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- Title
AN ECOLOGICAL BASIS FOR THE DEVELOPMENT OF SICULAR ANNULI IN LATE WENLOCK AND LUDLOW MONOGRAPTIDS.
- Authors
Whittingham, Misha; Spiridonov, Andrej; Radzevičius, Sigitas
- Abstract
The Mulde event of the Late Silurian greatly disrupted the diversity and morphology of planktic biota. Among the affected groups were the chitinous acritarchs, which experienced a major shift in diversity and size distribution, and the monograptid graptolites, which were reduced to just one species. A consequence of the bottlenecking of monograptids was the inheritance of thickened rings of chitin, referred to as "sicular annuli", by descendants of the surviving Pristiograptus dubius stem lineage. These annuli remain present throughout the Ludlow and Přidoli, fluctuating in numbers between 0 and 8 per sicula until the start of the Saetograptus leintwardinensis biozone, at which point the number of sicular annuli is fixed at 3 across all monograptid taxa. Especially curious is the presence of a short interval during the Colonograptus praedeubeli and Colonograptus deubeli biozones wherein almost all siculae are devoid of annuli, before regaining them again at the end of the interval. This ring-less interval is concurrent with a major drop in acritarch abundance and increase in acritarch size and evenness. The interpretation of acritarchs as a primary food source for graptolites may provide some explanation for the shifts in the number of sicular annuli in the latter group. The excess chitin consumed from an acritarch-heavy diet provides a structural and ecological basis for the development of sicular annuli. The increase in average size during the praedeubeli-deubeli interval would mean a reduction in surface area, and therefore consumed chitin, for an amount of acritarchs with the same nutritional value. This would have extinguished the consumption of excess chitin in this period, temporarily arresting the development of sicular annuli.
- Subjects
ACRITARCHS; CHITIN; GRAPTOLITES; SURFACE area; NUTRITIONAL value
- Publication
Geologija. Geografija, 2018, Vol 4, Issue 4, p184
- ISSN
2351-7549
- Publication type
Article