Arthropod Evolution

Abstract

The arthropods are a very diverse group of organismswhose evolution andinterrelation- ships have been vigorously debatedfor more than a century. Supporters of a monophyletic origin for the group rely heavily on the existence of numerous common features in the arthropod body plan. Their opponents, who must account for the extraordinary degree of convergent evolution inherent inanypolyphyletictheory, argue that all of these features are essentiallythe resultof asinglephenomenon, theevolution of a hard exoskeleton, and that arthropodization could easily have been repeated several times among thevarious ancestral groups. In the polyphyletic theory, therefore, the four dominant groups of arthro- pods (Trilobita, Crustacea, Chelicerata, and Insecta), as well as several smaller groups both fossil and extant, originatedfrom distinct, unrelated ancestors. The proponents of poly- phyly use evidence from comparative morphology (notably studies oflimb and mandible structure), comparative embryology (fate maps), and more recently the fossil record (which shows an abundance of arthropod types not easily assignable to already known groups). The monophyleticists claim, in turn, that these comparative embryological and morpho- logical studies are ofdoubtful value because of the methodology employed and assump- tions made. Overall,the current balance seems in favor of a monophyleticorigin for the arthropods. The uniting ofOnychophora,Myriapoda, andHexapoda as the clade Uniramia is highly questionable. Most modern authors agree that apparent similarities between onychophorans and members of theother two groups are due to convergent evolution. The Myriapoda, al- thoughincluding four rather distinct groups (Diplopoda, Chilopoda, Pauropoda, and Sym- phyla), arewidelythoughtto be monophyletic. For many years, myriapods were considered the sister group to the Hexapoda. However, recent research indicates thatmyriapods may be allied more closely to the chelicerates, and hexapods to crustaceans. Five distinct groups of hexapods occur: collembolans, proturans, diplurans, thysanurans, and winged insects. On the basisof their entognathous mouthparts and other synapomorphies the first three groups are placedinthe Entognathaand are distinct from thethysanurans and pterygotes which form the true Insecta