Comparison with Late Ordovician erratic sponges on Gotland
Although the Ordovician and Silurian sponge assemblages on Gotland are intermixed, their taxonomic compositions show conspicuous differences. The Ordovician assemblage from Baltica consists almost exclusively of lithistid demosponges of the suborder Orchocladina, with five families represented: Anthaspidellidae, Streptosolenidae, Astylospongiidae, Chiastoclonellidae and Hindiidae. The only representative of the Hexactinellida recorded is ‘Pyritonema’ sp. (M’Coy, 1850; Rhebergen et al. 2001), which most probably should be re-interpreted as an indeterminate root-tuft. Recently, Botting & Rhebergen (2011) described Haljalaspongia inaudita, collected in the westernmost part of Germany; this is the first hexactinellid sponge known from the Ordovician (Sandbian; Upper Ordovician) erratic blocks of silicified limestone originating in Baltic region.
The large majority of taxa in the Telychian assemblage are absent from the Ordovician assemblage. The assemblages have only six species in common, two of which are cosmopolitan (H. sphaeroidalis and Archaeoscyphia minganensis). The differences become even more apparent on generic level; the most frequent Ordovician genera, Aulocopium, Astylospongia, Carpospongia and Caryospongia, comprising more than 86 per cent of the specimens, are absent from the Silurian assemblage.
In contrast, the Silurian assemblage includes, in addition to the families in the Orchocladina, several genera of Rhizomorina, numerous spicules of hexactinellid sponges (of at least four species, and probably many more), as well as non-lithistid demosponges. The most frequent Silurian genus, Caryoconus (Rhebergen & van Kempen, 2002), comprising about 50 per cent of the assemblage, is apparently endemic (although it possibly occurs in Arctic Canada, as discussed in Systematic Palaeontology). The genus Archaeoscyphia is the second most frequent genus in the Telychian assemblage and is represented by seven species, but it is rare in the Ordovician Baltic assemblages and represented by only one or possibly two species.
These differences lead to the conclusion that the present assemblage does not represent a direct succession from the Ordovician fauna of the same region, but results from repopulation of the environment following a major extinction of sponges during the Late Ordovician biotic crises.
Comparison with Silurian assemblages in northern Canada
Diverse Silurian sponge assemblages have been described from Arctic and northwest Canada. Rigby & Dixon (1979) described a sponge fauna from the Upper Silurian (probably Ludlovian) Read Bay Formation, Somerset Island (Arctic Canada). Rigby & Chatterton (1989) described somewhat older (Wenlock and Ludlow) sponge associations from Baillie–Hamilton and Cornwallis Islands (Arctic Canada). De Freitas (1989) described orchocladinid sponges of Wenlockian age, as well as a more varied sponge fauna consisting of lithistids and hexactinellids of Ludlovian age (de Freitas 1991), both from Cornwallis Island. Rigby & Chatterton (1994, 1999) described a hexactinellid sponge and an assemblage of Wenlockian demosponges from the Mackenzie Mountains (Northwest Territories, Canada).
The striking similarities between the present assemblage and those from northern Canada encourage a more detailed comparison. Note that the hexactinellids in the Gotland assemblage are with two exceptions (unique to the assemblage) poorly known and are therefore left out of the discussion; hexactinellids are much more easily destroyed bio-stratinomically than lithistids. It is likely that the non-rigid skeletons of these Silurian hexactinellids were in most cases lost through disarticulation, and they may originally have constituted a much greater proportion of the community; the preserved assemblages may therefore be random samples of the same assemblage.
The lithistids mentioned in the assemblages discussed here are listed in Table 3. There is a striking similarity in the composition of occurring taxa: fourteen or fifteen of 22 species in the Gotland assemblage also occur in Canadian assemblages, which total 48 or 49; only seven or eight species are not known from Canada. The uncertainty relates to two specimens in the sponge assemblage from the Mackenzie Mountains (northwest Canada), which Rigby & Chatterton (1999) assigned tentatively to Astylospongia praemorsa. These specimens may need to be reassigned to Caryoconus Rhebergen & van Kempen, 2002), but examination of the internal canal system is needed.
All assemblages known from Northern Canada are considered to be significantly younger than the Telychian sponge association from Baltica. At first sight, these different ages may lead to the assumption of a migration from Baltica to Laurentia, but that is as yet premature. Further investigation and comparison of the associations is needed and may yield new information on palaeobiogeographical and palaeostratigraphical aspects of the distribution of Early Silurian sponges. More critically, additional faunas are needed to establish palaeobiogeographical patterns in Silurian sponges (Muir et al. in press), particularly from the Llandovery. Strong faunal overturn during the end-Ordovician interval, followed by the apparent homogeneity of faunas through the Silurian, across Baltica and Laurentia, suggests that a small number of taxa survived the extinction event, diversified and subsequently dispersed across at least some low latitude regions.
The Ordovician sponge association from Baltica probably disappeared during the Hirnantian (end-Ordovician) extinction events. Equatorial taxa may have been particularly sensitive to a cooling event, as the temperature regime to which they were adapted no longer existed, whereas taxa from higher latitudes may have survived by shifting to lower latitudes. It would be interesting to see whether the Silurian assemblage from Gotland is more similar to higherlatitude Late Ordovician assemblages, but the data necessary for comparison are not yet known. Both Aulocopium and Archaeoscyphia are present in the Middle Ordovician San Juan Formation of the Argentine Precordillera (Beresi & Rigby 1993), but this fauna is too old for useful comparison with the Telychian assemblage.
Aldridge et al. (1993) and Jeppsson (1998) summarized general characteristics of several events related to sea-level changes during the Llandoverian. In particular, their correlations of the chronostratigraphy, oceanic regime and sea-level curves with the standard conodont and graptolite biostratigraphy are important for this study, and their scheme has been reproduced in adapted form in Figure 11 (stratigraphy after Jeppsson 1998). The sponge assemblage lived during the Snipklint Primo Episode, the relevant period in this study. The M. spiralis Graptolite Biozone, with which the sponge fauna can be correlated, appears to be coeval with the P. amorphognathoides Conodont Biozone. Although graptolites and conodonts have not yet been recorded, the age has been established on account of associated a-critarchs (see geological setting and materials and methods sections, above).
Aldridge et al. (1993) characterized the Snipklint Primo Episode as an interval with increased humidity, which led to increased weathering and correspondingly increased deposition of argillaceous sediment; the interval also yields abundant and diverse communities, which lived in well-oxygenated ocean waters. This allowed ‘infaunal and epifaunal benthic organisms to survive in places where they were formerly rare, due to low levels of oxygen and/or food’ (Aldridge et al. 1993, p. 508). They considered this episode to indicate ‘the first return of full cycling of nutrients after the Ordovician glaciation, producing bonanza conditions, especially for plankton and plankton-dependant communities’ (Aldridge et al. 1993, p. 509). Sponges would have been directly encouraged by increases in plankton abundance due to their filter-feeding habit and probably underwent rapid expansion of faunal ranges and abundance, and perhaps also diversification.
Table 3. Comparison of Silurian demosponge assemblages from Gotland and northern Canada.
Some cosmopolitan species colonized the empty ecospace, in combination with new forms that radiated within a geographically restricted area. This may have occurred during a short period in the Late Telychian, when conditions ameliorated, but given the poor fossil record it is possible that the fauna had been evolving steadily since the end of the Ordovician. Rapid in situ diversification would explain the relatively large number of new taxa, and the endemic characteristics that are quite distinct from the local Ordovician community. It is possible, however, that the fauna diversified elsewhere and spread into Baltica as a coherent sponge community, as appears to have happened with the spread of the Baltic assemblage into Arctic Canada. Modern planktonic sponge larvae usually settle within a few days, a trait that hampers rapid dispersal. If the Gotland fauna migrated in from elsewhere, it is likely that this was not a rapid process, and therefore if the Late Telychian climatic conditions were important, the fauna probably evolved within the local area.
Fig. 11. Early Silurian oceanic changes (after Jeppsson 1998). The arrow indicates the Snipklint Primo Episode, coeval with the Monograptus spiralis Graptolite Biozone and the sponge fauna.
Taphonomy
Three aspects of the diagenesis of the sponges deserve brief discussion. Firstly, most of the cylindrical sponge bodies have been compressed laterally, sometimes to such a degree that spongocoels with a diameter up to 20–40 mm have been compressed to a slit. However, among the roughly 1000 specimens of the astylospongiid Caryoconus, fewer than five specimens show any compression, and where present it is slight. This difference is caused by the framework of the skeleton: spheroclones in astylospongiids were firmly fused, making the sponge body stiff and resistant to compression. In contrast, dendroclones in the ladderlike anthaspidellid structure were connected, but not fused, allowing the sponge body a certain degree of flexibility, and as a result they were easily compressed during compaction of the sediment.
Secondly, some of the tubular spongocoels of the anthaspidellids were filled with matrix. However, the spongocoels of many sponges embedded horizontally were filled with matrix in the lower part only, whereas the upper part remained as a cavity. During diagenesis, these cavities were filled with chalcedony, usually concentrically layered as in agate (Pl. 3, fig. 5). This implies that the chalcedony formed diagenetically early, as otherwise the void areas should have been compressed in most taxa.
Finally, this early silicification appears in some cases to have been extremely rapid. In the hexactinellid U. euplectelloides n. gen., n. sp., this transparent chalcedony surrounds a three-dimensional, articulated array of unfused spicules, which could only have been preserved prior to soft tissue decay. Translucent, slightly milky chalcedony forms discrete patches between the spicule layers, suggesting a distinction between soft tissue replacement and infilling of pore or canal space; similar material also infills canals in the best-preserved lithistid specimens, with a distinct boundary against the clear chalcedony that immediately surrounds the skeletal architecture. This appears to represent a previously unrecognized form of soft-tissue preservation. However, it may be restricted to sponges and be dependent on a high proportion of silica in the form of micro-scleres within the sponge tissue.
Palaeoecology and palaeoenvironmental interpretation
Specific fossil associations in the community are listed in Table 2. The combination of fine sediment substrate with a faunal assemblage including abundant sponges, relatively few stromatoporoids and tabulates, and only rare rugose corals, suggests a community in an outer platform area, with a relatively high sedimentation rate, but a rather firm, muddy seafloor. Disarticulation of hexactinellid and monaxonid sponges, together with shell coquina development, indicates at least occasional high-energy conditions, probably with near-constant agitation. The frequent monospecific occurrence of the brachiopod P. subrectus supports this conclusion, based on the environmental preferences described by Jin & Copper (2000) in their study of Late Ordovician and Silurian pentamerid brachiopods from Anticosti Island. P. subrectus is listed as one of the latest species recorded, confined to the Middle and Late Telychian, that is, the Pavillon Member of the Upper Jupiter Formation. The fauna of this mudstone unit is assigned to the Benthic Assemblage scale 4–5 (Boucot 1975). According to Johnson et al. (1991), coeval mudstones in Estonia correspond to a depth of BA 5, suggesting an absolute depth of about 50 m (Brett et al. 1993); this is in accordance with our interpretation of the depth of the Gotland assemblage.
Narbonne & Dixon (1984) described probably Ludlovian reefs on Somerset Island (Arctic Canada), including a sponge association (Rigby & Dixon 1979) that is comparable with the Telychian assemblage from Gotland. According to Narbonne & Dixon (1984), the reefs illustrate a consistent vertical zonation of lithology and fossil content. The basal ‘Crinoid Stage’ consisted mainly of crinoid debris, with an abrupt transition to the ‘Sponge Stage’, composed of mudstone and approximately 80 per cent by volume of lithistid sponges. In the upper part, the number of stromatoporoids, tabulates and corals increased, gradually developing into the third ‘Coral Stage’, in which these groups became predominant and the volume of sponges decreased considerably and eventually disappeared. These reefal mudmounds, some metres in height and 5–35 m in diameter, were surrounded by a halo of debris, in which the zonal structure of the reef was mirrored. According to Narbonne & Dixon (1984), the combination of very fine-grained matrix and fragile reef-builders indicated an environment of quiet water conditions, well below storm wave-base and with high turbidity. However, the reef-building organisms listed are capable of withstanding highly turbulent conditions, and a shallower water depth seems more likely.
The palaeoecology of the Telychian assemblage of Gotland may have followed a similar pattern, although the associated faunas do not support a directly analogous palaeoecology. Among several hundred fossils with adhering sediment, crinoids are limited to a single holdfast and fewer than ten columnals. Bryozoans are represented only by five specimens of a cryptostome species. Trepostome bryozoans, trilobites, molluscs and algae have not yet been recorded. This composition is in accordance with the ‘Sponge stage’ as described above, but the absence of crinoid debris suggests that if the community was equivalent, it was not part of a progressive sequence in the development of mudmound architecture.
Although the bedrock strata are not exposed on Gotland, there are similarities between the available loose blocks and the reefs described above. Both associations share the combination of very finegrained matrix and an abundance of fragile hexactinellid sponges. Assuming the sponges to be derived from the ‘Sponge Stage’, it may explain the paucity of crinoids and bryozoans from the basal ‘Crinoid Stage’, as well as the rare association of sponges with tabulates and stromatoporoids, as representatives of the ‘Coral Stage’. However, this presupposes that the source area for these blocks was restricted only to one stage of the mound development or that the rest of the deposit was not preserved in the pebble deposits. The latter scenario is possible in the light of the rapid silicification necessary to explain the taphonomy, discussed above. It is possible that the silicification affected only the part of the mound with abundant sponge bodies and spicules, which acted as both a silica source and nucleation. If that is the case, then the carbonate and mud-dominated parts of the mound sequence could have been destroyed during transport, or during one of the weathering stages.
Although no direct evidence of storm-generated structures could be observed, there are three lines of evidence, indicating substantial water movement. First, the numerous hexactinellid sponges were fully disarticulated except for rare articulated fragments, and isolated spicules have been transported and deposited in sheltered areas adjacent to sponge bodies, particularly in spongocoels, superficial grooves and constrictions. Secondly, almost all of the (sub-) cylindrical anthaspidellids, such as Archaeoscyphia spp., and the cylindrical rhizomorines, such as H. cylindricum and H. toftanum n. sp., have been preserved horizontally, whereas sponges embedded in quiet-water mud mounds should have been preserved vertically in many cases. Third, rare coquinas composed of fragments of brachiopods and sponges indicate winnowing and transport during episodes of sustained high energy. The lack of complete hexactinellid sponges also suggests that conditions were not quiet with occasional rapid burial, but rather were consistently agitated. This is indicative of conditions at or above normal wave base, perhaps with storm-influenced rapid sedimentation episodes, but with generally turbulent conditions.
Several aspects of autecology and synecology can also be investigated in the fauna, despite the lack of bedrock exposure. The frequent occurrence of P. subrectus Schuchert & Cooper, 1931 allows recognition of host preference. Association of Pentameroides with Caryoconus (Pl. 12, fig. 2) is about ten times more frequent than it is on anthaspidellids, and as yet has not been seen with Hindia or rhizomorine sponges such as Haplistion. The brachiopod in most cases presumably settled on a dead sponge body, using it as substrate, but some brachiopods seem to have settled on living sponge bodies, as the margins of the shell appear to have been overgrown by the sponge skeleton (Pl. 5, fig. 9; Pl. 8, fig. 3). There are even many examples of overgrowth to such a degree that the brachiopod is recognizable only in cross-sections of valves or septa (Fig. 12).
In contrast to the successful Early Ordovician radiation of the anthaspidellids, which apparently adapted rapidly to habitats in a range of depths from deep slope conditions to shallow water, the short duration of the Telychian sponge assemblage implies they had only limited potential for widespread colonization, or very specific environmental requirements. This limitation, whatever its cause, prevented them from becoming common over wide areas, although many genera survived through the Baltica–Laurentia region as a whole for most of the Silurian. It is possible that they could not compete effectively with more rapidly growing or physically more resistant organisms such as tabulates and stromatoporoids (Narbonne & Dixon 1984; Johns 1994).
Fig. 12. Example of overgrowth of the brachiopod Pentameroides subrectus by an orchocladinid, that is, Archaeoscyphia gislei (NRM Sp10204).
Conclusions
The sponge fauna described here is by far the most diverse fauna yet recorded from the Llandoverian worldwide and provides our best evidence yet for the evolutionary history of sponges following the end-Ordovician extinction episodes. The lithistid-dominated fauna is very different to the pre-existing Late Ordovician assemblage in the same area, implying a dramatic faunal overturn. In contrast, there is a striking similarity of the fauna to that seen in later Silurian deposits in Laurentia, indicating that the reestablished sponge faunas rapidly became widespread and successful.
Despite its diversity, the sponge assemblage described here disappeared or became extinct during the Late Telychian in this part of Baltica. Sponges are absent or have not been preserved in the overlying Visby Formation, apart from some rare specimens of H. sphaeroidalis in the Lower and/or Upper Visby Formation (Rhebergen 2005) that indicate that universal dissolution of skeletons was not responsible for their absence. It is possible that future examination of the ‘Red Layer’ may answer some questions, such as the relationship between the local extinction of the sponge assemblage and severe environmental perturbations, possibly including chemical changes in the water caused by volcanic ash input (Laufeld & Jeppsson 1976). In contrast to the fate of the siliceous sponges, coeval assemblages of tabulates, rugose corals and stromatoporoids continued through the Vis- by formations. This suggests that the cause of their decline may have been related to chemical factors (e.g. levels of dissolved silica), or physical factors (e.g. a switch to higher-energy conditions) that affected spiculate sponges more than other groups. The apparent reliance of the sponges on particular stages of reef-mound development may also be significant.
The fauna of non-lithistid demosponges and hexactinellids is also unusual. These taxa are rarely preserved in shallow-water, high-energy situations due to fragmentation of their skeletons, and so we have little understanding of their evolutionary history in this type of environment. The fauna described here includes rare preservation of delicate skeletal architecture, and even soft-tissue replacements by silica, and reveal a fauna that more closely resemble described Mesozoic assemblages than Palaeozoic ones. As discussed by Botting (2005), this suggests that shallow-water environments were critical to the evolution of derived morphologies and architectures in hexactinellids and non-lithistid demosponges, but their record is largely hidden by taphonomic biases. This fauna shows that even in turbulent environments such taxa can be preserved and further encourages the search for similar faunas in the Late Ordovician – Llandoverian interval, which appears to have been critical to the development of modern lineages.
Acknowledgements. – We are grateful to Mrs. Heilwig Leipnitz, Uelzen (Germany), for much discussion and support, and the loan of hundreds of sponges, as well as for the initial information about the sponge collection in Visby. This study would have been impossible without her indefatigable assiduity in collecting on Gotland. We are indebted to Christina Franzén and her staff and to the Late Valdar Jaanusson in the Department of Palaeozoology of the National Museum of Natural History (Stockholm), as well as to Sara Eliason in the County Museum of Gotland (Visby, Sweden), both for loan of material, valuable discussions and relevant information. Lennart Jeppsson contributed importantly by extended discussions and commenting on earlier drafts. Lars Karis and Linda Wickström (SGU, Uppsala) provided valuable data on the occurrence of the ‘Red Layer’. FR is also grateful to his late friend, Ulrich von Hacht, Hamburg, for cooperation, advises and inspiring discussions during many years. Zwier Smeenk (former Laboratory of Palaeobotany and Palynology, University of Utrecht, NL) did the indispensable acritarch research and examined residues of tens of dissolved samples, thus documenting the Silurian age of the sponges. FR thanks Harry Huisman (curator of the former Natuurmuseum Groningen NL), for identifying tabulates and stromatoporoids, as well as Ji-suo Jin (Department of Earth Sciences, University of Western Ontario, London, Canada), Madis Rubel (Geological Institute of the Technical University Tallinn, Estonia) for help in identifying brachiopod taxa. FR is also indebted to Andrzej Pisera (Institute of Paleobiology of the Polish Academy of Sciences, Warszawa, Poland) for loan of some specimens from Arctic Canada, advise and helpful discussions. We also thank Peter and Karin de Vries (Sappemeer, NL) for unlimited access to their collections and for loan of some important specimens, Saskia Kars (Institute of Earth Sciences, Free University, Amsterdam) for scanning and photo editing, as well as her colleague, Wynanda Koot, for cutting and preparing a number of slabs, Gerard Beersma (curator former Ecodrome, Zwolle NL) for help with photography, Gerrit Anninga (f) for cutting tens of sponges and Carien Hut (Emmen, NL) for preparation of one of the specimens. We appreciate the helpful comments of reviewer Ronald Johns, and the editor Svend Stouge, both of whom have helped to improve the manuscript. JPB is funded by the Chinese Academy of Sciences Fellowships for Young International Scientists Grant No. 2010Y2ZA03 and National Science Foundation of China, The Research Fellowship for International Young Scientists (Grant No. 41150110152).
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