Heterochrony describes acceleration, displacement and/or retardation of descendants’ development events compared to ancestral states and has often been cited as an important process to bring about morphological novelty. It was coined one and half centuries ago and has been discussed by both paleobiologists and biologists frequently ever since. Many types of fossil organisms preserve aspects of their development histories in their bones or shells that have been used for heterochrony analyses, with body size being used as a developmental age indicator, despite raised questions regarding this practice. For organisms whose hard structures consist of multiple chambers, or that contain growth lines, age information suggested by these structures independently can facilitate ontogenetic modeling. In this way, relations among size, shape and age can be established in order to document patterns of morphological development.
Morphological analysis of pseudoschwagerine fusulinids, a fossil foraminifera group that developed a morphologically novel spherical shell, along with their presumptive triticitid ancestors illustrates this approach to heterochrony analysis. Ontogenetic trajectory comparisons of four major pseudoschwagerine genera, as well as those of triticitid foraminifera, document relations between their shape, size and developmental ages. A complex of heterochronic patterns including peramorphic pre-displacement, hypermorphosis and acceleration characterize pseudoschwagerine development and appear to be responsible for the novel appearance of large, inflated fusiform and spherical tests in these Late Paleozoic benthic foraminifera. The morphometric approach employed in this investigation could be applied widely in the quantitative morphological studies of development histories in a variety of other fossil groups.
To analyze the heterochronic pattern embedded in the development of pseudoschwagerine fusulinids development, a set of 106 specimens belonging to four pseudoschwagerine genera and their presumptive ancestor, Triticites, have been included in this study. Their test morphology at each whorl-growing stage could be recognized in the axial thin sections and the test outlines were then digitized. In total 1396 test outlines representing ontogenetic form changes in the 106 specimens were acquired and each outline is represented by four Type 2 landmarks and 36 semi-landmarks.
The data file is present as .dat file, and contains X, Y coordinates of landmarks and semi-landmarks for all the 1396 test outline. The data were arranged in the file as 1396 matrixes with each composed of a title line presenting outline name and the following 40 lines of two numbers as X, Y coordinates. Abbreviated name of each specimen is used as part of the outline name and presented here in Table 1. The number in outline name represents the whorl-growing stage.
National Scientific Foundation of China, Award: Nos. 41772017, 91955201
National Key Research and Development Program of China, Award: No. 2018YFE0204201