Evolution in Monograptus priodon

Biometric analysis of the Silurian graptolite Monograptus priodon (Bronn) from Arctic Canada and the British Isles shows that the length of the sicula and the rhabdosome width at the level of theca 5 undergo a progressive increase from latest Llandoverian to late Wenlockian time. This change is attributed to sympatric evolution within the species. Monograptus flemingii (Salter) may have evolved elsewhere from priodon stock and migrated later into the two regions. Monograptus riccartonensis Lapworth probably ranges from latest Llandoverian through the Wenlockian, and may have been ancestral to the Ludlovian to Devonian Monograptus lineages.     

Punctuated stasis and collateral evolution in the Devonian lineage of Monograptus hercynicus

The changes in the sicula of Early Devonian monograptid graptolite populations show a pattern that is described as punctuated stasis: two longer periods of stasis separated by a transitional population of shorter duration between them. The pattern differs from punctuated equilibrium in that anagenesis is involved rather than cladogenesis. The pattern has been observed in several widely separated areas (Nevada, Thuringia, Poland, Ural Mountains) and is an example of collateral evolution in graptolites. In this case, the collateral morphologic change occurs relatively rapidly in a large, globally distributed taxon. The study permits accurate correlation of the horizon of appearance of Monograptus hercynicus Pemer in the upper delta Zone (conodont zonation) of late Lochkovian age (Devonian) on a global scale. The partial integration of these two biostratigraphies constrains correlations between the shelly, shallow-water biofacies and the deeper graptolitic biofacies. The pattern of punctuated stasis has been discovered in Lower Devonian strata of central Nevada, where 22 horizons in sequence were sampled. Statistical analyses were performed on 13 characters encompassing measurements of the sicula, thecae, and rhabdosome. Most of these characters do not exhibit sustained trends or punctuated changes. However, sicular width exhibits an abrupt increase over a short stratigraphic interval and is used to classify members of the group into four quantitatively defined morphotypes. This natural perturbation in the pattern of change is used to divide the lineage into paleospecies. This makes the segments of the lineage easy to identify and imparts stability to the classification and precision in biostratigraphy. The variation studies enable the ranges of the morphotypes to be used to further subdivide the stratigraphic record of the late Lochkovian. □ Graptolites, Devonian, Lochkovian, evolution, punctuated stasis, biostratigraphy, MONOGRAPTUS, phylogeny.     

Ultrastructure of the tuboid graptolite tubotheca

The wall of the tubotheca of the tuboid graptoliteEpigraptus eisenacki Mierzejewski has been studied with TEM. It was found that the tubothecal wall is composed of the cortical tissue which differs only slightly from the same tissue described in Dendroidea andMastigograptus sp. In addition a giant tubotheca inDiscograptus schmidti Wiman has been examined with SEM. The tubothecae studied seem to confirm a non-graptolite nature of the organisms living in them, while providing evidence that the tubotheca wall was a genuine graptolite structure overgrowing these organisms.     

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.     

A provisional classification of the graptolite Order Dendroidea

It is argued that a morphological, stratigraphical and evolutionary series can be established in the Middle and Upper Cambrian for the origin and early diversification of the graptolite Order Dendroidea, the main benthic order of the class Graptolithina. Rhabdosomal form is shown to be useful in classification down to generic level, and it is considered thatDendrograptus gave rise through more ordered species, sequentially toCallograptus andDictyonema;Desmograptus;Aspidograptus andAcanthograptus; and then to compoundAcanthograptus, Thallograptus andPalaeodictyota. Thus by the Upper Cambrian all the main genera of the families Dendrograptidae and Acanthograptidae were established. The latter family then became dominated by genera with compound stipes; whilst a minority of compound genera arose in the Dendrograptidae such asPseudocallograptus Skevington 1963; some groups deriving from the Dendrograptidae are separated as distinct families, namely the Pseudodictyonemidae and the Stelechocladiidae (Chapman et al. 1993).     

Collagen in the pterobranch coenecium and the problem of graptolite affinities

Amino-acid analysis of the extracellular skeleton (coenecium) of two pterobranchs (Hemichordata) Rhabdopleura normani and Cephalodiscus (C.) hodgsoni shows that both contain considerable quantities of collagenous material with relatively high hydroxyproline and low hydroxylysine levels. The appearance of the fibrous material in the skeleton of pterobranchs, although collagenous, differs from standard EM characteristics of collagen. The identification of the collagenous nature of the pterobranch skeleton with the presumed presence of collagen-like material in the periderm of fossil graptolites, taken in conjunction with other data, supports the hypothesis that both groups may be closely related phylogenetically.     

Revision of the Zone 13 graptolite biostratigraphy in the Marathon, Texas, standard succession and its bearing on Upper Ordovician graptolite biogeography

One of the long-standing problems in North American graptolite biostratigraphy is the distinct differences in assemblages of post-Climacograptus bicornis age between the classical graptolite sequences in the New York - Quebec and Marathon, west Texas, regions. These have been attributed either to faunal provincialism or to the presence of a major hiatus between the Woods Hollow and Maravillas formations in Texas. New collections from the key Marathon Picnic Grounds section contain diagnostic Late Ordovician graptolites that confirm the existence of a major stratigraphic gap below the Maravillas Formation. The lower Maravillas Formation (Zone 13) has a Late Ordovician, low-diversity Pacific Province graptolite fauna that includes the biostratigraphically diagnostic species Climacograptus nevadensis, C. tubuliferus, Orthograptus fastigatus and Dicellograptus ornatus. Zone 13 graptolite assemblages from the Marathon region correlate with the C. tubuliferus to D. ornatus zones in the Trail Creek, Idaho, succession, the Ea4-Bo2 interval in Victoria, the O. fastigatus Zone in the Canadian Arctic Islands, the O. quadrimucronatus to D. ornatus zones of the Canadian Cordillera, and the D. complanatus to D. anceps zones in Scotland. The hiatus between the Woods Hollow and Maravillas formations spans an interval corresponding to at least the Eal Ea3 interval in Australia, the C. americanus to upper A. manitoulinensis zones in the New York - Quebec succession, and the D. clingani and P. linearis zones in Scotland. These results agree with the magnitude of the hiatus previously indicated by conodont biostratigraphy. Late Ordovician graptolite distribution patterns in North America can be explained by an extension of Cooper, Fortey & Lindholm's (1991; Lethaia 24) Lower Ordovician graptolite biofacies model into the Upper Ordovician, which incorporates both lateral water-mass specificity and depth stratification. Using this model, we recognize in Laurentia two separate biofacies among tropical-zone Late Ordovician Pacific Province graptolite faunas, a cosmopolitan Oceanic biofacies, and a cratonic Laurentian biofacies. The lower Maravillas Formation graptolite fauna is clearly part of the Oceanic biofacies, whereas the coeval Appalachian faunas represent the Laurentian biofacies. □Graptolites, Ordovician, biostratigraphy, Texas, biofacies, biogeography.     

Utility and precision of Silurian graptolite biozones

The various past uses of the assemblage-zone and biozone concepts are reviewed and a modern definition of biozone is given. Silurian biozones are related to the recentideas on standard reference (graptolite) zones (SRZs) and primary biozones (PBs). Use of evolving lineages, interregna, numerical notation systems, and chronozones are discussed. Precision in correlation of the Silurian biozones is described in terms of the Ordovician-Silurian boundary and the Wenlock-Ludlow (Silurian) boundary. A number of recommendations are made with respect to all zonal concepts, essentially giving prime importance to the use of biozones. □ Biozones, standard reference zones, primary biozones, chronozones, graptolites.     

The graptolite correlation of the North American Upper Ordovician Standard

The North American Upper Ordovician reference standard, the Cincinnatian Series, contains rich shelly microfossil faunas in its type area in the Cincinnati Region but graptolites are uncommon in most of its shallow-water calcareous sediments. Consequently, the graptolite correlation of this key sequence has remained uncertain, in part, even controversial. A review of both previously published recently discovered graptolite Occurrences in the type Cincinnatian, combined with data from the important graptolite successions in Oklahoma New York-Quebec, has not only clarified the graptolite correlation of the Cincinnatian but also added new data on the morphology taxonomy, the vertical horizontal distribution, of several taxa. The information now at hand indicates that the Edenian Stage correlates with the C. spiniferus Zone, the Maysvillian Stage with the C. pygmaeus lower middle P. manitoulinensis Zone, the Richmondian Stage with the upper P. manitoulinensis , the D. complanatus , possibly part of the C. inuiti Zone. Comparison between graptolite conodont biostratigraphic evidence reveals no apparent conflict. Correlations are proposed between Upper Ordovician North American stages, graptolite conodont zones, successions in Texas, Oklahoma, Sweden, European graptolite zones, British series.     

Distinguishing between the Wenlock graptolite biozonal indicesCyrtograptus centrifugus andCyrtograptus murchisoni

The lower Wenlock biozonal indicesCyrtograptus centrifugus andCyrtograptus murchisoni, although easily separated as entire mature rhabdosomes, by the former’s tighter rhabdosome curvature proximally and the latter’s secondary cladia, are very similar proximally, the position of the sicular apex offering the most satisfactory basis for distinction. InC. centrifugus the apex reaches only to the aperture of th1, whilst inC. murchisoni it reaches to above the top of th1.     

湖南益阳早、中奥陶世的叶笔石属(Phyllograptus)

报道湖南益阳早、中奥陶世的叶笔石1属5种,包括Phyllograptus anna Hall,1865,Phyllograptus anna longus Ruedemann,1947,Phyllograptus anna ultimus Ruedemann,1904,Phyllograptus claviger Monsen,1937,Phyllograptus claviger acuminatus(Chen and Xia),1979。研究表明,该属主要产出于弗洛晚期至大坪中期,在早、中奥陶世笔石多样性演变事件第一阶段的后期(Corymbograptus deflexus带),叶笔石所代表的上攀伸展类型就已萌芽,且在第二阶段(Pseudophyllograptus angustifolius elongatus带-Isograptus victoriae divergens带)于均分笔石动物群中占据优势地位,对该动物群内部类群的演替起着承前启后的作用,同时也验证了笔石动物群自身的演替对于其多样性演变具有重要影响。     

The ultrastructure of the Crustoidea and the evolution of graptolite skeletal tissues

The Crustoidea are important in discussions of graptolite origins, because they represent a morphoecological type intermediate between the Rhabdopleurida and the Dendroidea. TEM and SEM studies of their rhabdosomes provide new data on the structural variation displayed by the graptolite periderm. Since the crustoid fusellar fabric does not differ markedly from the fuseller fabric of other graptolites and is dissimilar to that of pterobranchs, it is supposed that the fundamental graptolite fibril pattern was attained at the crustoid stage of graptolite evolution. The crustoid cortex is made of paracortex (the multiple deposition of sheets separated by an intersheet material in the form of condensed meshwork of fibrous material) and of pseudocortex (the accumulation of sheets and intersheet material devoid of fibrous character). The presence of sporadically genuine cortex is noted. Presumably the mechanisms of cortical fibrilogenesis were attained at the crustoid stage, but only in an incipient and incomplete form. Modes of periderm corticization arc discussed. Sheets, common elements in secondary deposits of pterobranchs and graptolites, arc compared, but some problems still remain unclear. ?Crustoidea, Pterobranchia, phytogeny, ultrastructure, cortex, morphogenesis, homology.     

Early Tremadocian (Ordovician) graptolite biostratigraphy and correlation

A revision of the early Tremadocian graptolites of the genus Rhabdinopora shows that Rhabdinopora proparabola from the Xiaoyangqiao section, Dayangcha, North China is the oldest species of the genus. Early Tremadocian species of Rhabdinopora can be differentiated easily by the construction of their nemata. The earliest species, Rhabdinopora proparabola, lacks a nema, while a multibranched nema is present in Rhabdinopora campanulatum (= Rhabdinopora parabola). The Rhabdinopora proparabola, Rhabdinopora campanulatum, Rhabdinopora flabelliformis (Anisograptus matanensis Biozone) and Rhabdinopora anglica biozones can be correlated worldwide. A short interval at the base of the Ordovician System does not contain any planktic graptolites.     

Phylogenetic analysis reveals that Rhabdopleura is an extant graptolite

A phylogenetic analysis of morphological data from modern pterobranch hemichordates (Cephalodiscus, Rhabdopleura) and representatives of each of the major graptolite orders reveals that Rhabdopleura nests among the benthic, encrusting graptolite taxa as it shares all of the synapomorphies that unite the graptolites. Therefore, rhabdopleurids can be regarded as extant members of the Subclass Graptolithina (Class Pterobranchia). Combined with the results of previous molecular phylogenetic studies of extant deuterostomes, these results also suggest that the Graptolithina is a sister taxon to the Subclass Cephalodiscida. The Graptolithina, as an important component of Early–Middle Palaeozoic biotas, provide data critical to our understanding of early deuterostome phylogeny. This result allows one to infer the zooid morphology, mechanics of colony growth and palaeobiology of fossil graptolites in direct relation to the living members of the clade. The Subdivision Graptoloida (nom. transl.), which are all planktic graptolites, is well supported in this analysis. In addition, we recognize the clade Eugraptolithina (nov.). This clade comprises the Graptoloida and all of the other common and well‐known graptolites of the distinctive Palaeozoic fauna. Most of the graptolites traditionally regarded as tuboids and dendroids appear to be paraphyletic groups within the Eugraptolithina; however, Epigraptus is probably not a member of this clade. The Eugraptolithina appear to be derived from an encrusting, Rhabdopleura‐like species, but the available information is insufficient to resolve the phylogeny of basal graptolites. The phylogenetic position of Mastigograptus and the status of the Dithecoidea and Mastigograptida also remain unresolved. □ Biodiversity, Cambrian, Hemichordata, Deuterostomia, Ordovician.     

Proximal structure and development in the Ordovician graptolite Parisograptus Chen and Zhang, 1996

The proximal development of Parisograptus Chen and Zhang is described from three–dimensionally preserved specimens. The unique development features an origin of proximal thecae like a string of pearls vertically upon each other on the reverse side of the rhabdosome with the dorsal sides of the initial stipes placed side by side. The development differs strongly from that found in the superficially similar Arienigraptus in which the first thecal pairs grow downwards side by side, even though the rhabdosome shapes are quite similar. It represents a first step towards the development of a completely biserial rhabdosome and eventually leads to the biserial, monopleural glossograptids.     

Middle Cambrian pterobranchs and the Question: What is a graptolite?

The presence of distinct fusellar structure is taken as evidence to include a number of fossils from the Middle Cambrian to the Lower Ordovician of North America and Europe with the Pterobranchia. The dome of the pterobranchs and the prosicula of the planktic graptolites are contrasted and evidence is given for the re‐assignment of a number of well known dendroid graptolites to the pterobranchs. A non‐destructive method is described to reveal fusellar development of delicate hemichordate exoskeletons from shales. Rhabdotubus robustus n. sp. from the Czech Republic and ? Cephalodiscus sp. from the Wheeler Shale of North America are described as new Middle Cambrian pterobranchs.     

Cephalodiscus graptolitoides sp. nov. a probable extant graptolite

A new species of pterobranch has been found that has features that could resolve the longstanding arguments by palaeontologists about how the fossil graptolites produced their elaborate extra-corporeal homes. Many complex and biologically highly unlikely solutions previously proposed can be simplified. This animal is probably classifiable as a graptolite and thus a living fossil, representing a group thought to be extinct for 300 million years.     

New insights into the paleobiogeography of the Early Ordovician graptolite fauna of northwestern Argentina

A multivariate analysis was used to determine the faunistic relationships of the northwestern Argentinean graptolite faunas of Floian age to faunas from Baltica, Avalonia, Laurentia and SW China. A statistical analysis at the species level of the five geographic regions for the Lower Floian was performed with the classic Jaccard's index. The resulting affinity dendrogram shows stronger relationships between Early Floian graptolite faunas of northwestern Argentina and those from Baltica, with less obvious similarities to Great Britain (Avalonia) faunas and only weak affinities to North American (Laurentia) and Chinese (SW China) graptolite faunas. The statistical analysis confirms the paleobiogeographic relationships previously observed in other areas of the Cordillera Oriental, and supports the hypothesis that during the Early Ordovician, northwestern Argentina was located at middle to high latitudes, corresponding to the Atlantic Faunal Realm of cold water graptolite biofacies. The studied graptolite material from the Quinilicán and Agua Chica sections is preserved in shales and siltstones interbedded with fine to medium-grained sandstones corresponding to the lower part of the Acoite Formation. The biostratigraphic implications of the associated graptolites are briefly discussed, and Trichograptus dilaceratus (Herrmann), Acrograptus gracilis (Törnquist), Expansograptus latus (T.S. Hall) and Corymbograptus v-fractus tullbergi (Monsen) are described from the Argentine Cordillera Oriental for the first time.