A radicle solution: morphology and biomechanics of the Eucalyptocrinites ‘root’ system

Eucalyptocrinites is one the most common and familiar mid‐Palaeozoic crinoids and is the exemplar of a form with dendritic radicular holdfasts. American museums have hundreds of specimens of Eucalyptocrinites holdfasts from the Silurian Waldron Shale of Indiana and Kentucky, USA. The radix (‘root’) system of Eucalyptocrinites can be described as comprised of links (branches) that meet at nodes. Measured values include the x‐ and y‐coordinates of the nodes, the distances of the nodes from the stem, the angles between branches, branch length and branch width. Rose diagrams show clearly that the root systems are not isotropic but have preferred orientations. Branch angles are highly variable, but cluster around 60 degrees. Branch lengths and distal branch widths are relatively constant, but branch width increases variably towards the column. The branching pattern can be modelled as a self‐similar (‘fractal’) structure. Several specimens labelled as Eucalyptocrinites show a distinct fivefold symmetry without branching and very likely represent a different taxon. The Eucalyptocrinites radix system, along with the probably stiff dististele, most likely functioned as a rigid plate that resisted rotational forces due to currents acting on the crown. Upstream radicles experienced tension, whereas downstream roots were compressed. This force distribution may explain the observed anisotropies in radix morphology. The ‘roots’ of Eucalyptocrinites and other crinoids have long been compared with the root systems of plants. Although there are superficial similarities, there are fundamental differences.     

Radial perforations in crinoid stems from the Silurian of Gotland

Radiating intracolumnal canals are a characteristic feature of large (diameter 10 mm or more) crinoid stems from the Silurian of Gotland. They are found in nodals as well as in internodals where the columnal height exceeds one millimetre. They were formed secondarily in the median and distal portions of crinoid stems with pseudocirriferous holdfasts. Intercolumnal canals are found in the distal parts of stems with true cirri regardless of the size of the stem. It is suggested that these canals played an important role in crinoid physiology. The crinoids are believed to have sustained a large proportion of their tissues through cutaneous digestion and uptake of dissolved substances from the surrounding sea water. The intra- and intercolumnal canals increased the surface of the axial canal in relation to volume. They provided a connection between the axial canal and the surrounding sea water, thus facilitating nutrient transport to the tissues.     

Silurian bolbozoids and cypridinids (Myodocopa) from Europe: pioneer pelagic ostracods

Abstract: Bolbozoid and cypridinid myodocope ostracods from France, the Czech Republic and Sardinia consist of four genera (one new: Silurocypridina) and at least ten species (eight new: Bolbozoe acuta, B. rugosa, B. parvafraga, Parabolbozoe armoricana, Silurocypridina retroreticulata, S. variostriata, S. calva and Calocaria robusta). They have a functional design, palaeogeographical distribution, depositional setting and faunal associates that suggest that they were swimmers in the water column, living in dim light, relatively deep environments, above near bottom dysoxic/anoxic conditions. These myodocope faunas include some possible pelagic species sensu stricto, with widespread palaeogeographical (transoceanic) distribution, and some species that occupied hyperbenthic niches and were capable of making short incursions to the sediment to scavenge on carrion. These data support the model that Silurian myodocopes were pioneer pelagic ostracods.     

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.     

Pseudoplanktonic lifestyle of the Triassic crinoid Traumatocrinus from Southwest China

Colonies of Traumatocrinus (Echinodermata, Crinoidea) attached to driftwood from the lower Upper Triassic (Carnian) Xiaowa Formation of the Guanling area (Guizhou, Southwest China) document a pseudoplanktonic lifestyle for this specialized offshoot of the otherwise benthic Middle Triassic Encrinidae. After the end-Carnian decline of Traumatocrinus, its ecological niche was subsumed in Norian by genera derived from another benthic family (Holocrinidae) with convergent morphological modifications. After the Toarcian (Lower Jurassic), this niche disappeared, possibly due to the emergence of wood-boring bivalves.     

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.     

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.     

Palaeozoic calcareous plankton: evidence from the Silurian of Gotland

In order to investigate whether or not equivalents to modern calcareous plankton existed in Palaeozoic times, extremely well-preserved successions have to be investigated. The Silurian strata on Gotland (Sweden) are exceptionally well preserved because they have not experienced deep burial conditions and tectonic stress, due to their position on the stable Baltic Shield. Scanning electron microscope investigations of polished, slightly etched rock surfaces revealed the presence of a variety of calcareous micro- and nannofossils. Among these organisms, many can be termed 'calcispheres' (60–100 µm in diameter), whereas others due to their size range (nannofossils) are informally termed herein as 'nannospheres' (10–25 µm in diameter). The systematic attribution of these fossils is unknown ( incertae sedis ). Mesozoic calcispheres are usually attributed to calcareous cysts of dinoflagellates following comparisons of ultrastructure with modern species. The abundance of different calcispheres in the Silurian sediments of Gotland and the observation that many of the calcareous microfossils occur in distinctly different facies as well as their spherical shape indicate that they probably belong to calcareous micro- and nannoplankton. We therefore conclude that calcareous plankton most probably existed already during the Palaeozoic, but can only be observed under conditions of exceptional preservation.     

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.      

Gonad pouches in a new clam-like monoplacophoran from the Silurian of Sweden

Beyrichidiscus fecundatus n. gen. n. sp. is described from the Upper Silurian of Gotland, Sweden. The minute, isostrophically coiled mollusc exhibits profound allometry and is considered to have been a monoplacophoran on the basis of functional interpretation of apertural structures. A clam-like, semi-infaunal mode of life is suggested. A pair of prominent lateral swellings in the adult shell are interpreted as reflecting the location of the gonads.     

Functional morphology of synarthrial articulations in the crinoid stem

Synarthrial fulcral ridges are found in crinoid columnals from the mid-Ordovician to the present and in all four subclasses. Similar articulations did not become common in the cirri until the Mesozoic. Synarthrial stem articulations fall into two broad groups. Type I articulations have a fulcral ridge in the centre of the articular facet. In elliptical ossicles this ridge corresponds to either the long (IA) or short (IB) axis of the facet. Although both are functionally similar, Type IA ossicles are more common. Type II articulations have an excentric fulcral ridge, parallel to either the long (IIA) or short (IIB) axis of the articular facet. Type IIA articulations are found in crinoid stems capable of coiling. Type II articulations are particularly common in the cirri of articulates and are well adapted for attachment to hard and soft substrates. Columnals with Type I articulations often have divergent fulcra, giving the stem flexibility in all directions, but this feature is not seen in cirri or in coiled stems, where it would impair normal functions. Only the cirri of isocrinids and comatulids are muscular, so the movement of columns with fulcra must be passive.     

Chitinozoan biostratigraphy of the basal Wenlock Series (Silurian) Global Stratotype Section and Point

Diverse and abundant assemblages of chitinozoans allow the recognition of the upper part of the Margachitina margaritana Biozone in the uppermost Llandovery and lowermost Wenlock series of the Hughley Brook section. The Cingulochitina bouniensis and Salopochitina bella biozones are identified in the lower part of the Buildwas Formation, Wenlock Series. The chitinozoan data indicate that the base of the Wenlock Series most probably correlates with a level in the upper centrifugus or lower murchisoni graptolite biozones. Chitinozoans also indicate that the base of the riccartonensis graptolite Biozone may occur within the Buildwas Formation and not the overlying Coalbrookdale Formation.     

Ecological aspects of a silicified bivalve fauna from the Silurian of Gotland

The silicified Wenlockian (Silurian) bivalve fauna from MÖllbos, Gotland, is part of a life assemblage. The vast number of shells show unusual phenomena, e.g. shell repair, pearl and tumour formation, etc. A number of shells contain epibionts and bored, round holes. Presumptive predators of the bivalve community are discussed. Size-frequency distribution of the two most abundant species possibly reflects age classes. The fauna, comprising eleven species, is dominated by deposit-feeders (90 %). They exhibit niche diversification, including at least three different feeding levels within the sediment.     

The Machaeridian Lepidocoleus Sarlei Clarke, 1896, from the Rochester Shale (Silurian) of New York State

Eight new specimens and the holotype of Lepidocoleus sarlei Clarke, 1896 from Lagersta¨tten deposits within the Rochester Shale (Silurian, Wenlock) of New York are described to elucidate the morphology, function and construction of the scleritome. The material exhibits an unusual type of dorsal hinge with a pronounced antero-posterior displacement between the two sides. Two specimens have an aberrant sclerite on the right side of segment eleven. One tightly enrolled specimen may demonstrate a method of defence with the stronger protected dorsal side oriented outwards. The presence of a segmented body plan is suggested, based on requirements for a functional spiral enrolment. Distinct anterior and posterior morphologies together with the pattern of rugae provide valuable help in reconstructing disarticulated lepidocoleids.     

The functional morphology and palaeoecology of the Dinantian brachiopod Levitusia

The functional morphology of Levitusia , based principally upon the type-species L. humerosa (J. Sowerby), is described and compared with living brachiopods. It is concluded that Levitusia species lived in tropical Dinantian seas with agitated waters and at shallow depths, perhaps no more than 50 m. The lithologies in which specimens are found support this view and indicate the need for some revision in our interpretations of some Dinantian sedimentary environments. Further evidence for the synonymy of L. christiani (de Koninck) in L. humerosa is presented.     

Aspects of crinoid palaeontology of the North Esk Inlier,Scotland (Silurian,Llandovery, Telychian)

Abstract: The described fauna of well‐preserved Llandovery (Telychian) echinoderms from the North Esk Inlier, including six crinoids, one echinoid and seven starfish species, is mainly allochthonous. Most of these taxa are known only from starfish beds, channel fill deposits probably representing submarine mass flows and preserving a biota probably derived from elsewhere, presumably shallower water. Only one crinoid species, Pisocrinus cf. campana Miller, is recognized as a common fossil away from the starfish beds and is a biostratigraphic marker for the base of the Wether Law Linn Formation, forming part of the Skenidioides‐Cyrtia Association. Crinoid columnals preserved perpendicular to bedding (that is, in putative life position) in Lamont’s bivalve bed, Deerhope Formation, are tentatively interpreted as being in situ by comparison with a similar occurrence in the Silurian of Arisaig, Nova Scotia. Two new species of crinoid are described, the cladid Dendrocrinus? sp. and the columnal morphospecies Pentagonocyclicus (col.) lamonti sp. nov.     

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.     

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.     

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.     

A sequence stratigraphical model for the Late Ludfordian (Silurian) of Gotland,Sweden: implications for timing between changes in sea level,palaeoecology, and the global carbon cycle

This paper investigates a time interval within the Late Ludfordian (Late Silurian), involving changes in faunal composition (the Lau Event), a major positive carbon isotope excursion (CIE), and contemporaneous sea-level changes in remote palaeo-basins. Based on the Silurian strata of Gotland (Sweden), we integrate sequence stratigraphy, carbon isotope stratigraphy, and platform-scale palaeoecological changes associated with this turbulent time period in Earth history. Three depositional sequences (sequences Nos. 1–3), including two separate periods of forced regression (falling stage systems tracts, FSSTs) are identified from outcrop and drillcore studies. The sequence stratigraphical framework is interpreted to reflect glacio-eustatic sea-level changes. The CIE starts at the onset of the initial FSST (sequence No. 1), just below the last appearance datum of the conodont Polygnathoides siluricus. The values increase through the ensuing lowstand and transgressive systems tracts (LST and TST) of sequence No. 2 and peak in the following highstand systems tract (HST). A second forced regression (FSST of sequence No. 2) took place in the lower Ozarkodina snajdri Zone. δ¹³C data are scarce from these siliciclastic strata, but inferably remain high. The δ¹³C values increase within the LST and earliest TST of sequence No. 3, before a decreasing trend starts within the early TST. δ¹³C values return to pre-excursion levels within the ensuing HST. The CIE is closely associated with an increase in stromatolites (mats and oncoids) across a wide range of depths and sedimentary environments, and correlations to other basins indicate a global increase in cyanobacterial activity. A drastic decline in level-bottom benthic faunas during the FSST of sequence No. 2 is, however, interpreted as a local response to the progradation of a delta complex (the Burgsvik Sandstone). Biological carbonate production replenishes rapidly within the TST of sequence No. 3, succeeding a thin LST dominated by reworked siliciclastics and chemically precipitated carbonates (ooids). The detailed relationship between the CIE and sea-level change presented herein is not fully consistent with previous reports on the CIEs associated with the lower Silurian Ireviken and Mulde events, respectively. Based on our facies analysis and sequence stratigraphical interpretation, two main mechanisms are suggested as responsible for the Late Ludfordian CIE: (1) a change in the riverine C-weathering flux towards the ¹³C end member following glacio-eustatically induced subaerial exposure of carbonate platforms throughout the tropics, and, (2) increased photosynthetic activity by benthic cyanobacteria exaggerating the δ¹³C values of precipitated carbonates.