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The ultrastructural diversity of the Middle and Late Jurassic serpulid tubes from the Polish Basin has been investigated. The inspection of 12 taxa representing the two major serpulid clades allowed for the identification of three ultrastructure types—irregularly oriented prismatic structure (IOP), spherulitic prismatic structure (SPHP), and simple prismatic structure (SP). Six of the studied species are single-layered and six species possess two distinct layers. Ultrastructural diversity corresponds to certain serpulid clades. The members of Filograninae have single-layered tube walls composed of possibly plesiomorphic, irregularly oriented prismatic structure (IOP). Two-layered tubes occur solely within the clade Serpulinae, where the external, denser layer is built of either the ordered spherulitic (SPHP) or simple prismatic microstructure (SP), and the internal layer is composed of irregularly oriented prismatic structure (IOP). Apart from phylogenetic signals provided by the tube ultrastructure, it can be used in analyzing paleoecological aspects of tube-dwelling polychaetes. Compared to the more primitive, irregularly oriented microstructures of Filograninae, the regularly oriented microstructures of Serpulinae need a higher level of biological control over biomineralization. The advent of the dense outer protective layer (DOL) in serpulids, as well as the general increase in ultrastructure diversity, was likely a result of the evolutionary importance of the tubes for serpulids. Such diversity of the tube ultrastructural fabrics allowed for maximizing functionality by utilizing a variety of morphogenetic programs. The biomineralization system of serpulids remains more complex compared to other tube-dwelling polychaetes. Physiologically more expensive tube formation allows for mechanical strengthening of the tube by building robust, strongly ornamented tubes and firm attachment to the substrate. Contrary to sabellids, which perform a fugitive strategy, an increased tube durability allows serpulids a competitive advantage over other encrusters. [ABSTRACT FROM AUTHOR]

Microconchid and anticalyptraeid tubeworms from the Middle Devonian of Morocco are described for the first time. The assemblage, coming from the locality of Madène el Mrakib (Mader Basin, eastern Anti-Atlas), consists of one microconchid species, Palaeoconchus sanctacrucensis, and one new anticalyptraeid species, Anticalyptraea madenensis. Both species encrusted uppermost Eifelian/lowermost Givetian brachiopod shells. P. sanctacrucensis was hitherto only known from the upper Emsian–upper Eifelian of the Holy Cross Mountains, Poland. Thus, its presence in the shelf habitats of northern Gondwana implies its migration from the southern margin of Laurussia during the transgression associated with the late Eifelian Kačák Episode. The encrustation patterns of the brachiopod shells by the Moroccan microconchids indicate that the brachiopods only served as favourable hard substrate for colonisation by the microconchid larvae. However, the occupation of different valves of the same brachiopod by microconchids and Anticalyptraea may imply that both encrusters potentially avoided direct competition for suspended food particles. Characteristics and position of Oichnus borings present on a few microconchids and anticalyptraeids indicate that they were not necessarily formed syn vivo and produced by predatory organisms. It is likely that they were drilled by some tiny suspension-feeders and may represent non-predatory domichnia. [ABSTRACT FROM AUTHOR]

Monospecific accumulations of ostracods, represented by spine‐bearing Janischewskya? sp. and a smooth‐shelled Cavellina? sp., were detected inside the body chamber, siphuncle and camerae of three cephalopod specimens (a nautiloid and two goniatites) from the Carboniferous (Mississippian) Cracoean reefs of North Yorkshire, UK. The ostracods occur as isolated valves packed together and are well‐preserved, with delicate spines of Janischewskya? sp. still intact on numerous specimens. Such a mode of ostracod preservation inside the cephalopod conchs and their paucity in the surrounding matrix outside the shells indicate that the valves were not concentrated due to sedimentary processes. Rather, the ostracods deliberately entered the empty cephalopod shells in order to seek sheltered habitats for moulting. In the case of the smooth‐shelled Cavellina? sp., the ostracod valves preserved inside a camera of the nautiloid have similar size, indicating that a synchronized mass moulting took place in this species. Additionally, the presence of putative eggs closely associated with the spine‐bearing Janischewskya? sp. valves inside a siphuncle of the nautiloid shows that the empty conchs could have also served as safe places for egg deposition. The present finds are thus the first examples of cryptic moulting behaviour in ostracods. They also imply that such behaviour in this group of arthropods has a long evolutionary history, at least since the Carboniferous, and potentially may persist in present day ostracods. [ABSTRACT FROM AUTHOR]