PHYLOGENY AND PALEOBIOGEOGRAPHY OF THE FAMILY NISUSIIDAE (CAMBRIAN RHYNCHONELLIFORM BRACHIOPOD) WITH SYSTEMATIC REVISION OF NISUSIIDS FROM SOUTH KOREA

The Nisusiidae, one of the most primitive brachiopod group, consists of Nisusia, Narynella, Eochoncha and Bellistrophia. Nisusia, which is the only group with a spinose shell and shows most widespread paleogeographic distribution, has been regarded as one of the key taxa in tracing the early evolution of rhynchonelliform brachiopods. This study aims to 1) re-evaluate South Korean nisusiids based on new collection; 2) to examine phylogeny of the Nisusiidae through cladistic analysis; and 3) to investigate the evolutionary and paleobiogeographic history.

Nisusia paucicostellata and Nisusiidae gen. sp. indet., are identified from the middle Cambrian strata of the Mungyeong Group exposed in the eastern part of South Korea. N. paucicostellata is characterized by a strophic and semiquadrate outline, dorsibiconvex lateral profile, rectimarginate anterior commissure and catacline to weakly apsacline ventral interarea. Nisusiidae gen. sp. indet. also has a strophic, biconvex shell with apsacline ventral interarea, but is distinguished in having a semirectangular outline and emarginate anterior commissure.

A data matrix of 27 characters and 26 taxa was analyzed with parsimony methods. For phylogenetic and paleobiogeographic interpretations, two trees were selected among 80 most parsimonious trees with 125 tree length, based on multiple stratigraphic congruence indices which are independent of character-based analysis. Both trees illustrate the presence of three monophyletic or paraphyletic groups recognized by three characters, development of spines, type of ventral interarea, and development of ventral convexity. The character state transformations show that the nisusiids may have evolved from strongly spinose to non-spinose forms through finely spinose condition and as they evolved, the ventral interarea changed from procline or catacline to apsacline state and the highest point of ventral valve moved anteriorly, from at umbo to mid-valve position.

In order to trace paleobiogeographic history of the nisusiids, cladogram-based analyses were performed. The calculated ancestral area index indicates that the nisusiids originated in Laurentia. Further analyses are pending to investigate how the nisusiids migrated into other paleocontinents, by vicariance or dispersal.