Seed ferns from the late Paleozoic and Mesozoic: Any angiosperm ancestors lurking there?

Abstract

Five orders of late Paleozoic-Mesozoic seed ferns have, at one time or another, figured in discussions on the origin of angiosperms, even before the application of phylogenetic systematics. These are the Glossopteridales, Peltaspermales, Corystospermales, Caytoniales, and Petriellales. Although vegetative features have been used to suggest homologies, most discussion has focused on ovulate structures, which are generally interpreted as megasporophylls bearing seeds, with the seeds partially to almost completely enclosed by the megasporophyll (or cupule). Here we discuss current information about the reproductive parts of these plants. Since most specimens are impression-compression remains, homologizing the ovulate organs, deriving angiospermous homologues, and defining synapomorphies remain somewhat speculative. Although new specimens have increased the known diversity in these groups, a reconstruction of an entire plant is available only for the corystosperms, and thus hypotheses about phylogenetic position are of limited value. We conclude that, in the case of these seed plants, phylogenetic analysis techniques have surpassed the hard data needed to formulate meaningful phylogenetic hypotheses. Speculation on angiosperm origins and transitional stages in these fossils provides for interesting discussion, but currently it is still speculation, as the role of these groups in the origin of angiospermy continues to be cloaked in Darwin's mystery.

The Pollination of Mid Mesozoic Seed Plants and the Early History of Long-proboscid Insects1,2,3

“…Cycads probably had an origin during the Pennsylvanian, based on several late Paleozoic taxa with cycad features (Mamay, 1976;Anderson et al, 2007;Taylor et al, 2009). They evidently were pollinated by insects, based on circumstantial evidence from Permian cone structure, surrounding vegetative glands, and other possible insect-reward tissues (Mamay, 1976;).…”

“…Most early angiosperm flowers were comparatively small in size (Dilcher, 1995;Crepet, 2008;Taylor et al, 2009;Thien et al, 2009) and may have developed from a preangiospermous ancestor possessing an ovulate longitudinal slit with access to pseudostigmatic secretions (Dilcher, 2010). This model is consistent with small, nonproboscid, fluid-feeding insects as the earliest pollinators of small-flowered, dullcolored, unscented angiosperms.…”

“…For ovules reported in Pentoxylon and Umkomasia, structures consistent with wind pollination, an additional mode could have been insertion of a short insect proboscis into the micropyle, providing a simple reward of pollination fluid with minimal modification of surrounding extraovular tissue. Gne-taleans such as Problematospermum were achenelike ovules that bore broad micropylar pappus tubes extending at least to 14 mm, at whose base originated enveloping, bracteate, papery structures and setose plumes (Krassilov, 2009;Taylor et al, 2009), possibly attracting mobile, winged pollinators.…”

“…Discussions of the magnitude and consequences of the event associated with the diversification of angiosperms historically can be contrasted with what is known of the immediate, preangiospermous world of the earlier Mesozoic flora dominated by extinct gymnospermous seed plants of seed ferns, cycads, conifers, bennettitaleans, and gnetophytes, as well as ferns, sphenopsids, and bryophytes (Willis & McElwain, 2002;Taylor et al, 2009). Similarly, speciose insect clades that interacted with these plants included lineages that either became extinct or were represented by closely related, but early groups of extant lineages (Grimaldi & Engel, 2005).…”

Tracing the early evolutionary diversification of the angiosperm flower

“…Morphological analyses that constrained Gnetale s to a position in conifers (Doyle, 2006(Doyle, , 2008 identifi ed fossil glossopterids , Pentoxylon , Bennettitale s and Caytonia as extinct outgroups of the angiosperms, but there is no consensus that any of these taxa are related to angiosperms (cf. Taylor and Taylor, 2009 ).…”