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

Citation

Guo, Junfeng et al. (2020), Geometric morphometric analysis of Protoconites minor from the Cambrian (Terreneuvian) Yanjiahe Formation in Three Gorges, South China, Dryad, Dataset, https://doi.org/10.5061/dryad.7wm37pvr8

Abstract

The Ediacaran to Cambrian transition is a critical interval of time during which major evolutionary changes occurred. Recently, abundant Protoconites minor have been recovered from the silty shales of the lower Cambrian Yanjiahe Formation (Terreneuvian, Fortunian – Stage 2) in the Three Gorges area of South China. These fossils represent an important ecological diversification of macroscopic organisms at the onset of the Cambrian. Protoconites minor is a probable cnidarian-grade organism preserved by carbon compression. Herein, geometric morphometric analyses are applied to crack out specimens of P. minor to reveal any cryptic morphological details that may have implications for their morphological diversity, ontogenetic processes, and taxonomic identification. These statistical analyses reveal a strong relationship between size and shape, which indicates that the overall shape of P. minor was mainly controlled by allometric growth. The smaller specimens are generally wider at the anterior, and more commonly have straight-sides. Larger individuals tend to be narrower at the anterior, with bending more common. Our analyses demonstrate that there are transitional forms between larger, strongly bent specimens and smaller, straight specimens, suggesting that the assemblage likely consists of a single species.

Methods

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.

Usage Notes

The upload files here are the input files used for all the analyses used in this manuscript.

Funding

National Natural Science Foundation of China, Award: 41890844

National Natural Science Foundation of China, Award: 41890840

National Natural Science Foundation of China, Award: 41472015

National Natural Science Foundation of China, Award: 41672009

National Natural Science Foundation of China, Award: 41621003

National Natural Science Foundation of China, Award: 41772010

National Natural Science Foundation of China, Award: 41720104002

National Natural Science Foundation of China, Award: 41930319

National Natural Science Foundation of China, Award: 41702010

Chinese Academy of Sciences, Award: Strategic Priority Research Program XDB26000000