- Email: [email protected]
A lower carboniferous seed compression from North Wales
Review of Palaeobotany and Palynology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
A LOWER CARBONIFEROUS SEED COMPRESSION FROM NORTH WALES J. M. PETTITT and W. S. LACEY
Department of Palaeontology, British Museum of National History, London (Great Britain) School of Plant Biology, University College of North Wales,Bangor (Great Britain)
ABSTRACT Pettitt, J. M. and Lacey, W. S., 1972. A Lower Carboniferous seed compression from North Wales. Rev. Palaeobot. Palynol., 14: 159-169. A new gymnospermous seed compression, Menaisperma greenlyii n. gen and n. sp., from the Lower Carboniferous Basement Beds (early Upper Visean) of the Menai Straits, North Wales, is described. The seed, studied by maceration and balsam transfer, has: (1) an entire integument; (2) a free nucellus; (3) nucellar stomata; and (4) a nucellar apex consisting of three lobes. These features together and the fact that it occupies a position more or less intermediate in time between the exclusively lagenostomalean seeds of the basal Mississippian (Tournaisian) and the lagenostomalean, trigonocarpalean and cardiocarpalean seeds of the Pennsylvanian make the affinity of Menaisperma of particular interest. The structural resemblance the fossil has to the seeds of the Tournaisian and Pennsylvanian is discussed. INTRODUCTION The seed described in this paper comes from the Lower Carboniferous Basement Beds in the Menai Straits, Caernarvonshire, North Wales. "The geology of the area was described by Greenly (1928). The same paper included a short account by Walton of the rather sparse macroflora, but the rich seed and miospore-containing bed was not found until much later." (Lacey, 1952.) Specimens of isolated seeds were extracted by bulk hydrofluoric acid digestion and by balsam transfer from a shale that occurs in thin lenticles where the Basement Beds outcrop by the Britannia Tubular Bridge near Bangor. Hibbert and Lacey (1969) have concluded from an examination of the miospore flora that the Basement Beds are of early Late Visean age. SYSTEMATIC DESCRIPTION Class Gymnospermae
Menaisperma Pettitt and Lacey n. gen. Type species." Menaisperma greenlyii Pettitt and Lacey n. sp.
160
J.M. PETT1TT AND W. S. LACEY
t
I
Fig. 1. Semidiagrammatic representation of the seed Menaispermagreenlyii showing the integument, the nucellus with the three apical lobes and the megaspore. The cellular cuticle surrounding the megaspore is the tapetal membrane. The scale line corresponds to 1 mm.
Menaisperma greenlyii n. sp. (Plates I - I V ; Fig. 1, 2.) Combined diagnosis: Seed compressions more or less ovate, 7 mm long, 4 mm wide. Point of abscission a conspicuous notch 1.5 mm wide in the integument, lntegumeni entire, thicker at micropylar than at chalazal end. Outer cuticle of integument thin, celi outlines indistinct. Nucellus free from integument except at extreme chalazal end. Nucellus cuticle robust, stomata present below the pollen chamber. Pollen chamber dome-shaped, apex elaborated as three small lobes. Megaspore large, thick-walled. Holotype: V. 56655. British Museum (Natural History), Department of Palaeontology Type locality: Menai Straits, Caernarvonshire side, near Britannia Tubular Bridge (Grid Ref.: SH 54170). Basement Beds (early Upper Visean Stage), Lower Carboniferous.
LOWER CARBONIFEROUS SEED COMPRESSION FROM NORTH WALES
161
PLATE I
Menaisperma greenlyii. Transfer preparation showing the integument (outlined with arrow heads), the nucellus and the megaspore. V. 56656; X 23.
162
J.M. PETTITT AND W. S. LACEY
\
I I
I
I I I I
\
\ 1t
\
\\ \
\ \\ \
Fig.2. Drawing of a nucellar stomate. The hatching shows the cuticular thickenings on the epidermal ceils at the edge of the stomatal pit and on the guard cells at the edge of the stomatal aperture. The nucellar epidermal cell outlines are drawn as continuous lines and the cell outlines on the underlying tapetal membrane as dashed lines (see also Plate IV, 1, 2). The scale line corresponds to 50 ~tm.
Description The integument. The form of the integument ofM. greenlyii is most clearly revealed in balsam transfer preparations (Plate I). These show the integument is entire and is thickest at the micropylar end of the seed, gradually decreasing in thickness towards the point of abscission at the chalazat end. The point of abscission is marked by a conspicuous notch about 1.5 mm wide in the integument. The tissues of the integument are not preserved, but the cuticle which covered the outer surface sometimes remains after oxidative maceration. The cuticle is a very delicate and continuous membrane (Plate II, 1 ; Plate IlL 1) and it bears faint outlines of isodiametric cells 6 6 - 1 4 5 / a m . A micropylar opening was not observed in any of the specimens, nor could the cuticle which would have covered the inner free face of the integument be distinguished. It is probable that this cuticle too is thin and has become inseparably adherent to the outer cuticle of the integument. It is perplexing that the integument is so poorly represented in these fossils, and the fact that the micropyle cannot be detected is almost certainly a consequence of this.
~lenaisperma greenlyii. Seeds released from the shale with hydrofluoric acid, treated with Schulze's ;olution (potassium chlorate in nitric acid) and dilute alkali. L. The most complete specimen showing the thin i n t e g u m e n t cuticle, the nuceUus cuticle and the megaspore. Holotype. V. 56655; × 16. ).. Specimen showing the dimensions of the pollen chamber. V. 56657; × 26. L In this specimen the megaspore m e m b r a n e is folded and the e x t e n t o f the nucellus cuticle can be seen. V. 56658; X 22.
?LATE II
rl ¢]
,d
r~ 3
¢3 ¢)
"3
r~
-)
164
PLATE
J M. PETTITT AND W, S. LA('I: Y
Ill
Menaisperma greenlyii. 1. Cuticle released by macerating a fragment o f the i n t e g u m e n t from the apex of a seed. The shaUow depression (arrow) m a y be the opening of the micropyle. V. 56659; X 120. 2. Lobes at the apex of the nucellus. Only two o f the three are visible. V. 56660; X 140.
LOWER C A R B O N I F E R O U S SEED COMPRESSION FROM N O R T H WALES
PLATE
165
IV
Menaisperma greenlyii. 1. Cellular outlines on the nucellus cuticle (arrow head) and on the underlying tapetal m e m b r a n e (arrow). V. 56661; × 420. 2. Stomate on the nucellus cuticle. Notice the cuticular thickenings; Xc.400. (The m e d i u m in which this cuticle was originally m o u n t e d deteriorated and during an a t t e m p t to replace it the stomate was destroyed). 3. Section showing the tapetal m e m b r a n e (arrow) closely covering the megaspore. V. 56662; × 1,850.
166
J. M. PETTITT
AND
W. S. LACEY
7%e nucellus. The nucellus cuticle in M. greenlyii extends almost to the base of the seed and its presence demonstrates that except at the extreme chalazal end the nucellus was free from the integument (Plate II, 3). The nucellus cuticle is considerably thicker than the outer cuticle of the integument, and above the level of the megaspore apex is differentiated as the covering of a dome-shaped pollen chamber (Plates I; II, 1, 2). The pollen chamber is a relatively massive structure about 1S-2 mm wide at its base and extending 2 mm or more above the megaspore apex with, at the summit, three rather thinly cutinized lobes 87-210 pm high and 87-2 12 pm wide (Plate III, 2). A few specimens show elongated cells 63-133 pm long and 7-22 pm wide on the pollen chamber cuticle. In one preparation of a piece of nucellus cuticle taken from beneath the base of the pollen chamber there are stomata (Plate IV, 2; Fig.2). The reniform guard cells of the stomata are obliquely orientated (with respect to the longitudinal axis of the seed) and slightly sunken below the general level of the epidermal cells which are as much as 340 pm in length and 42 pm in width. There are cuticular thickenings on the epidermal cells at the edge of the stomatal pit and on the guard cells at the edge of the stomata1 aperture (Plate IV, 2; Fig.2). The megaspore. The megaspore in M. greenlyii is about 4 mm long and 3 mm wide and has a thick wall. Closely covering the surface is a very much thinner tapetal cuticle (Plate IV, 1, 3) bearing a distinct cellular pattern. The cell outlines are 136-196 pm long and 14-35 grn wide with strong anticlinal walls commonly 3-8 pm, but sometimes 14 pm in thickness (Plate IV, 1; Fig.2). Discussion There are four features of M. greenlyii which warrant discussion. They are: (1) the nature of the integument; (2) the construction of the pollen chamber apex; (3) the occurrence of nucellar stomata; and (4) the affinity of the seed and its stratigraphical position. We have already mentioned that only a few of the specimens recovered from bulk digestion and oxidative maceration show the outer cuticle of the integument - Plate II, 1 is the most complete - and that none, with certainty, show the inner cuticle. The outer cuticle in Menaisperma is a delicate membrane and it is interesting that this contrasts, on the one hand, with the situation in the cordaites and on the other, compares with that in the Pennsylvanian pteridosperms (Pettitt, 1966). There is some temptation to suppose that this feature is significant to the extent that it indicates something of the constructural character of the integument. But it must be admitted there is no support from Pennsylvanian or Mesozoic seeds (Harris, 1954,1958) for such an assumption. Moreover, it is known that because of its relation to the environment the degree of development of a cuticular membrane is liable to considerable phenotypic variation (State, 1965). Comparisons between petrifaction and compression fossils are always equivocal to some extent since they seek to relate two forms of preservation between which, by the fact, there
LOWERCARBONIFEROUSSEED COMPRESSIONFROM NORTH WALES
167
cannot be more than limited correspondence. It is exceptional for any compression to reveal histological details except those that are reflected on the surface cuticles. Nonetheless, we would have expected at least a sclerotic tissue to have been preserved in the integument of Menaisperma and to have been disclosed in the transfers, but none could be detected. Notwithstanding Harris's (1958) comment on the danger of interpreting what a fossil does not show, we are inclined to stress the absence of fibrous cells in the integument of Menaisperma. In the present-day cycads and conifers, lignification of the sclerotesta does not begin until quite an advanced stage in ovule ontogeny (Quisumbing, 1925; Schnarf, 1937) and it could be that the absence of stony tissue in the integument of Menaisperma denotes nothing more or less than immaturity. The presence of an obviously well differentiated nucellus and megaspore in Menaisperma, however, suggests that this explanation is unlikely. The investigation failed to reveal the micropyle, but we conclude that for the seed to have functioned there must, of course, have been one. Perhaps the apparent absence of a micropyle in the fossil is explained by the innate nature of the integument and the way a tissue composed wholly of thin-walled cells would behave on compression. The micropyle would have been lined with a cuticle which was, in all probability, like the integument cuticle, very tenuous. It is clear that in this case it would be difficult in macerations to discriminate the one cuticle from the other. Presumably the three cutinized lobes at the summit of the pollen chamber in Menaisperrna were for pollen reception and were situated at the base of the micropyle (Fig.l). As such they are assumedly homologous with the salpinx of Tournaisian and Pennsylvanian seed petrifactions. There is, in Tournaisian seeds, compelling evidence which suggests an evolutionary relationship between the degree of development of the micropyle and that of the salpinx - a strongly developed salpinx is accompanied with a rudimentary micropyle and conversely (Long, 1961). This is generally interpreted as the responsibility for pollen capture being progressively transferred with time from the nucellus to the integument; a change necessitated by (and probably concurrent with) the evolution of the integument from a multipartite to an entire structure (Long, 1966). When compared to that in many Tournaisian seeds, the design of the salpinx in Menaisperma is simple. But from this it does not necessarily follow that the simplicity is the result of evolutionary reduction: it might be fundamental. Nucellar stomata are not unique to Menaisperma; they occur in the present-day cycads Zamiafloridana (Shapiro, 1951), Zamia pumula and Cycas pectinata (Pant and Nautiyal, 1963). The stomata in Z. pumula are confined to the basal region of the free part of the nucellus, in Z. floridana they are most numerous in the central region, whereas in C. pectinata they are found towards the micropylar end. The stomata in M. greenlyii were found on the nucellus cuticle just below the rim of the pollen chamber and none were found on the pollen chamber itself. So far as we are aware, nucellar stomata have not been seen in any other Palaeozoic seed and it is far from clear what significance can be attached to their presence in Menaisperma or, for that matter, in Zamia and Cycas. It is generally agreed that there are only two kinds of Carboniferous seeds - pterido-
168
J.M. PETTITT AND W. S. LACE~r
spermous and cordaitalean. The Carboniferous pteridosperm seeds can be distinguished o~i the basis of structure as either lagenostomalean (belonging in part or wholly to the Lyginopterideae) or trigonocarpalean (belonging in part or wholly to the Medulloseae: see Delevoryas, 1962). All Lower Carboniferous seeds - the majority are Tournaisian are identified as lagenostomalean and these exhibit considerable morphological diversity (see Long, 1966). There are no unequivocal records of pre-Pennsylvanian trigonocarpalea~ or cordaitalean (cardiocarpalean) seeds. The structural features permitting discrimination between lagenostomalean and trigonocarpalean seeds are well documented (see Smith, 1964) and embrace the symmetry, the vascularization, the relationship of the nucellus ~o the integument, the form of the integument and the complexity of the nucellar apex Now it is clear that a number of these will normally only be illustrated in petrifactions m which, indeed, the distinction was first recognized, and we have already mentioned the interpretational difficulties in comparing compressions with petrifactions. Thus. we are faced with assigningM, greenlyii to one or other of the seed groups ignorant of certain anatomical and morphological characters that together form the very basis of lhe difference between them. It will be recalled that the most obvious features ofM. greenlyii are the entire inte~ gument, the free nucellus and the simple nucellar apex, and to this list might be added or~. more, that of large size. Now, a free nucellus is a principal characteristic of the trigonocarpalean and cardiocarpalean seeds in the Pennsylvanian, and in the absence of any anatomical criteria, these two can be distinguished by their symmetry which is radial in the former and bilateral in the latter. Accordingly, on the occurrence of the separate nucellus alone, we speculate upon Menaisperrna being a Mississippian trigonocarpalean o.r cardiocarpalean ovule. The entire integument, the structurally simple nucellar apex and the large size of the seed are characters not inconsistent with this assignation. Further. since the arrangement of the lobes at the apex of the nucellus suggest that the seed was radially symmetrical, we are inclined to favour the pteridosperm alternative. But let us note that Smith (1964) believes radial symmetry was common also to the ovules of the ancestors of the cordaites. Whereas a free nucellusis common to trigonocarpalean and cardiocarpalean seeds in the Pennsylvanian, in the basal Mississippian it is found in seeds assigned to the lagenostomales. The difference is, however, in the Mississippian lagenostomales a free nucellus Is associated with an integument consisting of a number of separate lobes (Long, 1966). A seed of this construction is considered primitive and it is supposed that subsequenl evolutionary events included the integumentary lobes fusing completely so that in later, more advanced seeds little or no trace of them remained and the adnation of the integument to the nucellus. Thus, a seed with a multipartite integument and a free nucellus evolved into one with an entire integument adnate to the nucellus. Both kinds occur in lhe Tournaisian as do a number of forms recognized as intermediate (Long, 1966). Now the interest in Menaisperma in relation to all this is that it is early Late Visean in age and therefore occupies a position more or less intermediate in time between the lagenostomalean seeds of the basal Mississippian and those of the Pennsylvanian. Perhaps then
LOWER CARBONIFEROUS SEED COMPRESSION FROM NORTH WALES
169
M. greenlyii represents in the Visean a hitherto unknown situation in lagenostomalean seeds where nucellar adnation did not attend fusion of the integumentary lobes. It is clear that such a state differs not at all from the trigonocarpalean condition and, in fact, it has been suggested that the trigonocarpalean seed evolved in precisely this way (Long, 1966). REFERENCES Delevoryas, T. D., 1962. Morphology and Evolution of Fossil Plants. Holt, Rinehart and Winston, New York-Chicago-San Francisco-London, 189 pp. Greenly, E., 1928. The Lower Carboniferous Rocks of the Menaian region of Caernarvonshire. Q. J. Geol. Soc., 84: 382-439. Harris, T. M., 1954. Mesozoic seed cuticles. Svensk. Bot. Tidsk., 48: 281-291. Harris, T. M., 1958. The seed of Caytonia. Palaeobotanist, 7: 93-106. Hibbert, F. A. and Lacey, W. S., 1969. Miospores from the Lower Carboniferous Basement Beds in the Menai Straits region of Caernarvonshire, North Wales. Palaeontology, 12:420-440. Lacey, W. S., 1952. Additions to the Lower Carboniferous flora of North Wales. Congr. Av. Etud. Stratigr. Geol. Carb., 3, Heerlen, 1951, CR., 2: 375-377. Long, A. G., 1961. Some pteridosperm seeds from the Calciferous Sandstone Series of Berwickshire. Trans. R. Soc. Edinb., 64: 401-419. Long, A. G., 1966. Some Lower Carboniferous fructifications from Berwickshire, together with a theoretical account of the evolution of ovules, cupules and carpels. Trans. R. Soc. Edinb., 66: 345-375. Pant, D. D. and Nautiyal, D. D., 1963. Cuticle and epidermis of recent cycadaies. Leaves, sporangia and seeds. Senckenb. BioL, 44: 257-348. Pettitt, J. M., 1966. A Comparative Study of the Fructification Cuticles and Spores of Palaeozoic and Living Plants. Thesis, University of London, 189 pp. Quisumbing, E., 1925. Stony layer in seeds of gymnosperms. Bot. Gaz., 79: 121-195. Schnarf, K., 1937. Anatomie der Gymnospermen-Samen. In: K. Linsbauer, G. Tischler and A. Pascher (Editors), Handbuch der Pflanzenanatomie, 1I. Borntr~iger, Berlin, pp. 1-156. Shapiro, S., 1951. Stomata on the ovules ofZamia floridana. Am. J. Bot., 38: 47-53. Smith, D. L., 1964. The evolution of the ovule. Biol. Rev., 39: 137-159. Stace, C. A., 1965. Cuticular studies as an aid to plant taxonomy. Bull, Br. Mus. Nat. Hist. (Bot.), 4: 3-78.
A LOWER CARBONIFEROUS SEED COMPRESSION FROM NORTH WALES J. M. PETTITT and W. S. LACEY
Department of Palaeontology, British Museum of National History, London (Great Britain) School of Plant Biology, University College of North Wales,Bangor (Great Britain)
ABSTRACT Pettitt, J. M. and Lacey, W. S., 1972. A Lower Carboniferous seed compression from North Wales. Rev. Palaeobot. Palynol., 14: 159-169. A new gymnospermous seed compression, Menaisperma greenlyii n. gen and n. sp., from the Lower Carboniferous Basement Beds (early Upper Visean) of the Menai Straits, North Wales, is described. The seed, studied by maceration and balsam transfer, has: (1) an entire integument; (2) a free nucellus; (3) nucellar stomata; and (4) a nucellar apex consisting of three lobes. These features together and the fact that it occupies a position more or less intermediate in time between the exclusively lagenostomalean seeds of the basal Mississippian (Tournaisian) and the lagenostomalean, trigonocarpalean and cardiocarpalean seeds of the Pennsylvanian make the affinity of Menaisperma of particular interest. The structural resemblance the fossil has to the seeds of the Tournaisian and Pennsylvanian is discussed. INTRODUCTION The seed described in this paper comes from the Lower Carboniferous Basement Beds in the Menai Straits, Caernarvonshire, North Wales. "The geology of the area was described by Greenly (1928). The same paper included a short account by Walton of the rather sparse macroflora, but the rich seed and miospore-containing bed was not found until much later." (Lacey, 1952.) Specimens of isolated seeds were extracted by bulk hydrofluoric acid digestion and by balsam transfer from a shale that occurs in thin lenticles where the Basement Beds outcrop by the Britannia Tubular Bridge near Bangor. Hibbert and Lacey (1969) have concluded from an examination of the miospore flora that the Basement Beds are of early Late Visean age. SYSTEMATIC DESCRIPTION Class Gymnospermae
Menaisperma Pettitt and Lacey n. gen. Type species." Menaisperma greenlyii Pettitt and Lacey n. sp.
160
J.M. PETT1TT AND W. S. LACEY
t
I
Fig. 1. Semidiagrammatic representation of the seed Menaispermagreenlyii showing the integument, the nucellus with the three apical lobes and the megaspore. The cellular cuticle surrounding the megaspore is the tapetal membrane. The scale line corresponds to 1 mm.
Menaisperma greenlyii n. sp. (Plates I - I V ; Fig. 1, 2.) Combined diagnosis: Seed compressions more or less ovate, 7 mm long, 4 mm wide. Point of abscission a conspicuous notch 1.5 mm wide in the integument, lntegumeni entire, thicker at micropylar than at chalazal end. Outer cuticle of integument thin, celi outlines indistinct. Nucellus free from integument except at extreme chalazal end. Nucellus cuticle robust, stomata present below the pollen chamber. Pollen chamber dome-shaped, apex elaborated as three small lobes. Megaspore large, thick-walled. Holotype: V. 56655. British Museum (Natural History), Department of Palaeontology Type locality: Menai Straits, Caernarvonshire side, near Britannia Tubular Bridge (Grid Ref.: SH 54170). Basement Beds (early Upper Visean Stage), Lower Carboniferous.
LOWER CARBONIFEROUS SEED COMPRESSION FROM NORTH WALES
161
PLATE I
Menaisperma greenlyii. Transfer preparation showing the integument (outlined with arrow heads), the nucellus and the megaspore. V. 56656; X 23.
162
J.M. PETTITT AND W. S. LACEY
\
I I
I
I I I I
\
\ 1t
\
\\ \
\ \\ \
Fig.2. Drawing of a nucellar stomate. The hatching shows the cuticular thickenings on the epidermal ceils at the edge of the stomatal pit and on the guard cells at the edge of the stomatal aperture. The nucellar epidermal cell outlines are drawn as continuous lines and the cell outlines on the underlying tapetal membrane as dashed lines (see also Plate IV, 1, 2). The scale line corresponds to 50 ~tm.
Description The integument. The form of the integument ofM. greenlyii is most clearly revealed in balsam transfer preparations (Plate I). These show the integument is entire and is thickest at the micropylar end of the seed, gradually decreasing in thickness towards the point of abscission at the chalazat end. The point of abscission is marked by a conspicuous notch about 1.5 mm wide in the integument. The tissues of the integument are not preserved, but the cuticle which covered the outer surface sometimes remains after oxidative maceration. The cuticle is a very delicate and continuous membrane (Plate II, 1 ; Plate IlL 1) and it bears faint outlines of isodiametric cells 6 6 - 1 4 5 / a m . A micropylar opening was not observed in any of the specimens, nor could the cuticle which would have covered the inner free face of the integument be distinguished. It is probable that this cuticle too is thin and has become inseparably adherent to the outer cuticle of the integument. It is perplexing that the integument is so poorly represented in these fossils, and the fact that the micropyle cannot be detected is almost certainly a consequence of this.
~lenaisperma greenlyii. Seeds released from the shale with hydrofluoric acid, treated with Schulze's ;olution (potassium chlorate in nitric acid) and dilute alkali. L. The most complete specimen showing the thin i n t e g u m e n t cuticle, the nuceUus cuticle and the megaspore. Holotype. V. 56655; × 16. ).. Specimen showing the dimensions of the pollen chamber. V. 56657; × 26. L In this specimen the megaspore m e m b r a n e is folded and the e x t e n t o f the nucellus cuticle can be seen. V. 56658; X 22.
?LATE II
rl ¢]
,d
r~ 3
¢3 ¢)
"3
r~
-)
164
PLATE
J M. PETTITT AND W, S. LA('I: Y
Ill
Menaisperma greenlyii. 1. Cuticle released by macerating a fragment o f the i n t e g u m e n t from the apex of a seed. The shaUow depression (arrow) m a y be the opening of the micropyle. V. 56659; X 120. 2. Lobes at the apex of the nucellus. Only two o f the three are visible. V. 56660; X 140.
LOWER C A R B O N I F E R O U S SEED COMPRESSION FROM N O R T H WALES
PLATE
165
IV
Menaisperma greenlyii. 1. Cellular outlines on the nucellus cuticle (arrow head) and on the underlying tapetal m e m b r a n e (arrow). V. 56661; × 420. 2. Stomate on the nucellus cuticle. Notice the cuticular thickenings; Xc.400. (The m e d i u m in which this cuticle was originally m o u n t e d deteriorated and during an a t t e m p t to replace it the stomate was destroyed). 3. Section showing the tapetal m e m b r a n e (arrow) closely covering the megaspore. V. 56662; × 1,850.
166
J. M. PETTITT
AND
W. S. LACEY
7%e nucellus. The nucellus cuticle in M. greenlyii extends almost to the base of the seed and its presence demonstrates that except at the extreme chalazal end the nucellus was free from the integument (Plate II, 3). The nucellus cuticle is considerably thicker than the outer cuticle of the integument, and above the level of the megaspore apex is differentiated as the covering of a dome-shaped pollen chamber (Plates I; II, 1, 2). The pollen chamber is a relatively massive structure about 1S-2 mm wide at its base and extending 2 mm or more above the megaspore apex with, at the summit, three rather thinly cutinized lobes 87-210 pm high and 87-2 12 pm wide (Plate III, 2). A few specimens show elongated cells 63-133 pm long and 7-22 pm wide on the pollen chamber cuticle. In one preparation of a piece of nucellus cuticle taken from beneath the base of the pollen chamber there are stomata (Plate IV, 2; Fig.2). The reniform guard cells of the stomata are obliquely orientated (with respect to the longitudinal axis of the seed) and slightly sunken below the general level of the epidermal cells which are as much as 340 pm in length and 42 pm in width. There are cuticular thickenings on the epidermal cells at the edge of the stomatal pit and on the guard cells at the edge of the stomata1 aperture (Plate IV, 2; Fig.2). The megaspore. The megaspore in M. greenlyii is about 4 mm long and 3 mm wide and has a thick wall. Closely covering the surface is a very much thinner tapetal cuticle (Plate IV, 1, 3) bearing a distinct cellular pattern. The cell outlines are 136-196 pm long and 14-35 grn wide with strong anticlinal walls commonly 3-8 pm, but sometimes 14 pm in thickness (Plate IV, 1; Fig.2). Discussion There are four features of M. greenlyii which warrant discussion. They are: (1) the nature of the integument; (2) the construction of the pollen chamber apex; (3) the occurrence of nucellar stomata; and (4) the affinity of the seed and its stratigraphical position. We have already mentioned that only a few of the specimens recovered from bulk digestion and oxidative maceration show the outer cuticle of the integument - Plate II, 1 is the most complete - and that none, with certainty, show the inner cuticle. The outer cuticle in Menaisperma is a delicate membrane and it is interesting that this contrasts, on the one hand, with the situation in the cordaites and on the other, compares with that in the Pennsylvanian pteridosperms (Pettitt, 1966). There is some temptation to suppose that this feature is significant to the extent that it indicates something of the constructural character of the integument. But it must be admitted there is no support from Pennsylvanian or Mesozoic seeds (Harris, 1954,1958) for such an assumption. Moreover, it is known that because of its relation to the environment the degree of development of a cuticular membrane is liable to considerable phenotypic variation (State, 1965). Comparisons between petrifaction and compression fossils are always equivocal to some extent since they seek to relate two forms of preservation between which, by the fact, there
LOWERCARBONIFEROUSSEED COMPRESSIONFROM NORTH WALES
167
cannot be more than limited correspondence. It is exceptional for any compression to reveal histological details except those that are reflected on the surface cuticles. Nonetheless, we would have expected at least a sclerotic tissue to have been preserved in the integument of Menaisperma and to have been disclosed in the transfers, but none could be detected. Notwithstanding Harris's (1958) comment on the danger of interpreting what a fossil does not show, we are inclined to stress the absence of fibrous cells in the integument of Menaisperma. In the present-day cycads and conifers, lignification of the sclerotesta does not begin until quite an advanced stage in ovule ontogeny (Quisumbing, 1925; Schnarf, 1937) and it could be that the absence of stony tissue in the integument of Menaisperma denotes nothing more or less than immaturity. The presence of an obviously well differentiated nucellus and megaspore in Menaisperma, however, suggests that this explanation is unlikely. The investigation failed to reveal the micropyle, but we conclude that for the seed to have functioned there must, of course, have been one. Perhaps the apparent absence of a micropyle in the fossil is explained by the innate nature of the integument and the way a tissue composed wholly of thin-walled cells would behave on compression. The micropyle would have been lined with a cuticle which was, in all probability, like the integument cuticle, very tenuous. It is clear that in this case it would be difficult in macerations to discriminate the one cuticle from the other. Presumably the three cutinized lobes at the summit of the pollen chamber in Menaisperrna were for pollen reception and were situated at the base of the micropyle (Fig.l). As such they are assumedly homologous with the salpinx of Tournaisian and Pennsylvanian seed petrifactions. There is, in Tournaisian seeds, compelling evidence which suggests an evolutionary relationship between the degree of development of the micropyle and that of the salpinx - a strongly developed salpinx is accompanied with a rudimentary micropyle and conversely (Long, 1961). This is generally interpreted as the responsibility for pollen capture being progressively transferred with time from the nucellus to the integument; a change necessitated by (and probably concurrent with) the evolution of the integument from a multipartite to an entire structure (Long, 1966). When compared to that in many Tournaisian seeds, the design of the salpinx in Menaisperma is simple. But from this it does not necessarily follow that the simplicity is the result of evolutionary reduction: it might be fundamental. Nucellar stomata are not unique to Menaisperma; they occur in the present-day cycads Zamiafloridana (Shapiro, 1951), Zamia pumula and Cycas pectinata (Pant and Nautiyal, 1963). The stomata in Z. pumula are confined to the basal region of the free part of the nucellus, in Z. floridana they are most numerous in the central region, whereas in C. pectinata they are found towards the micropylar end. The stomata in M. greenlyii were found on the nucellus cuticle just below the rim of the pollen chamber and none were found on the pollen chamber itself. So far as we are aware, nucellar stomata have not been seen in any other Palaeozoic seed and it is far from clear what significance can be attached to their presence in Menaisperma or, for that matter, in Zamia and Cycas. It is generally agreed that there are only two kinds of Carboniferous seeds - pterido-
168
J.M. PETTITT AND W. S. LACE~r
spermous and cordaitalean. The Carboniferous pteridosperm seeds can be distinguished o~i the basis of structure as either lagenostomalean (belonging in part or wholly to the Lyginopterideae) or trigonocarpalean (belonging in part or wholly to the Medulloseae: see Delevoryas, 1962). All Lower Carboniferous seeds - the majority are Tournaisian are identified as lagenostomalean and these exhibit considerable morphological diversity (see Long, 1966). There are no unequivocal records of pre-Pennsylvanian trigonocarpalea~ or cordaitalean (cardiocarpalean) seeds. The structural features permitting discrimination between lagenostomalean and trigonocarpalean seeds are well documented (see Smith, 1964) and embrace the symmetry, the vascularization, the relationship of the nucellus ~o the integument, the form of the integument and the complexity of the nucellar apex Now it is clear that a number of these will normally only be illustrated in petrifactions m which, indeed, the distinction was first recognized, and we have already mentioned the interpretational difficulties in comparing compressions with petrifactions. Thus. we are faced with assigningM, greenlyii to one or other of the seed groups ignorant of certain anatomical and morphological characters that together form the very basis of lhe difference between them. It will be recalled that the most obvious features ofM. greenlyii are the entire inte~ gument, the free nucellus and the simple nucellar apex, and to this list might be added or~. more, that of large size. Now, a free nucellus is a principal characteristic of the trigonocarpalean and cardiocarpalean seeds in the Pennsylvanian, and in the absence of any anatomical criteria, these two can be distinguished by their symmetry which is radial in the former and bilateral in the latter. Accordingly, on the occurrence of the separate nucellus alone, we speculate upon Menaisperrna being a Mississippian trigonocarpalean o.r cardiocarpalean ovule. The entire integument, the structurally simple nucellar apex and the large size of the seed are characters not inconsistent with this assignation. Further. since the arrangement of the lobes at the apex of the nucellus suggest that the seed was radially symmetrical, we are inclined to favour the pteridosperm alternative. But let us note that Smith (1964) believes radial symmetry was common also to the ovules of the ancestors of the cordaites. Whereas a free nucellusis common to trigonocarpalean and cardiocarpalean seeds in the Pennsylvanian, in the basal Mississippian it is found in seeds assigned to the lagenostomales. The difference is, however, in the Mississippian lagenostomales a free nucellus Is associated with an integument consisting of a number of separate lobes (Long, 1966). A seed of this construction is considered primitive and it is supposed that subsequenl evolutionary events included the integumentary lobes fusing completely so that in later, more advanced seeds little or no trace of them remained and the adnation of the integument to the nucellus. Thus, a seed with a multipartite integument and a free nucellus evolved into one with an entire integument adnate to the nucellus. Both kinds occur in lhe Tournaisian as do a number of forms recognized as intermediate (Long, 1966). Now the interest in Menaisperma in relation to all this is that it is early Late Visean in age and therefore occupies a position more or less intermediate in time between the lagenostomalean seeds of the basal Mississippian and those of the Pennsylvanian. Perhaps then
LOWER CARBONIFEROUS SEED COMPRESSION FROM NORTH WALES
169
M. greenlyii represents in the Visean a hitherto unknown situation in lagenostomalean seeds where nucellar adnation did not attend fusion of the integumentary lobes. It is clear that such a state differs not at all from the trigonocarpalean condition and, in fact, it has been suggested that the trigonocarpalean seed evolved in precisely this way (Long, 1966). REFERENCES Delevoryas, T. D., 1962. Morphology and Evolution of Fossil Plants. Holt, Rinehart and Winston, New York-Chicago-San Francisco-London, 189 pp. Greenly, E., 1928. The Lower Carboniferous Rocks of the Menaian region of Caernarvonshire. Q. J. Geol. Soc., 84: 382-439. Harris, T. M., 1954. Mesozoic seed cuticles. Svensk. Bot. Tidsk., 48: 281-291. Harris, T. M., 1958. The seed of Caytonia. Palaeobotanist, 7: 93-106. Hibbert, F. A. and Lacey, W. S., 1969. Miospores from the Lower Carboniferous Basement Beds in the Menai Straits region of Caernarvonshire, North Wales. Palaeontology, 12:420-440. Lacey, W. S., 1952. Additions to the Lower Carboniferous flora of North Wales. Congr. Av. Etud. Stratigr. Geol. Carb., 3, Heerlen, 1951, CR., 2: 375-377. Long, A. G., 1961. Some pteridosperm seeds from the Calciferous Sandstone Series of Berwickshire. Trans. R. Soc. Edinb., 64: 401-419. Long, A. G., 1966. Some Lower Carboniferous fructifications from Berwickshire, together with a theoretical account of the evolution of ovules, cupules and carpels. Trans. R. Soc. Edinb., 66: 345-375. Pant, D. D. and Nautiyal, D. D., 1963. Cuticle and epidermis of recent cycadaies. Leaves, sporangia and seeds. Senckenb. BioL, 44: 257-348. Pettitt, J. M., 1966. A Comparative Study of the Fructification Cuticles and Spores of Palaeozoic and Living Plants. Thesis, University of London, 189 pp. Quisumbing, E., 1925. Stony layer in seeds of gymnosperms. Bot. Gaz., 79: 121-195. Schnarf, K., 1937. Anatomie der Gymnospermen-Samen. In: K. Linsbauer, G. Tischler and A. Pascher (Editors), Handbuch der Pflanzenanatomie, 1I. Borntr~iger, Berlin, pp. 1-156. Shapiro, S., 1951. Stomata on the ovules ofZamia floridana. Am. J. Bot., 38: 47-53. Smith, D. L., 1964. The evolution of the ovule. Biol. Rev., 39: 137-159. Stace, C. A., 1965. Cuticular studies as an aid to plant taxonomy. Bull, Br. Mus. Nat. Hist. (Bot.), 4: 3-78.