Silurian Forms Research Articles

Morphology and reconstruction of the retiolitines: Silurian graptolites of the Paraplectograptus lineage (Graptolithina)

The long ranging Silurian retiolitines belonging to the Paraplectograptus lineage (Spirograptus turriculatus to Cyrtograptus/lundgreni Biozone), represented by the genera Paraplectograptus Bouček & Münch, 1952 and Pseudoplectograptus Obut & Zaslavskaya, 1983 are described in detail based on well preserved, three-dimensional material from the Canadian Arctic and Poland. For the first time, reconstructions of their structure are attempted, and comparisons are made with the related genera, Stomatograptus Tullberg, 1883 and Retiolites Barrande, 1850. Ultrastructural details vary somewhat between the genera: the bandaging on the Paraplectograptus lineage and Pseudoplectograptus has a characteristic pustulose ornament, while that of Retiolites and Stomatograptus shows a surface of longitudinal striations. Prosicular preservation is generally incomplete, represented merely by the prosicular apex and prosicular rim. The thecal part of the Paraplectograptus and Pseudoplectograptus tubaria (rhabdosomes) possesses transverse rods, connecting rods, lateral apertural rods and lips. The ancora sleeve lists are variably developed but lack a dorsal/zigzag list, which is present in Stomatograptus and Retiolites. The Paraplectograptus lineage appears to mark an intermediate stage in the evolution of the retiolitines, from the early Silurian forms, with strong thecal and ancora sleeve frameworks (e.g. Retiolites, Stomatograptus), to late Silurian descendants with strong ancora sleeves, minimal thecal frameworks and pustular ornament, with no transverse rods.

Palynology and isotope geochronology of the Upper Ordovician–Silurian successions (Ghelli and Soltan Maidan Formations) in the Khoshyeilagh area, eastern Alborz Range, northern Iran; stratigraphic and palaeogeographic implications

Surface samples from the Ghelli and Soltan Maidan Formations in the Khoshyeilagh area of the eastern Alborz Range, northern Iran, were analyzed to determine the age and the stratigraphic relationship of these two units. The samples contained rich palynomorph assemblages, dominated by acritarchs (36 species recognized, distributed among 28 genera) and chitinozoans (nine species identified, distributed among seven genera). Cryptospores and scolecodonts as well as a few graptolite remains were also observed, although not studied in detail. Based on the restricted stratigraphic range of chitinozoan and acritarch species, a Late Ordovician (late Katian–Hirnantian) age is assigned to the Ghelli Formation. Palynological samples from the Soltan Maidan Formation yielded acritarch assemblages characterized by species commonly found in Upper Ordovician sediments together with typical middle Silurian forms. Considering reworking of the Upper Ordovician species, the age of the investigated part of the Soltan Maidan Formation is not younger than Gorstian (early Ludlow, early late Silurian). U–Pb zircon ages of 434.4 ± 6.4 Ma (Telychian, late Llandovery, late early Silurian) obtained for granitic clasts collected at the base of the Soltan Maidan Formation, are consistent with the inferred palynological age and indicate that granite emplacement, cooling, exhumation, erosion, transportation and deposition of its clasts took place in a time interval estimated to be 5 to 10 million year long. Based on the presence of diagnostic chitinozoan taxa, the Ghelli Formation can be assigned to the Armoricochitina nigerica, Ancyrochitina merga, and Spinachitina oulebsiri chitinozoan biozones. Chitinozoan assemblages reflect a clear palaeobiogeographic affinity with the previously defined ‘North Gondwana Domain’. The composition of acritarch assemblages also appears to be consistent with newly proposed hypotheses of a Late Ordovician phytoplanktonic biogeographical differentiation between a Laurentian/Baltica realm, and a Gondwanan realm to which the present assemblages belong. The presence of cryptospores together with taxonomically diverse chitinozoan, acritarch, and scolecodont assemblages, suggest a relatively shallow marine, platformal depositional environment for the Ghelli Formation.

Placoderms (Armored Fish): Dominant Vertebrates of the Devonian Period

Placoderms, the most diverse group of Devonian fishes, were globally distributed in all habitable freshwater and marine environments, like teleost fishes in the modern fauna. Their known evolutionary history (Early Silurian–Late Devonian) spanned at least 70 million years. Known diversity (335 genera) will increase when diverse assemblages from new areas are described. Placoderms first occur in the Early Silurian of China, but their diversity remained low until their main evolutionary radiation in the Early Devonian, after which they became the dominant vertebrates of Devonian seas. Most current placoderm data are derived from the second half of the group's evolutionary history, and recent claims that they form a paraphyletic group are based on highly derived Late Devonian forms; 16 shared derived characters are proposed here to support placoderm monophyly. Interrelationships of seven placoderm orders are unresolved because Silurian forms from China are still poorly known. The relationship of placoderms to the two major extant groups of jawed fishes—osteichthyans (bony fishes) and chondrichthyans (cartilaginous sharks, rays, and chimaeras)—remains uncertain, but the detailed preservation of placoderm internal braincase structures provides insights into the ancestral gnathostome (jawed vertebrate) condition. Placoderms provide the most complex morphological and biogeographic data set for the Middle Paleozoic; marked discrepancies in stratigraphic occurrence between different continental regions indicate strongly endemic faunas that were probably constrained by marine barriers until changes in paleogeography permitted range enlargement into new areas. Placoderm distributions in time and space indicate major faunal interchange between Gondwana and Laurussia near the Frasnian-Famennian boundary; closure of the Devonian equatorial ocean is a possible explanation.

The oldest articulated osteichthyan reveals mosaic gnathostome characters

The evolutionary history of osteichthyans (bony fishes plus tetrapods) extends back to the Ludlow epoch of the Silurian period. However, these Silurian forms have been documented exclusively by fragmentary fossils. Here we report the discovery of an exceptionally preserved primitive fish from the Ludlow of Yunnan, China, that represents the oldest near-complete gnathostome (jawed vertebrate). The postcranial skeleton of this fish includes a primitive pectoral girdle and median fin spine as in non-osteichthyan gnathostomes, but a derived macromeric squamation as in crown osteichthyans, and substantiates the unexpected mix of postcranial features in basal sarcopterygians, previously restored from the disarticulated remains of Psarolepis. As the oldest articulated sarcopterygian, the new taxon offers insights into the origin and early divergence of osteichthyans, and indicates that the minimum date for the actinopterygian-sarcopterygian split was no later than 419 million years ago.

A Middle Ordovician cephalopod fauna from Cuzco Province, southern Perú and its palaeobiogeographical significance

AbstractA small cephalopod assemblage collected during the 1930s by J. A. Douglas from the Middle Ordovician San José Formation of Cuzco Province is redescribed. Although small in number (four taxa) and poorly preserved, this assemblage contains a representative of the ellesmerocerid family Eothinoceratidae and a probable member of the Arionoceratidae (Orthocerida). One taxon may be closely related to Protocycloceras harringtoni Cecioni from northern Argentina. The presence of a particular group of eothinoceratids, here and elsewhere in South America indicate a link with western Gondwana. The relatively diminutive size of the arionoceratids suggests homeomorphy with those Silurian forms associated with a pelagic habitat and indicates a relatively offshore site for this assemblage. Copyright © 2006 John Wiley & Sons, Ltd.

OBITUARY: W. H. C. (Bill) RAMSBOTTOM (1926–2004)

Bill Ramsbottom, one of the greatest names in Carboniferous geology of the 20th century, not just in Britain but throughout the world, died at Kirby Malzeard in Nidderdale on the 31st October, 2004. Bill was born in Salford in 1926, but grew up in Yorkshire, a county that he loved. He graduated from Clare College, Cambridge, in 1947, and in the same year joined the Geological Survey in London. He completed his doctoral dissertation on Lower Palaeozoic crinoids at Chelsea Polytechnic College (University of London) in 1954. A monograph of Ordovician crinoids was duly published, but although some of the results of his work on Silurian forms also appeared in several journals, much important work on echinoderms from this period remained unpublished at the time of his death. He moved to the Leeds Geological Survey Office in 1958, and purchased a house in Collingham, near Wetherby, close to the home of W. S. Bisat, F.R.S., the pioneering worker on the Carboniferous ammonoids of northern England and their use in bio- and chronostratigraphy. This coincidental but happy juxtaposition of great minds proved of particular benefit to Bill since, because of the requirements of his official duties, the principal objects of his researches had changed from Lower Palaeozoic echinoderms to Carboniferous stratigraphy and biostratigraphy. As a …

Early terrestrial arthropods: a fragmentary record

The earliest unequivocal terrestrial fossils are uppermost Silurian (Přídolí) myriapods, presumed to be pioneer decomposers. Descendants of their marine ancestors may be discernible in the Cambrian lobopodAysheaia(recently challenged). Known euthycarcinoids are too late except as survivors from such a lobopod ancestry. Unique arthropods are also known from Cambrian and Ordovician lake deposits, but shed no light on origin of terrestrial forms. Among relevant arthropods, only scorpions have a continuous record from Silurian aquatic to Devonian terrestrial records (eurypterids are dealt with elsewhere in this volume). Respiratory organs are unknown in Silurian forms, but at least three types of gills may have existed by the Devonian, arguing for great diversification by then, and possible multiple colonization of the land. Trace fossils suggest some Devonian scorpions were amphibious, while development of a pre-oral tube indicates they were adapted for feeding out of water. Morphology of Silurian forms implies they were aquatic, solid feeders. The presence of Silurian fungivorous microarthropods is indicated by Ludlow faecal pellets containing fungal hyphae. The terrestrial decomposer niche was occupied, and soils therefore in existence, by then. Constant humid microclimates created by early land plants were important in enabling land colonization by arthropods otherwise susceptible to desiccation. Mamayev’s gravitational hypothesis provides an explanation for evolution of hexapody from uniramians clambering up and over early land plants. Plants of the Siegenian Rhynie Chert show pathological features that may indicate terrestrial plant-animal interaction by that time, as do other lines of circumstantial evidence. Energy costs incurred by plants producing anti-herbivore and anti-detritivore defences have therefore probably been significant in ecosystems since the Devonian. Recent major backwards extensions in time of terrestrial arthropods prove that the terrestrial geological record is still poorly sampled. Givetian Gilboa shows that, as with early plants, much can be hoped for from study of cuticle fragments. New finds should be sought in pre-Devonian equivalents of the Carboniferous Mazon Creek delta plain facies.

Silurian of Great Basin: ABSTRACT

The Silurian rocks in the Great Basin can be assigned to two distinct facies: an eastern carbonate facies and a western shale facies. The line separating the two facies, actually a narrow transition zone, trends northeast from Independence, California, through Beatty, Eureka, and Elko in Nevada. The rocks in the eastern carbonate facies are composed almost entirely of dolomite. The dolomite is heterogeneous, varying from fine- to coarse-grained, black to white, and from almost pure dolomite to very silty dolomite. The dolomite facies is one of the poorest systems for paleontological studies in the Great Basin. Fossils are scarce and where present are usually poorly preserved. A few localities yield silicified fossils that can be etched and studied in detail. Both Middle and Upper Silurian fossils are present in the dolomite. The faunas seem to be most closely allied to those of Australia and the Orient, and have little similarity to the Silurian forms in eastern North America. No Lower Silurian fossils have been End_Page 354------------------------------ recognized. Early references to Lower Silurian faunas in the Great Basin are Lower Silurian in the old European sense, that is, Ordovician. The rocks in the western facies are predominantly black shale with minor amounts of sandy and calcareous shale and rare interbeds of limestone. The fauna in the western shale facies is largely pelagic. Locally well preserved graptolites range from Middle to Late Silurian in age. The Upper Silurian shales locally contain eurypterids. End_of_Article - Last_Page 355------------

Fishes of the Jefferson Formation of Utah

In the lower part of the Jefferson series (Devonian) of northeastern Utah a rich fish fauna was found in 1928. The study of the fossils is far from complete, but six genera have been recognized. Dinichthys, Ptyctodus, and Aspidichthys are genera which range through Middle or Upper Devonian. Glossoidaspis, Cardipeltis, and Camptaspis are new genera closely related to Upper Devonian, Lower Devonian, and Silurian forms. A complete study of the fish fauna may furnish data for correlation of the Jefferson series.

In Defence of Bothriocidaris

Two rival hypotheses as to the origin of the Echinoidea are prevalent to-day. One of them postulates as the primitive form an organism built of a few large plates arranged in vertical series; the other demands that the earliest types should have been “scaly”, with many small plates which were regular only along the radii. Those who hold the former hypothesis see in the Ordovician Bothriocidaris a close approximation to their ideal; supporters of the latter claim that the Silurian genera Echinocystis and Palaeo1discus represent fairly well the ancestral qualities of the class. So long as Bothriocidaris is regarded as a true Echinoid, the Silurian forms lose much of their claim to be considered truly primitive; the chronological sequence is against them.

New Reconstruction of Lasanius

Introduction The earliest vertebrates of which we possess a workable knowledge occur in the Silurian, the Ordovician remains being extremely fragmentary. Of the Silurian forms none seem to be more primitive than the Anaspida, and among anaspidans the least specialized type is generally considered to be Lasanius, which, therefore, is of extreme interest as apparently the most primitive known fossil vertebrate. Traquair has described two species of this remarkable genus from the Downtonian of Lanarkshire, in Scotland— Lasanius problematicus and L. armatus .[3][1] Both are of small size, from two to fifteen centimeters in length, if reconstructions from fragments may be trusted for the larger figure. The common form, L. problematicus, is usually about five centimeters long. In his earlier paper (1898) Traquair published an outline restoration of the latter species, this being the reconstruction usually met in text-books. In 1905 he altered this slightly, making the animal abruptly larger back . . . [1]: #fn-1

VII.—The Classification of the Phacopidæ

In North and South America and in South Africa there is a considerable variety of types of the Dalmaniles-branch in the Devonian period which have in many cases received distinctive subgeneric names; but species retaining the typical characters of the Silurian forms persist at any rate in North America. Many of these Devonian forms show incomplete second lateral furrows on the glabella, these furrows not reaching the axial furrows and causing a partial coalescence of the two middle lateral lobes. This tendency towards the fusion of the first and second lateral lobes of the glabella is a departure from the perfect segmentation found in typical Silurian members of Dalmanites, and has caused Clarke to group all such forms together into the subgenus or section Synphoria. This type of structure, as Van Ingen has recently shown, is not unknown amongst the Silurian species of Dalmanites in America, but it finds its most pronounced development in Devonian time and occurs in the groups Coronura, Corycephalus, Odontocephalus, and Probolium, all of which are put by Clarke in the section Synphoria. The marginal ornamentation and different processes on the pygidium and head-shield on which these four groups have been founded are scarcely of the same structural importance as the modifications of the glabellar segmentation. As in other families, the spinosity of these forms is the symbol of a last expiring effort before extinction.

The British Silurian Species of Acidaspis

The genus Acidaspis has been a peculiarly unfortunate one in Britain. Several of the specific names which are in common use are manuscript terms, or but little better; and the species to which they are applied have never been described. It is impossible, therefore, without access to standard collections, to determine to what forms the names refer. Even the species which have been described have in many cases been imperfectly figured, and the result is endless confusion. The common English trilobite, A. coronata , has received abroad no less than three names, all of them different from ours; while in England, on the other hand, the foreign name A. crenata is often applied to a species which is quite distinct from the original A. crenata , and which in fact has never yet been found out of Britain. The disorder is worst among the Silurian forms, although these are much the most perfect. The Ordovician species are usually fragmentary, but the fragments have been fully described. It is the object of the present paper to attempt to reduce the specific terminology to some sort of order, and to rescue the common manuscript names from the obscurity in which such terms tend to become involved after a lapse of time. Only the Silurian forms are here described; the Ordovician species have been left in the hope that better material may be forthcoming in the future. Even the name of the genus itself is matter of controversy. Murchison employed the term Acidaspis in 1839

On the Upper Silurian Passage-beds at Linley, Salop

A zone of thin-bedded, often conglomeratic, sandstones, forming the passage-bed between two great formations, is frequently found to include within its series of deposits one or more hands of shell- or bone-breccia. Perhaps the most important of these zones is the one which lies between the mudstones of the Upper Silurian series and the lowest Old Red Cornstone, the most northerly extension of which forms the subject of this paper. This, from the number and variety of its organic contents, may almost be called a formation by itself. The continuance of Silurian forms of life is, however, too strongly marked to permit any division of it from that great series of deposits. On the other hand, its uppermost layers are allied, by physical constitution and fossil contents, so nearly with the overlying Old Red, that, although this new exposure at Linley adds to our knowledge of the series, it does not, in our opinion, enable us to draw a more definite boundary-line between the Silurian deposits and the Old Red Sandstone. In commenting generally upon these passage-beds, we cannot choose a more fitting preface than the following passage from ‘Siluria’:— “During the accumulation of nearly all the Silurian deposits of Britain, which were characterized by a certain fauna, the bottom of the sea was, to a wide extent, occupied by dark- and grey-coloured sediments. At the close of that period a great change occurred, over large areas, in the nature and colour of the submarine detritus. In and around the