Redlichia nobilis Walcott, 1905, the oldest trilobite in South Korea: age and morphologic restoration by strain analysis

摘要

Redlichia nobilis Walcott, 1905 occurs in the Gurangri Formation of the Mungyeong Group, a lithostratigraphic unit of the Cambrian-Ordovician Joseon Supergroup in the Taebaeksan Basin, South Korea. The R. nobilis Zone of the formation is correlated with the Lungwangmiaoan Stage and the middle part of the Duyunian Stage of North and South China regional chronostratigraphic scheme, respectively. It corresponds to middle Stage 4 of Cambrian Series 2 in international chronostratigraphic chart. R. nobilis, which is regarded as the oldest trilobite in South Korea, is estimated to be 512 to 510.4 million years old. The specimens of R. nobilis are preserved on the bedding plane as deformed due to structural movement occurred in the Mungyeong area. It is observed in the outcrop that the cleavage plane intersects the bedding plane almost at right angle, forming intersection lineation parallel to fold axis. The cleavage plane can be generally defined as the XY plane and the lineation as the Y axis of strain ellipse on the bedding plane, and thus, the bedding plane corresponds to the YZ plane. The strain analysis using Mohr circle was performed to estimate strain ratio of the deformed specimens. Strain ratio (R) of the deformed specimens calculated ranges from 2.18 to 2.20, with the average of 2.19. The value is considered strongly reliable because the strain ratio of a combined strain marker consisting of a deformed specimen and elliptical structure (probably concretion) around the specimen is 2.18, nearly identical to the R values of other deformed specimens. Morphologic restoration of shape and size was conducted under no longitudinal strain along Y axis because structures indicative of stretching or shortening along the lineation are not observed in the outcrop and those indicative of shortening along the Z axis and thickening along the X axis are observed in thin section. The deformed specimens were restored into an undeformed state using a digital graphic software by only applying the R value perpendicular to the lineation for each specimen. R. nobilis, based on its restored morphology, is characterized by having anteriorly diagonal preocular sutures that are about 120% more apart than palpebral sutures, a hypostome that is fused with rostral plate and has two pairs of short spines (one pair along the lateral margin and the other at postero-lateral corner), 15 thoracic segments with a long axial spine on the 11th (from the anterior) segment, and a small sub-elliptical pygidium with three axial rings and sagittal furrow.