Geometric Morphometric Analysis of Protoconites from the Lower Cambrian Yanjiahe Formation, Yichang, Hubei, China
1 School of Earth Science and Resources, Key Laboratory for the Study of Focused Magmatism and Giant Ore Deposits, MLR, Chang'an University, Xi'an 710054, China;
2 State Key Laboratory for Continental Dynamics, Department of Geology, Northwest University, Xi'an 710069, China;
3 Shaanxi Key Laboratory of Early Life and Environments, Northwest University, Xi'an 710069, China.
The Ediacaran to Cambrian transition is well known as a key geological time period, during which most major animal phyla appeared explosively. A number of macroscopic fossils have been discovered in silty shales in the lower Cambrian Yanjiahe Formation, representing an important fossil record of the Cambrian radiation.A large number of Protoconites have been recovered from the middle Yanjiahe Formation, which are especially diverse in size and shape. To explore their morphological space (morphospace) and to determine whether shape changes are associated with differences in size, we conducted geometric morphometric analysis. In total, 1048 specimens were randomly chosen for study of their size and shape. The (centroid) size distribution of overall specimens ranges from 5.24 to 50.8. A Shapiro-Wilks test indicated a right-tailed non-normal distribution (W=0.9885, p < 0.001; skewness=0.2683). Relative Warp (RW) analysis resulted in 70 relative warp (RW) scores, with RW 1 explaining 38.84%, RW 2 accounting for 34.92%, and RW 3 explaining 8.81% of the total morphological variation. To reveal whether the shape variance is related to ontogenetic allometry, we conducted multivariable linear regression analysis between the overall shape (generalized least-squares aligned by tpsRelw v. 1.65) variance and the log10-transformed centroid size using the software TpsRegr v. 1.45. The analysis indicated a strong relationship between size and shape (generalized Goodall F-test: F=235.856, p < 0.001; Wilks' Lambda test: λ=0.437, p < 0.001). The right-tailed non-normal distribution is typical across many different animal clades, implying that there was no taphonomic or sampling bias for these specimens. RW 1 is related to the width of the shell, suggesting that the main shape variation of Protoconites is the relative width of the anterior conical shell.The strong relationship between size and shape indicates that the shape of Protoconites was mainly controlled by allometric growth. Small-sized conical shells are generally wider in the anterior part, and commonly have straight shells. With increasing size, shells become relatively narrower in the anterior part, and more specimens are strongly laterally curved.