Equisetostachys sp.

Pott, Christian, 2021, First Record Of Intact Equisetalean Strobili From The Wealden (Lower Cretaceous) Of The Isle Of Wight, Southern England, Fossil Imprint 77 (1), pp. 43-52 : 45

Equisetostachys sp.

Pls 1, 2

D e s c r i p t i o n. The specimen under study comprises two compressed strobili preserved close to each other (Pl. 1, Fig. 1). The strobilus in front is preserved in plan view and slightly overlaps the other strobilus, which is preserved in side view. This arrangement, however, is probably caused taphonomically, as is the slight distortion of the strobili through compression of the specimens; no organic connection between the strobili is recognisable. The preserved portions of the roughly round to oval strobili are 25.6 mm and 23.6 mm long, respectively, and 22.6 mm and 25.4 mm wide. The surface is characterised by the heads of closely packed sporangiophores (Pl. 1, Figs 1, 2, 5; Pl. 2, Figs 1–4, 7). The sporangiophore heads are predominantly hexagonal in outline (Pl. 1, Figs 2, 5; Pl. 2, Figs 3, 4, 7), but several heads, especially in the apical regions of the strobili, are pentagonal in outline (Pl. 1, Figs 2, 5; Pl. 2, Figs 1, 2). All sporangiophore heads have a flat, central, embossed area (umbo) with a corresponding hexagonal or pentagonal outline (Pl. 2, Figs 3, 4, 6), from the vertices of which an embossed ridge extends to the margin of the sporangiophore head (Pl. 2, Figs 4, 6). Densely arranged, tiny scars characterise the surface area of the sporangiophore heads, probably representing the cellular structure (Pl. 2, Figs 5, 6, 8). Heads preserved in side view do not show any evidence of preserved sporangia, nor are spores discernible, not even with epifluorescence microscopy.

Undistorted sporangiophore heads are up to 4.2 mm wide and 3.2 mm long, others, equal in length and width, measure up to 3.4 mm across. The sporangiophore heads slightly decrease in size towards the apices of the strobili (Pl. 1, Figs 1, 5; Pl. 2, Figs 1, 2). The apical-most sporangiophore heads clearly do not constitute a sterile corona or apex such as is visible in the modern species Equisetum hyemale L. (Textfig. 2a–c), nor is there any sign of an annulus, specialised cells forming a collar at the base of the strobilus to aid dehiscence of the sporangiophores in modern Equisetum species ( Page 1972). However, this structure may be hidden by the uppermost leaf sheath of the fertile shoot ( Text-fig. 2 ). In the rear strobilus, such a cluster of elongate, cuneate, up to 5-mm-long microphylls is preserved on its reverse side (Pl. 1, Figs 3, 4), which corresponds to the location of any potential annulus and the lowermost sporangiophore heads.

G e n e r i c a s s i g n m e n t. The strobili are assigned to the fossil-genus Equisetostachys that has been erected for isolated sphenophyte strobili consisting of whorls of peltate sporangiophores, and which has been used extensively for such strobili from Mesozoic and Cenozoic deposits ( Halle 1908, Brick 1938, Taylor et al. 2009). The complete absence of any characteristic, and therefore easily recognisable, articulated shoots, whorled leaves and branches in the Wessex Formation does not exclude sphenophyte remains to be found, and consequently renders these finds probably the first reports of equisetalean remains from this stratum. No other intact sphenophyte strobili have previously been reported from the English (and German) Wealden, so the presumed affinity of these new strobili relies entirely on gross morphology. The shape, size and close-packed sporophylls (probably in whorls) show remarkable agreement in all visible details with those of several extant ( Text-fig. 2 ) and fossil Equisetites / Equisetum species (see discussion below); therefore, the assignment to Equisetostachys seems most appropriate.

Referral to a particular species of Equisetites or Equisetum will only be possible if relevant sterile material or sporangiophores with spores are discovered and prove conspecific with the strobili. Differences between the two genera have been extensively discussed (e.g., Harris 1961, Watson and Batten 1990, Stanich et al. 2009), and revealed that identification of diagnostic characters, which clearly distinguish fossil Equisetites species from extant Equisetum , remains unresolved. Many of the fossil representatives of these genera have been suggested as belonging to the same evolutionary lineage leading to the extant horsetails. However, Equisetites lyellii displays features which suggest it belongs to a previously unknown, extinct sub-genus ( Watson and Batten 1990). Therefore, the generic identity of species assigned to Equisetites is probably not as ambiguous and arbitrary as is sometimes suggested.

Figures

Text-fig. 2. Strobili of modern Equisetum hyemale L. in different stages of expansion/maturation. Living specimens from near Billerbeck in Westphalia, Germany. Scale bars 5 mm.