Geological Background and Locality

During the Ordovician, the Baltica paleocontinent migrated from the temperate to the subtropical realm [21], [22]. The climatic change resulted in an increase of carbonate production and sedimentation rate on the shelf during the Middle and Late Ordovician. In the Upper Ordovician the first carbonate buildups are recorded, emphasizing a striking change in the overall character of the paleobasin [23].

The total thickness of the Ordovician in Estonia varies from 70 to 180 m [23]. The Ordovician limestones of Estonia form a wide belt from the Narva River in the northeast to Hiiumaa Island in the northwest [23]. In the Middle Ordovician and early Late Ordovician, the slowly subsiding western part of the East-European Platform was covered by a shallow, epicontinental sea with little bathymetric differentiation and an extremely low sedimentation rate. Along the extent of the ramp a series of grey calcareous - argillaceous sediments accumulated (argillaceous limestones and marls), with a trend of increasing clay and decreasing bioclasts in the offshore direction [22].

The material studied here was collected from the Hirmuse Creek (Fig. 1) and Alliku Ditches (Fig. 1) of Sandbian age (Haljala Stage) (Fig. 2). Hirmuse Creek is located in Maidla parish of Ida-Viru County. Clayey and skeletal limestones with interlayers of marls are exposed on the creek bed and in its banks. The fossil assemblage includes algae (Mastopora), brachiopods (Clinambon, Leptaena, Platystrophia, Apartorthis, Porambonites, Pseudolingula), conulariids, gastropods (Lesueurilla, Holopea, Bucanella, Pachydictya, Megalomphala, Cymbularia), ichnofossils (Amphorichnus, Arachnostega), sponges, receptaculitids (Tettragonis), rugosans (Lambelasma), bryozoans, and asaphid trilobites. The Alliku Ditches are located in Harju County near the village of Alliku. Clayey limestones with interlayers of marls are exposed here. The fauna includes algae, brachiopods, bryozoans (Aluverina, Annunziopora, Batostoma, Ceramoporella, Coeloclema, Constellaria, Corynotrypa, Crepipora, Dekayella, Diazipora, Diplotrypa, Enallopora, Esthoniophora, Graptodictya, Hallopora, Hemiphragma, Homotrypa, Homotrypella, Kukersella, Lioclemella, Mesotrypa, Monotrypa, Nematopora, Nematotrypa, Oanduella, Moyerella, Pachydictya, Phylloporina, Prasopora, Proavella, Pseudohornera, Rhinidictya, Stictoporella), echinoderms, gastropods, ostracods, rugosans, receptaculitids and trilobites according to Rõõmusoks [24].

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Figure 1. Locality map.

Location of Hirmuse Creek and Alliku Ditches in North Estonia.

https://doi.org/10.1371/journal.pone.0099455.g001

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Figure 2. Stratigraphic scheme.

The Middle and Upper Ordovician in Estonia. Location of Osprioneides kampto borings. Modified after Hints et al. (2008).

https://doi.org/10.1371/journal.pone.0099455.g002

No permits were required for the described study, which complied with all relevant Estonian regulations, as our study did not involve collecting protected fossil species. Three described bryozoan specimens with the Osprioneides borings are deposited at the Institute of Geology, Tallinn University of Technology (GIT), Ehitajate tee 5, Tallinn, Estonia, with specimen numbers GIT-398-729, GIT 665-18 and GIT 665-19.

Taxonomic Affinity of the Borings and the Possible Trace Maker

The borings in these bryozoans resemble somewhat Petroxestes known from Late Ordovician bryozoans and hardgrounds of North America [14]. Both are of unusually large size for Ordovician borings, and both have oval-shaped apertures. However, in Petroxestes the aperture width is much greater than the boring’s depth. In contrast, the depth of the borings in bryozoans is much greater than their apertural width. Unlike Petroxestes, the Sandbian borings examined here have a tapering terminus and somewhat sinuous course. The axial ratio of Petroxestes borings aperture (major axis/minor axis) is also much greater than observed in these borings.

The other similar large Palaeozoic boring is Osprioneides, which is known from the Silurian of Baltica, Britain and North America [13]. We assign borings in the bryozoans studied here to Osprioneides kampto because of their similar general morphology. They have a single entrance, an oval cross section, and significant depth similar to Osprioneides kampto. Their straight, curved to somewhat sinuous shape also resembles that of Osprioneides. Both Osprioneides and these borings in bryozoans have a tapered to rounded terminus.

Most likely the Osprioneides trace maker was a soft-bodied animal similar to polychaete worms that used chemical means of boring as suggested by Beuck et al. [13]. This is supported by the slightly curved to sinuous course of several borings and their variable length. The presence of a tapered terminus in Osprioneides means bivalves were very unlikely to have been the trace makers.

Paleoecology and Taphonomy

Osprioneides borings were made post mortem because the growth lamellae of the bryozoan do not deflect around the borings. There are also no signs of skeletal repair by the bryozoans. Several Osprioneides borings truncate other Osprioneides borings that were likely abandoned by the trace maker by that time. Similarly, empty Osprioneides borings were colonized by Trypanites trace makers. This indicates that the Osprioneides borings may have appeared relatively early in the ecological succession. Overturning of the bryozoan zoaria can explain the occurrence of Osprioneides borings apertures on both upper and lower surfaces. There is no sign of encrustation on the walls of the studied Osprioneides borings, suggesting relatively rapid burial of the host bryozoans shortly after the Osprioneides colonization.

It is likely that Osprioneides trace makers were suspension feeders similar to the Trypanites animals due to their stationary life mode [25]. Bryozoan skeletons may have offered them protection against predators and a higher tier for suspension feeding. Previously known host substrates of Osprioneides comprise stromatoporoids and tabulate corals. This new occurrence of Osprioneides borings in large bryozoans shows that the trace maker possibly selected its substrate only by size of skeleton because the traces are not found in smaller fossils. However, they are not found in any Ordovician hardgrounds that provide more area than do the bryozoan colonies. Wyse Jackson and Key [20] suggest that large bryozoan colonies were exploited by borers because they would have been easy to bore into.

Ordovician Bioerosion Revolution

Morphological diversification was not the only result of the Ordovician Bioerosion Revolution. Most of the large bioerosional traces of the Paleozoic had their earliest appearances in the Ordovician [7], [8]. The earliest known large borings are those of Gastrochaenolites from the Early Ordovician of Baltica [4], [17]. Later, during the Late Ordovician, large Petroxestes borings appeared in North America. At the same time the Osprioneides borings described here appeared in Baltica. Thus the Ordovician was also the time of great increase in quantities of hard substrate removed by single trace makers. The biological affinities of Ordovician Gastrochaenolites are not known [8], but it may have been a soft-bodied animal. The Late Ordovician Petroxestes was almost certainly produced by the facultatively boring bivalve Corallidomus scobina [26]. Boring polychaetes were the likely Osprioneides trace makers, which is suggested by the somewhat sinuous shape of some borings. This indicates that more than one group of animal was involved in the appearance of large bioerosional traces during the Ordovician Bioerosion Revolution. Increased predation pressure [27] was most likely the driving force behind the infaunalization of larger invertebrates such as the Osprioneides trace makers in the Ordovician. On the other hand in echinoids, for example, infaunalization was presumably the result of colonization of unoccupied niche space [28].

Paleobiogeography

Osprioneides is a relatively rare fossil compared to the abundance of Trypanites in the Silurian of Baltica [29]. In the Silurian, Osprioneides borings also occur outside of Baltica. They are known from the Llandovery of North America and Ludlow of the Welsh Borderlands [30]. Osprioneides is presumably absent in the Ordovician of North America because Ordovician bioerosional trace fossils of North America are relatively well studied [15], [16]. Thus, it is possible that the Osprioneides trace maker originated in Baltica or elsewhere and migrated to North America in the Silurian. This may well be connected to the decreased distance between Baltica and Laurentia (the closing of the Iapetus Ocean) and the loss of provinciality of faunas in the Silurian.