Review of Palaeobotany and Palynology 111 (2000) 103–109 www.elsevier.nl/ locate/revpalbo An unusual process structure in Tresarcus, a new acritarch genus from the Ordovician of O ¨ land, Sweden Rosaria Tropepi * , Cristiana Ribecai Universita ` degli Studi di Pisa, Dipartimento di Scienze della Terra, via S. Maria 53, I 56126 Pisa, Italy Received 12 August 1999; accepted for publication 7 March 2000 Abstract Tresarcus, a new monospecific acritarch genus, is established along with the type-species Tresarcus suecicus sp. nov. It was found in samples from the upper Arenig ( Lenodus sp. A conodont Zone) of the Hagudden and Horns Udde sections, O ¨ land, Sweden. The three-arched process tip is a peculiar characteristic of Tresarcus. This structure recalls, from a morphological point of view (though at a different dimensional scale), the four-arched termination of the epidermal hairs that ornate the adaxial (upper) surface of the assimilating leaves of Salvinia natans, Adanson, 1785 (Pteridophyta). This species has an aquatic habit and the four-arched structure as the tip of the epidermal is interpreted as a device that favours buoyancy by trapping air on the leaf surface. A similar function is hypothesized for the three-arched processes of Tresarcus. © 2000 Elsevier Science B.V. All rights reserved. Keywords: acritarchs; O ¨ land; Ordovician; Pteridophyta; Sweden; Tresarcus 1. Introduction O ¨ land. The stratigraphic location of the palynifer- ous samples is based on conodonts, and the cono- dont biostratigraphic scale established by Bagnoli The isle of O ¨ land is located in the Baltic sea, offshore of southern Sweden. Biostratigraphic and Stouge (1997) is followed throughout this paper. researches on Arenig–Llanvirn acritarchs have been carried out in northern O ¨ land, Sweden during the 1990s (Righi, 1991; Ribecai and Tongiorgi, 1995, 1997; Playford et al., 1995; Stouge et al., 2. Sections and samples 1995; Ribecai, 1996). During the same years, the conodont biostratigraphy of the same area and Samples used in this study were collected from sections has been studied in detail (Stouge the Hagudden and Horns Udde sections, in north- and Bagnoli, 1990; Stouge et al., 1995; Bagnoli ern O ¨ land (Fig. 1). and Stouge, 1997, in preparation). The aim of this study is to describe a new, 2.1. Hagudden section unusual acritarch observed in the Arenig–Llanvirn boundary interval from two sections of northern This section is located in northern O ¨ land, 5 km south of Byxelkrok (16°57∞03◊E–57°14∞02◊N ); it is * Corresponding author. 5.54 m thick, and is composed (from bottom to 0034-6667/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S0034-6667(00)00020-8