Silurian Leptaeninae (Brachiopoda) from Gotland, Sweden

Strophomenid brachiopods belonging to the generaLeptaena andLepidoleptaena are described from the uppermost Llandovery — Ludlow succession of Gotland, Sweden. In Gotland,Lepidoleptaena comprises the single speciesL. poulseni, andLeptaena includes four species:L. rhomboidalis, L. sperion, L. depressa andL. parvirugata n. sp.L. depressa shows a considerable amount of morphological variation, which is recognised in the two new subspeciesLeptaena depressa visbyensis n. ssp. from the Llandoverian — Wenlockian Visby Formation, andL. depressa lata n. ssp. from the mid-Wenlockian Slite Group. The distribution of the different species is largely substrate-dependent, withLeptaena rhomboidalis andLepidoleptaena poulseni adapted to high-energy environments with firm substrates, whereas the remaining species preferred fine-grained substrates in low-energy environments.      

Enigmatic polychaete jaws from the Silurian of Gotland, Sweden

The jaws of the new polychaetaspid polychaete, Oenonites? honki, from the Silurian of Gotland, Sweden, differ from most Palaeozoic polychaete jaws. They exhibit enigmatic microstructural features in that they appear rough and give a corroded, or weathered impression. The altered microstructure of the jaws suggests a jaw chemistry and/or composition differing in some way from that of the co‐occurring polychaete taxa. The jaws appear to have limited preservational potential and/or were particularly susceptible to secondary processes, resulting in microstructural alteration. Commonly, a row of distinct pits occurs on the outer face, especially of the first right maxillae (MIr). Because these pits are interpreted as associated with the dentary, the term ‘denticle marks’ is suggested. The pits may be the result of primary or secondary physical wear, or, more probably, secondary chemical alteration of localized mineral deposits. The primary function of such mineral deposits was to harden those parts of the surface that were exposed to great stress. The restricted occurrence of O.? honki, coupled with occasional increases in abundance (especially in the Halla Formation, unit b), indicates a preference for shallow marine, high‐energy environments, particularly in reefal pockets with calcilutitic sediments. Highest frequency coincides with faunas characteristically containing a few labidognath species also displaying high frequencies.     

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 Högklint 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.     

Cementing strophomenide brachiopods from the Silurian of Gotland (Sweden): Morphology and life habits

Dorsal valves of the cementing strophomenide brachiopods Leptaenoidea silurica Hedström and Liljevallia gotlandica Hedström are described for the first time, and it is shown that both these species could also live ambitopically. The lower Wenlock Scamnomena rugata (Lindström) represents young individuals of the ambitopic variant of Leptaenoidea silurica, and is placed in synonymy, resulting in the valid name for the taxon being Leptaenoidea rugata. The range of this species now spans the whole Wenlock, from the upper Visby Formation to the Klinteberg Formation, and possibly even into the Ludlow. Ambitopic gerontic specimens of L. rugata develop very thick shells, in which the ventral valves have strong curvature, and become deeper not by geniculation but by successive mantle retractions and subsequent re-growth, in a way similar to that of atrypides. This shape was probably an adaptation to “floating” on softer substrates. The thickened gerontic dorsal valves have well-developed lophophoral support, showing the shape of the lophophore, which comprises two branches that curve inwards and then backwards; the lophophore was probably ptycholophous and similar to that in living members of the Thecideidina. Ambitopic specimens of Liljevallia could grow to a much larger size than cementing forms, where the dorsal valves have very large, posterior-facing cardinal process lobes and deeply impressed muscle fields and anterior scars. The presence of a ventral process and long, posteroventrally elongated cardinal process lobes, and the absence of dental plates reveals that Liljevallia was probably an early member of the Douvillinidae and is thus removed from the Leptaenoideidae.     

FTIR characterisation of the chemical composition of Silurian miospores (cryptospores and trilete spores) from Gotland, Sweden

To better understand the biological affinities of cryptospores, micro-FTIR (Fourier transform infrared) spectroscopy analysis has been carried out on isolated specimens from the Upper Silurian of Gotland. The geobiochemical results have been compared to spectra of trilete spores, chitinozoans and leiospheres from the same sample. The palynomorphs are all very well preserved as attested by their pale yellow to orange colour indicative of a low thermal maturity. Micro-FTIR spectroscopy indicates that cryptospores display very similar spectra to those of the trilete spores, which are composed of sporopollenin characterised by absorption bands from aliphatic C-H in methylene (CH₂) and methyl (CH₃) groups, aromatic (C=C and C-H) groups and C=O groups of carboxylic acids. The sporopollenin composition of the cryptospore wall observed here is additional evidence demonstrating the embryophytic affinity of the cryptospores. In addition, several variations in other bands in the spectra of the different miospore morphospecies are evidenced and may be linked to their biological affinity or palaeoecological history.     

Silurian calcispheres (Calcitarcha) of Gotland (Sweden): Comparisons with calcareous dinoflagellates

Scanning electron microscope examinations of polished and etched surfaces of sediments from the Silurian carbonate platform of Gotland, Sweden, revealed the presence of numerous, morphologically diverse “calcispheres” (Calcitarcha). Some of these spherical calcareous microfossils display wall structures that are surprisingly similar to those of calcareous dinoflagellate cysts. In analogy to the interpretation of the biological affinities of Palaeozoic acritarchs as cysts of organisms that might have been the ancestors of organic-walled dinoflagellates, the Calcitarcha from Gotland can be compared and may possibly be related to organisms that may have been the ancestors of calcareous cyst-producing dinoflagellates that so far have not been observed before the Late Jurassic.     

Silurian craniide brachiopods from Gotland

Seven craniide brachiopod genera are described from the Silurian (Wenlock–Ludlow) of Gotland, including one new genus and five new species. The new genus and species Thulecrania septicostata is unique among Silurian craniides as it possesses solid spines. The new species Lepidocrania multilamellosa is the first Silurian record of this poorly known Permian genus. The problematic North American Propatella Grubbs, 1939 , was originally described as a gastropod, but the new species Propatella palmaria from Gotland shows that it is a craniide with sutured hollow spines of a type not previously recorded from Silurian craniides. The dorsal valves of the new species Valdiviathyris? bicornis are remarkably similar to those of the type species and represent the first possible Palaeozoic record of this poorly known extant craniide. This first systematic study of craniide brachiopods from the Silurian of Gotland shows that the diversity is relatively high as compared to other known Silurian craniide faunas, but a more thorough comparison is not possible due to the lack of data from most parts of the world. The new data from Gotland support the view that the craniides were not affected by the end‐Ordovician extinction.     

Early Palaeozoic diversification of chitons (Polyplacophora, Mollusca) based on new data from the Silurian of Gotland, Sweden

Isolated, well-preserved silicified sclerites of an unusually diverse Silurian paleoloricate assemblage from Gotland preserve morphological features that are important in interpreting palaeobiology. The typically granular dorsal ornament is comparable with Recent chitons, and is hence possibly linked functionally with sensory aesthetes. Ventral structures particularly in thickened shells indicate major muscle attachment sites sub-apically or, equivalently, beneath the rim of the apical area, and also marginally. In early chitons such as Cambrian Matthevia, deep ventral cavities represent comparable sub-apical sites. Three Gotland genera with an unusual, holoperipheral shell growth style apparently represent plated aplacophorans (cf. Acaenoplax), which coexisted with paleoloricate chitons in shallow inshore carbonate shelf environments. Sclerite features of all the Gotland genera are discussed together since they share most characteristics. The new family Heloplacidae includes those genera and Acaenoplax, which in a preliminary cladistic analysis form a sister group to other Lower Palaeozoic paleoloricates. Multiplated skeletons in paleoloricates and this group of aplacophorans represent parallel evolution of dorsal armour, which in chitons resulted in overlapping, articulating sclerites. The diversity of the mid-Silurian Gotland assemblage is examined against early evolutionary diversification of polyplacophorans, aplacophorans, and in relation to the overall record of Palaeozoic paleoloricate and neoloricate chitons. Peaks in diversity in early Ordovician, mid-Silurian and early Carboniferous times correspond to periods with widespread development of low latitude carbonate shelves. Neoloricates, apparently with an additional shell layer that contributed articulatory plates, appeared in the Devonian from where the fossil record remains poorly known.     

Acritarch distribution along an inshore–offshore transect in the Gorstian (lower Ludlow) of Gotland, Sweden

To better understand palaeoecological signatures in Palaeozoic acritarch assemblages, the distribution of palynomorphs has been quantitatively studied in eight localities from the Gorstian, lower Ludlow (Late Silurian) of Gotland, Sweden. The localities are situated along an inshore–offshore transect comprising shallow marine lagoonal environments to distal shelf facies. Process-bearing acritarchs and sphaeromorphs constitute the main components within the palynomorph assemblages. The lateral distribution of palynomorphs exhibits characteristic features at three different levels as follows. (1) With regard to the overall composition of the palynomorph assemblages, the abundance of process-bearing acritarchs increases towards the distal shelf, while the abundance of sphaeromorphs decreases. (2) At the generic level, the acritarchs Micrhystridium and Dilatisphaera are more abundant in the proximal facies, while Evittia, Percultisphaera and Oppilatala are more common in distal environments. (3) At an infrageneric level, Micrhystridium morphotypes with shorter processes are mainly present in proximal environments, while those with longer and ramified processes occur in more distal shelf environments. The palynomorph distribution along the inshore–offshore transect highlights the potential of acritarchs and prasinophytes as palaeoenvironmental indicators.     

Patterns of sclerobiont colonization on the rugose coral Schlotheimophyllum patellatum (Schlotheim, 1820) from the Silurian of Gotland,Sweden

Analysis of mushroom-shaped rugose corals Schlotheimophyllum patellatum (Schlotheim, 1820) from the Silurian (Upper Visby Beds, Lower Wenlock, Sheinwoodian) of Gotland, Sweden, showed that they were colonized on both the upper (exposed) and lower (cryptic) sides by a variety of encrusting and boring (sclerobiont) biotas, represented by 10 taxa and at least 23 species. Bryozoans and microconchid tubeworms, the most abundant encrusters, dominated on the cryptic undersides of the corals, while the dominant endobionts responsible for Trypanites borings overwhelmingly dominated the exposed surfaces. Except for cnidarian sphenothallids, which were exclusive colonizers of the underside of only one coral host, no other encrusters could be referred to as obligate cryptobionts. Because the upper surface of these corals was likely covered by soft-tissues during life, in specimens lifted off the sea-floor sclerobionts must have settled on the cryptic sides first. They could colonize the upper side only after the coral’s death, unless it was covered by sediment as could be the case in some flat specimens. With time, the space on the underside of the coral skeleton may have progressively been filled by sediment as well, precluding further colonization by sclerobionts. In that respect, the colonization patterns of these corals by encrusters and borers were controlled by the complex interplay of environmental factors, sclerobiont dynamics and coral growth in a given Silurian habitat. Compared with Silurian stromatoporoid hosts, the sclerobiont diversity and abundance noted on the Schlotheimophyllum corals may be regarded as representative for the Silurian as a whole.     

Microplankton changes through the early Silurian Ireviken extinction event on Gotland, Sweden

This paper documents and analyses the extinction and origination patterns of acritarchs and prasinophyte algae at the Llandovery/Wenlock boundary transition in the Lower Silurian on the island of Gotland, Sweden. Closely spaced samples were collected from two parallel sections: Lusklint 1 and Lickershamn 2, spanning the upper part of the Lower Visby Beds and almost all of the Upper Visby Beds (i.e. the uppermost Llandovery and lowermost Wenlock).

At least eight extinctions affecting the conodont record have been reported at these levels. This turnover (named the Ireviken Event) has been interpreted as an example of the change from a P to an S climate state, reflecting large changes in the ocean/atmosphere system.

The palynomorph data show a significant turnover in the phytoplankton, with most of the extinctions at the end of the event (85.3% in the top 4 m of the Lusklint 1 section). The originations are more numerous than the extinctions and they are distributed through the whole of the Ireviken Event. There is an uneven distribution across the event with more originations in the Lower Visby Beds forming a convex pattern.

Comparison of these data to other palynological studies suggests that there was a slightly higher number of migrations out of Gotland than in. The P and S model does not successfully explain all the changes recorded, but is the model most inclusive of all the climatic variables available at this time.     

A New Family Of Calcareous Microfossils From The Silurian Of Gotland, Sweden

Scanning electron microscopy of polished, slightly etched rock surfaces of samples from the Silurian of Gotland, Sweden, revealed the presence of excellently preserved calcareous microfaunas and microfloras. This paper illustrates several morphotypes of calcareous micro-organisms of unknown biological affinity, attributed to the new family Ovummuridae. Specimens of Ovummurus duoportius Minoura and Chitoku, a problematical microfossil, which was first described from the Upper Pennsylvanian of Kansas, USA, were found in addition to comparable micro-organisms attributed to the following new taxa: Minourella gotlandica gen. et sp. nov., Arouxina pluricellata gen. et sp. nov. and Samtlebenella circumcamerata gen. et sp. nov. In addition, a few other problematical micro-organisms, probably belonging to the same family, are described under open nomenclature. The enigmatic microfossils are compared with other known calcareous organisms, their possible biological affinities are discussed, and the different stages of diagenetic alteration and destruction are illustrated.     

Proetid trilobites from the middle Telychian (Llandovery,Silurian) Ningqiang Formation,Shaanxi, China

Four proetid trilobite species, Chuanqianoproetus guizhouensis Yin in Yin and Li, 1978, C. shuangheensis Wu, 1977, C. sp., and Latiproetus guizhouensis Yin in Yin and Li, 1978, are described from the Ningqiang Formation in Ningqiang County, Shaanxi, China. They are middle Telychian (Llandovery, Silurian) in age. Chuanqianoproetus and Latiproetus are both regarded as members of the subfamily Warburgellinae; the former has been considered to be a junior subjective synonym of Astroproetus, but on the basis of its morphology we argue here that they represent two independent genera. The poorly sorted bioclastic grainstones containing trilobite sclerites may indicate their deposition in turbulent conditions on an open marine platform.     

Crinoid holdfasts from the Silurian of Gotland

Crinoid holdfasts occur throughout the Silurian sequence of Gotland, with a marked concentration to reefs and immediate reef surroundings. Four morphological groups are recognized: (1) Discoid holdfasts. (2) Cirriferous holdfasts, represented by (a) Rhizocrinus -like holdfasts, (b) large radices with stout, often branching cirri, and (c) rhizoid holdfasts, formed by a complicated net of pseudocirri. (3) Creeping stems attached to the substrate by thin strands of stereome. (4) Coiling stems. Discoid attachments, cirriferous holdfasts (types b and c) and coiling stems show little preference to substrate and adapt morphologically to conditions at hand. Rhizoid holdfasts display the greatest variation and apparently occur within many different groups of crinoids. Rhizocrinusttke holdfasts have been found only in quiet-water deposits, while creeping stems were concentrated to more turbulent environments. Coiling stems were epizoic, attached to rugose corals, bryozoans, crinoid stems and similar supports. Growth, mode of life (attached or free), settling and fixation of larvae, and relation to substrate and other organisms ate discussed.     

Silurian Chitinozoa from the type Wenlock and Ludlow of Shropshire,England

Chitinozoa recorded from the Wenlock and Ludlow from the type areas in Shropshire and other localities in the Welsh Basin are listed, and the ranges of 35 taxa of stratigraphic value plotted.     

Silurian acritarchs from the type Wenlock and Ludlow of Shropshire,England

Acritarch taxa are recorded and listed from the Wenlock and Ludlow from the type areas in Shropshire, England, and other localities in the Welsh Basin of South Wales and the Welsh Borderland of England. The ranges of 141 taxa of stratigraphic value are plotted. Five new genera are described: Hogklintia, Lepto brachion, Rhaco brachion, Salopidium and Wrensnestia. 36 new species are described, and 32 new combinations proposed. Seven acritarch zones are recognised in the Wenlock and Ludlow.     

Two micromorphic bivalves from the Silurian of Gotland

Two new micromorphic bivalve species from the Silurian of Gotland are described and assigned to the new genusKenzieana, viz.Kenzieana lata andKenzieana angusta. Kenzieana exhibits morphological features characteristic of some unrelated extant species; above all a conspicuous antero-posterior flattening of the shell. Probably it was a siphonate suspension feeder with a reclining mode of life.     

Scutelluid trilobites from the Silurian of New South Wales

Abstract: Twenty‐six species (nineteen of them formally named) of scutelluid trilobites belonging to eight genera are described from limestones of the lower Wenlock to Ludlow Mirrabooka Formation and its stratigraphical equivalents in the Orange district of New South Wales. Two genera, Borenoria and Mirrabooka, are new. Japonoscutellum, previously employed mainly for several species from Japan, is a globally widely distributed genus in rocks of middle Llandovery to Ludlow age and includes a number of species previously assigned to Kosovopeltis. Japonoscutellum is remarkably diverse in the faunas from Orange where it is represented by eleven species, one of them known also from Kazakhstan. Flexiscutellum and Tosacephalus are regarded as junior synonyms of Decoroscutellum and Kosovopeltis, respectively. The homology of various scutelluid cephalic structures is discussed, in particular the cephalic borders and border furrows that have routinely been misidentified; the term ‘bolus’ is introduced for the variably developed swelling that is enclosed by S1 on the glabella of most scutelluids. New species are Australoscutellum iota, A. microps, A. pulex, A. trica, Borenoria cirrita, Decoroscutellum planes, Eoscutellum annosum, Illaenoscutellum tryo, Japonoscutellum borenorense, J. crassum, J. diascetum, J. gephyricum, J. guttulatum, J. serangicum, J. strabo, Kosovopeltis trepida, K. xynon and Mirrabooka harryi. Some of these trilobites show strong affinity with species from the Kurosegawa Terrane of south‐western Japan and the Chu‐Ili Terrane of Kazakhstan, as previously reported also for the effaced Illaenina from the same limestones. Australoscutellum and Mirrabooka are presently known only from eastern Australia. The new genus Securifrons is proposed for several species previously assigned to Kosovopeltis or Eokosovopeltis from the Sandbian or Katian to Llandovery of North Greenland, Canada, Scotland, South China and New South Wales.     

A 10,000-year temperature record from Gotland,Sweden

The sediment of Lake Tingstdäde Träsk, a shallow Chara-dominated lake onm the island of Gotland in the middle of the Baltic, was analyzed as to its ^1*O and ¹³C content. The lake carbonate (Chara lime) was precipitate due to the assimilation of CO₂ by Chara during the summer months. The isotope curves give a very detaile record of the temperature fluctuations from 10, 700 to 1000 B.P. The depth scale was converted to absolute age via numerous identified levels. The ¹⁸O curve was converted to temperature (mean summer temperature of the lake) via the relation between measured lake temperature and corresponding isotopes composition of the water. The corresponding mean air temperature was calculated. The temperature record shows: (1) a low temperature during the Younger Dryas Stadial: (2) a very rapid rise at the Pleistocene/Halocene Halocene boundary; (3) a rapid rise through the early Holocene with present-day values reached at about 9, 300 B.P.; (4) a Holocene optimum of 1.5-1.2°C higher temperature than today, ending with a 2°C temperature fall at the Subboreal/Subatlantic transition at about 2500 B.P.; and (5) a fluctuating Holocene temperature with warm and cold peaks at the same chronological levels as known eustatic and global climatic fluctuations.     

The formation of micritic limestones and the development of limestone-marl alternations in the Silurian of Gotland,Sweden

Summary Micritic limestone-marl alternations make up the major part of the Silurian strata on Gotland (Sweden). Their position on the stable Baltic Shield protected them from deep burial and tectonic stress and allowed the preservation of early stages of burial diagenesis, including lithification. In the micritic limestones certain characteristics have been preserved (e.g., pitted microspar crystals, sharp boundaries between microspar and components, lack of deformation phenomena) that offer insights into their formation. We suppose the formation of these micritic limestones and limestone-marl alternations to be based on a rhythmic diagenesis within an aragonite solution zone (ASZ) close below the sediment surface. The micritic limestones are the product of a poikilotopic cementation of carbonate muds which consisted of varying portions of aragonitic, calcitic and terrigenous matter. Their microspar crystals show the primary size and shape of the cements lithifying the original carbonate mud. Dissolution of aragonite in the marls provided the carbonate for the lithification of the limestones. By cementation, the limestone beds evaded further compaction. The marls, which already underwent a volume decrease by aragonite depletion, lacked cement and became more and more compacted due to increasing sedimentary overburden. Although field observations show that primary differences in material influence the development of limestone-marl alternations they are not required for their formation.