Geographic differentiation and speciation in Cerion–a preliminary discussion of patterns and processes

Cerion's exuberant morphological diversity (600 described 'species') combined with the extreme rarity of reproductive isolation among morphotypes (only one unambiguous case of sympatry in the Bahamas) has long made this genus an object of fascination for evolutionists and of frustration for taxonomists. We have pursued an integrated approach of genetic and morphometric study based on field investigations of ecology and biogeography in the Bahamas. Cerion's morphotypes are not distributed haphazardly, but show definite patterns of correlation with habitat and geography. Although all morphotypes interbreed, hybrid zones tend to be narrow and characterized by highly local genetic anomalies–unique alleles present in neither parental population. Different patterns of covariance in ontogeny, and habitat preferences, also indicate mat the morphotypes are distinctive, non-amalgamating entities (despite little difference in the frequencies of structural genes among them) that may be called species once a definition based on strict reproductive isolation is abandoned. Variation in structural genes, anatomy and morphology is non-concordant, but orderly for each criterion. Similar morphologies are often polyphyletic and evolved repeatedly as one possible ontogenetic route within a developmental program common to all Cerion. Although we cannot always distinguish among competing causes for observed patterns, we can establish genetic, morphological, anatomical, and biogeographic criteria for decisions when adequate evidence is available. The species of Cerion will be reduced by more than an order of magnitude from a list currently described.     

Evolutionary patterns in biogeography and host plant association: 'taxonomic conservatism' in Lophopidae (Hemiptera, Fulgoromorpha)

Abstract.  The association between the Lophopidae (Hemiptera, Fulgoromorpha) and their host plants was studied within a phylogenetic framework. Host plant use was optimized on Lophopidae phylogeny and the most parsimonious hypothesis is presented. This hypothesis describes the evolution of host plant use by the Lophopidae, and postulates the ancestral plant family used. This scenario is discussed within the biogeographical evolutionary context of the fulgoromorphan families, and is corroborated by information from both insect and host plant fossils. The association of the Lophopidae and their host plants is made by comparing the angiosperms and Lophopidae phylogenies, demonstrating at this level of comparison that the insects show 'taxonomic conservatism' for their host plants.     

Stable carbon isotopes and the metabolism of the terrestrial Devonian organism Spongiophyton

Devonian fossils of Spongiophyton have been identified as a terrestrial evolutionary intermediate between algae and vascular land plants on the basis of their dichotomously branched, tubular morphology, thick cuticles, and the scattered distribution of surface pores. Our understanding of their physiology is, however, severely limited, but may be increased through the use of stable carbon isotope measurements. One such study led to the hypothesis that Spongiophyton carried a carbon isotope (δ¹³C) signature characteristic of lichens ( Jahren et al., 2003 Geology 31 , 99–102). Here, we outline three difficulties with accepting this idea and report independent isotopic measurements of Spongiophyton fossils from Canada and Ghana. Our results show that the isotopic discrimination of analogous tissues of Spongiophyton, extant lichens, liverworts and mosses are statistically indistinguishable. We suggest therefore that claims to have definitively identified lichen metabolism are premature, and cannot be sustained.     

Cladistics of the genera of Mesochorinae (Hymenoptera: Ichneumonidae)

Abstract. A cladistic analysis is presented for the genera of the ichneumonid subfamily Mesochorinae. Nine genera are recognized as valid. Previous work on larval morphology is reviewed and a number of larvae are newly illustrated and discussed. Four new synonyms are proposed: Oncocotta, Piestetron, Plectochorus and Rhaibaspis are all synonymized with Mesochorus , and Stictopisthus is placed back as a synonym of Mesochorus. Four new genera are described: Artherola from South Africa, Chineater from Chile, Thamester from Japan, and Varnado from Morocco. A new key is provided to the mesochorine genera of the world.     

Belemnite battlefields

Mass accumulations of belemnite rostra ('belemnite battlefields') are common in Mesozoic sediments, and accumulations of belemnoids are also known from older rocks. Many Recent teuthid species suffer mortality immediately after spawning, and some authors have suggested that belemnite accumulations record a similar phenomenon. Conversely, it is clear that many belemnite battlefields actually formed in an environment of net sediment loss, with current alignment and winnowing. A proposed model for the production of belemnite battlefields involves five possible pathways: post-spawning mortality, catastrophic mass mortality, predation concentration, stratigraphical condensation, and resedimentation. Although accumulations that have not been reworked are rare, it is possible to recognise the generating agents of belemnite accumulations through their intrinsic features. A genetic classification of belemnite battlefields, based on the identified pathways, is tested through field examples in Britain and Antarctica, as well as through published examples. □ Belemnites, fossil accumulations, palaeoecology, Mesozoic, Antarctica, Yorkshire.     

Ultrastructure of the loop of terebratulide brachiopods

The folded and twisted calcareous ribbon, forming both the ascending and descending lamellae of the loop of Waltonia inconspicua (Sowerby), is a two-layered structure consisting of a wedge of regularly stacked secondary layer fibres that overlie a thin layer of non-fibrous calcite (herein termed brachiotest). On one surface, that facing into the mantle cavity, secondary fibrous mosaic predominates, but smooth, finely banded brachiotest occurs as a narrow marginal lip upon which secondary layer fibres proliferate and progressively overlap. This growing edge of the ribbon is secreted by long, folded epithelial cells with digitate extensions to their apical plasmalemmas, which are distinguishable from the cuboidal epithelium-secreting fibres and their membranous sheaths. The other surface, facing the body cavity and the brachial coelom, consists entirely of roughened brachiotest exhibiting prominent banding that is aligned parallel to the growing loop edge. This surface is overlain by microfilamentar epithelium acting as a holdfast for the connective tissue frame of the lophophore. The other edge of the ribbon consists of truncated sections of both secondary-layer fibres and brachiotest which bear signs of resorption consistent with the degenerated state of the associated epithelium. Growth of the Waltonia loop is controlled by these localized processes of secretion and resorption of the fibrous and brachiotest layers and is typical of all terebratulides so far studied. The brachiotest is not homologous with the non-fibrous primary shell secreted at the valve margin. □ Brachiopoda, Articulata, Terebratulida, ultrastructure, lophophore, loop.     

Phytophagous British insects and mites and their food-plant families: total numbers and polyphagy

The Phytophagous Insects Data Base (PIDB) was used to summarize information about 6933 species of British insects/mites and their food-plant families. Total species in 183 insect families on 127 vascular plant families were correlated with numbers of plant species in each family in Britain and Europe, accounting for 41.8% of the variation. Families with trees had more insects, while ferns and aquatic, uncommon and ephemeral plants, particularly orchids, had fewer. Seven individual insect families were analysed separately: Aphididae were more closely correlated with plant species numbers than were the more polyphagous Geometridae. Agromyzidae were uncommon on families with trees. Of insect species 75.8% fed on only one plant family and 10.1% were recorded on two families. Species feeding on three families or more were considered to be polyphagous. In the 30 largest insect families, eight had species feeding on only one plant family, while 12 had more than 20% of their species polyphagous. Proportions of family-restricted insects were high on Pinaceae, Gramineae and Compositae, but low on Corylaceae, Primulaceae and Tiliaceae. More pest species were polyphagous. Polyphagy was related to large larvae, larvae overwintering, wood-feeding, ephemeral food sources, poor dispersal abilities and omnivory. Some polyphagous species might be actively evolving in exploitation of vacant niches, leading to later progressive specialization to food-plants.     

Femoral ontogeny and locomotor biomechanics of Dryosaurus lettowvorbecki (Dinosauria, Iguanodontia)

Ontogenetic changes in femoral morphology and locomotion were analysed in the iguanodontian dinosaur Dryosaurus lettowvorbecki using cross-sectional data and applying principles of beam theory. The results presented here suggest that locomotor ontogeny in D. lettowvorbecki was more complicated than has generally been recognized. The percentage cortical area (a measure of the relative amount of bone) increases abruptly over a relatively short period during early ontogeny and then remains uniform during subsequent increases in body size. Modifications in cross-sectional shape also occur with increasing size, as demonstrated by differences in second moment of area ratios. The patterns of change in these properties indicate that the orientation of mechanical loadings acting on the femur of D. lettowvorbecki differed at various stages of growth and development. It is suggested that the alterations in femoral architecture described here reflect a shift from quadrupedality to bipedality early in the ontogeny of this animal.