THE EURYPTERID ADELOPHTHALMUS SIEVERTSI (CHELICERATA: EURYPTERIDA) FROM THE LOWER DEVONIAN (EMSIAN) KLERF FORMATION OF WILLWERATH, GERMANY

Abstract: The eurypterid Rhenopterus sievertsi Størmer from the Lower Devonian of Germany is redescribed. Based on new morphological data, including the possession of prosomal limbs of Adelophthalmus‐type and spatulae on the genital operculum, the species is transferred to Adelophthalmus Jordan, in Jordan and von Meyer and thus is the oldest representative of this geographically and stratigraphically widespread genus. Eurypterus? trapezoides Størmer is recognized as a junior synonym of A. sievertsi.     

Fossilized ontogenies: the contribution of placoderm ontogeny to our understanding of the evolution of early gnathostomes

Placoderms, representing phylogenetically more inclusive jawed vertebrates and successive sister taxa to crown‐group gnathostomes, are critical to our understanding of character evolution within the crown‐group (chondrichthyans + osteichthyans), including developmental characters. Early ontogenetic stages of placoderms are generally poorly known, although some exceptional faunas preserve both embryonic (e.g. from the Gogo Formation, Western Australia) and post‐embryonic individuals (the Miguasha Formation, Canada; Lode Formation, Latvia; Merriganowry Formation, Gogo Formation, Australia). Information provided by these ontogenies is relevant to questions of placoderm taxonomy and phylogeny, but also to broader questions pertinent to vertebrate evolution as a whole, for example, evolution of bone development, evolution of the axial skeleton and evolution of reproduction.     

Eubrachythoracid arthrodires with tubular rostral plates from Gogo, Western Australia

Two monotypic genera of tubular-snouted eubrachythoracid arthrodires are described from the Upper Devonian Gogo Formation of Western Australia as Rolfosteur gen nov. and Tubonnsus gen. nov. They are most closely related to Camuropiscis Dennis & Miles, also from the Gogo Formation, and all three are therefore placed in the family Camuropiscidae nov. Rolfosteus and Tubonasus are jointly the sister-group of Camuropiscis .     

A New Carcinosomatid Eurypterid From The Upper Silurian Of Northern Vietnam

A new carcinosomatid eurypterid, Rhinocarcinosoma dosonensis sp. nov., and Hughmilleria sp., are described from the Dô Son Formation of the Dô Son Peninsula, northern Vietnam. R. dosonensis is characterized by podomere 7 of prosomal appendage VI produced into an anterodistal spine, a metastoma with a cordate posterior margin, and an indented opisthosomal differentiation (i.e. preabdomen–postabdomen margin is concave). The Dô Son Formation was originally interpreted as Late Devonian (Givetian/Frasnian) in age but the unit containing the eurypterid assemblage is now considered much older (Late Silurian). A deltaic palaeoenvironmental setting is interpreted from biotic associations and sedimentological evidence.     

Eubrachythoracid arthrodires from Gogo, Western Australia

Two monotypic genera of coccosteoid eubrachythoracid arthrodires are described from the Upper Devonian Gogo Formation of Western Australia as Compagopiscis gen. nov. and Gogopiscis gen. nov. They are most closely related to Torosteus Gardiner & Miles, also from the Gogo Formation, and all three are placed in the family Plourdosteidae Vezina, 1990; Compagopiscis and Gogopiscis are jointly the sister-group of Torosteus.     

A pachyosteomorph arthrodire from Gogo, Western Australia

A new monotypic genus of eubrachythoracid arthrodire is described from die Upper Devonian Gogo Formation of Western Australia as Incisoscutum ritchiei gen. et sp. nov. It is at die pachyosteomorph level of organization, having no contact between the plates of die flank and those of the ventral shield behind the pectoral fin. It is interchangeable widi Coccosteus spp. and a number of earlier described Gogo forms in a sketch dadogram for arthrodires, but its phylogenedc relationships are unknown. Well preserved material of die scapulocoracoid throws new light on die structure of this element in arthrodires, particularly widi respect to variations in die pattern of canals and foramina.     

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.     

A eubrachythoracid arthrodire with a snubnose from Gogo, Western Australia

A new eubrachythoracid arthrodire, Simosteus tuberculatus gen. et sp. nov., is described from the Upper Devonian Gogo Formation of Western Australia and its structure and relationships are discussed. It is related to the trematosteids, leiosteids and hadrosteids on the evidence of 'fused' preorbital and postnasal bones.     

New trace fossil evidence for eurypterid swimming behaviour

We describe a recently discovered trace fossil from a eurypterid Konservat‐Lagerstätte in the upper Silurian Tonoloway Formation of Pennsylvania, and formally describe contemporaneous traces from the Williamsville Formation Lagerstätte of Ontario. The traces from both localities are assigned here to Arcuites bertiensis igen. et isp. nov. Based on comparisons with previously described eurypterid trackways, neoichnological experiments, and the co‐occurrence with eurypterid remains, Arcuites is interpreted as having been made by the swimming leg (sixth prosomal appendage) of swimming juvenile to adult eurypteroid eurypterids, and represents the first unambiguous trace fossil evidence for eurypterid swimming behaviour. The morphology of Arcuites indicates that eurypteroid eurypterids swam using drag‐based rowing, whereby the animal propelled itself forward by moving its oar blade‐like swimming paddles in an in‐phase backstroke. Arcuites morphology also indicates that the eurypteroid swimming appendage had a greater degree of movement than was previously suggested, and a revised rowing model is proposed. Differences in the abundance of A. bertiensis in the Tonoloway and Williamsville formations suggest a bathymetric control on eurypterid swimming behaviour and trace production. The association of Arcuites with eurypterid body fossils in both units indicates that these Lagerstätten were autochthonous assemblages and provides additional evidence for eurypterid inhabitation of shallow subtidal marine environments in the late Silurian.     

PALEOMERUS-MEROSTOME OR MEROSTOMOID

The relation between segmentation and length of the head tagma in some groups of arthropods is discussed and used for a tentative assignation of Paleomerus to Merostomoidea Størmer, 1944. The relationships between Strabopidae Gerhardt, 1932, and Paleomeridae are discussed. A new specimen of P. hamiltoni gives some idea of the exterior surface of the exoskeleton.     

A putative tyrosine phosphorylation site of the cell surface receptor Golden goal is involved in synaptic layer selection in the visual system

Golden goal (Gogo) is a cell surface protein that is crucial for proper synaptic layer targeting of photoreceptors (R cells) in the Drosophila visual system. In collaboration with the seven-transmembrane cadherin Flamingo (Fmi), Gogo mediates both temporary and final layer targeting of R-cell axons through its cytoplasmic activity. However, it is not known how Gogo activity is regulated. Here, we show that a conserved Tyr-Tyr-Asp (YYD) tripeptide motif in the Gogo cytoplasmic domain is required for photoreceptor axon targeting. Deleting the YYD motif is sufficient to abolish Gogo function. We demonstrate that the YYD motif is a phosphorylation site and that mutations in the YYD tripeptide impair synaptic layer targeting. Gogo phosphorylation results in axon stopping at the temporary targeting layer, and dephosphorylation is crucial for final layer targeting in collaboration with Fmi. Therefore, both temporary and final layer targeting strongly depend on the Gogo phosphorylation status. Drosophila Insulin-like receptor (DInR) has been reported to regulate the wiring of photoreceptors. We show that insulin signaling is a positive regulator, directly or indirectly, of YYD motif phosphorylation. Our findings indicate a novel mechanism for the regulation of Gogo activity by insulin signaling-mediated phosphorylation. We propose the model that a constant phosphorylation signal is antagonized by a presumably temporal dephosphorylation signal, which creates a permissive signal that controls developmental timing in axon targeting.     

Gogo Receptor Contributes to Retinotopic Map Formation and Prevents R1-6 Photoreceptor Axon Bundling

Background

Topographic maps form the basis of neural processing in sensory systems of both vertebrate and invertebrate species. In the Drosophila visual system, neighboring R1–R6 photoreceptor axons innervate adjacent positions in the first optic ganglion, the lamina, and thereby represent visual space as a continuous map in the brain. The mechanisms responsible for the establishment of retinotopic maps remain incompletely understood.

Results

Here, we show that the receptor Golden goal (Gogo) is required for R axon lamina targeting and cartridge elongation in a partially redundant fashion with local guidance cues provided by neighboring axons. Loss of function of Gogo in large clones of R axons results in aberrant R1–R6 fascicle spacing. Gogo affects target cartridge selection only indirectly as a consequence of the disordered lamina map. Interestingly, small clones of gogo deficient R axons perfectly integrate into a proper retinotopic map suggesting that surrounding R axons of the same or neighboring fascicles provide complementary spatial guidance. Using single photoreceptor type rescue, we show that Gogo expression exclusively in R8 cells is sufficient to mediate targeting of all photoreceptor types in the lamina. Upon lamina targeting and cartridge selection, R axons elongate within their individual cartridges. Interestingly, here Gogo prevents bundling of extending R1-6 axons.

Conclusion

Taken together, we propose that Gogo contributes to retinotopic map formation in the Drosophila lamina by controlling the distribution of R1–R6 axon fascicles. In a later developmental step, the regular position of R1–R6 axons along the lamina plexus is crucial for target cartridge selection. During cartridge elongation, Gogo allows R1–R6 axons to extend centrally in the lamina cartridge.     

A new camuropiscid arthrodire (Pisces: Placodermi) from Gogo, Western Australia

A new camuropiscid arthrodire, Latocamurus coulthardi gen. et sp. nov., is described from the Upper Devonian Gogo Formation, Western Australia. Latocamurus , known from two complete specimens, is recognized as a camuropiscid by its narrow, spindle-shaped armour, deep postnasal plates participating in the orbits, preorbital plates which meet mesially, cheek unit firmly sutured to skull roof, posterior check plates tightly interconnected and much reduced, and the robust durophagous dentition. It is characterized by its downturned snout, broad, flat rostral plate, and narrow, deep parasphenoid. It is placed phyletically as the plesiomorphic sister taxon to all other camuropiscids which are more derived in having, inter alia , an anterior lateral plate which anteriorly contacts the anterior ventrolateral plate and pointed rostral plates. The family Camuropiscidae Dennis & Miles 1979b is redefined to incorporate features of the new genus. Camuropiscids and Incisoscutum are closely related by features of the postnasal plate and cheek.     

New durophagous arthrodires from Gogo, Western Australia

Three new monotypic genera of eubrachythorarid arthrodires with crushing toothplates are described from the Upper Devonian Gogo Formation of Western Australia as Bruntonichthys multidens gen. et sp. nov., Bullerichthysfascidens gen. et sp. nov. and Kendrickkhthys cavemosus gen. et sp. nov. Their structure and relationships are discussed within the framework provided by a sketch cladogram. All three are interchangeable with Coccosteus spp. but their phylogenetic relationships are not known in any detail. However, Kendrickkhthys does appear to be immediately related to Dinomylostoma from North America.     

The transmembrane protein Golden goal regulates R8 photoreceptor axon-axon and axon-target interactions

During Drosophila visual system development, photoreceptor (R) axons choose their correct paths and targets in a step-wise fashion. R axons with different identities make specific pathfinding decisions at different stages during development. We show here that the transmembrane protein Golden goal (Gogo), which is dynamically expressed in all R neurons and localizes predominantly to growth cones, is required in two distinct steps of R8 photoreceptor axon pathfinding: Gogo regulates axon-axon interactions and axon-target interactions in R8 photoreceptor axons. gogo loss-of-function and gain-of-function phenotypes suggest that Gogo mediates repulsive axon-axon interaction between R8 axons to maintain their proper spacing, and it promotes axon-target recognition at the temporary layer to enable R8 axons to enter their correct target columns in the medulla. From detailed structure-function experiments, we propose that Gogo functions as a receptor that binds an unidentified ligand through its conserved extracellular domain.     

On the Emsian (Lower Devonian) arthropods of the Rhenish Slate Mountains: 2. The synziphosurineWillwerathia

Willwerathia laticeps (Størmer 1936) originally described as a eurypterid is reinterpreted as a synziphosurine belonging to Family WeinberginidaeRichter &Richter 1929. Recently collected material from the locality, from which the destroyed holotype comes, suggests thatWillwerathia possessed ten opisthosomal tergites, deduced from all available arrays of disarticulates.Willwerathia is the largest synziphosurine yet discovered with a carapace approximately 90 mm across. The occurrence of synziphosurines with eurypterids mirrors previously described preservational associations.     

A primitive eubrachythoracid arthrodire from Gogo, Western Australia

A new eubrachythoracid arthrodire, Hanytoombsia elegans gen. et sp. nov., is described from the Upper Devonian Gogo formation of Western Australia. Its structure and relationships are discussed within the framework provided by a new sketch cladogram for arthrodires. H. elegans is morphologically similar to Coccosteus cuspidatus , but its relationships among primitive eubrachythoracids are unknown.