Redescription and palaeoecological significance of the bryozoan Hippoporidra patagonica (Pallaroni, 1920) in the San Julián Formation (late Oligocene) of Santa Cruz province, Argentina

Abstract

Hippoporidra is a cheilostome bryozoan that predominantly encrusts gastropod shells inhabited by hermit crabs. The genus has a Pan-Atlantic distribution and its fossil record ranges from the Eocene to Holocene. Cellepora patagonica Pallaroni is here transferred to Hippoporidra and its autozooids and avicularia are described for the first time based on scanning electron microscopy. Hippoporidra patagonica can be distinguished from the remaining species of the genus by the peculiar shape of its interzooidal avicularia and the morphology of its orifice, which is semicircular, with a straight proximal border. In addition, many specimens have adventitious avicularia located on a suboral umbo. The record of Hippoporidra patagonica (Pallaroni) in the Gran Bajo Member of the San Julián Formation (late Oligocene) in Patagonia confirms the presence of this genus at the southern tip of South America and indicates a Pan-Atlantic distribution during the Cenozoic.

苔藓动物属Hippoporidra是一种唇口类苔藓虫，主要被覆寄居蟹居住的腹足类贝壳。该属呈泛大西洋分布，其化石记录范围从始新世到全新世。 Cellepora patagonica Pallaroni在此被归入Hippoporidra，它的独立个体和鸟头器通过运用扫描电子显微镜而首次被描述。 Hippoporidra patagonica与该属的其余种的区别在于其个体间鸟头器的奇特形状和口孔的形态特征。其口孔呈半圆形，具有一个直的近端边界。此外，许多样品具有位于口腹壳顶的异位口器。 Hippoporidra patagonica（Pallaroni）在Patagonia地区的San Julian组（渐新世晚期）Gran Bajo段的记录证实了该属在南美洲南端的存在，并标示它在新生代期间的泛大西洋分布。

Key words:

• Hippoporidra patagonica
• Cheilostomata
• pagurization
• late Oligocene
• San Julián Formation
• Santa Cruz
• Patagonia
• Argentina.

Acknowledgements

Special thanks to Paul Taylor and Marcelo Sánchez-Villagra for help with the literature; to Maria Cristina Bonzi for her assistance in the search for specimens in the collection of Dipartimento di Scienze della Terra dell’Ambiente e della Vita (DISTAV) and for bibliographic advice; and to Marian Tanuz, for her help to access the fossils in the collection of the UBA. Emanuela Di Martino, Pierre Moissette and Stephen McLoughlin are acknowledged for their constructive comments on an earlier version of this paper. This research was supported by CONICET through a postdoctoral fellowship to LMP. We are grateful to Fabián Tricárico who operated the Scanning Electron Microscope at MACN. We acknowledge financial support by CONICET [PIP 2013-2015 No. 00247 to JLG] and ANPCyT [PICT 2012-1043 to JLG and PICT 2012-1726 to MG and GP].