Ursula Toom

This study documents previously unknown tax-onomic and morphological diversity among early Palaeozoic crinoids. Based on highly complete, well preserved crown material, we describe two new genera from the Ordovician and Silurian of the Baltic region (Estonia) that provide insight into two major features of the geological history of crinoids: the early evolution of the flexible clade during the Great Ordovician Biodiversification Event (GOBE), and their diversification history surrounding the end-Ordovician mass extinction. The unexpected occurrence of a highly derived sagenocrinid, Tintinnabulicrinus estoniensis gen. et. sp. nov., from Upper Ordovician (lower Katian) rocks of the Baltic palaeocontinent provides high-resolution temporal , taxonomic and palaeobiogeographical constraints on the origin and early evolution of the Flexibilia. The Silurian (lower Rhuddanian, Llandovery) Paerticrinus arvosus gen. et sp. nov. is the oldest known Silurian crinoid from Baltica and thus provides the earliest Baltic record of crinoids following the aftermath of the end-Ordovician mass extinction. A Bayesian 'fossil tip-dating' analysis implementing the fossilized birth–death process and a relaxed morphological clock model suggests that flexibles evolved c. 3 million years prior to their oldest fossil record, potentially involving an ancestor–descendant relationship (via 'budding' cladogenesis or anagenesis) with the paraphyletic cladid Cupulocrinus. The sagenocrinid subclade rapidly diverged from 'tax-ocrinid' grade crinoids during the final stages of the GOBE, culminating in maximal diversity among Ordovician crinoid faunas on a global scale. Remarkably, diversification patterns indicate little taxonomic turnover among flexibles across the Late Ordovician mass extinction. However, the elimination of closely related clades may have helped pave the way for their subsequent Silurian diversification and increased ecological role in post-Ordovician Palaeozoic marine communities. This study highlights the significance of studies reporting faunas from undersampled palaeogeo-graphical regions for clade-based phylogenetic studies and improving estimates of global biodiversity through geological time.

A new species of a possible thecate scyphozoan Byronia jaegeri sp. nov. is described from the late Katian of Estonia. This new taxon is distinct as it lacks the longitudinal ornamentation present in other Byronia species. Phosphanullus is a phosphatic attachment disk, which has been considered to be a junior synonym of Byronia. We report qualitative energy dispersive analysis and scanning electron microscopy of the composition of B. jaegeri, demonstrating that the tube is organic in composition and has a lamellar microstructure. The compositional differences of Phosphanullus and Byronia do not support their synonymy. It is likely that Phosphanullus belongs to a closely related but distinct phosphatic group of thecate scyphozoans, due to their morphological similarity, but differing composition.

– The earliest known macroborings (Trypanites) from Baltica occur in early Cambrian phosphatized siltstone pebbles from Kopli quarry in Tallinn, Estonia. Trypanites borings also occur in Furongian phosphatized siltstone pebbles in northern Estonia. The intensity of bioerosion on these Cambrian pebbles is low compared to analogue substrates from Ordovician deposits of Baltica. These bored phosphatized siltstone pebbles show that bioerosion of hard substrates occurred in relatively cold climate epicontinental seas during Cambrian time.

The earliest known endobiotic rugose corals are recorded in the Katian of Estonia. Multiple rugosans were partially embedded in colonies of the cystoporate bryozoan Ceramopora intercellata Bassler, 1911, leaving only their apertures free on the bryozoan growth surface. Bodophyllum sp. and Lambelasma sp. are rugosans that formed a symbiotic association with C. intercellata which may have been mutualistic. Rugosans presumably benefitted from growth within the stable substrate provided by the bryozoan, while bryozoans presumably benefitted by protection against some types of predators. Symbiosis between rugosans and the bryozoan Ceramopora intercellata was most likely facultative.

The diversity of Silurian microconchids is still poorly understood. Here, a new microconchid tubeworm species, Palaeoconchus wilsoni, is described from the Silurian (Ludlow) encrusting rugose corals from Estonia (Saaremaa Island) and a brachiopod shell from Sweden (Gotland). In Estonia, the microconchids are a dominant constituent of the encrusting assemblages, associated with cornulitids, Anticalyptraea, auloporids, trepostome bryozoans, hederelloids and enigmatic ascodictyids. It is notable that these Silurian encrusting assemblages are clearly dominated by tentaculitoids (microconchids, cornulitids and Anticalyptraea) which very often co-exist on the same coral host. Morphologically similar microconchids and Anticalyptraea may have exploited a more similar ecological niche than the straight-shelled cornulitids. However, the clear predominance of microconchids over Anticalyptraea in the communities may indicate that this genus was a less effective competitor for food than microconchid tubeworms.

The Päärdu hardground from the Telychian (Rumba Formation) of western Estonia is sparsely encrusted (0.4% of the studied surface) by possible tabulate corals, sheet-like bryozoans and discoidal echinoderm holdfasts. Both the upper and cryptic sides of the hardground are intensely bioeroded by Trypanites borings. The taxonomic composition of the Päärdu hardground association is rather different from the characteristic Silurian association in being dominated by tabulate corals, while bryozoans and echinoderms played a minor role in the association. The Päärdu hardground is more sparsely encrusted than common for the Late Ordovician and Silurian hardgrounds, but this may be a characteristic feature of the hardgrounds of Baltica. The Päärdu hardground is important among the Silurian hardgrounds because it has unusually low encrustation combined with high bioerosion.

The earliest known rugosans attached syn vivo to vertical stems occur in the late Silurian of Saaremaa, Estonia. These rugosans display vertical to subvertical attachment scars and are more common in the Ludfordian than in the Pridoli. The unknown hosts provided a higher tier for the feeding, making the association beneficial for the rugosans. Several rugosans were themselves syn vivo encrusted by bryozoans and unknown endobiotic tubicolous organisms, possibly cornulitids. Estonian rugosans appear to have been host size selective and preferred substrates of certain size. Silurian symbiotic rugosans are more often endobionts in stromatoporoids than epibionts on the vertical stems.