FOSSIL PLANTS/Gymnosperms 445 Figure Two hypotheses of how living gymnosperms are re lated to angiosperms (flowering plants) Morphological data favour the anthophyte hypothesis, whereas molecular data favour the gne pine hypothesis Figure Frond of the progymnosperm Archaeopteris hibernica (Upper Devonian, Kiltorkan, Kilkenny, Ireland) phylogenetic trees the fossils would in fact fall out However, it seems likely that whichever hypothesis ultimately wins out, Early Palaeozoic gymnosperms such as Calamopityales, Hydraspermales, Lyginopteridales, and possibly Medullosales (Table 1) will remain basal in a phylogenetic sense to both angiosperms and to the more derived living gymnosperms Also, it is plausible that extinct Mesozoic groups such as Pentoxylales, Bennettitales, and possible Caytoniales are more closely related to angiosperms than they are to other gymnosperms From a phylogenetic perspective, angiosperms are in reality little more than a morphologically divergent group of gymnosperms Gymnosperm Origins Gymnosperms originated from a grade of Late Devonian plants called the progymnosperms The bestknown exemplar is Archaeopteris This plant had large fern-like fronds, some of which are known to have been well over m in length (Figure 2) Like ferns, Archaeopteris produced spores rather than seeds However, unlike true ferns, the fronds were borne on woody branches In this respect, Archaeopteris resembled gymnosperms Some species of Archaeopteris were undoubtedly large trees, as trunks of the distinctive wood (Callixylon) are known to exceed several metres in diameter This extraordinary extinct plant therefore possessed a unique combination of characteristics that are intermediate between gymnosperms and early relatives of the ferns It shows that the woody architecture that characterizes gymnosperms evolved before other defining features, such as the ovule The earliest gymnosperms are found in Upper Devonian sediments of Europe and North America These are known from plant fragments such as dispersed seeds and seed-bearing branches (Figure 3) as well as a handful of more completely known plants Elkinsia polymorpha (Hydraspermales) from the Upper Devonian of West Virginia, USA, is one of the more completely characterized forms Petrified ovules in which cellular anatomy is preserved show gymnosperm features alongside other features that differ from modern forms Specifically, many early ovules lacked a completely formed integument (enveloping tissue layer), so they are sometimes called ‘preovules’ Some also had additional specialization to aid fertilization (pollen capture) In many, ovules were borne on the ends of branches in ‘cupules’ The earliest gymnosperms were small woody shrubs that would have had a fern-like appearance (Figure 4)