Abominable Mystery: The Origin of Flowers and the History of Pollination
Land plants are the sister taxon to the freshwater green algae Charales (stoneworts), and together are nested among freshwater green algae. Charales have "stems" which are multinucleate single cells, not homologous to the vascular stems of tracheophytes (see below). Charales reproduce by sending flagellated sperm cells in the water, and indeed spend their entire life cycle in the water.
In contrast the most primitive living embryophytes ("bryophyte-grade" forms) reproduce by releasing spores into the air, which land on wet surfaces, producing a generation of tiny plants that makes sex cells. The sperm are carried along by raindrops or over wet surfaces, fertilizing the egg cells, which grow into an adult plant which can generate spores.
The oldest possible embryophyte remains are the so-called "cryptospores" of the Cambrian and Ordovician: these are spores which are identical in form to basal crown-embryophyte spores, but could conceivably be from some aquatic stem-embryophyte taxa (i.e., that lived in the water but spread spores through the air to colonize new water habitats). There are also some possible thalloid (multicellular, tissue-grade but no definite organ systems) algal from the Cambrian and Ordovician which are conceivably from some terrestrial basal plants. Also present in the Ordovician, however, are tissue of organ-grade embryophytes.
They basal modern land plants (liverworts, hornworts, and true mosses: see cladogram above) are all built very low to the ground, lacking significant support or the means to transport fluids up into tall tissues. However, they do contain two major adaptations required by all land plants:
The next major phase of embryophyte evolution is Tracheophyta, the vascular plants. These plants show new adaptations:
The oldest tracheophytes are the paraphyletic grade of "rhiniophytes", known first from the Early Silurian. These primitive land plants show stems, branches, sporangia (spore-generating organs), but no leaves.
The next major adaptation of plants is:
The primitive Silurian and Early Devonian leafed plant Baragwanathia is a member of the Lycophyta (clubmosses). Lycophytes (in the form of lycopsid "scale trees") were one of the first very successful land plants, eventually producing trees up to 30 m tall. However, they still reproduced by spores and thus needed to live in environments that were at least seasonally wet.
The leaves of lycophytes ("microphylls") are very small and simple. The next major adaptation in the plant conquest of land were large complex vascular leaves. One of the first branches of the Euphyllophyta ("good leaf plants") are the Monilophyta (ferns and horsetails). Ferns still reproduce by spores, in a manner similar to "bryophyte-grade" plants. Ferns are actually quite diverse, producing "classic" ferns, tree ferns (in which many stems growing next to each other support the tall structure), and horsetails (a major group of ground cover plants in wet environments, and during the late Paleozoic a major group of tree-sized plants).
The last major advance in the first wave of plant colonization of land is the development of wood (tough tissue with bark and the ability to grow in thickness as the plant grows in height). The oldest members of Lignophyta (woody plants) still reproduced with spores: this differs from all living woody plants which are also members of Spermatophyta (seed plants). Early lignophytes are present in the Devonian, and were the trees that dominated the Devonian forests.
Seeds appeared among lignophytes during the Devonian. The seed plants (Spermophyta) today include the non-flowering Acrogymnospermae and the flowering Angiospermae. The term "gymnosperms" refers to the paraphyletic grade of non-angiosperm spermatophytes: this includes the acrogymnosperms, the non-flowering stem-angiosperms, and a set of basal clades collectively called "pteridosperms" ("seed ferns", because of their leaf shape).
In spermatophytes, the old spore plant life cycle is replaced with a new system. In the gymnosperm condition, the adult plant produces male cones (that produce pollen, spread primarily by wind), which will land in the female cones, which fertilized the egg, to produce a seed. The seed has a cover which allows it be deposited on much drier land than any spore plant could colonize. Thus, seed plants were able to colonize essentially all habitats on land except for the driest deserts and coldest polar regions.
Seed plants (technically Spermatophyta) reproduce by pollen (containing the sperm cells) fertilizing specialized female organs containing the egg cells: the embryo produced is protected by a seed. The oldest seed plants are Devonian. Current evidence points to the living non-flowering seed plants (conifers, cycads, ginkgos, gnetophytes) forming a monophyletic clade Acrogymnospermae instead of the traditional interpretation of these "gymnosperms" are a paraphyletic grouping with respect to the flowering plants. There definitely do appear to be fossil gymnosperms (that is, non-flowering seed plants) which are more closely related to flowering plants than to any living group of gymnosperm regardless of the monophyly or not of acrogymnosperms. The major groups of fossilizing seed plants are:
The basic angiosperm life cycle hinges on co-evolution with animals:
Fossil Record of Angiosperms: There is a substantial record of Cenozoic angiosperms, and of Late Cretaceous angiosperms, too. This includes pollen, flowers, fruit, wood, and indeed all body parts. For many decades the oldest angiosperm fossils (the characteristic pollen and leaves) were from the far-off land of Prince Georges County, Maryland! (and and nearby in Virginia).
The Yixian Formation Lagerstätte of northeastern China has greatly increased our knowledge of Early Cretaceous angiosperm diversity, just as it did for feathered dinosaurs, early mammals, and so forth. The first taxon to show up, Archaefructus, was a water plant (something like a waterlily without the pads). But more species have shown up since: the early eudicot Leefructus, Hyrcantha, bean-like Sinocarpus, Callianthus, and Chaoyangia (which confusingly has the same name as an Early Cretaceous Chinese bird!). These show that the divergences among the early angiosperms has already occurred, so the origin of crown-group Angiospermae is even earlier.
Recent study of the environmental distribution of early angiosperms, we see that they first show up in lakes and braided stream deposits, and move out from there to floodplains and then further upland, so that by the mid-Cretaceous they make up the majority of plant species in most terrestrial environments. (However, studies of well-preserved forest stands show that they represented a minority of the biomass
Some have suggested that they have body fossils of angiosperms back in the Jurassic, and pollen similar to angiosperms as far back to the Middle Jurassic. But are these REALLY from crown-group angiosperms? Or might they be more basal?
Anthophyta: Flowers and Stems: Acrogymnospermae is the living sister taxon to Angiospermae, but the divergence had to have been back in the Carboniferous (since that is when the oldest acrogymnosperms show up.) So there should be a series of extinct outgroups in the stem-group to crown-Angiospermae. The clade name for the total-group for angiosperms is Anthophyta: Angiospermae and all taxa closer to it than to Acrogymnospermae.
And, in fact, there are a number of extinct outgroups to angiosperms, that show at least some flowering plant similarities. These include Schmeissneria of the Early and Middle Jurassic; the Caytoniales; Sanmiguelia of the Late Triassic; and the diverse Bennettitales (or Cycadeoidea) of the Triassic through Cretaceous. In these forms there are often some sort of flower-like structure (inflorescence), but often it has either male or female parts, not both. In some (like some bennettitalians) the fertilized seed produce a seed-like cover. So just as with the origins of birds (where bird-like traits show up step by step up the phylogeny towards the origin of crown-Aves), the traits associated with crown-group Angiospermae actually appear during a long stem of now-extinct forms.
Also, just as we see a sequence of regulatory genes associated with the different steps of feather formation, we see a set of regulatory controls in modern flower development. And the protoflowers of the earlier anthophytes are similar to the protofeathers of non-avian dinosaurs, representing some but not all the genetic component seen in the modern forms.
But who was fertilizing the protoflowers? It turns out that the mecopteran insects (today represented by the scary-looking carnivorous scorpionflies) served that role. These early scorpionflies did not have biting jaws, but instead butterfly-like probing snouts.
By the Late Cretaceous many modern clades of angiosperms were present (mangolias, rose-relatives, maples, etc.). Also during this time the first grass appears. Grasses include not only the stuff that grows in lawns and meadows, but all the grain-producing plants (wheat, barley, etc.), as well as bamboo. Their flowers are extremely small, and they are often wind-pollinated rather than by the help of insects.
Grasses grow from the base of the leaf rather than the tip. They often have little bits of silica in them to persuade herbivores not to eat them. Recent discoveries in both Laurasia and Gondwana: the latter were found in titanosaur sauropod coprolites! So at least some Cretaceous dinosaurs were grass eaters. However, grasses seem to have been relatively rare in the Mesozoic, and did not form grasslands until much later. Ground cover in the later Mesozoic was a mixture of ferns and herbaceous angiosperms.
For more about the story of grasses and grasslands, see these lecture notes.
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