Evolutionary trends in the ornamentation of Cambrian solenopleuropsine trilobites

A phylogeny of the Cambrian solenopleuropsine trilobites is constructed to evaluate the importance of differential speciation, extinction and developm ental constraints in the evolutionary history of the clade. The transformat ion of the Pardailhaniu-Solenopleuropsis (Manublesia)-Solenopleuropsis (Sol enopleuropsis) lineages occurred during the mid Caesaraugustian-early Langu edocian interval, with all measured morphological variables exhibiting cont inuous and gradual changes. Several evolutionary trends show tendencies tow ards an increase in tuberculation and its complexity on the anterior part o f the cephalon. A mid Caesaraugustian anagenetic tendency is related to the influence of constant environmental conditions, whereas a late Caesaraugus tian-early Languedocian cladogenetic tendency seems to have been an adaptat ive response to unstable environments. The Solenopleuropsinae clade has a well-preserved continuous fossil record over its full biogeographical range in the Mediterranean region. The sympat ric evolution in centrally located populations (Mediterranean area) follows the model of phyletic gradualism, whilst a peripheral population in Avalon ia gave rise (by allopatric speciation) to a distinct species, characterize d by a variation in the relative size of the palpebral lobes, which may be evidence of a parallel evolution in the trends of ornamentation.