Alethopteris, STERNBERG, 1825

Šimůnek, Zbyněk & Cleal, Christopher J., 2018, Early Occurrence Of A Pennsylvanian-Age Medullosalean Frond Similar To Alethopteris Pseudograndinioides In The Intra-Montane Basin Of Bohemia, Fossil Imprint 74 (1 - 2), pp. 37-44 : 38-39

Alethopteris cf. pseudograndinioides ZODROW et CLEAL, 1998

Pls 1, 2

D e s c r i p t i o n. This species is represented here by just a single broken pinnule preserved as part and counterpart (Pl. 1, Figs 1, 2). The specimen is 10 mm wide and preserved for a length of 18 mm, with a linguaeform to slightly barreled shape, and rounded to slightly obtuse apex; the base of the pinnule is not fully preserved but shows what may be an incipient basiscopic auricle, and the acroscopic side is somewhat constricted. A prominent midvein extends for most of the pinnule length. Fine lateral veins are emitted from the midvein at an acute angle, arch broadly along most of their length, to meet the pinnule margin at about rightangles. The lateral veins fork three or four times at an acute angle, producing a marginal vein density of ca. 40 per cm.

Adaxial epidermis was thinly cutinised, but relatively large fragments could be obtained (Pl. 1, Figs 3–6). The cells of the intercostal field were isodiametric, polygonal, 45–75 mm in size. The costal field was formed by elongated polygonal to tetragonal cells that were 20–30 mm wide and 50–100 mm long. No hairs or trichome bases were observed.

Abaxial epidermis was thinly cutinised and there was a distinct differentiation between the cells of the costal and intercostal fields (Pl. 1, Fig. 7, Pl. 2, Fig. 3). The costal field, covering the lateral veins, was formed by elongate tetragonal, trapezoidal or rarely polygonal cells, 40–110 mm long and 15–30 mm wide. The intercostal fields had more irregularly shaped randomly oriented polygonal cells 30–75 mm long and 15–30 mm wide. The anticlinal walls were straight or bent.

Structured holes (Pl. 1, Fig. 8) occur rarely in the costal field of the abaxial epidermis. They are round and around 25 mm in diameter.

Anomocytic stomata were restricted to the intercostal fields of the abaxial surface and were randomly oriented (Pl. 2, Fig. 3). The guard cells were sunken and surrounded by 5 to 6 adjacent cells (Pl. 2, Figs 2, 4). The guard cells were reniform with projections in the polar areas, 32–38 mm long and 7–12 mm wide. The exterior (dorsal) walls of the guard cells had strong thickened lamellae and radial striae except in the polar regions (Pl. 2, Fig. 4). Moreover, the guard cells were strongly cutinised around the pores and at the contact areas with adjacent cells. The central part of the dorsal walls bore characteristic radial striae. The stomatal density (SD) varied from 100 (costal field included) to 130 stomata per mm 2 and the stomatal index from 15 to 18.

C o m p a r i s o n s. Alethopterids are generally rare in the upper Radnice Member ( Němejc 1936, Šimůnek 1988, 2007, Šimůnek and Cleal 2002). The only species to be previously reported from the tuffites above the “Z-tuff” are possible fragments Alethopteris lonchitica STERNBERG; it is better documented from the “Z-tuff” itself. However, the latter species tends to have more elongate linguaeform pinnules, and lateral veins that are less curved, rather denser and fork fewer times (usually only once or occasionally twice). The shorter pinnules in A. lonchitica can develop somewhat biconvex pinnules, but here the pinnules tend to have a more acute apex, in contrast to the more rounded apex in the presently described specimen. A. lonchitica has much denser stomata (370 per mm 2) on the abaxial surface and are surrounded by a ring of subsidiary cells ( Barthel 1962, Šimůnek 1996, 2007).

The only other alethopterid to be recorded from the upper Radnice Member is Alethopteris distantinervosa R.H.WAGNER, emended by Šimůnek and Cleal (2002). The latter has very similar shaped pinnules, but the veins are much less dense (typically 18‒27 per cm on the pinnules margin) and tend to fork at a wider angle. Cuticles of A. distantinervosa from the Whetstone Horizon described by Šimůnek (1988) are broadly similar to those obtained from the Upper Lubná Coal specimen in the general configuration of the epidermal cells, but differ in having trichomes on the adaxial surface and papillae on the abaxial surface.

The type of Alethopteris nemejcii R. H.WAGNER originated from the lower Radnice Member and has pinnules of similar size and similar vein density, but which are clearly more subtriangular with an acute apex than in the currently described specimen .

Němejc (1936) and Šimůnek (2007) reported Alethopteris serlii (BRONGNIART) GÖPPERT from the lower Radnice Member, although in the lowland paralic basins it does not appear until late Asturian times (Crenulopteris acadica Zone – e.g. Zodrow and Cleal 1998, Cleal et al. 2010). However, A. serlii has generally longer and more slender pinnules, and the lateral veins often tend to be more flexuous ( Wagner 1968, Zodrow and Cleal 1998). The stomatal density is similar in both species but in A. serlii they are orientated more parallel to the veins ( Zodrow and Cleal 1998).

In floras of similar age in the lowland paralic basins of Euramerica, there is some comparison with Alethopteris corsinii BUISINE. However, the pinnules tend to be rather more slender and lanceolate, and the veins are straighter, less forked and with a lower density (ca. 30 per cm on the pinnule margin).

Among early Westphalian alethopterids of Euramerica, only three species are notably abundant ( Buisine 1961, Šimůnek 1996). Alethopteris decurrens (ARTIS) ZEILLER is clearly quite different having much more slender, often almost needle-like pinnules and widely forked lateral veins. A. decurrens can also be distinguished by having adaxial epidermal cells that are not so markedly differentiated between the costal and intercostal fields and the guard cells of the stomata lack the thickened lamellae and radial striae ( Šimůnek 1996). Alethopteris davreuxii (BRONGNIART) ZEILLER has notably more lanceolate pinnules, and veins that are more flexuous, often verging on the pseudo-anastomosed, and somewhat less dense (30–36 per cm on pinnule margin); A. davreuxii can also be distinguished by having papillate stomata ( Šimůnek 1996). Alethopteris urophylla (BRONGNIART) GÖPPERT has less barrel-shaped pinnules, the smaller ones tending to an obliquely subtriangular form ( Wagner and Álvarez-Vázquez 2008); and the stomata are smaller and oriented parallel to the veins ( Šimůnek 1996).

The closest comparison to the currently described specimen is in fact a species normally found in upper Asturian floras (C. acadica Zone) – Alethopteris pseudograndinioides ZODROW et CLEAL. It also has linguaeform to slightly barrelled pinnules, a well-marked midvein, and broadly curved lateral veins that fork two or four times. The most notable difference is the rather higher vein density (in A. pseudograndinioides it is usually 20–34 per cm – see Zodrow and Cleal 1998). It is possible that this merely represents an extreme end-member of intra-specific variation but, given its very early stratigraphical occurrence (A. pseudograndinioides has not previously been reliably reported from below the Crenulopteris acadica Zone – see Zodrow and Cleal 1998) we think this unlikely. Hence, we have named it Alethopteris cf. pseudograndinioides.

The cuticles of this Czech specimen also resemble the types of Alethopteris pseudograndinioides (from the upper Asturian Sydney Mines Formation , Cape Breton , Canada) in the shape and the dimensions of the epidermal cells. The stomatal guard cells of the Canadian specimens were a little smaller and narrower than in the described specimen. The Canadian specimens also show evidence of stomata that were more parallel oriented than in the described specimen. The stomatal density is a little higher in the Canadian specimen – 250 stomata per mm 2, whereas the studied specimen has only some 100–130 stomata per mm 2 .