Silurian Of Gotland Research Articles

Mesophotic vs. shallow water reefs: ecosystem connectivity in the Silurian of Gotland

The Lower Silurian Visby Beds (ca. 430 Ma) exposed on the island of Gotland (Sweden) yield the oldest so far described MCE’s (mesophotic coral ecosystems). Several basic questions concerning these ecosystems, including their extent, positioning, and particularly their spatial relation and affinity with the shallow water reefs, remain unanswered. We describe a deeply incised erosional channel infilled by redeposited detrital and biodetrital material including corals and stromatoporoids, which represents a unique example of a fossil depositional structure connecting shallow- and deep-water reef environments of that time. The mesophotic environment is embodied by limestones and marls, in which the channel is incised, while the shallow water habitats are represented by the redeposited material infilling the channel. The erosional channel contains material derived from hypothetical adjacent shallow-water reefal settings, which indicates that it served as an interconnecting zone between shallow- and deep-water benthic communities. The vertical connectivity of the shallow-water reefs and the adjacent mesophotic ecosystems results in a significant taxonomic overlap between these communities. This is especially important when taking into account that the Visby Beds are the oldest example of a fossil mesophotic ecosystem described so far. This indicates that the development of the earliest mesophotic ecosystems should not be attributed solely to gradual expansion of corals and stromatoporoids into greater depths, but probably took place also by means of redistribution of shallow water taxa during high-energy sedimentary events.

Detecting initial aragonite and calcite variations in limestone–marl alternations

AbstractLimestone–marl alternations are commonly used for high‐resolution cyclostratigraphic studies and palaeoenvironmental reconstructions, but diagenetic studies indicate that not all limestone–marl alternations reflect genuine differences in the initial sediment composition driven by environmental changes. Differences in the ratios of diagenetically inert trace elements between limestones and marls indicate changes affecting the terrigenous fraction of the precursor sediment. Contrarily, limestone–marl alternations without these differences can be the product of: (i) variations in CaCO3 input (aragonite, calcite); (ii) distortion of the latter by diagenetic CaCO3 redistribution; or (iii) diagenetic CaCO3 redistribution in a homogeneous precursor sediment. The aim of this study is to provide a method to differentiate these cases and to identify variations in the proportion of calcite and aragonite in the precursor sediment composition. The model of differential diagenesis assumes that the concentration of diagenetically inert elements is inversely proportional to the amount of redistributed CaCO3. Consequently, the difference between ratios of diagenetically inert elements from two adjacent beds is a measure for CaCO3 redistribution. This is quantifiable by the vector length between ratios from two adjacent beds. The approach is illustrated here by evaluation of a case study from the Silurian of Gotland, Sweden. Trace elements were compared according to their solubility during diagenesis. All elements bound to clay minerals or calcite show similar patterns of vector length, while vector length of elements which fit into the aragonite lattice, and are diagenetically mobile, differ. The vector length approach provides a tool to test the diagenetic origin of limestone–marl alternations, to identify initial variations in CaCO3 input and to test a limestone–marl alternation’s suitability for cyclostratigraphic analyses.

Bryozoan‐rich stromatolites (bryostromatolites) from the Silurian of Gotland and their relation to climate‐related perturbations of the global carbon cycle

ABSTRACTBryozoan–stromatolite associations (bryostromatolites) formed conspicuous reef structures throughout the Sheinwoodian (Wenlock) to Ludfordian (Ludlow) stratigraphy on Gotland but have not been described so far. They are mainly composed of encrusting bryozoans forming a complex intergrowth with porostromate and spongiostromate microbes and are different from the abundant stromatoporoid–coral–algal reefs with respect to their composition. In the bryostromatolite different growth stages can be identified. The observed succession can be taken as evidence for cyclic environmental changes during reef formation. Stenohaline reef‐dwelling organisms, such as echinoderms, sponges, corals and trilobites, indicate fully marine salinities. Ten localities exposing bryostromatolites were discovered. Individual bryostromatolites are small with few decimetres up to one metre in size, and occur solely in shallow marine areas. Common features of these reefs on Gotland are cauliflower‐like growth, a high bryozoan diversity, a high abundance of phosphatic fossils and components such as bryozoan pearls and inarticulate phosphatic brachiopods, enhanced bioerosion, Palaeomicrocodium crusts, vadose silt and gypsum pseudomorphs. The high abundance of Palaeomicrocodium, as well as the alternation with other crust‐forming contributors, suggest that it could have been formed directly at the palaeo‐sea surface, probably in times of minor but high‐frequency sea‐level fluctuations. Vadose silt and pseudomorphs after gypsum in reef cavities indicate subaerial exposure shortly after reef growth. The high amount of phosphatic components indicates a high nutrient input, probably by dust. All bryostromatolites were formed in times of strongly elevated δ13C values. The unusual combination of sedimentological and palaeoecological features, as well as their occurrence exclusively during strong positive δ13C excursions, are evidence that the bryostromatolite development responded to climatic/oceanographic changes, which may have played an important role in reef control.

The oldest deep-boring bivalves? Evidence from the Silurian of Gotland (Sweden)

Compared to modern counterparts, bioerosion is rare in Paleozoic reefs, especially macro-bioerosion. The unique and enigmatic Silurian reefs from Gotland (Sweden), composed of bryozoans and microbial laminates, show evidence of a large amount of bioerosion. The samples contain Trypanites trace fossils, as well as a large number of undescribed macroborings. Small articulated bivalve shells are preserved in some of these macroborings, identified from thin-sections. Three-dimensional images from micro-computed tomography (microCT) reveal an additional bivalve, which is occupying a bioerosion trace. This specimen is possibly contained in a different boring that can be classified as possibly clavate-shaped. Furthermore, evidence of nestling, such as a subsequent modification of the ichnofossils, the presence of bivalves that are much smaller than the trace, or the presence of additional specimens, is missing; therefore, it is most likely that the bivalves made the borings. This is evidence for the existence of deep-boring bivalves in the Silurian.

Coralline red algae from the Silurian of Gotland indicate that the order Corallinales (Corallinophycidae, Rhodophyta) is much older than previously thought

AbstractAguirrea fluegelii gen. et sp. nov. (Corallinales, Corallinophycidae, Rhodophyta) is described from the mid‐Silurian of Gotland Island, Sweden (Högklint Formation, lower Wenlock). The holotype is of dimerous construction and includes a uniporate conceptacle with a sporangium, thus providing evidence that taxa of the Corallinales/Corallinaceae existed at least 300 million years earlier than previously documented. Aguirrea fluegelii cannot be unequivocally placed in any of seven currently recognized lineages/subfamilies/groups of the Corallinaceae as not all diagnostic characters are preserved, and thus is accorded incertae sedis status within the family Corallinaceae and order Corallinales. Extant evolutionary history studies of Corallinophycidae involving molecular clocks now require updating using new calibration points to take account of the much earlier unequivocal mid‐Silurian record of uniporate conceptacle‐bearing taxa of Corallinales/Corallinaceae as well as the parallel record of Graticula, a genus attributed to the Sporolithales.

Coralline red algae from the Silurian of Gotland indicate that the order Corallinales (Corallinophycidae, Rhodophyta) is much older than previously thought

Coralline red algae from the Silurian of Gotland indicate that the order Corallinales (Corallinophycidae, Rhodophyta) is much older than previously thought

Coralline red algae from the Silurian of Gotland indicate that the order Corallinales (Corallinophycidae, Rhodophyta) is much older than previously thought

Coralline red algae from the Silurian of Gotland indicate that the order Corallinales (Corallinophycidae, Rhodophyta) is much older than previously thought

Earliest ontogeny of the Silurian orthotetide brachiopodCooliniaand its significance for interpreting strophomenate phylogeny

Well-preserved juvenile specimens of the orthotetide brachiopod Coolinia pecten (Linnaeus, 1758) from the Silurian of Gotland, Sweden, demonstrate evidence of a planktotrophic larval habit. Larval shell morphology indicates the absence of a pedicle sheath: this character is otherwise typical of derived billingsellides, strophomenides and productides, which form the conventional strophomenide clade. The presence of a rudimentary colleplax structure in the larval shell of Colinia suggests instead a phylogenetic link to chiliate brachiopods and the enigmatic genus Salanygolina. This relationship suggests an early divergence of rhynchonellate and strophomenate brachiopods.

Mass occurrence of the large solitary rugose coral Phaulactis angusta at the boundary Lower/Upper Visby Formation in the Silurian of Gotland, Sweden: palaeoecology and depositional implications

The boundary between the Lower and Upper Visby formations on Gotland (Sweden), which roughly correlates with the Llandovery–Wenlock boundary, is characterised by a mass occurrence of the large solitary rugose coral Phaulactis angusta. This coral-rich layer can be followed for some 50 km along the north-west coast of Gotland. The mass occurrence coincides with the onset of a pronounced, globally recognised δ13C and δ18O excursions in the lower Sheinwoodian. Altogether 31 specimens of P. angusta from eight localities were examined. We demonstrate that the corals preferred to grow in an upright position. Attachment structures were not observed, and therefore a liberosessile life strategy is suggested. Some corallites show an abrupt change in growth direction accompanied by a decrease in diameter. These features mostly occur in later growth stages and may be explained by disturbances in the growth due to toppling or tilting of the corals. Many specimens were able to change their growth axes, but finally died after such an event. Most specimens in the outcrops show a flattened upper surface, which is interpreted as a result of post-mortem abrasion. Epibiontic bryozoan encrustation took place mostly while the corals where alive, whereas stromatoporoid encrustation and Trypanites borings occurred post-mortem. The repeated growth, tilting and erosion of the corallites of P. angusta demonstrate that the Phaulactis layer represents a condensed interval, characterised by multiple pulses of sediment accumulation and erosion.

Radiomagnetotelluric mapping of marlstone and limestone in the Silurian bedrock of Gotland

The Silurian of Gotland is composed of a series of stacked carbonate platforms. Airborne electromagnetic measurements show areas with high electrical resistivity, dominated by dense limestone, and areas with lower resistivity, dominated by marlstone. On Gotland, three main regional, up to 80 m thick, limestone-dominated bedrock packages overlie marlstone-dominated strata. The thickness of the limestone and the subsurface relationship between limestone and marlstone was investigated in the area of Ale–Ardre on central Gotland by use of the radiomagnetotelluric (RMT) method. A significant difference in resistivity between the limestone (1000–10 000 Ωm) and the marlstone ( < 100 Ωm) renders favorable RMT conditions. Four profiles, up to 4.3 km long, were measured across the upper part of the Klinteberg Formation and lower part of the Hemse Group (late Wenlock–early Ludlow). The results show a distinct change in resistivity at 50–80 m depth, correlating with the transition between biostromal–biohermal limestone and shelf marlstone–wackestone lithofacies, as observed in boreholes. The measurements were used to create resistivity models that make it possible to map the subsurface lateral relationship between limestone- and marlstone-dominated sequences. The results also provide important information on distribution and bedding, which give supporting information regarding the sequence stratigraphic model for the Klinteberg Fm–Hemse Group in the Ala–Ardre–Sigsarve area. The RMT method utilized in this study proved to be a very useful method for mapping subsurface bedrock in carbonate-dominated geological settings such as those on Gotland.

A new ophiocistioid (Echinodermata) from the Silurian of Gotland, Sweden

From the early Silurian of Gotland, Sweden, the new ophiocistioid goniodont Linguaserra franzenae sp. nov. is described, figured and discussed within the Linguaserridae Reich & Haude, 2004. The evolutionary history of linguaserrid ophiocistioids is briefly discussed in relation to other members of the Ophiocistioidea. This new echinoderm species not only represents the first documented occurrence of this group from the Llandovery but also establishes the stratigraphically oldest record of goniodonts in the world.

A complete reconstruction of the hyolithid skeleton

Hyolithids are a group of Paleozoic lophotrochozoans with a four-pieced skeleton consisting of a conch, an operculum, and a pair of lateral ‘spines' named helens. Both the conch and operculum are relatively well known and, to a certain extent, have modern analogues in other lophotrochozoan groups. The helens, on the other hand, are less well known and do not have clear modern analogues. This has hindered the knowledge of the complete morphology of the hyolithid skeleton, as well as other aspects of hyolithid biology, such as the organization of soft parts, and their ability to move. The material studied herein, consisting of disarticulated skeletal elements from the Silurian of Gotland, Sweden, illustrates a complete developmental sequence of a hyolithid species and includes the first complete, three-dimensionally preserved helens. Our material confirms that helens were massive skeletal elements, whose growth started proximally with the deposition of a central, coherent lamella. Further shell accretion took place around this lamella, but followed a particular accretion pattern probably constrained by the presence of marginal muscle attachment sites on the proximal-most portion of the helens. These muscle attachment sites were ideally located to allow a wide range of movements for the helens, suggesting that hyolithids may have been relatively mobile organisms.

A new view on Nematothallus: coralline red algae from the Silurian of Gotland

Abstract: The thalloid carbonaceous fossil Nematothallus Lang, 1937, has been widely interpreted as an early Palaeozoic land‐plant, despite the absence of a convincing modern analogue. Exceptionally well‐preserved nematophyte cuticle from the Late Silurian Burgsvik Sandstone Formation, Gotland provides additional insight into the organism’s anatomy, phylogenetic affiliations and ecology. Because this material exhibits additional characters not present in the type material we assign it to Nematothallopsis gotlandii gen. et sp. nov. The organism was constituted of a close‐packed layer of palisade‐like filaments covered by a cuticle that bears a characteristic pseudocellular pattern on its inner surface. Apertures in this cuticle are often encircled by a ring of multicellular filaments, which are sometimes associated with spheroidal, spore‐like entities. In the light of the conspicuous similarity of the palisade layers to the pseudoparenchymatous tissue of coralline red algae, and of the filament‐fringed apertures to their reproductive conceptacles, we reconstruct the Nematothallopsis organism as an extinct rhodophyte and re‐evaluate the putative terrestrial habit of cuticular nematophytes in general.

An acid-free method of microfossil extraction from clay-rich lithologies using the surfactant Rewoquat

Marine rocks characterized by high clay content provide excellent conditions for fossil preservation, particularly for organic-walled microfossils such as retiolitid graptolites and chitinozoans. Nevertheless, the phyllosilicate minerals, which constitute the clay component, make microfossil extractions difficult. The problem results from the tendency of phyllosilicates to form aggregates in low pH values, as standard methods of microfossil extraction employ acids for rock digestion. Consequently, the use of acids for clay-rich rocks is often inefficient and time-consuming. We propose a method of rock disintegration using the surfactant Rewoquat and compare it with two commonly applied approaches: digestion in buffered acetic acid and in HCl-HF. Using examples from the Mulde Brick-clay Member from the Silurian of Gotland and the Daleje Shale from the Devonian of the Prague Basin we observed that disintegration in Rewoquat was faster (days) than digestion in acid (months), and allowed to recover calcareous in addition to organic-walled fossils. The yield and preservation was comparably good, except for conodonts, which were strongly etched after using HCl-HF. Retiolitid graptolites recovered using Rewoquat were preserved in 3D and showed a lower degree of fragmentation. The fossil content of the residue obtained using Rewoquat was higher due to dispersion of clay aggregates. For observation of delicate fossils we recommend to coat the sample with the surfactant. Application of Rewoquat can reveal the most delicate forms and growth stages, and thus provide a better insight into the ontogeny, autecology, and body size distributions of a number of fossil groups.

Scale morphology and squamation of the Late Silurian osteichthyan Andreolepis from Gotland, Sweden

The origin of osteichthyans (bony fishes and tetrapods) dates back to the Late Silurian, but the early evolution of the group is poorly understood. Andreolepis is one of the oldest known osteichthyans, but exclusively documented by detached and fragmentary dermal microremains. A large data-set of Andreolepis scales from the Silurian of Gotland has been used to explore the scale morphology on different parts of the body. Landmark-based geometric morphometrics together with comparative anatomy and functional morphology has allowed 10 morphotypes to be identified and incorporated into a squamation model, in which scales are allocated to anterior-mid lateral flank scales, posterior lateral flank scales, caudal peduncle scales, pectoral peduncle scales, dorsal flank scales, dorsal fulcral scales, caudal fulcral scales, ventral flank scales, medioventral scales and cranial scales. The scale morphology and squamation pattern of Andreolepis may be primitive for the Osteichthyes and thus informative about the acquisition of the osteichthyan body plan.

Silurian oncocerid Octamerella (Cephalopoda) from Gotland, Sweden

An actinosiphonate deposit with radial lamellae occurs in the siphuncle of several genera classified within the Oncocerida. Unique structural preservation of the connecting ring and the actinosiphonate deposit is here described in the oncocerid Octamerella sp. from the Silurian of Gotland, Sweden. The connecting ring of Octamerella is of the Nautilus type. It consists of an outer porous spherulitic–prismatic layer and an inner thin fibrous organic layer, the latter destroyed by diagenesis. The actinosiphonate deposit consists of two, structurally different, spherulitic–prismatic sublayers. The thin outer sublayer has a fragile porous structure and low mechanical strength, similar to the outer spherulitic–prismatic layer of the connecting ring. The inner spherulitic–prismatic sublayer that forms the actinosiphonate lamellae has a more rigid, less porous structure and higher mechanical strength than the outer sublayer. Each lamella is longitudinally traversed by a calcareous central plate that has a solid structure and consists of thin acicular crystallites that are orientated parallel to the long axis of the plate. The secretion of the actinosiphonate deposit probably began immediately after a new siphuncular segment was completed. The connecting ring together with the actinosiphonate deposit functioned as a reservoir and transport route for the cameral liquid improving the buoyancy regulation.

Strophomenidae, Leptostrophiidae, Strophodontidae and Shaleriidae (Brachiopoda, Strophomenida) from the Silurian of Gotland, Sweden

Twelve species of Brachiopods are described from the Silurian of Gotland, six furcitellinines and six “strophodontids.” One is new—Strophodonta hoburgensis n. sp. The furcitellinines are moderately common and diverse in the lower part of the succession, but the last species disappears in the middle Hemse beds (~middle Ludlow). Three genera are represented: Bellimurina, Pentlandina and Katastrophomena, with the species and subspecies B. wisgoriensis, P. tartana, P. loveni, P. lewisii lewisii, K. penkillensis and K. antiquata scabrosa. Most of the taxa are confined to low energy environments, but P. loveni was evidently specialized for the high energy reef environments of the Hogklint Formation. B. wisgoriensis displays environmentally induced morphological variability in developing strong, frilly growth lamellae in high-energy environments. The “strophodontids,” although belonging to three different families, share a common morphology consisting of denticles along the hinge line, a semi-circular outline, unequally to finely costellate ornament and, most importantly, a concavo-convex profile with both valves of the same curvature, enclosing a very small body chamber. Two leptostrophiids are generalists, occurring in both high- and low-energy environments and with long stratigraphical ranges [Mesoleptostrophia filosa; latest Llandovery through the entire Ludlow. Brachyprion (Brachyprion) semiglobosa; latest Llandovery to latest Wenlock]. The third leptostrophiid (Brachyprion (Erinostrophia) walmstedti) is short ranged and occurs in low-energy environments in the latest Llandovery. The species belonging to the Strophodontidae (Strophodonta hoburgensis n. sp.) and Shaleriidae [Shaleria (Janiomya) ornatella and S. (Shaleriella) ezerensis] occur only in high-energy environments and have a short range within the late Ludlow.

Coralline red algae from the Silurian of Gotland indicate that the order Corallinales (Corallinophycidae, Rhodophyta) is much older than previously thought

Coralline red algae from the Silurian of Gotland indicate that the order Corallinales (Corallinophycidae, Rhodophyta) is much older than previously thought

Coralline red algae from the Silurian of Gotland indicate that the order Corallinales (Corallinophycidae, Rhodophyta) is much older than previously thought

Coralline red algae from the Silurian of Gotland indicate that the order Corallinales (Corallinophycidae, Rhodophyta) is much older than previously thought

Origins and biomechanical evolution of teeth in echinoids and their relatives

Abstract: Echinoid teeth are without doubt the most complex and highly specialized skeletal component to have evolved in echinoderms. They are biomechanically constructed to be resilient and tough while maintaining a self‐sharpening point. Based on SEM analysis of isolated tooth elements collected primarily from the Ordovician and Silurian of Gotland, we provide a detailed structural analysis of the earliest echinoderm teeth. Eight distinct constructional designs are recognized encompassing various degrees of sophistication, from a simple vertical battery of tooth spines to advanced teeth with multiple tooth plate series and a reinforced core of fibres. These provide key data from which we reconstruct the early stages of tooth evolution. The simplest teeth are composed of stacked rod‐like elements with solid calcite tips. More advanced teeth underwent continuous replacement of tooth elements, as a simple self‐sharpening mechanism. Within echinoids tooth design was refined by evolving thinner, flatter primary plates with buttressing, allowing maintenance of a sharper and stronger biting edge. Despite the obvious homology between the lanterns of ophiocistioids and echinoids, their teeth are very different in microstructural organization, and they have evolved different self‐sharpening mechanisms. Whereas echinoid teeth evolved from a biseries of mouth spines, ophiocistioid goniodonts evolved from a single series of mouth spines. Rogeriserra represents the most primitive known battery of tooth elements but its taxonomic affinities remain unknown.