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Abstract
Silurian environmental changes during approximately 15 million years of retiolitid (Graptolithina) evolution resulted in the progressive reduction and possibly disappearance of their rhabdosome. A strong decrease in retiolitid colony size is also observed. This results from an increase in the sicula size and a decrease in the number of thecae during their evolution. Plectodinemagraptusgracilis Kozłowska-Dawidziuk, 1995 from the hemiaversus/aversus and leintwardinensis biozones, lower Ludfordian of the East European Platform of Poland, is the youngest known retiolitid. It has a very small rhabdosome comprising a very simple ancora umbrella with reduced lateral lists, free nema and ventral walls of two parallel rods with prominent loops per theca. The ancora sleeve seems to be extremely reduced. The rhabdosome with two pairs of thecae reaches about 2 mm in length. It is parallel sided, as in some larger Ludlow forms, e.g. Plectograptus (Moberg & Törnquist, 1909) and Semiplectograptus Kozłowska-Dawidziuk, 1995. P. gracilis is a clear example of the latest stage of the reduction in the retiolitid colony and the last known form to appear before their extinction, which coincided with the leintwardinensis event.
Acknowledgements
A.K. acknowledges financial support from the Ministerstwo Nauki i Szkolnictwa Wyższego, decision number 499/N-NSERC/2009/0, Poland and International Geoscience Programme Project 591 (IGCP 591). The authors thank Mike Melchin, Jan Zalasiewicz, and anonymous reviewer for valuable comments on the manuscript. Aleksandra Hołda-Michalska (ZPAL) is thanked for the improvement of the figures and Grażyna Matriba (ZPAL) for technical assistance.