Assessing temporal transition between microgranular and hyaline tests of calcareous microplankton during the Late Jurassic

Calcareous microplankton increased in abundance during the latest Jurassic, coinciding with the increase in abundance of calcareous nannofossils and with the onset of deposition of pelagic calcareous oozes. However, the timing and causes of the shift from microgranular tests of the earliest microplankton (chitinoidellids) to hyaline tests of calpionellids are obscured because the ultrastructure of two-layered praecalpionellids that occur during the Tithonian is poorly documented. Here, we investigate the ultrastructure of chitinoidellids and praecalpionellids from Upper Tithonian deposits in the Western Carpathians. We show that (1) the chitinoidellid microgranular layer is formed by elongated, euhedral, densely-packed, nanometric needles rather than by fragments of calcareous nannofossils, (2) two-layered chitinoidellids (Semichitinoidella) are formed by an internal microgranular layer (identical to that of Chitinoidella) and by an external hyaline prismatic layer, and (3) two-layered ..., , , # Assessing temporal transition between microgranular and hyaline tests of calcareous microplankton during the Late Jurassic

[https://doi.org/10.5061/dryad.g1jwstr06](https://doi.org/10.5061/dryad.g1jwstr06)

## Description of the data and file structure

The measurements of crystal length and width (in micrometers) and the length/width ratio in test layers of three genera (Semichitinoidella, Chitinoidella, Praetintinnopsella). Each specimen is characterized by a unique combination of sample ID (bed number) and specimen ID. The length and width was measured on ten crystals in each specimen (and each specimen is thus represented by ten rows). Source refers to the imaging method (SEM - scanning electron microscopy, BSE - back-scattered electrons). File name refers to the image number and Preservation refers to the quality of specimen preservation (poor - tests are markedly affected by diagenetic alteration).Â