WEST COAST AND WORLD: BIOLOGY OF SIZE IN COSMOPOLITAN AGNOSTID TRILOBITES

Biometrics of agnostid trilobites from the Early Paleozoic West Coast complement data from 109 European agnostids to establish a size cline for Early Ordovician agnostids of the cosmopolitan genera Geragnostus and Glyptagnostus. Similar increases of maximum size with latitude are demonstrable from the literature on Late Cambrian cosmopolitan species. Various data sets illustrate size increases of between 1.6 and 2.2 times over approximately 70 degrees of paleolatitude. Explanations for size clines in modern marine invertebrates have focused on fecundity, fluid dynamics, and food supply. Agnostid size clines may shed light on history of accreted terrains, climate changes in the Early Paleozoic, and origins of new agnostid taxa. In other aspects of agnostid biology of size: 1) The proportion of growth subsequent to the meraspid stage exceeds that of polymerids, in spite of the small number of total instars and the conservative allometry in agnostid holaspid stages. 2) Agnostid hypostoma are large in proportion to the cephala and the overall carapace; but that proportion varies, being larger, for example, in Middle Cambrian Great Basin specimens of Peronopsis than in published examples of Late Cambrian Agnostus pisiformis of Sweden. Considering the importance of hypostoma for feeding modes, this variability should inspire caution in assuming that all agnostids shared a single mode of life or life history strategy. 3) The small size of agnostids has fueled a century of speculation on the probable paedomorphic origin of the taxon. While this generalization remains to be established, heterochronic origins of lesser taxa from species with known ontogeny may be corroborated – for example, in the Late Cambrian Great Basin Psuedagnostus communis, for which a vexing array of geographic variants may best be explained by heterochrony.