Linguliformea

Abstract: Small shelly fossils are common in sediments of Early Cambrian age and include the earliest common representatives of metazoan animals with mineralized hard parts. The group includes fossils of very different morphology, composition and ultrastructure. They seem to represent skeletal remains of numerous animal groups, the biological affinities of which are largely unresolved. However, the wide geographic range of many forms has the potential to enhance biostratigraphic and palaeogeographic resolution in the Early Cambrian. The late Early Cambrian sequence of North-East Greenland has yielded an assemblage of more than 88 species of small shelly fossils, brachiopods and trilobites, indicative of a middle Dyeran age (Botoman equivalent). The recovered fossils include a number of species that are known from other Early Cambrian palaeocontinents, and particularly strong ties to late Early Cambrian faunas of Australia are documented. The many cosmopolitan taxa thus identified suggests a close juxtaposition of palaeocontinents at this time. The systematic affinity of many of these small shelly fossils is poorly understood, partly because of their fragmentary nature and poor preservation. However, new data from North-East Greenland improves our understanding of the function and biological affinity of certain taxa. Collections of the problematic fossil Mongolitubulus from North and North-East Greenland exhibit characters indicative of a defensive function as spines of bivalved arthropods, while species of the problematic genus Triplicatella represent the opercula of an unknown tubular shell, probably related to orthothecid hyoliths. The bivalved fossil Mickwitzia from North-East Greenland combines characters of linguliform brachiopods and sclerites of Micrina, a non-bivalved problematic form (halkieriid) from Australia. The combination suggests that Mickwitzia is a stem group brachiopod and strengthens arguments for a halkieriid ancestry of the brachiopod phylum.

Abstract: The distribution of embryonic and larval mantles is documented in linguliform and craniiform brachiopods Criteria are presented for identifying these mantle types The mantle type is related to planktotrophic and lecithotrophic larval life history patterns In the Linguliformea and Craniiformea, all Lower Palaeozoic families with adequate preservation had larval mantles, indicating the presence of a planktotrophic larva Heterochronic changes in the time of mantle origin, from the larval to the embryonic stage of development, has occurred several times In the Lingulidae this change appears to have taken place at about the time the family originated in the Devonian and has been retained in extant genera The family Discinidae has also retained a planktotrophic larval stage from the Lower Palaeozic to the present The extant genus Crania in the Craniidae has a short-lived lecithotrophic larva that lacks a mantle Through the Lower Jurassic, this family had planktotrophic larvae with a larval shell During the Upper Jurassic, genera with a lecithotrophic larva that lacked a larval shell began to appear; however, the last genera in this family with a planktotrophic larva and a larval shell did not become extinct until the Tertiary