Paleobiology of agnostid trilobites : functional morphology, growth, and evolution

Agnostid trilobites are a group of Cambro-Ordovician arthropods having unique features of morphology, function, growth, and evolution. Middle Cambrian agnostids from western Utah appear to have been pelagic filter-feeders capable of independent locomotion and defensive enrollment. The specially-modified hypostoma was probably adapted for the creation and exploitation of turbulence during feeding. Reduction in number of appendage pairs may have resulted in increased reliance on labral secretions and appendage modification for feeding. A quantitative analysis of shape changes during ontogeny reveals that agnostids display the least degree of ontogenetic change among Cambro- Ordovician groups. Evolutionary rates in agnostids appear to differ significantly from those of other trilobites. Rates of generic origination among agnostids were low during the Late Cambrian and Early Ordovician--an effect attributable to freer gene flow among pelagic populations. The question of which taxon is closest phylogenetically to the agnostids has not been adequately answered. Previous hypotheses suggesting a close evolutionary relationship between agnostids and eodiscids are inadequate because the similarities upon which these models have been based are especially susceptible to convergence or parallelism. The search for taxa sharing derived traits with agnostids should be broadened to include the entire spectrum of early arthropods.