Enrollment and coaptations in some species of the Ordovician
Transcription
Enrollment and coaptations in some species of the Ordovician
Enroll ment and coa p tat ions in s o m e s p e eie s o f the Ordovi c ian tril obite genus Placoparia JEAN-LOUIS HENRY AND EUAN N .K. CLARKSON Henry, J .-L. & Clarkson, E . N . K. 1 9 74 1 2 1 5 : Enrollment and coap tations in some species of the Ordovician trilobite genus Plac oparia. Fossils and Stra ta, No. 4, pp . 8 7 - 9 5 . PIs. 1 -3 . Oslo ISSN 0 3 0 0-949 l . ISBN 8 2- 00-049 63-9 . The species Pl. (Placoparia ) cambriensis, Pl. (Coplac oparia ) tournemini and Pl. (Coplacoparia ) borni, described by Hammann ( 1 9 7 1 ) 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 ) b orni 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 coap tative structures sensu Cuenot ( 1 9 1 9 ) , are also to be observed in Pl. (Placoparia ) zipp ei and Pl. (Hawleia ) grand is, from the Ordovician o f Bohemia. However, in the species tournemini and borni, additional depressions appear on the anterior c ephalic border, and the coap tations evolve towards an increasing complexity. From Pl. (Placoparia ) cambriensis, an ancestrai form with a wide geographic distribution, two distinc t populations se em to have individu· alised by a proc ess of allopatric speciation; one of these, probably neotenous, is represented by Pl. (Coplacoparia ) tournemini (Massif Armoricain and Iberian Peninsula) , the o ther by Pl. (Placoparia ) zippei (Bohemia) . Geo graphic isolation may b e indirectly responsible for th e appearanc e of new coap tative devic es. jean-L ouis Henry, Laboratoire de Paleontologie et de S tra tigraphie, Institu t de Geologie de l'Universite, B.P. 25 A. F-3 5 03 1 Rennes Cedex France, et Cen tre na tio nal de la Recherche scien tifiqu e (R. C.P. N° 240) ; Euan N. K. Clarkson, Grant Institu te of Geology, University of Edinburgh, West Mains R o ad, Edinburgh EH 9 3 j W, Sco tland 1st A ugust, 1 9 73. The genus Placoparia is common in some Ordovician formations of the Ma:>sif Armoricain, but 5 0 far no detailed account of the speeies occurring in Brittany has yet been published. The only recent studies were made by Cavet & Pillet ( 1 9 64, 1 9 6 8 ) , who described and figured a Placoparia occurring in nodular shales of Llanvirnian age in the synclinorium of Redon-Ancenis , which in 1 9 6 8 was only provisionally assigned to the speeies zippei. In 1 8 7 6 , De Tromelin & Lebesconte (pp . 6 3 6 -6 3 7 ) recognized two speeies among the speeimens found in the Palaeozoic synclines south of Rennes : Placoparia tourneminei (Rouault) and Pl. zippei (B oeck) . which differ essentially in that they have respectively 1 1 and 12 thoracic segments. Later on, in the thesis he devoted to the Ordovician and the S ilurian of the Crozon peninsula, Kerforne ( 1 9 0 1 :5 3 , 1 7 8 - 1 7 9 ) mentioned one form only : Placoparia tourneminei; the author added that he never collected any Pl. zipp ei, "dont l'aire geographique n'est p as assez grande en Bretagne pour qu 'on puisse s'en servir comme fossile caract e ristique". 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 ( 1 9 7 1 a, 1 9 7 1 b , 1 9 7 1 c ) 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 presen t paper is that established by Il upe ( 1 9 5 3 ) . From rear to front the glabellar furrows are numbered S l , S 2 , S 3 , and the corresponding lateral lobes L 1 , L 2 , L3. The word coaptation , first introduced by the French biologist Cuenot in 1 9 1 9 , 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 b een 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 1 7 98 to TGR ° N 1 9 6 6 . The letter "a" following a sample number indicates an internal mould, the letter"b " an extemal mould. THE SPECIES OF THE GENUS PLA COPARIA 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 ( F ig. l). They have been collected from lutites or from areno-lutites with siliceous nodules, generally known as "schistes a Calym e nes" . 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 1 9 7 0) . Let us add that the use of the expression "Armorican S andstone" 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 outcrop s ( there is an almost total lack of continuous sections) justifies the adoption of three different denominations replacing that of "schistes cl. Calym e nes" : Postolonnec Formation for the Crozon peninsula in the West, B as-Couyer Formation in the North of Rennes, for the region of Menez-B elair studied recently by Paris ( 1 9 7 1 , 1 9 7 2) , and Andouill e Formation for the northem side of the B assin 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. S outh 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 B rittany , namely Pl. (Placoparia ) carnbriensis Hicks, Pl. (Coplacoparia ) tournemini ( Rouault) , Pl. (Coplacoparia ) borni Hammann, have already been described from Spain by Hammann ( 1 9 7 1 a ) . Pl. (Pla coparia ) r;amb riensis 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. P a i eO l O i" q u e Gra n i t , s Socl. m o n et lll.ns indiffer.ncl. '0,0, K m ,--_________----, Fig. 1. Geographical distribution, as known at present , of th e speeies Pl. (Placoparia) cam briensis (*), Pl. (Coplacoparia) tournemini ( - ) and Pl. (Coplacoparia) b orni (e) in th e 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 8 5 ", 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. F irstly , 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 5 0 to 60 much larger deep cavities, roughly oval or roun d 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 p resent 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 excep ted, 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 lob es L I , L2, L3 , but their dimensions increase on the frontal glabellar lobe (Pl. 1 : 1 ; Pl. 3 : 1 ) . The anterior cephalic border is gently arched and its breadth (sag. ) remains more or less uniform over its whole length ; the granulation which covers it is identical with that which ornaments the frontal lobe of the glabella. Librigenae h ave not been found in the material from Brittany, though Hammann ( 1 9 7 1 a, Pl . 1 :9a-b) illustrates a pyritised enrolled specimen with the librigenae in place. 89 The specimens discovered by Cavet & Pillet ( 1 9 64 ) in the nodular shales of Ligne ( syncli norium of Redon-Ancenis ) are certainly Pl. (Placoparia) cambriensis. The typical ornamenta tion of the fixigenae , and the presence of 1 2 thoracic segments and of 3 clear rings on the pygidial axis, make this indisputable. The specimens of the Ordovician shales of Sion-Ies-Mines (Loire-Atlantique ) , mentioned by De Tromelin & Lebesconte ( 1 8 7 6 ) as Placoparia z ippei, must also be assigned to the species cambriensis. In the synclinorium of Martigne-Ferchaud, in Laille, S aint-Senoux, and Sion-Ies-Mine s, Pl. (Placoparia ) cambn'ensis has been collected at the base of the Traveusot Formation, in black shales containing Didy mograp tus cf. b tfidus. In addition to this grap tolite there occur, in the locality named "cote 8 5 " , Trinodus sp . and Pseudosphaerex ochus (Paterasp is) sp . Pl. (Placoparia ) cambriensis is also present at the same level in the syncline of Reminiac (Quete 1 9 7 5 ) . Finally , in siliceous nodules from the Ligne region, the species is found together wi th Pricy clopyge b inodosa and Ormathops cf. atavus (Cavet & Pillet 1 9 6 8 ) . Apart from the genus Trinodus, all these trilobites occur in the S arka Formation (Llanvirn ) in B ohemia (Marek 1 9 6 1 ; Havl i c ek & Van e k 1 9 66 ) . Ormathops nicholsoni and o. alatus also accompany Pl. (Placoparia ) cambriensis and Pricyclopyge binodosa in the H o p e Shales (Lower Llanvirn ) of Shropshire (Whittard 1 9 6 6 ; Dean 1 9 6 7 ) . Though the determ ination of the Didy m ograptus is uncertain owing to distortion and poor preservation, all other criteria lead us to conclude that the shales containing Pl. (Placoparia ) cambriensis belong, in the Massif Armoricain, to the Lower Llanvirn. In B rittany, the species cambriensis has only been recorded, so far, in the Palaeozoic synclines south of Rennes and at the extreme eastern end of the Median synclinorium ( region of Saint-Denis-d 'Orques : locality called "Butte du Creux" ) . When Rouault ( 1 8 4 7 :3 2 0 , 3 2 6 ) first cited Caly mene tournem ini, he wrote tournemini, and not tourneminei. Whatever the origin of this specific name may be, and even if there has been an incorrect transliteration, there is no need to amend and modify the name originally given by Rouault. Pl. (Coplacoparia ) tournem ini is the most common species of this genus in the whole Median synclinorium and in the Palaeozoic synclines south of Rennes. It differs from Pl. (Placoparia ) cambriensis in the number both of its thoracic segments ( I l instead of 1 2 ) , and in the clearly individualized rings in the pygidial axis (4 instead of 3 ) . Moreover, the external edges of the librigenae each bear 12 deep cavities, and on the lateral parts of the anterior cephalic border there are two depressions expanding transversely lengthwise; they are a constant feature in all the individuals examined (Pl. 1 :6-8 ) . Though the large cavities of the fixigenae remain clearly visible, the y are more faintly impressed than those of Pl. (Placoparia ) cambriensis. The very small pits, however, are similar in size and distribution to those o f that speeies. Finally, it will be noted that the granules covering the anterior border of the cephalon are smaller, but far more numerous, than those which ornament the frontal glabellar lobe (Pl. 1 :8 ; Pl. 3 :4) . The hypostoma has b een found "in situ" in severai specimens from Traveusot ( IGR N ° 1 8 7 3b , Pl. 1 : 1 4 ; IGR N ° 1 8 8 3 a-b; IGR N ° 1 89 1 b , Pl. 1 : 1 3 ) . It is oval-shaped; its m aximum breadth, measured at the level of the triangular anterior wings, is som ewhat greater than its maximum length. The hypostomal suture is a curve, slightly convex towards the front. The median b ody, strongly curved, has neither an anterior furrow nor distinet maculae , but it dis plays laterally, half-way down its length, two extremely faint short notches obliquely directed rearwards and inwardly ( Pl . l : 1 4) . At the level of these two notehes, just where the posterior furrow disappears, the lateral edges of the hypostoma swell into two projecting protuberances. The narrow border constituting the buccal edge carries p erhaps one or two p airs of tiny p oints ( ? ) . I n the Crozon Peninsula, Pl. (Coplacoparia ) tournem ini app ears, according to Kerforne ( 1 9 0 1 :5 3 ) , above the Kerarvail Member. At about 30 m in thickness under the Marro lithus bureaui Zone, this species is still represented by a few specimens only, then it becomes extinct. Its vertical range appears to be identical in b oth the B as-Couyer Formation (Menez-B elair ) and the Andouille Formation (Bassin de Laval ) . To the south of Rennes, Pl. (Coplacoparia ) tour nem ini is present in the middle and upper part of the Traveusot Formation, in micaceous areno-Iutites which are of unknown thickness but which overlie black lutites containing Pl. (Placoparia ) cambriensis. The areno-Iutites of Traveusot en Guichen contain a few Glyp tograp tus, unfortunately incomplete and poorly preserved, which m akes their precise determination impossible; the se Glyptograp tus might b elong to the species teretiuscu lus, or to a new species ( information communicated by V. J aanusson ) . Overlying the Traveusot Formation in ascend ing sequence are the Chatellier Sandstones, about 50 to 60 m thick, then the Shales of Riadan whose trilobite fauna (very similar to that collected from the top of the Postolonnec F, orm a tion in the Finist e re) shows strong affinities with the fauna known in the Liben Formation in B ohemia (Middle and Upper Llandeilo , and Lower Caradoc ; c f. Williams & al. 1 9 7 2 : Fig. 2 for 90 - 7- - �Eg·�·: � Upper boun dary P o s t olon n e c [10 of the Formation E=�I bi� \ � a \ 2 o ...J W O Z <t ...J ...J _ . _ . _,-, Kerarvoil \ \ \ \ .... \ - \ Member \ T � \ \ \ - '1�-== /' /' / / 2 r-4 :...J /' / / / / / / I ? ? 2 ? ? r=�i� �--� z o:: 7 Z <t ...J ...J ';����. / / 2 :; Z w o:: <t / ? /' Forma tion ? 3 ---./ Traveusot U p p e r b oun dary ....... _ of the A n douille Formation o f the B a s - Courer Formation I 1 ...J <.!) Be 3 \ k Eæ 8 ·- - - - - - - - - \ \ _ _ f::::::1 4 O ? 83 Upper b ou n d a r y o:: w _ _ b � U p p e r b o u n dary o f the � _ - [�=�1 g �f C �� m §h d ".:�-:·.:'':';'.)';\:· 1 L :; �; .... O "' t U n d ar Y Pos tolonnec Formatlun ? �S�· ..... B � ��� Low r f ndOry Bos - Cauyer Formation A n d ouilfe Formation c ���� R L o kt e r of b o u !1 dary Tr a v e u s o t tha Fo r m a trun o Fig. 2. Vertical distribution, as known at present, of the sp ecies Pl. (Placoparia) cam briensis ( 1 ) , Pl. (Copla coparia) tournemin i ( 2 ) , Pl. (Coplacoparia) b o rn i ( 3 ) , Marrolithus bureau i (4) . A. Postolonnec section, Crozon p eninsula. B. Menez-Belair region. C . Bassin de Laval. D. Martigne- F erchaud synclinorium . a. Armorican Sandstone Formation (Gres armoricain) . b . Sandstone - shale alternations. c. Lutites. d. Areno-lutites. e . Oolitic b eds. f. Lenticular beds of calcareous sandstone . g. Kerarvail sandstone. h. Sill. i. Thick sandsto ne b ed. j . Sandstones of the Kermeur Formation. k. Sandstones of the Saint-Ger main-sur-Ille Formation. l . Sandstones of the Chatellier F ormation. further information) . In the absenee of unequivocal criteria and since the sedimentary sequence is apparently conformable, it would seem for the moment that to attribute p art of the Forma tions of Postolonnec ( the Marrolithus bureaui Zone included) and of Traveusot to the Lower Llandeilo remains the only logical possibility . In stratigraphie sequence Pl. (Coplacoparia ) b o rni succeeds Pl. ( Coplacoparia ) tournemini, but this trilobite has never been reported, as far as we know, in the synclinorium of M artigne Ferchaud. It is well represented, however, throughout the Armorican Median synclinorium, where it is found in association with N. (Neseuretus) tristani, Plaesiacomia oehlerti, Croz o nas pis struvei, Morgatia hupei; it is still abundant in the Marrolithus bureau i Zone, but then dis appears suddenly. One of the remarkable features of Pl. (Coplacoparia ) borni is the presence of 4 semicircular depressions on the inn er edge of the anterior cephalic border, a border which is 91 covered with extremely dose-set granules (PL 3 :7 - 8 ) . On the fixigenae the large cavities are barely visible and not numerous, but , as can be observed in the speeies tournemini, a few deep cavities remain on the outer region of the fixigenae. The distribution of the tiny pits remains as in the preceding speeies. ENROLLMENT AND COAPTATIVE STRUCTURES An abundance of well preserved samples , and partially or totally enrolled speeimens, has made it possible for us to describe in detail the coaptative devices of Pl. (Coplacoparia ) tour nemini and of Pl. ( Coplacoparia ) borni, two speeies in which the enrollment is exactly alike. When an individual trilobite enrolled, the rounded distal ends of the thoracic pleurae fitted into weU marked cavities, each with a smooth bottom and set in a spiral on the lateral borders of the cheeks. These small depressions, from point W onwards, have been numbered from 1 to 1 2 on each border (Pl. 2 :6) . From re ar to front they are first located sub-ventrally, then they gradually become marginal. It is into the two foremost cavities of the librigenae that the blunt tips of the first two ribs of the pygidium come and fit. The tip s of the second pair of ribs fit into the lateral depressions of the anterior cephalic border, whereas the tip s of the third and fourth pairs of pygidial rib s lock into the preglabellar furrow (Pl. 2 :8- 1 0) . It will be noted that the posterior border of the fixigenae bears distally a wide and short spine which fits, at the beginning of the enrollment, into a corresponding depression located on the first thoracic pleura (Pl. 2 : 1 1 - 1 2 ) . Hammann ( 1 9 7 l a, Pl. 1 :9a-b ) has given two excellent photographs of an enrolled Pl. (Pla coparia ) cambriensis. Despite appearances, the position of the pygidium in respect to that of the cephalon at the end of the enrollment, does not differ, in that speeies, from that ob served in Pl. ( Coplacoparia ) to urnemini and Pl. ( Coplacoparia ) b o rni. Indeed, from the lateral view of the speeimen figured by Hammann, it may b e seen that the distal ends of the thoracic pleurae ( from the fifth to the eleventh) are not in contact with the lateral m argin of the cheek. Such incompletely enrolled speeimens are frequently ob served ( Barrande 1 8 7 2 , Pl. 8 :3 3 ; Prantl & S naj dr 1 9 5 7 , Pl. 1 ( 3 1 ) :5 ; Hammann 1 9 7 1 , Pl. 2 : 1 3 a-b ; Pl. 2 :7 in the present paper) , not only in the genus Placoparia but also in Colp o coryphe (unpublished) . Incomplete enroll ment might be explained, in certain cases at least, by a partial relaxation of the museulature after death, together with rapid burial in the sediment. Pl. (Placoparia ) zippei (Dobrotiva Formation) was also cap able of enrollment, as Prantl & Snajdr ( 1 9 5 7 , Pl. 2 ( 3 2) : 1 5 ) have figured the east of one cheek revealing, on the lateral border, a few depressions which are very similar to those observed on Pl. (Coplacoparia ) tournemini and Pl. (Coplacoparia ) borni. We have not examined any enrolled individual o f Pl. (Placoparia) zippei, b u t there i s n o reason t o assurne that i n this speeies the enrollment differed from that of Ordovician forms from B rittany. The same applies to Pl. (Hawleia ) grandis ( Liben and Letna Forrnations ) i n which the lateral margins o f the cheeks also bear depressions (B arrande 1 8 7 2 , Pl. 8 :43-44 ) . The samples of Pl. (Hawleia ) pran tli (Ashgill of Poland) figured by Kielan ( 1 9 5 9 , Pl. 2 3 :4-6) are p oorly preserved and it is therefore impos sible to comment upon the presenee or the absenee of coap tative structures. � EVOLUTION OF THE GENUS PLA COPARIA DURING THE LLANVIRN AND THE LLANDEILO The most ancient representative of the genus so far known is Pl. (Placoparia) cambriensis. The lateral margin of the cheeks belonging to the speeimen figured by Hammann ( 1 9 7 1 a, Pl. 1 :9b ) seems to reveal very slight depressions in the anterior p art ;.. These seem to be present also, and similarly located, on an external mould coming from the Sarka Formation in B ohemia. If the existence of these b arely excavated depressions were confirmed by the exam ination of more abundant material it would reinforce the suggestion that PI. (Placoparia ) zippei and Pl. (Coplacoparia) tournemini derived from Pl. (Placoparia) cambriensis. Indeed, in the two Llandeilian speeies the depressions in to which the distal ends of the thoracic pleurae interlock are already quite dear. But on the anterior border of Pl. (Coplacoparia) tournem ini two additional excavations are present. These coaptative structures , like the ones already (described in the case of Ordovician and Silurian tr ) 92 described in the case of Ordovician and Silurian trilobites (Clarkson & Henry 1 9 7 3 ) , are rem ark ably regular features of the adults throughout the whole time range of the speeies. The same applies to Pl. (Coplacoparia) borni in which the cephalon bears, besides the two deepening lateral cavities, four new depressions located on the inner edge of the anterior cephalic b order, and not on the outer edge as figured by Hammann ( 1 9 7 1 a : Fig. 3 ) . It is as though the blunt tips of the pygidial ribs, which come to rest in the preglabellar furrow when enrollment is complete, were imprinted into an exoskeleton which was still soft; yet researches upon the ' coaptations of contemp orary invertebrates have revealed that such mutual moulding of the coap ting parts during ontogeny cannot be invoked as an explanation, and that the coap tative devices are genetically inherited (Tetry 1 9 6 9 ; Sahuc 1 9 69 ) . In the Armorican Median syndinorium , Pl. (Coplacoparia) b orn i succeeds Pl. (Coplaco paria) tournemini from which it probably derived, but the two trilobites may p erhap s have coexisted for a short while, since they are encountered together in the same locality ( founda tions of the house named "les Atlantes" in Postolonnec) . One of the cranidia collected (IGR N ° 1 8 5 1 b , Pl. 3 :6) shows an interesting peculiarity : although it has most of the characteristic s of the speeies b orni, the total number of depressions on the anterior border is 4 rather than 6. The two small hollows located immediately on each side of the plane of symmetry are missing. This "absence" originates neither in the distortion it underwent - which is very faint - nor in the preservation - which is excellent since the granulation is perfectly preserved. This specimen could either be representative of an intermediate stage , indicating a gradual transition from Pl. (Coplacoparia) tournemini to Pl. (Coplacoparia) b o rni, or else a hybrid, signifying the inter fecundity of two distinct populations. ParalleI with the increasing complexity of the coaptative structures throughout time , and perhaps in dose connection with this evolution, some characters already observed by Curtis ( 1 9 6 1 ) and by Hammann ( 1 9 7 1 a) undergo gradual changes. Thus one may observe the de crease in number and size of the small pits on the fixigenae , the increase of the degree of cu.rvature and of the length of the pygidial rib s, the increase of the density of the granules on c B A Fig. 3. Pl. (Placoparia) cam briensis (A) , Pl. (Coplacoparia) tournemini ( B ) , Pl. (Coplacoparia) b orn i ( C ) . Reconstruction of t h e cranidia. 93 the anterior cephalic border of the species tournemini and born i. Along the contact surfaces between two coapting parts , this increase of the granulation has been observed, without any exception, in all the Ordovician trilobites of B rittany having coaptative devices (Clarkson & Henry 1 9 7 3 ) . In aur estimation, this corroborates the ide as of S ahuc ( 1 9 6 9 ) on the direct part played by mechanical rubbing and pressure in the elaboration of the coap tative structures. In B ohemia, during the Llandeilo and the Caradoc (Dobrotiva Formation, Liben and Letna Forrnations) , the coaptative devices of Pl. (Placoparia) z ippei and Pl. (Hawleia) grandis do not seem to undergo any improvement. In contrast to what is observed in the species from Brittany, no depressions.appear on the anterior border of the cephalon whose granulated ornamentation, at least in the case of Pl. (Placoparia) zippei, is not different from that of the glabella. It is difficult to show for the moment, as direct evidence is lacking, that the two species Pl. (Coplacoparia) tournemini and Pl. (Placoparia) z ippei, with their limited geographi cal distribution, constituted two strictly contemporaneous populations, but it is logical to assurne that the y are both derived, through allop atric speciation, from Pl. (Placoparia) cambriensis, a widely distributed ancestral form , since it is known in Great B ritain, in Spain, in B ohemia, and in Brittany. As Devillers ( 1 9 7 3 :3 2 ) puts it : " . . . ei partir d'une population ancestrale divergent progressive ment des populations entre lesquelles s' e tablit une barri e re, geographique, physiologique ou autre qui, en supprimant les croisements, empeche le brassage genetique. Dans chaque population s'accumulent, p eu li peu , des caracteres qui vont accentuer sa physionomie particuliere et contribueront, avec l'isolement gen e tique qui progresse, a lui donner le statut d'espece" . Confirmatory evidence from further observations might give addi tional weight to some of the ideas recently expressed by Eldredge ( 1 9 7 1 , 1 9 7 3 ) regarding allopatric speciation in Palaeozoic invertebrates. In a paper published recently (Clarkson & Henry 1 9 7 3 ) , we wrote that the development of of predators ( cephalopods) might be an indirect cause of the increasing complexity of the coaptative structures in some of the Ordovician and Silurian trilobites. This suggestion remains reasonable, but, as Hammann ( 1 9 7 1 a:66) wrote, geographical isolation, leading to particular modes of life in a given region, must not be forgotten. Whereas enrollment is of ten considered as a defence reaction against predators , it may also be interpreted as a reaction of the animal liv ing in shallow waters liable to rapid variations of ecological factors ( temperature , salinity, etc . . . . ) . In a temporarily isolated palaeogeographical province (Massif Armoricain and Iberian Peninsula) , an unfavourable and unstable environment might be indirectly responsible for the appearance of new morphological devices among the representatives of genus Placoparia. Such a p ossibility is not unreasonable since the trilobite faunas known in the Spanish and Armorican Ordovician are poor; they only contain a limited number of genera and species. Moreover, in some fossiliferous localities from B rittany , where the exoskeletons are often complete (Traveu sot for instance) , the proportion of enrolled individuals is always very high. The presence of 1 1 thoracic segments only in Pl. (Coplacoparia) tournemini and Pl. (Coplacoparia) borni, as compared with the other known species which have 1 2 , rem ains to be explained. During ontogeny , the posterior elements of the exoskeleton are the last to become differentiated and the growth is directed forwards . This is clear from a number of ontogenetic series, particularly that of Breviscu tellum (Meridioscu tellum), as Feist ( 1 9 7 0 ) has shown. There is no reason to suppose that growth and development in Placoparia was any different. Thus to be equivalent to Pl. (Placoparia) camb riensis, the pygidium of the tournemini and b o rn i species would have to release an additional segment towards the thorax , one pair of pleurae then simultaneously originating in the posterior extremity of the pygidium . It may therefore be assumed that Pl. (Coplacoparia) tournem ini and Pl. (Coplacoparia) b orni consti tute a neotenous line in which there occurs" . . . un retard dans le developpement des struc tures corporeHes relativement a celui des organes reproducteurs" (Hup e 1 9 5 3 :8 5 ) . To assurne that evolution along this line resulted from arrested development, as R. Feist (personal communication ) has suggested to us, appears to b e the most plausible explanation. In Brittany, the biozones of the species cam briensis, tournemini, and b o rni, such as are illustrated in Fig. 2 , certainly could be improved upon, but they are still very useful in an area where the outcrops are scarce and where continuous sections are lacking. ACKNOWLEDGEMENTS. - We wish to thank Pro fessors and Drs. R. F eist (MontpelIier) , V . Jaanusson ( Stockholm ) , J.-C. Lefeuvre ( Rennes) L . Marek (Praque ) and H. Tintant (Dij o n ) , and also the palaeontologists of Brest and Rennes for their advice and assistance . 94 RE FERENCES Barrande, J. 1 8 7 2 : Syst e m e silurien du cen tre de la Bo heme. I: Reeherehes paleon to logiques. SuppLement au Vol. 1 ( Trilo b ites, Crustaees divers et Poissons ) , I-XXX, 1 -6 4 7 et Atlas (Planches) , 1 -3 5 . Prague-Paris. Cavet , P. & Pillet, J. 1 9 64 : Contribution a I 'etude stratigraphique et paleontologique du synclinal d' Ancenis (Massif armoricain) . Bull. soe. geo!. Franee 5, 3 1 8 - 3 2 9 . Paris. 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Senekenbergiana L e thaea 52, 5 3 - 7 5 . Frankfurt am Main. Hammann, W . 1 9 7 1 b: Stratigraphische E in teilung des spanischen Ordoviziums nach Dalmanitacea und Cheirurina (Trilobita ) . Mem. Bur. R eeh. Geol. Min. (B. R . G.M.) 73, 2 6 5 - 2 7 2 . Paris. Hammann, W. 1 9 7 1 c : Zur Synonymie vo n Plaeoparia (Coplacoparia) tourneminei ( Rouault 1 84 7 ) . Seneken b ergiana L e thaea 52, 5 8 9 - 5 9 0 . Frankfurt am Main. Havi lcek, V . & Van e k, J . 1 9 66 : The biostratigraphy of the Ordovician of Bohemia. S b o rn. geo!. Ved, paleont. 8, 7 -69 . Praha. Hupe, P. 1 9 5 3 - 1 9 5 5 : Classification des Trilobites. A nn. Palfont. 3 9, 6 1 - 1 6 8 . Paris 1 9 5 3 . - A nn. Paleont. 41, 9 1 -304. Paris 1 9 5 5 . Kerforne, F . 1 9 0 l : E tude d e la region silurique o ecidentale de la presqu 'ne de Crozon. These, 2 3 4 pp. Rennes. Kielan, Z. 1 9 5 9 : Upper Ordovician trilobites fro m Poland and some related forms from Bohemia and Scandi navia. Palaeont. Polonica 1 1, 1 - 1 9 8 . 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Paris. Sahuc, L.J.M. 1 9 6 9 : Coaptations chez le grillon domestique. Bull. b iol. Franee Belgique 1 03, 9 - 1 3 0 . Paris. Tetry, A. 1 9 6 9 : Out i Is animaux et vegetaux. Science Progres Decouverte 3 4 1 6, 48 1 -49 0 . Paris, Dunod ed. Trornelin, G . de & Lebesconte, P. 1 8 7 6 : E ssai d 'un catalogue raisonne des fossiles siluriens des departements de Maine-et-Loire, de la Loire-Inferieure et du Morbihan, avec des observations.sur les terrains p aleozoiques de l 'Ouest de la France. C. R. 4eme Sess. A ss. franc. A vane. Sei. (A . F. A . S.), 60 1 -66 1 . Nantes. Whittard, W . F . 1 9 66 : The Ordovician trilobites of the Shelve Inlier, West Shropshire. Monogr. Palaeon togr. Soe. 1 1 9, 2 6 5 - 3 0 6 . London. Williams, A., Strachan, 1., Bassett, D.A. , Dean, W.T., Ingham , J.K., Wright, A.D., Whittington, H . B . 1 9 7 2 : A correlation of Ordovician rocks in the British Isles. Geo!. Soe., Spee. R eport 3, 1 - 7 4 . London. 95 EXPLANATION OF PLATES Plate 1. The fo llowing figures are based on latex replicas : 1 , 3 , 5 - 1 1 , 1 3 - 1 4. Figs. 1 - 4. Placoparia (Placoparia) cambriensis Hicks. Fig. 1. Cranidium, dorsal view. IGR N V l 69 0b . x l O . Figs. 2-3. Pygidium, dorsal and oblique posterior views . IGR N O 1 9 3 9 a-b . x 1 6 . (approx . ) . Fig. 4. Cranidium , dorsal view. IGR N ° 1 69 2 . x 1 0 . All the specimens come from the Traveusot Formation. "Cote 8 5 " near Laille, Ille-et-Vilaine. Figs. 5- 1 0. Placoparia (Coplacoparia) to urnemini ( Rouault) . Fig. 5. Cephalon partially shown , in lateral oblique view; not e the depressions on the lateral border of the librigena. IGR N O 1 89 4b . x8 (approx. ) . Traveusot F ormation. Traveusot e n Guichen, Ille-et-Vilaine. Fig. 6 . Two cranidia, dorsal view. IGR N O 1 8 54b. x 1 0 . Postolonnec Formation. Morgat, Crozon peninsula, Finistere. Fig. 7. Incomplete c ephalon and three thoracic segments, dorsal view. IGR N° 1 8 2 3 b . x 5 . Andouille F ormation ( ? ) . Butte du Creux near Saint-Denis-d 'Orques, Sarthe . Fig. 8. Cranidium, dorsal view. IGR N O 1 8 !J 2 b . x 1 4 (approx . ) . l'ostolon nec Formation. Foundations of the house named "les Atlantes " , Postolonnec , Crozon peninsula. Figs. 9- 1 0. Incomplete pygidium, dorsal and posterior views. IGR N O 1 89 9 b . x 7 . Traveusot Formation. Traveusot en Guichen. Fig. 1 1. Placoparia (Coplacoparia) sp. Hypostoma. IGR N O 1 8 5 6b. x 6 . Postolonnec F ormation. Morgat, Crozon peninsula. Figs. 1 2 - 1 4. Placoparia (Coplac oparia) tournemini. Fig. 1 2. Hypostoma. IGR N O 1 5 8 1 a. x6. Bas-Couyer Formation. Bas-Couyer en Melesse, Ille-et-Vilaine. Figs. 1 3 - 1 4. Hypostoma "in situ ". IGR N O l 89 1 b ( Fig. 1 3 ) and IGR N O l 8 7 3 b ( Fig. 1 4) . x8. Traveusot Formation. Traveusot en Guichen. Plate 2. The following figures are based on latex replicas : 2 , 6 - 8 , 1 2 . Figs. 1 -5. Placoparia (Coplacoparia) borni Hammann. Fig. 1 . Cranidium, dorsal view. IGR N U 1 8 3 2 . x6. Andouille F ormation. Les Monneries, Andouill e , Mayenne. Fig. 2. Incomplete cranidium, dorsal view. IGR N O 1 8 5 3 b . x 6 . Postolonnec Formation. Postolon nec , Crozon e eninsula. Figs. 3 -5. Pygidium in dorsal, posterior and lateral views. IGR N O 1 8 0 6 . x 5 . Andouill e , Formation (Marrolithus bureaui Zone ) . La To uche, Andouill e , Mayenne . Figs. 6-8. Placoparia (Coplac oparia) tournemini. Fig. 6. Incomplete c ephalon partially shown, frontal oblique view; note the depressions (from the fifth to the twelfth ) on the lateral border of the librigena. IGR N O 1 9 6 6 b . x 1 0 . Traveusot Formation. Traveusot en Guichen. Fig. 7. Incompletely enrolled specimen, dorsal view . IGR N ° 1 8 2 2b . x4. Andouill e Formation ( ? ) . Butte du Creux near Saint-Denis-d'Orques . Fig. 8. Incomplete, but fully enrolled speeimen, partially shown in dorsal view. IGR N ° 5400b. x6 (approx. ) . Bas-Couyer F ormation. Bas-Couyer en Melesse. Figs. 9- 1 1 . Placoparia (Coplacoparia) borni. Figs. 9- 1 0. Fully enrolled specimen, lateral and dorsal views. IGR N O 1 8 0 5 a. x 3 . 5 (approx . ) . Fig. 1 1 . Enrolled speeimen partially shown, in lateral view , with the distal extremities of five thoracic pleurae fitting into corresponding depressions of the lateral border of the librigena. IGR N O 1 8 1 8 . x l O . Andouill e Formation (Marrolithus bureaui Zone ) . La Touche, Andouill e . Fig. 1 2. Placoparia (Coplacoparia) tournemini. Part of an enrolled specimen, lateral view. IGR N O 5400b (see Flg. 8 ) . x l O. B as Couyer Formation. Bas-Couyer en MeJesse. Plate 3 . T h e following figures are based on latex replicas : 1 -9 . Figs. 1 -3. Pla coparia (Placoparia) cam briensis. Fig. 1 . Part of a cranidium showing the anterior cephalic border. IGR N O 1 690b (see P l . 1 : 1 ) . x 1 5 . Fig. 2. Fixigena o f the same speeimen. x 1 5 . Fig. 3. Incomplete cranidium partially shown ; t h e distortion i s very faint. I G R N O l 8 2 1 b. x 1 5 (approx . ) . Butte d u Creux near Saint-Denis-d'Orques. Figs. 4 - 5 . Placoparia (Coplacoparia) tournemini. Fig. 4. Enlargement of part of a cranidium showing the two depressions and the extreme tubercu lation on the anterior cephalic border. IGR N O 1 8 52b (see Pl. 1 :8 ) . x 2 0 . Fig. 5. Dorsal oblique view of a fixi gena. IGR N O 5400b (see Pl. 2 :8 , 1 2 ) . x 1 0 . Fig. 6. Placoparia (Coplacoparia) cf. b o rni. Enlargement of a cranidium partially shown; the anterior cephalic border bears o nly four depressions. IGR N o 1 8 5 1 b. x 1 5 . Foundations o f the house named "les Atlantes " , Postolonnec, Crozon peninsula. Figs. 7-- ,] 0. Placoparia (Coplacoparia) b o rni. Figs. 7-8. Two incomplete cranidia partially shown, dor sal views; note the depressions located on the inner edge of the anterior c ephalic border. IGR N O 1 8 5 5 b ( F ig. 7) and IGR N O 1 8 5 3b ( Fig. 8 ) . x 1 5 . Postolonnec Formation. Foundati ons o n the house named "les Atlantes" , Postolonnec, Crozon peninsula. Fig. 9. Enlargement of a fixigena belonging to the specimen IGR N O 1 8 5 5 b (see Fig. 7 ) . x 1 5 . Fig. 1 0. Part of a cranidium in dorsal view showing the six depressions on the anterior cephalic border. I G R N O 1 8 1 3 . x 1 8 (approx. ) . Andouille Formation. Les Monneries, Andouille, Mayenne. 96 Plate 1. Plate 2. Plate 3.