MALFORMATION IN NANNOPLANKTON SPECIES FROM THE PALEOCENE-EOCENE THERMAL MAXIMUM: A STUDY OF EXPANDED RECORDS FROM SHELF SECTIONS IN MARYLAND AND NEW JERSEY

Expanded records of the Paleocene-Eocene thermal maximum (PETM) from the Atlantic Coastal Plain in Maryland and New Jersey provide a unique opportunity to document the response of the shallow ocean and its inhabitants to abrupt climate change. The PETM is characterized by a transient group of nannoplankton whose range is restricted to the associated carbon isotope excursion (CIE). Many of these so-called excursion taxa belong to the genus Discoaster, a large group characterized by multi-rayed specimens. The origin of the Discoaster excursion taxa is debated, and, since their phylogeny is unknown, these taxa have previously been classified as discrete species. Our observation of excursion Discoaster taxa in the early part of the PETM from the shelf sections provides a unique and refined view of their morphology, phylogeny and relationship to environmental change. Discoaster shows three progressive stages of morphological response, starting with inter-ray irregularity of spacing, followed by ray-tip deformation and ray malformation, and, ultimately, ray loss. Tip deformation and ray malformation and loss characterize the dominant excursion taxa, Discoaster anartios and D. araneus. Finally, detailed observations suggest that D. anartios may have morphed into D. araneus via progressive malformation and ray loss. Inter-ray irregularity begins in specimens from near the onset of the PETM, but ray-tip deformation and ray malformation and loss begin close to the peak of the CIE and extend through the early part of the recovery.

Our results suggest that the excursion taxa are not separate species, but rather morphological variants of existing species of Discoaster. The extent of malformation is indicative of decreased calcification, potentially as a result of ocean acidification. Like modern coccolithophores, PETM malformation is highly taxon-selective; excursion Discoaster constitute a minor component of the nannoplankton assemblage, with a broad range of species showing excellent preservation and no apparent morphological change during the PETM. Discoaster is thought to have inhabited the deeper photic zone near the nutricline, and we postulate that the excursion taxa are adaptations to undersaturated waters within an expanded oxygen minimum zone.