ONTOGENETIC VARIATION ON CRANIDIAL SHAPE OF THE LATE MIDDLE CAMBRIAN (CAMBRIAN SERIES 3) TRILOBITE JIULONGSHANIA ACALLA

A large number of complete cranidiums together with numerous disarticulated sclerites of the late middle Cambrian (Cambrian series 3) trilobite Jiulongshania acalla Walcott, 1905 were collected from the Gushan Formation at the Tangwangzhai section in Shandong Province, North China. Jiulongshania acalla is characterized by a subtrapezoidal cranidium with conical to triangular glabella, wide fixigenae and preglabellar field. The cranidial morphology shows a wide range of variation and can be divided into different morphotypes, but it is unclear whether the morphological variations were related to the ontogenetic change during growth. In this study, we analyzed the morphological and ontogentic changes in the cranidial shape of J. aculla to gain insight into the morphogenetic processes during the development of trilobites. Classic morphometrics and geometric morphometrics methods were used to evaluate the variability in the cranidium. Five variables of cranidium were measured for morphometric analysis, including the total cranidial length and width, the maximum glabellar length (including the occipital ring) and width, and the length of cranidial frontal area. In addition, 5 landmarks were selected to optimize the half shape of the cranidium and at the same time retain a reasonable sample size.

In this study, we found the following observations: (1) allometric growth of the preglabellar and glabellar area, (2) elongation of cranidium during the deveopmental process, (3) gradual increase in the length of cranidial frontal area with respect to glabellar length, and (4) progressive expansion of the maximum glabellar width relative to the total cranidial width. Our results clearly show that the relative growth rates of different regions of cephalon was disparate during growth. Fortey and Owens (1999) have proposed that the variation of trilobite exoskeletal morphology, particularly the cephalon, might be related to the feeding mode of trilobites. Thus,we speculate that the natant trilobites may extend the preglabellar field during growth when the hypostome detaches from the doublure. It is likely that there was a change in feeding habit between larvals and adult stages in the development of Jiulongshania acalla.