Jurassic radiation of large Branchiopoda (Arthropoda: Crustacea) using secondary structure-based phylogeny and relaxed molecular clocks

摘要

The phylogenetic relationships of large Branchiopoda have been one of the most controversial issues in the studies of Crustacea. This study attempts to tackle systematic problems concerning "conchostracan" and anostracan evolution, by analyzing ribosomal RNA sequences of both mitochondrial (12S) and nuclear genes (18S) . Phylogenetic trees are reconstructed with these data from 27 gener-a belonging to 18 families, using maximum parsimony procedures, maximum likelihood, and Bayesian analysis coupled with Markov chain Monte Carlo (MCMC) techniques under mixed and doublet models. The results suggest that Diplostraca (Conchostraca+ Cladocera) form a monophyletic group, with Spincaudata and Cladoceromorpha being sister groups and Laevicaudata a basal lineage of Diplostraca; Noto-straca and Diplostraca together form the monophyletic Phyllopoda. Within Diplostraca, Conchostraca is paraphyletic in contrast to the traditional systematic classification. Relaxed molecular clock analysis, using multiple fossil calibration points, indicates that the conchostracan three genera—Imnadia, Caenestheriella, and Caenestheria, emerged during Jurassic; most anostracan groups, such as Polyartemiinae, Chirocephalinae, Branchinectidae, and Streptocephalidae, experienced an adaptive radiation during Jurassic, and probably related with the fragmentation of the Pangea supercontinent.