A NEW EARLY ORDOVICIAN CONODONT GENUS FROM THE SOUTHERN MONTAGNE NOIRE, FRANCE

Abstract:  Two species of a new Tremadocian (Early Ordovician) conodont genus from the Saint Chinian Formation of the southern Montagne Noire, France, are erected: Hammannodus sarae gen. et sp. nov. and Hammannodus juliae gen. et sp. nov. They were found within a single storm-induced limestone nodule interbedded with offshore shales belonging to the regional Shumardia ( C .) pusilla (trilobite) Biozone, and to the Paltodus deltifer deltifer (conodont) Subzone. This conodont record is associated with the episodic development of carbonate productivity in temperate waters of the Montagne Noire platform, a process absent in neighbouring platforms of north-west Gondwana. The apparatus is composed of five coniform pyramidal elements occupying P and S positions and one bicostate element in the M position, having three or two sharp costae, respectively, with a subtriangular basal outline.     

Constructional morphology of strombid gastropods

The extreme diversity in shell shape of strombid gastropods is interpreted as the result of three independent factors: (1) The terminal growth pattern of the Strombidae allows the circumvention of geometric constraints on shell morphology found in gastropods with continuous or periodic growth patterns. (2) Shell morphology in the Strombidac is adaptive in epifaunal locomotion, burrowing. infaunal or semi-infaunal habits, and passive protection from predators. Specialization for one of these functions often conflicted with the others. thus bringing about a forced 'choice' among mutually exclusive morphological characters. (3) Conservatism in life habits and anatomy of the soft parts has allowed the multiple evolution of extreme shell morphologies, as well as the secondary return to relativcly unspecialized morphologies. □ Constructional morphology, functional morphology. growth. behaviour. evolution, locomotion, burrowing, predation, exoskeleton. shell. Mollusca. Gastropoda. Strombacea. Strombidae.     

Environmental factors affecting patterns of distribution and co‐occurrence of two competing raptor species

Habitat selection is a complex process, that is affected by several factors, including habitat characteristics, environmental conditions, and both intra‐ and interspecific interactions. We analysed habitat preferences of two top avian predators, Peregrine Falcon Falco peregrinus, a medium‐sized diurnal raptor, and Eagle Owl Bubo bubo, a large nocturnal raptor. These two species are known to compete for preferred nest‐sites, and proximity to cliffs with Eagle Owls may reduce Peregrine breeding output through predation of young Falcons. We investigated the environmental factors affecting occurrence and coexistence of the two species and the potential role of habitat suitability in favouring co‐occurrence in 3519 km² of the central pre‐Alps of Italy, where the two species breed on cliffs and sometimes co‐occur on the same cliff. Peregrines settled on long, steep and favourably orientated cliffs in woodland landscapes close to urban areas. Eagle Owls settled on topographically similar cliffs, but in lower rainfall areas compared with cliffs occupied by Peregrines and cliffs unoccupied by either species. Sites where the two species co‐occurred were characterized by more horizontally extended cliffs compared with sites of exclusive occurrence of each species. An analysis of relative habitat suitability revealed that sites where the two species co‐occurred had the highest predicted probability of occupancy for both species, suggesting that those sites should be regarded as high‐quality sites. Breeding productivity of Eagle Owls was negatively affected by the co‐occurrence of Peregrines, whereas the effect of Eagle Owl proximity on Peregrine productivity varied according to cliff suitability for the Peregrines. Habitat selection had fitness consequences for Eagle Owls because breeding productivity increased with cliff length. Environmental conditions, particularly climatic factors, could allow the widespread coexistence of these competing raptors at the landscape scale, whereas at the local scale co‐occurrence could take place only on larger cliffs. These were preferred sites for both species, presumably because breeding at such sites offsets the costs of settling close to the competitor species.     

Shell biomechanics in the bivalve Laternula

The ligament in Laternula is modified into a complex mechanical structure composed of rigid and flexible parts with multiple connection points. Rather than allowing the valves to move with respect to each other, this ligament tends to keep them immobile. Providing space inside the shell for the modified ligament requires a secondary increase in shell curvature of the umbones. This is achieved by the shell flexing by muscular contraction and stiffening while flexed through the construction of a rigid buttress. A vertical slit in each valve, produced by secondary resorption and breakage of shell material, confines mechanical stress to the posterior slope of the shell. □ Mollusca, Bivalvia, Laternulidae, Holocene, Indo-Pacific, functional morphology.     

Barbatia mytiloides and the evolution of shell torsion in arcid pelecypods

Barbatia mytiloidrs (Brocchi) (Arcidae, Arcinae) from the Neogene of northwestern Italy shows a variable but significant amount of dextral shell torsion about the hinge axis. This phenomenon represents a morphological and functional convergence with the sinistrally torted ark Trisidos . Polyphyletic evolution of shell torsion in mud-dwelling, semi-infaunal descendants of epibyssate Barbatia is probably due to ventilation problems that forced these forms to lie with the posterior commissure horizontal. Depending on which valve happened to lie lowermost in the non-torted reclining or semi-infaunal ancestors, shell torsion became established in dextral or sinistral direction.     

Geometric and functional constraints on bivalve shell morphology

The range of shell morphologies available to bivalves is constrained by the geometric properties of coiled shells, and by two contrasting functional necessities: positioning the umbones at a distance from each other, to allow an adequate amount of shell gape, and limiting linear growth of the axial shell margin, in order to prevent the ligament from being rapidly stretched beyond its elasticity limits. These necessities are achieved, or circumvented, in one or more of the following ways: (1) evolving a range of inequivalve adaptations, (2) allowing a large amount of interumbonal growth, while simultaneously adopting a ligament that quickly breaks and is continuously replaced during ontogeny, (3) adopting an outward curving ligament which flexes along its entire width, thus effectively placing the actual pivoting axis of the valves dorsally to the axial shell margin, (4) substituting the ligament with diductor muscles, (5) stopping growth before shell coiling reaches half whorl, (6) decreasing the whorl expansion rate throughout ontogeny, (7) orienting the coiling axes of the umbones at an angle to each other and to the hinge axis.     

Silurian bivalves

Liljedahl, L. 1994: Silurian nuculoid and modiomorphid bivalves from Sweden. Fossils and Strata 33 .     

Experiments on the frictional properties of terrace sculptures

Artificial terrace patterns and molluscan shells were dragged on and within sorted sandy sediments, and the differential force necessary to move in the forward versus backward direction was measured. As expected, terraces display different frictions when moving in opposite directions. However, no obvious relationship in frictional properties emerged between the studied ranges of terrace heights and sediment grain-sizes. Rather, friction is affected by the shape of sediment grains. Terraces with a very low inclination of their leading sides are more effective than terraces of similar height but with steeper leading sides, in spite of the closer packing of terraces allowed by the latter character. Experimental results appear to be considerably influenced by minute changes in sediment compaction (especially in water-permeated sediments), depth of burial, and orientation of the test objects. □ Burrowing sculptures, fiction, functional morphology, experimental biology, biomechanics.