Palaeobiology and taphonomy of early problematic fossils

Sammanfattning: Extinct organisms that cannot be accommodated in any extant phylum are generally referred to as "problematic fossils". They are more numerous in progressively older strata and constitute an important part of the late Proterozoic and early Palaeozoic biota. This thesis is concerned with a disparate collection of problematic fossils from these periods of geological time, including cornutes, vase-shaped rnicrofossils and hyolithids. All of them are exceptionally preserved, a fact that has allowed detailed morphological studies. Some of them are preserved in a complex manner and the study of their taphonomy was essential to decode the palaeobiological information they contain.Cornutes are a group of metazoans with an echinoderm-like endoskeleton. They have played a central role in the debate concerning deuterostome phylogeny and have been interpreted as either early chordates (calcichordates) or basal echinoderms. A re-evaluation of deuterostome comparative morphology in the light of recent molecular and developmental data suggests that chordates originated from an enteropneust-like ancestor and that cornutes lie within a monophyletic echinoderm clade. A new cornute genus is described from the Tremadoc of England; its phylogenetic position and morphology is intermediate between a Cambrian and an Arenig genus.The oldest fossils studied, the Neoproterozoic vase-shaped rnicrofossils (VSMs), are small vesicles with a single opening and a distinct morphology. The discovery of a membranous compartment containing a spherical cell inside the vesicle of the VSMs from the Visingsö Group, Sweden, indicates that these enigmatic microfossils do not represent in themselves an encystment stage. Instead the vesicle is likely to constitute the lorica of a protist or the "fruiting body" of a multicellular alga. The assemblage of VSMs from the Elbobreen Fm, Spitsbergen shows that this group was more diverse than previously recognised.Hyolithids are a group of bilaterians with a calcareous external skeleton consisting of a conical conch, an operculum and a pair of "appendages" called helens. While the conch and operculum are relatively well known, the structure, orientation and function of the helens have remained controversial. Muscle scars occur on the conch and operculum and have been central to discussions about the organisation of hyolithid soft anatomy. The structure, growth and life position of the helens have been studied and the muscle insertion pattern in different hyolithid species compared. Hyolithids shared a common muscular system consisting of muscle bundles involved in the articulation of the four skeletal elements. The helens were solid, grew by accretion and extended out of the conch curving ventrally. While the arrangement of muscle scars with respect to the helens suggests they could have been used to propel the animal over the substrate, their general morphology and orientation indicates an additional function of stabilisation.