Spores and land-plant evolution

Review of Palaeobotany and Palynology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

SPORES A N D L A N D - P L A N T E V O L U T I O N WILLIAM G. CHALONER Department of Botany, University College, London (Great Britain)

(Received August 30, 1966)

SUMMARY

Because of their occurrence in relatively large numbers and in a wide range of rock types, spores offer a means of checking the evidence from macrofossils as to the course of early vascular plant evolution. Palynology offers several types of evidence in this connection viz.: (1) The time of first appearance of triradiate spores. (2) The rate at which distinct genera of spores appear. (3) The rate of spore size divergence (i.e., the rise of heterospory). (4) The approach of heterospory to the seed habit. The spore record suggests a relatively small number of land plant types first appearing in the Silurian, with steadily increasing diversity through the Devonian. This diversity is accompanied by a divergence in spore size groups through the Devonian into two increasingly clearly defined size classes by the Carboniferous. The closest approach to the seed habit within the Devonian appears to be represented by a Cystosporites tetrad. The pollen grains of some of the earliest Carboniferous gymnosperms appear to have germinated via the proximal, triradiate mark (i.e., they were prepollen); if found dispersed, they would be indistinguishable from triradiate isospores or microspores. There appears at present to be no way of recognising the earliest pollen grains, as such, among dispersed spores. The palynological evidence is consistent with a Silurian origin of land plants followed by a relatively slow diversification, rather than an early Lower Palaeozoic or Precambrian origin suggested by some authors.

INTRODUCTION The successful use of fossil spores as stratigraphic indices has tended to make palynology, especially of the older rocks, a primarily geological excercise. Many palaeobotanists have been inclined to ignore spores as a source of palaeobotanical information, and to concentrate on the more informative macrofossils. Rev. Palaeobotan. Palynol., 1 (1967) 83-93

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The purpose of this paper is to review some of the general conclusions that may be arrived at concerning early land plant evolution from the study of their dispersed spores. Among palaeobotanists there is still great controversy about the timing and nature of the early evolution of the land flora. AXELROD(1959) reviewed the situation some years ago and advances the hypothesis that vascular plants must have originated polyphyletically in the Precambrian, with the appearance of seed plants in the Lower Palaeozoic. He holds that the later and slower appearance of diversified vascular plants in the macrofossil record represents their migration into environments in which fossilization is favoured, from habitats in which it occurred less readily. Axelrod leaned considerably on the reports of vascular plant spores, some of which were regarded as pollen, from Lower Palaeozoic rocks. The opposite point of view has been summarised by STEWART(1960) who favours a monophyletic origin of vascular plants, probably occurring in the Silurian, with the sequence represented in the macrofossil record corresponding substantially with the true state of affairs. I want briefly to consider which of these two opposed points of view seems to be most closely in agreement with the evidence offered by Devonian microfossils. I do not intend to discuss the now extensive and contentious literature on supposed land-plant spores of pre-Silurian age. This l attempted to review two years ago at the International Botanical Congress (CHALONER, 1964), and I believe the situation has changed little since then. The first well authenticated---and repeated-record of clearly triradiate spores is in the Silurian. This situation has been commented on succinctly by HOFFMEISTER(1959), and more recently by the Russian worker VOLKOVA(1962), who regards the Devonian as the earliest record of clearly triradiate spores. I believe that on the evidence at present available we have no basis, in either microfossils or macrofossils, for supposing that vascular plants existed before the Silurian. I have accordingly examined the published data on Devonian spores to try to elucidate the rate at which new kinds of spores appear, and the time and nature of origin of heterospory and the seed habit.

RATE OF DIVERSIFICATION OF SPORES

In Fig.l, I have plotted the first and last appearance of spore genera within the Silurian and Devonian, as a crude measure of the rate of appearance of new Fig.1. A table of the first and last appearance of Devonian spore genera through the stages of the Devonian. (For sources, see the section "References" of this paper). Any Carboniferous or subsequent record is used as a basis for extrapolating the range of a genus from its first appearance in the Devonian to the end of the period. It is here suggested that the known ranges of spore genera represent a measure of the rate of diversification of land plants through the Devonian. 84

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Punctatisporltes Ambltlsporltes .Lophot..rlletes L.eK)triletes Colamosporo Retusotriletes Gronulatlsporltes Emphanisporites ChellnosporQ Trlk~ites Butlotisporit¢s ,~ictyotrlkltes nozcmotriletes 5amarisporites L~,cospora C,rratriradttes Murospora CAuarnptozonotriletes roraspora Rhabdoslx)rlt es Planls~rltes Acanthotrktes Apiculatislx~is Cycla~ranisl)orites DibolisporRes Vcrrucosls~or i t ~ Camptotrktrs Ccmvolutispora R~iculatiqlorites I:~rforosporit(~ Densoslx~rites Vallatis~rltes Cadlospora Crospedispora Archaeozonotriletes Tholisporit¢s ¢rotril itcs Calyptosporitcs Grandlspora GemlnosFora Diaphanospora Ancyrospora Corystlsporltes Anapiculatisporltcs I-lystrlcosl~ritcs AclnOSlporites Phylloth¢cot riletes Ralstrickla Biharis#oritcs Apk;ullrct uslspora Sptnozonotriletcs L~loz onot r iletes Aneurosporo Cincturasporites Lophozonotr ilctes Dlatomozonotriletes Hymenozonotriletes Cymbosporites Nikitinsporites Archoeotriletes Enigmophytosporo Brochotriletcs Heliosporit,s Carncrozonotrilctes Triongulotisporites Cystosporites Lagenoisporites Logenicula At cho¢operisoccus A zonomonoletcs Knoxisporit~s Canthospora Pulvinispora

EI.F

IS.

GIV,

!A.

FAM

i

i

Fig.1. (Legend see p.84.) Rev. Palaeobotan. Palynol., 1 (1967) 83-93

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kinds of spore-bearing plants. I do not claim to have exhausted the literature on the subject, and indeed the diagram is no doubt already out of date in terms of the earliest (or latest) record of this or that genus. I only plead that the purpose of the table is palaeobotanical and not primarily stratigraphic; if it is incomplete, as I am sure it is, then at least I believe that the omissions are more or less randomly distributed. There is certainly no particular bias in the compilation of the data, unless it be that Russian literature is poorly represented against Western, on account of the more ready accessibility to me of the latter. I chose to plot the number of genera rather than species because the tacit agreement of what constitutes a reasonable spore genus seems to show greater consistency than in the case of spore species. Even so, it is evident that some workers, for example NAUMOVA(1953) and WINSLOW (1962) take a much broader generic concept than others such as ALLEN (1965), FRANKE (1965) and RICHARDSON(1960, 1962, 1965). I have accordingly tried to bring such work to a comparable level of generic treatment. Naumova, for example, recognises only 15 genera in her large assemblage of spores from the Frasnian; while no western worker has expressly reviewed her flora in terms of spore genera currently recognised outside Russia, it is evident that comparable spores have been placed in from two to four times as many genera as those that she recognises. As an opposite case to this, FRANKE (1965), in his valuable study of an Early Emsian assemblage, recognises 24 genera, of which 11 are new. I believe that a broader generic concept would allow all of these 11 new genera to be placed in existing genera, already known from comparable or later horizons. I cite these two extreme cases as a reminder of what we all know, namely, that the recognition even of the limits of a spore genus, is a very subjective matter, and that data based on the collation of generic records is itself no less subjective. Fig.1 is based on records of spores from 24 papers by 23 authors or coauthors dealing with spores from the Silurian to Famennian (see list of references). The variation in the intensity with which the different stages of the Devonian have been examined for plant microfossils is of course one of the weaknesses of an analysis of this type. The paucity of work in, say, the Famennian compared with that in the Givetian or Frasnian is probably reflected in the results plotted. Aside from the omissions resulting from ignorance or inaccessibility of literature, a number of interesting Devonian spore records have had to be omitted from the plot because of uncertainty of the stage within the Devonian represented. On these grounds I have for example had to omit SEN'S (1958) interesting study of Nathorst's megaspores from Bear Island, and those that I obtained (CHALONER,1959)from the Late Devonian of Ellesmere Island. It should also be noted that 1 used any Carboniferous or later records of a genus as a basis for extrapolating its range from its first occurrence to the end of the Devonian. Fig.1 forms the basis of the histograms in Fig.2 showing the number of genera of spores reported from the Silurian and successively through the stages 86

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60-

I

I

O~ 40,

E •~ 20.

(Sil.)

Ge.

Si.

Em.

El.

Gi.

I Mio-

Fr.

Fa.

(Wes.)

Fig.2. A histogram of the numbers of genera of spores known from each of the stages of the Devonian. The total Silurian and the Westphalian records (the latter based on Poror~ and K~,re, 1955, 1956) are also shown. The numbers of those genera which contain species with a mean size exceeding 200 # are shown shaded, as megaspores ("mega-") in contradistinction to miospores (microspores and isospores) with a smaller mean size ("mio-"). of the Devonian. This rises from the three genera of indisputably triradiate spores from the Silurian (DowNIE, 1963; HOFFMEISTER, 1959; NAUMOVA, 1953), to 56 in the Frasnian. The slight fall-off in the Famennian may well reflect the lack of study already referred to, rather than a genuine decline in any source plants. Taken at its face value, the evidence presented here certainly seems consistent with a relatively late diversification of spore-producing land plants, initiated in the Silurian and progressing steadily through the Devonian. In view of the fact that these fossil spores potentially represent the whole of the vascular plant flora (together with bryophytes and possibly other groups) the rate of appearance of new genera is if anything surprisingly slow. This may be partly explained by supposing that some of the simpler genera of spores (e.g., Leiotriletes) are equivalent to several genera of parent plants. It cannot be denied that this picture could result from the sort of process that Axelrod postulates; that the appearance of new forms merely represents their migration into a region where their spores are getting into the microfossil record for the first time. But aside from other implausibilities of this hypothesis, a consideration of spore size-changes makes this very unlikely.

CHANGES IN MEAN SIZE OF SPORES Fig.3 is a histogram of the changing frequency of the different classes of mean size of spore species. This is based on the same sources of information as Fig.1 and 2, but as no limits of taxa are at stake, all spore species are included regardless of their generic assignation. The single size measurement used for each species is either the mean given by the author of the species, or if no mean is cited, then the arithmetic mean of the quoted range. I have used a logarithmic scale for the species number in order to emphasize the low frequencies in those size classes Rev. Palaeobotan. Palynol., 1 (1967) 83-93

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1 I&

GIVETIAN

SIEGENIAN

GEDINNIAN [SILURIAN] 200

500

1occ

,500

2000/

Fig.3. Histograms of the frequency of numbers of species arranged in classes of mean spore size. The vertical axis of each histogram is logarithmic. It is here suggested that the progressive increase in mean spore size towards and across the arbitrary 200 1’~line represents the rise of heterospory. By the Upper Carboniferous the number of species around the 200 p line decreases, and a clear segregation of large megaspores from microspores, isospores and pollen (represented mainly by the size classes below 200 /x) is evident in the histograms.

Fig.4. A histogram to show the number of species in successive stages of the Devonian (and, for comparison, in the Westphalian), which have a size range extending across the 200 p line. These probably represent plants with only weakly differentiated heterospory, or in which the size range of megaspores and microspores overlaps. By the late Devonian and in the Carboniferous, the number of these species falls off, with increasing segregation of mega- and microspore size among heterosporous plants.

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where there are any species at all, while drawing attention away from the large numbers of species at all horizons in the very small size classes. This series of histograms shows a progressive increase in maximum spore size through the Lower and Middle Devonian, and then an eventual segregation into small spores and megaspores--the origin, in fact, of heterospory. The arbitary demarcation line at 200# between megaspores and small spores (proposed by ZERNDT, 1934, and widely adopted by other workers) has also been shown on the histograms. For the sake of comparison with the Devonian results, I have also plotted the corresponding data for the Westphalian, based on the species given in POTONI~ and KREMP(1956, 1957). Perhaps the most significant feature of these histograms is the progression of species size towards and then across this 200# line. In this connexion the species with a mean size between 200# and 300# are especially significant. None occur before the Emsian; from then on a considerable number of species fall in the range 300#-500#, but then these "small megaspores" diminish in frequency, and by the Westphalian there is significantly, a distinct gap at the 200# level, but with a large number of species with mean size ranging from 300# to over 2,000#. The histograms of Fig.3, based on the mean sizes of spore species, take no account of the interesting cases where within a single spore species the size range of the population extends across the 200# line. The histogram in Fig.4 shows the number of species in each stage of the Devonian for which this is the case. They first appear in the Siegenian, and increase in number into the Givetian where they reach a maximum. From then on the number falls, and even in the well-studied Carboniferous (Westphalian) spore assemblages, only two such species are known. The simplest explanation of this rise is that these spore species ranging across the 200# limit probably represent, at least in part, plants in process of differentiating from homospory to heterospory; plants which may in fact have been heterosporous, but in which the size range of mega- and microspores overlapped. The overall picture presented by the spores seems to be consistent with a progression from a phase early in the Devonian in which plants with a low degree of heterospory are succeeded by those with a much more pronounced size difference between megaspores and microspores. Of course a "background" of homosporous plants prevailed from the Devonian into the Carboniferous, but from our knowledge of the macrofossils it seems likely that by the Westphalian, heterosporous and seed-bearing plants far outweighed the homosporous plants in number and ecological status. By the Upper Devonian large megaspores, apparently of freesporing plants, already exceed 1,000# in diameter. But they do not attain the very large dimensions of the biggest Carboniferous megaspores, which only appear after contemporaneous plants were already producing seeds. It might still be argued, with Axelrod, that the Early Devonian microfloras are simply under-representations in the spore record of a pattern of highly diversified plants ranging from homosporous to seed-bearing forms. But the progressively Rev. Palaeobotan. Palynol., 1 (1967) 83-93

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changing pattern of spore size makes this look very improbable. The spore record suggests not merely an orderly increase in the range of spore form through the course of the Devonian, but the simultaneous evolution from homospory to heterospory. It may also be noted that here, as elsewhere in the fossil record, the macrofossil remains probably greatly under-represent the diversity of heterosporous plants. We know of only two genera of heterosporous Devonian plants, of which we have fairly secure knowledge of the systematic position: the one, Cyclostigma, an arborescent lycopod, and the other Archaeopteris, a pteropsid, both of Upper Devonian age. Two others, Enigmophyton and Barinophyton are known, but their systematic position is uncertain. This compares with eighteen genera of Devonian plants for one or more species of which the spore contents are known, which are apparently homosporous. It is interesting in this respect to compare these figures with the number of genera in the Devonian which qualify as megaspores on the basis of the 200# arbitrary demarcation line. On this basis we know of sixteen dispersed spore genera within the Devonian which qualify as megaspores.

T H E ONSET OF SEED F O R M A T I O N A N D R E C O G N I T I O N OF P O L L E N

While the progression from homospory to heterospory shows clearly in the dispersed spore record, the production of seeds--and hence the attainment of the status of pollen by the corresponding microspores-- is less clear. There are three significant steps in the progression from free-sporing heterospory to the seed, which concern the megaspore. Firstly the entire megasporangium production is reduce to a single tetrad; then one member of the tetrad develops at the expense of the remaining three. Finally the whole megasporangium is enclosed in an integument in some form, in which it is retained when shed. The megaspore tetrad described as Cystosporites devonicus (CHALONERand PETTITT, 1964) has at least one of these attributes; there is very pronounced size differentiation within the tetrad. We cannot prove that the tetrad represents the entire contents of a megasporangium, much less that it was integumented. But the nearest equivalent structure known from the Carboniferous is the seed megaspore contained within the integumented Lepidocarpon megasporangium. C),stosporites devonicus seems to represent the nearest approach to a seed that has yet been found in the Devonian. The recognition of pollen in the Devonian is on even more uncertain ground. There is only one feature botanically which separates a microspore from a pollen grain, that is, the functioning o f the corresponding megaspore within its integumented sporangium as a seed. Unfortunately this seed habit is not reflected directly in any way in the microspore. Among the earliest known Carboniferous pollen grains (those of some of the pteridosperms) are forms which are indistinguishable 90

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from triradiate isospores. These are the so-called pre-pollen which, although they functioned as pollen grains, none the less seem to have germinated through a proximal triradiate suture. We only know them to be pollen grains because their parent plants are known to be seed-bearing. An attribute of many living pollen grains is that their germination occurs away from the proximal pole, either distally or equatorially, or through one of a number of variously dispersed apertures. But this migration of the germinal aperture away from the proximal pole seems to have been a later and incidental development in pollen. As we have seen in the pteridosperms, its origin did not coincide with the development of the seed (and hence pollen status) as such. In any case, distal germination is not in itself an infallible criterion of a pollen grain. Some living bryophytes show distal germination of their isospores. As far as I know, no Devonian spores show clear evidence of distal germination (in the way that some Carboniferous forms such as Florinites do). But from what has been said, it is evident that this is at best a very limited piece of negative evidence. It simply indicates that on present knowledge, this relatively advanced type of pollen is not yet known in the Devonian. If pollen is actually represented among known Devonian small spores, then it must be in the state of proximally-germinating pre-pollen. Some workers have considered the presence of an air sac, and particularly one showing an infrareticulum, as a criterion for recognising pollen in contradistinction to spores. While it is true that among living spores and pollen, air sacs are characteristic of many conifer pollen grains, a number of fossil microspores and isospores also possess them. In the Carboniferous, the saccate spore genera Endosporites and Spencerisporites are known to be lycopod microspores and isospores respectively. The saccus of Endosporites has been regarded by some as having internal ornament, and by others external. In thin sections of the saccus, sculptural elements seem to be equally developed both internally and externally, but in nothing like the pronounced degree of some living conifer pollen. There is certainly no basis whatever for regarding the presence of a saccus as a unique attribute of pollen. Even if a pronounced infrareticulum is demonstrated for a Devonian spore, the case for its being a pollen grain is still only a rather tenuous argument by analogy with living conifers. The most plausible candidate in this category is the apparently monolete spore Archaeoperisaccus which NAUMOVA (1953) compared with gymnospermous pollen. But I think that an equally close match can be found with the Triassic monolete saccate spore Aratrisporites, which agrees closely with the microspores of the lycopod Lycostrobus. It might be added that there are many Devonian acritarchs known which have a membranous extension at the periphery which gives a superficially saccus-like appearance. This possible source of misinterpretation should perhaps be considered in weighing up the acceptability of Palaeozoic inaperturate and apparently saccate "pollen", especially if the assemblage contains other acritarchs. To sum up, the dispersed spores of the Devonian are consistent with a

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progressive differentiation o f spore m o r p h o l o g y t h r o u g h the course o f Late Silurian and D e v o n i a n time, coupled with the origin and d e v e l o p m e n t o f heterospory during the Middle and Late D e v o n i a n . Th e nearest a p p r o a c h to a seed is seen in a Cystosporites tetrad; there is n o t h i n g which can be regarded as convincing pollen before the Carboniferous, a l t h o u g h it must be accepted that the earliest pollen occurring dispersed w o u l d p r o b a b l y pass unrecognised as such.

REFERENCES ALLEN, K. C., 1965. Lower and Middle Devonian spores of north and central Vestspitsbergen. Palaeontology, 8: 687-748. AXELROD, D. I., 1959. Evolution of the psilophyte paleoflora. Evolution, 13: 264-275. BALME, B. E., 1962. Upper Devonian (Frasnian) spores from the Carnarvon Basin, Western Australia. Palaeobotanist, 9: 1-10. BALME,B. E. and HASSELL,C. W., 1962. Upper Devonian spores from the Canning Basin, Western Australia. Micropaleontology, 8: 1-28. CHALONER,W. G., 1959. Devonian megaspores from Arctic Canada. Palaeontotogy, 1: 321-332. CHALONER, W. G., 1964. An outline of Pre-Cambrian and Pre-Devonian microfossil records, evidence of early land plants from microfossils. Proc. Intern; Botan. Congr., l Oth, Edinburgh, 1964, pp. 16-17 (abstract). CHALONER,W. G. and PETTITT, J. M., 1964. A seed megaspore from the Devonian of Canada. Palaeontology, 7: 29-36. CHALONER,W. G. and STREEL,M., 1967. Lower Devonian spores from South Wales. Argumenta Palaeobotan., 1: 12-24 (in press). DOWNIE, C., 1963. "Hystrichospheres" (acritarchs) and spores of the Wenlock Shales (Silurian) of Wenlock, England. Palaeontology, 6: 625-652. FRANKE, F., 1965. Mikrofossilien eines Unterdevonischen Brandschieferprofils nahe Miinstereifel. Inaugural Dissertation, University of Berlin, Berlin, 86 pp. HOFFMEISTER,W. S., 1959. Lower Silurian plant spores from Libya. Micropaleontology, 5: 331-334. McGREGOR, D. C., 1960. Devonian spores from Melville Island, Canadian Archipelago. Palaeontology, 3: 26-44. MOREAU-BENO1T,A., 1966. l~tude des spores de D6vonien inferieur d'Avrille fie Fl6chay), Anjou. Rev. Micropal(ontol., 4:215-232. MORTIMER, M. G. and CHALONER,W. G. Devonian megaspores from the Wyboston borehole in Bedfordshire, England. Palaeontology, in press. NAUMOVA, S. N., 1953. Spore and pollen assemblages of the Upper Devonian of the Russian platform. Tr. Inst. Geol. Nauk, Akad. Nauk S.S.S.R., Geol. Set., 143(60): 1-204 (in Russian). PETT~TT, J. M., 1965. Two heterosporous plants from the Upper Devonian of North America. Bull. Brit. Museum, Geol., 10: 83-92. PII~RART, P., 1964. D6couverte de megaspores et miospores dans le Givetien de Roncqui6res (Brabant, Belgique). Bull. Soc. Beige GOol. Pal~ontol. Hydrol., 73: 82-100. PLAVFORt), G., 1962. Lower Carboniferous microfloras of Spitsbergen, 1. Palaeontology, 5: 550-618. PLAVFORD, G., t963. Lower Carboniferous microfloras of Spitsbergen, 2, Palaeontology, 5: 619-678. POTONI~, R. und KREMP, G., 1955. Die Sporae dispersae des Ruhrkarbons, ihre Morphographie und Stratigraphie, mit Ausblicken auf Arten anderer Gebiete und Zeitabschnitte, 1. Palaeontographica, B, 98: 1-136. POTONI~, R. und KREMP, G., 1956. Die Sporae dispersae des Ruhrkarbons, ihre Morphographie und Stratigraphie, mit Ausblicken auf Arten anderer Gebiete und Zeitabschnitte, 2. Palaeontographica, B, 99: 85-191.

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RICHARDSON, J. B., 1960. Spores from the Middle Old Red Sandstone of Cromarty, Scotland. Palaeontology, 3: 45-63. RICHARDSON, J. B., 1962. Spores with bifurcate processes from the Middle Old Red Sandstone of Scotland. Palaeontology, 5: 171-194. RICHARDSON, J. B., 1965. Middle Old Red Sandstone spore assemblages from the Orcadian Basin, northeast Scotland. Palaeontology, 7: 559-605. SCOTT, D. L. and ROUSE, G. E., 1961. Perforosporites, a new genus of plant spores from the Lower Devonian of eastern Canada. J. Paleontol., 35: 977-980. SEN, J., 1958. On the megaspores described by Nathorst from the Upper Devonian of Bear Island. Geol. F6ren. Stockholm Fdrh., 80: 141-148. STEWART, W. N., 1960. More about the origin of vascular plants. Plant Sci. Bull., 6(5): 1-5. STREEL, M., 1964. Une association de spores du Giv6tlen inf6rieur de la Vesdre, a Go6 (Belgique). Ann. Soc. G~ol. Belg., Bull., 87(7): 1-30. TAUGOURDEAU-LANTZ,J., 1960. Sur la microflore du Frasnien inf6rieure de Beaulieu (Boulonnais). Rev. Micropal~ontol., 3: 144-154. VIGRAN, J. O., 1964. Spores from Devonian deposits, Mimerdalen, Spitsbergen. Skrifter Norsk Polarinst., 132: 1-33. VOLKOVA, N. A., 1962. Precambrian spores from the Dniester region. Dokl. Akad. Nauk S.S.S.R., 142:893-895 (in Russian). WINSLOW, M. R., 1962. Plant spores and other microfossils from Upper Devonian and Lower Mississippian rocks of Ohio. U.S., Geol. Surv., Profess. Papers, 364: 1-93. ZERNDT, J., 1934. Les megaspores du bassin houiIler polonais. Bull. Acad. Polon. Sci., Trav. G~ol., 1: 1-56.

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