BULLETIN DE L’INSTITUT ROYAL DES SCIENCES NATURELLES DE BELGIQUE SCIENCES DE LA TERRE, 74-SUPPL.: 73-80, 2004 BULLETIN VAN HET KONINKLIJK BELGISCH INSTITUUT VOOR NATUURWETENSCHAPPEN AARDWETENSCHAPPEN, 74-SUPPL.: 73-80, 2004

The mode of life of entomozoacean and the Myodocopid Mega-Assemblage proxy for hypoxic events

by Jean-Georges CASIER

C asier , J.-G., 2004 - The mode of life of Devonian entomozoacean Introduction ostracods and the Myodocopid Mega-Assemblage proxy for hypoxic events. Bulletin de l’Institut royal des Sciences naturelles de Belgique, Sciences de la Terre, 74 suppl.: 73-80, 2 text-figs., 1 PL, Bruxelles- Ostracods are ecologically sensitive , and Brussel, December 15, 2004- ISSN 0374-6291. their study provides valuable information about environ­ mental changes, particularly in the Palaeozoic. For ex­ ample, studies of more than 40,000 ostracods across the Frasnian - Famennian (F/F) boundary from several sec­ Abstract tions worldwide confirms that the upper Devonian event was one of the largest Phanerozoic extinctions for ostra­ The mode of life of entomozoacean ostracods, which belong to thecods. Approximately 75 percent of all marine Myodocopida, is controversial. It is generally assumed that they are species went extinct close to the F/F boundary(L e t h i e r s pelagic, more precisely planktonic or nektoplanktonic, and that their & C a s i e r , C a s i e r & L e t h i e r s , presence is indicative of deep environments. However, a purely mor­ 1999a; 2001), as a result phological study is not sufficient to determine their mode of life. Their of sea-level changes(C a s i e r & D evleeschouwer , 1995) relationship with the substrata and other , and their response to and a hypoxic event (C a s i e r , 1987a). The survival of major environmental changes must be taken into account. These data several species demonstrates that very shallow marine confirm a swimming mode of life for all the entomozoaceans, and suggest that a nektobenthic lifestyle, in poorly oxygenated, but not environments and surface waters were not affected during necessarily deep, environments is more probable than a planktonic the extinction(C a s i e r , 2003). mode of life. The presence of a Myodocopid Mega-Assemblage proxy Entomozoacean ostracods played an important role for hypoxic events, seems more useful that the differentiation of an “ Entomozoid ecotype” . during the Late Devonian mass extinction. However, their mode of life is controversial. Entomozoaceans are usually Key-words: Ostracods - Entomozoacea - Devonian - Mode of life -regarded as pelagic, more precisely planktonic or nekto­ Palaeoecology. planktonic. The goal of our paper is to demonstrate that a nektobenthic mode of life in poorly oxygenated environ­ ments is more probable for this group of ostracods. The Super-family EntomozoaceaP r i b y l , 1951 belongs Résumé to the Order MyodocopidaS a r s , 1866. Entomozoaceans are characterized by relatively large carapaces with Les Entomozoacea constituent une super-famille d’ostracodes apparte­ weakly calcified sub-equal valves (Fig. 1 and PI. 1, Figs nant à l’ordre des Myodocopida. Ils ont en commun une taille relative­ ment grande, des valves égales peu calcifiées, et une ornementation 9, 10, 12-15 show some species of entomozoaceans), the souvent comparée à des empreintes digitales. Elle leur confère une presence in some species of an adductorial sulcus (= nu­ grande valeur biostratigraphique. La plupart des ostracodologistes chal furrow), and a ribbed ornamentation frequently com­ estiment qu’ils étaient pélagiques et plus précisément planctoniques ou necto-planctoniques, et que leur abondance dans les sédiments pared to fingerprints (seeO l e m p s k a , 1992, for a detailed témoigne par conséquent d’une grande profondeur de dépôt. Mais study of their shell structure). Entomozoaceans were l’étude morphologique seule ne suffit pas pour déterminer leur mode abundant from the to the Upper . de vie. Leur relation avec le substrat et avec les autres organismes, ainsi que leur comportement lors des changements environnementaux ma­ jeurs doivent aussi être pris en considération. Ils montrent que les Entomozoacea étaient plutôt necto-benthiques et opportunistes dans Previous statements concerning the mode of life of des milieux pauvres en oxygène dissous, mais pas nécessairement entomozoaceans profonds. Un Mega-Assemblage à Myodocopida caractérisé par la présence d’Entomozoacea et (ou) de Cypridinacea (= Assemblage V de C a s ie r et al., 1985;C a s ie r , 2003) peut servir d’indicateur pour les M a t e r n (1929) considered the majority of entomozoa­ événements hypoxiques dans le Dévonien. ceans to be active swimmers. K u m m e r o w (1939) and Mots-clefs: Ostracodes - Entomozoacea - Dévonien - Mode de vie - R a b i e n (1954) compared entomozoaceans to modem Paléoécologie. Halocypridacea and surmised a nektonic or planktonic 74 Jean-Georges CASIER

(1987a,b, 1988c) arrived to the conclusion that entomo­ zoaceans were probably nektobenthic, and adapted to survive in poorly oxygenated, but not necessary deep, environments. However this statement was not supported by G r o o s -U f f e n o r d e & S c h i n d l e r (1990), for whom a nektobenthic mode of life for entomozoaceans cannot be assumed because they have not been affected during global events, particularly the Upper Kellwasser Event in the Late Devonian. Their abundance in the Upper Kellwasser Horizon, which according toG r o o s -U f f e ­ n o r d e & S c h i n d l e r (1990) lacks benthos, and their rapid evolution are supplementary arguments against a nekto­ benthic lifestyle. Conversely, and also in the Proceedings of the Tenth International Symposium on Ostracoda, W i l k i n s o n & R i l e y (1990) arrived at the conclusion that entomozoaceans present in five marine horizons in the Namurian of Northern England were nektobenthic com­ Fig. 1 — Richterina (Volkina) zimmermanni ( V o l k , 1939). ponent of the dysaerobic community. The fauna asso­ An example of streamlined entomozoacean in late­ ciated with the Namurian entomozoacean in northern ral, dorsal and anterior view. Matagne Formation. England represents low diversity high-abundance assem­ Boussu-en-Fagne, Belgium. Collection IRScNB blages typical of a late Palaeozoic dysaerobic environ­ n° a l211. x50. ment, and conclusivelyW i l k i n s o n & R i l e y (Ibid.) con­ sidered the entomozoacean an integral opportunistic va­ grant nektobenthic component of the dysaerobic assem­ blage, colonizing during brief spells when the oxygen life habit for this group. On the contrary, the discovery of level was suitably high. thick shelled silicifted carapaces of entomozoaceans led Furthermore in the Proceedings of the same sympo­ B lumenstengel (1965, 1973) to suggest that they were sium, B e c k e r & B l e s s (1990) considered C a s i e r heavily calcified and consequently that they were benthic ( 1987a,b)’s argument that entomozoaceans may have a organisms. B e c k e r (1971) discovered a highly calcified nektobenthic, nearshore lifestyle to be inconclusive. They carapace of entomozoacean in the Belgian Frasnian, anddid not observe mixed Eifelian and entomozoacean as­ also concluded that some entomozoaceans displayed a semblages, and consider this to be evidence that the benthic mode of life.K o z u r (1972) arrived to the con­ entomozoaceans did not live in near-shore environments. clusion that they were benthic psychrosphaeric ostracods. Finally, O l e m p s k a ( 1992) suggested that the extremely Referring to T r i e b e l (1941), G r ü n d e l , (1962, 1965) fragile thin ornamentation over the whole carapace of and B e c k e r (1976),G o o d a y (1983) summarised the entomozoaceans is apparently inconsistent with a benthic morphological evidence for a planktonic life habit formode of life, and that the extremely high flanges may be entomozoaceans: 1. the presence of a thin, weakly calci­ an adaptation to a planktonic life habit. However, for fied, often streamlined carapace; 2. a convex ventral several species with rather heavily calcified shells and margin, and 3. the tendency of the carapace to be rounded relatively short flanges,O l e m p s k a (1992) suggested that with a high surface-volume ratio. In addition,G o o d a y an epiplanktonic as well as a planktobenthic life habit is (Ibid.) suggested that the presence of flange-like ribs of conceivable. It is herein suggested that the presence of some species may increase the surface area of the shell longitudinal canals in the carapace of some entomozoa­ and retard its sinking velocity, and that their worldwide ceans may have assisted in the exchange of respiratory distribution, together with their abundance in facies gases and such a function could have been important for which are often virtually devoid of other fossils, are entomozoaceans in stagnant and possibly dysaerobic supplementary arguments for a planktonic existence. waters. However, G o o d a y (Ibid) noted that the carapace mor­ phology of entomozoaceans provides no clear evidence for either a planktonic or benthic existence, and he con­No argument relevant for a benthic mode of life for cluded that both these life habits may have been repre­ the entomozoaceans sented within the group. He compared some species of entomozoaceans with modem species belonging to theNo particular evidence points conclusively to a genuine Thaumatocyprididae and considered that there is no con­ benthic mode of life for the entomozoaceans. Generally, vincing morphological basis for rejecting the hypothesisthe principal argument for a benthic life habit is the that at least some entomozoaceans were benthic or nek­presence of well calcified and silicifted carapaces in some tobenthic. specimens, which has led some authors to suggest that After the study of entomozoaceans present in the Ma­ some or all entomozoaceans possessed a heavy, thick tagne black shale Formation, in Belgium,C a s i e r calcified carapace. The epigenesis in silicon dioxide is Devonian entomozoacean ostracods and the Myodocopid Mega-Assemblage proxy for hypoxic events 75 frequent during the fossilisation, and there is no doubt North America are limited to rare species, even though all that the heavy calcification of rare carapaces is related to these regions were close during the Late Devonian. Sev­ the same phenomenon. The heavy calcified carapace eral indisputable benthic species belonginge.g. to the discovered by B e c k e r (1971) in the access path to the podocopid ostracods show more widespread geographic Lion quarry, Belgium, is probably an example ofEnto- distributions compared to that of entomozoacean species! moprimitia (E.) concentrica (M a t e r n , 1929), a very The abundance of entomozoaceans in facies which are abundant species in the Belgian late Frasnian. Such dis­ often virtually devoid of other fossils is another false coveries prove only that the carapaces of entomozoaceansargument. Entomozoaceans are always associated with were relatively thick, a feature demonstrated recently other ostracods or animals. For example, in the Matagne by O l e m p s k a (1992). This author confirmedL a n g e r black shale Formation of Belgium, and in the Cypridie- (1973)’s statement that the carapace of entomozoaceans nen Schiefers of Germany, entomozoaceans were always consists of a thin outer layer which formed the ornamen­associated with palaeocopid ostracods belonging to the tation, and a thicker more complicated inner layer. The Hollinacea and the Kirkbyacea, and also with bivalves ornamentation on the upper surface of the inner layer wasand brachiopods, including lingulids(S a r t e n a e r , 1974; generally erroneously interpreted to represent the orna­ R a b i e n , 1954). In the more oxygenated environment of mentation on the internal surface of the valve(O l e m p s k a , the upper part of the Valisette Formation, formerly ex­ Ibid.). In other words, the "internal” moulds of authorsposed in the Senzeille section, Belgium, entomozoaceans showing the ornamentation corresponds really to exfoli­are associated with diverse fauna including brachiopods, ate carapaces. It is undoubtable that only the outer layer solitary corals, and Eifelian ecotype ostracods, including of the carapaces of entomozoaceans were weakly calci­ podocopids, platycopids, metacopids and palaeocopids fied, an adaptation to a swimming life habit. The flange­ (C a s i e r , 1989). Recently we have also demonstrated that like ribs strengthening the carapace displayed by some entomozoaceans are mixed with numerous ostracods of species belonging to the genusRichterina confirm this. the Eifelian ecotype in the late Frasnian Upper Kellwas­ One argument for a genuine benthic mode of life for ser Horizon of the Schmidt quarry parastratotype in the entomozoaceans, would be the absence of a rostral in­Kellerwald, Germany (C a s i e r & L e t h i e r s , 1 9 9 8 ) , con­ cisure through which swimming appendages can protrude trary to G r o o s -U f f e n o r d e & S c h i n d l e r ( 1 9 9 0 ) ’ s state­ (G o o d a y , 1983). R a b i e n (1954) suggested that the ante- ment. ro-ventral curvature of the carapace observed in some The third argument for a nektoplanktonic mode of life entomozoaceans might correspond to such a structure, butis the rapid evolution of the entomozoaceans. Their ap­ G o o d a y (1983) demonstrated that the same curvature parently rapid evolution is in reality linked to their dis­ observed in the recent thaumatocypridids bears no ob­ tinctive fine ornamentation. Ostracods are generally of vious relationship to the first and second antennae, thepoor biostratigraphic value in the Palaeozoic since criter­ swimming appendages. However, the absence of a rostral ia of determination are insufficient. They are generally incisure is not an argument for a genuine benthic mode ofsmooth, and due to poor preservation and difficulties of life, because that structure lacks in numerous recent extraction, internal structures of carapaces (hinge, muscle swimming ostracods(G o o d a y , Ibid.). scars...) are only very rarely observable. In addition their morphology is highly controlled by environments and convergent species are frequent. For these reasons numer­ No argument relevant for a nektoplanktonic mode of ous species and genera display very long stratigraphie life for the entomozoaceans ranges. For example, the podocopid genusBairdia Me C o y , 1844, is recognized from the to The weakly calcified, sometimes streamlined carapaces the Recent! In the Devonian, only the thlipsuraceans, e.g.: Richterina (V.) zimmermanni (V o l k , 1939) (see with their strong ornamentation, the spiny ostracods of Fig. 1), occasionally with postero-dorsal spines (possibly the Thuringian ecotype ofB e c k e r (in B a n d e l & B e c k e r , used for the balance of the valves and for the stabilisation 1975), and the entomozoaceans with their delicate orna­ of the carapaces in e.g. some species ofEntomoprimitia mentation comparable to finger prints, have a biostrati­ and Franklinella), with flange-like ribs (strengthening the graphic value. However, without ornamentation, the carapace and used probably for aerofoilsin e.g. Richte­ number of species distinguishable among the entomozoa­ rina (R.) goodayi O l e m p s k a , 1992), with longitudinal ceans would be considerably reduced, and consequently canals (lightening the carapace; seeO l e m p s k a , 1992, their apparent rate of evolution would be comparable to fig. 8), and with antero-ventral spurs (a structure incom­ the rate of evolution observed for the majority of other patible with a benthic mode of life in the genusFrankli­ ostracods! nella), indicate a swimming life habit for entomozoa­ ceans, but not necessary a nektoplanktonic mode of life. A worldwide distribution of entomozoaceans would be A nektobenthic mode of life in dysaerobic environ­ an argument for a nektoplanktonic mode of life, howeverments is the most probable for entomozoaceans this affirmation is overstated: several species are known from Belgium to the Russian Platform, and North Africa The presence of homoplasic species in the entomozoa­ but the relationships with Southern France, China andceans (e.g. some Entomoprimitia resemble paraparchita- 76 Jean-Georges CASIER

cean ostracods) is an argument in favour of a relationshipP r e a t , 2003, fig. 3) which is now dominated by filter- with the substrate(C a s i e r , 1992). O l e m p s k a (1992) ob­ feeding metacopids and palaeocopids. The preponder­ served an outermost ridge in some entomozoaceans, an­ance of filter-feeding relative to deposit-feeding ostra­ other interesting example of convergence. This ridge is cods is considered to signal the start of an anoxic trend comparable to the adventral structure, which continues (W h a t l e y , 1991; L e t h i e r s & W h a t l e y , 1994), and it is round the entire valve margin in the palaeocopids, and a most likely related to the increase in water-depth, which, nektobenthic life habit is frequently assumed for repre­ in turn, would have been responsible for the termination sentatives of this group.G r o o s -U f f e n o r d e (1984) ob­ of the reef development as was postulated on basis of served that “ Entomoprimitia” sandbergeri (M a t e r n , detailed sedimentological analysis (B o u l v a i n & H e r - 1929) displays such a structure, and suggested that the b o s c h , 1996); 3. So extensive was the hypoxia at the species be excluded from the entomozoaceans and moved Valisettes Formation - Matagne Formation boundary, that to the palaeocopids! ostracods belonging to the benthic Assemblage III dis­ As mentioned before, in the Matagne black shale For­ appeared abruptly, and were replaced in the black shales mation, Belgium, and the Cypridienen Schiefers, Ger­ of the overlying Matagne Formation by opportunistic many, entomozoaceans were always associated with pa- entomozoaceans and by cypridinaceans (Assemblage V laeocopid ostracods belonging to the Flollinacea and theo fC a s i e r , 1987a; see alsoC a s i e r & P r e a t , 2003, fig. 3 ). Kirkbyacea, and also with bivalves (Buchiola) and bra­ These myodocopid ostracods abruptly disappeared 3.5 m chiopods (Lingulids, Chonetes, Ripidiorhynchus, Ryo-below the Frasnian - Famennian boundary in the Neuville carhynchus etc...), and it is indisputable that these taxa railway section, during the peak of anoxia, even though lived in the same environment. The mode of life of Palaeophilomedes neuvillensis C a s i e r , 1988a, an undis- lingulids is well known because they are still alive, and putable nektoplanktonic cypridinacean, survived. The the life habit of chonetaceans is easy to imagine because ostracod recovery in the Neuville railway section is they displayed spines for the fixation on a muddy sub­ marked by the successive entry of Franklinella sigmoi­ strate. A nektobenthic mode of life is assumed for some dalis M u l l e r -S t e f f e n , 1964, má “Entomoprimitia" palaeocopids (H a r t m a n n , 1963; B e c k e r & B l e s s , sandbergeri (M a t e r n , 1929). The most important argu­ 1990). In fact, all these organisms were adapted to sur­ ment for a nektobenthic mode of life is that the entomo­ vive in poorly oxygenated environments and this was the zoaceans are affected by extinction events, contrary to the case for entomozoaceans. Their weakly calcified cara­ suggestion ofG r o o s -U f f e n o r d e & S c h i n d l e r (1990). pace might be an adaptation to aid survival in poorly Further evidence is provided by the study of ostracods oxygenated, calm waters. O l e m p s k a (1992) suggested in the upper Kellwasser Horizon, close to the Frasnian - that the presence of longitudinal canals observed in the Famennian boundary, in the Schmidt quarry parastrato- carapace of some entomozoaceans may have assisted in type (Kellerwald, Germany). In this horizon, the relative exchange of respiratory gases and such a function could proportions of ostracods belonging to the Eifelian eco­ have been important for entomozoaceans in dysaerobictype and to the entomozoaceans reflect variations in the waters. oxygen content of bottom waters, as suggested by varia­ In short-term, dysoxic conditions, entomozoaceanstions of the total organic carbon contents (TOC)(C a s i e r were poorly diversified. For instance, in the lower Fras­ et al., 1999). Entomozaceans undoubtedly became extinct nian (punctata conodont Zone) observed in the during the Late Devonian mass extinction in the Schmidt Sourd’Ave section in Belgium, and in the Tempiute quarry. Mountain in Nevada, only one species belonging to the Finally, O l e m p s k a (2002) also displayed that entomo­ genus Franklinella has been recorded (C a s i e r , 1987b; zoaceans became extinct close to the Frasnian - Famen­ C a s i e r in S a n d b e r g et al., 1997). However, when such nian boundary in the Plucki section in the Holy Cross conditions persisted for a long time, for example in the Mountains, Poland displaying their persistent anoxic de­ Belgian Matagne Formation or in the German Cypridie­ position in thelinguiformis-triangularis interval (B o n d et nen Schiefers, entomozoaceans displayed a gradual di­ al„ 2004). versification, inducing the development of large forms, inconsistent with a nektoplanktonic mode of life. Our recent study of ostracods present in the late Fras­ The Myodocopid Mega-Assemblage nian of Belgium confirms this life habit(C a s i e r , 2003). The following assemblages are recognized on the south­On the basis of the carapace morphology,B e c k e r (in ern flank of the Neuville Massif, within the Dinant Basin B a n d e l & B e c k e r , 1975) differentiated three ostracod ( P I . 1): 1. In the Valisettes Formation enclosing the reef ofecotypes (= Ökotypen) for the Devonian, including the the Beauchâteau quarry, the ostracod fauna is largely “ entomozoid ecotype” . Since then, cypridinacean ostra­ dominated by deposit-feeding thick shelled podocopids cods have been discovered in the Frasnian of Germany indicative of well oxygenated water conditions between (B u g g i s c h et a i, 1983; B e c k e r & B l e s s , 1987), o f fair-weather and storm wave bases; 2. In the upper part of Belgium (C a s i e r , 1988a), of France(L e t h i e r s & C a s i e r , the Valisettes Formation outcropping in the Neuville rail­ 1995), and of Poland(O l e m p s k a , 2002) (Fig. 2 and PI. 1, way section, ostracods belong to the same assemblage Fig. 11 show a Cypridinacea). Considering that the super­ (Assemblage III of C a s i e r , 1987a; see alsoC a s i e r & families Cypridinacea and Entomozoacea both belong to Devonian entomozoacean ostracods and the Myodocopid Mega-Assemblage proxy for hypoxic events 77

ambiguity, especially as the usage of the term ecotype seems to be totally inappropriate in this case. In fact the term ecotype describes a group of plants, more rarely of animals, within a single species, and adapted genetically to a particular habitat but able to cross freely with other ecotypes of the same species(A b e r c r o m b i e et al., 1980)! For the same reason, we propose to differentiate an Eifelian Mega-Assemblage constituted of several assem­ blages (Assemblages 0 to III in C a s i e r , 1987a; see also C a s i e r & P r é a t , 2003, fig. 3) and a Thuringian Mega-

Fig. 2 — Palaeophilomedes neuvillensis CASIER, 1988. An Assemblage (= Assemblage V Ibid.). example of Cypridinacea. Specimen discovered by T . B e c k e r , in the La Serre coupeC in the Montagne Noire, France. Collection University of Paris 6 n° P6M 1836. xl6.5. Conclusions

A purely morphological study is not sufficient to deter­ mine the mode of life of entomozoacean ostracods. Their the Myodocopida, and, sharing the opinionB ofe c k e r & relations with the substrate and with other animals, and B l e s s (1987) that during the Devonian they lived in the their response to major environmental changes must been same environments, we suggested it more appropriate to taken into account. These data confirm a swimming life replace the “ entomozoid ecotype” by a “myodocopid habit for the entomozoaceans, and suggest that a nekto­ ecotype” (C a s i e r , et al., 1985). This was an error of benthic mode of life in poorly oxygenated environments, judgment. The recent study of ostracods present in the is more probable than a planktonic or nektoplanktonic Neuville railway section (C a s i e r , 2003) has shown that mode of life for this group. on the contrary to entomozoaceans, cypridinoid ostracods The distinction of a “Myodocopid Mega-Assem­ were not affected during the Late Devonian mass extinc­ blage” characterising poorly oxygenated water seems tion. Moreover, unlike entomozoaceans, cypridinoid os­preferable and more useful that the differentiation of an tracods are sometimes found in environments devoid of “Entomozoid ecotype” or a “ Myodocopid ecotype” , other ostracods and fossils. Consequently we suggest that since the term “ ecotype” seems inappropriate. More­ not all cypridinaceans and entomozoaceans shared the over, species belonging to the cypridinaceans and the same life strategy during the Devonian: entomozoaceans entomozoaceans did not necessarily share the same en­ were nektobenthic and opportunists in poorly oxygenated vironment. This is proved by the fact that cypridinaceans environments, whereas all or a part of the cypridinaceans are sometimes found alone (probably during period of lived in an upper layer of water, maybe close to the intense anoxia) and by the fact that entomozoaceans surface. Consequently, and by definition, they were not became extinct during the late Frasnian event whilst members of the same ecotype, and the “ entomozoid cypridinaceans were unaffected by this extinction. ecotype” of B e c k e r (in B a n d e l & B e c k e r , 1975) re­ mains valid but not as emended by the same author in 2001 in order to include the cypridinaceans. In order to distinguish assemblages, a “ Myodocopid Mega-Assem­ Acknowledgments blage” characterized by the presence of entomozoaceans Many thanks to Ewa Olempska (Warsaw), and to David Bond (Leeds) and (or) cypridinaceans (= Assemblage V ofC a s i e r et al., for having kindly reviewed this paper. The research has been supported 1985; see also fig. 3 in C a s i e r & P r é a t , 2003) is used in by the F.R.F.C. n° 2.4501.02 project of the Belgian “ Fonds National de this case, rather than an ecotype, since this removes la Recherche Scientifique”

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P l a t e 1

Generalized section of the Frasnian - Famennian boundary beds and their ostracod faunas in the type area (Dinant Basin, Belgium). Most representative ostracod species recovered are shown in successive faunas. The succession of assemblages around the Frasnian - Famennian boundary (= FFB) suggests that the Late Devonian mass extinction was principally linked to a regressive-regressive cycle, and to an anoxic event. The transgression was progressive and culminated below the FFB as suspectedJ o hby n s o n et al. (1985); the regression was more rapid and important. O.M.Z. = oxygen minimum zone. See also comments in the text. More details about these ostracod faunas can be found C in:a s ie r & D evleeschouwer (1995) for ostracods collected in the Sinsin section (Figs. 1 -8 ); C a s ie r (2003) for ostracods collected in the Neuville railway section (Figs. 9-22);C a s ie r (1988b) for ostracods collected in the Beauchâteau quarry (Figs. 23-30). The survival of several species and among them ofOvatoquassilites avesnellensis (L e t h ie r s , 1973) (=Fig. 5) and of the cypridinaceanPalaeophilomedes neuvillensis C a s ie r , 1988 (Fig. 11 ) shows that very shallow marine environments and surficial waters were not touched during the Late Devonian mass extinction. Ostracods are deposited in the Department of Palaeontology (section Micropalaeontology) of the Royal Belgian Institute of natural Sciences. 80 Jean-Georges CASIER

SENZEILLE Historic ASS. I F.F.B. very shallow FORMATION

F.F.B.

ASS. V opportu nistic 8 MATAGNE

Extinction-\ FORMATION

O.M.Z.

ASS. V opportu nistic

VALISETTES

FORMATION

ASS. Ill shallow

P l a t e 1