Journal of the Geological Society, London, Vol. 147, 1990, pp. 725-728, 7 figs. Printed in Northern Ireland

Further evidence for diversity in late Silurian land vegetation

U. FANNING',D. EDWARDS' & J. B. RICHARDSON* l Department of Geology, University of Wales College of Cardiff, Cardiff CFl3YE, UK 'Department of Palaeontology, British Museum (Natural History), London SW75BD, UK

Abtraet: Small coalified plant fragments from basal Pridoli (Silurian) strata at Perton near Hereford, England comprise isotomously branching smooth axes terminating in vertically elongate sporangia. The latter, whichoccasionally bifurcate, arecharacterized by prominent, distally concentrated, spinous emergences and contain trilete, retusoid, smooth-walled isospores. The plants are placed in a new genus and species, Caia langii. Comparisons are made with a number of Silurian and Lower Devonianplants with elongate sporangia and particularly with Horneophyton. A hypothesisis developed that spines on sporangia had a nutritive function. The spores are discussed in terms of other records of late Silurian-Early Devonian in situ spores.

It is not without irony that the very strata in Wales and the Systematic palaeobotany Welsh Borderland that Roderick Murchison insisted were devoid of plants have yielded the most extensive record to Incertae sedis date of early land vegetation. The exposure in the Downton Genus CAIA gen. nov. Group of PertonLane near Hereford was not visited by Murchison, but is now well known to palaeobotanists as the Type species. Caia langii sp. nov. type locality of the genus Cooksonia Lang (1937), the oldest known land plant of pteridophyte aspect. In the abundant Derivation of name. From caia, Latin for cudgel, referring but fragmentary plant and animal fossils that crowd certain to the shapeof the sporangia. bedding planes, the onlyhigher plant taxa originally recorded were Cooksonia pertoni and Hostinella sp. (Lang Diagnosis. Plant in which distal region comprises smooth 1937). This contrasts markedly with the diverse rhyniophyt- isotomously branchingaxes with terminalsometimes oid assemblage described more recently from approximately bifurcating sporangia.Sporangia consistently longer thm coeval strataat Freshwater East, Dyfed (Edwards 1979). wide, with parallel sides and obtuse tips. Homosporous; However, new collections from Perton Lane have yielded a spores trilete, laevigate and retusoid. number of plants of rhyniophyteaspect, including subspecies of C. pertoni (Fanning et al. 1988), a new genus Remarks. Sporangiain Rhynia,Aglaophyton, Sporogonites with spiny discoidal sporangia(Fanning et al. 1990), new and Salopella are fusiform and unbranched. The genus Caia species of Salopella Edwards & Richardson,and further may be closer to simply branching forms of Horneophyton, plants with differentkinds of vertically elongate terminal but fossils assigned to the new genus lack any anatomical sporangia, one of which is described here as a new genus. data diagnostic of the latter. Steganotheca has an abruptly truncated sporangium with an apical thickening. Locality data and material Caia langii sp. nov. The fossils occur near the base of the Rushall Beds, just Figures 1-7 above the local equivalent of the Ludlow BoneBed Derivation of name. After W. H. Lang. (Squirrel1 & Tucker 1960). Ostracodes and spores indicative of a Pridoli age are present at this level (Kaljo & Klaaman Diagnosis. As for genus. Unbranched sporangia 2-3 times 1982; Richardson et al. 1981), the spores belonging to the longer than wide, 1.38-3.13 mm long (f = 2.07, n = 10) and lower part of the tripapillatus-spicci;la spore assemblage 0.58-1.25 mm wide (f = 0.82, n = 10) with parallel sides and zone of Richardson & McGregor (1986). obtuse apices. Sporangia bear less than 10 conical Wehave recently described elsewhere thenature of emergences with rounded apices anddecurrent bases, fossil plant preservation and the techniques employed, when 0.18-0.30mm high (f = 0.24, n = lO),0.13-0.25 mm in reporting on a new genus from the samehorizon (Fanning et diameter at base (2 = 0.18, n = 10) and 0.05-0.12 mm wide al. 1990). The present description is based on 24 coalified when parallel-sided (f = 0.08, n = 8); emergences mostly compressions. However, in those sporangia with well- concentrated on distal one-third of sporangium.Branched preserved emergences the coalified material is granular and sporangia of overall height 0.75-2.75 mm (f = 1.72, n = 4) readily fragments, whereas when emergences are apparently and 0.63-2.25 wide beforebranching (f = 1.50, n = 4). notpresent or arerepresented by traces of coal tightly Lobe length abovedichotomy 0.30-1.13 mm (f = 0.95, adhering to the sediment, the body of the sporangium often n = 4) and lobe width 0.28-0.75 mm (f = 0.52, n = 4). Axes remains intact, and can be lifted away easily, revealing an 0.18-0.75 mm wide (f = 0.34, n = 14) when parallel-sided. orange stained matrix beneath. Isospores 23-32 pm in diameter (f = 26 pm, n = 20) with 725

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sporangiumfound measured 1.38 mm high and0.75mm wide, the largest 3.13 mm high and 1.25 mm wide. In thefour bifurcatingspecimens, the two sporangial lobes are parallel-sided, and their lengths are less than that of the unbranched sporangia. They have similar apices (Figs 3 & 4). Theirsubtending axes tapermarkedly before becoming parallel-sided, and thus it is difficult to determine the position of sporangium-axisthe junction on morphological criteria.However, the coalified material of the sporangium is usually thicker than that of the axis. Seventeen sporangia, including branched examples, are characterized by emergences,the majority of which are

incomplete (Figs 1, 2 & 3).They vary in the 3 number present,but extrapolation from a number of specimens indicates that there were probably few in the living plant. The maximum seen on anyone specimen is seven. The emergencesare mostclearly evident when preserved in profile on the sides of the sporangium. When complete such anemergence has a slightly asymmetrical, proximally attenuated, basal portionthat tapers into a parallel-sided region of approximately equal length. The apex is bluntly rounded.The emergences were probably circular in cross-section. Thedecurrent basesand a few intact specimens indicatethat theemergences were directed distally,except in one casewhere the long axisis perpendicular theto sporangium surface. Incomplete examplesare variable in shape ranging fromtriangular bases to sharply truncated outgrowths. Traces of emergences can sometimes be detected on the Fig. 1. Elongated, unbranched, terminal sporangiumof Cain lungii, surface coalifiedof compressions and as coal-filled with parallel sides, obtuse tip and distal concentrationof well depressions on the underlying matrix when the sporangium preserved sporangial emergences,NMW 89.38G.1; scale bar= 0.5 mm. Fig. 2. Isolated sporangium of C. lungii with sporangial emergences seen in profile and, where granular coalified material removed, penetrating the matrix (represented by coal-filled depressions), NMW 89.386.2; scale bar= 0.5 mm.

circular-irregular ambs. Trilete with narrow, low, even lips, c. 0.5 pm wide,confluent with narrowa curvatural ‘ridge’/fold of thesame width.Exine laevigate, less than 1pm thick, usually crumpledand showing taper-pointed folds.

Hofofype. NMW 89.38G.l.,Department of Geology, National Museum of Wales, Cardiff, Fig. 1.

Type horizon. RushallBeds, Downton Castle Sandstone Formation, Downton Group, Pridoli Series.

Type locality. PertonLane Quarry, Perton Village, Stoke Edith, near Hereford (SO 5971 4035).

Description. Of the 24 fertile specimens collected, 20 have elongateand unbranched sporangia with more or less parallel sides and obtuse tips (Figs 1 & 2). In 12 examples the subtending axis increased gradually in width below the sporangium so that the junction is difficult to recognize and height measurements are equivocal. They are more precise Fig. 3. Specimen of C. lungii in which the body of the bifurcating where the junction is marked by a constriction or there is an sporangium was removed intact and destroyedfor in situ spores; abrupt change in the thickness of coalified material. In two faintly preserved sporangial, emergences are seen in profile and cases, thesubtending axis is parallel-sided and slightly penetrating the matrix beneath,NMW 89.38G.3; scale bar= narrowerthan thesporangium. Six specimensare 0.5 mm. Fig. 4. Bifurcating sporangiumof C. lungii lacking represented by isolatedsporangia (Fig. 2). The smallest sporangial emergences,NMW 89.38G.4; scale bar is0.5 mm.

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Pi. 5. Reconstruction of the terminal part of C. lungii; scale bar is 0.5 mm. Pi. 6. Scanning electron micrographof in situ, trilete,laevigate, retusoid spore of C. langii NMW 89.386.5; scale bar is 7.5 pm. Fig. 7’. Scanning electron micrograph of in situ, folded spore of C. Lungii, with granular surface, NMW 89.386.6; scale bar is 8.5 pm.

is removed (Fig. 2). In unbranched sporangia, emergences Lang (1920) originally described the sporangia of this are usually concentrated in the distal third of the species as bifulcate or cylindrical structures with broad flat sporangium with rare examples towards the base. Limited apical ends, and considered such organization unique among evidence suggests that they are inserted in a spiral. A typical bryophytes and vascular plants. Should such three- arrangement is shown in the reconstruction (Fig. 5). dimensionally preservedpreservedplantsbe as Seven specimens (including a branching one) appear to compressions they would certainly be comparable in gross lack any emergences (Fig. 4). These are of the preservation morphology to the new plant described here, if not in size. type where the sporangium lifts off intact and emergences Further similarities include the lack of distinction between are very poorly preserved or missing. At present they are sporangium and subtending axis, sporangial lobes of varying included in the new species because they are similar in gross length (Eggert 1974), and the presence of distally tapered shape and possess similar spores. Further investigation may appendages on the sporangium wall (El-Saadawy & Lacey warrant the erection of a new species. 1979). The spores of H. lignieri are considerably larger than In all specimens, the subtendingaxes aresmooth and those of Caia and are often associated in tetrads. However, lack both distinguishing featuresand anatomicaldetail. both plants produced spores of similar structure. The spores Branching is isotomous. A maximum of two successive of Horneophyton have been compared with thosein the divisions was observed and these show that branching was dispersed spore genera Emphanisporites, Retusotriletes and not in one plane. The longest specimenfound measured Apiculireturispora (Bhutta 1973; Eggert 1974) although 5.9 mm. Gensel (1980), following a review of these accounts and the The spores removed from the sporangia have more or personalobservation of Kidston and Lang’s original less circular to irregular ambs and were originally elliptical material, concluded that the spores of Horneophyton were in cross-section. They are laevigate, thin-walled (exine less referable to Apiculiretusispora, an apiculate retusoid genus. than 1 pm thick) andare often folded. Theirtriradiate Since Eggert (1974) believed that branched sporangia were marks are frequentlyindistinct and have low, even lips typical of Horneophyton and questioned the existence of c. 0.5 pm wide, which are confluent with a narrow unbranched forms, a view supported by Bhutta (1973) and curvatural fold of thesame width. Theyrange from El-Saadawy & Lacey (1979), and as the majority of our 23-32 pm in diameter (Fig. 6). Occasionally, scanning specimens are unbranched and we have not observed electron micrographs show a granular spore surface, but this anatomical data, we conclude that our specimens should not may be a degradational feature (Fig. 7). Most of the spore be placed in Horneophyton. masses recovered from sporangia appear quite ‘clean’, but Recent investigations on Lower Devonian assemblages some are covered by a thinmicrogranulate ‘sac’. These from the Welsh Borderland reveal that branched sporangia retusoid sporesbelong to the dispersed spore genus are not uncommon. An example has been illustrated from Retusotriletes (Naumova) Richardson. Gedinnian specimens the Gedinnian age locality at Targrove (Edwards & Fanning of R. cf. triangulatus (Streel) in Richardson & Lister 1969 1985). In situ spores of the majority of such specimens unite have a more rigid wall than the in situ Silurian spores, and them with Salopella, a genuserected for unbranched show a darkened/thickened triangular area at the proximal fusiform sporangia (Edwards & Richardson 1974). Although pole Traces of such a proximal feature are occasionally seen the spores isolated from Caia may be similar to those in Caia spores. The Lower Devonian dispersed spores are derived from certain species of Salopella (see Edwards & sometimesenclosed by a loose,sculptured, possibly Richardson 1974); our plant is clearly distinguished from perisporal layer (Richardson & Lister 1969, pl. 37, fig. 5). this genus by the shape of the sporangiumapex and the presence of sporansial emergences. Wetherefore assign Afinities. The new plants are characterized by vertically them to a new genus. elongate, distally spiny, terminal sporangia that sometimes On morphological criteriathey conform to Banks’ bifurcate. The only published example of the latter feature (1975) concept of the Rhyniophytina. However, we have in an early land plant is in Horneophyton lignieri Barghoorn failed to show that they were vascular and SO describe them & Darrah from the Rhynie Chert of Scotland. Kidston & informally as rhyniophytoid.

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General discussion Finally, the wall of in situ spores of zosterophyllophytes and trimerophytes is often two layered although the nature of theouter layer is debatable(Gensel & White 1983). Sporangial spines Similar organization has not yet been recorded in Silurian in Sporangial emergences are rare in early land plants, and so situ examples, although the microgranulate sac-like it is quite striking that we have discovered them recently in structure reported here in Caia may be the remnant of a less two late Silurian rhyniophytoids fromthe same locality well developed/less precisely developed structure, covering (Fanning et al. 1990). The discussion we present (Fanning et the spore mass as a whole rather than individual spores. al. 1990) for sporangial spines in that new plant is equally pertinent here and we remain equally as uncertain of their Fanning gratefully acknowledges receipt of an NERC post-graduate adaptive significance. Inboth plantsthey were probably CASEstudentship. We thank L. Nortonfordrawing the quite rigid structures, and concentrated on the most distal reconstruction shown in Fig. 3. part of the sporangium. We postulate that they may have had a protective or nutritional role. We return to the latter possibility here.The smooth axial organization typifying earlyrhyniophytoids coupled with anapparent lack of References intercellular spaces in the sterome and infrequent stomata BANKS, H.P. 1975. Reclassification of Psilophyta. Taxon, 24, 401-13. (work in progress in Cardiff) are well documented structural BHUTTA,A.A. 1973. On the spores (includinggerminating spores) of features that may have limited photosynthetic productivity. Horneophyton(Hornea) lignieri (Kidstonand Lang) Barghoorn and In the Silurian rocks of the Anglo-Welsh region we have Darrah (1938). Pakistani Journal of Botany, 5, 45-55. EDWARDS,D. 1979. A late Silurian flora from the Lower Old Red Sandstone found only one specimen with axial emergences (Edwards of south-west Dyfed. Palaeontology, 22, 23-52. 1979), although these became more common in the - & FANNING,U. 1985. Evolution and environment in the late Devonian and may well haveincreased surface areafor Silurian-earlyDevonian: the rise of the pteridophytes. Philosophical photosynthesis. However,the terminalsporangia of Transactions of the Royal Society of London, B, 309, 147-165. -& RICHARDSON,J. B. 1974. Lower Devonian (Dittonian) plants from the rhyniophytoids would have been an energy-demanding sink Welsh Borderland. Palaeontology, 17, 311-324. somedistance fromthe mainphotosynthetic sources and -, FANNING,U. & RICHARDSON,J. B. 1990. In situ spores in early land would have necessitated the development of ametabolite plants from Wales and the Welsh Borderland: a review. Proceedings of transport system. Wehave no evidence forthe latter in the Geologists' Association (in press). rhyniophytoids,although the distinctive zonesurrounding EGGERT,D. A. 1974. The sporangium of Horneophyton lignieri (Rhyniophytina). American Journal of Botany, 61,405-413. the xylem in Rhynia gwynne-vaughanii and Aglaophyton EL-SAADAWY,W. EL-S. & LACEY,W. S. 1979. The sporangia of major suggests that such systems had evolved by Siegenian Horneophyton lignieri (Kidstonand Lang) Barghoorn and Darrah. times (see e.g. Satterthwait & Schopf 1972). The production Review of Palaeobotany and Palynology, 28, 137-144. of sporangia with spines that are an immediate, if putative, FANNING,U,, EDWARDS, & D. RICHARDSON, J. B. 1990. A new rhyniophytoid from the late Silurian of the Welsh Borderland. Neues lahrbuch fur increased source of metabolite would eliminate the Geologie und Palaontologie, Abhandlungen. (In press). problemsrelating to transport. The absence of such large -, RICHARDSON,.I. B. & EDWARDS,D. 1988. Cryptic evolution in an early emergences on sporangia of youngerplants suggest that land plant. Evolutionary trends in plants, 2, 13-24. these prototypes were not successful, perhaps because of GENSEL,P. G. 1980. Devonian in situ spores: a survey and discussion. Review energy demands relating totheir production, or totheir of Palaeobotany and Palynology, M, 101-32. -& WHITE,A. R. 1983. The morphology and ultrastructure of spres of interference with dispersal. The light-collecting problem was the EarlyDevonian trimerophyte Psilophyton (Dawson) Hueber and solved most efficiently by increased branching capacity and Banks. Palynology, 7, 221-33. eventually the evolution of leaves. KAUO,D. & KLAAMAN,E. (eds) 1982. Ecostratigraphy of the East Baltic Silurian. Academy of Science of the Estonian SSR, Institute of Geology. Spores Valgus, Tallinn, 109 pp. KIDSTON,R. & LANG,W. H. 1920. On Old Red Sandstone plantsshowing Laevigate retusoid spores made their first appearence in the structure, from the Rhynie Chert Bed, Aberdeenshire. Part 11. dispersed spore recordin thelate Wenlock/early Ludlow Additional Notes on Rhynia gwynne-uaughanii Kidston and Lang; with and became more common and widespread in the Siegenian descriptions of Rhynia major, nsp., and Hornea lignieri, n.g., n.sp. Transactions of the Royal Society of Edinburgh, 52, 603-27. andlate Devonian. We havedescribed above the earliest LANG,W. H. 1937. On the plant-remains from the Downtonian of England record of their occurrencein situ. Spores with similar and Wales. Philosophical Transactions of the Royal Society of London, morphology with smooth and sculptured exines have been B, 227, 245-91. extracted from morphologically diverseplants in thelate RICHARDSON,J. B. & LISTER,T. R. 1969. Upper Silurianand Lower Silurian and Early Devonian. They include rhyniophytoids Devonian spore assemblagesfrom the WelshBorderland and South Wales. Palaentology, 12, 201-52. (i.e. Salopella allenii, Cooksonia crassiparietilis, cf. C. -MCGREGOR, D. C. 1986. Silurian and Devonian spore zones of the Old caledonica, and Caia langii), rhyniophytes (i.e. R. Red Sandstone Continent and adjacent regions. Bulletin of the gwynne-vaughanii, H. lignieri and Renalia hueberi), and Geological Survey of Canada, 364, 1-79. zosterophyllophytes(i.e. Zosterophyllumllanoveranum, 2. -, RASUL,S. M. & AL-AMERI,T. 1981 . Acritarchs, miosporesand correlation of the Ludlovian-DowntonianandSilurian-Devonian cf.fertile, Z. yunnanianum, Discalis longistipa, Oricilla boundaries. Review of Palaeobotany and Palynology, 34, 209-24. bilinearis, Sawdonia ornata, S. acanthotheca and Rebuchia SAITERTHWAIT,D. F. & SCHOPF,J. W. 1972. Structurally preserved phloem ovata (Edwards et al. 1990)). Spores may thus provide zone tissue in Rhynia. American Journal of Botany, 59, 373-76. further evidence for derivation of zosterophyllophytes from STEWART,W. N. 1983. Paleobotany and the evolution of plants. Cambridge University Press, Cambridge. therhyniophytes/oids (e.g. Stewart 1983), but since Caia SOUIRRELL,C. H. & TUCKER,E. V. 1960. The geology of the Woolhope inlier has elongate sporangia it is unlikely to be directly related to (Herefordshire). Quarterly Journal of the Geological Society of London, the zosterophylls. 116, 139-85.

Received 22 September 1989: revised typescript accepted 1 November 1989.

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