An ontogenetic analysis of changes in morphology during phylogeny of some Lamprocyrtis spp. from deep sea sediments

The ontogenetic patterns of skeletal development that may account for variations in morphology of the mature skeleton during evolution of Lamprocyrtis heteroporos to L. nigriniae and L. heteroporos to L. gamphonycha have been determined using scanning electron microscopy. Ontogenetic patterns of development are inferred from examination of mature stages exhibiting features characteristic of rim growth or bridge growth. The characteristic differences in pore size and shape between the cephalo-thoracic segments (round pores with thick rims) and third segment (polygonal pores with thin, bar-like rims) in mature specimens of L. heteroporos are explained by differences in ontogenetic growth patterns. A process of rim growth during deposition of the first two segments produces the nearly circular pores and wide rims in the cephalo-thoracic regions. This is followed by bridge-growth during formation of the third segment producing large, polygonal pores with thin bars. The septal ring at the juncture of the second and third segment is the terminal apertural rim deposited at the orifice of the second segment before bridge growth commences. The gradual shift toward more polygonal pores throughout the skeleton and the loss of the septal ring during evolution toward L. nigriniae is explained by increasing amounts of bridge growth occurring during morphogenesis of the cephalo-thoracic segments. This is further exemplified by the bridge-like buttresses supporting the offset apical spine in L. nigriniae. These analyses of ontogenetic growth patterns combined with classical methods of analyzing mature skeletons of fossil radiolaria may yield significant additional information in elucidating phylogenetic pathways and discriminating among species.