Derek J. Siveter

Brood care in a Silurian ostracod

An exceptionally preserved new ostracod crustacean from the Silurian of Herefordshire, England, preserves eggs and possible juveniles within its carapace, providing an unequivocal and unique view of parental brood care in the invertebrate fossil record. The female fossil is assigned to a new family and superfamily of myodocopids based on its soft-part anatomy. It demonstrates a remarkably conserved egg-brooding reproductive strategy within these ostracods over 425 Myr. The soft-tissue anatomy urges extreme caution in classifying ‘straight-hinged’ Palaeozoic ostracods based on the carapace alone and fundamentally questions the nature of the shell-based Palaeozoic ostracod record.

The Cambrian Fossils of Chengjiang, China

The Cambrian fossils of Chengjiang, China , The Cambrian fossils of Chengjiang, China , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

New evidence on the anatomy and phylogeny of the earliest vertebrates

We report the discovery of a new agnathan specimen from the Lower Cambrian Chengjiang Lagerstätte of China and thereby provide new evidence on the myomeres (V–shaped), the branchial apparatus (gill filaments and arches), the dorsal fin and the gonads (24–26) of the earliest vertebrates. The new specimen and the co–occurring Myllokunmingia fengjiaoa and Haikouichthys ercaicunensis represent a single species, which is a primitive member of the crown group craniates (vertebrates) and post–dates the origin of the myxinoids (hagfish). The origin of the vertebrate clade is at least as old as Early Cambrian.

The arthropod Offacolus kingi (Chelicerata) from the Silurian of Herefordshire, England: computer based morphological reconstructions and phylogenetic affinities

The small, non–biomineralized, three–dimensionally preserved arthropod Offacolus kingi Orr et al. from the Wenlock Series (Silurian) of Herefordshire, England, is re–evaluated, and the new family Offacolidae erected. This new study is based on specimens which have been serially ground, reconstructed by computer and rendered in the round as coloured models. Offacolus possesses a prosomal appendage array similar to that of Limulus, but also bears robust and setose exopods on appendages II–V which are unlike those found in any other arthropods. Opisthosomal appendages are similar in number and morphology to the book–gills of Limulus. Cladistic analysis places Offacolus basally within the Chelicerata, as a sister taxon to the eurypterids and extant chelicerates, but more derived than the Devonian Weinbergina.

A Silurian sea spider

Pycnogonids (sea spiders) are marine arthropods numbering some 1,160 extant species. They are globally distributed in depths of up to 6,000 metres, and locally abundant; however, their typically delicate form and non-biomineralized cuticle has resulted in an extremely sparse fossil record that is not accepted universally. There are two opposing views of their phylogenetic position: either within Chelicerata as sister group to the euchelicerates, or as a sister taxon to all other euarthropods. The Silurian Herefordshire Konservat-Lagerstätte in England (∼ 425 million years (Myr) bp) yields exceptionally preserved three-dimensional fossils that provide unrivalled insights into the palaeobiology of a variety of invertebrates. The fossils are preserved as calcitic void in-fills in carbonate concretions within a volcaniclastic horizon, and are reconstructed digitally. Here we describe a new pycnogonid from this deposit, which is the oldest adult sea spider by ∼35 Myr and the most completely known fossil species. The large chelate first appendage is consistent with a chelicerate affinity for the pycnogonids. Cladistic analyses place the new species near the base of the pycnogonid crown group, implying that the latter had arisen by the Silurian period.

Silurian Bio-Events

In the history of Silurian biota and ecosystem as a whole no “big” catastrophes occurred like the one at the Ordovician-Silurian boundary. Yet it was not a quiet period either. There were established 15 more or less remarkable bio-events, among others the most severe extinction of conodonts and acritarchs in the very beginning of the Wenlock (Ireviken Event), the Great Crisis or lundgreni Event among graptolites in the Homerian and the middle Ludfordian Event comprising many lineages of vertebrates, graptolites, conodonts and corals. Most remarkable diversity rises of the Silurian biota were in the late Rhuddanian, in the Telychian and in the early Gorstian. Both extinctions and originations were in good correlation with the global sea-level curve, but the effect has to be interpreted as an integrated process partly triggered by development of early Silurian glaciation and climate in general.

Three‐dimensional preservation of a non‐biomineralized arthropod in concretions in Silurian volcaniclastic rocks from Herefordshire, England

Three‐dimensional preservation of a non‐biomineralized arthropod occurs in carbonate concretions in a volcaniclastic deposit from the Wenlock Series of Herefordshire, England. Specimens are preserved in calcite that co‐precipitated with framboids and polyhedra of pyrite. The texture of the calcite indicates that it is a void infill. It forms a cast of the external surface of the arthropod, having precipitated after decay of even the most recalcitrant biological tissues. Incorporation of the fossils into concretions ensured their long term preservation but was not, at least in most examples, responsible for preventing potential collapse and occlusion of voids in the interval between the decay of tissues and the precipitation of calcite. The precipitation and/or accumulation of clay minerals adjacent to specimens during decay was important in this process, as were possibly the geotechnical properties of the ash itself. Limited dolomitization of the calcite around the edges of the fossils and in the matrix of the concretion occurred at a later stage.

A new phyllocarid (Crustacea: Malacostraca) from the Silurian Fossil–Lagerstätte of Herefordshire, UK

A new three–dimensionally preserved arthropod, Cinerocaris magnifica, from the Wenlock Series (Silurian) of Herefordshire, UK, is described and assigned to the Phyllocarida (Crustacea). The description and reconstruction are based on specimens that have been serially ground, reconstructed by computer and rendered in three dimensions as coloured virtual models. Cinerocaris magnifica displayed the tagmosis characteristic of phyllocarids, with eight thoracic and seven abdominal somites, terminating in a telson with furca. The remarkable preservation of the appendages makes this the earliest completely known malacostracan crustacean. Two pairs of antennae (the first with two flagella) were followed by a mandible and first maxilla, each with a slender palp–like ramus. The second maxilla consisted of a limb stem with endites and an endopod that tapered distally. There was no exopod. The thoracopods comprised a limb stem with six or seven endites, an arrangement previously known only in entomostracans, and an endopod with about five endites. Flap–like outer rami correspond to an exopod and epipods. The pleopods bore two long slender oar–blade–like rami. Cladistic analysis places C. magnifica as a plesion within the Echinocaridina. It provides critical evidence of the limb morphology of an early malacostracan, which will be important in understanding crustacean evolution.

A Silurian armoured aplacophoran and implications for molluscan phylogeny

The Mollusca is one of the most diverse, important and well-studied invertebrate phyla; however, relationships among major molluscan taxa have long been a subject of controversy. In particular, the position of the shell-less vermiform Aplacophora and its relationship to the better-known Polyplacophora (chitons) have been problematic: Aplacophora has been treated as a paraphyletic or monophyletic group at the base of the Mollusca, proximate to other derived clades such as Cephalopoda, or as sister group to the Polyplacophora, forming the clade Aculifera. Resolution of this debate is required to allow the evolutionary origins of Mollusca to be reconstructed with confidence. Recent fossil finds support the Aculifera hypothesis, demonstrating that the Palaeozoic-era palaeoloricate ‘chitons’ included taxa combining certain polyplacophoran and aplacophoran characteristics. However, fossils combining an unambiguously aplacophoran-like body with chiton-like valves have remained elusive. Here we describe such a fossil, Kulindroplax perissokomos gen. et sp. nov., from the Herefordshire Lagerstätte (about 425 million years bp), a Silurian deposit preserving a marine biota in unusual three-dimensional detail. The specimen is reconstructed three-dimensionally through physical–optical tomography. Phylogenetic analysis indicates that this and many other palaeoloricate chitons are crown-group aplacophorans.

Carbon isotope excursion near the Wenlock-Ludlow, (Silurian) boundary in the Anglo-Welsh area

Oxygen and carbon isotope analyses have been performed at close sampling intervals on rocks from several exposures that show the boundary between the Wenlock and Ludlow Series in the classic Silurian areas of Wales, the Welsh Borderland, and the West Midlands of England. Our data indicate a monotonic decline in δ 13 C from positive to negative values across the boundary between the Much Wenlock Limestone Formation (Wenlock Series) and the Lower Elton Formation (Ludlow Series), and correlatives, in all sites investigated. A slightly earlier negative excursion is also present in the upper Wenlock strata of two of these localities. Both negative δ 13 C excursions appear to be contemporaneous with episodes of increased ocean ventilation and eustatic sea- level lowstand postulated by Kemp. At least one, and possibly both, of the late Wenlock carbon isotope depletions can also be related to the decline in pelagic graptolite diversity. The distribution of the carbon isotope depletion in both carbonate-platform and off-shelf facies implies that this trend is unlikely to have been caused by local diagenetic alteration. Isotope chemostratigraphy may also provide an extra correlation tool that can be integrated with traditional biostratigraphic methods for Silurian strata.