ONTOGENY OF TRIMEROCEPHALUS-LELIEVREI (TRILOBITA, PHACOPIDA), A REPRESENTATIVE OF THE LATE DEVONIAN PHACOPINE PAEDOMORPHOCLINE - A MORPHOMETRIC APPROACH

A detailed morphometric approach based on size and on outline analyses has been used on an exceptionally well-preserved assemblage of silici fied trilobite exuvia, recovered from a Late Devonian limestone from s outheastern Morocco. The material comprises a series of late larval to postlarval growth stages belonging to a single phacopine species, Tri merocephalus lelievrei Cronier and Feist, 1997. Plurimodality of size distribution has allowed us to discriminate postlarval instars. Distin ct dimensional classes of isolated parts are obtained using the intert ooth distances on the posterior pygidial margin and the internotch dis tances in the cephalic vincular furrow, which are functionally linked during trilobite enrollment. Morphometric analysis of development perm itted demonstration of progressive shape change in agreement with onto genetic ordination and a comparison of the timing of size and shape ch anges. The main shape changes appear to occur early in development, an d once the ''adult'' morphology is obtained, size increases significan tly. The growth rate during ontogeny is estimated by analogy with exta nt deep-sea crustaceans. Exponential size increase resulting from cons tant duration of intermolt periods may be regarded as a life history s trategy to compete in a nutrient-impoverished offshore environment. Th e particular phacopine mode of molting, which involves the opening of the neck joint after ankylosis of the facial sutures, occurred in Trim erocephalus lelievrei between the first two postlarval instars, later than in its ancestor. Trimerocephalus lelievrei occupies an intermedia te position within the phacopine paedomorphocline as indicated by the delayed onset of ankylosis.