All of the studied material has been registered into the fossil worm collections of the Natural History Museum, London. After initial study using optical microscopes, selected specimens were examined with a low−vacuum scanning elec− tron microscope (LEO VP−1455). This enabled back−scat− tered electron imaging of uncoated specimens, including large substrates encrusted by small tubeworms. A few speci− mens were detached from their substrates using a blade and embedded in epoxy resin. These were either thin sectioned, or polished and etched with 1% acetic acid for a few seconds to enable SEM of skeletal ultrastructures.
Morphology
The striking homeomorphy between microconchids and spi− rorbid polychaetes is reflected in the frequent misidentifica− tion of Ordovician–Jurassic microconchids as Spirorbis (Tay− lor and Vinn 2006). Both groups have small, spirally coiled, calcareous tubes that attach to hard or firm substrates (Fig. 1). Tightness of coiling may vary between species in each group, and the apertures of the tubes sometimes become elevated above the substrate by upward growth during late ontogeny. Mass recruitment in dense aggregations characterises both groups and they very often occur as opportunistic foulers of organic substrates.
Set against these morphological and ecological similari− ties are contrasts that indicate a radical dissimilarity in the mode of tube formation and betray the different phylum− level affinities of microconchids and spirorbids. Microcon− chid tubes have a closed origin with a bulb−like initial cham− ber (Fig. 3A2), whereas tubes of spirorbid polychaetes have an open origin without a bulbous initial chamber. The tube ultrastructure of microconchids is microlamellar. In contrast, that of spirorbids usually comprises finely prismatic crystal− lites that lie in various orientations and are never arranged to form laminae. Microconchids have either pseudopunctate tubes, in which the laminae contain regular inflections, or punctate tubes penetrated by pores up to 20 μm in diameter (Figs. 2A2, D, E2, 3A3, 4E2). Spirorbids are never pseudo− punctate (Taylor and Vinn 2006) and, although the tubes of Paradexiospira and some species of the subfamily Januinae may contain pores called alveoli (Rzhavsky 1994), these are much larger than the punctae of microconchids and have rounded edges. There are usually only two lateral rows of al− veoli, unlike the punctae in microconchids which are spread evenly over the entire tube surface.
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