Cooksonia sp.

Libertín, Milan, Kvaček, Jiří, Bek, Jiří & Štorch, Petr, 2018, Plant Diversity Of The Mid Silurian (Lower Wenlock, Sheinwoodian) Terrestrial Vegetation Preserved In Marine Sediments From The Barrandian Area, The Czech Republic, Fossil Imprint 74 (3 - 4), pp. 327-333 : 330-332

Cooksonia sp.

Text-fig. 5a–c

L o c a l i t y.Loděnice, Špičatý vrch – Barrandovy Jámy.

H o r i z o n. Monograptus belophorus Biozone, Motol

Formation, middle Sheinwoodian, Wenlock, Silurian.

M a t e r i a l. Specimens, NM-D 550, NM-D 554, NM-D

557a, b, National Museum, Prague.

D e s c r i p t i o n. The largest specimen (NM-D 554) is 75 mm long and 3.2 mm broad in its basal part ( Text-fig. 5a ). Its axes are three times isotomously branched. In the lower proximal branches, one branch subsequently becomes dominant. This means that the branches are isotomous with overtopping. They do not form a pseudomonopodium, because branches of higher orders are uniform. Branching is comparatively regular; segments branch off at acute angles of 30° to 40°, and range from 7 mm to 25 mm in length ( Text-fig. 5a ). The sporangium (3 mm in diameter) is loosely attached (attachment being visible only in polarized light Text-fig. 5a ).

There are two further specimens (NM-D 550, NM-D 557a, b; Text-fig. 5b, c ) showing putative terminally-borne sporangia (3–3.8 mm in diameter). The specimens are branched isotomously three times, with overtopping. The precise mode of attachment of sporangia is obscured by the sporangia

themselves covering critical areas. Specimen NM-D 550 shows a 25 mm long terminal axis. Sporangia are well-preserved in its left part. Specimen NM-D 557a and its counterpart NM-D

557b show several putative sporangia forming one group, loosely attached and of moderate preservation. The axis has

the same general form as NM-D550 and NM-D554.

D i s c u s s i o n. The above described fossil plant material identified as Cooksonia sp. differs from Cooksonia barrandei ( Libertín et al. 2018) in having upright axes and slender terminal parts of subtending axes and sporangia without a rim. Additionally, terminal parts of Cooksonia sp. branches are shorter than C. barrandei, and their branching is more frequent.

The specimens described by Libertín et al. (2002) resemble Cooksonia sp. rather than C. barrandei in the absence of an apparently procumbent basal part of its axis and having terminal axes only slightly widened without any pronounced sporangial rim. C. bohemica differs from Cooksonia sp. in having a wider branching angle, short subtending axes and kidney-shaped sporangia ( Schweitzer 1980).

Cooksonia sp. also resembles Concavatheca banksii from the Lochkovian of England ( Habgood et al. 2002, Morris et al. 2012) in having similarly shaped terminal axes subtending sporangia. According to Morris et al. (2012), Concavatheca differs from Cooksonia in having, “extension of the sporangial cavity into the distally widening subtending axis as exemplified by changes in the shape of the epidermal cells”. This character is not possible to distinguish in our material, although the shapes of subtending axes of both Concavatheca banksii and Cooksonia sp. are similar. The holotype of Concavatheca banksii is a charcoalified mesofossil, while our material consists of compressions. Due to different preservation states of the two species, direct and detailed comparisons cannot be made. However, there are more distinguishing characters between the above mentioned taxa, particularly in sizes of their axes and sporangia. Three subspecies of Cooksonia pertoni [C. pertoni ssp. pertoni , C. pertoni ssp. synorispora from Pridoli ( Fanning et al. 1988) and C. pertoni ssp. reticulispora from Lochkovian ( Habgood et al. 2002)] described from Great Britain differ from Cooksonia sp. particularly in their smaller size. Cooksonia hemisphaerica from the Lochkovian of England ( Lang 1937), recorded also from the Pridoli Series of the Barrandian area ( Obrhel 1962), differs from Cooksonia sp. in both its size and in the shape of the sporangia, which are hemispherical. Cooksonia sp. differs from Cooksonia crassiparietilis from the Early Devonian of Kazakhstan ( Yurina 1969) in having a wider branching angle (60°) and more elliptically shaped sporangia. Cooksonia sp. from the uppermost Homerian of Tipperary in Ireland ( Edwards et al. 1983) differs from our material in size; their axes are only about 0.2 mm in diameter and having slightly widened axes subtending sporangia ( Morris et al. 2012).

The dispersed spore assemblage prepared from the same sediment samples bearing Cooksonia fossils provided at least five other trilete spore types, suggesting further diversity of cooksonioid and similar plants. The dispersed spores will be described in a separate paper.

The Cooksonia fossils described here come from what was then the southern hemisphere, where the peri-Gondwana terranes and microcontinents, including those of the present Bohemian Massif, were situated at mid latitude ( Cocks and Torsvik 2006). The rocks containing these fossils are products of near-shore marine sedimentation associated with volcanic islands, the largest of which formed today’s Svatý Jan Volcanic Centre ( Havlíček and Štorch 1990, Kříž 1991).

Figures

Text-fig. 5. Cooksonia sp., Loděnice, Špičatý vrch – Barrandovy Jámy, coll. National Museum, Prague, the Czech Republic. a. Three times isotomously branched axis with one preserved sporangium, NM-D 554 (scale bar 10 mm); b. Terminal part of three times isotomously branched axis with loosely attached sporangia, detail of Text-fig. 5a; c. Schematic drawing of terminal part of axis bearing several sporangia, NM-D 550; d. Terminal part of axis bearing several sporangia, NM-D 550 (scale bar 10 mm); e. Terminal part of branch with loosely attached putative sporangia forming one group, NM-D 557a (scale bar 10 mm); f. Fragmentary sporangium, detail of Text-fig. 5d (scale bar 10 mm).