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
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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
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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.