Mesozoic seed ferns: Old paradigms, new discoveries1

Mesozoic seed ferns: Old paradigms, new discoveries1

Taylor, Edith L

TAYLOR, E. L., T. N. TAYLOR (Department of Ecology and Evolutionary Biology, and Natural History Museum and Biodiversity Research Center, 1200 Sunnyside Ave., University of Kansas, Lawrence, KS 66045), H. KERP (Forschungsstelle für Paläobotanik, Westfälische Wilhelms-Universität Münster, Hindenburgplatz 57, D-48143 Münster, Germany), and E.J. HERMSEN (Department of Ecology and Evolutionary Biology, and Natural History Museum and Biodiversity Research Center, 1200 Sunnyside Ave., University of Kansas, Lawrence, KS 66045). J. Torrey Bot. Soc. 133: 62-82. 2006.-Mesozoic seed ferns represent a grade of gymnospermous plants whose affinities remain problematic. The three major orders recognized today include the Caytoniales (Triassic-Cretaceous), Peltaspermales (Carboniferous-Triassic) and Corystospermales (Triassic-Cretaceous). A number of genera described from Mesozoic rocks have also been included broadly in the Mesozoic seed ferns, but their frequency, distribution, and affinities render their assignment to specific orders equivocal. The morphotypes in the three principal orders have been important in phylogenetic analyses of seed plants and have been implicated as angiosperm progenitors at various times in the past. All three groups were originally described only from compression/impression fossils, but anatomically preserved corystosperms are now known from Argentina and Antarctica. Since their original description, the geographic and stratigraphic ranges of all three major groups have been expanded, and they are now known from both the northern and southern hemispheres. An additional order of Mesozoic seed ferns, the Petriellales, has been described from the Triassic of Antarctica. This paper will summarize our current knowledge of the Mesozoic seed ferns and comment on the phylogenetic position of several orders, focusing especially on permineralized and compressed corystosperms from the Triassic of Antarctica. Recent studies of well-preserved material from the central Transantarctic Mountains have provided information about the three-dimensional morphology and anatomy of pollen organs and ovulate cupules, as well as the first evidence of the attachment of reproductive organs to the parent plant. These discoveries offer new information that can be used in phylogenetic analyses to provide increased resolution of seed plant relationships.

Key words: Antarctica, Caytoniales, Corystospermales, Cretaceous, Gondwana, Jurassic, Peltaspermales, permineralized peat, pteridosperms, seed plants, Triassic, seed ferns.

Unlike the Carboniferous seed ferns, the socalled Mesozoic pteridosperms have never been considered to represent a natural group, but rather have been regarded historically as separate orders (T. Taylor and Taylor 1993) or families (Thomas 1933, Anderson and Anderson 1985, Stewart and Rothwell 1993), with their uncertain hierarchical position underscoring their uncertain affinities. The three major groups, Caytoniales, Corystospermales and Peltaspermales, were established in the early part of the last century (Thomas 1925, 1933; Harris 1937), and have been variously interpreted as angiosperme, pre-angiosperms and gymnosperms. Although the three orders are now known in some detail, their phylogenetic positions and relationships to other seed plants remain problematic. The Mesozoic seed ferns are often treated together in textbooks (e.g., Stewart and Rothwell 1993, T. Taylor and Taylor 1993), but only their Mesozoic age and the presence of megasporophylls bearing ovules have traditionally united them. Now, even these characteristics are no longer completely applicable.

The concept of the seed ferns was originally circumscribed by Oliver and Scott (1904) for Paleozoic fossils with fern-like foliage and foliar borne seeds. The Pteridospermae was erected using dispersed, permineralized organs of the Iyginopterids (Lyginopteridales), but soon afterward encompassed all of the other Carboniferous seed fern groups. Where the foliage is known, all of these groups have fern-like fronds, so the concept of the Paleozoic seed ferns was expanded when Gould and Delevoryas (1977) demonstrated definitively that the Permian Glossopteridales, with entire, strap-shaped leaves, bore seeds on a modified leaf or megasporophyll. Within the Mesozoic seed ferns, only the Corystospermales combine the original characters of fern-like fronds and ovules borne on modified leaves or cupules. The Caytoniales have ovules within leaf-like cupules, but have palmately compound leaves with entire margins and reticulate venation. Although some of the Peltaspermales have fern-like fronds, the nature of the ovule-bearing structures is less uniform and more difficult to homologize.

Historically the existence of Mesozoic seed ferns was suggested long before Thomas’ circumscription of the groups. Schenk (1867) noted that some of these Mesozoic plants appeared to be intermediate between gymnosperms and ferns, and Antevs (1914) echoed this idea in his description of a peltasperm pollen organ. Since the initial delimitation of the orders, there have been numerous reports of new organs assigned to these groups, but despite several phylogenetic analyses, the position of each of the orders is still unresolved (see, e.g., Nixon et al. 1994, Doyle 1996, Axsmith et al. 2000). Since most of the organs are preserved as compression/impression fossils, there have been different interpretations of the three-dimensional morphology of the reproductive organs, and therefore different coding of characters in phylogenetic studies. In this paper, we present a review of the current understanding of the major groups that have traditionally been considered to be Mesozoic seed ferns, in light of more recent discoveries based on permineralized material from Gondwana.

Caytoniales. The Caytoniales were initially described by H. Hamshaw Thomas (1925) from well-preserved compressions with cuticles from the Jurassic of Cayton Bay in Yorkshire. Because of their common occurrence at this site, he included Sagenopteris Presl. leaves, Caytonia Thomas cupules, and Caytonanthus Harris pollen organs (originally Antholithus) in the new order. Based on this co-occurrence and the presence of Caytonanthus pollen on the stigmatic lip of Caytonia, Thomas considered these organs as parts of the same plant. Because of the organization of the ovule-bearing cupules and leaves with reticulate venation, he regarded the group as representing early angiosperms.

FOLIAGE. The leaves of the Caytoniales are placed in the morphogenus Sagenopteris and are quite distinctive; the taxon includes palmately compound leaves consisting of three to five lanceolate leaflets, each up to 7.0 cm long (Fig. 1; Harris 1964). Each leaflet has a central midrib, with lateral veins that initially branch dichotomously; subsequent branching forms a complex, reticulate venation pattern with anomocytic stomata (Barbacka and Boka 200Oa). Harris (1972) reconstructed the plant that bore Sagenopteris leaves as a small tree, based on compressed woody axes with attached bud scales from the Cayton Bay site.

POLLEN ORGANS. The pollen bearing organ, Caytonanihus Harris (1937), consists of a slender axis bearing flattened, lateral branches (pinnae; Fig. 6, 14); these are borne in a subopposite-opposite arrangement (Osborn 1994). Laterals branch irregularly and each ultimate segment bears one to three elongate synangia about 1.0 cm long, which have been variously interpreted as sessile or fused (Osborn 1994). Each synangium is made up of 3-4 pollen sacs (originally called locules) that are arranged around a central zone of tissue. They bore small (~25 µm in diameter) monosulcate, bisaccate pollen grains, which conform to the morphotaxon Vitreisporites Leschik if found dispersed (Fig. 2, 19) (Balme 1995). A number of studies have examined the ultrastructure of different species of Caytonanthus pollen grains (e.g., Krassilov 1977, Pedersen and Friis 1986, Zavada and Crepet 1985, Osborn 1994). The pollen wall is tectate-alveolate laterally and the sacci show an extensive network of endoreticulations which extend into the lumen of the saccus. Some have suggested that the grains are protosaccate (Pedersen and Friis 1986), but others have interpreted them as eusaccate, since the endoreticulations do not appear to be attached to the corpus wall (Zavada and Crepet 1985, Osborn 1994). Germination is presumed to have been distal, based on the presence of a broad sulcus and a thinner exine layer on that pole (Osborn 1994). To date, nothing is known about the transfer of pollen.

OVULATE ORGANS. The genus Caytonia includes reproductive organs that exhibit a slender axis about 5 cm long, bearing sub-oppositely arranged cupules, each recurved toward the main axis (Fig. 3, 15; Thomas 1933). At the point of attachment is a lip-like projection which Thomas interpreted as a stigmatic surface since he found pollen grains adhering to this area. Cupules are approximately 4.5 mm in diameter and produced between 8 and 13 orthotropous ovules along the midrib (Fig. 4). The stigmatic interpretation was subsequently discounted by Harris (1940) when he documented pollen grains inside the cupule and associated with the ovules. As a result of Harris’ work, the gymnospermous nature of the Caytoniales was affirmed. Subsequent work by Reymanówna (1973) uncovered delicate, cuticular canals that extended from the micropyle of each ovule to the lip of the cupule (Fig. 5). Pollen grains were apparently transported to the ovules via these canals, perhaps within a pollination droplet. This work showed conclusively that the seed-bearing unit Caytonia was a cupule, not a carpel, and that it functioned like a gymnosperm. It is not known whether swimming gametes or pollen tubes were produced.

Since the early descriptions of the Caytoniales, which were based on specimens from the Jurassic of Europe and Greenland, their range has been extended geographically and stratigraphically. Sagenopteris is worldwide in its distribution and has been described from Late Jurassic and Early Cretaceous floras of the northern and southern hemispheres (e.g., Herbst 1966, Bose and Banerji 1977, LaPasha and Miller 1985, MacLeod and Hills 1991, Boyd 1992), including Antarctica (e.g., Gee 1989, Rees 1993, Cantrill 200Oa). Although Sagenopteris-type leaves are relatively widespread, reproductive organs are far rarer, and it is probable that this morphotaxon may have been borne by more than one type of plant. Ovulate reproductive organs have been described from the Jurassic of Europe (e.g., Lundblad 1948, Reymanownaea Barbacka and Boka 2000b) and Australia (Clifford 1998), the Cretaceous of Siberia (Krassilov 1977) and the Antarctic peninsula (Barale et al. 1995). The latter flora was originally described as Triassic, but other analyses (Rees and Smellie 1989), and U-Pb dating of zircons (Loske 1988) suggest a Cretaceous age. Rees (1993) described the first example of Caytonanthus from the southern hemisphere, from Botany Bay on the Antarctic peninsula. The microsporophylls were associated with Sagenopteris leaves, which showed considerable morphological variability, including lobed leaflets. However, due to the presence of intermediate forms, the leaves were placed in a single species, S. nilssoniana.

Archangelsky (1963) described tripinnate leaves, which he named Ruflorinia sierra, and associated cupules of Ktalenia circularis from the Upper Cretaceous Ticó flora, Anfiteatro de Ticó Formation (Cladera et al. 2002) of Argentina. Taylor and Archangelsky (1985) examined the cuticular anatomy of these two taxa and suggested their assignment to the Caytoniales. Ktalenia is similar to Caytonia in overall morphology, but is smaller (3-4 mm wide) and contains only one or two ovules per cupule. More recently, Villar de Seoane (2000) has detailed cuticular structure in a new foliage species, Ruflorinia papillosa, also from Santa Cruz province, Argentina.

CAYTONIA AND THE ORIGIN OF THE ANGIOSPERMS. The earliest phylogenetic analyses of seed plant relationships, as well as some more recent studies, considered the cupule of Caytonia to be homologous to the anatropous, bitegmic ovule of the angiosperms (e.g., Crane 1985, Doyle and Donoghue 1986, 1987, 1992). This interpretation considers the outer wall of the Caytonia cupule homologous with the outer integument of the angiosperm ovule through a hypothetical reduction in the number of ovules per cupule to one (Gaussen 1945, Doyle 1978). When this assumption is coded for Caytoniales in phylogenetic analyses, the Caytoniales are often resolved sister to the anthophytes (Bennettitales, Gnetales, angiosperme, Pentoxylon), which form a monophyletic group, the anthophyte clade (e.g., Doyle and Donoghue 1987). Nixon et al. (1994) suggested that this assumption of homology affected the outcome of at least some previous analyses. Based on a reexamination of cupules of Caytonia, they coded the cupule as unitegmic and orthotropous. In their most parsimonious trees that included fossils (analyses I and III), Caytonia often formed a monophyletic group with the glossopterids outside of the ginkgophytes, and the Caytoniales did not place sister to the angiosperms in any trees (Nixon et al. 1994). Rothwell and Serbet’s (1994) analysis also placed Caytoniales sister to the glossopterids. Later, Doyle (1996) published revised seed plant analyses in which Caytonia was often resolved sister to the angiosperms. According to this work, synapomorphies shared by the angiosperms and Caytonia in some trees included: flat guard cells, pinnate megasporophylls, anatropous cupules (bitegmic ovules), and loss of nucellar vasculature (Doyle 1996).

The results from more recent analyses utilizing molecular sequence data from extant seed plants challenge the monophyly of the extant anthophytes; some suggest that the extant gymnosperms are monophyletic, or at least the Gnetales are not the extant sister group to the angiosperms (e.g., Bowe et al. 2000, Chaw et al. 2000, Soltis et al. 2002, Burleigh and Mathews 2004). However, even analyses based only on molecular sequence data have failed to stabilize on a single set of relationships among major groups of extant seed plants (Rydin et al. 2002). The position of Caytonia among the seed plants and interpretation of the homology of the cupule thus remains in dispute. It should be noted that coding of Caytonia (Caytoniales) in the analyses above is based on organ taxa thought to represent different parts of the whole plant, not just the ovulate organ.

Corystospermales. Since their initial description in 1933, the corystosperms have become perhaps the best-known group of Mesozoic seed ferns. Thomas (1933) circumscribed the group (as family Corystospermaceae) from the Molteno Formation (Upper Triassic) of South Africa. He described forking fronds (Dicroidium Gothan), ovulate organs (Umkomasia, Pilophorosperma, Spermatocodon) and microsporangiate organs (Pteruchus), which were united based on close association in the same beds, cuticular similarities, and the presence of bisaccate pollen grains in the cupules and within the pollen chamber of one of the seeds.

FOLIAGE. Dicroidium Gothan is the most common foliage type in Gondwana during the Triassic and the typical bifurcating fronds are found on all Gondwanan continents (Fig. 7, 16). Townrow (1957), based on a detailed analysis of cuticle, included all fronds with bifurcating foliage in Dicroidium, but the range of morphologies within this morphotaxon is very large, ranging from simple (non-pinnate) to bipinnate (Fig. 18) and possibly tripinnate fronds, with entire, pinnatifid, and needle-like pinnules. Most authors today segregate Johnstonia Walkom and Xylopteris Frenguelli, which are differentiated based on their overall morphology and venation architecture (see, e.g., Retallack 1977, Petriella 1979, Baldoni 1980). Gnaedinger and Herbst (1998, 2001), based on their extensive studies of Upper Triassic floras in Chile and Argentina, also include Diplasiophyllum Frenguelli and Zuberia Frenguelli as corystosperm foliage taxa, although Retallack (1977) suggests they should be included in Dicroidium, based on his studies of Australian floras. Anderson and Anderson (1983) named 17 subspecies and 14 forms within Dicroidium, but later elevated all the subspecies to species (Anderson and Anderson 1989). Pigg (1990) described the first anatomically preserved Dicroidium foliage, from the Middle Triassic of Antarctica (Fig. 17). She detailed pinnule variation within a single frond (Fig. 20), which ranged from entire margins basally to lobed more distally. Rachis anatomy compared in complexity with that in some extant cycads (Fig. 21).

Northern hemisphere foliage that has been assigned to the corystosperms includes Pachypteris Brongniart, an unbranched bipinnate-tripinnate frond, which is known throughout the Jurassic of Europe (e.g., Thomas 1954, Barbacka 1994, Cleal and Rees 2003), and Thinnfeldia Ettingshausen. Based on cuticular anatomy, however, Doludenko (1971; Doludenko et al. 1998) suggested that Thinnfeldia is conspecific with Pachypteris and should be considered a later synonym. Although Pachypteris is primarily known from the Jurassic of the northern hemisphere, it has also been described from the Jurassic (e.g., Townrow 1965, Baldoni and Barale 1996) and Cretaceous of Gondwana (e.g., McLoughlin 1996, Cantrill 2000b). Pachypteris had been considered a probable corystosperm based on its association with Pteroma (Harris 1964), a pollen organ similar to Pteruchus, but Townrow (1965) formally placed it in the group based on stomatal similarities and association with Pteruchus petasatus in the Rhaeto-Liassic of Tasmania (Townrow 1965). Since then, the foliage (described as Thinnfeldia) has been found associated with both Pteruchus pollen organs and Umkomasia ovulate structures in the German Jurassic (Kirchner and Müller 1992).

STEMS AND PLANT HABIT. Thomas (1933) found no anatomically preserved stem material associated with the Molteno plants, but the Ischigualasto Formation (Upper Triassic) in Argentina has yielded permineralized stems assignable to Rhexoxylon Bancroft emend. Archangelsky et Brett which co-occur with Dicroidium fronds (Archangelsky 1968); additional descriptions have confirmed this association. Rhexoxylon has unusual anatomy, consisting of several cycles of vascular tissue. The cycles are variously constructed of both centripetally and centrifugalIy developed secondary xylem, depending upon the species (Artabe et al. 1999). The wedges of secondary xylem are separated by areas of parenchyma that are continuous with the large pith (Fig. 13) (Archangelsky and Brett 1961, Brett 1968). This structure has prompted several authors to suggest that Rhexoxylon had a scrambling habit rather than an upright trunk (e.g., Wallon 1923). However, the description of very large diameter specimens, such as R. brasiliensis (47 × 37 cm; Herbst and Lutz 1988) and R. brunoi (71 × 58 cm; Artabe et al. 1999) brings this theory into question; perhaps some species were scramblers or leaners and others were upright (see also Brett 1968). Artabe et al. (1999) have summarized the anatomy of the known species of Rhexoxylon and noted that differential cambial activity accounts for the differences in numbers of cycles of vascular tissue and amount of parenchymatous tissue in these axes. Based on material from the Ischigualasto Formation, Petriella (1978, 1981, 1983) reconstructed the Dicroidium plant with a Rhexoxylon-type trunk to be a medium-sized, unbranched tree with a habit similar to that of a modern tree fern (Fig. 12).

Although Rhexoxylon was originally described from southern Africa (Bancroft 1913), the age and locality of this material was uncertain. There have been subsequent reports from various horizons which seem to range from the Triassic to the Early Jurassic (Anderson and Anderson 1983); e.g., from Zimbabwe (Seward and Holttum 1921, Walton 1956); from the upper Beaufort and Stormberg Groups in South Africa (Walton 1923); and from Mali (Walton 1956), but most are based on only one or two specimens. Walton (1923) included Antarcticoxylon Seward in Rhexoxylon, even though this single specimen from southern Victoria Land, Antarctica does not exhibit the characteristic alternating wedges of wood and parenchyma; the species was later restored to Antarcticoxylon by Archangelsky and Brett (1961). As a result, some of the older reports of Rhexoxylon from southern Africa may include specimens that do not conform to this genus (e.g., Walton 1925). In addition, Anderson and Anderson (1983) note that Rhexoxylon has never been found associated with Dicroidium in South Africa; it is also found in the Red Beds, which are dated as Early Jurassic, where Dicroidium does not occur. To our knowledge, Rhexoxylon has not been reported from Australia, and there is only one fragmentary report from the Middle Triassic of Antarctica (Taylor 1992).

Meyer-Berthaud et al. (1992, 1993) described an additional type of corystosperm stem from the Fremouw Formation (Middle Triassic) of the central Transantarctic Mountains (Taylor et al. 1989). Kykloxylon fremouwensis includes small leafy stems, each with a ring of primary bundles surrounding a central pith. Leaf bases still attached to the stems exhibit the same characteristic secretory cavities that occur in Dicroidium fremouwensis leaves from the same site (Fig. 17) (Pigg, 1990); the presence of these distinctive secretory cavities has been used to unite all the organs of the plant (Taylor 1996; Klavins et al. 2002). Unlike Rhexoxylon, these axes contain dense secondary xylem and prominent growth rings (Fig. 11) (E. Taylor and Taylor 1993; Cúneo et al. 2003), as well as axillary branching (Meyer-Berthaud et al. 1993). Del Fueyo et al. (1995) described large woody stems from an in situ, Middle Triassic fossil forest in the Gordon Valley, central Transantarctic Mountains. Primary xylem was not preserved in these stumps, so they could not be correlated with existing wood morphotaxa and were placed in a new genus, Jeffersonioxylon. Stumps were up to 61 cm in diameter and the trees were reconstructed to have reached a maximum height of ~31 m (Fig. 10) (Cúneo et al. 2003). Although del Fueyo et al. (1995) noted anatomical similarities with podocarpaceous wood, the trunks of Jeffersonioxylon were surrounded by and rooted in shales containing abundant Dicroidium fronds (Cúneo et al. 2003). Based on their analysis of the depositional environment of these stumps and leaves, Cúneo et al. (2003) concluded that the Dicroidium leaves and Jeffersonioxylon trunks were produced by the same plant, so this large forest tree correctly belongs in the Corystospermales (see Table 1). Retallack (1977) has also reconstructed some Dicroidium plants from the Middle Triassic of eastern Australia as woodland trees.

Meyer-Berthaud et al. (1993) presented anatomical evidence that Dicroidium leaves in Antarctica were seasonally deciduous, and this has been supported by sedimentologic data (Cúneo et al. 2003). Based on all these data, the ‘Dicroidium plant’ in Antarctica has been reconstructed as a ~20-30 m tall forest tree, with dense, pycnoxylic wood and axillary branching producing a bushy crown (Taylor 1996, Taylor et al. 2000, Cúneo et al. 2003). Although the habit of the trunk was no doubt similar to modern conifers, these large trees bore deciduous Dicroidium fronds.

Another stem that has been placed in the corystosperms is Tranquiloxylon Herbst and Lutz (1995). This axis, unlike Rhexoxylon, has a continuous ring of secondary xylem surrounding the primary xylem and pith. However, unlike Kykloxylon, the outer xylem is lobed, with wedges separated by areas of parenchyma. In the northern hemisphere, Harris (1983) described the stem that bore Pachypteris papillosa foliage as somewhat fleshy, up to 50 mm in diameter, with the surface covered by leaf bases and epidermal blisters. He suggested that it was probably shrubby in its habit, somewhat like modern sumac (Rhus sp.).

Based on current evidence, there were at least two different plant habits represented by the Gondwana corystosperms: the unbranched, tree-fern-like plant from the Ischigualasto Formation of Argentina (Petriella 1978), and the larger, more conifer-like plant from the Fremouw Formation of Antarctica (Meyer-Berthaud et al. 1992, Taylor 1996). These two reconstructions may reflect paleogeographical differences between western (South America and South Africa) and eastern Gondwana (East Antarctica and Australia) (Meyer-Berthaud et al. 1993). At present, the habit of the plants that bore Dicroidium leaves in Australia, South Africa, and India is unknown.

POLLEN ORGANS. Although there are differences in vegetative morphology in the corystosperms, the reproductive morphology appears to be fairly consistent throughout the Triassic of Gondwana and even into the Jurassic of Europe (Kirchner and Müller 1992). The most widely distributed pollen-bearing unit, Pteruchus Townrow is known throughout Gondwana and is fairly uniform in its morphology, consisting of a narrow central axis (usually

There has been disagreement on the exact morphological nature of Pteruchus, with some describing it as an axis bearing microsporophylls, and others as a rachis with pinnae or pinnules attached to it. The discovery of anatomically preserved specimens from Antarctica (Yao et al. 1995) showed that it represents a fertile branch bearing helically arranged microsporophylls. The microsporophyll is leaflike, with a basal stalk and a distal flattened head (Fig. 23). Although Pteruchus has not yet been found attached to axes bearing Dicroidium leaves, Yao et al. (1995) found the same secretory cavities in the microsporophyll and pollen sac wall (Fig. 24) that had previously been described from D. fremouwensis leaves (Pigg 1990) and K. fremouwensis stems (Meyer-Berthaud et al. 1992, 1993), thus confirming the reconstruction of these Antarctic specimens as a single plant. On other continents, compressed Pteruchus has been widely found associated with Dicroidium fronds, for example, in South Africa (Thomas 1933), Tasmania (Townrow 1965), India (Pant and Basu 1973), Antarctica (Cantrill et al. 1995), and South America (Petriella 1980, Guerra-Somer et al. 1999).

Harris (1964) described Pteroma from the Yorkshire Jurassic as the probable pollen organ of Pachypterls papillosa. Pteroma pollen sacs are more deeply embedded in the microsporophyll than those of Pteruchus and are arranged on the abaxial surface in either two rows or in an elongated oval. However, Harris notes that this material is very poorly preserved, and there were few specimens recovered. He particularly notes that it is impossible to discern the arrangement of the laterals in these specimens, so that it could represent a Pteruchus. The pollen is identical to that recovered from all known species of Pteruchus.

OVULATE ORGANS. Like the pollen organs, the ovulate reproductive organs of the corystosperms are also remarkably similar throughout Gondwana. Thomas (1933) delimited three genera, Umkomasia, Spermatocodon and Pilophorosperma, based on branching and cupule morphology as well as cuticle structure. Holmes (1987) synonymized Pilophorosperma with Umkomasia based on the preservation of new material from Australia, along with Karibacarpon Lacey (Lacey 1976). Spermatocodon was based on very fragmentary specimens and is poorly known (Thomas 1933). Umkomasia is the most widely known morphogenus and consists of a main axis that bears lateral branches, each with one or more pairs of uniovulate, helmetshaped and recurved cupules (Fig. 9). The ovules are enclosed except for an extruded, bifid micropyle, which extends beyond the distal margin. Thomas (1933) noted that the lateral branches were subtended by bracts, but Holmes (1987) illustrated some specimens without bracts and suggested that they may have been shed at some point. Thomas (1933) noted that some cupules were divided longitudinally by a deep cleft. As in Pteruchus, there has been disagreement over the arrangement of the laterals on the main axis, due to the preservation of the axes as flattened compressions and impressions. Thomas interpreted the cupular unit (branch with cupules) as a pinna, believing it was homologous with a leaf and thus flattened, and Holmes (1987) described the cupules as opposite or alternate in their arrangement. Klavins et al. (2002) described permineralized Umkomasia from Antarctica and determined that the main axis is a determinate branch with helically arranged cupules, based on axis anatomy and lateral trace production. The bilobed cupules each contain two ovules, but the two most distal cupules are unlobed and uniovulate (Fig. 9, 27). They suggest that ovule number may vary with position on the branch and number of lobes with developmental stage. Based on this material, and a re-examination of Thomas’ type material, they emended the generic diagnosis to reflect helical arrangement of laterals and the character of either one or two ovules per cupule (Klavins et al. 2002). The same type of secretory cavities that were present in the leaves of Dicroidium fremouwensis (Pigg 1990), the pollen organ Pteruchus fremouwensis (Yao et al. 1995) and the stem Kykloxylon fremouwensis (Meyer-Berthaud et al. 1992, 1993) were also present in the axes, cupule wall and integument of Umkomasia resinosa (Klavins et al. 2002). Aside from organic connection, the best paleobotanical evidence for attribution of dispersed organs is anatomical, and this attribution is more secure when the anatomical evidence is a unique feature, such as these secretory cavities with a one-cell thick epithelial layer. In this case, the reconstruction of the Dicroidium plant from the Middle Triassic Fremouw Formation of Antarctic follows the same method employed by Oliver and Scott (1904) in their reconstruction of the Paleozoic lyginopterid plant, the first accepted evidence for the seed ferns.

Compressed and impressed fossils of Umkomasia are known from all Gondwana continents except India, although both Dicroidium and Pteruchus occur there (e.g., Bose et al. 1990), and it has also been described from the Jurassic of Germany (Kirchner and Miiller 1992). In the Late Triassic of Antarctica, lateral branches bearing compressed Umkomasia cupules (Fig. 30) have been found attached to axes that also bear fronds of Dicroidium, providing the first evidence of actual organic attachment of these two morphotaxa (Axsmith et al. 2000). This species, U. uniramia, consists of a whorl or pseudowhorl of 5-8 uniovulate stalked cupules (Fig. 28, 37) that are borne subapically on a short shoot; D. odontopteroides fronds are attached to the main axis above the level of the short shoots (Fig. 22, 26). This dimorphic shoot system provides additional evidence for the gymnospermic affinities of these plants. Several authors have questioned the attachment evidence in these Antarctic plants on the basis that the fossil shoots are too robust to bear leaves (Anderson and Anderson 2003) and that the leaves are not attached to short shoots (Holmes and Anderson 2005). Holmes and Anderson (2005) suggest that the attached Umkomasia may belong to another leaf type such as Heidiphyllum or Taeniopteris, although these leaf types are very rare at this site. In modern plants with dimorphic shoots, such as Ginkgo biloba, leaves are attached to both short and long shoots; at the tip of each stem, leaves are attached to this year’s new growth (i.e., a long shoot), and more proximally, leaves are attached to short shoots which grew from the axils of previous years’ leaves. The morphological differences between U. uniramia and other species of Umkomasia, such as the diameter of the main axis and the arrangement of the cupule stalks in a whorl or pseudowhorl, rather than a helix, may simply be a consequence of the growth of these plants at very high latitudes (70-75° S paleolatitude) with short growing seasons.

PHYLOGENETIC POSITION. Generally, the corystosperms are represented in seed plant phylogenetic analyses as a single composite terminal coded for the characteristics representing various detached organs assigned to Corystospermales (e.g., Crane 1985, Doyle and Donoghue 1992 and others cited therein; Rothwell and Serbet 1994, Doyle 1996). Nixon et al. (1994) state that their terminal represented the reconstruction of “Corystospermum”, a terminal definition based on South American corystosperm organs; this definition was also followed by Albert et al. (1994). Other analyses, such as Crane (1985), included both northern and southern hemisphere corystosperms, as well as some incompletely known taxa, e.g., Pteroma. Possibly as a result, there is conflict as to the position of this group in various phylogenetic analyses (compare, e.g., Crane 1985, Doyle and Donoghue 1986, 1987, 1992; Albert et al. 1994, Nixon et al. 1994, Rothwell and Serbet 1994, Doyle 1996). Rothwell and Serbet (1994) included characteristics of Petriellaea Taylor et aJ. (1994) in their coding of the corystosperms, because at the time it was thought to represent a corystosperm, based on preliminary data (Taylor and Taylor 1987); Petriellaea was eventually placed in its own order (see below). Currently, the position of the Corystospermales remains unresolved within the seed plants.

Petriellales. The Middle Triassic permineralized peat from the Fremouw Formation also yielded a second anatomically preserved ovulate organ, Petriellaea Taylor, del Fueyo et Taylor, a cupulate structure that bears multiple ovules (Fig. 31, 32) (Taylor et al. 1994). The cupules are attached at the ends of an axis that dichotomizes once; further attachment to a main axis or parent plant is not known. Based on dehisced cupules in the permineralized peat, they were probably borne in clusters (Taylor et al. 1994). The cupule is leaflike in its anatomy and completely encloses five or six small sessile ovules that are triangular in cross section. The ovules are attached to the adaxial surface of the megasporophyll, and some contain megagametophytes (Fig. 33). Since this structure was unlike any known group of Mesozoic seed ferns, it was placed in its own order, the Petriellales (Taylor et al. 1994). The affinities of Petriellaea are not currently known, but neither the cupule wall nor the integument of the ovules contains any of the distinctive secretory cavities that have been found in the various organs of the Dicroidium plant from this Middle Triassic peat. Petriellaea’s morphology is interesting as the cupule is formed by the ontogenetic or phylogenetic “folding” of the megasporophyll from tip to base (Fig. 38). While this morphology is unlike the angiosperm conduplicate carpel, it does resemble the cupule of Caytonia. However, the attachment of Petriellaea cupules at the tip of branches and the apparent clustering of cupules is unlike the arrangement of cupules in Caytonia.

Peltaspermales. The Peltaspermales were established by Thomas (1933, as Peltaspermaceae), and were originally known from the Upper Triassic of South Africa (Thomas 1933) and Greenland (Harris 1937), based on associated foliage, pollen organs, and seedbearing parts. In Thomas’ work, foliage and ovulate organs were given the name Lepidopteris. Although the circumscription of the group has been expanded since, it broadly includes bi- to tri-pinnate frond-like foliage and seed-bearing organs that consist of megasporophylls or cupular discs bearing ovules on their abaxial surface. The cupules are somewhat thick walled, showing wrinkles in compression specimens, and are best described as cupulate heads or discs (Harris 1937) since they do not enclose the ovules. Today the group is known from the Upper Carboniferous to the Triassic and has a worldwide distribution.

FOLIAGE. Lepidopteris Schimper was originally described from Europe, but is now known from both the northern and southern hemispheres (e.g., Townrow 1956; 1960). It is the most common peltasperm foliage genus in Gondwana, where it has been described from Triassic rocks on all continents (e.g., Baldoni 1972, Holmes 1982, Anderson and Anderson 1985, Banerji 1997, McLoughlin et al. 1997, Gnaedinger and Herbst 1998, Retallack 2002) The bipinnate frond bears subopposite to alternate pinnae and alethopteroid pinnules (Fig. 43). Intercalary pinnules (zwischerfiedern) are attached to the rachis between the primary pinnae and are characteristic of the genus (Townrow 1957, 1960). Originally described from the Upper Triassic of Europe and the Arctic (e.g., Antevs 1914, Harris 1932, Lundblad 1950), the genus is also known from Permian rocks in Europe (e.g., Barbacka 1991). Townrow (1960) reconstructed Lepidopteris foliage with ovulate and pollen organs of Peltaspermum Harris (1937) and Antevsia Harris, respectively, based on the presence of small blisters on the epidermis of all parts; these epidermal features and the intercalary pinnules make the genus distinctive.

Callipteris Brongniart non Bory is a widespread morphogenus of foliage from the northern hemisphere that is also placed in the peltasperms. It was originally used as a biostratigraphic marker for the Lower Permian in Europe (Kerp 1982, 1988), but is now known from Carboniferous, Permian and Triassic rocks of Europe (Kerp 1988), Russia (Meyen and Migdisova 1969), North America (Remy et al. 1980; Dimichele et al. 2004), and the Arctic (Vasilevskaya 1987). The generic name has been very broadly applied in the past; a number of the European species have been reexamined and placed into other morphogenera of callipterid foliage, such as Rhachiphyllum Kerp, Lodevia Haubold and Kerp, Arnhardtia Haubold and Kerp, Sphenocallipteris Haubold and Kerp, and Dichophyllum Elias (Kerp 1988; Kerp and Haubold 1988). Kerp and Haubold (1988) placed CaIlipteris conferta-type foliage and the reproductive organs associated with it (both ovulate and pollen organs) in the morphogenus Autunia Krasser (Fig. 41), and formally assigned these fossils to the Peltaspermaceae. Callipteris conferta, the most common species, has bi- or tripinnate fronds with alethopteroid pinnules.

Other foliage taxa assigned to the peltasperms include Tatarina Meyen from the Russian platform (Meyen and Gomankov 1980), Scytophyllum Bornemann (renamed Dellephyllum by Doweld, 2001) which is known from both Russia and Argentina (e.g., Zamuner and Artabe 1990), Vittaephyllum (Dobruskina 1975), and Pursongia-like leaves (Naugolnykh 2001).

POLLEN ORGANS. The pollen organs of the peltasperms were originally correlated with Lepidopteris foliage by Antevs (1914), based on the presence of the distinctive blisters on the epidermis of both structures. Antevs examined cuticle from a large number of fronds of L. ottonis and noted that the cuticle structure clearly indicated seed plant affinities, rather than that of ferns. He suggested that these plants belonged to some Mesozoic successor of the Paleozoic seed ferns (Antevs 1914). Harris (1937) erected the genus Antevsia for these structures, since their original name, Antholithus Nathorst (1908), was not a genus per se, but a term used for any fossil “flowers”. Townrow (1960) emended Antevsia, based on material from South Africa and Argentina, as a bipinnate, planar microsporophyll with alternately arranged primary branches (pinnae). Primary branches are divided to form two to five ultimate branchlets, each of which bears four to twelve sessile pollen sacs in two rows on the abaxial surface (Fig. 44). The rachis and primary branches exhibit blister-like swellings. Pollen grains are oval, monosulcate, and small (30-40 µm long), with smooth walls (Fig. 40) and correspond to the dispersed taxon, Cycadopites Wodehouse (Balme 1995).

OVULATE ORGANS. The morphotaxon PeItaspermum Harris (1937) includes the most commonly encountered ovulate organs (megasporophylls) of this group and has a worldwide distribution. Harris originally defined the taxon as consisting of an axis bearing alternate branches, which dichotomized several times and bore terminal, peltate discs (Fig. 45) with ovules on the lower surface (Fig. 39). The genus was emended by Townrow (1960) to consist of a central axis bearing alternate branches in a single plane. Each branch terminates in a lobed, cupulate disc or head. In P. thomasii Townrow (1960) each cupulate head is attached at its margin to the stalk, instead of centrally (peltate) as in P. rotula Harris (1937). In the latter species, 10-12 ovules are attached to the lower surface or the disc, while P. thomasii bears just two ovules per head. All species exhibit the characteristic blisters on the rachis and primary branches. As additional species have been described, however, the concept of the genus has broadened considerably (see, e.g., Gomankov and Meyen 1979).

Kerp (1988; Kerp and Haubold 1988) included the genus Autunia Krasser in the PeItaspermaceae. Originally a morphotaxon for ovuliferous organs, the genus was emended to include foliage of Callipteris conferta, ovulate organs (Fig. 34, 35), and pollen organs previously assigned to Pterispermostrobus Slopes. The ovulate organs of this Permian plant consist of helically arranged, bilaterally symmetrical megasporophylls which are attached to the axis by a marginal petiole. They are flabelliform and leaf-like, with one, or perhaps two, ovules attached to the abaxial (lower) surface. The pollen organs are small, cup-shaped structures constructed of up to nine elongate pollen sacs that are fused only at the base, originally described as Pterispermostrobus gimmianus by Remy (1954) and correlated with Callipteris by Barthel and Kozur (1981). Barthel later described the same type of pollen organ attached to Arnhardtia scheibei foliage (Barthel 2001). Pollen is bisaccate and of the Vesicaspora type (Kerp 1988). Callipterianthus Roselt (1962) (Fig. 36) is another type of peltasperm pollen organ from the Permian of Europe.

Our understanding of the basic morphology of peltasperm ovulate organs was further confounded by the description of a reproductive organ associated with Supaia-type fronds from China (Wang 1997). This structure was described as a new species of Autunia and consists of a cone bearing wedge-shaped megasporophylls, which are described as both peltate and bilaterally symmetrical, as seemingly contradictory condition. Dispersed peltate structures show distal lobes, although none of the attached structures have this morphology. Although the diagnosis states that ovules were attached to the adaxial surface, no ovules were illustrated or described. Wang (1997) compares this Autunia with a pollen organ described by Meyen (1982) as Peltaspermum?.

As noted above, Lacey (1976) delimited a new morphogenus of ovulate organs from Zimbabwe (then Rhodesia) as Karibacarpon. Holmes (1987) included these specimens in Umkomasia, based on the discovery of additional specimens from New South Wales, Australia (Holmes and Ash 1979). However, it is unknown whether the original specimens were re-examined or this change was based only on the new material. Retallack (1977) noted that Karibacarpon appeared to share more characters with peltasperms than with corystosperms, and Lacey (1976) noted that both Dicroidium and Lepidopteris occur at the same locality as Karibacarpon, so perhaps this taxon also represents a peltasperm reproductive organ, rather than an Umkomasia.

The amount of morphological variation within the ovulate organs of the peltasperms is large. In fact, the morphological variability in Peltaspermum alone suggests that more than one taxon, and perhaps more than one group of plants may currently be included in this morphotaxon. Gomankov and Meyen (1979) summarized the known variation when they described P. buevichae from Russia. They noted that Peltaspermum heads could be radially or bilaterally symmetrical; the individual heads or discs could be loosely arranged on the primary axis or grouped together into compact heads; and the number of ovules varied from two (P. thomasii Townrow) to 14 (P. buevichae Gomankov and Meyen 1979). Since these ovulate organs have been correlated with various morphotaxa of leaves, based either on association or on epidermal similarities, it is likely that the genus will be subdivided as more complete plants become known. Peltasperm ovulate organs first appear in the latest CarboniferousEarly Permian of the northern hemisphere (Kerp et al. 2001), and both the disc-like megasporophylls (e.g., Naugolnykh and Kerp 1996; Liu and Yao 2000), and bilaterally symmetrical forms (e.g., Kerp 1988) are present at this time. Although Kerp et al. (2001) reported Peltaspermum in a mixed Gondwana-Cathaysia-Angara flora from the Lower Permian of Morocco, typical Lepidopteris foliage does not appear in Gondwana until the Triassic (Retallack 2002).

PHYLOGENETIC POSITION. Peltaspermales are usually represented in cladistic analyses by a single composite terminal coded for characteristics representing various detached organs assigned to the Peltaspermales (Crane 1985. Rothwell and Serbet 1994), or a single terminal represented by a reconstruction (e.g., Peltaspermum in Doyle and Donoghue 1986, 1987, 1992; Doyle 1996). In the analyses of Nixon et al. (1994) and Albert et al. (1994), the peltasperms are represented by two composite taxa, Lepidopteris (Peltaspermum of other analyses) and Tatarina. These terminals represent, respectively, reconstructions of Late Triassic peltasperms and Late Permian Angaran forms, based on vegetative and reproductive organs found in association with these leaf taxa. In the strict consensus of all most parsimonious trees found in Nixon et al.’s analysis I (angiosperms not treated as a single terminal) and the three total evidence analyses of Albert et al. (1994), the positions of Lepidopteris and Tatarina are unresolved relative to one another and to certain other seed plant taxa and clades. In the strict consensus of trees found for analysis HI of Nixon et al. (1994) (angiosperms treated as a single terminal), Lepidopteris is resolved as sister to the Corystospermales, and Tatarina is resolved sister to a clade composed of the anthophyte taxa (including Pentoxylon), Cordaites, and the extant gymnosperms excluding the cycads. These results suggest, perhaps unsurprisingly, that the Peltaspermales may not be a monophyletic group, and additional fossil descriptions which have appeared since these analyses were done appear to support this conclusion.

Conclusions. Although our knowledge of the so-called Mesozoic seed ferns has increased tremendously in the last 10-15 years, we are still a long way from our level of understanding of the Carboniferous seed ferns. The major reason for this knowledge gap is preservation. Since the Carboniferous pteridosperms are known from both compressions and permineralizations (coal balls), in many groups we have evidence of entire life histories from development to reproductive biology, including a pollination droplet (Rothwell 1977) and microgametophyte (Millay and Eggert 1974). In the three major Mesozoic groups, we have evidence for pollination syndrome only in the Caytoniales, as represented by the presence of pollen grains in the tubes within the Caytonia cupule. As yet, we do not know how pollen was moved from the pollen organs to the ovules, even in the Caytoniales. In some cases, i.e., the peltasperms, two different types of pollen have been found associated with pollen organs which exhibit the same basic morphology. In most cases, we do not know how or where reproductive axes were attached to the parent plant.

Adding to the systematic confusion within the Mesozoic seed ferns is the fact that some plants have been reconstructed based on a single co-occurrence of organs, rather than on a consistent, repeatable association, and groups have been reconstructed for phylogenetic analyses using organs from different formations, different continents, and even different time periods. It is clear that the three major orders of Mesozoic seed ferns are more diverse than their original concepts and, in some cases, this increased diversity has not yet been well accommodated within the current taxonomy. This is particularly true if these plants are classified (including Paleozoic groups) within a single order, the Pteridospermales, as has been done in the past. It is important that taxonomic assignments and plant reconstructions be based on careful analysis of all the available evidence, including morphology, cuticular structure, and where available, internal anatomy, and that these data be analyzed within a phylogenetic framework. Although the seed ferns were established one hundred years ago (Oliver and Scott 1904), the Mesozoic groups were not delimited until 25-30 years later, and were not understood to be seed ferns, rather than angiosperms, for some years after their establishment. However, in recent years, there has been a renewed focus on the Mesozoic seed ferns from both northern and southern hemispheres. More data have become available on plant habit, paleoenvironment and systematics, and we are rapidly moving towards a more global understanding of the distribution and evolutionary relationships of these interesting seed plants.

1 This contribution is dedicated to the memory of Wilson N. Stewart-teacher, colleague and friend-for his many contributions to our understanding of the seed ferns. This material is based upon work supported by the National Science Foundation (OPP 0126230, 0229877).

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Edith L. Taylor2 and Thomas N. Taylor

Department of Ecology and Evolutionary Biology, and Natural History Museum and Biodiversity Research Center, 1200 Sunnyside Ave., University of Kansas, Lawrence, KS 66045

Hans Kerp

Forschungsstelle für Paläobotanik, Westfälische Wilhelms-Universität Münster, Hindenburgplatz 57, D-48143 Münster, Germany

Elizabeth J. Hermsen

Department of Ecology and Evolutionary Biology, 1200 Sunnyside Ave., University of Kansas, Lawrence, KS 66045

2 Author for correspondence: eTAYLOR@KU.edu

Received for publication July 15, 2005, and in revised form September 26, 2005.

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