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

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

Mendoza

Congreso; 4th International Palaeonological Congress; 2014

International Palaeontological Association

As diverse, abundant and worldwide distributed, the trinucleoids constitute an opened window to describe the Earth during the Ordovician. Several formations of the Middle Ordovician from the Precordillera Argentina revealed preserved fauna 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 protaspid stage (larval) to holaspid (adult). This material is therefore remarkable to compare ontogeny and phenotypic modularity between closed species. Morphological changes occurring during the development of two Bancroftolithus species, B. pozensis and B. hughesi are described from geometric morphometric methods. Thank 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. Then the identification of the interdependence provides a clear pattern between the morphological features during ontogeny. Phenotypic integration is controlled by the function and/or the 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 an environmental constrain may lead an ontogenetic trajectory. The trilobites studied being 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 the 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.