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What Is the Boundary for the Quaternary Period and Pleistocene Epoch? the Contribution of Turnover Patterns in Large Mammalian C

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What Is the Boundary for the Quaternary Period and Pleistocene Epoch? the Contribution of Turnover Patterns in Large Mammalian C Quaternaire, 18, (1), 2007, p. 35-53 WHAT IS THE BOUNDARY FOR THE QUATERNARY PERIOD AND PLEISTOCENE EPOCH? THE CONTRIBUTION OF TURNOVER PATTERNS IN LARGE MAMMALIAN COMPLEXES FROM NORTH-WESTERN MEDITERRANEAN TO THE DEBATE Ⅲ Maria Rita PALOMBO* ABSTRACT Assuming that the Quaternary has to be recognized as a formal chronostratigraphic/geochronological unit (having a Sub-Erathem/Sub-Era rank, as recommended by the International Commission on Stratigraphy (ICS), or better a System/ Period rank, as suggested by several scientists), what boundary should be chosen? Should the lower boundary of the Quaternary coincide with the base of the Gelasian Stage (2.6 Ma) as proposed by the ICS? If so, should the Quaternary and Pleistocene lower boundaries be the same or should be different? To contribute to the debate, the Villafranchian large mammal fossil record from the North-Western Mediterranean region have been revised in order to correlate the diversity and structural dynamics of reconstructed faunal complexes with the changes in environmental conditions occur- ring from the Middle Pliocene to the Early Pleistocene. According to the results obtained, two major faunal renewals are detectable at the transition from the early to middle Villafranchian [~2.7-2.5 Ma, about at the time of the transition from the Middle (Zanclean Stage) to the Late Pliocene (Gelasian Stage)], and from the middle to late Villafranchian (~2.0-~1.9 Ma, shortly before the official Plio-Pleistocene boundary). The faunal renewal from the early (V1) to the middle (V2) Villafranchian faunal complexes (that means from the MN16a “zone”/Triversa FU to MN16b “zone”/Montopoli FU) is linked to the Middle Plio- cene climate worsening, in turn related to the onset of bipolar glaciations followed by glacial-interglacial cycles of moderate amplitude (orbital pe- riodicity of 41 ka). The resulting increase in aridity and more intense seasonality caused the disappearance of several forest-dwelling taxa, especially small carnivores and arboreal-scansorial taxa, whereas new large grazers, mixed feeders or even browsers appeared. This renewal (al- ready called the “Equus-elephant event”) can be regarded as a true turnover phase, due to the high percentage of last and new appearances, and to the important ecological structural changes in faunal complexes, involving mainly the herbivore guild. These faunal changes indicate that forests or woodlands gradually gave way to more open environments (including Artemisia steppe) alternating with warm-temperate deciduous forests. More- over, this event can be considered as the starting point for a dispersal phase leading to a progressive standing richness increase during the following Pliocene (middle Villafranchian, V3). Around 2.0-1.8 Ma (late Villafranchian, V4), despite the extinction of some small browsing and grazing ruminants, diversity notably increased due to the progressive appearance of a number of carnivores. Indeed, the so-called “wolf-event” involved several large and small Carnivora, such as the powerful scavenger Pachycrocuta brevirostris, the jaguar-like Panthera gombazsoegensis,andcoo- perative foraging canids. On the other hand, minor phyletic adjustments and some new appearances (especially grazers) affected herbivore guild. This relatively long dispersal phase, and correlated moderate turnover pulses, seems to be less important than the early/middle Villafranchian renewal phase, especially as far as France and the Italian peninsula are concerned. As a result, taking into account the importance of faunal renewal at the Middle to Late Pliocene transition, it seems more reasonable to extend the base of the Pleistocene downwards from 1.81 Ma (official Plio/Pleistocene boundary) to 2.6 Ma (base of Pliocene Gelasian Stage). Accordingly, the base of the Gelasian seems to be the most appropriate lower boundary for both the Quaternary Period and Pleistocene Epoch. Key-words: Quaternary, Pleistocene, Large mammals, North Western Mediterranean. VERSION FRANÇAISE ABRÉGÉE QUELLE LIMITE POUR LE QUATERNAIRE ET LE PLÉISTOCÈNE ? L’APPORT DE L’ÉTUDE DU RENOUVELLEMENT DES FAUNES À GRANDS MAMMIFÈRES DE LA MÉDITERRANÉE NORD OCCIDENTALE DURANT LE PLIOCÈNE MOYEN ET SUPÉRIEUR ET LE PLÉISTOCÈNE INFÉRIEUR INTRODUCTION La plupart des chercheurs conviennent que le Quaternaire devrait être reconnu comme une unité chronostratigraphique/géochronologique formalisée, Sub-Erathème/Sub-Ere, comme cela a été recommandé par la Commission Internationale pour la Stratigraphie (ICS) (cf. Clague, 2005, 2006) ou mieux Système / Période, comme suggéré par plusieurs chercheurs. Mais, il n’existe pas encore d’accord au sujet de cette unité. La limite inférieure du Quaternaire devrait-elle coïncider avec la base de l’étage Gélasien (2.6 Ma), comme cela a été proposé par l’ICS ? Si oui, les limites in- férieures du Quaternaire et du Pléistocène devraient-elles coïncider ou bien devraient-elles être différentes ? * Dipartimento di Scienze della Terra, Università “La Sapienza”, CNR – Istituto di Geologia Ambientale e Geoingegneria, Piazzale A. Moro, 5 – 00185 ROMA, Italy. E-mail : [email protected] Manuscrit reçu le 08/07/2006, accepté le 09/10/2006 36 Pour contribuer au débat et en rappelant que le « Pléistocène » et le Quaternaire ont été créés en s’appuyant sur des donnés soit paléontologi- ques, soit climatiques (ex. Desnoyers, 1829 ; Lyell, 1833, 1839 ; Reboul, 1833 ; Agassiz, 1840 ; Forbes, 1846), les faunes à grands mammifères de la région méditerranéenne nord-occidentale ont été révisées dans le but d’envisager quels ont été les changements les plus importants des complexes fauniques (fluctuations de la diversité et changement de la structure) qui se sont produits durant le Pliocène moyen et supérieur et le Pléistocène in- férieur, lorsque d’importants changements climatiques ont modifié l’environnement. Le renouvellement des faunes et de la structure des paléocommunautés et leurs rapports avec les changements globaux du climat ont été considérés. En particulier, la dynamique des deux plus importants « turnovers » du Villafranchien, l’“elephant-Equus event” (Lindsay et al.,1980) (qui s’est déroulé grosso modo entre la fin du Zancléen et le début du Gélasien), et le « wolf event » (Azzaroli, 1983 ; Palmqvist, 1999 ; Sardella & Palombo, 2007), auparavant placé au début du Pléistocène, a été analysée dans le but d’envisager lequel des deux était le plus remarquable, en préci- sant si et de quelle façon le climat et les changements de l’environnement avaient entraîné le renouvellement des faunes. En effet, écologistes et également évolutionnistes ont débattu depuis longtemps du rôle que les changements du climat ont et ont eu sur l’évo- lution de la faune, et il semble que nous n’ayons pas encore trouvé d’accord. Les modèles les plus importants – tel que le « Stationary model »(Ro- senzweig, 1975) et les théories qui privilégient le rôle de l’environnent (Vrba, 1992, 1995 ; Brett & Baird, 1995), ou par contre les modèles et les théories qui regardent comme plus important la compétition intra- et inter-spécifique (Red Queen Hypothesis, Van Vallen, 1973 ; Bell, 1982 ; Pro- thero 2004) ; ou, enfin les hypothèses dont la vision est un peu plus complexe (“Coevolutionary disequilibrium Model”, Graham & Lundelius, 1984) ou stochastique (Court Jester Hypothesis, Barnosky 2001) – semblent être à la fois confirmés ou réfutés par les donnés paléontologiques. D’autre part, si le climat et les changements de l’environnement ont causé le renouvellement des faunes, soit par migration, soit par apparition et ex- tinction, les variations dans la structure des paléocommunautés devraient être en accord avec ces changements du climat. Dans ce but, nous avons analysé les turnovers et les variations en richesse, diversité et structure écologique, des complexes faunistiques à grands mammifères du Pliocène moyen et supérieur et du Pléistocène inférieur de la zone méditerranéenne nord occidentale. MATÉRIAUX ET MÉTHODE Les listes fauniques de 112 faunes locales (LFAs) d’Espagne, de France et d’Italie, les plus riches ou les plus irréfutables au point de vue chronologique, ont été révisées aussi bien que la taxonomie et la distribution chronologique des espèces (fig. 1, tab. 1 et 2). Des complexes faunisti- ques proches des « paléocommunautés » ont été déterminés également à l’aide d’analyses multivariées (analyses de cluster), permettant de recon- naître des complexes fauniques (FCs) cohérents au point de vue chronologique et écologique (biochrones). Ces complexes peuvent être regardés comme des « block of coordinated stasis »(sensu Brett & Baird 1995 ; Bret et al., 1996), puisqu’on assume que, pendant le temps qu’ils renferment, aucun turnover ne se produisit (cf. Palombo, 2005, sous presse et références bibliographiques citées). Les méthodes de Harper (1975) et Foote (2000) ont été utilisées pour calculer la richesse standardisée et la diversité, celles de Torre et al. (1999) et Foote (2000) pour calculer l’index de turnover entre deux FCs successives et les taux d’apparition et d’extinction dans chaque FC respectivement. RÉSULTATS L’analyse de similitude montre l’évidence d’une séparation plus importante entre le LFAs du Villafranchien inférieur et moyen qu’entre celles du Villafranchien moyen et supérieur. Les valeurs des indices de turnover calculés à la transition entre deux FCs successives confirment le caractère progressif du renouvellement qui se développe durant le Pliocène supérieur. Les taux d’apparition et d’extinction soulignent d’un côté ce turnover, et de l’autre
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