A latitudinal gradient in biodiversity has existed since before the time of the dinosaurs, yet how and why this gradient arose remains unresolved. Here we review two major hypotheses for the origin of the latitudinal diversity gradient. The time and area hypothesis holds that tropical climates are older and historically larger, allowing more opportunity for diversification. This hypothesis is supported by observations that temperate taxa are often younger than, and nested within, tropical taxa, and that diversity is positively correlated with the age and area of geographical regions. The diversification rate hypothesis holds that tropical regions diversify faster due to higher rates of speciation (caused by increased opportunities for the evolution of reproductive isolation, or faster molecular evolution, or the increased importance of biotic interactions), or due to lower extinction rates. There is phylogenetic evidence for higher rates of diversification in tropical clades, and palaeontological data demonstrate higher rates of origination for tropical taxa, but mixed evidence for latitudinal differences in extinction rates. Studies of latitudinal variation in incipient speciation also suggest faster speciation in the tropics. Distinguishing the roles of history, speciation and extinction in the origin of the latitudinal gradient represents a major challenge to future research.

ABSTRACT-We describe the morphology of Semionotus, focusing on the Semionotus elegans group from the Newark Supergroup of eastern North America. Our description is based largely on specimens from the Boonton Formation (Early Jurassic) of New Jersey because they are particularly well-preserved and include good material of both the dermal skeleton and the endoskeleton. A single anamestic suborbital distinguishes Sernionotus from its sister-genus Lepidotes. We restrict the Semionotidae, defined by the presence ofdorsal ridge scales between the nape and dorsal fin as well as a large posteriorly directed process on the epiotic, to two genera, Semionotus and Lepidotes. We restrict the Semionotiformes, defined by four characters and five character losses, to the Lepisosteidae, Macrosemiidae, and Semionotidae. Our study of Semionotus and previous work on Watsonulus suggest new interpretations of characters and character polarities. These data support the hypothesis that the Semionotiformes as we define them are more closely related to teleosts than either is to Amia. Analysis of the same data using PAUP suggests an equally parsimonious hypothesis that the Semionotiformes and Amia form a monophyletic group that, in turn, forms the sister-group to teleosts.

The diet and habitat of eight danionin species (Danio and Devario) from 18 sites in India and Thailand are reported. At every site, habitat characteristics, including pH, light penetration, water temperature, current speed, stream dimensions, canopy cover and substratum type, were recorded. Danionins were found primarily in warm (24-35°C), moving water of moderate clarity and pH (6Á6-8Á2) and there were significant differences among species in water temperature, pH and current speed. Deep-bodied Devario species were generally associated with faster water currents than more slender-bodied Danio species. Gut content analyses of 327 individuals representing 17 populations showed that insects were the primary food resource for the eight Danio and Devario species. Crustaceans, fish scales, algae and detritus were also important supplements for particular species. Stable isotope data from syntopic species indicated long-term differences in consumption of terrestrial v. aquatic insects in one of two population pairs.

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The bowfin (Amia calva) is a ray-finned fish that possesses a unique suite of ancestral and derived phenotypes, which are key to understanding vertebrate evolution. The phylogenetic position of bowfin as a representative of neopterygian fishes, its archetypical body plan and its unduplicated and slowly evolving genome make bowfin a central species for the genomic exploration of ray-finned fishes. Here we present a chromosome-level genome assembly for bowfin that enables gene-order analyses, settling long-debated neopterygian phylogenetic relationships. We examine chromatin accessibility and gene expression through bowfin development to investigate the evolution of immune, scale, respiratory and fin skeletal systems and identify hundreds of gene-regulatory loci conserved across vertebrates. These resources connect developmental evolution among bony fishes, further highlighting the bowfin’s importance for illuminating vertebrate biology and diversity in the genomic era.