Geometric morphometric analysis of Protoconites minor from the Cambrian (Terreneuvian) Yanjiahe Formation in Three Gorges, South China

All 1048 specimens (see supplementary material) of P. minor utilised for this study occur as carbonised films preserved on bedding surfaces (Figure 2). Obviously broken or extensively damaged specimens have been excluded from the analysis. Collection number “CH” indicates fossils from the Dingjiaping section. “Y” indicates fossils from the Yangjiachong section. Digital images were taken with a light camera (Canon EOS70D) under both natural light and with an external light source in Chang’an University, Xi’an, China. Specimens are housed in the collections at the School of Earth Science and Resources, Chang’an University. Geometric morphometric analyses required application of a combination of landmarks and sliding semilandmarks. Landmarks were used to cover static homologous anatomical loci that can be found unambiguously on every specimen (Zelditch et al., 2004), while sliding semilandmarks were used to cover curves where no homologous point can be precisely defined (Bookstein, 1997a, b; Gunz et al., 2005; Mitteroecker and Gunz, 2009; Gunz and Mitteroecker, 2013). In total, three Type II landmarks (Zelditch et al., 2004) and 34 sliding semilandmarks, were applied to each specimen using the software TpsDig2 v. 2.26 (Rohlf, 2010a; Figure 3.1-2). A sliders file was generated using the software TpsUtil v. 1.74 (Rohlf, 2011a). Relative Warp (RW) analyses were carried out using the software TpsRelw v 1.74 (Rohlf, 2010b) using the Generalized Procrustes Analysis superimposition method (Figure 3.3; Zelditch et al., 2004; Gunz and Mitteroecker, 2013; Chen et al., 2016). The centroid is a measure of size that is mathematically independent of shape (Zelditch et al., 2004), and was calculated using the mean of all the landmarks’ coordinates automatically using TpsRelw v 1.74. Multivariable linear regression analyses were conducted using TpsRegr v 1.45 (Rohlf, 2011b). Data was also transferred to the Palaeontological Statistics Program (PAST) (Hammer et al., 2001) for simple linear regression. Some specimens exhibit left or right curvature. As P. minor was not mineralized, it is possible that extent of curvature is a taphonomic artefact. To control for bias associated with direction of curvature, we have reflected some of the images so all specimens curve in the same direction.