Head Shield Research Articles

The earliest osteostracan Kalanaspis delectabilis gen. et sp. nov. from the mid-Aeronian (mid-Llandovery, lower Silurian) of Estonia

ABSTRACTThe Osteostraci are an early clade of jawless vertebrates widely regarded as the sister group of all jawed vertebrates. Despite their prominent role in the Silurian to Devonian marginal marine ecosystems, the early evolutionary history of this subclass remains poorly understood. Here we report the discovery of a head shield (part with broken right margin and counterpart) of Kalanaspis delectabilis gen. et sp. nov. from the Kalana Quarry in Central Estonia. Reliably dated to the Pranognathus tenuis Conodont Zone, Raikküla Regional Stage, mid-Aeronian, mid-Llandovery, the specimen is about 10 million years older than the previously known oldest record of the Osteostraci. Kalanaspis delectabilis gen. et sp. nov. is identified as belonging to the new Family Kalanaspididae fam. nov., Order Ateleaspidiformes. It has a relatively large dorsal head shield, which is somewhat wider than long, with very small and very closely placed orbits, short prepineal length, one pair of notably long lateral fields, and gently undulating posterior margin. It lacks a posteromedian dorsal ridge as well as cornual processes and pectoral sinuses. No impressions of nasohypophyseal and pineal features were detected. The preservation of the specimen is unusual; the energy-dispersive spectroscopy (EDS) elemental analysis revealed that calcium phosphate, the original major constituent of the dermal skeleton of vertebrates, has disappeared and has been substituted by carbonaceous matter.SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVPCitation for this article: Tinn, O., and T. Märss. 2018. The earliest osteostracan Kalanaspis delectabilis gen. et sp. nov. from the mid-Aeronian (mid-Llandovery, lower Silurian) of Estonia. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2017.1425212.

The functional head of the Cambrian radiodontan (stem-group Euarthropoda) Amplectobelua symbrachiata

BackgroundSegmental composition and homologies of the head of stem-group Euarthropoda have been the foci of recent studies on arthropod origins. An emerging hypothesis suggests that upper-stem group euarthropods possessed a three-segmented head/brain, including an ocular segment (protocerebrum) followed by the deutocerebrum with associated antennae/raptorial limbs and the tritocerebrum, while in the lower stem, head structures of Radiodonta are wholly associated with the protocerebrum and its preceding part. However, this hypothesis is incompletely tested because detailed knowledge on the head components of radiodontans is patchy, and informative articulated specimens are lacking for many taxa. Amplectobelua symbrachiata is the most common radiodontan species in the Chengjiang biota (ca. 520 Ma), normally known as isolated frontal appendages. Here we present detailed descriptions of new articulated specimens that elucidate the morphology and function of its head structures, and discuss their implications for hypotheses about euarthropod cephalic organisation.ResultsIn addition to a central oval head shield, A. symbrachiata also bears a pair of P-elements connected by an elongated rod. The mouth consists of sets of smooth and tuberculate plates, in contrast to the typical radial oral cones of other radiodontans. Previously identified ‘palm-like teeth’ are located external to the mouth in the posterior head region, and are interpreted as segmental gnathobase-like structures (GLSs) associated with at least three reduced transitional flaps in a one (pair)-to-one (pair) pattern, consistent with an appendicular nature. Comparisons with other panarthropods show that GLSs are morphologically similar to the mandibles and other gnathobasic mouthparts of euarthropods, as well as to the jaws of onychophorans, indicating their functional integration into the feeding activities of A. symbrachiata.ConclusionsThe functional head of A. symbrachiata must include the reduced transitional segments (and their associated structures), which have been identified in several other radiodontans. This functional view supports the idea that the integration of segments (and associated appendages) into the head region, probably driven by feeding, occurred along the euarthropod stem-lineage. However, the number of reduced transitional segments varies between different groups and it remains uncertain whether GLSs represent proximal or distal parts of appendages. Our study is the first description of appendicular structures other than the frontal appendages in the functional head region of radiodontans, revealing novel feeding structures in the morphological transition from the lower- to the upper- stem-group of Euarthropoda.

First evidence for the occurrence of Unapertura (Crustacea, Branchiopoda, Anomopoda, Chydoridae) in North America based on subfossil remains

We recovered subfossil chydorid cladoceran remains, which morphologically resemble the remains of Unapertura latens, from the surface sediments of two small lakes (TK-32, TK-18) located in the Central Arctic Treeline region, Northwest Territories, Canada. This species had previously been described only from lakes in Europe, primarily in southern Finland. We compared head shields and postabdomens from Canadian and Finnish sites and found them to be morphologically identical. The occurrence of Unapertura remains in North America suggests that this group of chydorids may be much more widely distributed than previously reported.

A new species of Tropidophorus Duméril & Bibron, 1839 (Squamata: Sauria: Scincidae) from Sarawak, East Malaysia (Borneo).

A new species of the genus Tropidophorus is described from Putai, upper Baleh, Kapit districts, Sarawak, East Malaysia (Borneo). Tropidophorus sebi sp. nov. is diagnosable from congeners from Borneo by the combination of the following characters: head shields present, dorsal and lateral scales smooth; parietal scales in two pairs; supraciliaries eight; supraoculars four; supralabials seven; infralabials four; postmental undivided; longitudinal scale rows 58; ventrals 53; transverse scale rows at midbody 34; subcaudals 98; preanals enlarged, single; and subdigital lamellae of Toe IV 19. In addition, we determine the phylogenetic position of this species within the Tropidophorus group based on mitochondrial markers, and present a key to identification of the known Bornean species in the genus.

New morphological information on, and species of placoderm fish Africanaspis (Arthrodira, Placodermi) from the Late Devonian of South Africa.

Here we present a new species of placoderm fish, Africanaspis edmountaini sp. nov., and redescribe Africanaspis doryssa on the basis of new material collected from the type locality of Africanaspis. The new material includes the first head shields of Africanaspis doryssa in addition to soft anatomy for both taxa. Hitherto Africanaspis was entirely described from trunk armour and no record of body and fin outlines had been recorded. In addition the first record of embryonic and juvenile specimens of Africanaspis doryssa is presented and provides a growth series from presumed hatchlings to presumed adults. The presence of a greater number of juveniles compared to adults indicates that the Waterloo Farm fossil site in South Africa represents the first nursery site of arthrodire placoderms known from a cold water environment. The preservation of an ontogenetic series demonstrates that variation within the earlier known sample, initially considered to have resulted from ontogenetic change, instead indicates the presence of a second, less common species Africanaspis edmountaini sp. nov. There is some faunal overlap between the Waterloo Farm fossil site and faunas described from Strud in Belgium and Red Hill, Pennsylvania, in north America, supporting the concept of a more cosmopolitan vertebrate fauna in the Famennian than earlier in the Devonian.

A new crustacean from the Herefordshire (Silurian) Lagerstätte, UK, and its significance in malacostracan evolution.

Cascolus ravitis gen. et sp. nov. is a three-dimensionally preserved fossil crustacean with soft parts from the Herefordshire (Silurian) Lagerstätte, UK. It is characterized by a head with a head shield and five limb pairs, and a thorax (pereon) with nine appendage-bearing segments followed by an apodous abdomen (pleon). All the appendages except the first are biramous and have a gnathobase. The post-mandibular appendages are similar one to another, and bear petal-shaped epipods that probably functioned as a part of the respiratory–circulatory system. Cladistic analysis resolves the new taxon as a stem-group leptostracan (Malacostraca). This well-preserved arthropod provides novel insights into the evolution of appendage morphology, tagmosis and the possible respiratory–circulatory physiology of a basal malacostracan.

Investigation into the effects of two component grout properties on surface settlements

Ground displacement is an inevitable problem in the excavation and construction of tunnels and underground spaces near the surface of the earth. In urban areas, this displacement can affect surface and subsurface constructs. In fact, there is no possibility of boring underground space and the installation of support systems, simultaneously. Therefore, some ground displacement will happen at the level of the tunnel and eventually this displacement will also lead to surface settlement. Surface settlement becomes more important by reducing the depth of the tunnel. When settlements greater than the limit value are created they can damage surface and subsurface constructs. Therefore, identification of factors affecting the settlement of the tunnel is important and one of these factors is grouting behind the segmental lining of the tunnel. In tunneling by the shield, due to differences in the diameter of cutter head and tail shield, empty space is created around the segmental lining of the tunnel. This void is usually the ratio of the cross sectional area that is being drilled in soil and soft ground and will cause problems such as settlement of surface, displacement and damage to the lining of the tunnel and unsuitable sealing. Therefore, this empty space must be filled immediately after the drilling operation. There are different methods to fill any given empty space. One of them, in excavation by EPB-TBM, is the injection of two component grout with suitable pressure to the back of the segment (simultaneously with moving ahead of the shield of the excavator). Grout used must have the appropriate performance and mechanical properties. There is still no certain standard for the compressive strength of two component grout and the compressive strength of grout proposed is variable with different values in projects and researches. In this study, grout mechanical properties such as compressive strength, modulus of elasticity and Poisson's ratio were studied. Thus, more tests were performed with different proportions of ingredients and finally the effect of each of the components on the properties were examined. Then, to study the effect of the grout properties on surface settlement until introduction of the appropriate amount and compressive strength of two component grout a numerical modeling of Tabriz subway line 2 by FLAC3D software were completed. For this purpose, the results of the laboratory tests for numerical modeling of grout were used in software. Finally, with regard to the obtained numerical results, the conclusion that excessive increased resistance were came and thereby the amount of cement of grout will only result in additional costs and in terms of settlement control has little effect. Therefore, create a grout with resistance as the resistance of the soil is sufficient to fill the empty space and surface settlement control (part of the settlement due to lack of suitable filler material occurs).

Investigation of Hall effect on the performance of magnetohydrodynamic heat shield system based on variable uniform Hall parameter model

There has been a resurgence in the field of magnetohydrodynamic (MHD) flow control in the past 20 years. An increasing demand for sustained hypersonic flight and rapid access to space, along with numerous mechanical and material advances in flight-weight MHD technologies, has aroused renewed interest in this subject area. As a novel application of MHD flow control in the thermal protection field, MHD heat shield system has been proved to be of great intrinsic value by lots of researchers in recent years. Although its theoretical feasibility has been validated, there are many problems that remain to be further investigated. Among those problems, the Hall effect is a remarkable one that may affect the effectiveness of MHD flow control. Considering the fact that it is not sufficient to evaluate the Hall effect by merely using the chemical reaction model implemented in the nonequilibrium flow simulation to calculate the Hall parameter, a parametric study is conducted under the assumption of simplified uniform Hall parameter. First, coupling numerical methods are constructed and validated to solve the thermochemical nonequilibrium flow field and the electro-magnetic field. Second, a series of numerical simulations of the MHD head shield system is conducted with different magnitudes of Hall parameter under two magnetic induction intensities (B0=0.2 T, 0.5 T). Finally, the influence of Hall effect on the performance of MHD heat shield system is analyzed. Results indicate that Hall effect is closely related to the wall conductivity. If the vehicle surface is regarded as an insulating wall, the heat flux variation is co-determined by varying the Lorentz forces within the boundary layer and the shock-control effect. Compared with the one neglecting the Hall effect, the heat flux with Hall effect is slightly mitigated as the increase of Lorentz forces in the boundary layer dominates when the stagnation magnetic induction intensity B0 equals 0.2 T. However, the heat flux is increased when B0 equals 0.5 T, because the decrease of shock stand-off distance dominates which increases the gas temperature outside the boundary layer. Moreover, in this case the larger the Hall parameter, the higher the heat flux will be. As for the conductive wall, the performance of MHD heat shield system becomes worse with the increase of Hall parameter, and while it is equal to or higher than 5.0, this system loses its effectiveness.

Varying degradation of subfossil Daphnia longispina during the past 250 years and the discovery of fossil helmet-type head shields: preliminary results

<p>Zooplankton are regarded as a good indicator of environmental change, but comprehensive monitoring programs including zooplankton are uncommon and only rarely extend over longer periods of time. A part of the zooplankton community can be reconstructed using palaeolimnological methods, yet challenges remain. For example, cladoceran subfossil remains preserve selectively in sediments. In particular, the remains of <em>Daphnia</em> spp. are known to usually exhibit poor level of preservation; the reasons for this are still unclear. In the rural Lake Kivijärvi, located in central Finland, <em>Daphnia</em> subfossil remains preserve extraordinary well and multiple fossil components are found. However, the preservation level is not uniform and exhibits directional change throughout the sediment record. To investigate the changes in <em>Daphnia</em> preservation in lake sediments, we graded caudal spines from 20 fossil sediment samples into three taphonomic groups. A dataset of sediment geochemistry, diatom-inferred lake water pH, predation indices, and the catchment land use history was used to assess the environmental history of our study lake. In Lake Kivijärvi, the most significant change in <em>Daphnia</em> preservation seems to correspond best with the historical fishing activities. Additional explanatory variables include forestry in the catchment area, and pH, which, however, had contradicting effects on the preservation of <em>Daphnia</em> remains in this study. Finally, a fossil <em>Daphnia longispina</em> helmet type head shield derived from the lake sediment is presented for the first time.</p>

A crustacean with eumalacostracan affinities from the Early Devonian Hunsrück Slate (SW Germany)

AbstractWe describe a new fossil crustacean from the famous Early Devonian Hunsrück Slate, Germany. Other crustacean fossils from this Lagerstätte are of the typical phyllocarid type. The new fossil genus and species Hohensteiniella engeli, represented by a single specimen, exhibits a typical shrimp‐like habitus. H. engeli therefore represents the first crustacean with eumalacostracan affinities from the Hunsrück Slate. Most prominently preserved is the biflagellate antennula, which measures about two‐thirds of the length of the main body. A series of stenopod‐like thoracopods enforce its shrimp‐like appearance. The head shield bears a short rostrum, but appears to have been rather short posteriorly. It remains unclear whether the new species forms an ingroup to Eumalacostraca or is its sister group, as certain key features are not preserved. Possible ingroup affinities may lie with the syncarid lineage (due to the short head shield) or decapod lineage (due to anterior thoracopods being shorter than posterior ones). Representatives of the eumalacostracan lineage are rare in the Devonian and are in most cases younger than H. engeli. Only Archangeliphausia spinosa is slightly older. The specimen of H. engeli is preserved associated with small spherical structures that could represent eggs. We discuss the possibility that H. engeli might have performed a type of parental care.

A new subspecies of Ovalona setulosa (Megard, 1967) (Cladocera: Anomopoda: Chydoridae) from the Caribbean coast of Colombia.

A new subspecies of Ovalona setulosa (Megard, 1967) (Cladocera: Anomopoda: Chydoridae) is described based on material from Laguna Navío Quebrado (La Guajira Department), Colombia. Ovalona setulosa arangureni ssp. nov. differs from the nominative subspecies in: (1) major head pores position on head shield; (2) posterior margin of labral keel without clusters of short setules; (3) more elongated postabdomen, with length/height ratio about 2.7-3; (4) male postabdomen with straight dorsal margin in whole distal portion. In outer morphology and morphology of postabdomen, O. setulosa arangureni ssp. nov. can be confused with O. glabra (Sars, 1901), but, like nominative subspecies, differs from it by interrupted connection between major head pores. Our data suggest that diversity of Aloninae species with limited distribution areas in South America is underestimated yet, and areas north and south of Amazon basin can have different local species.

A new species of freshwater turtle of the genus Elseya (Testudinata: Pleurodira: Chelidae) from the Northern Territory of Australia.

The genus Elseya has had a checkered taxonomic history, but is now restricted to species characterized by an alveolar ridge on the triturating surfaces of the jaw. The Australian forms were once regarded as a single widespread species extending from the Mary River of south-eastern Queensland to the Fitzroy River of north Western Australia, but a number of Australian species have now been identified based on a combination of molecular and morphological data-Elseya dentata, E. irwini, E. lavarackorum and E. albagula. The genus is represented in New Guinea by E. branderhorsti, E. novaeguineae, E. schultzii, and E. rhodini. One additional Australian taxon first identified in 1981 and subsequently established as a distinct taxon by molecular studies, is described here. It is a large chelid turtle that can be distinguished from all other Australian members of the genus Elseya by the distinctive cream or yellow plastron, free of the dark streaking, blotches or suffusing present in other species; an extensive bridge with little or no abrupt angle between the bridge and the ventral surface of the plastron; a head shield broken into a series of small plates rather than a single unit; flat uncornified temporal scales; and a narrower, less robust skull. Osteologically, it can be distinguished from Elseya dentata by the contact of the vomer and the pterygoids. The carapace is typically a light to medium brown in color whereas the carapace of Elseya dentata is typically dark brown to almost black in color. Distribution is the Mary, South Alligator, East Alligator, Goyder and Mann River drainages of the north east of the Northern Territory, Australia. It does not appear to be in sympatry with any other member of Elseya. It is, however, in sympatry with three species of Chelodina, at least two species of Emydura, Myuchelys latisternum and Carettochelys insculpta.

An embryological perspective on the early arthropod fossil record.

BackgroundOur understanding of the early evolution of the arthropod body plan has recently improved significantly through advances in phylogeny and developmental biology and through new interpretations of the fossil record. However, there has been limited effort to synthesize data from these different sources. Bringing an embryological perspective into the fossil record is a useful way to integrate knowledge from different disciplines into a single coherent view of arthropod evolution.ResultsI have used current knowledge on the development of extant arthropods, together with published descriptions of fossils, to reconstruct the germband stages of a series of key taxa leading from the arthropod lower stem group to crown group taxa. These reconstruction highlight the main evolutionary transitions that have occurred during early arthropod evolution, provide new insights into the types of mechanisms that could have been active and suggest new questions and research directions.ConclusionsThe reconstructions suggest several novel homology hypotheses – e.g. the lower stem group head shield and head capsules in the crown group are all hypothesized to derive from the embryonic head lobes. The homology of anterior segments in different groups is resolved consistently. The transition between “lower-stem” and “upper-stem” arthropods is highlighted as a major transition with a concentration of novelties and innovations, suggesting a gap in the fossil record. A close relationship between chelicerates and megacheirans is supported by the embryonic reconstructions, and I suggest that the depth of the mandibulate-chelicerate split should be reexamined.

Enalikter aphson is an arthropod: a reply to Struck et al . (2014)

<i>Enalikter aphson</i> is an arthropod: a reply to Struck <i>et al</i> . (2014)

The oldest notostracan (Upper Devonian Strud locality, Belgium)

AbstractA new notostracan crustacean, Strudops goldenbergi gen. et sp. nov., is described from the well‐preserved terrestrial arthropod fauna of the Upper Devonian of Strud, Belgium. The fossil notostracan bears a close resemblance to modern notostracans in possessing a large, simple head shield covering almost half of the whole body, a set of phyllopodous thoracic appendages and a legless posterior abdomen with a telson bearing a caudal furca. The differentiation and relative size of mouthparts and limbs suggest that these specimens are all adults. The notostracans described herein are the earliest clear members of the total group Notostraca.

Palaeolimnological evidence of Bosmina morphotypes appearance in the late Holocene

Lacustrine sediments retain Cladocera (Crustacea) subfossils (carapaces, ephippia, head shields, and postabdomens) from the Holocene, which allows reconstructing past aquatic communities and environmental changes. In late Holocene (Subatlantic) fossils, among many Cladocera species, we noticed the appearance of several Bosmina ( Eubosmina) species in lakes surrounding the Baltic Sea. These species were never recorded in the early Holocene. The appearance of these species is a record of very recent speciation, which took place during the last millennium in lakes surrounding the Baltic Sea. Contemporary data show co-occurrence of several Bosmina ( Eubosmina) species ( longispina, kessleri, reflexa, ruehei, coregoni, gibbera, thersites). These species are not a product of invasion or introgressive hybridization, but in fact, they are probably a product of speciation. Here, we present the palaeolimnological evidence of this speciation from the only one species, Bosmina ( Eubosmina) longispina, found in lacustrine sediments from the last interglacial and glacial periods.

A New Species of Wolf Snake (Serpentes: Colubridae: Lycodon ) from Berhampur, Ganjam, Odisha, India

A new species of the wolf snake is described from Odisha, India. Morphologically, the new species is most similar to Lycodon jara , but differs by its well defined collar. The collar is white in hatchlings and white with yellow blotches on the dorsal side in juvenules and adults. The snout is projected beyond the lower jaw (vs. not beyond in L. jara ). The post-oculars are one or two (vs. 2 always in L. jara ). Twin spots are present on each scale of the body except the 6 – 7 rows of scales in the collar region (vs. each scale in L. jara ). The color of twin spots is white in hatchlings and yellow in adults in the new species. The yellow tinges on the head shields are much less as compared to L. jara in the adults of the new species.

Ephemeroporus quasimodo sp. nov. (Crustacea: Cladocera: Chydoridae), a new species from the Brazilian Cerrado.

A new species of Chydoridae cladoceran (Crustacea: Cladocera: Chydoridae) is described from the Cerrado (a Brazilian tropical savanna ecoregion), Central Brazil. Ephemeroporus quasimodo sp. nov. differs from other species of the genus in a high dorsal keel on carapace and posterior part of the head shield, which resembles a hunchback. Parthenogenetic females, ephippial females and males are described, including details of trunk limb morphology. The species is endemic to shallow wetlands found in the Cerrado, where pristine conditions are preserved. Some questions on genus morphology and taxonomic status of Ephemeroporus species occurring in Brazil are discussed.

Biodiversity of the Silurian osteostracans of the East Baltic

ABSTRACTThe sculpture and histology of osteostracan head shields and trunk squamations from the Silurian of Estonia are described, illustrated and used for the identification of disarticulated microremains collected from outcrop sections and numerous drill cores in Estonia and Latvia over the last 40 years. The scattered osteostracan material contains thousands of specimens of scales and shield fragments. The sculpture and histology of species of the previously identified eight genera in the region (Tremataspis,Dartmuthia,Saaremaaspis,Oeselaspis,Aestiaspis,Thyestes,Procephalaspis, Witaaspis) are redescribed, together withAteleaspis, found in Estonia for the first time. The sculpture on the cornua of several taxa is described for the first time. The new taxaTremataspis perforatasp. nov.,Dartmuthia procerasp. nov.,Eldaaspis mikliigen. et sp. nov.,Tahulaspis ordinatagen. et sp. nov.,Tahulaspis praeviagen. et sp. nov.,Meelaidaspis gennadiigen. et sp. nov. andOhesaareaspis ponticulatagen. et sp. nov. are established, based upon sculpture and histological characteristics of the exoskeleton. The supraoral field with denticles of that field are described for the first time inOeselaspis pustulata, as is the supraoral plate with buccal denticles inMeelaidaspis gennadiigen. et sp. nov. Thin sections of all taxa (except ofWitaaspis) have been studied. This work sheds light on East Baltic osteostracan biodiversity from the Maasi Beds of the Jaagarahu Stage, Sheinwoodian, lower Wenlock up to the Ohesaare Stage, upper Přidoli, Silurian.

Taxonomic revision of buchanosteoid placoderms (Arthrodira) from the Early Devonian of south-eastern Australia and Arctic Russia

The ‘buchanosteid’ placoderms are best known from the Early Devonian of Australia, but also occur in China, Russia, Central Asia and the Middle East. Here we rediagnose the type species Buchanosteus confertituberculatus (Hills 1936) from the type locality at Buchan, Victoria, in the light of new material of both head and trunk shields. The superfamily Buchanosteoidea Denison, 1978 is redefined to unite taxa that share a similar skull roof with separate rostro-pineal (ethmoid) bone, and postethmoid skull pattern characterised by a large trapezoidal nuchal, strap-like short and broad preorbitals, large subrectangular centrals, small postorbitals not contacting the paranuchals, and large, elongate marginal plates. The Family Buchanosteidae is redefined on skull roof and parasphenoid shape and trunk armour features as a monotypic family within the Buchanosteoidea. A new family (Parabuchanosteidae nov.) includes taxa with the posterior lateral plate overlapping the anterior dorsolateral plate externally. Two new buchanosteids are described, Richardosteus barwickorum gen. et sp. nov., from Burrinjuck, south-eastern Australia, and Urvaspis lithuanica gen. et sp. nov., from Severnaya Zemlya, Russia.