Family Lycophoriidae (Brachiopoda) from the Ordovician of Baltoscandia

The family Lycophoriidae from the Middle Ordovician of Baltoscandia with one monotypic genus and five species is revised. New inner structures were revealed and diagnoses of the family, genera, and species were specified in the study of cross sections and microstructure of the shell. The third prismatic layer, complex dental plates, and socket ridges with short pointed lateral outgrowths were recorded. The family Lycophoriidae cannot be assigned with confidence to any known brachiopod order.     

AnOistodus venustus-like conodont species from the Middle Ordovician of Baltoscandia

Ordovician conodont specimens resemblingOistodus venustus Stauffer, 1935 have been reported from many areas. There is increasing evidence, however, that several lineages with homeomorphic conodont elements have erroneously been referred to one and the same species. I have investigated Baltoscandian conodont elements of this kind in order to find out about their origins and phylogenetic relationships with morphologically comparable elements from other areas. A natural grouping of finds from the Middle and lower Upper Ordovician of Baltoscandia is here described as belonging to a new species of a new genus,Venoistodus balticus n. gen., n. sp. The new species probably evolved paedomorphically fromDrepanoistodus forceps (Lindström, 1955) in the Early Ordovician.     

Biodiversity curves for the Ordovician of Baltoscandia

Biodiversity curves for the Ordovician of Baltoscandia show a substantial increase in taxonomical diversity through the period, as seen also in global data sets. A database of 10,340 records of first and last appearances of species at different localities in the region has been analysed using simple species counts, and partly validated with resampling methods. While the biodiversity curve for all fossil groups combined is probably reasonably accurate except for an unknown scaling constant, taxonomical or geographical subsets may not be sufficiently well sampled to allow precise measurement of their species counts through time. The analysis shows a major diversification event commencing in the middle Arenig (Ibex-Whiterock boundary), and more limited diversification events in the Llanvirn, Caradoc and Ashgill. The Scandinavian (Norwegian and Swedish) biodiversity curves are broadly correlated with major changes in sea level, with low biodiversity at highstands and high biodiversity at lowstands, although this pattern is not clear for all fossil groups. In the Arenig, graptolites and trilobites appear to have higher diversities at high sea levels, while the brachiopods and ostracodes show higher diversities at low sea level. As a consequence, the Arenig diversification is delayed for the latter two groups until the upper end of the interval.     

Morphology,ontogeny and affinities of the Hirnantian triplisiid brachiopod Streptis undifera from Baltoscandia

New silicified topotypic material of the Upper Ordovician Streptis undifera (Schmidt, 1858) from the stratotype of the Porkuni Regional Stage in Estonia provides important data on triplesiid morphology and ontogeny, which has substantial implications for our understanding of the affinity of this group of brachiopods. In particular, the new material shows that the early ontogeny of Streptis includes evidence for a cicatrix attachment and colleplax‐like structure in the ventral valve. It is likely also that a lecithotrophic feeding habit evolved in triplesiidines sometime in the Ordovician, which sets them apart from all other known strophomenates. A neotype is selected among the specimens of Streptis undifera from the type locality in Porkuni. In Baltoscandia, Streptis undifera appears first in Norway in late Katian below the strata with the pentamerid brachiopod Holorhynchus. In Estonia, Streptis undifera is an index species of the early Hirnantian and occurs in association with stromatoporoid‐coral reefs and related inter‐reef deposits, which overly the strata with Holorhynchus. In Baltoscandia, the distribution areas of Streptis and Holorhynchus are more or less identical in spite of some differences in age.     

Aspects of carbonate sedimentation in the Ordovician of Baltoscandia

In the Baltoscandian epicontinental sea the normal type of Ordovician carbonate sediments was a mixture, in various proportions, of skeletal sand and carbonate mud. In some marginal areas sediments of Bahaman type were deposited during the latest part of the Viruan Epoch, and such sediments were widely distributed from the very latest Ordovician onwards. It is suggested that deposition mostly took place in a temperate climatic zone and that the bahamitic deposits indicate an increase of temperature to subtropical or tropical level.
The succession of the Ordovician lithofacies belts in Baltoscandia is mostly roughly symmetrical with regard to grain size, but asymmetrical as to the distribution of terrigenous mud. The position of the boundary between the terrigenous and carbonate mud belts may have been a result of competition in the material transport from the west and from the east. The direction of the lithofacies belts is not parallel to the axis of the Caledonian geosynclinal zone. The predominance of carbonate sediments in the Baltoscandian Ordovician epicontinental sea probably does not reflect particularly good conditions for production of carbonate material, but rather, a very low supply of terrigenous matter.
In the Central Baltic area changes of sea level were probably largely opposite to those in Västergötland. This indicates differential isostatic movements in the Baltoscandian cratonic shield during parts of the Ordovician Period.     

A calcichordate interpretation of the new mitrateeumitrocystella savilli from the ordovician of Morocco

A new mitrate (stem-group craniate sensuJefferies 1979) is described from the Ordovician (Llandeilo) of Morocco and an anatomical and functional interpretation on the basis of the calcichordate theory is given. It is characterized by its small size, marked asymmetry of the anterior part of the head, and extensive resorption of the skeleton.     

Middle Ordovician cephalopod biofacies and palaeoenvironments of Baltoscandia

During the Middle Ordovician cephalopods became an important part of the macrofauna of the Baltoscandian carbonate platform. The earliest cephalopod abundance peak was reached during the early Darriwilian, within the Kunda Stage Yangtzeplacognathus crassus and Lenodus pseudoplanus conodont zones. In sediments of this time interval large orthoconic cephalopods often occur in masses with more than one specimen per square‐meter on bedding surfaces. The assemblages are characterized by the strong dominance of often large endocerids. In proximal depositional settings coiled tarphycerids and other cephalopod groups are an important additional component. In the most distal settings orthocerids are the most important secondary component. Correspondence Analysis of assemblages throughout Baltoscandia revealed three distinct biofacies, which here are termed Orthocerid, Proterovaginoceras and Anthoceras Biofacies, respectively. The biofacies reflect differences in depth and proximity to the shoreline and are consistent with the Baltoscandian Confacies Belts. Spatial changes in absolute abundance and taxonomic composition indicate increased original cephalopod population densities and habitat expansion within the Y. crassus and L. pseudoplanus conodont zones. A nearly coeval abundance peak in a similar facies in South China indicates supraregional causes of the mass occurrence, probably reflecting a globally increased nutrient availability in the water column during the Darriwilian.     

Early ordovician reefs from Argentina: Stromatoporoid vs stromatolite origin

Summary Late Arenigian biohermal reef mounds and biostromes within the shallow-marine platform facies of the upper San Juan Formation of the Precordillera (Western Argentina) represent a new Early Ordovician reef type. The meter-sized reefs are dominated byZondarella communis n.g. n. sp. The new taxon is characterized by domical, bulbous and laminar morphotypes exhibiting growth layers and thin horizontal and vertical as well as intermingled skeletal elements included within different sets. The fossil maybe compared with stromatolites and stromatoporoids but an interpretation as primitive stromatoporoids is favoured.     

Species-diversity patterns in some middle ordovician communities from California - Nevada

Certain fossil communities may be recognized in collections from Early—mid-Middle Ordovician age rock units in the Inyo Mountains, California, and the Bare and Ranger Mountains in adjacent Nevada. These dominantly brachiopod—tribolite communities lived in several different, shallow shelf-sea environments. The communities appear to have maintained their diversity and evenness within relatively narrow limits for as long as the major aspects of the environments in which they lived, remained little changed (periods of about 1–2 m.y.). Some taxa occur in several communities (and are called wide-niched), and some occur in only a single community (and are called narrow-niched). Certain taxa that occur in large numbers in a single community appear to have been far more tolerant of particular environmental conditions than were associated taxa in the same community that are represented by few individuals. Counts of such abundant taxa that are restricted to a single community (and are thus considered narrow-niched) compared with counts of the individuals of those taxa found in several communities (and thus are considered wide-niched) indicate that the taxa highly tolerant of particular environmental conditions may be represented by more individuals than are the wide-niched taxa.     

Morphology,faunas and genesis of ordovician hardgrounds from Southern Ontario,Canada

We have used associations of different microfacies to define facies (or microfacies associations) which form reasonably well-defined sequences, which we infer, from analogies with recent and ancient carbonate environments, to have been deposited in a shelf environment characterized by small-scale topographic differentiation into shoal, slope and basinal environments.Shoal environments are characterized by typically cross-bedded, well-sorted bioclastic sands, with intershoal areas consisting of interbedded bioclastic sands and heavily bioturbated finer-grained carbonates.Slope and “basinal” environments are typically represented by “proximal” and “distal” cycles respectively. These we compare with deposits of carbonate ramp bypass channels, and with the more thoroughly studied deep-water clastic submarine fans. Many of the strong variations in environmental energy in these proximal and distal cycles can be attributed to migration of channels on the fans and the effect of funnelling of storm surges down the channels.Although hardground morphology and faunas are mostly related to local effects such as intensity of scouring, time of exposure, topographic differentiation of the surface and other factors, differing hardground types tend to be found in different environments. Smooth and rolling hardgrounds occur in the deeper distal environments, where the beds were subject to only slight scour and often limited exposure before renewed sedimentation. Hummocky and undercut hardgrounds are characteristic of the middle parts of proximal cycles, where they developed marginally to the main bypass channel, and in intershoal areas. Both these areas are sites of intermittent sedimentation and moderate turbulence, where cemented beds may be exposed for some time in environments optimal for attached benthos. These hardgrounds usually contain the most diverse hardground biotas. Pebbly and reworked hardgrounds occur in coarse, basal units of proximal cycles, which are interpreted as the grain-flow fillings of the central parts of bypass channels, though isolated examples occur in intershoal areas and in the higher parts of proximal channels. These hardgrounds contain low-diversity faunas, reflecting the stresses imposed by intermittent or constant abrasion; though some contain more diverse faunal assemblages formed after redeposition.     

Phylogenetic relationships of Early–Middle Ordovician ostracods of Baltoscandia

Phylogenetic analysis of the Early and early Middle Ordovician (Tremadoc and Arenig) ostracod species of Baltoscandia suggests a polyphyletic origin for the suborder Beyrichiocopa. Binodicopes, leiocopes and eridostracans are excluded from the beyrichiocopide clade. An independent origin from the basal ostracods is suggested for the binodicopes and eridostracans. The palaeocopes form a strongly supported monophyletic clade. Within this suborder, the ctenonotellid and the tetradellid families together form a monophyletic clade. The tetradellids are paraphyletic, being a stem-group for the ctenonotellids. Nanopsis nanella , the earliest known ostracod from the Tremadoc, is a basal palaeocope. The early eridostracans Conchoprimitia and Incisua , with their uncomplicated carapace morphology, might be the most primitive ostracods.     

Selenopeltis (Trilobita) from Brittany and its distribution in the ordovician

The distribution of the genus SelenopeltisHawle & Corda throughout the Selenopeltis Province shows the presence during the Arenig of northern and southern species «groups, and a Llandeilo migration of the latter is identified within the Median Armorican synclinorium. New material of S. macrophthalmus (Kloucek) is described from Brittany together with S. gallicus gallicus nov. sp., nov. subsp. and S. gallicus irroratus nov. sp., nov. subsp.     

The type species of the ordovician trilobitesymphysurus: systematics,functional morphology and terrace ridges

The type of the widespread Ordovician trilobiteSymphysurus, S. palpebrosus Dalman, 1827, is redescribed from the type Swedish material; its distribution is documented.Symphysurus is one of very few trilobite genera to be found in both Ordovician Baltica and Gondwana, in more peripheral sites relative to the platform areas.S. palpebrosus has complex coaptative devices to ensure tight enrollment. Analysis of the functional morphology shows that it spent at least part of its life with thorax and pygidium buried in the sediment in the bumastoid stance, although it may have fed outside its burrow. Terrace ridges are well developed on certain parts of the cuticle; their positioning and geometry shows that only a few of them could have functioned in direct engagement with the sediment. Those associated with petaloid thoracic facets may have permitted respiration in the enrolled condition.     

The oldest bryozoans of Baltoscandia from the lowermost Floian (Ordovician) of north-western Russia: two new rare,small and simple species of Revalotrypidae

We describe two new species of esthonioporate bryozoans, Revalotrypa inopinata sp. nov. and R. yugaensis sp. nov., from the Lower Ordovician, south-east of Lake Ladoga, north-western Russia. The colonies of the two species are very small and were extracted from limestone nodules found in the lower part of glauconitic sandstone in the Joa Member (lowermost Floian). Revalotrypa inopinata sp. nov. and R. yugaensis sp. nov. are the oldest known bryozoans from Baltoscandia. Study of the two species includes scanning electron microscopy (SEM) imaging and X-ray microtomography (micro-CT). The palaeogeographic distribution of the oldest known bryozoans from the Lower Ordovician and the advantages and disadvantages of micro-CT for the study of Palaeozoic bryozoans are discussed. The authorship of Revalotrypidae, which has been inconsistently allocated in the literature, is discussed.     

The gonorynchiform fish Dastilbe from the lower Cretaceous of Brazil

We examine the preservation, autecology and morphological variation for several characters of the Cretaceous gonorynchiform fish Dastilbe from the Lower Cretaceous of Brazil and Africa. More than 83 specimens were examined. We test species validity using characters of the caudal endoskeleton and meristic counts of fin-rays vs length. Evidence provided by fossilized soft tissues and slabs containing large individuals 'freeze framed' in the process of swallowing smaller prey meals, show that Dastilbe was predatory, at least as adults, as well as cannibalistic. Dastilbe was probably an anadromous fish tolerant of hypersalinity and in Araripe was subjected to frequent mass mortality events. Observations of the otic region indicate that the lagenar statolith is consistently larger than the saccular statolith, hence revealing a primitive actinopterygian condition. For the first time, a lagenar statolith from Dastilbe has been cleaved to expose putative annuli-like ridges. Our results clearly show that there is a wide degree of morphological plasticity of the endoskeleton coupled with wide meristic variation, and as such, overall length, fin-ray count and even absence or presence of caudal diastema are not suitable criteria for species recognition in Dastilbe . New specimens from the Crato Formation (Aptian) and statistical tests suggest rejection of all species of Dastilbe erected subsequent to Jordan (1910). All Brazilian specimens of Lower Cretaceous Dastilbe can be assigned to the single species D. crandalli Jordan. The African D. batai Gayet is also placed within D. crandalli     

Early growth-stages and classification of orthoceridan Cephalopods of the Darriwillian (Middle Ordovician) of Baltoscandia

Five apices of orthoceridan cephalopods from the early Middle Ordovician Holen Limestone of Öland, Sweden that where collected in the late 19th Century by G. Holm provide information on cephalopod evolution in the early Palaeozoic. The apices belong to specimens of the genus Hedstroemoceras Foerste, 1930 and Archigeisonoceras Chen, 1984. The apices are small in comparison with apices of other cephalopods of the Ordovician; the initial chambers of the shells of both genera are hemispherical and approximately 1 mm and 1.5 mm in cross-section diameter, respectively. The apical 2–3 mm of the shell are free from growth-lines and possess no cicatrix, though distinct longitudinal wrinkles are present. There is a slight variability of siphuncle position during early growth in Archigeisonoceras. It can be shown that the structure of the connecting ring of Hedstroemoceras is similar to that of other Orthocerida. Additionally, the hemispherical apex of Lituites perfectus Wahlenberg, 1821 gives evidence for the orthoceridan affinity of lituitidans. The investigation shows that early Middle Ordovician Orthocerida display a characteristic connecting ring structure, a characteristic apex morphology and variable siphuncular positions that differs significantly from other cephalopods of the Ordovician. Based on this evidence it is concluded that a small spherical apex is an autapomorphy of the Orthocerida. Moreover, this evidence supports a splitting of the order Orthocerida in two taxa of different affinities. The Orthocerida sensu stricto comprises orthocones with a tubular siphuncle nearly without endospiphuncular deposits, and a spherical apex. Embryonic shell, orthoceridan ancestry, orthoceridan classification.     

The US distribution of physicians from lower income countries

Introduction

Since the 1960 s, the number of international medical graduates (IMGs) in the United States has increased significantly. Given concerns regarding the effects of this loss to their countries of origin, the authors undertook a study of IMGs from lower income countries currently practicing in the United States.

Methods

The AMA Physician Masterfile was accessed to identify all 265,851 IMGs in active practice in the United States. These were divided by state of practice and country of origin. World Bank income classification was used to identify lower income countries.

Results

128,729 IMGs were identified from 53 lower income countries, constituting 15 percent of the US active physician workforce. As a percentage of the workforce, West Virginia (29%), New Jersey (27%), and Michigan (26%) had the most IMGs from lower income countries, and Montana, Idaho, and Alaska (all less than 2%), the least. The countries with the greatest loss of physicians to the United States per 100,000 population were the Philippines, Syria, Jordan, and Haiti.

Discussion

The reliance of US medicine on physicians from lower income countries is beneficial to the United States both clinically and economically. However, it results in a loss of the lower income country''s investment in the IMG''s education. We discuss possible mechanisms to compensate the lower income countries for the medical education costs of their physicians who immigrate to the US.