G E Budd

..Why animals appear so late in the fossil record is still unclear, but the recent trend to embrace rising oxygen levels as being the proximate cause remains unproven and may even involve a degree of circularity...

..These findings may further help unravelling the phylogenetic position of the Priapulida within the Scalidophora and hence contribute to the elucidation of the nature of the ecdysozoan ancestor...

..abstract:..

..Of the two splice variants of Ubx/Antp only one could be isolated from myriapods, representing a possible further synapomorphy of the Myriapoda...

..It was therefore assumed that the expression of col in myriapods would reflect the ancestral state like in crustaceans and chelicerates, i.e. absence from the premandibular/intercalary segment and hence no function in its formation...

..We also characterised Wnt gene expression in the latter three species, and further investigated expression of these genes in the beetle Tribolium castaneum...

..A complementary source of data is the discovery of fossils from several spectacular Cambrian faunas. These fossils form well-characterized groupings, making the broad pattern of Cambrian arthropod systematics increasingly consensual...

..In particular, understanding how morphology can evolve may provide the critical link between the ecological and morphological networks that are currently the intense focus of evolutionary investigations...

..This analysis points to the requirement for a careful application of systematic methodology before explanations are sought for alleged patterns of constraint and flexibility...

..Evolutionary functional morphology has been relatively little considered in the context of the evolution of development, but may play an important role in defining the framework within which this evolution occurs...

..The latter appendage has been reduced in all crown-group euarthropods. Its most likely relic is as a component of the labrum. These fossils thus tie together results from disparate living groups (onychophorans and euarthropods)...

..These features amplify and clarify the diversity of known anomalocaridid morphology and provide insight into the origins of important arthropod features, such as the head shield and respiratory exites...

..The reversed hth and exd patterns in chelicerates and myriapods are therefore an apomorphy for this group, the Myriochelata, lending support to the Myriochelata and Tetraconata clades in arthropod phylogeny...

..The lack of the traditional expression in the posterior compartment of segments reported earlier in onychophorans is discussed, and we suggest that onychophorans may have acquired two copies of engrailed with different functions...

..Evidence is presented to further support the presence of a terminal mouth in the ground plan of the Onychophora and, hence, an acron may not exist in the arthropod clade...

..The proposed mechanism is similar to the mechanism used to establish the AP boundary in Drosophila wings and ventral pleura...

..All of these forms of inference are fundamental for comparative biology and of immediate importance to the practice of evolutionary developmental biology...