ABNORMAL ACRITARCHS: A TERATOLOGICAL CASE FROM THE LLANDOVERY/WENLOCK BOUNDARY IN THE AMAZON BASIN, NORTHERN BRAZIL

Abnormal acritarchs include forms that present morphological anomalies. According to Le Hérissé, 1989, they occur in several intervals of the Silurian System, but become particularly abundant around the Llandovery/Wenlock Series boundary. The documentation of abnormal acritarchs is still relatively poor in the palynological literature. Cramer, in 1968, was the first to mention a possible case of acritarch malformation in Veryhachium europaeum from the Silurian Maplewood Shale of New York, although he did not assign any chronostratigraphic or paleoecological significance to it. Le Hérissé, in his 1989 study of the Silurian acritarchs from the Visby and Slite Formations of Gotland (Sweden), considers different hypotheses in order to explain anomalies observed in the processes of some species. He concludes that variations in water luminosity and temperature could have brought about pathological effects on the Gotland acritarchs. Furthermore, he suggests that the noticeable occurrence of abnormal acritarchs near the Llandovery/Wenlock boundary could be related to such global phenomena as widespread volcanic activity, paleomagnetic inversions, or the impact of extraterrestrial bolides. In the present work, abnormal acritarchs are recorded for the first time in Llandovery/Wenlock boundary sediments of the Pitinga Formation (Trombetas Group) in the Amazon Basin, northern Brazil, reliably dated by chitinozoans. This new documentation includes some species described by Le Hérissé from Gotland, like Veryhachium sp., Ammonidium microcladum, and Salopidium granuliferum. Teratological representatives of these taxa share common features, as they all bear unusually thin walls and inflated, deformed processes. Such anomalies in Amazon Basin Silurian acritarchs are interpreted as the effect of stressing environmental conditions on the marine organic-walled microphytoplankton. Possible causes may include changes in seawater temperature and salinity, probably related to the sharp sea-level fluctuations that characterize the Llandovery - Wenlock transition on a global scale.