Varied development of trunk segmentation in three related Upper Devonian phacopine trilobites

In many arthropods, the development of the trunk region is a complex part of post-embryonic development. Consideration of fossil ontogeny provides an additional source of data and a broader evolutionary perspective on the evolution of arthropod body patterning. Here, I examine the development of the thoraco-pygidial exoskeleton of three related phacopine trilobites from the Upper Devonian according to the integrated ontogenetic scheme proposed by Hughes et al. Pygidial shields assigned to three ontogenetic series gave us the opportunity to further explore the evolutionary pattern of the trilobite segmentation. The analysis showed a different mode of development in two of three species and thus reveals variability between the related taxa. Comparison of the boundaries of different aspects of ontogenesis ratifies the diversity of the segmentation process among trilobites and even among related phacopine species. Results include (i) in a synarthromeric trunk condition recorded to date, there has consistently been a preceding ‘equilibrium’ phase for the late meraspid pygidium and (ii) two developmental modes, i.e. both hypoprotomeric development and synarthromeric development, occur contemporaneously in closely related taxa. Such developments suggest that aspects of segmental development such as segment accretion and segment articulation were able to vary in a labile manner.     

Late Devonian carbon isotope stratigraphy and sea level fluctuations, Canning Basin, Western Australia

The Upper Devonian reef complexes of the Canning Basin contain some of the world’s best exposed, continuous stratigraphic sections through the Frasnian-Famennian boundary. The facies distribution and composition of these reef complexes record interactions among sea level changes, sediment supply, ocean chemistry, and paleoecology. Changes in relative sea level produced spatial shifts in reef platform development and regional changes in sediment supply that can be correlated across facies boundaries using a combination of sequence stratigraphy, biostratigraphy, and carbon isotope stratigraphy. During the lowstand interval below the Frasnian-Famennian boundary, the reef margin advanced down the reef slope in shallow-water environments, and siliciclastics locally dominated in the marginal slope environment. Compilation of a broad late Frasnian to early Famennian sequence stratigraphic framework for the Canning Basin demonstrates that transgressive intervals correlate to positive carbon isotopic excursions within the basin. These isotopic shifts also can be correlated to time-equivalent positive carbon isotopic excursions reported from transgressive intervals in Europe. Thus, the late Frasnian transgressions in the Canning Basin were primarily eustatic rather than tectonic in origin, and positive carbon isotopic signatures of the Kellwasser horizons are globally correlative.     

A quantitative comparison of the ontogeny of two closely-related Upper Devonian phacopid trilobites

The best insight into the development of Devonian phacopids has been obtained from Trimerocephalus lelievrei Crônier & Feist, 1997, a Famennian phacopine from Morocco, where changes in size and shape have been quantified. In this study, a morphometric approach has been used: (1) to retrodeform and then establish patterns of morphological variation in a well preserved but tectonically deformed assemblage belonging to another phacopine species Weyerites ensae (Richter & Richter, 1926), a Famennian phacopine from Thuringia, and (2) to establish patterns of developmental and evolutionary changes within two closely related species: Weyerites ensae and Trimerocephalus lelievrei. The method of retrodeformation using a set of discrete points presumed to be homologous on all studied individuals, has demonstrated that the next analyses are possible on the retrodeformed material as compared to the undeformed material. Morphometric analysis based on outline analysis has permitted demonstration of progressive shape change in agreement with ontogenetic ordination and a comparison of changes in size and shape in Weyerites ensae. The main changes in shape appear to occur in the meraspid period, whereas increase in size takes place mainly in the holaspid period. This pattern, already reported for Trimerocephalus lelievrei, can be generalized for phacopine trilobites from the Late Devonian. Moreover, the comparison of the two ontogenetic trajectories has shown that most of the differences are related to 'structural' changes, probably linked to a relative pre- post-displacement. The results suggest that ecological adaptation may be studied by examining the changes in development that occur within species through time and space.     

Environmentally controlled phyletic evolution, blindness and extinction in Late Devonian tropidocoryphine trilobites

The Middle and Upper Devonian carbonate succession of the Montagne Noire, Southern France has been precisely zoned by an unbroken sequence of conodont zones. Stratigraphic control is excellent, and has allowed evolutionary changes in tropidocoryphine trilobites, which occur throughout the succession, to be directly established. The tropidocoryphine had been a stable group for some 40 million years prior to the Middle Givetian. In their last few million years, however, they underwent rapid evolution and exhibit some striking transformations of the cephalon and the regression and virtual disappearance of the eye within a relatively short space of time. They also show a marked diminution in size, and lose their original relief so that the glabella becomes virtually flush with the surface. There are two separate lineages, both of which show eye-reduction and subsequent blindness. In the earlier lineage Tropidocoryphe (Longicoryphe)-Erbenicoryphe, the main features of the ancestral rootstock are conserved and the stable, strongly divergent anterior sutural pattern of the cephalon remains the same. The eye, however, became reduced to a slightly convex surface lacking lenses, only indistinctly defined. Erbenicorphe is confined to well-oxygenated facies, and probably lived as a shallow burrower within the sediment; it became extinct in the early Frasnian. The second lineage T. (Longicoryphe)-Pterocoryphe- Pteroparia shows a remarkable backward migration of the suture, which progressively swings posteriorly in successive species spanning four Frasnian conodont zones (about three million years duration). At the same time the eye progressively degenerates so that the last forms are blind. Sutural migration and eye reduction are not genetically linked, however; the unusual form of the cephalon and suture probably resulted from an adaptation to the euxinic environment in which Pterocoryphe originated. The loss of the eye resulted from the adoption of an endobenthic habit in Pteroparia which descended from the ancestral Pterocoryphe but which had migrated to an oxygenated facies. Eye-reduction is therefore parallel in the two lineages, but superimposed upon a different original cephalic configuration. The last Pteroparia became extinct when the late Frasnian fauna became overwhelmed by the first pulse of the ‘Kellwasser Event’ (probably an anoxic overturn). The evolving characters, through virtually all observed steps, show progressive unidirectional change without sudden breaks or saltations. Such unidirectional evolution is an adaptive response to constant long-lasting environmental influences. □Evolution, Devonian trilobites, France, gradualism, eye reduction.     

Probable predation on Upper Cambrian trilobites and its relevance for the extinction of soft-bodied Burgess Shale-type animals

The lower Rabbitkettle Formation of northwestern Canada is a monofacial Upper Cambrian unit of variably calcareous, argillaceous siltstone and fine-grained sandstone with rare bioclastic grainstone, deposited on a gentle slope below fair-weather wave base with no discernible fluctuation in water depth. The trilobite fauna is a mixture of pandemic agnostoids and Laurentian polymeroids, including protaspides and meraspides, and individuals are disarticulated, non-abraded and mostly oriented convex-upward. Bioclasts are interpreted as in situ elements affected only by weak bottom currents and storm-induced turbulence. A major proportion of the larger (≥5 mm across) polymeroid cranidia and pygidia in the lower part (Marjuman) of the formation are broken; large thoracic segments are often broken at the axial furrow and some broken free cheeks occur, but essentially no broken agnostoids or hypostomes were observed. Trilobites are not broken in upper beds (Steptoean), above the base of the Glyptagnostus retculatus Zone. Physical breakage cannot be dismissed entirely, but most damage is interpreted to be due to size-selective predation, possibly through lethal blows similar to those delivered by some extant stomatopod crustaceans. A possible culprit may be an animal akin to Yohoia , known from the Middle Cambrian Burgess Shale. The distribution of attacked trilobites serves as a proxy for the presence and disappearance of soft-bodied carnivores. In the Rabbitkettle Formation, it suggests that Burgess Shale-type animals may have persisted into the Late Cambrian but suffered extinction at the Marjuman-Steptoean 'biomere' event when most trilobite species vanished.     

Concepts and analysis of mass extinction with the late Ordovician event as an example

Twelve groups of fossils, including graptolites, brachiopods, nautiloids, trilobites, corals, crinoids, bryozoans, conodonts, ostracods, gastropods, chitinozoans, and acritarchs expired in different but substantial magnitude and global extent during the late Caradoc to latest Ashgill. It indicates a multiple‐episodic mass extinction containing the possible Prologue (late Caradoc), Climax episode (Rawtheyan) and Epilogue (late Hirnantian). The main causes of this mass extinction are recognized as a global sea‐level lowering in the climax and remarkable rapid rise at the final, and global cooling. The Chinese data, especially from the South China Paleoplate, are evaluated first. They are significant for explaining this global bioevent.     

A New Eurypterid (Chelicerata: Eurypterida) from the Upper Devonian Gogo Formation of Western Australia, With A Review of the Rhenopteridae

A new eurypterid, Rhenopterus waterstoni sp. nov., is described from the Gogo Formation (Frasnian, Upper Devonian) of Western Australia. This species is distinguished from related forms by the tuberculation of the anteriormost tergite and crenulated posterior margins of the carapace and opisthosomal segments. It is the only eurypterid specimen known from the Gogo Formation, the most complete eurypterid from Australia, and also the youngest representative of Rhenopterus in the fossil record. Structures retrieved from between the prosomal-opisthosomal juncture comprise polygonal tubes 30–40 μm in diameter, which are interpreted as sarcomeral sheaths of muscular tissue. Rhenopterus is reviewed: R. latus Størmer, 1936 is synonymized with R. diensti Størmer, 1936 as it is here recognized as a female sexual dimorph; R. maccarthyi (Kjellesvig-Waering, 1934) is an orthocone nautiloid.     

Onshore expansion of benthic communities after the Late Devonian mass extinction

A detailed ichnological analysis of the Upper Devonian–Lower Mississippian Bakken Formation of sub‐surface Saskatchewan and the partially coeval Exshaw Formation of Alberta indicates the presence of an anomalous ichnofacies gradient. The distal Cruziana Ichnofacies, which in rocks of other ages is restricted to lower‐offshore facies, here ranges from this setting to the lower shoreface. No archetypal Cruziana Ichnofacies is present in these deposits. This pattern is interpreted as resulting from the differential effects of the Late Devonian mass extinction in shallow‐water ecosystems. The onshore expansion evidenced by ichnological data is consistent with the pattern displayed by the body‐fossil record, which indicates a re‐invasion of shallow‐water environments by the Palaeozoic evolutionary fauna during the Late Devonian and into the Early Carboniferous. The ichnofauna studied is overwhelmingly dominated by deposit feeders, with suspension feeders being notably absent, further underscoring the importance of trophic type as a selectivity trait during mass extinctions.     

New ischnacanthid acanthodians from the Early Devonian of Australia, with comments on acanthodian interrelationships

Rockycampacanthus milesi n.gen., n.sp. is described from a single jaw from the Rocky Camp member of Lower Devonian Buchan Group, E Victoria. Rockycampacanthus differs from other ischnacanthiforms in having large multicuspidate teeth with dual rows of secondary cusps forming a posteromesial flange, a mesial tooth row beginning opposite the fourth cusp of the main tooth row, and in the gnathal bone being deepest in the anterior half. Taemasacanthus erroli n. gen., n. sp. is described from several jaw bones from the Lower Devonian Murrumbidgee Group, New South Wales. Taemasacanthus has a well developed posterolabial flange with secondary cusps developed, vertical rows of denticles on the cusps of the main tooth row and a well developed mesial tooth row separated from the main row by a prominent ridge. The labial face of the jaw has a circular ridge which may have supported labial cartilages. The complex mandibular joint in climatiforms, acanthodiiforms and some primitive sharks differs from the simple jaw articulation of ischnacanthids. It is suggested that ischnacanthids are the plesiomorphic sister group to climatiforms plus acanthodiiforms. The interrelationships of ischnacanthids, climatiforms and acanthodiforms are discussed.     

Ordovician conodont biostratigraphy of the Willara Formation in the Canning Basin,Western Australia

The conodont fauna from the Willara Formation, a carbonate-dominated stratigraphic unit widely distributed in the subsurface Canning Basin of Western Australia, is represented by 41 species, including a new species, Erraticodon neopatu Zhen n. sp. The Jumudontus gananda and Histiodella altifrons biozones are recognized in the lower and upper parts, respectively, of the Willara Formation. Deposited primarily in shallow nearshore settings, the Willara Formation is characterized by the occurrence of predominantly long-range coniform species of Triangulodus, Scalpellodus, Drepanoistodus, Drepanodus, and Kirkupodus. Several widely distributed age-diagnostic species, including Histiodella altifrons, Histiodella holodentata, Histiodella serrata, and Jumudontus gananda, serve as keys for biostratigraphic analysis and correlation. Our study also shows that the basal and top boundaries of the Willara Formation are diachronous across the basin, extending from the middle Floian (Oepikodus communis Biozone) to middle Darriwilian (Histiodella holodentata-Eoplacognathus pseudoplanus Biozone). This contribution provides crucial new biostratigraphic data for precise correlation of the Willara Formation with its time equivalents regionally and internationally.     

Stratigraphic distribution of selected acritarchs in the Ordovician subsurface,Canning Basin,Western Australia

Palynological investigation of 80 core samples from four boreholes, drilled through the Lower to Middle Ordovician marine sections of the central-northeastern Canning Basin, northwestern Australia, reveals diverse acritarch associations including a number of stratigraphically significant species. The stratigraphic succession studied, embracing the Willara Formation and conformably overlying Goldwyer Formation, has been independently dated as early Arenig to Llanvirn by conodont faunas (Oepikodus communis through Phragmodus-Plectodina zonal interval). Nine morphologically distinctive acritarch species have relatively narrow vertical ranges within the study interval and likely constitute serviceable palynostratigraphic indices for the Ordovician in and possibly beyond the Western Australian study area. Three new species, Comasphaeridium setaricum, Polyancistrodorus kunzeanensis, and Stelomorpha calix, are established; and one new combination, Baltisphaeridium variocavatum (Playford and Martin, 1984), is proposed. Other stratigraphically significant species include Petaloferidium comptum Playford and Martin, 1984 and Peteinosphaeridium sp. cf. P. exornatum Tongiorgi et al., 1995; together with Dorsennidium symmetricum (Lu, 1987) Sarjeant and Stancliffe, 1994, Peteinosphaeridium coronula Yin et al., 1998, and Striatotheca rarirrugulata (Cramer et al., 1974) Eisenack et al., 1976, which are found for the first time in Australia. The vertical distribution, and hence the stratigraphic utility, of each of the above species is documented through the studied sections.     

Avian evolution, Gondwana biogeography and the Cretaceous-Tertiary mass extinction event

The fossil record has been used to support the origin and radiation of modern birds (Neornithes) in Laurasia after the Cretaceous-Tertiary mass extinction event, whereas molecular clocks have suggested a Cretaceous origin for most avian orders. These alternative views of neornithine evolution are examined using an independent set of evidence, namely phylogenetic relationships and historical biogeography. Pylogenetic relationships of basal lineages of neornithines, including ratite birds and their allies (Palaleocognathae), galliforms and anseriforms (Galloanserae), as well as lineages of the more advanced Neoves (Gruiformes, (Capimulgiformes, Passeriformes and others) demonstrate pervasive trans-Antarctic distribution patterns. The temporal history of the neornithines can be inferred from fossil taxa and the ages of vicariance events, and along with their biogeographical patterns, leads to the conclusion that neornithines arose in Gondwana prior to the Cretaceous Tertiary extinction event.     

A new species of eastmanosteid arthrodire (Pisces: Placodermi) from Gogo, Western Australia

A new species of Eastmanosteus Obruchev is described from the Upper Devonian Gogo Formation of Western Australia as E. calliaspis sp. nov. It is at the pachyosteomorph level of organization having no contact between the plates of the flank and those of the ventral shield behind the pectoral fin. Well preserved material of the parasphenoid and scapulocoracoid throws further light on the structure of these elements in arthrodires. The genus Eastmanosteus is reviewed.     

The influence of denitrifying seawater on graptolite extinction and diversification during the Hirnantian (latest Ordovician) mass extinction event

A continuous trench exposure within the uppermost type Vinini Formation at Vinini Creek, Roberts Mountains, Nevada, provides an unparalleled opportunity to examine the fate of graptolites, prominent Paleozoic zooplankton, during most of the Hirnantian mass extinction event. On the basis of a detailed biostratigraphic and sedimentological dataset, the relatively complete extinction record is examined in the context of ecological constraints, and it is found to reflect an ecological collapse driven by glacio-eustatic sea-level fall and associated changes in oceanic circulation. Diverse graptolite populations of the Dicranograptidae-Diplograptidae-Orthograptidae (DDO) fauna, which flourished in denitrifying waters within the oceanic oxygen-minimum zone (OMZ) during sea-level highstand, largely vanished with the loss of these conditions during glacio-eustatic sea-level fall. However, populations of one clade, the normalograptids, which inhabited the oxygenated waters of the photic zone, not only survived but diversified. These survivors gave rise to rapid recolonization and diversification with re-establishment of the oxygen-minimum and denitrifying conditions during post-Hirnantian sea-level rise. This ecological model also applies globally to other well-documented coeval stratigraphic intervals, representing both oceanic and platform sea settings.     

Biostratinomy and functional morphology of enrollment in two Middle Devonian trilobites

Although it is common knowledge that many trilobites enrolled, behavioral and functional aspects of enrollment are not at all well understood. Taphonomic details portrayed by enrolled trilobites in the Middle Devonian Hamilton Group (New York State) indicate that enrollment was a complex and morphologically constrained behavior. The trilobites Phacops rana (Green) and Greenops boothi (Green) are frequently enrolled in Hamilton strata; biostratinomic data indicate two very different enrollment postures. Interlocking morphologies (coaptative devices) and apodeme structure and disposition indicate that these postures reflect specific behaviors which involved interaction between tergal structures, inferred musculature, and the substratum. Phacops enrolled by burrowing forward and down into the sediment; dorsal muscles, attached to prominent articulating half-rings, imbricated the thorax such that each lappet overlapped the next posterior segment and locked into a posterior pleural facet. The pygidium was brought into place as the posterior segments of the thorax were placed into vincular notches along the lateral margin of the ventral cephalon. The pygidium locked with the cephalic vincular furrow to complete ‘perfect sphaeroidal’ closure. Greenops enrolled with the cephalon in an upright position at the sediment surface; a submarginal furrow on the ventral surface of the pygidium received the anterior rim of the cephalon. Relatively narrow articulating half-rings limited pleural rotation. Segments were loosely locked into narrow facets at the anterior margin of the next posterior lappet. In spite of rudimentary lappet and half-ring structures, Greenops displays an elaborate system of thoracopygidial muscles which involved dorsoventral and longitudinal attachments along the thorax and into the pygidium. Phacops, in contrast, displays very poorly developed apodemes which occur in the thorax only. Longitudinal muscle strength was likely less important during Phacops enrollment than is evident for the Greenops enrollment procedure. Conversely, Phacops clearly relied to a great degree upon competent closure devices which are poorly developed in Greenops. Biostratinomic data reveal different enrollment behaviors which reflect the function of different enrollment-related morphologies present in each species.     

Late Devonian Acritarchs from the Gneudna formation in the Western Carnarvon basin,Western Australia

The Gneudna Formation is a Late Devonian(Frasnian) sequence of marine calcareous sediments that occurs in the Carnarvon Basin, Western Australia. The present palynological study is based upon subsurface silty strata from a borehole (Pelican Hill or Bibbawarra Bore) that was drilled early this century near the western coastal limit of the Carnarvon Basin.The subject strata have previously been attributed to the Gneudna Formation on lithostratigraphic grounds. They contain a rich and varied assemblage of marine microphytoplankton (acritarchs), associated with trilete miospores of which Geminospora lemurataBalme, 1962 is the dominant form. Forty-seven species of acritarchs are recognizable in the palynoflora, which corresponds very closely with that described recently (Playford & Dring, 1981) from the Gneudna Formation in the vicinity of its type section on the opposite (eastern) side of the Carnarvon Basin. The apparently parochial complexion of the Gneudna acritarch suite is probably illusory, insofar as early Late Devonian acritarchs have not been studied extensively or intensively from either the northern or southern hemispheres.The following new species of acritarchs areformally instituted herein: Elektoriskos villosa, Lophosphaeridium pelicanensis, and Pterospermella tenellula.     

Estuarine fish and tetrapod evolution: insights from a Late Devonian (Famennian) Gondwanan estuarine lake and a southern African Holocene equivalent

The Waterloo Farm lagerstätte in South Africa provides a uniquely well‐preserved record of a Latest Devonian estuarine ecosystem. Ecological evidence from it is reviewed, contextualised, and compared with that available from the analogous Swartvlei estuarine lake, with a particular emphasis on their piscean inhabitants. Although the taxonomic affinities of the estuarine species are temporally very different, the overall patterns of utilisation prove to be remarkably congruent, with similar trophic structures. Significantly, both systems show evidence of widespread use of estuaries as fish nurseries by both resident and marine migrant taxa. Holocene estuaries are almost exclusively utilised by actinopterygians which are overwhelmingly dominated by oviparous species. Complex strategies are utilised by estuarine resident species to avoid exposure of eggs to environmental stresses that characterize these systems. By contrast, many of the groups utilising Devonian estuaries were likely live bearers, potentially allowing them to avoid the challenges faced by oviparous taxa. This may have contributed to dominance of these systems by non‐actinoptergians prior to the End Devonian Mass Extinction. The association of early aquatic tetrapods at Waterloo Farm with a fish nursery environment is consistent with findings from North America, Belgium and Russia, and may be implied by the estuarine settings of a number of other Devonian tetrapods. Tetrapods apparently replace their sister group, the elpistostegids, in estuaries with both groups having been postulated to be adaptated to shallow water habitats where they could access small piscean prey. Correlation of tetrapods (and elpistostegids) with fish nursery areas in the Late Devonian lends strong support to this hypothesis, suggesting that adaptations permitting improved access to the abundant juvenile fish within the littoral zone of estuarine lakes and continental water bodies may have been pivotal in the evolution of tetrapods.