Osmunda LINNAEUS

Kvaček, Zlatko & Martinetto, Edoardo, 2016, Foliage Accumulations Of Osmunda Lignitum (Osmundaceae) In The Oligocene Of Northern Italy And Western Germany, Fossil Imprint 72 (3 - 4), pp. 131-139 : 133-134

Osmunda LINNAEUS

Subg. Plenasium (C. PRESL) C. PRESL

Osmunda (Plenasium) lignitum (GIEBEL) STUR

Pl. 1, Figs 1–3, Pl. 2, Figs 1–5, Pl. 3, Figs 1–2

1857 Pecopteris lignitum GIEBEL , p. 303, pl. 2, fig. 2a. 1870 Osmunda lignitum (GIEBEL) STUR , p. 5. 2006 Pronephrium stiriacum (UNGER) KNOBLOCH & KVA-

ČEK; Winterscheid, p. 61, pro parte, pl. 24, fig. 4. 2016 Osmunda lignitum (GIEBEL) STUR ; Winterscheid and

Kvaček, p. 46, fig. 2A.

M a t e r i a l s t u d i e d: Sterile fern foliage with preserved cuticle structure, collections of the National Museum in Prague (NM) housed under the inventory numbers K 2761–62 (Cassinelle) and K 2758–2760, K 2765a, b (Niederpleis-1).

Fragmentary compressions of pinnae 12–15 mm wide or slightly wider, never as complete lengths, coarsely dentate, composed of oval apiculate sub-opposite pinnules, fused to 2/3 of the length; midrib of pinna stout and straight, venation of pinnules free, midribs at an angle of 40°–50° to the main midrib, which reaches the apex, of almost the same thickness as other veinlets, secondary veins straight to slightly bent, often forked, craspedodromous, in 3–6 sub-parallel pairs, basal ending in the angular sinus, other bent along the margin (Pl. 1–2). Adaxial side cuticles of medium thickness, smooth, anticlinal walls Ω-shaped undulate demarcating irregularly polygonal, slightly elongate cells ca. 30–40 µm wide and 60–100 µm long, cuticle of abaxial side (obtained from Niederpleis-1 material only) much thinner and delicate, non-modified cells similar to those of adaxial side, stomata anomocytic, subparallel, widely elliptic, 50 µm long, 30 µm wide, stomatal ledges slightly bent, close together, stomatal slit linear (Pl. 3, Figs 1–2).

For the typification and synonymy see Barthel (1976).

D i s c u s s i o n. Previously studied foliage of O. lignitum with preserved cuticle structure (e.g. Mathiesen 1965, Walther 1967, Barthel 1976, Mai and Walther 1985) provided details of pinnules, in particular venation (free catadromous) and epidermis cell structure (undulate anticlinal cell walls, stomata anomocytic, roundish, only on abaxial leaf side, longitudinally as well as irregularly orientated and subparallel) which does not differ from the data obtained from the presently described material. It thus proves the affinity of this sterile fern foliage to the subgenus Plenasium , namely to the similar living species Osmunda banksiifolia (C. PRESL) KUHN (Holttum in Chandler 1963, Mathiesen 1965), which differs only by slightly larger cells of the adaxial epidermis as well as larger stomata in the abaxial epidermis (Pl. 3, Figs 3–4). Our material does not permit comparisons of the whole plants including fertile parts.

We can expand the European distribution as reviewed by Barthel (1976) and Collinson (2001) based on new records from the North Bohemian Eocene at Kučlín ( Kvaček 2002, Kvaček and Teodoridis 2011), the Rhineland Oligocene, Germany (Altenrath – Winterscheid and Kvaček 2016, Niederpleis-1 – Winterscheid 2006: described as Pronephrium stiriacum , present study – Pl. 2, Figs 1–5, Pl. 3, Figs 1–2) and the new record from the Oligocene of Italy, which has also the above characteristics (Pl. 1, Figs 1–3).

Not all fossils ascribed so far to O. lignitum have in our opinion been correctly determined. In addition to the excluded records listed by Barthel (1976: 440) we also excluded dispersed cuticles from the Rhineland Miocene, which were assigned to Osmunda lignitum but which differ in stomatal details. Stomata in Osmunda are simple, anomocytic, broadly elliptic in outline, orientated subparallel and in cases of optimal preservation retaining polar T-pieces ( Mathiesen 1965: text-fig. 5b, Walther 1967: pl. 4, figs 1–3, Barthel 1976: pl. 73, fig. 9, Walther in Mai and Walther 1985). Those wrongly assigned to Osmunda lignitum from the Miocene of the Zülpich Mine ( Schultz 1962) differ in the thickened cutinized ring ( Schultz 1962: pl. 5, figs 4–5, 7) and stomatal I-pieces ( Schultz 1962: pl. 5, fig. 6) and belong to dispersed dicotyledonous cuticles (cf. Litke 1966: as NFu 23, Kilpper 1969: determined as Illicium ceriferum WEYLAND et al. ). This observation may also apply to the abaxial cuticle from the Regis Mine with stomata illustrated by Kräusel and Weyland (1950: pl. 2, fig. 4, text-fig. 3 right), in which the stomata are irregularly orientated, as pointed out by Walther (1967).

Most of the records of Osmunda (Plenasium) lignitum in Europe have been found in the Eocene (e.g. Bournemouth, Geiseltal, Haselbach, Kučlín) and Oligocene (e.g. Bovey Tracey, Seifhennersdorf, Eger, Cornești-Aghireș) and only exceptionally in the lower Miocene (Jutland). The dating of some of the records previously assigned to the Miocene, namely Seifhennersdorf ( Barthel 1976), was rectified and falls correctly into the Oligocene ( Walther and Kvaček 2007). The new occurrence at Niederpleis-1 requires a more accurate independent dating. According to the pollen data ( Von der Brelie et al. 1981) the strata with Osmunda are of late Oligocene age (see Winterscheid 2006).

This fern belongs to wetland ferns ( Collinson 2002) and undoubtedly represents a paleo-subtropical element, at least in Europe, contrary to species belonging Osmunda subg. Osmunda of warm-temperate to temperate autecology. The accompanying vegetation corresponds to paratropical to subtropical broad-leaved forests dominated by evergreen Lauraceae , Hamamelidaceae , Fagaceae and palms, with accompanying deciduous Arcto-tertiary elements. The chronostratigraphical distribution of Osmunda lignitum in Europe is suggestive of the situation in East Asia, where this fern did not cross the Palaeogene/Neogene boundary and was replaced by another species of the Plenasium subgenus, O. totangensis (COLANI) GUO ( Tian et al. 2016) , probably synonymous with Osmunda bromeliaefolioides MATSUO (cf. Tanai 1970). The scarcity of frost-sensitive Osmunda lignitum in the European Neogene may explain the gap in circumboreal distribution of O. lignitum in the Cenozoic over most boreal Asia, namely the former Soviet Union and boreal North America.

Presl (1831) was the first botanist who recognized this group of ferns in the collections of Czech explorer Tadeus Haenke, collected in Luzon, Philippines. Tadeus Haenke never returned to Europe, but from South America, he sent his collection of plants to Hamburg, where Kašpar Sternberg subsequently learned about them, and arranged for them to be shipped to the Prague museum. The type specimen of Plenasium was later on transferred to the collection of the Charles University ( PRC). The extant species of Osmunda subg. Plenasium have been interpreted in various ways, partly with the aid of fossil records ( Bobrov 1967, Bomfleur et al. 2015). Our study may only add confirmation of the evolutionary as well as ecological stasis of the fossil Osmunda subg. Plenasium since the Palaeogene.