. The trilobites figured in this paper come from two large geological domains: the Median Armorican synclinorium and the synclinorium of Martigne-Ferchaud (Fig. l). They have been collected from lutites or from areno-lutites with siliceous nodules, generally known as "schistes a Calymenes". The latter, varying in thickness from about 200 to 400 m, lie over the Armorican Sandstone Formation which probably dates back to the Arenig (Deunff & Chauvel 1970). Let us add that the use of the expression "Armorican Sandstone" for the region of Brittany does not imply, in our meaning, that the lower or the upper limit of this lithological unit is everywhere synchronous. In the Median synclinorium, the very discontinuous character of the outcrops (there is an almost total lack of continuous sections) justifies the adoption of three different denominations replacing that of "schistes cl. Calymenes": Postolonnec Formation for the Crozon peninsula in the West, Bas-Couyer Formation in the North of Rennes, for the region of Menez-Belair studied recently by Paris (1971, 1972) , and Andouille Formation for the northem side of the Bassin de Laval in the East. In the Crozon peninsula some quartzites forming broad layers having an overall thickness of about 14 m, appear within the Postolonnec Formation and constitute the Kerarvail Member. South of Rennes, in the synclinorium of Martigne-Ferchaud, we shall refer to the "schistes a Calymenes" under the name of Traveusot Formation. These different lithological units are described in detail and their names defended in a publication which is now in press. The three species of genus Placoparia identified with certainty in the Ordovician of Brittany, namely Pl. (Placoparia ) carnbriensis Hicks, Pl. (Coplacoparia) tournemini (Rouault), Pl. (Coplacoparia) borni Hammann, have already been described from Spain by Hammann (1971a). Pl. (Placoparia) r;ambriensis has been found in the Lower Llanvim of the Iberian peninsulaj the observations that may be added to Hammann's excellent description need only be minor 88 N " Secondotr. PaieOlO i" que Granit,s monetlll.ns Socl. indiffer.ncl. ,-- Km _________ ----,,'0,0 Fig. 1. Geographical distribution, as known at present, of the speeies Pl. (Placoparia) cambriensis (*), Pl. (Coplacoparia) tournemini ( - ) and Pl. (Coplacoparia) borni (e) in the Massif Armoricain. ones. The specimens from the Massif Armoricain are identical with those from Spain, but the few cephala collected near Laille, in the locality named "cote 85", are remarkably well preserved and make it possible to describe the most minute ornamentation of the exoskeleton. Three distinct elements of surface sculpture are evident. Firstly, all parts of the cephalon are densely perforated with tiny pits; these are largest and most numerous on the regions bordering the cephalic furrows and near the posterior border and near edge of the occipital ring. Secondly, the fixigenae are pitted with about 50 to 60 much larger deep cavities, roughly oval or round in shape. Irregularly distributed, and differing even on the two fixigenae of one single cephalon (Pl. 1 :4), these cavities are scattered over a trapezoidal surface separated from the dorsal gla­ bellar furrows and the posterior furrows of the cheeks by a margin ranging from 0.3 to 0.4 mm in breadth. It will be noted that the most distinct cavities lie dose against the facial suture, on the external lateral portions of the fixigenae. Three similar cavities are present near the anterior glabellar border, in the plane of symmetry of the cephalon. Thirdly, between the cavities the test carries a fine and dense granulation. The whole cephalon is covered, the furrows excepted, with granules: the se are of small size on the occipital ring and on the posterior borders of the cheeks, as well as on the lateral lobes LI, L2, L3, but their dimensions increase on the frontal glabellar lobe (Pl.
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