Since Darwin threw down the gauntlet -- calling the origin of the flowering plants an "abominable mystery" -- the genesis of the angiosperms has been one of the signal grails of paleobotany. A number of possible ancestors have been proposed, only to be rejected when new material is found or conflicting interpretations of homology are made. In this lab, we will be studying several paraphyletic clades, including Caytonia, the corystosperms, peltasperms, Callistophyton, glossopterids, and the angiosperms and their closest relatives (gnetophytes and bennettitaleans). We consider them together because these groups include once and future contenders for the angiosperm sister group.
Traditional systematic schemes have not helped to clarify relationships among these Mesozoic seed plants. Some of the taxa: Caytonia, corystosperms, glossopterids, peltasperms, Callistophyton, Pentoxylon, and bennettitaleans are commonly included in the "seed ferns" (Pteridosperma), due to the position of the reproductive structures on leaves or leaf homologs, or assigned their own order within the seed plants. As you recall, both the "Pteridosperma" and the "Gymnosperma" are grades, not clades, and this classification is evolutionarily inappropriate. Thus, looking at these plants together will give you a concrete sense for the difficulty facing scientists working to unravel the relationships -- and classifications -- among Mesozoic seed plants and flowering plants.
Lately, a number of phylogenetic analyses have attempted to resolve the relationships of the seed plants. Most of these have been specifically directed towards sorting out the origin of the angiosperms (see Loconte & Stevenson 1990 for a summary; Doyle & Donoghue 1993). In fact, more energy has been put into debating the phylogenetic relationships of the flowering plants than into any other group of vascular plants. This probably reflects Darwin's frustration. It also certainly relates to the tremendous diversity and ecological importance of angiosperms in the Tertiary. Nevertheless, it is not entirely clear what group of seed plants are most closely related to flowering plants, although the gnetophytes appear to be the living sister group to the flowering plants. Every author has his or her own opinion in this matter, and a number of different topologies have been proposed (compare Nixon 1994 with Crane 1985). Much of the difficulty stems from the way that most Mesozoic taxa are preserved as compression/impressions, revealing no anatomical details and leaving many homologies unclear.
In their quest, paleobotanists have tended to focus on how the angiosperm double integument was derived from ovulate structures of other fossil seed plants. All of the Paleozoic and Mesozoic seed plants in this lab all (except Callistophyton and the peltasperms) show some kind of enclosure of their ovule by a cupule outside of the integument itself. Some authors (e.g. Crane 1985; but see Nixon 1994) have interpreted the cupule as homologous with the outer integument of the bitegmic ovule of the flowering plants. This interpretation led to the proposal of several different groups of fossil taxa as their immediate sister: Caytonia, corystosperms, glossopterids, Pentoxylon, or even Carboniferous "seed ferns". Others argue that the derived characters in the ovulate structures of all these putative pre-angiosperms represent an evolutionary grade more than a clade, perhaps having developed as a convergence of form (homoplasy) in response to some factor in the Mesozoic environment. Therefore the angiosperm second integument may be related to only a few of these -- if any. So, there are two problems of homology that you should keep in mind while reading the descriptions of the putative sister taxa of the flowering plants: (1) Are the cupules of the various groups homologous to each other, and (2) are any/all of these homologous with the angiosperm second integument? What evidence do you need to resolve this problem? Is it even possible?
The cladogram in the Virtual Laboratory is based primarily on the cladistic analysis by Crane (1985), which treats the taxa in this lab as members of a monophyletic clade, based on the suggested homology of all cupule-type structures and the angiosperm carpel. However, you might want to compare with e.g. Nixon (1994), Doyle & Donoghue (1993) and decide for yourself which evolutionary scenario you find more likely.
So, with apologies for paraphyly, let's look at the plants: