Acetabularia Lamouroux, 1812

Barattolo, Filippo, Ionesi, Viorel & Ţibuleac, Paul, 2019, A new polyphysacean alga from the Miocene of Romania and its biomineralization, Acta Palaeontologica Polonica 64 (1), pp. 85-100 : 87-88

Genus Acetabularia Lamouroux, 1812

Type species: Madrepora acetabulum Linnaeus, 1758 ; Recent, Mediterranean Sea.

Remarks.— The alga consists of a cylindrical stalk terminating, at maturity, with a cap made by a whorl of fertile rays. At the inner end, each fertile ray bears two small convex cushions, one above (corona superior) and one below (corona inferior), and communicates with the stalk by means of a small vestibule.

The presence of both superior and inferior coronae or of the corona superior only (i.e., absence of corona inferior) is a character of generic importance. Among extant genera, Acetabularia possesses both superior and inferior coronae, while Parvocaulis , Chalmasia , and Halicoryne lack the corona inferior ( Berger et al. 2003; Berger 2006).

From a structural point of view, coronae have been variously interpreted as (i) expansions of the central stem, (ii) primary laterals together with the basal part, or (iii) as secondary laterals ( Dumais and Harrison 2000).

According to Berger et al. (2003), morphological and developmental characters of the cap, and molecular data 18 S rDNA), allow Acicularia schenckii to be placed in the Acetabularia that also includes Polyphysa peniculus , a species lacking a corona inferior, the absence of which is supposed to be a derived feature ( Berger et al. 2003). These authors concluded that the genus Acetabularia can be split into two subgenera, Acetabularia , containing only A. acetabulum , and Acicularia including all other species. The former subgenus develops cap rings with congenitally fused cap primordia, the latter subgenus has unfused cap primordia.

As discussed below, Acicularia (with type species Acicularia pavantina ) should be restricted to fossil aggregates of cysts only. Therefore, extant species of Acetabularia with unfused cap primordia are better referred to the sectio Acetabuloides Solms-Laubach (1895), with type species Acetabularia kilneri , rather than to the fossil genus Acicularia as proposed by Berger et al. 2003, because extant Acetabularia schenckii is not the type species of the genus Acicularia .

Genera included: Several Cenozoic polyphysacean remains showing evidence of coronae (see review in Deloffre and Granier 1992): (i) Andrussow (1887) established A. miocenica from the Miocene of the Kerch Peninsula and near Sevastopol (Crimea). Later Solms-Laubach (1895) moved the taxon to Acicularia and renamed it Acicularia andrussowii Solms-Laubach, 1895 because of the homonymy with Acicularia miocaenica Reuss, 1861 . Solms-Laubach (1895: 11) observed Andrussow’s (1887) material and confirmed the occurrence of double coronae. Here the original attribution to Acetabularia is followed: the name A. miocenica Andrussow, 1887 is maintained and a lectotype is designated ( Andrussow 1887: text-fig. 1); (ii) Squinabol (1902) described A. chiavonica from Oligocene beds cropping out along the river Chiavon. This is a classical middle Oligocene fossiliferous horizon (with fishes, crustaceans, and palms) near Vicenza (Northern Italy; Giusberti et al. 2014). The thin calcified wall shows evidence of both coronae, and fertile rays display no evidence of intracellular mineralization. The specimen in Squinabol (1902: text-fig. 2) has been selected as lectotype by Deloffre and Granier (1992); iii) Acicularia transylvana Bányai and Morellet, 1936 from the Sarmatian near Satul Nou ( Romania) is preserved with a calcareous sheath and mineralized fertile rays, and structures related to coronae are also present ( Génot 1987). The presence of intracellular mineralization allowed Bányai and Morellet (1936) to attribute the taxon to Acicularia . The species resembles Acetabularia miocenica Andrussow, 1887 , but according to Bányai and Morellet (1936) mineralization between the fertile rays is much thinner, the distal ends of the fertile rays are more or less rounded, and gametangia are not arranged in a single row on the upper and the lower side of gametophores. The taxon is here moved to the genus Acetabularia and maintained the name A. transylvana ( Bányai and Morellet, 1936) . The specimen corresponding to the drawings by Bányai and Morellet (1936: fig. 2) is selected as lectotype ( Deloffre and Granier 1992); (iv) a fourth new species is described here.