MY ABSTRACTS 2
Lerosey-Aubril, R. and R. Feist. 2003. GSA annual meeting, Seattle (02-05 November)
Our recent discoveries of both latest Devonian and earliest Carboniferous proetide larvae allow us to propose a new hypothesis to explain the survivorship of this group during the end-Devonian Hangenberg Event. The comparison of Lower Carboniferous larvae with latest Devonian ones clearly demonstrate that the size-range of planktonic larvae, very narrow in oldest larvae, widened. Some Lower Carboniferous planktonic larvae are 12.5% smaller than the smallest pre-Hangenberg ones, whereas others are 12.5% larger than the largest pre-Hangenberg ones. This extension of the planktonic size-range suggests that the Lower Carboniferous proetide trilobites may have had two planktonic larval stages which considerably increased the duration of the planktonic period, thus delaying the onset of metamorphosis. In the context of overall spread of hypoxic bottom waters poisoning benthic habitats in end-Devonian outer-shelf seas, the observed modification of the early ontogenetic life cycle might have constituted an advantageous strategy for survivorship and post-event recovery. Indeed the opportunity for planktonic larvae to migrate further and to reach other, potentially non-poisoned, habitats could have enabled these trilobites to survive the Hangenberg cisis.
Finally, it must be emphasized that proetide trilobite larvae from the Famennian and Lower Carboniferous share a particular morphology lacking in other trilobites such as phacopids. Indeed, proetide planktonic larvae are very bulbous and smooth whereas, after metamorphosis, benthic ones are slightly vaulted and possess well defined furrows that are clearly homologous with major adult features. In contrast, earliest larval stages of phacopid trilobites are disc-shaped and of low relief, already provided with adult-like anteriorly diverging proto-cephalic axial furrows; these forms might well have been already adapted to bottom life excluding a long preceeding planktonic period. We speculate that the absence or short duration of any planktonic larval stage was the ultimate cause of the extinction of the Phacopida at the end-Devonian Hangenberg Event.
Feist, R., Klapper, G. and R. Lerosey-Aubril. 2003. GSA annual meeting, Seattle (02-05 November)
The late Emsian middle Member of the Bissounel Formation (Montagne Noire, France) has yielded a highly diverse silicified trilobite fauna. Among the thousands of disarticulated sclerites discovered, two protaspid specimens were found that display morphological characteristics of a planktonic larval stage of a calymenid trilobite. The presence of calymenid larvae within a non-calymenid trilobite fauna is discussed. The two specimens are very similar to the first planktonic larval stage of the Ordovician Flexicalymene senaria (Conrad, 1841), and are of comparable size. This indicates that, as in F. senaria, a second planktonic larval stage might have occurred in the ontogeny of this undetermined calymenid. Thus, some post-Ordovician calymenids might have retained the ancestral developmental strategy with a two-stage planktonic period. The discovery of these two protaspides constitutes the first evidence of a calymenid trilobite in the Devonian of the Montagne Noire.
R. Lerosey-Aubril. 2004. PalAss annual meeting, Lille (17-20 December)
New silicified material from the Middle Famennian of Morocco, the Late Famennian of Germany, and the Early Carboniferous of France substantially increase our knowledge of proetoid larvae. Both anaprotaspides and metaprotaspides illustrate that proetoid trilobites possess a unique early ontogeny, remaining highly conservative from the Ordovician to the Early Carboniferous. Accordingly, they support the view that Proetida may be composed of two different clades. The new data also enable the evolution of anaprotaspis size in proetoids from the Ordovician to the Lower Carboniferous to be broadly depicted. In particular, two marked increases of larval sizes are recognised. The first one may have occurred around the Silurian/Devonian boundary, but its potential implication on proetoid evolutionary history is unknown. On the other hand, the substantial sizes reached by Late Famennian and Early Carboniferous anaprotaspides suggest the acquisition of an extended planktonic period in the early ontogeny of proetoids. The possible implications of this developmental strategy on the survivorship of these trilobites during the end Devonian Hangenberg biocrisis or, alternatively, on the colonization of deep water benthic habitats, are discussed.
Feist, R. and R. Lerosey-Aubril. 2006. Rencontre pour la valorisation et la préservation du patrimoine paléontologique, Marrakech (27-29 Avril)
Au cours de ces trente dernières années, l’exploitation de l’exceptionnel patrimoine paléontologique marocain a contribué de façon significative au développement économique de certaines régions du pays (e.g. Erfoud et ses environs). Toutefois, la commercialisation des fossiles a pris de telles proportions que l’on est aujourd’hui en droit de se demander si un épuisement rapide de certains gisements n’est pas à craindre dans les années à venir. Celui-ci aurait pour conséquence non seulement le tarissement d’une manne financière vitale pour un nombre notable de marocains, mais également la disparition irrémédiable de sources d’informations de premier choix pour les historiens de la vie que sont les paléontologues. En tant que scientifiques, nous nous sentons particulièrement concernés par la préservation du patrimoine fossilifère marocain. Toutefois, notre fonction ne nous autorise qu’à jouer le rôle de conseillers dans ce débat, en rappelant notamment l’intérêt d’un tel patrimoine pour la communauté scientifique internationale. C’est ce que nous tâcherons de faire à travers l’exemple d’un groupe particulièrement affecté par la commercialisation abusive de fossiles, les trilobites du Dévonien.
Après une courte présentation de l’évolution de la diversité des trilobites marocains au cours du Dévonien, nous présenterons un certain nombre d’exemples visant à démontrer l’intérêt scientifique des faunes à trilobites découvertes dans ce pays. En premier lieu, nous aborderons l’importance en biostratigraphie de certaines espèces qui, compte tenu de leur vaste répartition à travers le monde et de leur grand nombre dans les gisements marocains, permettent de déterminer, directement sur le terrain, l’appartenance de certains niveaux aux zones standards à conodontes. Nous soulignerons également l’intérêt des trilobites dévoniens pour les reconstitutions paléogéographiques. En particulier, nous rappellerons l’opportunité unique, offerte par les faunes à trilobites du Dévonien marocain, d’étudier l’histoire géodynamique de la marge Nord-Gondwanienne, cette dernière étant principalement représentée à l’heure actuelle par les terrains marocains. Enfin, nous expliquerons comment la richesse et l’exceptionnelle qualité de préservation des faunes à trilobites du Maroc peuvent nous permettre d’aborder des aspects peu connus de l’évolution de ces organismes. Notamment, nous décrirons comment les découvertes de spécimens entiers, si fréquentes dans les gisements dévoniens du Maroc, facilitent la réalisation d’études morpho-fonctionnelles, contribuant ainsi de façon notable à l’accroissement de nos connaissances sur les modes de vie des trilobites et les adaptations morphologiques associées. De même, nous évoquerons les études ontogénétiques réalisées à partir de matériels finement silicifiés découverts au Maroc, ainsi que l’intérêt de telles études pour comprendre les mécanismes régissant l’évolution biologique. Enfin, nous montrerons comment la très bonne qualité et la richesse des matériels fossiles du Dévonien du Maroc ont permis l’étude de tendances évolutives majeures chez les trilobites, telles que la réduction du complexe visuel chez les Phacopidae ou l’apparition d’un troisième tagme et ses modifications chez les Scutelluinae.
En conclusion, nous nous interrogerons sur la possibilité d’une entente entre paléontologues, commerçants et législateurs. Notamment, nous suggérons aux responsables de la préservation du patrimoine paléontologique marocain d’encourager l’instauration d’un système de collaboration entre scientifiques et commerçants qui, sous leur tutelle, permettrait un réel échange de compétences. Dans un tel système, les scientifiques, par des déterminations et des datations, pourraient aidés les commerçants à valoriser leurs produits alors que ces derniers autoriseraient l’étude et le dépôt dans une institution de recherche marocaine de certains spécimens d’importance scientifique notable. Ce système aurait pour avantage de réconcilier impératifs scientifiques et économiques ainsi que de rétablir une certaine communication entre les diverses parties concernées par l’exploitation des gisements fossilifères marocains.
Feist, R., P., Morzadec & R. Lerosey-Aubril. 2007. First International Palaeobiogeography Symposium, Paris (10-13 July)
Devonian trilobites are stenotopic benthic inhabitants of epicontinental shallow seas. They are unable to cross large oceans mainly because of their extremely short planktonic period in precocious evolution before metamorphosis to the adult-like benthic state of development. Thus, they display finer biogeographic sensitivity than many other benthic biotas and have great potential for discriminating oceanic barriers between major continental entities. However, testing kinetic models with trilobites that concern the dynamic relations between main Gondwana and Gondwana derived continental fragments such as Carinthia and the Armorican Terrane Assemblage on one hand, and the rheno-hercynian margin of Avalonia on the other is problematic during Early and mid-Devonian times. Indeed, biofacies control of contrasting substrate conditions severely constrains diversity and evolution of trilobite communities in space and time. The silici-clastic rhenish Magnafazies with neritic, poorly diversified, acastid and homalonotid trilobites and the calcareous off-shore bohemian (“Hercynian”) Magnafazies with highly diversified proetids, scutelluids, lichids, harpetids, phacopids, calymenids and dalmanitids can be distinguished . In the rheno-hercynian belt the entire Early Devonian is characterised by exclusively “rhenish” trilobites that originate from Late Silurian homalonotid and acastid communities widespread in the Welsh borderland and in the Holy Cross Mountains (Polish Baltica). Appearance of “bohemian” elements coincides with the first carbonate deposits no earlier than in the latest Emsian, and these become increasingly admixed to the rhenish communities thereafter. Gondwana related terranes are characterised by bohemian trilobites that are exclusively present in entirely calcareous sequences such as developed in the Carnic Alps, the Barrandian and in allochthonous blocs, amalgated to the southeastern margin of the rheno-hercynian belt, the Harz, as well as in the Ligerian domain. All other Gondwana derived continental fragments south of the Rheic suture, i.e. Ibero-armorican arc, Montagne Noire, Moroccan Central Meseta and main N-Gondwana exhibit, in the Early Devonian, sequences of alternating siliciclastic and pure limestone deposits with mixed rhenish and bohemian trilobite associations. Thus rhenish trilobites are the unique elements common to both the rheno-hercynian belt of Avalonia and Gondwana related terranes. Fluctuation in the degree of affinity to be observed in this group may serve to discuss the nature and timing of geodynamic plate movements. In this regard, the highest rate of common genera (78%) and species (28%) is reached in the Lochkovian and Pragian (55% versus 17%) which is in agreement with the postulated early closure of the Rheic at that time. Thereafter, the affinity rate fluctuates with 50% of common genera in the lower Emsian and 73% in the upper Emsian whereas the specific rates are extremely low (0% and 6%). In mid-Devonian times common genera drop to 17% in the Eifelian and 0% in the Givetian whereas no species is in common. The decrease coincides with an increase of bohemian elements in the rhenish domain. This trend is not in contradiction with the opening of a narrow rheno-hercynian ocean at that time prior to general convergence and faunistic cosmopolitism in the Late Devonian.
