Heterochrony of the Late Cambrian olenid trilobites from the Machari Formation, Yeongwol, Korea: implications for biostratigraphy and intercontinental correlation

摘要

This study deals with the heterochrony of two Late Cambrian olenid trilobites,Olenus asiaticus Kobayashi, 1944 from theGlyptagnostus reticulatus Zone andHancrania brevilimbata Kobayashi, 1962 from theHancrania brevilimbata Zone, recovered from the Machari Formation in Yeongwol, Korea. Comparison of ontogenetic developments ofOlenus asiaticus andHancrania brevilimbata led to consider that the two species are phylogenetically related: viz.,Hanerania brevilimbata is a probable descendant ofOlenus asiaticus. The ontogenetic trajectories of the two species are closely comparable in the protaspid and early meraspid stages, but deviate in the late meraspid and holaspid stages. The morphological changes with growth in theOlenus asiaticus-Hancrania brevilimbata lineage may have resulted from combined heterochronic processes: i.e., the reduction of preglabellar field in the descendant is attained by paedomorphic progenesis, whereas the expansion of glabella is a consequence of peramorphic acceleration. Of these, the progressive reduction of preglabellar field has been known to occur in other parts of the world and thus is assumed to be useful in improving the biostratigraphic correlation among widely separated paleogeographic provinces, such as Baltica and Avalonia. TheGlyptagnostus reticulatus Zone of the Paleozoic Gondwana can be correlated with theOlenus gibbosus Subzone of Avalonia and Baltica, while theHancrania brevilimbata Zone of Korea with theO. attenuatus andO. dentatus subzones of Baltica and the lower part of theO. cataractes Subzone of Avalonia.