Ontogenetic trajectories and phenotypic integration of several trinucleoids (Trilobita) from Argentina

BIGNON, A.; WAISFELD, B.G; VACCARI, N. E

Mendoza

Congreso; 4thInternational Palaeontological Congress: The History of Life: A view from the Southern Hemisphere.; 2014

International Palaeontological Association

Diverse, abundant and with a worldwide distribution, the trinucleoids constitute an opened window to describe the Earth during the Ordovician. Several formations of the Middle Ordovician from the Argentinian Precordillera have revealed a trinucleoid fauna preserved in silica. This exceptional preservation allows a precise and quantitative description of their ontogeny. Indeed, a vast scale of developmental instars is known in these species, from the protaspid (larval) to holaspid (adult) stages. This material is therefore remarkable to compare ontogeny and phenotypic modularity between close species. Morphological changes occurring during the development of two Bancroftolithus species, B. pozensis and B. hughesi are described using geometric morphometric methods. Thanks to these quantitative data we are able to compare the development of these species in order to recognize modifications of the ontogenetic trajectories. A preliminary analysis revealed a parallelism between the two species trajectories but at a different rhythm. Indeed, ontogenetic changes occurring during the meraspid and holaspid period of B. pozensis are significantly slower than the other species. Moreover the same change of trajectory happens during the early meraspid period for B. pozensis but only at the late meraspid for B. hughesi. The same trend is recognised in separate analyses for cephala and pygidia. The identification of this interdependence provides a clear pattern between the morphological features during ontogeny. Phenotypic integration is controlled by function and/or development. So understanding this organization is crucial to identify environmental constraints applied on a particular morphological area. These quantitative methods provide a precise framework of developmental relationships between morphological characteristics and between species. The results obtained here will be useful to study how environmental constraints may influence ontogenetic trajectory. As the trilobites studied are epibenthic, their survival depends on hydrodynamic conditions. Consequently, their shape is influenced by the water flow parameters. Thus we will model in 3D the trinucleoid morphology and analyse the water motion around the exoskeleton using Computational Fluid Dynamics simulations. Comparing hydrodynamic performances between juveniles and adults, we will be able to understand the control applied by the water flow dynamics on ontogenetic trajectory. Indeed, size changes during development imply a modification of hydrodynamic constraints. Ontogeny must consider these changes for the survival. The precise description of the developmental framework with the knowledge of the hydrodynamic constraints variation will offer an exceptional example of the control of an extrinsic parameter on ontogeny.