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Tubeworms form an important part of the modern marine fauna. They were also common in the geological past. We discovered a new genus and species of tubeworms from the latest Ordovician of Estonia. These tubeworms grew on the lithified sea floor during the time of the end-Ordovician mass extinction. Our discovery helps better understand and reconstruct the marine life during this extraordinary time interval. A new cornulitid genus and species, Porkuniconchus fragilis new genus and species, is here described from the Ärina Formation (Hirnantian, Porkuni Regional Stage) of northern Estonia. This new taxon differs from most cornulitids by having a fusiform ornamentation pattern that is somewhat similar to that of Kolihaia. All studied specimens are attached to a carbonate hardground. The hardground fauna is by abundance and encrustation area dominated by cornulitids. Other encrusters are represented only by a single sheet-like cystoporate bryozoan. The cornulitid specimens represent different growth stages, which suggest that the hardground was continuously colonized by cornulitid larvae. The high encrustation density indicates that the studied hardground may have represented a high-productivity site in the Hirnantian of the Baltic Basin.

Three species of trepostome bryozoans formed syn vivo associations with the Cornulites in the Late Ordovician of Estonia. Cornulites sp. and Mesotrypa excentrica presumably formed a true symbiotic association. This is the first known case of symbiosis between cornulitids and bryozoans. It is not known whether this symbiotic association was obligatory of facultative for the cornulitid, but it was facultative for the bryozoan. In this association cornulitids may have competed for the food with bryozoans and the association may have been parasitic. The remaining associations between cornulitids and bryozoans were accidental. Most common skeletonized endobionts of the Ordovician bryozoans were not cornulitids, but conulariids and rugosans.

Endobiotic tentaculitoids formed symbiotic associations with tabulates, heliolitids, rugosans, bryozoans, crinoids, stromatoporoids, and chaetetids from the Late Ordovician to the Carboniferous. The Ordovician was dominated by coral hosts, but there was a shift from mostly coral-based associations to stromatoporoid-based associations in the early Silurian. Specialization increased during the evolution of tentaculitoid symbiosis. In the Devonian, specialized symbiotic endobiont genera appeared which did not occur separately from their hosts.

Abundant calcareous tubeworms have been found in both shallow platform and deep basin deposits after the end-Permian mass extinction in the Cili area, South China. Tubeworms from the microbialites deposited on the shallow platform appear to be cone-shaped tubes with diameters ranging from 0.5 to 1.8 mm (mean 1.1 mm), while those attached to Claraia, the most abundant bivalve fossil preserved in the deep basin deposits after the mass extinction, are planispiral tubes with smaller diameters (0.5–1.5 mm, mean 0.9 mm). The calcareous tubeworms are identified as Microconchida (Tentaculita) according to the typical laminated sheet texture of the tubeworms found on the shallow platform. The difference in morphology between the cone-shaped tubeworms found in the microbialites and the planispiral tubeworms attached to Claraia in deeper water deposits may be related to differences in how fast the surrounding sediments were accumulating. Bacterially mediated precipitation of calcium carbonate led to rapid accumulation of the microbialites that forced the tubeworms to grow upward so as to keep up with the rate of microbialite growth and led to the cone-shaped tubes found there, whereas the slowly accumulating sediments surrounding the tubeworm-encrusted Claraia led to the development of the planispiral forms in basin deposits. Calcareous tubeworms found in the shallow platform and colonizing the shells of bivalve Claraia in basin deposits indicates calcareous tubeworms, as a significant disaster form, should have benefited from the opening of ecological space by the extinction of most marine invertebrates. Widespread oceanic anoxia has long been considered to be one of the extraordinary conditions after the end-Permian mass extinction. Tubeworm fossils flourishing in basin deposits within the short interval near the Permian-Triassic boundary implies that the deepwater environment immediately after the end-Permian mass extinction may not have been as anoxic as previously thought.

The use of sclerobiosis as a tool for paleoenvironmental and paleoecological research is undermined by a lack of comparable methods for sclerobiont data collection and analysis. We present a new method for mapping sclerobiont distributions across any host, and offer an example of how the method may be used to interpret sclerobiont data in relation to host orientation. This approach can also be used to assess the suitability of beds and fossil material for paleoenvironmental reconstruction. A sample of 150 encrusted dorsibiconvex atrypide brachiopods were selected from six beds in the Waterways Formation (latest Givetian — Early Frasnian; Alberta, Canada). The dorsal and ventral valves of each brachiopod were photographed. Sclerobiont taxa were mapped onto the photographs, and the maps were used to create stacked images with each of the 25 brachiopod specimens from each bed. Based on the life orientation of dorsibiconvex atrypides, three zones were designated on the host: the post mortem zone, (only available to sclerobionts after death and reorientation of the host); the shaded zone (brachial valve, excluding the post mortem zone); and the exposed zone (ventral valve). Randomization simulation results indicate that all beds likely exhibit non random encrustation patterns, and corroborate the hypotheses that: (1) much of the encrustation occurred while the hosts were alive, and (2) these beds and fossils have experienced little physical reworking or transport and would be suitable for paleoenvironmental analysis. Mapping sclerobionts across hosts can serve as a unifying method to increase the recognition and use of sclerobiosis in paleontological studies.

A new microconchid tentaculitoid, Microconchus utahensis new species, is described from the Lower Triassic (Spathian) Virgin Formation of two localities (Hurricane Cliffs and Beaver Dam Mountains) near St George, Utah. This small encrusting tubeworm, previously referred to erroneously as Spirorbis, has a laminated shell microstructure containing minute pores (punctae). The population from deeper water facies of the Beaver Dam Mountains is more abundant than that from Hurricane Cliffs and the tubes are significantly larger in size. Although represented by only one species (M. utahensis), microconchids are by far the most dominant component of the otherwise impoverished sclerobiont assemblage of the Virgin Formation, which also includes rare cemented bivalves and probable foraminifers. Whereas the remainder of the Virgin fauna is quite diverse, the low diversity of encrusters suggests a slow recovery from end-Permian mass extinctions. Indeed, more typically Mesozoic sclerobiont assemblages dominated by cyclostome bryozoans and serpulid polychaetes did not appear until the Late Triassic, probably Rhaetian.

Tentaculitoid microconchid tubeworms from Devonian (uppermost Emsian-upper Givetian) deposits of the Holy Cross Mountains, Poland, include three new species from stratigraphically well-constrained lithological units: Polonoconchus skalensis n. gen. n. sp., Palaeoconchus sanctacrucensis n. sp. and Microconchus vinni n. sp. The microconchids inhabited fully marine environments during transgressive pulses, as is evidenced from facies and associated fossils. Polonoconchus skalensis n. gen. n. sp. and Palaeoconchus sanctacrucensis n. sp. inhabited secondary firm- to hard-substrates in deeper-water, soft-bottom environments. They developed planispiral, completely substrate-cemented tubes and planispiral tubes with elevated apertures, which is indicative of environments where sedimentation rate is low but competition for space (by overgrowth) may be high. Microconchus vinni n. sp., on the other hand, developed a helically coiled distal portion of the tube as a response to a high sedimentation rate. As the taxonomic composition of Devonian microconchids is poorly recognized at both regional and global scales, this new material contributes significantly to our understanding of the diversity of these extinct tube-dwelling encrusters.

An earliest Famennian (Late Devonian) record of tentaculitoids (an extinct 'class' of small calcareous conical shells) preserved as palynomorphs is documented from Sosnogorsk in the Komi Republic of Russia. These were preserved in considerable abundance in a near-shore shallow lagoon subjected to marine flooding with euxinia. Four forms of nowakiid tentaculitoids plus related aberrant forms are documented. They represent evidence for survival of the nowakiids into the earliest Famennian and hence post-date the Frasnian-Famennian mass extinction. It is hypothesised that the Frasnian and younger occurrence of tentaculitoids as palynomorphs may relate to a changing balance of carbonate and organic matter in their shells driven by the environmental conditions of the Frasnian-Famennian mass extinction.

Conoidal shells of Cornulites celatus n. sp. occur commonly within host coralla of Propora confertaMilne-Edwards and Haime, 1851, sensu lato, from the Laframboise Member of the Ellis Bay Formation (Ashgill: Upper Ordovician) at Pointe Laframboise on western Anticosti Island. Examples have also been found at the same locality in the tabulate corals Paleofavosites sp., Acidolites arctatusDixon, 1986, and A. compactusDixon, 1986, and the stromatoporoid Ecclimadictyon sp., but not in other associated tabulate coral species. Growth interference between the shells and their hosts indicates a commensal relationship. C. celatus apparently had a more limited paleoenvironmental range than its principal coral host species, which occurs abundantly elsewhere on the island without its endobiotic partner. The diagnosis of Cornulites is emended to include forms having a two-layered shell wall with a distinctive outer layer consistently preserved as prismatic calcite. This new species extends the known stratigraphic range of cornulitids in commensal relationships with corals and stromatoporoids from the Silurian back to the Upper Ordovician.