Are the traditional classes of arthropods natural ones? Recent advances in palaeontology and some considerations on morphology

Abstract

The present paper considers the problems of evaluating some important morphological characters both in fossil and living arthropods as examples of the difficulty of assessing the significance of all characters of the Arthropoda for the purpose of phy‐logenetic reconstruction. As both the fossil and the molecular evidence point to a splitting of the main branches of the metazoans well in the early Vendian or even earlier, and of the arthropods during the Cambrian, this must be carefully considered in the assessment of all sorts of evidence. Moreover the fossil evidence clearly indicates that, as far as the arthropods are concerned, there was a slow acquisition of the arthropod characters in different lineages. I argue, on the evidence of a few selected and important characters (such as basic morphology of the appendages, tagmosis, development of tracheal systems and of the overall evolution of the general morphology of selected groups) that convergent and parallel evolution was common and that, especially when functionally important characters are concerned, extremely similar or even identical structures were repeatedly and independently evolved. Therefore, I argue that an assessment of the phylogenetic significance of characters is indeed possible, but that it requires the simultaneous assessment of all functionally related characters and apparatuses and of their adaptive significance, and furthermore that no standard rules may apply, all the more so the more the characters considered are functionally important. Finally, just as a working hypothesis, I suggest a new phylogenetic arrangement of arthropod taxa.