intraspecific variation of taeniate bisaccate pollen within permian glossopterid sporangia, from the...

Intraspecific Variation of Taeniate Bisaccate Pollen Within Permian Glossopterid Sporangia,from the Prince Charles Mountains, AntarcticaAuthor(s): Sofie Lindström, Stephen McLoughlin and Andrew N. DrinnanSource: International Journal of Plant Sciences, Vol. 158, No. 5 (Sep., 1997), pp. 673-684Published by: The University of Chicago PressStable URL: http://www.jstor.org/stable/2474926 .

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Int. J. Plant Sci. 158(5):673-684. 1997. ? 1997 by The University of Chicago. All rights reserved. 1058-5893/97/5805-0020$03.00

INTRASPECIFIC VARIATION OF TAENIATE BISACCATE POLLEN WITHIN PERMIAN GLOSSOPTERID SPORANGIA, FROM THE PRINCE CHARLES MOUNTAINS, ANTARCTICA

SOFIE LINDSTROM,* STEPHEN McLOUGHLIN,t AND ANDREW N. DRINNANI t

*Division of Historical Geology and Palaeontology, University of Lund, Solvegatan 13, S-223 62 Lund, Sweden; and tSchool of Botany, University of Melbourne, Parkville, Victoria 3052, Australia

Permineralized sporangia from Late Permian sediments of the Amery Group in the Prince Charles Mountains, East Antarctica, are assigned to Arberiella sp. cf. A. africana Pant and Nautiyal. These sporangia contain between 2000 and 3000 taeniate, saccate pollen grains that are predominantly haploxylonoid bisaccate and referable to the palynotaxon Protohaploxypinus limpidus (Balme and Hennelly) Balme and Playford. However, the sporangia also contain greater than 4% of diploxylonoid bisaccate forms comparable to Striatopodocarpidites cancellatus (Balme and Hennelly) Hart 1963, together with sporadic monosaccate and trisaccate grains that, if found dispersed, would be assigned to several different pollen form genera. Morphometric analysis of in situ bisaccate pollen grains and taeniate bisaccate pollen in the dispersed palynoflora indicates that in situ grains occupy only the smaller end of the total size range. The tendency for in situ grains to cluster into two different size groups may reflect differential predispersal expansion of the corpus. The in situ pollen grains are variable in most qualitative and quantitative features used for taxonomic discrimination of dispersed taeniate bisaccate pollen, and this may lead to unreliable estimates of Late Permian floristic diversity if an overly restrictive species delimitation scheme is used.

Introduction

Taeniate bisaccate pollen are strikingly abundant in Permian-Triassic strata in most parts of the world and are particularly common in Southern Hemi- sphere assemblages. They were produced by a range of gymnosperm groups including conifers, peltas- perms, glossopterids, and possibly corystosperms (Meyen 1987). Dispersed taeniate bisaccate pollen in Permian Gondwanan sediments have been attributed chiefly to glossopterid gymnosperms. They have been assigned to a wide range of form genera and species on the basis of a suite of quantitative and qualitative morphological parameters, including corpus and saccus dimensions, total breadth of the grain, and number, size, and orientation of the taeniae. To date, few workers have studied pollen preserved in situ within sporangia to gain an insight into the true intraspecific morphological variability of glossopterid pollen. An understanding of intraspecific pollen variability in late Palaeozoic plants is important for assessing plant diversity through time; analysis of plant diversity through time has been un- dertaken in some cases (Retallack 1995) using counts of species richness reported in systematic studies of dispersed pollen and spore assemblages. Understanding the intraspecific variability of pollen is also critical when assessing first and last appear- ances of plant groups and community diversity trends across major extinction events such as that recognized at the Permian-Triassic boundary (Knoll 1984; Retallack 1995). Studies of intraspecific pollen variability also offer potential insights into the types of morphological/structural characters that will

'Author for reprint requests.

Manuscript received November 1996; revised manuscript received April 1997.

be useful for taxonomic definition of dispersed pollen.

Sporangia have been found attached to glossopterid bracts of Eretmonia- and Glossotheca-type by several authors (Surange and Chandra 1975), but no in situ pollen grains have been recovered from these sporangia (Balme 1995). All records of in situ pollen are from detached sporangia that have been referred to Arberiella and are most certainly of glossopterid affinity (Balme 1995). Principally on the basis of their in situ pollen, which has been compared to dispersed forms from several form species and genera (table 1), Arberiella sporangia have been assigned to several species by various workers. Critical examination of the illustrations provided in previous studies indicates that some taxonomic assignations re- quire revision, and we have referred forms to alternative taxa herein (table 1).

This article describes well-preserved Permian glossopterid sporangia from Antarctica that contain abundant taeniate bisaccate pollen of variable form, and compares and contrasts the in situ glossopterid pollen with the range of morphotypes represented in dispersed pollen from the same bed. In the light of the intraspecific variability of glossopterid pollen, this study also considers the utility of dispersed taeniate bisaccate pollen species for stratigraphic correlation and age determination and assesses the key morphological characters for taxonomic differentiation of Gondwanan Permian taeniate bisaccates.

Material and methods Studied sporangia were preserved as permineralizations in

a single silicified peat horizon within the Upper Permian Bainmedart Coal Measures, northern Prince Charles Moun- tains, Antarctica. The silicified peat layer (<40 cm thick) caps a prominent coal seam in the lower part of the Bain- medart Coal Measures. It is exposed over a strike length of around 3 km and grades laterally into nonsilicified coals or

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LINDSTROM ET AL.-IN SITU GLOSSOPTERID POLLEN 675

siliceous sandstones. No volcanigenic deposits are associated with the permineralized peat.

The permineralized peat layer contains macro- and microfossils derived dominantly from glossopterid and cordaitalean gymnosperms but also incorporates fern, lycopod, fun- gal, and insect coprolite remains (Neish et al. 1993; McLoughlin and Drinnan 1996; Weaver et al., 1997). The chert horizon represents a permineralized litter deposit of an in situ peat swamp community and is immediately overlain by a 15-25-m-thick lacustrine deposit of limonitic and sid- eritic shales and sandstones (Dragons Teeth Member of Fielding and Webb [1996]). A Late Permian (Australian pa- lynofloral Stage 5) age is tentatively ascribed to the lower Bainmedart Coal Measures (Balme and Playford 1967; Kemp 1973; Playford 1990).

Sporangia were extracted from the peat by bulk digestion of the chert samples in cold HF for 3-5 d. The undigested organic matter contained a range of plant remains dominated by glossopterid and cordaitalean foliage, and wood frag- ments. Abundant fine, organic detritus was removed from the samples by sieving with a 200-Rlm nylon mesh. The pa- lynomorph content of the collected fine matter was pro- cessed and concentrated using standard palynological tech- niques, and strew slides were prepared of the dispersed spore-pollen assemblage.

Whole sporangia were removed from the coarse fraction using a needle or fine hairbrush then mounted on stubs and prepared for scanning electron microscopy. Ten additional sporangia were selected for examination of their pollen contents. These sporangia showed no signs of dehiscence and did not possess any adhering spores or pollen on their ex- ternal surfaces. These sporangia were delicately broken open using a fine needle, and the contained pollen mass was teased apart and mounted on an optical microscope slide for observation.

Each macerated sporangium was examined to assess the general range of pollen morphology. Four sporangia were selected for detailed measurement and assessment of a range of quantitative and qualitative criteria traditionally used for the taxonomic differentiation of dispersed pollen species. The range of bisaccate pollen represented in the dispersed assemblages from the peat layer was analyzed according to the same criteria. Pollen morphological terminology used herein is that of Foster (1979) and Traverse (1980).

The pollen grains were examined and photographed in light microscopy, using an Olympus BH2 microscope with a BH2-UCD universal condenser and Olympus automatic photomicrographic system PM-lOADS, and Tech-Pan film at 200 ASA. Specimens are lodged in the Commonwealth Palaeontological Collections, Canberra, Australia, under the administration of the Australian Geological Survey Organi- sation.

Results SPORANGIA

Complete glossopterid sporangia extracted from the permineralized peat are of essentially identical morphology. All sporangia were found detached from other plant organs. They are typically ovoid, ellipsoid, spindle-shaped, or reniform, 700-1200 (average 900) ,um long, and 300-600 (average 450) ,um wide and deep. The sporangial wall is one cell thick and com- prises narrow (10-30 pLm wide), elongate (200-300 ,um long) cells aligned roughly parallel to the long axis

of the sporangium, giving it a striate, or in some cases slightly sinuous striate, appearance (fig. 1A). Dehis- cence of the sporangia occurs along lines of weakness between these longitudinal files of cells (fig. 1B). In some sporangia, large sections of the sporangial wall may become detached, exposing the pollen contents (fig. 1C, E). Each sporangium contains ca. 2000-3000 pollen grains.

POLLEN

The pollen contents of the 10 sporangia are very consistent and fairly well preserved, although pits in the exine caused by mineral growth are quite common. All in situ pollen possess a saccus or sacci and a taeniate corpus. One macerated sporangium contained 2656 pollen grains of which 2540 were haploxylonoid (ca. 96%), 109 diploxylonoid (ca. 4%), six monosaccate (ca. 0.2%), and one trisaccate (ca. 0.03%).

Measurements were carried out on pollen from four randomly selected sporangia to document the ranges of a series of key taxonomic characters. If found dispersed, the two major morphotypes would be referred to the fossil form taxa Protohaploxypinus limpidus (Balme and Hennelly) Balme and Playford 1967 (fig. 2D) and Striatopodocarpidites cancellatus (Balme and Hennelly) Hart 1963 (fig. 3A). A few specimens with obliquely orientated taeniae would be assigned to Protohaploxypinus diagonalis Balme 1970 (fig. 3K). Rare trisaccate pollen grains (fig. 4E) would be referred to species of Crustaesporites Leschik emend. Jansonius 1962, and monosaccate forms (fig. 4A-D) would be placed in Striomonosaccites Bharadwaj 1962.

GENERAL DESCRIPTION. Pollen grains bisaccate, occasionally trisaccate or monosaccate, taeniate; haploxylonoid to diploxylonoid (figs. 2-4). Corpus circular, oval to rhomboidal, usually distinct, although indistinct in some haploxylonoid specimens (fig. 2D) and often outlined by a narrow rim in diploxylonoid specimens (fig. 3F, I). Cappa finely infrapunctate; di- vided into five to 10 transverse taeniae, separated by narrow clefts 0.5-2 ,um in width. Taeniae well defined, usually continuous over full breadth of corpus (figs. 2B, 3C), 2-6 ,um wide; commonly parallel-sid- ed to lensoid, but occasionally wedge-shaped (figs. 2H; 3D, I). Taeniae of haploxylonoid grains appear smooth and more translucent than the denser and often wrinkled taeniae in the diploxylonoid forms. Cap- pula with parallel, convex or concave margins, 0.5- 14 ,um wide, often bordered by longitudinal, intexinal folds. Sacci of bisaccate grains greater than semicir- cular to near-circular in outline, distally inclined, commonly discrete, but occasionally continuous along one or both lateral margins of corpus. Sacci infrareticulum with polygonal brochi, 0.5-3 ,um in diameter, often radially elongate near sacci bases in diploxylonoid specimens (fig. 3H, J). Many diploxylonoid specimens have radially arranged constriction folds toward the distal saccus attachment (fig. 1D, G).




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Total breadth of 172 bisaccate pollen grains 36(47)56 pLm. Corpus breadth 14(27)35 Fm, corpus length 17(30)40 Fm. Saccus breadth 17(23)28 pLm, saccus length 22(30)41 pLm.

Comparison with previously described fossils Although detached from other plant organs, the

sporangia are clearly assignable to Arberiella on the basis of their gross morphology, distinctive unicel- lular striate walls, and pollen content. Arberiella incorporates striate pollen sacs known to occur attached to a range of glossopterid reproductive organs (Er- etmonia, Glossotheca, Nesowalesia, and Squamella; Pant 1977). In Eretmonia, Glossotheca, and Squa- mella, numerous sporangia are borne terminally on delicate branching threads attached to the midline of spathulate or rhomboid bracts with dichotomous or reticulate venation. Nesowalesia is interpreted as a cup-shaped or disklike reproductive structure with sporangia densely distributed over the inner surface of the cup. The type of reproductive organ that produced the isolated Antarctic sporangia has not been determined as the sporangia probably detached readi- ly from the parent organs during life or during dis- solution and sieving of the peat samples.

Of the three formally defined species of Arberiella (table 1), one (A. thomasii) is represented only by the shape of aggregated pollen masses and lacks a preserved sporangial wall. The other species, A. africana and A. vulgaris, are almost identical in gross sporangial morphology and are distinguishable only by the nature of their enclosed pollen. In the absence of in situ pollen, the Antarctic sporangia would be indis- tinguishable from other Arberiella specimens recorded throughout the Gondwanan Permian (Arber 1905; Sen 1955, 1956; Pant 1958; Pant and Nautiyal 1960; Cridland 1963; Pant and Bhatnagar 1973; Gould and Delevoryas 1977; Zavada 1991). The in situ pollen of the Antarctic specimens is similar in morphology to that recovered from A. africana by Pant and Nau- tiyal (1960), although the Antarctic forms show a greater range of gross dimensions. The Antarctic sporangia are here referred to Arberiella sp. cf. A. africana.

Discussion The large number of pollen per sporangium, the

numerous sporangia borne on individual fructifica-

tions (Pant 1958; Surange and Chandra 1975), and the substantial size of the parent plants (Gould and Delevoryas 1977) indicate that glossopterids were heavy pollen producers, and the abundance of their fossilized pollen in dispersed assemblages may to some extent present an amplified indication of the parent plants' representation in the Permian flora. Nevertheless, both macro- and microfossil studies indicate that glossopterids overwhelmingly dominated the extensive lowland mire floras of Gondwana throughout most of the Permian (Plumstead 1969; Re- tallack 1980; Chandra 1992).

The pollen recovered from the Antarctic Arberiella sporangia, if found dispersed, could be distributed among at least four form genera (Protohaploxypinus, Striatopodocarpidites, Striomonosaccites, and Crus- taesporites) and at least five form species. Morpho- logical variability of in situ pollen and spores has also been recorded for other plant groups including lyco- phytes (Thomas 1987), ferns (Laveine 1969; Maithy 1974), corystosperms (Townrow 1962; Melendi and Scafati 1987), and conifers (Van Konijnenburg-van Cittert 1971). The morphological diversity evident in the in situ pollen indicates that many past treatments of dispersed pollen may have resulted in excessive taxonomic splitting on the basis of insufficiently consistent qualitative characters and/or inadequately de- limited quantitative characters. The variability of pollen within sporangia studied here may also indicate that aberrant grains recovered from fructifications and previously attributed to natural contaminants (Rigby and Chandra 1990) may simply represent end-members of a morphologically variable in situ pollen type.

Pollen within Arberiella sporangia have a reticulate mesh flanking the internal surface of the saccus wall. In most pollen, the reticulum bears mesexinous radial threads and rods that connect the saccus wall to the corpus (figs. iF, 2H). Transmission electron microscopy of South African pollen from Arberiella sporangia by Zavada (1991) supports this structural inter- pretation, and the saccus structure is most similar to the "protosaccate" condition of Scheuring (1974). However, in some pollen, the threads and rods do not completely fill the saccus, as a result of either dete- rioration or incomplete development, thus giving the grains an apparently "eusaccate" condition. Conti- nuity of the sacci around the corpus is also variable

Fig. 1 Scanning electron micrographs of glossopterid sporangia (Arberiella) and pollen obtained from macerations of sporangia. A, Spo- rangium showing longitudinally sinuous bands of cells comprising the sporangial wall; X 100; CPC34303. B, Sporangium showing initial stages of dehiscence along weaknesses between longitudinal files of wall cells; X 100; CPC34304. C, Sporangium with a portion of the sporangial wall removed showing the mass of enclosed bisaccate pollen; X 100; CPC34305. D, Enlargement of two pollen grains in distal view showing the typical (expanded) Protohaploxypinus limpidus-type pollen (left) and the atypical (constricted) Striatopodocarpidites cancellatus-type pollen (right). Note folding at the base of the sacci characteristic of forms with an unexpanded corpus; X 1400; CPC34306. E, Enlargement of a portion of the sporangium in fig. 1C showing a range of tightly packed, taeniate, bisaccate pollen; X 500; CPC34305. F, Two broken pollen showing sacci with a well-developed reticulum flanking the internal sacci walls and variably developed radial rods and threads (arrow) extending through the sacci lumina; X 1100; CPC34305. G, Proximal view of a diploxylonoid S. cancellatus-type pollen grain showing unexpanded corpus with wrinkled taeniae; X 1750; CPC34305. H, Proximal view of a haploxylonoid P. limpidus-type pollen grain showing fully expanded corpus with relatively smooth microfoveolate/infrapunctate taeniae; X 1750; CPC34307.




E B

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Fig. 2 Light micrographs of the range of haploxylonoid pollen types from macerations of Arberiella sporangia; all specimens X 1300. Specimens are identified by specimen number, slide number, and England-finder coordinates. A, Protohaploxypinus limpidus-type pollen grain, proximal focus; CPC34337, Arb 9, U44/2. B, Weakly diploxylonoid P. limpidus-type pollen grain, proximal focus; CPC34334, Arb 8, R33/4. C, P. limpidus-type pollen grain, proximal focus; CPC34333, Arb 7, Q35/4. D, P. limpidus-type pollen grain, distal focus. Note the indistinct corpus; CPC34338, Arb 9, J33/2. E, P. limpidus-type pollen grain, proximal focus. Note the lateral connection of the sacci along the lower corpus margin; CPC34315, Arb 1, S31/4. F, P. limpidus-type pollen grain showing a slight asymmetry in sacci length, medium focus; CPC343 16, Arb 1, W23/4. G, P. limpidus-type pollen grain, distal focus. Note the narrow cappula; CPC34332, Mrb 2, J41/3. H, P. limpidus-type pollen grain, proximal focus. Note the very narrow cappula and infrasacci rods and threads; CPC34323, Mrb 2, N37/4.




(figs. 2, 4) such that, in some pollen, the distinction between a bisaccate, trisaccate, and monosaccate condition as a diagnostic character becomes meaningless. The morphological continuum evident between monosaccate and bisaccate grains within a single sporangium may reflect aberrant or incomplete ontogenetic development of some grains and signal potential phy- logenetic affinities between predominantly protobi- saccate Permian glossopterids and Carboniferous pro- to- to eumonosaccate pollen-producing groups such as the Cordaitales, Coniferales, and Callistophytales (Balme 1995).

Glossopterids have a macrofossil record that begins at or near the Carboniferous-Permian boundary and terminates at or near the Permian-Triassic boundary (Retallack 1980; Anderson and Anderson 1985). However, some Gondwanan basins contain taeniate, monosaccate to bisaccate pollen (traditionally attributed to glossopterids) in sediments as old as the early Westphalian or Namurian (Powis 1984; Jones and Truswell 1992). These occurrences may indicate an earlier (pre-Permian) origin for glossopterids. Alter- natively, such pollen may have been produced by the precursors of glossopterids or allied plants. From a developmental perspective, there would appear to be few structural changes required, e.g., constriction of the saccus and development of additional proximal clefts on the corpus, to pass from Upper Carbonifer- ous, bilaterally symmetrical, monolete, monosaccate grains (e.g., Potoniesporites elongatus Jones and Tru- swell 1992) to early taeniate, monosaccate and bisaccate pollen (e.g., Striomonosaccites sp. and Proto- haploxypinus sp. cf. P. goraiensis of Jones and Truswell 1992).

Mono- and trisaccate specimens are rare constitu- ents of the studied sporangia and are therefore regarded as aberrant forms. Clarke (1965) surmised an equivalent natural relationship between bisaccate and rare trisaccate, taeniate grains found in dispersed pollen assemblages from the British Permian on the basis of similar aberrant forms occurring among bisaccate pollen produced by extant plants (Van Campo-Duplan 1947; Martin 1959). However, he continued to refer the aberrant trisaccate forms to Crustaesporites as he considered their recognition as a distinct genus in dispersed assemblages "an inevitable consequence of a morphographic treatment" (Clarke 1965, p. 331).

Diploxylonoid specimens in the sporangia are generally somewhat smaller than the haploxylonoid forms, and this size difference can mainly be attributed to the smaller corpus dimensions of the former group (fig. SA, C). Both the corpus wall and the taeniae of the diploxylonoid grains generally appear thicker than in the haploxylonoid forms (figs. 2, 3), indicating that the corpora of the diploxylonoid forms are not fully expanded. The often wrinkled appearance of the taeniae in the diploxylonoid specimens (figs. 1G; 3A, G) also indicates incomplete expansion of the corpus. In many diploxylonoid specimens, the taeniae are of different widths, often with one or more

wedging taeniae (fig. 3D, I). This may indicate that different parts of the corpus have not expanded si- multaneously. The number of taeniae on the corpus varies by up to 100% (5-10) between grains, and this feature appears to be independent of corpus dimensions. The width of the cappula varies from 0.5 to 14 ,um, reflecting the preservational orientation of the sacci in relation to the corpus. The sacci appear to be more or less fully expanded in all specimens, although there are sporadic examples of grains with sacci of unequal dimensions (fig. 3I). The only no- table difference between the sacci arrangement of the haploxylonoid and diploxylonoid forms appears to be the presence of constriction folds (figs. 1D, G; 3J) and/or radially elongated brochi near the sacci bases on the latter (fig. 3H). Most of the diploxylonoid bisaccate grains found in the sporangia are interpreted as incompletely developed forms, but some specimens (fig. 3B, C, L) appear to be fully expanded and are, hence, regarded as morphological end-members to the more common haploxylonoid forms. This does not necessarily imply that all dispersed diploxylonoid grains are incompletely expanded. However, the presence of thick, wrinkled taeniae, constriction folds in the sacci, and elongate brochi at the bases of the sacci in such grains may be useful indices for recognizing incompletely expanded pollen.

From the key morphographic parameters (fig. 5), it is evident that the greatest variation in the gross morphology of taeniate, bisaccate pollen grains is in the corpus dimensions. It is interesting to note that the in situ pollen grains appear to plot in two clusters on the basis of corpus dimensions (fig. SA, C). If the diploxylonoid forms are regarded as not completely expanded specimens, then the segregation of the two clusters may indicate that the corpus expands in a rapid pulse during the final stages of development, leaving few intermediate forms. The relationship between the total breadth and the respective saccus dimensions, however, appears to be linear (fig. SB, D).

Although a wide range of morphotypes are evident among pollen from the glossopterid sporangia, on most key quantitative parameters, the in situ pollen define a cohesive population when plotted against the range of dispersed taeniate bisaccate pollen from the same sediments (fig. 5). The dispersed pollen clearly incorporate a much greater morphological range and were probably derived from several plant taxa. Sig- nificantly, pollen assigned to various species of Stria- topodocarpidites tend to plot away from specimens assigned to Protohaploxypinus species when corpus dimensions are used as distinguishing parameters (fig. SA, C). The identification of incompletely expanded forms assignable to S. cancellatus and fully expanded forms referable to P. limpidus from the same sporangia indicates that many of the dispersed forms referable to various Striatopodocarpidites species may represent unexpanded forms of a range of Protohap- loxypinus species. However, the lack of discrete clus- tering within quantitative plots of the populations of




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wK L Fig. 3 Light micrographs of the range of diploxylonoid pollen types from macerations of Arberiella sporangia; all specimens X 1300.

Specimens are identified by specimen number, slide number, and England-finder coordinates. A, Striatopodocarpidites cancellatus-type pollen grain, proximal focus. Note the atypical transversely elongated corpus, the thick corpus wall, and the wrinkled appearance of the taeniae; CPC34324, Arb 2, J31/2. B, S. cancellatus-type pollen grain showing a great number of taeniae and a narrow cappula, medium focus; CPC34328, Arb 4, 038/4. C, S. cancellatus-type pollen grain, proximal focus. Specimen with only five taeniae; CPC34325, Arb 2, J34/3. D. S. cancellatus-type pollen grain, proximal focus. More strongly diploxylonoid specimen, with thick corpus, narrow cappula, and wedging taeniae; CPC34329, Arb 4, Q33/2. E, S. cancellatus-type pollen grain, proximal focus. Note the very narrow cappula; CPC34339, Arb 9, U41/2. F, S. cancellatus-type pollen grain showing a thick corpus and wrinkled appearance of the taeniae, medium focus; CPC343 17, Arb 1, G35/3. G, S. cancellatus-type pollen grain, proximal focus. Strongly diploxylonoid specimen, with thick corpus and wrinkled taeniae.




Fig. 4 Light micrographs of aberrant monosaccate and trisaccate pollen types from macerations of Arberiella sporangia; all specimens X 1300. Specimens are identified by specimen number, slide number, and England-finder coordinates. A, Circular to transversely elliptical Striomonosaccites-type pollen grain, medium focus; CPC34320, Arb 1, Z28/2. B, Striomonosaccites-type pollen grain, medium focus. Note the discrete saccus disjuncture in the lower part of the photograph; CPC34335, Arb 8, M42/2. C, Striomonosaccites- type pollen grain, medium focus. Note the discrete saccus disjuncture in the lower part of the photograph; CPC34330, Arb 4, Z43/2. D, Striomonosaccites-type pollen grain, proximal focus. Specimen with lobed saccus resembling a trisaccate state; CPC34336, Arb 8, Q36/4. E, Crustaesporites-type trisaccate pollen grain, distal focus; CPC34321, Arb 1, X27/3.

dispersed pollen prohibits assignation of natural alli- ances between any two Striatopodocarpidites and Protohaploxypinus species other than S. cancellatus and P. limpidus.

Dispersed pollen grains referred to P. limpidus and S. cancellatus are widespread and common in Permian strata of the Southern Hemisphere, having been reported from all the individual Gondwanan continents except South America, but they have also been reported as far north as Svalbard in the Northern Hemi-

sphere (Mangerud and Konieczny 1993). In the Gond- wanan Permian, these two form taxa commonly occur together, although in most areas the first occurrence of P. limpidus usually precedes the first appearance of forms assigned to S. cancellatus (Utting 1976, 1978; Kyle 1977; Calver et al. 1984; Backhouse 1991, 1993). Considering the quantitative relationship between the P. limpidus and the S. cancellatus forms found in situ in this study, it seems that the probability of finding specimens of the former taxon in dispersed assem-

CPC34340, Arb 9, M34/4. H, S. cancellatus-type pollen grain, distal focus. Note the narrow cappula and the constricted sacci bases with elongate brochi; CPC34342, Arb 10, E41/4. I, S. cancellatus-type pollen grain, proximal focus. Specimen with wedging and discontinuous taeniae; CPC34341, Arb 9, T47/2. J, S. cancellatus-type pollen grain, distal focus. Note the constriction folds near the sacci bases; CPC34318, Arb 1, H28/2. K, Protohaploxypinus diagonalis-type pollen grain, proximal focus. Note the diagonal arrangement of the taeniae; CPC34326, Arb 2, W35/2. L, S. cancellatus-type pollen grain, distal focus. Weakly diploxylonoid specimen with a wide cappula; CPC34319, Arb 1, U42/3.




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Total breadth (jim) Total breadth (jim)

Fig. 5 Graphic comnparison of selected mnorphological characters for in situ pollen (plus signs) comnpared to dispersed pollen of Protohap- loxypinus-type (diamnonds) and Striatopodocarpidites-type (open circles) fromn the samne bed. A, Corpus breadth vs. total breadth. B, Saccus breadth vs. total breadth. C, Corpus length vs. total breadth. D, Saccus length vs. total breadth.

blages is much greater than that of finding examples of the latter, especially in strata deposited in the ear- liest Permian when the glossopterids were not yet the most dominant plant group.

If the size range (total length vs. total breadth) of the pollen grains from the Arberiella sporangia is compared to the size ranges of some different dispersed populations of P. limpidus and S. cancellatus (fig. 6), it becomes evident that the in situ pollen cover a much smaller size range than that normally accepted for those two form taxa. This may indicate that these two form taxa are composite groups of pollen grains derived from reproductively distinct glossopterid species, and this would in turn complicate stratigraphic and geographical interpretations.

The quantitative and qualitative characters normally used for grouping dispersed taeniate bisaccate pollen into existing form taxa appear to have permitted a

greater degree of species differentiation than that which would be recognized if taxa were based on the range of pollen forms found within individual sporangia (i.e., within a reproductively discrete taxon). How- ever, there also appears to be a substantial degree of variation among in situ pollen populations from the detached Arberiella sporangia (fig. 7). While some es- tablished pollen form species are almost certainly based on unexpanded, aberrant, or irregularly orientated examples of more common pollen types, natural associations will not become fully apparent until morphographic analyses are carried out for pollen from a wide range of glossopterid sporangia. Until then, gross dimensions and the ratios of the principal morphographic characters (corpus size, saccus size, cappula width, and number of taeniae) together with some qualitative characters (continuity of sacci around the corpus, presence of intexinal folds flanking the cap-




60- ~~~ ~2b4

H40 I (5 12a z W lb

O2 - .. . .. . .......a 0

0 20 40 60 80 100 TOTAL BREADTH (,um)

Fig. 6 Plots of the gross morphological range of Protohaploxy- pinus limpidus- and Striatopodocarpidites cancellatus-type pollen recorded from selected dispersed Permian Gondwanan assemblages compared to in situ pollen from the Antarctic sporangia. Ja, S. cancellatus, Sydney Basin, Australia (Balme and Hennelly 1955); lb, P. limpidus, Sydney Basin, Australia (Balme and Hennelly 1955); 2a, S. cancellatus, Blair Athol and Bowen Basins, Australia (Foster 1979); 2b, P. limpidus, Blair Athol and Bowen Basins, Australia (Foster 1979); 3, in situ bisaccate pollen, Prince Charles Mountains, Antarctica (this study).

pula, orientation of the taeniae) will remain the principal criteria for differentiating between various groups of dispersed pollen.

Not all ancient plants may have registered the same degree of morphological variability in their pollen and

spores as glossopterids. Sporangia of many fossil plants have yielded microspores and pollen of relatively uniform morphology (Balme 1995, and refer- ences therein). Such differential morphological variation of palynomorphs and other detached organ taxa between plant groups poses problems for assessing trends in past floristic diversity (Knoll 1984; Retallack 1995). This is especially the case for analysis of ter- restrial plant diversity across major extinction events such as that at the Permian-Triassic boundary. If analyses were restricted to counting the numbers of plant form taxa from dispersed assemblages across this boundary then the impression might be gained that a large number of genera and species of taeniate bisaccate pollen-producing plants disappeared at the close of the Palaeozoic Era in Gondwana. However, if this apparent diversity of glossopterid pollen were largely a function of excessive taxonomic splitting of dispersed assemblages then the magnitude of the extinction of the gymnosperm flora might be artificially in- flated.

Acknowledgments This study was funded by an Australian Research

Council large grant to A. N. Drinnan, an Australian Research Fellowship to S. McLoughlin, and a Swedish Natural Science Research Council Postdoctoral Fellow- ship to S. Lindstrom.

120

100-

E 80-

0 t) 60 Jl ,

H~~~~~H

0

0 20 40 60 80 100 120 140 160 180 TOTAL BREADTH (,im)

Fig. 7 Plots of the gross morphological ranges for in situ glossopterid pollen recorded by previous workers compared to the present study. A, Elliptical pollen sacs (Sen 1955); B, pollen sacs (Sen 1956); C, Arberiella africana (Pant 1958; Pant and Nautiyal 1960; Townrow 1962); D, Arberiella vulgaris (Pant and Nautiyal 1960; Townrow 1962); E, Arberiella sp. (Cridland 1963); F, A. vulgaris (Pant and Bhatnagar 1973); G, Arberiella thomasii (Pant and Bhatnagar 1973); H, Arberiella sp. (Gould and Delevoryas 1977); 1, A. vulgaris (Rigby and Chandra 1990); J, Arberiella sp. (Zavada 1991); K, Arberiella sp. cf. A. africana (this study).




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intraspecific variation of taeniate bisaccate pollen within permian glossopterid sporangia, from the...

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