PHYLOGENY AND ONTOGENY OF THE MISSISQUOIIDAE

The family Missisquoiidae is an important member among the Late Cambrian and Early Ordovician trilobites. Eleven genera had been assigned to this family; they are Hardya, Tasmanocephalus, Parakoldinioidia, Pseudokoldinioidia, Missisquoia, Lunacrania, Macroculites, Rhamphopyge, Paranumia, Tangshanaspis, and Fuzhouwania. Various opinions have been expressed on the generic membership, mostly depending on how to define the type genus Missisquoia. This study attempts to review the systematics of the Missisquoiidae with cladistic method and to examine its paleobiogeographic and stratigraphic distribution in regard to the phylogenetic relationship. Four representative phenetic morphotypes are recognized: they are Missisquoia, Tangshanaspis, Parakoldinioidia, and Pseudokoldinioidia morphotypes. Some Middle and Upper Cambrian trilobites display similarities to different missisquoiid taxa, suggesting that the family may not be a monophyletic group. The palaeogeographical and stratigraphical distribution of the missisquoiid morphotypes suggest that the missisquoiids, if monophyletic, would have first evolved in Gondwana and then appeared, through dispersal or vicariance, in Laurentia. These hypotheses are cladistically tested. A data matrix of 26 species (missisquoiids and non-missisquoiids) x 42 characters is built. Three major clades are discovered by the analysis, [[Tangshanaspis clade + Parakoldinioidia clade] + Pseudokoldinoidia clade]. The concept of Missisquoiia is restricted to only two species, Missisquoia typicalis and Missisquoia cyclochila. The nested pattern of the three clades does not correspond to the observed stratigraphic and paleobiogeographic distribution data. Protaspides of Tangshanaspis depressa were discovered in northwestern Canada. Their morphology are of a proetid, not of a corynexochid, type, which seems to contradict the generally-accepted opinion that the Missisquoiidae is a member of the Corynexochida. However, the most derived position of T. depressa in the tree topology demonstrates that its protaspid morphology cannot be generalized.