Wu et al.—Chinese Permian–Triassic chonetid taxonomy
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Figure 7. Linear regression of shell width/length ratio to length of Tethyochonetes pygmaea from bed 9, and Fusichonetes nayongensis from bed 24 of the Talung Formation in the Xinmin section.
significant difference in shell size among the 15 species that have previously been assigned to the two genera (Fig. 5; D=0.2849, p=0.0796), the difference in the width/length ratio between the two genera is statistically significant (Fig. 6; D=0.7765, p<0.001), suggesting a possibility of distinguishing the two genera based on their shell outline or shape. To further investigate whether shell outline varies con-
Figure 5. Graph of shell length to width of previously recognized Tethyochonetes and Fusichonetes species, plus some Fusichonetes nayongensis specimens from the Xinmin section.
sistently with shell size, we used a subset of our dataset with specimens collected from discrete beds of the Xinmin section. This sub-dataset contains the measurements of shell length and width from 174 specimens representing what would be con- sidered as T. pygmaea using traditional qualitative taxonomy from bed 9, and 26 specimens representing F. nayongensis from bed 24. As shown in Figure 7, there is no significant linear correlation between the width/length ratio and length of the shell for either of them, suggesting that the shell shape or outline, approximated by the width/length ratio, is a rather stable feature because it changes little with shell size. This would translate to mean that the shell shape does not fluctuate much with ontogeny and, therefore, could be used as a reliable parameter for the comparison of other morphological features. The result from CATPCA is summarized in Figure 8, with
Figure 6. Graph showing the relationship between the total number of costallae and the shape (here measured by the width/length ratio) of Tethyochonetes and Fusichonetes shells (data points are comprised of measurements from literature and some Fusichonetes nayongensis specimens from the Xinmin section).
the 15 species previously assigned to the two genera arranged in a biplot defined by the first two principle components (PC). PC 1 explains 25.3% of the total variance while PC 2 accounts for 16.7%. In the biplot, the 15 variables (see Table 1 for abbre- viations) are projected as vectors whose lengths vary according to their variance accounted for (i.e., the longer the variable line in the plot, the larger the variance it accounts for). It is evident from the figure that the positive side of PC 1 holds F. nayongensis, T. soochowensis, and part of T. quadrata, T. chaoi, T. guizhouensis, T. wongiana, and T. cheni. These specimens are represented by having a significantly larger lateral margin angle (LMA) coupled with a large umbonal region width (URW). Next to this, specimens of F. nayongensis and T. soochowensis, located on the negative side of PC 2 in the
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