THE TAXONOMIC SIGNIFICANCE OF THE TERM PSILOPSIDA AND THE EVOLUTIO ARY TRENDS OF THIS PLANT PHYLUM

F. NE:\1E]C Prague, Czechoslovakia

ABSTRACT Lower and Middle Devonian there are forms, The taxonomical significance of the term which by their very primitively developed Psilopsida is discussed in this article. As a special axis and leaves can be considered either as of plant phylum the Silurian-Devonian Psilopsida psilophytic nature or as some of the most pri• sensu stricto are regarded as an independent inter• mitive pteridophytic types as the ancestors mediary cryptogamic vascular type standing be• tween the Bryophyta and all true pteridophytic of the last ones. The boundary between the plant phyla and exhibiting its own peculiar evo• Psilopsida and the four named pteridophytic lutionary trends. The opinion is suggested that plant phyla seems, therefore, very vague and we have to investigate these special evolutionary indefinite. The respective chapters in vari• trends quite independently and irrespectively of the evolution of the single pteridophytic phyla. ous text books dealing with the Psilopsida Several of these trends are indicated. and with other very archaic Silurian-Devo• nian plants are the best proof of this state of our present taxonomical conceptions (see e.g. in the new edition of Potonie• THEprimitiveexistencevascularof stillplantsmore archaicthan theor Gothan's "Lehrbuch der PaHi.obotanik" by pteridophytic phyla Lycopsida, Pterop• H. Weyland, 1953). But I believe that sida, Sphenopsida and Psygmophyllopsida* there are many circumstances, which are was recognized about the year 1915 by R. already suggesting to us some quite different Kinston and W. H. Lang. As a special group, opinions or an utterly different attitude to which is most abundantly distributed in the this whole taxonomical problem. Silurian-Devonian beds, they were later called First of all we know at present not only Psilophitineae or Psilopsida and as plant spores of a type commonly spread in the pte• types not yet differentiated into the well• ridophyticvascularplantsalready up from the known fundamental morphological units, Cambrian, but we became newly acquainted viz. leaves, axis and roots, they caused also with the impressions of twigs of a typical considerable revolution in our morphologi• pteridophytic plant of the Middle Cambrian, cal conceptions as regards the nature of the i.e. Aldanophyton antiquissimumKryst., found bodies of all vascular plants and even in East Siberia, which reminds very strongly of the Bryophyta. Most of the botanists several Devonian primitive lycopodioid types, have regarded them, on account of their especially the well known Drepanophycus. very primitive morphology, as the direct We must, therefore, assume that rather archaic starting point of the evolution of all main or primitive pteridophytic types with more or types of true Pteridophyta, and lately also as less developed leaves must have existed much the starting point of the reducing processes earlier than ever expected before, and that which presumably led to the origin of the Bry• they existed perhaps in subordinate position ophyta. No doubt just for these reasons the along with the primitive undifferentiated vas• whole term of Psilopsida up to the present day cular types, the psilophytic types, already long is not yet as clearly defined from the taxo• ago before the Upper Silurian or Lower Devo• nomical point of view as are the four named nian period, where their origin mostly was as• pteridophytic plant phyla (LYCOPSIDA,PTE• sumed. That means, of course, that vascular ROPSIDA,SPHENOPSIDAand PSYGMOPHYLLOP•types, exhibiting bodies not yet well differen• SIDA). But in the very ancient floras of the tiated into axis, leaves and roots, must have been of still earlier origin, and that such *The phylum of Psygmophyllopsida was defined primitive undifferentiated (i.e. psilophytic) by me (originally under the name of Psygmo• types, which we know from the Lower and phyllineae ) in 1950 for vascular cryptogamic plants related morphogenetically most nearly with the Middle Devonian where already many repre• Sphenopsida but having non-articulated stems and sentatives of the pteridophytic phyla are to be spirally arranged leaves. found, must be regarded as mere survivors of 17 18 THE PALAEOBOTANIST

a rather large phylum, which had its period principles than as we are accustomed to do of maximum devel.opment already accom• in the consideration of the well known pteri• plished long ago before the Devonian period. dophytic phyla. We have to take into con• It is then also very probable that many pteri• sideration that the representatives of this dophytic types showing differentiated leaves, psilophytic phylum, which kept their primi• evolved from rather similar ancestors quite tive organization, had to attain the same parallelly along with other such primitive objectives as regards the adaptation for life types, which never succeeded in producing conditions under terrestrial environments by any typical leaf or root organs. Accordingly quite different arrangement or equipment true ancestors of both these types, i.e. psilo• of their bodies, than the morphologically phytic types as well as the pteridophytic ones, very much differentiated representatives of are at present perhaps quite unknown. the pteridophytic phyla, for surviving in the Hence it is clear enough that the Silurian-De• life competition for longer geological periods. vonian psilophytic or psilophytoid types can• We have thus to investigate within this phy• not be regarded as true ancestral types of any lum some special evolutionary trends, which recognized pteridophytic type. Most likely are in no relations with the elaboration of the same may be presumed also as regards the the true pteridophytic types. origin of the moss plants, because the oldest In this light many of the Silurian-Devonian ever discovered representatives of this evolu• psilophytic primitive plants appear indisput• tionary branch, the carboniferous liverworts ably as plant types, which underwent, on and mosses, exhibit specifically no strange account of special life conditions, a kind of features from either morphological or ana• evolution by reduction. It is especially in the tomical point of view, if compared with the genera like Thallomia and Sciadophyton where living forms. we have to observe an analogous tendency as Taking into consideration all these circum• in various thalloid liverworts, the formation stances, I presume that we have to pick out of of flat, creeping thalli, rather broad in Thll• the whole Silurian-Devonian plant assem• lomia with sporangia submerged into the blage a whole series of very primitive plants, thalloid tissue and narrow in Sciadophyton. which as a parallel group to the true Pteri• Special adaptation for life on wet and muddy dophytes as well as to the Bryophytes evolved coastal soils by means of reduction is here long before the Devonian time quite indepen• evident. Even such plants like Rhynia, Hick• dently and more or less simultaneously, per• lingia, Cooksonia which on the one hand exhi• haps even from the very starting point when bit some certain original archaic and primitive some alga-like plants began their invasion of features (e.g. sporangia terminal on most of the dry land, penetrating from their original their twigs, without being arranged in some acquatic environment. According to my special inflorescences), show on the other mind only such types of the Silurian-Devo• hand several features which point to special nian floras, i.e. intermediary in habit between adaptations for some peculiar life conditions, the mosses and true pteridophytes, are to be viz. only rounded cylindrical undifferentiated classified as true Psilopsida. All the other telomoid twigs without any indication of ones showing undoubted relations, be it from some special arrangement for making the the morphological oranatomical point of view, best use of the solar radiation, and fastened with the lycopodioid, sphenopsid or filicinean together into dense tufts. Such type of types, must be regarded as true representa• twigs or leaves are to be found at present tives, even though very primitive, of some of especially in several swamp or water plants, these pteridophytic types. Their eventual e.g. our common rushes - funcus. relations to the true psilophytic Silurian• Psilophytic plant types, which exhibit a Devonian types must really be regarded as more or less normal mesophytic appearance unusually remote. or at least not directly a swampy or aquatic The second logical consequence of these one, are no doubt represented by such types examinations is that in resolving various like Taeniocrada, Zosterophyllum, Gosslingia, taxonomical and phylogenetical problems Bucheria, Hedeia, Yarravia, Psilophyton and concerning the representatives of this most finally also by Pseudosporochnus and Astero• primitive and most archaic vascular plant xylon, of which the last two genera, compared phylum, we have to investig'!-te the morpho• with the other named types, are in many res• genetic evolution of their organs from a quite pects of a much more progressive character. different point of view and on quite different In the first named plant types we see a clear NEMEJC - THE TAXONOMIC SIGNIFICANCE OF THE TERM PSILOPSIDA 19

differentiation of branches into sterile synte• the morphological point of view are relative lomic systems with often flattened ultimate chiefly to the assimilating telomes (resp. syn• telomoid twigs, or with twigs preserved by a teloms), or to the sporangia bearing synte• dense cover of hairs which are often glandu• loms and in several rather rare cases also to lous, and into syntelomaid fertile systems the sporangia. The reduction features, as far with thin cylindrical and always naked twigs, as mentioned, are no doubt related to special bearing terminal sporangia. In several gen• adaptations to some unusual life condi• era these fertile systems IOClklike some mare tions. Most of the abave-mentioned pecu• or less panicle-like inflorescences, e.g. Taenio• liarities represent no doubt features indicating crada, Psilophyton; in other ones a clear ten• the trends of evolution in various directions dency to the formation of some spike-like or of this highly primitive vascular phylum. If corymboid fructifications by means of a very observed in detail, they very often remind of regular reduction or even abartion of the telo• the several features characterizing different moid stalks of the single sp::>rangiais evident, pteridophytic phyla and where we find them. e.g. spike-like in Zosterophyllum, Gosslingia always in various rather characteristic and and Bucheria, corymbaid in Hedeia. This in stable combinations, characteristic of each extreme cases, where the syntelomic main pteridophytic phylum. On the contrary, in axis has been extremely reduced or even abor• various psilophytic genera, families or orders, tive, led finally to such fructifications as found mostly only one such fundamental progres• in Yarravia : sparangia crowded round the sive feature was realized, without being top of a telomaid twig into a globular bad y; combined with some other ones. None of in Psilophyton pubescens Kr. et W., we find the hitherto recClgnizedtrue psilophytic plant finally, fork-like, once divided corymboid clus• types could therefore lead, in the evolutionary ters of very shortly stalked spClrangia, as in sense, to any of the representatives of the several sp2cies of the genera Aneurophylon higher pteridophytic phyla. Moreover, we or Protopteridium. In the genusPseudosporo• have alsa seen in several genera of the Psilop• chnus we see another special tendency: the sida organs exhibiting mass-like characters. formation of a big stem and the cruwding to• The Psilopsida thus appears as an interme• gether of the thin telomaid branchlets into diary plant phylum between the Bryophyta several large but still rather irregularly bran• and all the pteridophytic phyla. All features, ched frond systems. A still more progressive which the Psilopsida have in cornman either type is no doubt the genus Asteroxylon, which with the mosses or with the pteridJphytic otherwise is provided by equally primitive types, cannot be marphogenetically brought fructification and roat (mere primordial in direct connection with the same or similar rhizomes) systems as in Taeniocrada and allied features in these two phyla. They only genera. In Asteroxylon we see already small represent mere morphological convergences, leaflets reminding the short linear leaves of i.e. features gained quite independently, the club masses, with distinct stomata, which without any real kindred relatiol13 either evidently are not of telomoid provenance, to the moss plants or to any pteridophytic but representing mJst likely only assimilat• type. ing excrescences, not unlike the leaflets in Accordingly we have to divide our Psilop• the moss plants. A quite special type of sida phylum, as far as known from the Silu• the psilophytoid plants are H orneophyton rian and Devonian, into the following syste• and the imperfectly recognized genus Spo• matic groups: rogonites, which (at least Horneophyton) P.3ILOPHYTA exhibit the same primitive external appear• Phylum: Psilopsida ance as Rhynia and several allied types. 1. Class: Horneophytineae Here we may observe at the same time (i) Order: Horneophytales several features' common with the moss Horneophytaceae plants, especially the columella within their (H orneophyton sporangia, in Sporogonites, also the neck-like Bargh.) sterile portion at the base of the sporangia. Sporogoni taceae All such data undoubtedly indicate that the (Sporogonites Haile) group or phylum of Psilopsida, in the narrow 2. Class: Psilophytineae sense as recognized, exhibits besides some (i) Order: Thallomiales distinctly visible reduction features a large Thallomiaceae (Thallo• number of progressive characters, which from mia Heard etJones) 20 THE PALAEOBOTANIST

(ii) Order: Sciadophytales (vi) Order: Loganellales Sciadophytaceae (Scia• Loganellaceae dophyton Kr. et W.) (Loganella Stolley) (iii) Order: Rhyniales (vii) Order: Pseudosporochnales Rhyniaceae (Rhynia Pseudosporochnaceae Kidst. et Lang, Hick• (Pseudosporochnus lingia Kidst. et Lang Pot. et Bern) Cooksonia Lang. (viii) Order: Psilophytales (iv) Order: Taeniocradales Psilophytaceae (Psilo• Taeniocradaceae (Tae• phyton Dawson) niocrada Goepp., Hi• 3. Class: Asteroxylineae manthaliopsis Zal.) (i) Order: Asteroxylales Zosterophyllaceae (Gos• Asteroxyla-:eae (Aster- slingia Heard, Zos• oxylon Kidst. et terophyllum Lang, Lang) Bucheria Hoeg.) All these systematic groups must be re• Hedeiaceae (Hedeia garded from the point of view of the Cooks.) phylogeny as rather parallel to all, even the Yarraviaceae (Yarra• most primitive, pteridophytic types, e.g. via Lang et Cooks.) Aldanophyton, Drepanophycus, Hyenia, (v) Order: Scougophytales Barradeina, Swalbardia, Protopteridium). Ac• Scougoph ytaceae( Scou• cordingly, there exists absolutely no direct gophyton H. et G. relationship between these two kinds of Termier) plant types.

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