THREE-DIMENSIONAL LANDMARK ANALYSIS OF TWO LATE CAMBRIAN (FURONGIAN) TRILOBITES, ASIOPTYCHASPIS SUBGLOBOSA AND QUADRATICEPHALUS ELONGATUS

Silicified specimens of two ptychaspidid trilobites, Asioptychaspis subglobosa (Sun, 1924) and Quadraticephalus elongatus Kobayashi, 1935, from the middle Furongian Hwajeol Formation, Korea, are well preserved and retain their original convexity. A. subglobosa occurs immediately below Q. elongatus, and based on phylogenetic analysis, A. subglobosa is considered an ancestral sister taxon of Q. elongatus. Three-dimensional coordinates of 30 landmarks were obtained for 40 cranidia of A. subglobosa and 22 cranidia of Q. elongatus including immature forms. Results of principal component analysis (PCA) using 2D landmark data indicated that the differences between A. subglobosa and Q. elongatus were related to the width of the fixigena near the eyes and the width of the glabella. However, PCA using 3D landmark data revealed that a more significant difference lies in the convexity, particularly the convexity of the glabella, evident even from the morphologically immature stages. The growth trajectories of the two species do not overlap with each other, making it difficult to use heterochrony to describe their evolutionary relationship. The spatial aspect of the morphological difference appears to be due to heterotopy. Nonetheless, the migration paths of the landmarks during ontogeny are similar in both species, suggesting that they are phylogenetically closely related.