Dynamic ecophenotypy in the Silurian Monograptidae (Graptolithina)

The monograptids from the Wenlock and Ludlow (mid- to late Silurian) of the palaeotropical Baltic Basin exhibit thickened ring structures (sicular annuli) over their initial phase of growth. Appearing before the lundgreni extinction event, they persisted throughout the remainder of the Silurian, fluctuating in number over that period. To better understand the mechanisms controlling their development and variation, counts of sicular annuli were taken from three well cores in Lithuania, compared between species in each sample and compared with contemporaneous gamma ray data, accompanied by the stable isotope (delta C-13), and acritarch diversity data. Mean counts of annuli fluctuated greatly over the studied interval, but showed negligible variation between species, indicating that the trait is ecophenotypic. The fluctuation in annulus presence aligned with variations in fourth- and fifth-order cycles derived from the gamma ray trends, which represent significant sea level fluctuations, delta C-13 ratios, and primary productivity, suggesting that annuli were more plentiful in high-stand states which are associated with the wetter climate and more productive conditions, whereas dryer, less productive conditions were not conducive to annulus development. In light of this evidence, we hypothesise that the action of upwelling as a result of intensified storm events during wetter periods would have encouraged phytoplankton blooms, increasing construction of annuli. These results show the potential utility of sicular annuli in the study of Silurian climate change and give new insights into graptolite palaeoecology.

Automated summary

The research found that the counts of sicular annuli in monograptids fluctuated greatly over the Silurian period, but showed negligible variation between species, indicating that this trait is ecophenotypic. The presence of annuli fluctuated in alignment with variations in sea level fluctuations, delta C-13 ratios, and primary productivity, suggesting that annuli were more plentiful in high-stand states associated with wetter climates and more productive conditions. The hypothesis is that upwelling as a result of intensified storm events during wetter periods would have encouraged phytoplankton blooms, increasing the construction of annuli.