AN APPROACH TO THE PROBLEM OF AND CLASSIFICATION IN THE STUDY OF SPORAE DISPERSAE

D. C. BHARDWAJ Birbal~i Institute of Palaeobotany, Lucknow

ABSTRACT ( 1950 ) system, the American workers prefer The present state of disagreement among that of Schopf, Wilson and Bentall's ( 1944), workers with regard to the taxonomy and classi• in Germany, an amplification of Ibrahim's fication of Sporae dispersae has been reviewed. ( 1933) system is mostly in vogue and the The taxonomic categories for Sporae dispersae have Russian workers classify according to Nau• been defined and practicability of organ- concept over the form-genus concept for the circum• mova's (1937) system. This tendency to scription of spore taxa has been discussed. stick to one or the other of these systems, none of which excels the others, has uncons• ciously led to thwart the evolution of one such INTRODUCTION system of classification for the Sporae dis• persae which may be universally followed. Of late, in view of the greater applied use of DETAILEDand pollen,studyrecoveredof the dispersedfrom sedimen• Sporae dispersae in coal and oil prospecting, tary strata, dates back to Reinsch the need for a universally accepted standard ( 1881) when he described some of them from system of classification for these is being coals of the Carboniferous age. Thereafter, increasingly felt so that assimilation of data for several decades, these dispersed micro• from all over the world can be more easily remains attracted occasional attention of achieved. palaeobotanists and petrologists. However, it is only since the early thirties that the THE PROBLEM British, German, Russian, American and Indian palaeobotanists as well as geologists, All the systems of classification proposed realizing the importance of the Sporae dis• in the past, in spite of their diversities, tend persae in geological dating and stratigraphical to fall into two groups depending upon the correlation of sedimentary strata, set them• differences in their taxonomic treatment of selves to a serious study of these micro• spore taxa: remains. This increased research activity 1. Artificial ( morphologic) systems which has resulted into very significant and substan• consider the morphologic characters of tial contributions on the study of Sporae spores and pollen as the basis of specific, dispersae of various ages and from many parts generic and suprageneric grouping of the world. (BENNIE & KIDSTON,1886; IBRAHIM, Prompted by a desire for exactitude and 1933; NAUMOVA,1937; ERDTMAN,1947; clarity in description of the Sporae dispersae, KNOX,1950; PANT,1954and POTONIE& spore workers have always adopted some KREMP,1954). system of taxonomy and classification to 2. Natural (phylogenetic) systems which present their results. A number of these emphasize circumscription of spore systems are a result of original thought but genera on the analogy of higher taxa others are a sequel to synthesis of the then ( Orders-Classes) of the natural system prevalent older conceptions. Each of these of classification of ( SCHOPF,1938; efforts on systematics of Sporae dispersae has SCHOPF,WILSON& BENTALL,1944). its merits and also shortcomings. However, The real problem among palynologists in spite of the latter these efforts are highly today is to decide about the suitability of commendable inasmuch as they have been any of the two approaches between which able to single out the numerous difficulties most of the opinions tend to segregate. besetting the evolution of an ideal system. However, to choose between the two is not From a perusal of the current literature on so easy unless we discuss the usefulness of Sporae dispersae it becomes evident that the one over the other in the light of scientific as British spore workers mostly follow Knox's well as practical needs and the limitations 3 4 THE PALAEOBOTANIST

imposed by the nature of the material hand• Study of Sporae dispersae reveals these floral• led. breaks readily. It is for the detection of such ASPECTS IN THE STUDY OF SPORAE floral-breaks and for fine stratigraphic cor• DISPERSAE relation of coal seams as well as other strata, that this branch of palaeobotany is proving The study of Sporae dispersae has twofold to be of great practical value. applications, i.e. botanical and geological. From the studies of Mason (1936) and The botanical aspect concerns primarily, with others it is now well known that floral changes the taxonomy and phylogeny of spores and are caused by the evolution or migration of pollen and secondarily with ecology in so far such constituent species which, in view of as these plant microremains are representa• their specialized needs of the environmental tives of the flora of that area. The geologi• factors, behave as the borderline species in cal aspect of stratigraphical correlation and the population. Thus to detect the floral dating of strata depends upon the applied changes in a fossil flora it is imperative that use of the botanical knowledge so gained, the fossils of the borderline species should be especially that which concerns the changes distinguished from the other species. The in the distribution of Sporae dispersae or same holds good in the study of Spora; dis• in other words their parent floras, during persae and implies a taxonomic treatment geological time. which aims at distinguishing the spores be• Speaking of the botanical aspects of taxo• longing to each species and genus of the nomy and phylogeny, Sporae dispersae have parent flora. Hence it can now be concluded limited value in view of their dispersed nature, that the primary need in the scientific study because nothing definite can be stated about of Sporae dispersae is to define the taxonomic the identity of their parent plants. As a principles which may enable circumscription. matter of fact, whatever we know today about of the small, spore taxa as nearly equivalent the phylogeny of some of the dispersed spores to natural plant species and genera as possible. has been a contribution from botany or palaeobotany rather than vice versa and it is TAXONOMIC CATEGORIES FOR SPORAE doubtful if an independent study of Sporae DISPERSAE dispersae would ever contribute anything Spore-Species more tangible than probabilities. However, for ecological studies, Sporae dispersae have A scientific definition of a plant speCIes proved to be of real help especially in Tertiary could be that it is a group of morphologi• ecology where Pollen-analysis, in conjunction cally and physiologically similar individuals with the study of plant megafossils, has which are biologically isolated from those always enhanced accuracy of data and there• of the other closely related species. Among by its interpretation. This success of pollen Sporae dispersae, in view of their isolated studies of Tertiary strata is in a large mea• state, a fossil spore-species can be defined sure due to the possibility of correctly identi• only as a group of morphologically ( i.e. mor• fying the parent genera or even species of the phographically) similar individuals. The dispersed spores and pollen. other features such as physiological similarity The application of the study of Sporae dis• and biological isolation are beyond any persae to stratigraphical (geological) prob• means to establish in fossils. Thus, in effect, lems especially of the pre-Tertiary strata the only important basis of defining spore• depends on the detection of floral changes species is the morphography of spore fea• which have occurred in an area during the tures as these are actually evidences of course of time due to migration and evolution evolutionary morphology. of the species constituting its vegetation. Theoretically, a species cannot be strictly Floral changes are normally imperceptibly genetically homogeneous population because gradual, but occasionally they are accentuat• no individual can contain all the hereditary ed due to biotic or environmental factors and, factors which exist in allelic pairs and series. accompanied with migration of species from But we know that in nature, barring excep• and to the area, cause significant change in tions, mostly one set of either of the pair of the composition of succeeding floras, thus allelic characters tend to be represented in a interpolating floral-breaks in the vegetational species. The more usual characters must, history of the area. These floral-breaks are therefore, be accepted as the characters to used for delimiting stratigraphic horizons. define a spore-species. BHARDWAJ - TAXOXOJI,IY AND CLASSIFICATION OF SPORAE DISPERSAE 5

Variation within a Spore-species - The well. The size is a valuable variable in characters tend to vary, within a species some miospores of Lycopsida but ofless consequence varying more but others less. From my for their megaspores, of indeterminate utility work ( BHARDWAJ, 1952) on the spores of the in spores of Sphenopsida and of problematic living genus Anthoceros ( L.) Prosk., the work use in the taxonomy of gymnospermous of Knox ( 1950) on the spores of Lycopo• pollen grains. dium, Phytloglossum, Selaginetla and Isoetes, In other features, e.g. height and width Stokey & Atkinson (1956) on Osmunda• of emergences as well as that of rays, the ceae and Wagner ( 1952) on Diellia, I have range of quantitative variation in a species been able to infer that in Cryptogams the is within definable limits. For delimitation morphographic characters of mature spores, of species within a genus it is important within a natural species, vary quantitatively to work out the range of variation and but the range of variation is within definable the statistical mean of each variable, to limits. Likewise, as far as known to me, enable comparison with that of the other this generalization holds good in the case of comparable species. Unless as pore worker pollen grains of Phaneogams as well. has a rational conception of these varia• It is worth pointing out here that while tions, he is very likely to split or lump writing these generalized observations, I am together natural species. not oblivious to the many exceptions noted Stratigraphical and geographical separa• so far by many colleagues, such as the tion - An important consideration while occurrence of monolete as well as trilete delimiting species is the stratigraphical and spores in Onoclea sensiblis and others or the geographical separation. Usually plant spe• occurrence of abortive or giant spores in cies remain unchanged only for short dura• many natural fern-hybrids or the produc• tions in geological time-scale and so to anti• tion of large and small spores as in N otho• cipate a long vertical range for a species may laena afjinis. According to Wagner ( 1952 ) lead to taxonomic error. Likewise contem• some fern species, known to be apogamous, porary species recorded from two widely such as Pteris cretica and others often show separated or even contiguous regions, having several size classes in spores. Likewise a no common floral history, are more likely to number of similar, exceptional inconsisten• be different. In such cases a very critical cies in the phanerogamic pollen grains are morphographical study is necessary which also on record. In my opinion the more usual may reveal differences otherwise not dis• features should be considered as more impor• cernible. tant in the taxonomy of fossil spores and pol• len rather than the less common exceptions. Spore-Subgenus To ascertain the variation range for one of the variables such as the size, within The status of a subgenus in the syste• species of Sporae dispersae, I carried out a matics of living plants has very frequently large number ofabsolute size measurements of been ambiguous and for this reason this random specimens of several unquestionably taxon has rarely received unanimous ap• distinct species of cryptogamic miospores proval of taxonomists wheresoever attempts ranging in size from 15 to 120 fl. and some have been made to subdivide genera in this megaspores, from a coal seam in the Saar. way. It is for such reasons that there are This study not only revealed the range of size many cases on record where subgenera have variation within these species but also a ultimately been raised to the rank of genera notable fact that this range shows more or abolished to end a controversy. Likewise than proportionate increase as the size of in Sporae dispersae a taxonomic treatment, the spore increases, e.g. in spores of 20 fl. envisaging creation of subgenera, is equally size, the range of variation may be only vulnerable or rather more so because here a 25 per cent of absolute size, but in larger taxonomist has recourse only to the morpho• spores, e.g. of 100 fl. average size up to 40 per logical characters of spores. And an attempt cent and in megaspores, it may be 100 per to group together two sets of species, with cent or even more. These figures are only qualitatively different association of charac• to give an approximate idea about the pos• ters only because they agree in one charac• sible range of size variation within a species ter which might have appeared in both of and should neither be taken as absolute nor them due to parallel evolution, is bound to universally applicable for other variables as be hazardous. 6 THE PALAEOBOTA 1ST

Spore-Genus circumscribed round a type and comprise only species of similar kind, thereby more or less In recent years, with better understanding approximating to a grouping of closely related of the nature of plant species and the species as in a natural genus or a family. modes of speciation, our concept of the Form-genera, on the other hand, are broadly various supraspecific taxa, especially the defined including all forms, remotely related genus, has become more precise. In older or even unrelated and thus represent highly times the genus-concept, as traced by Bartlett artificial groupings. It is as well worth while ( 1940 ), consisted merely in a tendency to to remark here that form-genus concept is group together plants of distinguishable but older than the organ-genus concept as the similar kind under one name. This morpho• former owes its inception to the earlier graphical concept continued through several decades of palaeobotanical thought when centuries till Darwin's ideas of evolution limitation of the knowledge about the phy• added phylogenetic background to the mor• logeny of plants necessitated groupings of phography, emphasizing the morphological convenience, based on similarities in super• circumscription of a plant genus. Such an ficial characters. The organ-genera represent outlook on the genus-concept has not changed the modern trend where more and more effort in the post-Darwin time. A modern defini• is made to create generic groupings as nearly tion of the genus by Buxbaum ( 1951 ) says homogeneous and closely phylogenetic as that the genus is (in words of JUST, 1953), possible. "the sum total of species belonging to a As already discussed above, all natural phylogenetic unit recognized as such by the plant genera are morphologically circum• unity of its morphological type (generic scribed, their phyletic homogeneity being types)". With this definition it becomes accepted on the basis of the morphographic clear that an agreement in the important congruity of their constituent species. Like• morphological features between the type• wise in Sporae dispersae where the nature of species and other constituent species is an the parent plants is uncertain, the morpho• important criterion of the modern genus• logy of spores alone is the means to a phyletic concept and that is what Greenman ( 1940 ) grouping. The concensus of opinion by paly• also said that "the species of a genus nologists tends to recognize at least as much must conform in all essential morphological non-plasticity of the morphologic characters characters to those of the type species of in spores as among other conservative organs the genus". Such a genus, whose constituent of plant sporophytes. Therefore, it can be species cohere round a type by the identity readily surmised that among Sporae dispersae of their morphological characters, represents morphologic circumscription of a genus, in one evolutionary tendency and is a phylo• faithful conformity to the type-concept, will genetically homogeneous, natural genus. In automatically lead to more or less natural this connection it appears worthwhile to genera which in terms of palaeobotanical point out that in spite of such a clear under• taxonomy can be calleclorgan-genera. These standing of the genus-concept there are spore organ-genera in exceptional cases may many genera in the plant kingdom, especially represent spores of a number of closely related among Lower Cryptogams which are still natural genera instead of one, as has also not natural genera (sensu strictu) but are been noted among living plants where some• composite, being groupings of convenience times spores of a sub-family or even a family and any analogy from these while circum• are of the same type. Likewise instances are scribing spore genera, should preferably be also not unexpected when a morphologic avoided. Apparently these composite taxa spore genus may represent only a section of are only awaiting detailed morphological and some natural plant genus whose spores cytological study before they can be justly might have varied from the rest but the other divided into natural genera. gross morphology of the sporophyte would In palaeobotanical taxonomy, in view of have continued to be like the rest. Irrespec• the fragmentary and difficult nature of mate• tive of this, for our practical purposes such rial available for study, delimitation of spore organ-genera are the ultimate ideal as genera has been more lax and two kinds of these are the only possible phylogenetically genera, i.e. organ-genera ( organo-genera ) and homogeneous groupings that can be achieved form-genera (forma-genera) are officially re• on the basis of factual study of Sporae cognized. The former are morphologically dispersae. BHARDWAJ - TAXONOMY AND CLASSIFICATION OF SPORAE DISPERSAE 7

Form-genus vs. Organ-genus in the Taxo• as such a genotype can represent only one nomy of Sporae dispersae - If we compare evolutionary tendency and not all. It is the history of systematics of Sporae dis• for such an anachronism that I have never persae to that of palaeobotany or botany been able to understand how form-genus in general, we find close correspondence in concept and the type concept can be faith• the way form-genus concept has gradually fully co-ordinated. For the same reason in led to organ-genus concept. The history the genus Triletes (Reinsch) Schopf, the of systematics of Sporae dispersae dates large number of constituent species are not back to Reinsch (1881) when he created similar to its genotype T. Reinschi (Ibr.) the genus Triletes to include all rounded Schopf. Actually in the system of Schopf or triangular microscopic remains having et al., the main emphasis in defining the spore a triradiate structure on one of their faces. genera is not on the type concept but on Subsequently H. Potonie (1893) createu gross phylogeny. that its constituent species Sporites to include all dispersed pterido• belong to one plant group each. phytic plant spores. Likewise Seward ( 1914) Now the question arises as to which instituted Pityosporites for the coniferous concept, i.e. of the form-genera or the organ• pollen of the Abietineae-type. As these genera, suits us better for our needs. It years constituted the beginning of the study has already been made clear in the preceding of dispersed plant spores, lack of adequate pages that the study of Sporae dispersae has knowledge about them necessitated broad two applied aspects, i.e. botanical ( floristic) groupings of convenience and these taxa and geological (stratigraphic). Consider• were thus of the nature of form-genera. In ing the botanical aspect we can well imagine 1933, Ibrahim introduced type method of that form-genera, in view of their artificial circumscribing spore genera and created a nature and composition, are likely to over• number of morphographic genera. Sub• emphasize similarities between floras of sequently Naumova (loc. cit.) , Erdtman various regions even if their virtual con• (loc. cit.), Knox ( loc. cit.) and Pant (loc. gruity is only remote. Organ-genera, on cit.) as well as numerous others have also the other hand, being homogeneous and created morphographic genera although they natural, can depict the real floral affinities, did not take cognizance of the type method. neither over-emphasizing nor underrating Potonie and Kremp (loc. cit.) have strictly similarities or dissimilarities. If the flora adhered to type method and have circum• of two regions shows similar association of scribed narrow morphographic genera whose organ-genera, it is presumable without any constituent species mostly agree to the type. doubt that the two are related and if other• Schopf (1938), Schopf, Wilson and Ben• wise, then unrelated. Among related floras tall (1944), Dijkstra (1946) and some how close the two may be is finally deter• others have prefered to maintain Triletes mined by the agreement or non-agreement among the older form-genera after restricting among the species. it as the genus to include all triradiate For stratigraphical purposes form-genera megaspores presumably of lycopod origin can rarely be of use as index-fossils because and assigning a type to this emended genus. they have enormous vertical range usually Apart from this Schopf et al. have redefined extending through several geological periods. a number of other validly described genera As their horizontal distribution tends to and created a few new ones, all having the become artificially wide, they are equally status of form-genera yet with generic types. useless for inter-regional correlations. Organ• The purpose of the type concept in circum• genera are the only supraspecific category scribing a genus, as far as understandable which being phyletic can correctly depict to me, is to enable grouping of such cons• the evolutionary changes in the floras and, tituent species which purport to be links of being smaller units, are better able to indicate a single evolutionary chain and they are the ecological changes in the floras. Spore supposed to show same association of organ-genera are thus of real importance for qualitative characters. In a form-genus practical purposes. In another of my papers which represents one or more plant classes, ( in this volume) dealing with the systema• it is apparent that a large number of evolu• tics and stratigraphy of the Saar Coal tionary tendencies may be included so that Measures, the successful applied use of spore to designate anyone form as the type, re• organ-genera for stratigraphical horizoning presenting the whole genus, is not possible is well established. 8 THE PAL.1\EOBOTANIST

It can be argued by those who want to easily observable morphographic character• retain form-genera in the systematics of istics. The need is, however, to interpret Sporae dispersae that if the form-genera are these characters on grounds of comparative not suitable for stratigraphical deductions, morphology and thereafter to circumscribe their species may well serve the need. higher categories on the basis of agreement in Natural spore-species usually have very phylogenetically more important characters. short vertical and horizontal range and are In this way the spore organ-genera, small as of real importance only for correlation of they are, can be grouped together into series coal seams or other strata within a basin. (equivalent to Families), the latter into Usually they are neither capable of defining subdivisions, divisions and so on, on the basis stratigraphic horizons within the epochs of phyletic nearness of the various types of nor are of use for inter-regional correlations. morphographic characters, whose elucidation However, it is another question if one is possible from a comparative study of employs composite species which, being Sporae dispersae and in situ spores from fossil equivalent to organ-genera, will serve very and living spore-bearing organs. Such ad• well to define stratigraphical horizons, but vanced studies are necessary to make the in such cases the absence of natural spore• suprageneric groupings phyletic thereby lead• species will undermine successful correlation ing to a phylogenetic system of classification of coal seams and other strata. for Sporae dispersae. It should be apparent from this discussion that natural spore-species and spore organ• CONCLUDING REMARKS genera are indispensable for the successful academic as well as applied study of Sporae Opinions can hardly be divided if I were dispersae, and that form-genus concept has to say that the ideal system of classification outlived its utility for the present needs of for Sporae dispel'sae can be that alone which palynologists and should be dispensed with. is most phylogenetic. To achieve such an As such, it is necessary to redefine the existing objective an easier course is to trace the spore genera, as far as possible, into narrowly phylogeny from species upwards through circumscribed, morphological organ-genera. morphologically circumscribed species and genera that can be grouped into suprageneric SUPRAGE ERIC CATEGORIES categories equivalent to families, orders, etc., based on facts of comparative morpho• Systematics of Sporae dispersae does not logy. Such a taxonomic procerlure will end merely after diagnosing species and eventually lead to a classification, more or genera but also raises the question of less corresponding to the natural system of arranging the genera into a system of supra• classification used for plants in general. No generic taxa as in a system of classifica• doubt, such a system will stand parallel to, tion. rather than be merged with, the natural So far most of the authors of morphological system of plants, still this will be the closest systems have suggested suprageneric cate• possible approach to it in view of the dif• gories on the basis of similarities in some ficult nature of Sporae dispersae.

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