Introduction

The continental Triassic of the Transantarctic Mountains has yielded rich, diverse, and in some cases exceptionally well-preserved plant fossils that certainly rank among the most intensely studied and well-known early Mesozoic floras worldwide. Large collections of Triassic impression/compression floras and permineralized peat deposits have been accumulated since the first discoveries of plant fossils from Antarctica in the early 20^th century. The Triassic vegetation of Antarctica consisted of a diverse array of plant groups, including horsetails, various fern and seed fern families, cycads, ginkgophytes, and conifers (see, e.g., Taylor and Taylor 1990; Taylor et al. 1993; Axsmith et al. 2000; Phipps et al. 2000; Ryberg et al. 2008; Hermsen et al. 2009; Decombeix et al. 2010; Escapa et al. 2010). In spite of this diversity based on macrofossils, palynofloras document that the high-latitude ecosystems of Antarctica were inhabited by an even wider spectrum of plants, many of which still lack macrofossil evidence. For instance, abundant lycophyte spores have been reported from various regions of East Antarctica, including spores of supposed pleuromeialean, isoetalean, and selaginellalean affinity (Kyle 1977; Kyle and Schopf 1982; Farabee et al. 1990; Taylor et al. 1993; McLoughlin et al. 1997; Askin and Cully 1998). With the exception of a few isolated sporangia (McLoughlin et al. 1997), however, documented evidence of lycophyte macrofossils is lacking. This may be explained by the generally low preservation potential of the small, presumably herbaceous plants, either because the plants themselves had a much lower fossilization potential or because they existed in habitats that were not conducive to fossilization. Alternatively, lycophyte macrofossils may simply have been overlooked or misidentified.

This paper describes the first macrofossil evidence for lycophytes in the Triassic of the Transantarctic Mountains. The material consists of an incomplete strobilus and several isolated sporophylls and sporangia that co-occur with abundant megaspores. Also present on the slab are several specimens of the enigmatic leaf Mesenteriophyllum serratum Sixtel, 1961. Mesenteriophyllum Sixtel, 1961 has so far been reported from the Triassic of the Kyrgyz Republic and the Taimyr Peninsula, and is currently regarded as a gymnosperm leaf of unknown affinity. By contrast, the material from the Transantarctic Mountains provides compelling evidence that M. serratum in fact represents sterile foliage of a pleuromeialean lycophyte.

Fig. 1.

Geographic location (A–C) and lithological column (D) of the plant-bearing succession at “Alfie's Elbow”; arrow indicates the position of the bed from which the present material was collected. Lithological column after Axsmith et al. (2000).

Institutional abbreviation.—PCUK, Paleobotanical Collection at the Natural History Museum and Biodiversity Institute, University of Kansas, Lawrence, USA.

Material and methods

The material was collected from an outcrop in the Shackleton Glacier region (central Transantarctic Mountains) that is informally known as “Alfie's Elbow” (Fig. 1A–C) (Axsmith et al. 2000). The section is dated as Late Triassic based on palynomorphs (Askin and Cully 1998). The entire collection of plant fossils from this locality comprises >500 slabs, most of which contain Dicroidium Gothan, 1912 and Heidiphyllum Retallack, 1981 foliage, as well as associated axes and reproductive organs, including Umkomasia Thomas, 1931, Pteruchus Thomas, 1933, and Telemachus Anderson, 1978.

The plant fossils described in this paper occur on a single block of silty, medium- to dark-grey, carbonaceous mudstone with abundant plant fragments (PCUK T5568). Plant fossils are preserved as impressions and coalified compressions. Cuticles and palynomorphs could not be recovered because the material is too highly altered. The plant fossils show little contrast to the surrounding matrix, which rendered photographic documentation challenging. Macroscopic images were taken under cross-polarization or oblique lighting to enhance contrast and surface details. Specimens were further analysed using a Leica MZ16 stereomicroscope. Microscopic images were taken with a Nikon DS-5M digital camera.

Systematic palaeontology

Fig. 2.

Pleuromeialean lycophyte reproductive structures from the Upper Triassic of the “Alfie's Elbow” site, central Transantarctic Mountains. A. Articulated fragment of a strobilus. Individual sporophylls can be recognized by the longitudinal striations on the abaxial surface (compare Fig. 2B, C; see Fig. 3). B. Isolated sporophyll, abaxial surface. C. Same sporophyll as Fig. 2B, but after manual preparation, showing globose adaxial sporangium (s) beneath the sporophyll. D, E. Sporangia. F. Cluster of megaspores. G. Megaspores showing trilete marks and curvaturae (arrows). H–J. Distal faces of megaspores showing variation in surface ornamentation or preservation, including verrucate, conate, and echinate patterns.

Fig. 3.

Interpretative drawing of the strobilus fragment in Fig. 2A, showing the loose, helical arrangement of sporophylls and sporophyll scars.

Discussion

The present material constitutes the first unequivocal macrofossil evidence of lycophytes in the Triassic of the Transantarctic Mountains. The remains of reproductive organs closely resemble those of Pleuromeia, in particular Pleuromeia rossica and Pleuromeia jiaochengensis (Neuburg 1960; Wang and Wang 1982). This is remarkable, because Pleuromeia has previously not been reported from strata younger than early Middle Triassic (Retallack 1975, 1997; Mader 1990). Moreover, pleuromeialean lycophytes are regarded by many authors as xero- and halophytic plants inhabiting either coastal lagoonal or ephemeral (semi)desert environments (e.g., Mägdefrau 1931; Retallack 1975, 1980, 1997; Wang and Wang 1982; Mader 1990; Fuchs et al. 1991; Meng 1996). Co-occurring leaf remains are assigned to Mesenteriophyllum serratum and represent the first record of Mesenteriophyllum from the Southern Hemisphere, and only the third report of this genus. Mesenteriophyllum has long been considered endemic to the Triassic Madygen Biota of the Kyrgyz Republic (Sixtel 1961; Dobruskina 1995), but has recently also been reported from the Lower Triassic of the eastern Taimyr Peninsula (Sadovnikov 2008). Including the fossils from East Antarctica described here, Mesenteriophyllum is now known from northern temperate regions and northern and southern high latitudes (Fig. 6).

Fig. 4.

Pleuromeialean lycophyte leaf Mesenteriophyllum serratum Sixtel, 1961 from the Upper Triassic of the “Alfie's Elbow” site, central Transantarctic Mountains. A. Two overlying leaf fragments recognizable primarily by the distinct serrate appearance of the leaf margins. B. Detail of A showing serrate appearance of margins. C. Detail of leaf margin with alternating transverse ridges and furrows, and preserved cell pattern. Note longitudinal striae (s), circular holes on central leaf portion (some marked with arrows), and vertical alignment of transversely elongated epidermal cells (e) on leaf margin. D. Detail of leaf margin with cellular preservation, showing transversely elongated cells aligned in vertical rows. E. Detail of longitudinal striae on central leaf portion. F, G. Orthogonal pattern of epidermal cells in central leaf portion, visible only in places where carbonized cuticle layer is lifted off.

Evidence for lycophyte affinities of Mesenteriophyllum serratum. —Mesenteriophyllum has been regarded as a gymnosperm leaf of uncertain affinities (Sixtel 1961, 1962; Dobruskina 1995). Leaf morphology and anatomy of the present material provide evidence that Mesenteriophyllum represents sterile foliage of an early Mesozoic lycophyte. The most striking evidence is the wrinkled or toothed appearance of the leaf margins which is due to a regular pattern of alternating transverse ridges and depressions (Fig. 3A–C). This feature has been previously interpreted as resulting from desiccation of a fleshy, probably succulent leaf lamina (Sixtel 1962). However, we suggest that this peculiar morphology is the result of the collapse of internal air chambers after burial, with the ridges reflecting the transverse septa of the leaf. This is a feature that has so far only been reported from fossil lycophyte leaves (see Brown 1939). For comparison, virtually identical structures are seen in the lacunate sporophylls of Isoetes choffatii Saporta, 1894 (Saporta 1894; see Teixeira 1948: pl. 26: 4), Isoetites serratus Brown, 1939 and Isoetites horridus (Dawson, 1883) Brown, 1939 (Brown 1939: figs. 5, 6, 1958: figs. 10, 12), Isoetites phyllophila Skog, Dilcher, and Potter, 1992 (Skog et al. 1992: figs. 1–8, 10), Isoetes ermayiensis Wang, 1991 (Wang 1991: pl. 2: 3–7) and cf. Isoetites (Pigg 2001: figs. 2, 3). All of these fossils are presumably closely related to extant Isoetes or even considered congeneric. Air chambers in the leaves of Triassic Pleuromeiales have not yet been demonstrated, but this may be due to taphonomic conditions. The genus Pleuromeia was established for casts of stems and rhizophores that are usually preserved in sandstones (e.g., Grauvogel-Stamm 1999). This rather coarse-grained clastic depositional setting is unlikely to preserve features of the internal anatomy of co-occurring foliage in detail. Nevertheless, Mägdefrau (1931) noted the presence of transverse wrinkles in some well-preserved leaves of Peuromeia sternbergii (Münster, 1839) Corda, 1852, which were later tentatively interpreted as collapsed air chambers (Grauvogel-Stamm 1999).

Fig. 5.

Pleuromeialean lycophyte leaf Mesenteriophyllum serratum Sixtel, 1961 from the Triassic Madygen Formation, Kyrgyz Republic. Modified from Sixtel (1961). Leaf width up to ∼12 mm.

Leaves of Pleuromeia are strap-shaped to elongate-lanceolate and may be over 1 cm wide and 10 cm long (e.g., Mägdefrau 1931; Kon'no 1973; Grauvogel-Stamm 1999). This is well within the dimensions reported for Mesenteriophyllum serratum (Sixtel 1961, 1962). Moreover, Mesenteriophyllum was originally described as without lateral veins, which corresponds to a microphyllous organization (Sixtel 1961, 1962). The cuticle of the Antarctic specimens has a distinctive surface ornamentation composed of continuous, fine longitudinal striae (Fig. 3C, E). A similar surface pattern is an important diagnostic character for many extant species of Isoetes (Rolleri and Prada 2007), as is the rectangular arrangement of the epidermal cells (Skog and Hill 1992; Skog et al. 1992; Rolleri and Prada 2007). Even though cuticle and epidermal patterns are preserved over large portions of the leaf, stomata are apparently absent, which is a typical feature for certain extant and fossil Isoetales (Pfeiffer 1922; Skog and Hill 1992; Rolleri and Prada 2007). The densely spaced, small circular holes on the central portion of the leaf are interpreted as hair bases or remnants of broken papillae. It is interesting to note that densely papillate leaf surfaces are known to occur in sterile sporophyll portions of fossil Isoetales as well (e.g., Brown 1958: fig. 9).

Further evidence for a lycophyte affinity of Mesenteriophyllum serratum is the close association with pleuromeialean remains at all known localities. In the Triassic Madygen Formation, Mesenteriophyllum occurs together in greater abundance only in strata that also yield Annalepis-type sporophylls and partially articulated strobili (Sebastian Voigt and Jörg Schneider, Freiberg, and Philippe Moisan, Münster, personal communications 2009). In the Fadyu-Kuda Formation, eastern Taimyr Peninsula, Mesenteriophyllum is found in a single horizon associated with abundant Pleuromeia remains (Sadovnikov 2008). In the present assemblage from Alfie's Elbow, Mesenteriophyllum and isolated pleuromeialean sporophylls and sporangia co-occur on a single slab, but are otherwise absent.

Fig. 6.

Known palaeogeographic distribution of Mesenteriophyllum Sixtel, 1961 in the northern and southern hemisphere Triassic. Palaeogeography after Golonka (2007).

Palaeoecology.—The plant-bearing beds at Alfie's Elbow are interpreted as overbank deposits of a low-energy braided-stream system (Axsmith et al. 2000). The majority of specimens from Alfie's Elbow are articulated Dicroidium fronds that occur in medium-grey, fine-grained siltstones and shales (Fig. 1D) representing crevasse splay deposits (Axsmith et al. 2000). Dicroidium plants commonly formed riparian vegetation on well-drained levees fringing major channel margins (e.g., Cairncross et al. 1995; Cúneo et al. 2003). It appears that the lycophytes described here may have occupied a different habitat within the river system. Although the entire collection at the University of Kansas Natural History Museum was investigated during this study, lycophyte remains and Mesenteriophyllum occur only on a single slab (∼25 × 20 cm in size). The lithofacies of the lycophyte-bearing bed differs from that of the rest of the collection. The rock matrix is crumbly and comparatively dark, indicating higher content of organic material with abundant plant fragments. We interpret this facies as representing a deposit of a stagnant water body, perhaps in a swampy overbank environment, with relatively high accumulation of organic remains and less frequent clastic input. The close association of the various lycophyte organs on a single slab may indicate parautochthonous deposition within this particular facies. We therefore assume that the Mesenteriophyllum-producing lycophyte inhabited a backswamp environment. There may be some support for this hypothesis in the fact that stomata are absent. In extant Isoetes non-stomatiferous leaves typically occur in aquatic or semiaquatic species (Rolleri and Prada 2007). For instance, Isoetes andicola (Amstutz, 1957) Gómez, 1980 is a peat-forming species that lacks stomata, even though it has a very thick cuticle that is impermeable to CO₂ and H₂O vapour (Keeley et al. 1984). Carbon dioxide uptake in Isoetes andicola is entirely via the root system that is embedded into a substrate of highly decomposed, CO₂-rich peat (Keeley et al. 1984).

Conclusions

The present study identifies the first macrofossil candidates that can be related to the diverse array of lycophyte spores in the Upper Triassic of Antarctica. In the present instance, lycophyte reproductive organs and associated foliage occur in a particular, carbonaceous sedimentary facies that otherwise contains only fragmented and rather poorly preserved gymnosperm remains. In order to obtain a more complete understanding of the Late Triassic high-latitude vegetation of Antarctica, we suggest that future collecting should also focus on those sedimentary facies that have up to now only yielded sparse or fragmentary plant fossils.

On the basis of this material we suggest that Mesenteriophyllum serratum represents foliage of a pleuromeialean lycophyte. The Antarctic fossils demonstrate that pleuromeialean leaves possessed several characteristics that are similar to those of fossil Isoetes and Isoetites, including air chambers, rectangular-shaped epidermal cell patterns, and the absence of stomata. These morphological and anatomical characters also offer another argument for close affinities between early Mesozoic and typically Cenozoic-extant rhizomorphic lycophytes.

Details of the co-occurring fertile organs indicate affinities to Pleuromeia rather than Annalepis or related taxa. If future studies confirm this tentative interpretation, this would greatly expand the stratigraphic and palaeoecological ranges of Pleuromeia. The Late Triassic Antarctic lycophytes apparently existed in swampy overbank environments of the humid high-latitude ecosystems of southern Gondwana, and may have contributed to peat accumulation and coal formation. Although the picture is far from complete, we hypothesize that during the Middle Triassic Pleuromeia may have radiated into high-latitude ecosystems adapting to peat-forming environments, where it persisted as a relictual element well into the Late Triassic, having long disappeared from its typical lower-latitude habitats.