«There have been many proposals of candidates for the ancestors or closest relatives of angiosperms. Some of the currently more frequently cited ...»
A General view of the reproductive organ. Note the multiple cupules physically connected to the same axis (GBM1, SFLBG). Bar = 1 cm. B Detailed view of the cupules helically arranged around the axis. Note the cupule stalks (black arrows) and the stub of another broken one suggesting a helical arrangement along the axis (a). Bar = 1 mm. C Diagram of a longitudinal section of a caytonialean cupule, showing cupule stalk, basal cupule opening, and seeds/ovules inside the cupule (from Wang 2010, courtesy of JSE) then separated from the outside by post-fertilization plugging of the canals (Harris 1933, 1940, 1964; Reymanowna 1973; Krassilov 1977a; Nixon et al. 1994). These characters clearly place Caytonia in gymnosperms rather than angiosperms.
It is generally believed that the Caytonia cupule is derived from megasporophyll that has become folded or recurved transversely, unlike the conduplicate carpel in angiosperms which is folded longitudinally (Taylor et al. 1994; Doyle 2008; Taylor and Taylor 2009). The pollen organ, Caytonanthus, moreover, has 3–5 microsporangia in a group, unlike tetrasporangiate stamen in angiosperms (Nixon et al. 1994;
Frohlich and Parker 2000). Nonetheless, Caytonia remains one of the most favored candidates for angiosperm ancestry (Krassilov 1977b; Hill and Crane 1982; Crane 1985; Doyle and Donoghue 1986a; Doyle 1998, 2006; Taylor et al. 2006b; Taylor and Taylor 2009). Since an angiospermous ovule usually has double integuments and the ovule of Caytonia is thought to be unitegmic, the cupule of Caytoniales is thought to be the equivalent of the outer integument in angiosperms (Crane 1986;
Nixon et al. 1994; Doyle 2006, 2008). A credible relationship between Caytoniales and angiosperms clearly remains speculative until a reasonable interpretation of the de novo origin of either the carpel or outer integument is evidenced by fossils, plus reduction of seed number to one per cupule appears in the fossil record (Nixon et al.
1994; Rothwell and Serbet 1994; Rothwell et al. 2009; Taylor and Taylor 2009;
Soltis et al. 2004).
The Chinese specimen of Paracaytonia (Caytoniales), unequivocally demonstrates that the arrangement of cupules along the axis is spiral rather than pinnate (Wang 2010; Fig. 2.5), suggesting that the so-called rachis is a true axis rather than 12 2 Suggested Angiosperm Ancestors a true rachis, as had been commonly thought (Doyle 2006, 2008; Taylor and Taylor 2009). This new information is important because many former interpretations of Caytonia are based on the assumed pinnate foliar nature of the whole organ, which is supposed to expand and turn into a conduplicate carpel (Doyle 2006, 2008; Taylor and Taylor 2009). Thus the Chinese material widens the gap between Caytoniales and angiosperms.
Bennettitales range from the Middle Triassic to Late Cretaceous in age, including two families: the Cycadeoidaceae (with stout trunks and bisporangiate reproductive structures) and Williamsoniaceae (with slender, branching trunks, and either bisporangiate or monosporangiate strobili). Their reproductive organs have been documented from North America, Europe, Greenland, India, and China (Wieland 1899a, b, c, 1901, 1911, 1912; Harris 1944, 1967, 1969; Ye et al. 1986; Pedersen et al. 1989b; Nixon et al. 1994; Sun et al. 2001; Li et al. 2004; Crane and Herendeen 2009; Rothwell et al. 2009; Friis et al. 2009). The orthotropous ovules, sometimes with elongated funiculi, are interspersed with sterile interseminal scales on a conical ovulate receptacle at the center of their reproductive structures (Crane and Herendeen 2009; Rothwell et al. 2009). Outside of this structure, if bisexual, are microsporophylls bearing pollen sacs on their adaxial surfaces containing monocolpate pollen grains. Outermost are several whorls of bracts resembling the tepals of angiosperms (Nixon et al. 1994; Crane and Herendeen 2009; Friis et al. 2009;
Rothwell et al. 2009).
The Bennettitales have been considered to be possible ﬂowering plant ancestors because of their bisexual ﬂower-like reproductive structures (Arber and Parkin 1908; Crane 1985; Doyle and Donoghue 1987; Nixon et al. 1994). The ovulate structure of Bennettitales is thought to be similar to an angiosperm carpel (Arber and Parkin 1907; Crane 1985; Doyle and Donoghue 1987). The presence of oleanane in Bennettitales adds further evidence to its possible relationship to angiosperms (Taylor et al. 2006a). Bennettitales, Gnetales and angiosperms are frequently grouped together in the anthophyte clade (Crane 1985; Doyle and Donoghue 1986a, b, 1987). These three groups share the minimized development of the gametophyte, together with rapid fertilization and embryogenesis after pollination (Pedersen et al. 1989b). Based on the similarities of seeds, Friis et al. (2009) proposed that Bennettitales, Erdtmanithecales, and Gnetales be grouped as the socalled BEG clade within a wider clade anthophyte. However, this interpretation faces some challenge due to possible mismatch of pollen in the reconstruction of Erdtmanithecales (Rothwell et al. 2009; Tekleva and Krassilov 2009). In addition, placement of Bennettitales in anthophytes is also questionable because of contradictory analyses and the lack of a character set for extinct taxa (Rothwell and Stockey 2002). The spatial arrangement of interseminal scales and ovules of Bennettitales appears to be too derived to be ancestral for carpels of angiosperms.
The gap between Bennettitales and angiosperms is no less narrower than that between Caytoniales and angiosperms.
2.9 Problematospermum 13
2.8 Umkomasia The Corystospermales is a group of plants of worldwide distribution that ﬂourished from the Late Permian to Middle Jurassic (Zan et al. 2008; Taylor and Taylor 2009).
One kind of female organs in Corystospermales is called Umkomasia (Fig. 2.6).
Based on strong evidence of association in the ﬁeld, it is believed that its pollen organ is Pteruchus, which produces bisaccate pollen grains. The connected leaf is Dicroidium (Axsmith et al. 2000; Taylor and Taylor 2009). It is thought to be mainly distributed on the Gondwanan continents (Holmes 1987; Zan et al. 2008), but recent progress in palaeobotany ﬁnds more evidence of Umkomasia in Laurasian (Germany and China) (Kirchner and Müller 1992; Zan et al. 2008). The main axis of Umkomasia is borne at the apex of a short shoot, bears numerous lateral cupulebearing axes arranged spirally or in whorls (Axsmith et al. 2000; Taylor et al. 2006b;
Zan et al. 2008; Fig. 2.6). Each lateral axis bears pairs or whorls of stalked, recurved, helmet-like cupules. Unlike Caytoniales, each cupule of Umkomasia contains only one or two ovules, and its curved biﬁd micropyle usually protrudes beyond the cupule opening. The abaxial position of the ovules separates Corystospermales from angiosperms and Petriellales as well as Caytoniales, which bear adaxial ovules (Klavins et al. 2002; Taylor and Taylor 2009). Detailed comparison indicates that Umkomasia is unlikely to be an ancestor of angiosperms (Axsmith et al. 2000;
Klavins et al. 2002), although Pteruchus (Corystospermales) is favored as a candidate for angiosperm ancestry by the Mostly Male Theory based on developmental genetics (Frohlich and Parker 2000; Frohlich 2003).
Fig. 2.6 Umkomasia and its details. A Reconstructed branch bearing a pair of cupules. B Longitudinal section showing cupule surrounding seeds with protruding micropyles.
Gray color stands for vascular bundles
2.9 Problematospermum Problematospermum is reported from the Middle Jurassic to Lower Cretaceous of Kazakhstan, Mongolia, and China (Fig. 2.7). It includes seeds with ﬁlamentous hairy appendages and an apical projection, all of which may fall off when mature.
The elongated oval seed body has a truncated tip and a pointed base, with spikes in 14 2 Suggested Angiosperm Ancestors Fig. 2.7 Seeds of Problematospermum ovale (PB21392, NIGPAS). A Complete seed. Bar = 2 mm. B Straight apical projection. Bar = 1 mm. C Seed with ﬁlamentous appendages (arrow).
Bar = 1 mm. D Seed body with ﬁlamentous appendages attached (arrow). Bar = 1 mm rows and ﬁles. Its apical projection is straight, with a central canal. Its seed coat is composed of epidermal cells and two types of scleriﬁed cells. Inside the seed coat is food storage tissue of parenchyma. This plant fossil has frequently been classiﬁed as an angiosperm or proangiosperm (Krassilov 1973a, b, 1977a, 1982; Liu 1988;
Wu 1999). However, recent work indicates that these conclusions are inconclusive and that this plant may well bridge gaps among several groups in seed plants (for further details, refer to Wang et al. 2010).
2.10 Dirhopalostachyaceae Dirhopalostachyaceae (the Upper Jurassic to Lower Cretaceous) has been regarded as a group of proangiosperms by Krassilov (1977a). The reproductive organ is comprised of helically attached elliptical to obovate cupules each dehiscing along a ventral suture. Each cupule has an elongated beak-like extension and a ventral suture, containing a single seed (Krassilov 1977a). Based on cuticular features, it is related to Nilssonia-type leaves (Krassilov 1975, 1977a). Krassilov (1977a) believes that Dirhopalostahys may have been derived from Beania by the involution of the ovuliferous shield. Based on similarities in beak, suture, external rib pattern, and/or leaf venation, Krassilov (1984) relates Dirhopalostachys to the capsule of the angiosperms Trochodendrocarpus (1977a) and Kingdonia. Little is known about the pollination/fertilization of this plant (Krassilov 1984), therefore it is hard to known whether or not it is truly an angiosperm.
2.12 Pentoxylales 15
2.11 Ktalenia The ovule-baring structure named Ktalenia (Fig. 2.8) from the Cretaceous (Aptian) of Argentina may be the youngest one among the so-called seed ferns, which occurred at the time of the angiosperm radiation (Taylor and Archangelsky 1985).
Its foliage is Ruﬂorinia. The cupules are sessile, spherical in form, recurved, with their openings pointing downward, and oppositely or suboppositely arranged along the axis. Unlike Caytonia, there are only one or two orthotropous seeds per cupule, with a distal nucellar beak (Taylor and Archangelsky 1985). Interestingly, Ktalenia demonstrates a nearly complete enclosure of ovules. Besides its abaxial ovules, Ktalenia is as challenging as Caytoniales to be considered as an angiosperm ancestor. Furthermore, the pre-Aptian megafossils of angiosperms, such as Chaoyangia, Archaefructus, Sinocarpus, and Callianthus (Duan 1998; Sun et al. 1998, 2001, 2002; Leng and Friis 2003, 2006; Wang and Zheng 2009, see also Chaps. 5 and 6), reduce the probability for Ktalenia to give rise to angiosperms, if the latter are monophyletic.
Fig. 2.8 Reconstructed Ktalenia. A Fertile axis bearing cluster of bracts at left and cupule at right.
B Longitudinal section of a cupule containing two ovules.
C Longitudinal section of a cupule containing one ovule
2.12 Pentoxylales Pentoxylon (Pentoxylales) is named after the ﬁve wedges of secondary xylem in transverse section, resembling a cut orange, that characterize its stem. It is a Gondwanan taxon ﬂourishing from the late Early Jurassic to Early Cretaceous in India, Australia, New Zealand, and Antarctica (Hughes 1994; Biswas and Johri 1997; Cesari et al. 1998; Bonde et al. 2004). It diversiﬁed during the Jurassic. Its foliage type is Nipaniophyllum with an epidermal cuticle bearing syndetocheilic stomata. The pollen organ, Sahnia, produces psilate monocolpate pollen grains.
The ovule-bearing structures, Carnoconites, are clustered into a mulberry-like cone attached to the apex of a stalk, which in turn is attached to a short shoot apex (Nixon et al. 1994; Biswas and Johri 1997). Each cone comprises about 20 orthotropous, unitegmic ovules with their micropyles facing away from the cone axis (Nixon et al.
1994; Biswas and Johri 1997). Pentoxylon is unisexual, and thus differs from the bisexual reproductive structures of some other anthophytes. This group is regarded as isolated even within gymnosperms (Biswas and Johri 1997). Therefore it may be too specialized to be an ancestor of angiosperms.
16 2 Suggested Angiosperm Ancestors
2.13 Summary Among these candidates for angiosperm ancestry, none of their evidence is sufﬁciently convincing to have become widely accepted. The main reasons are the same for all. Besides the lack of an angiosperm-like taxon (living or fossil) intermediate between these candidates and known angiosperms, they are either too derived to be an ancestor for angiosperms or lack convincing evidence of angiospermy (angioovuly) (see Chap. 3). These plants, at least, require further effort to be related to angiosperms.