Velumousspora, a megaspore genus from the Namurian of northern England

A new megaspore genus Velumousspora is described from Namurian (Stenozonotriletes triangulus-Rotaspora knoxi (TK.) concurrent range zone (E2a – E2b(pars) goniatite zonation) sediments from the Stainmore outlier, northern England.

shales in the Mousegill Beck section. Diagnosis. Small trilete megaspores approximately circular in outline consisting of three layers forming the spore wall. The middle layer, forms a distinct, laevigate 'central body' with a * distinct trilete mark of simple suturae. The outer layer is characterised by closely spaced folds or plications and extends beyond the equatorial margin of the 'central body'. This layer appears less dense (light colour) than the 'central body' and small verrucate-rugulate elements may be present, especially apparent at the margin. The two layers appear only to be in any contact in the area of the trilete mark of the 'central body'. N o convincing trilete structure is seen on the outer layer of the spore wall of most specimens, although on some, folds or ridges occur which may represent or simulate a trilete mark. No curvaturae or contact areas are distinguishable either on the outer layer or on the central body. On some specimens a further layer can be distinguished within the 'central body'. Remarks. The presence of two, in some instances three, distinct layers of material forming the wall of entire specimens presents problems of interpretation and use of nomenclature for the different layers. It is tempting to consider the outermost layer of the wall as being a perispore or perisporial as Dijkstra (1946, p. 3) considered the outermost layer of the wall of Triangulatisporites PotoniC & Kremp, 1954 to be. If it is accepted that a perispore is developed at a late stage in spore maturation, perhaps after the separation of the spore mother-cell, the absence of a distinct trilete structure on the outer layer would support such an interpretation. However, many workers have expressed uncertainty on the question of a perisporial layer being present in fossil material, e.g. Erdtman (1969) suggested the use of a neutral term sclerine; Pokrovskaya (1 9.50) (see also Kremp,196.5) claimed it is usually decomposed in the fossil condition; Harris (195.5) pointed out the need for reference to the ontogeny of the spore (impossible in the fossil condition) and Grebe (1971) recommended that the term should not be used in the description of fossil material. At the present time I favour the view expressed by Krutzsch ( 1 959) that it is more useful to speak of an outer, middle and inner layer.
The separation of the outer layer from the middle layer is clearly indicated by the folding due to compression of both layers. In addition I have been able to peel the outer layer from the 'central body' using a finely pointed needle and scalpel. Specimens where the outer layer has been removed could be assigned to Calamospora Schopf, Wilson and Bentall 1944, and such specimens, together with others, where the outer layer has been partly removed, were found in the organic residue.
Velumousspora is similar to Triangupapillaesporites Spinner, 1982 in size and in the possession of a threelayered spore wall. The middle layer in both instances forming a 'central body'. However, Triangupupilluesporites is characterised by the presence of papillae on the 'central body' and lacks the plicated outer layer of the spore wall. Granulamembranispora Spinner, 1983 is similar in form and shape to Velumousspora but it lacks the dense plications on the outermost layer of the spore wall.
(PI. 1, figs. 1-7) Derivation of Name. Named in honour of Dr. B. Owens, palynologist, Institute of Geological Sciences, Great Britain, who introduced the writer to the material. Holotype. P1. 1, fig. 1, slide No. M21513, stored in the reference collection, Micropalaeontology Laboratory, Geology Department, Sheffield University. Description. Size and Shape: Small trilete megaspores, circular to oval in compressed outline, originally spherical in shape, varying between 420p and 910p in maximum diameter (30 entire specimens, measured in a hydrous medium), mean 570p. A distinct 'central body' can be distinguished, circular to oval in outline, sometimes folded, varying between 205p and 5 3 5 p in maximum diameter, mean 320p (based on 30 specimens measured in a hydrous medium). External to the 'central body' is a layer of the spore wall which extends beyond the margin of the 'central body', irregular in outline due to compressional folding. On damaged specimens this surrounding layer may be partially removed. Haptotypic Structures: The most distinctive evidence of the trilete mark is seen on corroded specimens where three simple suturae can be seen on the 'central body' (see PI. 1, figs. 1, 4). On the holotype, the suturae are approximately 1 Sop in length, straight to slightly curved in outline. On entire specimens the trilete mark is i-distinct. On some specimens (viewed by reflected light or S.E.M.) ridges can be seen which may represent the position of the trilete mark (see P1. 1, fig. 6). The positions of the contact areas and curvaturae are not clearly defined. Exine Structure and Ornamentation: The spore wall consists of three layers, although in some specimens only the two outer layers can be distinguished. The outer layer is characterised by closely spaced folds or plications and has a verrucate-rugulate type of ornament. Elements measured vary between 6p and 30p in maximum diameter. At the margin the tops of the elements appear rounded. The elements are densely spaced and may simulate a vermiculate pattern. The middle layer of the spore wall forms the 'central body' of the spore. This layer appears darker in colour (denser) than the outer layer, is laevigate and is approximately 2 5 p thick (as measured in optical section). The middle and outer layers appear to be in contact in area of the trilete mark on the 'central body'. On some specimens a further layer can be distinguished within the 'central body', thin and folded. Remarks. The nature of the relationship of the outer layer of the spore wall to the 'central body' is problematical. If it is accepted that the outer layer is not perisporial, then the characters of the outer layer simulate in many respects a velum, as defined by Couper & Grebe  Fig. 7 Part of specimen illustrated in fig. 6, illustrating plications of outer layer of spore wall ( x 500). Spinner (IY61, p. 5). However, Grebe later (lY71, p. 16) redefined the term with emphasis on "multiple points of contact between the exine layers" which, in view of the relative ease by which the outer layer separates cleanly from the central body, casts doubts on such an interpretation being made. Neves & Owens ( 1 966, p. 33Y) introduced the term 'camerate' for separation of layers within spore walls, but these authors were concerned with spores in which only two layers were recognised, for which they used the terms 'intexine' and 'exoexine'. In the present instance it is probably most acceptable t o use the term 'cavate' as employed by Harris (1955, p. 25) indicating a separation of the outermost layer o f the sporc wall. V . owvnsii sp. nov. resembles Triungupupilluesporites biscratus Spinner, I Y82 in overall size and in having a thrcc layered spore wall, but the outer layer o f V . owrn.\ii is different t o T . biserutus in being highly plicated o r folded and the 'central body' lacks the biserially arranged papillae which characterises T .
hisvrtrtus. (;ranulunzemhruni.sporu dodgesontownii Spinner, l Y 8 3 is similar in form to V . owensii, but the o u t e r layer of the sporc wall lacks thc plications which c h ; m c t erise V . o w>rri.sii.