DYNAMIC ECOPHENOTYPY IN THE MONOGRAPTIDAE (GRAPTOLITHINA) CORRELATES WITH CHANGES IN THE LATE SILURIAN CLIMATE

The monograptids of the Wenlock and Ludlow (late Silurian) exhibit thickened ring structures (sicular annuli) intermittently over their initial phase of growth. These annuli appear in paleotropical assemblages like the Baltic Basin and Canadian High Arctic just before the lundgreni extinction event and persist throughout the remainder of the Silurian, but exhibit long-scale variations in occurrence over that period. The goal of this study was to quantitatively determine whether the fluctuation of sicular annuli was tied to taxonomy, and if not, what environmental factors might instead explain that fluctuation. In order to achieve that goal, counts of sicular annuli were taken from three well cores in Lithuania and compared between species in each sample. Those counts were also compared with contemporaneous stable isotope (δ¹⁸O and δ¹³C), gamma ray and acritarch diversity data from the same region. We find negligible variation in annulus counts between species, indicating that the trait was most probably ecophenotypic and not determined by taxonomy. Concurrent fluctuation with acritarch diversity, gamma ray and stable isotopes showed that sicular annulus counts were positively correlated with temperature, sea level, and primary productivity. In light of this evidence, we hypothesize that the action of upwelling as a result of intensified monsoons and tropical cyclones during warmer, wetter periods would have encouraged phytoplankton blooms, resulting in a nutritive excess for the filter-feeding monograptids and additional lateral deposition of shell material. These results show the potential utility of sicular annulus counts as a proxy for the frequency of temporary upwelling events like tropical storms, and shed new light on the paleoecology of these ancient creatures.

This study was supported by the grant S-MIP-19-15.