EVIDENCE FOR POLYMORPHISM OF PALAEO FLORA FROM JURASSIC BEDS IN THE TABBOWA BASIN SRI LANKA

Sedimentary rocks of Jurassic age are found in few small, faulted, basins in the crystalline rock terrain of Sri Lanka, in the northwestern part of the country. Presence of plant fossils in some of the sedimentary rocks in those basins are well known and therefore, the present study was carried out mainly in one of these sedimentary basins; Tabbowa, focusing identification of the morphology of macrofossils in view of determining their systematic affinity to palaeo-environment. The observed characteristic features based on the characteristic venation distribution, pinnule margin and shapes of pinnules, were compared with those of local and international type specimens and with published literature, pertaining to various palaeo-climates. The fossil record of Cladophlebis sp. belonging to the family Osmundaceae are known as one of the oldest plant group since Permian period and as representing high diversity in the Early Jurassic. Detailed observations indicated that a many of the Cladophlebis spp. are assigned to different species belong to Jurassic: C. zeylanica Sitholey 1944, C. indica (Oldham & Morris) Sahni and Rao 1933, C. australis (Morris) Seward 1904 and C. antarctica Halle 1913 even in different countries. However, varying formats of description, varying terminology and the lack of systematic uniformity have often hampered identification of those species correctly. The present study and its identification of morphological and, (though limited) taxonomical features of the studied samples suggest that all species are closely related and possibly originated from one classic species, Cladophlebis denticulata Brongniart (1828) Fontaine 1889, known as a polymorphisms or intra-specific variability of its evolutionary process. In addition, this paper provides the morphological characters vis a vis taxonomic assignment and palaeoecological significance of the genus Cladophlebis.


INTRODUCTION
About 10% of the landmass of Sri Lanka consists of sedimentary rocks and at few localities, belonging to Jurassic age. Found in some faulted, basins in the Northwestern part of the country. Tabbowa basin is the most known among those. Presence of plant fossils, that are present in some of the rocks in Tabbowa basin are less studied though some workers have studied geology, mineralogy and geochemistry (Kehelpannala, 1997;Cooray, 1984;Pitawala et al., 2003;Ratnayake and Sampei, 2015;Chang et al., 2017) at several times. Of the studies on Palaeobotany and Palynology, Seward and Holttum (1922), Wayland, (1925), Deraniyagala (1939) and Sitholey (1944) have made significant descriptions on the basic knowledge of fossil plants within the basin. However, since1944, there had been no recorded systematic investigations on Palaeobotany or Palynology of the Tabbowa basin with a few exceptions by Edirisooriya and Dharmagunawardhane in 2013, 2014, 2016 In view of the above, the present work is an integrated approach to study the plant fossils in Tabbowa sedimentary basin of Sri Lanka and to investigate the mode of deposition, their features in relation to intra-specific variability in view of reconstructing the environments and climate during the period when these flora were prevailing.
During Early Jurassic the Gondwana beds were dominated by ferns represented by four families as Osmundaceae, Matoniaceae, Dipteridaceae and Dicksoniaceae . One of the well-known species, Cladophlebis Brongniart (1828) of the family Osmundaceae is common in Gondwana beds of Sri Lanka, India, Australia and Antarctica (Taylor et al., 1983).
The family Osmundaceae is most primitive among the ferns of the Filicales, dating back to Late Paleozoic in the geological history (Bomfleur et al, 2017).
Foliage of the fossil Osmundaceae, however, is not well known. The absence of fertile structures attached to foliage particularly in ferns makes it difficult to separate taxonomically. Therefore, many fossil leaves of Osmundaceae have been placed by the investigators in Cladophlebis; a form genus of sterile Osmunda-like foliage (Brongniart, 1849). However, Osmundaceous foliage did not morphologically change in the Jurassic period but their fertile fronds have changed mostly in pre-Jurassic period (Harris, 1969). This family is taxonomically important because morphological investigation of these fossil plants indicated more than one form of individual species in a single species or in the single population, which is known as polymorphism. Evidence for the polymorphism of Cladophlebis was found in the Tabbowa plant fossils for the first time, in this study. This paper reports the evidences found, discusses the environment of deposition and the fossil records bridging the gaps of fossil evidence from Sri Lanka in support of its position in the Gondwana. However, the diversity and distribution pattern of this family is yet to be investigated.

GEOLOGICAL BACKGROUND OF THE AREA
Around 90% of the bedrock in Sri Lanka consists of Precambrian crystalline rocks. Jurassic, Miocene and Quaternary strata are largely confined to a thin belt of exposures in the north and northwest of the island (Herath, 1986; Figure 1a). The fossiliferous strata sampled in this study are exposed close to the spillway of an irrigation reservoirthe Tabbowa Tank (8° 04ʹ 21ʺ N, 79° 56ʹ E) -within the Tabbowa Basin ( Figure 1b). The Tabbowa Basin is a small graben previously considered to host only Jurassic strata. The basin extends over an area of only a few square kilometers in the Northwestern Province of Sri Lanka (Wayland, 1925;Cooray, 1984). The graben developed in a crystalline basement terrain and hosts nearly 1000 m of strata consisting mostly of grey to reddish shales, mudstones, siltstones and arkosic sandstones (Cooray, 1984).  Wayland, 1925;Cooray, 1984;Cooray and Vitanage, 1991).

MATERIALS AND PROCEDURES
Some of the fern fossil specimens of genus Cladophlebis were collected from the Early Jurassic deposits in the Tabbowa basin ( Figure 1). The specimens were preserved in Mudstone. The frond fragments were preserved as impressions and small amounts of organic material were occasionally observed but cuticles were not observed. The morphological features of most of the preserved specimens were well defined. Samples were observed under low power microscope under incidence light and the specimens were photographed using standard macrophotographic techniques with low angle illumination. Fossil specimens were identified based on morphological features and specific comparisons were made with the material stored in the Natural History Museum of London, Victoria National Museum, Melbourne and the specimens recorded from Australia, India and Antarctica.
Morphological terminology used by Seward (1900), Sitholey (1944), Douglas (1969) were followed and classifications and amendments as proposed by Harris (1961) were used in the present study. Wherever complete leaves were preserved the length and the width, thickness of rachis, were measured and the length /width relationship was calculated. All illustrated Sri Lankan specimens were lodged in the Herbarium, National Botanical Department, Royal Botanical Garden, Peradeniya, Sri Lanka (PDAtype) and at Museum, Department of Geology, University of Peradeniya (GMU-holo type), for future reference. Specimens were numbered as -Tabbowa Mudstone Collection-  Description: Sterile pinna fragments with rather stout pinna axis up to 1-1.5 mm wide. Length of the rachis 7 mm and up to 1.0 -1.5 mm wide. Rachis is distinctly thick, smooth, and rounded, bears alternate pinnules or subalternate. Frond shape is lanceolate. Pinnules are typically alternate, or subopposite (A-D), katadromic branching arrangement arising on basiscopic side of the rachis at an angle of 40 0 -50 0 . Pinnules straight to falcate or deltoid in shape, tapering slightly towards the apex. Pinnule margins distinctly entire but in some cases can be serrate (C). Pinnule apices are generally acute to sub-acute or blunt but occasionally obtuse. Pinnules are inserted laterally to the rachis by entire width of the base. Pinnule lamina contiguous near base while rest of the pinnule is separated. Acroscopic base of pinnule usually expanded. Basiscopic base of pinnule is slightly decurrent. Pinnule length varying from 10 mm to 11 mm, basal width 1 mm to 3.0 -5.0 mm. Pinnule length to basal width ratio is 3:1 or less than 3.0.
Midrib is prominent extending up to apex and strongly attached to rachis lateral side or upper-surface. Primary veins are straight arise on rachis at an angle to 60 0 -75 0 . Secondary veins are arising an angle of approximately 30 0 -48 0 to midrib. Secondary veins in apical region of pinnule remaining simple, unforked. Vein density varies from 8-9 pairs on one half of the pinnule.

MORPHOTAXON:
Cladophlebis australis (Morris) Seward 1904 (Pl.16,   Length of the frond variable, incomplete and actual size unknown. Fronds are sterile (A, B, C & D). Rachis is distinctly thick, smooth, and rounded, bears alternate pinnules though distal most apical pinnules bear sub-opposite (A). Frond shape is conical-lanceolate, apex long pointed rounded. Pinnules are inserted lateral to the rachis by entire width of their basiscopic base (A-D). Pinnule margins indented, rarely overlapping. Acroscopic base of pinnule usually expanded. Basiscopic base of pinnule is slightly decurrent, making asymmetrical base. Pinnule length varying from 3 mm to 15 mm, basal width 3 mm to 5 mm.
Midrib is prominent extending up to apex and strongly attached to rachis lower-surface. Primary veins are straight arise on rachis at an angle to 45 0 . Secondary veins usually branching near midrib making small angle and in apical region of pinnule remaining simple, unforked. Vein density varies from 10 to 12 per 10 mm (A-D) on one half of the pinnule. Description: Sterile frond fragment, compound and once pinnate. Length of the incomplete frond is up to 6 mm and the width is about 2.5 mm. Length of the rachis 6.5 mm and up to 1.0 -1.5 mm wide. Rachis is thick, smooth, and prominent, bears alternate pinnules though distal most apical pinnules bear sub-opposite. Frond shape is lanceolate.
Pinnules are typically alternate at the basal region and sub-opposite at the apical region. Pinnules are katadromic arising on basiscopic side of the rachis at an angle of 50 0 -70 0 . Pinnules falcate or cuneate in shape, tapering slightly towards the apex. Pinnule margins distinctly serrate / denticulate but in some cases can be entire in the upper part. Margins near parallel base to acute apex. Pinnule apices are generally acute to sub-acute. Pinnules are inserted laterally to the rachis and typically connected by lamina. Acroscopic base of pinnule usually expanded. Basiscopic base of pinnule is slightly decurrent or contracted therefore pinnule base is asymmetrical. Pinnule length varying from 8.0 mm to 12 mm, basal width 0.8 mm to 1.0 mm. Pinnule length to basal width ratio is 2.9-4.0 Midrib is prominent extending up to apex and strongly attached to rachis lower-surface, slightly sunken on upper surface. Primary veins are straight arise on rachis at an angle to 40 0 -46 0 in central region of pinnule. Secondary veins typically forking once. Secondary veins in apical region of pinnule remaining simple, un-forked (distal most two or three pairs of veins). Vein density varies from 10 to 12 on one half of the pinnule (5-8 pairs). Cladophlebis denticulata Brongniart; Kimura,sp. 13, pl. 3, fig. 4; pl. 4, figs 3,6, pl.5, fig.1, pl. 12, fig. 10 fig. 4, pl. 7, figs 1, 2, text- fig. 3D.

MATERIAL EXAMINED: Specimen No.
A-TB Cl 1, B-TB Cl 2 , TBCl 1 as follows : Figure 6, A-B All are stored in a Museum, Department of Geology . Description: A well preserved bipinnate, fragmented sterile pinnae. 21.3 mm long, contains nearly 10 complete pinules. The rachis up to 1.5 -2.5 mm wide and smooth.
Pinnules are arise at 45 0 -65 0 angle on pinna rachis. They are typically alternate (A & B) or sub-alternate. Also elongated pinnules are triangular shaped with acute or sub-acute apex, and relatively wide bases, or rarely rectangular pinnules with rounded apex. The adjacent pinnule bases are in contact with each other. Pinnule bases expanded acroscopically and slightly contracted basiscopically. Therefore pinnule base is being asymmetrical.
Margins are serrate or entire. Margins are near-parallel distance from base apex then converging to acute apex. Pinnules are cuneate or falcate can be seen at the middle of pinnae and straight towards the pinnae apex. Width of the largest pinnule at the base is about 6.0 mm to 3.0 -4.0 mm. Their length ranges from 6 to 17.5 mm and the basal width ranges from 2.8 to 6.5 mm. Ratio of pinnule length to basal width 2.1 to 2.7 (typically 1.5 to < 3 .
Venation is distinct, pecopterid type, the midrib straight, persisting nearly to apex of the pinnule; Primary veins straight and reaching to the margin. Then secondary veins arise 30 0 -35 0 angle at one third of distance from primary veins. Secondary veins are forking once symmetrically, keeping 0.3-0.4 mm interval between secondary veins.
Lowermost secondary vein arising on basiscopic side of pinnule, typically forking twice.
The number of secondary veins (twin) varies from 9 to 10 on one half of the pinnule, most specimens have 10-12 veins, and vein density at the pinnule margin is 5-9 /cm, (typically 5-6/cm).
In particular, morphological data showing comparative features of different species indicates that Cladophlebis denticulata in all probability is an ancestral taxon of the four species, C. zeylanica, C. indica, C.australis and C.antactica.
All specimens are recognized as Cladophlebis by its shape, relatively narrow and elongated pinnules with dentate, serrate, or entire margins, comparatively thick main rachis, Cladophlebis venation pattern, secondary veins once forked, Katadromic pinnule arrangement. Pinnule length to basal width ratio between 5-3.

COMPARISON AND DISCUSSION
Present study describes Cladophlebis denticulata (Brongniart 1828) Fontaine 1889 from the Tabbowa basin, based on fossil plant specimen, noted that there were a several taxa of Cladophlebis characterized by, falcated shape and slightly dentate or serrate pinnule margins and venation distribution such as Cladophlebis C. indica Oldham & Morris) Sahni and Rao 1933, C. antarctica Halle, 1913, C. zeylanica Sitholey, 1944 australis (Morris) Seward 1904 (Table  1 and Figure 7).
Cladophlebis denticulata (Brongniart, 1828) Fontaine 1889 is a very common species in the Early Jurassic and well distinguishable due to its elongated pinnules with dentate margins, acute or sub-acute apices, and rachis ridges. From super-gondwana cluster countries including Sri Lanka, all specimens were found in a large number, enough to show the variability within this species. The most variable characteristics are the shape, size of pinnules, ratio of pinnules and the pinnule arising angle. The genus Cladophlebis contains numerous species distinguished on morphological differences.
Cladophlebis denticulata var. C. zeylanica and C. indica has the smallest pinnules of all species. It corresponds to the dentate/serrate forms of C. denticulata from zeylanica and indica (Figure 2-B and Figure 3-C) in its rounded apices, sub-alternate or sub-opposite arrangement of the pinnules, serrate pinnule margins, and number of bifurcations of secondary veins. Other morphological features slightly differ in these taxa, such as pinnule length /width and number of secondary veins at the pinnule margin The difference between Cladophlebis antarctica and C. denticulata is very slight. The material from Antarctica has slightly lower density of secondary veins (5-8) and a larger angle of arising of secondary veins (usually 50°-80°), whereas the way of bifurcation of secondary veins, length/ width ratio, and the margins, apex, and base of pinnules correspond in the two taxa. Although C. australis bifurcation of secondary veins once only at the most basal veins. Halle (1913) noticed very close similarity between Cladophlebis denticulata (Brongniart) Fontaine 1889 and C. Antarctica Halle. Therefore, he left these species separate, but  suggested that C. antarctica Halle and C. denticulata (Brongniart, 1828) Fontaine 1889 were not different species, they suggested a fusion of species with emended diagnosis. However, the emended diagnosis given earlier by Harris (1961) seems to be sufficient. The differences between C.denticulata (Brongniart) Fontaine 1889 compared with C. antarctica Halle is presented in Table 1 fit into the range given by Harris (1961). The number (or density) of secondary veins and their bifurcations in the examined material also show intermediate forms.    However, high morphological variability of this C. denticulata and suggested that Cladophebis zeylanica might be a "naked" form of C. denticulata or small form of C. indica. The differences between them details of the pinnule margins and the secondary veins (Table 1 and Figure 7); other morphological features of all these taxa resembled, such as the length /width ratio, type of bifurcation, and angle of arising of secondary veins. The specimens described by Figure 2, 3 and 4 are similar to range of features observed in C. denticulata. The entire margin in C. denticulata corresponds to C. zeylanica ( Figure 2, A, C), C. indica ( Figure 3A, B), and C.australis ( Figure 4A), though the entire and serrate margins both occur even within the same pinnule. Some of the difference between alternate pinnae and the opposite pinnae, value of length and the width, ratio values, apex types, these features do not consider as a truly taxonomical features as they highly variable in taxa according to the position of pinnules, age of the taxon and conditions can be changed by the preservation . Halle (1913) described C. antarctica including to the "group denticulata", suggested that "it is better to separate distinct smaller species than to keep one large collective species" . However, in this reason there were numerous form of species with very similar, sometimes almost the same morphology. This effects as a problem with the identification of a new taxon, which may show slight differences, thus again justify creating as a new species. For the above four species also the same situation has been faced. In such similar forms as mentioned above should be considered as one species with differences that are the result of natural intra-species polymorphism of the genus Cladophlebis.
It is worth noticing that all discussed forms usually occur in the same localities with C. denticulata (Brongniart, 1828) Fontaine 1889 and their easily identifiable morphology may be the result of environmental circumstances. Present study suggests that species of Cladophlebis distributed in Sri Lanka, India, Australia and Antarctica e. g. C. zeylanica Sitholey, 1944 (Sri Lanka), C. indica (Oldham and Morris) Sahni and Rao 1933 (India), C. australis (Morris) Seward 1904 (Australia) and C. antarctica Halle, 1913 (Antarctica) are closely related with the species, C. denticulata Brongniart (1828) Fontaine 1889 although they have their own specific characters. Different features are examined in relation with intra-specific variability commonly observed in ferns. We have concluded that many characteristics which are used for species determination of this genus are inter related with classic species Cladophlebis denticulata distributed in Early Jurassic beds of Sri Lanka, India, Antarctica and Australia. The significance of these plant fossils have also been discussed in relation to palaeoecological and palaeoenvironmental conditions during Early Jurassic.

PALAEOECOLOGYAND PALAEO-ENVIRONMENTAL SIGNIFICANCE
The state of preservation of macroremains, collectively proves that most of them are autochthonous in origin and the conditions were suitable for preservation of plant fossils. The dominance of large undamaged fragments, fronds preserved in branching state, relatively large fragments of leaves, occurrence of underground rhizomes with thin roots in growing positions demonstrate in situ preservation of plant fossils. It is widely believed that osmundaceous ferns preferred usually wet habitats in the vicinity of riverbanks, lake banks, and marshes or understorey of forests growing in swampy areas (Gee, 1989;Barbacka and Bodor 2008). These sites were periodically flooded and were characterized by higher moisture.
Moreover, the low degree of frond fragmentation suggests relatively undisturbed and stable conditions, and short transport to the sedimentary basin.
The Tabbowa sediments where the C. denticulata and C. zeylanica collected are mostly fluvial fresh water deposits with a significant amount of terrestrial higher plants (Edirisooriya andDharmagunawardhane, 2013, Ratnayake andSampei, 2015).
Different Cladophlebis species with uniform morphological characters show similar patterns of preservation. Especially Cladophlebis denticulata has relatively narrow and elongated pinnules with dentate margins. Though these types of comparatively variable morphological characters polymorphism) are results of their micro-habitat which must have been a harsher or dry environment.

CONCLUSION
Present study reveals that comparative morphological features of all five Cladophlebis species known from Early Jurassic sediments of Gondwanaland viz., C. zeylanica (Sri Lanka), C. indica (India), C. australis (Australia) and C. antarctica (Antarctica) demonstrate that they are all closely related ally of the species C. denticulata (Sri Lanka) found in Tabbowa basin, Sri Lanka. However, taxonomical significance could not be revealed clearly due to limited number and poor status of preservations of the fossils. Morphological variations in the four species Cladophlebis may have possibly developed due to the effect of environmental conditions in their micro habitat in the Early Jurassic during their evolution. Observed morphological characters can be attributed to the result of polymorphism of Cladophlebis denticulata. As such Cladophlebis denticulata may be considered as the classic form of the family Osmundaceae and survived with morphological variation and adaptation during Early Jurassic in the Gondwanaland. However, further studies with a larger number of samples and solid statistical analysis are required for firm confirmation.