The Upper Tremadocian (Ordovician) graptolite Bryograptus: taxonomy, biostratigraphy and biogeography

Abstract:  The taxonomy, biostratigraphical and palaeogeographical distribution of the Lower Ordovician graptolite genus Bryograptus is evaluated. Bryograptus is recognized as a distinct triradiate anisograptid with a multiramous, pendent rhabdosome. The species of the genus Bryograptus can be interpreted as shallow water faunal elements with a strongly limited biogeographical distribution to the Atlantic Faunal Realm. Bryograptus is restricted to a narrow interval in the Upper Tremadocian, the Bryograptus Biozone of Scandinavia and South America (Argentina), making it a taxon with a high potential for precise biostratigraphical correlations. The proximal end development can be used to differentiate the genus Bryograptus from other pendent multiramous graptoloid genera with a homoplastic rhabdosome development. Characteristics of the proximal end development and structure easily differentiate these genera in relief specimens, but not in flattened material.     

Genetically controlled cortical tissue deposition inNormalograptus scalaris (Hisinger, 1837)

A specimen ofNormalograptus scalaris (Hisinger) from the Llandovery of Dalarna, Sweden, shows a highly asymmetrical rhabdosome in which one stipe was abandoned at an early stage in the growth of the colony. The surface of the rhabdosome is thickly covered by cortical tissue, but the remaining dorsal side of the second stipe is left free. Even though the symmetry of the colony is strongly distorted, it survived for a considerable time. It is suggested that this unusual development involved genetic control of cortical tissue deposition in the graptolite colony.      

Development of limbs in urodeles and the origin of tetrapod limbs

On the basis of studies on serial sections of larval Ranodon sibiricus limbs and published data, the hypothesis of the origin of tetrapod limbs from the biserial archipterygium is proposed. The mesomeres of the central axis of the biserial fin correspond (in proximodistal direction) to the humerus, ulna, ulnare, all carpalia distalia, metacarpale 1, and phalanges of the first digit in the forelimb of caudate amphibians and to the femur, fibula, fibulare, tarsalia distalia, metatarsale 1, and phalanges of the first digit in the hind limb. The preaxial elements of the zygopodium and autopodium, which are positioned proximal to the digital arch, correspond to the preaxial rays of the biserial fin, and digits 2–5 correspond to its postaxial rays. As the fin transformed into the limb, the central axis curved preaxially, forming the digital arch and resulting in partial reduction and fusion of preaxial rays.     

HIRNANTIAN (LATEST ORDOVICIAN) GRAPTOLITES FROM THE UPPER YANGTZE REGION, CHINA

Abstract:  The Upper Yangtze region yields a Hirnantian (latest Ordovician) graptolite fauna that includes 41 species assigned to 13 genera. This fauna is particularly important for understanding the Late Ordovician mass extinction event because it is the most diverse known from this interval. In addition, it records the survival, well into the Hirnantian, of many taxa of the Dicranograptidae-Diplograptidae-Orthograptidae (DDO) fauna, which was previously regarded as having gone extinct at the beginning of the Hirnantian. Taxa exhibiting six different astogenetic patterns, including taxa with reclined stipes, scandent, biserial, full-periderm and 'archiretiolitid' rhabdosome forms occur in the lower Normalograptus extraordinarius-N. ojsuensis Biozone. In contrast, in the upper N. persculptus Biozone only four genera remain, all but one of which are Normalograptidae: scandent and biserial taxa with Pattern H astogeny. Normalograptids are the dominant form of the succeeding, lower Rhuddanian, faunas. The Yangtze faunas also document the early expansion of normalograptids coeval with the decline of the DDO fauna. Many previously identified species considered endemic to China have been synonymized; 24 of the 41 species recorded here have been recognized elsewhere. No new taxa are described.     

On the rate of budding and proximal attenuation of the graptoloid rhabdosome

Earlier models of the morphogenesis in graptoloid colonies can be improved by taking into account the rate of growth and budding. It is assumed that both these factors are controlled by a specific function of the morphogen, here called for convenience the blastogen , and both are responsible for the attenuation of the proximal part of the rhabdosome. The permanence of this attenuation may theoretically be explained by the universal advantage of differentiation between the proximal and the distal part of the colony. Some aspects of possible adaptive significance of such colony organization are discussed. □ Graptoloid colonies, morphogenesis, mathematical model.     

Ordovician Paracrinoids from the Baltic: Key Problems of Comparative Morphology of Pelmatozoan Echinoderms

The erect feeding appendages of paracrinoids, brachioles of typical blastozoans and arms of crinoids are morphologically similar in their terminal growth, biserial cover plates, and pinnulation. This is attributed to the inducing effect of the radial ambulacral canal on their growth mode. The uniserial brachioles of Laurentian paracrinoids are homologous to the biserial brachioles of the Baltic Achradocystites and Heckerites, and those of other blastozoans. Based on this assumption, the two Baltic genera, which have a brachiole system plesiomorphic for paracrinoids, and a similar morphology of the theca, are assigned to this class. Brachiolars in brachioles are a new development, homologous to the flooring plates of the food groove and, where present, are the continuations of these plates beyond the theca. The uniserial brachioles of Laurentian paracrinoids evolved from the biserial brachioles as a result of a gradual shift of brachiolars in the neighboring rows and their subsequent fusion in pairs. Brachials in crinoidal arms are a new development that evolved as distal serial growth of radial plates under the induced influence of the incipient radial canals emerging from the closed vestibular cavity, which was an ontogenetic innovation in crinoids. The transformation of a nonorganized small-plated theca into a large-plated, and completely or partly symmetrized theca, or vice versa is possible and results from accelerated or retarded growth of some plate generation in relation to the growth rate of the theca.     

Bohemograptus papilio sp. nov. three-dimensionally preserved monograptid (Graptolithina) with an adaptation to retard sinking,from the Upper Silurian Poland

An unexpected new form of Bohemograptus with unusual morphological structure has been found in the upper part of the Bohemograptus praecornutus Biozone, Upper Ludlow of the Mielnik borehole, Poland. Bohemograptus papilio sp. nov. has the characteristic rhabdosome for Bohemograptus, in addition to extraordinary paired latero-dorsal additions, not comparable to any processes typical for Bohemograptus thecal openings. The new structure, a patagium, is located in the proximal part of the rhabdosome, and is built from main rods dividing repeatedly into thinner lists with significant remnants of membranes spread between them. Another new structure is the virgellarium, a membranous addition to the virgella. Membranes of the patagium and virgellarium are not built from microfusellar tissue as additions in other bohemograptids. It is suggested that the membranous additions were orientation devices, constructed to retard the sinking of the colony in the water column.     

Reconstruction of the proximal ends of retiolitid rhabdosomes (Graptolithina) from the Upper Wenlock and the Lower Ludlow

Detailed study of the ultrastructures on the cortical lists of retiolitids occurring after the lundgreni event has been undertaken. The extremely well preserved material from the Go?dap borehole of Poland (Baltica) enables classification of clathrial lists into two types—with single and double seams. Significantly different seam development is observed between two groups of retiolitids occurring before and after the lundgreni event. The study of seams and increments on the lists enables reconstruction of the membrane arrangement of the rhabdosomes. Reconstruction of the proximal ends, the arrangement of the ancora umbrella, and the layout of the proximal ventral and lateral orifices of five genera, Neogothograptus, Holoretiolites, Plectograptus, Spinograptus, and Quattuorgraptus n. gen., is presented. The new genus Quattuorgraptus is separated from Spinograptus because of the different development of the ancora umbrella and the width of the lateral walls of the rhabdosome.     

Lateral inhibition in the astogeny of conical multistiped graptolites

Conical multistiped graptolites (in particular Rhabdinopora) grow in patterns which can be understood using a lateral inhibition model based on diffusion of nutrients or pheromones around the colony. Many aspects of rhabdosome morphology are explained with this model. These include branching and the distribution of branching points, the morphology of bifurcations, the almost constant spacing of stipes, the high rate of branching in early stages, the variation in expansion rate, and regeneration patterns after damage. For the first time, a full three-dimensional computer simulation of graptolite growth has been accomplished, assuming biologically justified processes instead of formal growth rules. The simulations indicate that dissepiments were instrumental in controlling rhabdosome shape, but not sufficient to keep its horizontal section perfectly circular.     

Structure and Secretion of the Graptolite Prosicula, and its Application for Biostratigraphical and Evolutionary Studies

Studies on graptolite taxonomy and phylogeny in recent years have placed great emphasis on the proximal development of the rhabdosome, particularly the presence or absence of a virgella and early thecal growth patterns. As the prosicula was the earliest part of the graptolite skeleton to be secreted, it may also reveal fundamental information about evolutionary relationships within the Graptoloidea. The prosiculae from a variety of Ordovician taxa ranging in age from Tremadoc to Caradoc have been examined using a combination of light microscopy, SEM and TEM. Parameters investigated include the overall morphology, transition into the nema, pattern of longitudinal ridges and spiral line. Taxa show a change from early Tremadoc graptoloids which have a low diaphragm, prominent spiral line and lack longitudinal ridges, through late Tremadoc and early Arenig taxa which have longitudinal cortical bandages or spiralled, paired longitudinal ridges, into later Arenig and Llanvirn forms which have simple longitudinal ridges and indistinct spiral line and diaphragm. With additional work at higher stratigraphical levels, graptolite prosiculae may prove to be useful biostratigraphically when more complete material is absent, such as in palynomorph preparations from subsurface studies.     

Population structure of graptolite assemblages

Graptolite rhabdosomes display a diverse suite of morphologies. The range of morphotypes present within most moderate- to high-diversity assemblages from the Ordovician and Silurian is similar, despite the different taxonomic composition of the faunas at different times. Survivorship analyses of graptolite faunas from the Ordovician and Silurian demonstrate strong similarities in the mortality rates of unrelated graptolites of similar functional morphology. They also show a strong correlation between decreasing mortality rates among more mature colonies with increasing rhabdosome complexity. This similarity in both functional morphology and life history of graptolites suggests that they lived within a very stable planktic community structure.     

Limb morphogenesis in the branchiopod crustacean, Thamnocephalus platyurus, and the evolution of proximal limb lobes within Anostraca

Crustacean limbs exhibit highly diverse morphologies. One major route of diversification is in the number and position of branches arising from the proximal part of the limb. Here I describe development of larvae of the branchiopod crustacean, Thamnocephalus platyurus and describe in detail the development of the thoracic limbs. The thoracic limbs bear proximal branches both medially and laterally. The most proximal branches on either side (gnathobase and pre-epipod) show a similar developmental history: they develop via fusion of two rudiments into a single adult branch. However, phylogenetic analysis suggests that the developmental fusions have distinct evolutionary histories. In one case (gnathobase), the developmental rudiments reflect the ancestral adult morphology of two distinct branches. In the other (pre-epipod), the rudiments are an apparent novelty within the Anostraca and develop into two adult structures in only a single derived family.     

A new species of deepwater snake-eel,ophichthus aphotistos, with comments onneenchelys retropinna (anguilliformes: Ophichthidae) from taiwan

Ophichthus aphotistos, a new species of snake-eel, subfamily Ophichthinae, is described from three specimens trapped in deep water (700–800 m) off southwestern Taiwan. It is most similar toO. cruentifer but differes from it and other elongate, deep water species ofOphichthus in its vertebral number (158–162), posterior dorsal-fin origin, biserial dentition, and dark coloration. A poorly known species,Neenchelys retropinna Smith &; Böhlke, is reported from the South China Sea for the first time.     

The identity of the Ordovician (Darriwilian) graptolite Fucoides dentatus Brongniart, 1828

Abstract: The biostratigraphically important Middle Ordovician (Darriwilian) biserial graptolite Fucoides dentatus Brongniart, 1828 is redescribed and illustrated from its type material and from additional specimens collected at the type locality – Lévis, Quebec, Canada. It is referred to Levisograptus gen. nov., which includes also the austrodentatus group of early axonophoran graptolites. The species has previously been confused with a younger, mid‐Darriwilian species, now referred to as Eoglyptograptus gerhardi sp. nov., and recognized as the type species of the genus Eoglyptograptus Mitchell. Both species can be differentiated easily by their respective proximal development types and show nonoverlapping biostratigraphical ranges. Levisograptus dentatus (Brongniart) is an important biostratigraphical index species in the early Darriwilian. Eoglyptograptus species are found in the higher Darriwilian and are biogeographically restricted to the Atlantic Faunal Realm.     

The muscle scar in cavellinids and its importance for the phylogeny of platycope ostracodes

The adductor muscle scars of cavellinids from the Visean (in one case from the Upper Devonian) of the European part of the RSFSR have revealed a significant diversity in their morphology. Aggregate, multiserial with up to five series, bi- to triserial and nearly biserial muscle scars existed simultaneously. The verticaliserial muscle scars were initiated by a biserial larval group which in the course of evolution may have passed, due to neoteny, into the adult stage. Together with Cytherella sp. from Syria, the new data show that the genus Cytherella appeared not in Triassic nor Jurassic time, hut originated among cavellinids possibly in the Early Carboniferous. A new genus is Borovitchella , with the type species B. egorovi n. sp.     

Relation of the mental foramen to teeth on the mandibles belonging to the Byzantium period

Fifty mandibles belonging to the Byzantium period were studied in order to determine to racial differences about of the localization of the mental foramen according to the mandibular teeth. Mandibles of adult males without missing teeth, proximal decay and proximal attrition were used. On the right side, the most common position of the mental foramen was between the first and second lower teeth (50%), whereas on the left side, the most common position was in line with the longitudinal axis of the lower second premolar tooth (46%). Because of materials including mostly Alpine's and a few of Mediterranean subraces, two different positions of the mental foramen were determined in high similar ratios.     

Hindlimb patterning and mandible development require the Ptx1 gene

The restricted expression of the Ptx1 (Pitx1) gene in the posterior half of the lateral plate mesoderm has suggested that it may play a role in specification of posterior structures, in particular, specification of hindlimb identity. Ptx1 is also expressed in the most anterior ectoderm, the stomodeum, and in the first branchial arch. Ptx1 expression overlaps with that of Ptx2 in stomodeum and in posterior left lateral plate mesoderm. We now show that targeted inactivation of the mouse Ptx1 gene severely impairs hindlimb development: the ilium and knee cartilage are absent and the long bones are underdeveloped. Greater reduction of the right femur size in Ptx1 null mice suggests partial compensation by Ptx2 on the left side. The similarly sized tibia and fibula of mutant hindlimbs may be taken to resemble forelimb bones: however, the mutant limb buds appear to have retained their molecular identity as assessed by forelimb expression of Tbx5 and by hindlimb expression of Tbx4, even though Tbx4 expression is decreased in Ptx1 null mice. The hindlimb defects appear to be, at least partly, due to abnormal chondrogenesis. Since the most affected structures derive from the dorsal side of hindlimb buds, the data suggest that Ptx1 is responsible for patterning of these dorsal structures and that as such it may control development of hindlimb-specific features. Ptx1 inactivation also leads to loss of bones derived from the proximal part of the mandibular mesenchyme. The dual role of Ptx1 revealed by the gene knockout may reflect features of the mammalian jaw and hindlimbs that were acquired at a similar time during tetrapod evolution.     

Origin of the Retiolitidae: insights from a new graptolite genus from the early Silurian of Arctic Canada

Isolated specimens of Hercograptus introversus n. gen., n. sp. show features characteristic of the Retiolitidae, such as partial periderm reduction, and extended ancora lists and a reticulum that are attached to the thecal margins, on a rhabdosome that, in most other respects, resembles Pseudorthograptus. This newly discovered graptolite provides support for the hypothesis that Pseudoretiolites is the first-evolved retiolitid, since they share a similar ancora type and unique thecal style. It is also clear that the suite of features that normally distinguish the Retiolitidae did not all evolve simultaneously. It is proposed that development of a complete thecal framework of lists, permitting complete periderm reduction, should be the character used to distinguish the Retiolidae from the Petalolithidae. Unusual apertural modifications, such as strong introversion, lateral lappets, and a short, free dorsal wall suggest that Hercograptus may be an offshoot of the lineage leading from Pseudorthograptus to Pseudoretiolites.