Paleobotany I: Land plants



From octopup.org
We care about land plant, or embryophyte, evolution for many reasons:



Medullosa from UC Berkeley Museum of Paleontology
One pervasive technical difficulty: Plants tend to shed parts while living and to fall apart on death. Also, different parts have different preservation potential. Consequently, paleobotonists are vexed by synonymy of ancient plant taxa, whose leaves, stems, trunks, reproductive structures, roots, and rhizomes may all be known by different names.

Plant origins


from American Journal of Botany
We must revisit an issue that we took up at the beginning of our survey of the diversity of fossil forming organisms: Endosymbiosis.

Plant phylogeny:

Adaptations to life on land:

The closest sister taxa to land plants are the paraphyletic grade group of multicellular green algae known as charophytes

Charophytes:


In order for plants to colonize the land, they had to overcome the challenges of retaining water, exchanging gasses, supporting their bodies, and reproducing out of water. Five adaptations facilitated their response:


from The BBC

from Biology Reference

Alternation of Generations:



based on Crane et al., 2004.

Fossil record:

Conventionally, we site the appearance of unambiguous plant macrofossils in the Early Silurian as the start of the land plant record. Gensel, 2008 has revealed microfossils including:

The absence of other early body fossils testifies to the problematic fossil record of basal non-woody embryophytes. Indeed, for some groups, the actual fossil record occupies only a small fraction of their likely time range.

Embryophyte phylogeny: Apparently, in the ancestral embryophyte, the gametophyte generation was emphasized and the sporophyte generation was relatively ephemeral. Because of the gametes dependency on water, such plants could only thrive in moist environments. This general pattern persists throughout a "bryophyte" grade of non-tracheophyte embryophytes.

Their roster:

  • Marchantiophyta - liverworts (Late Dev. - Rec.) Land plants whose gametophytes form low, rounded, lobate leaf-like structures that attach to the substrate with unicellular rhizoids (similar to root hairs in higher plants.) Marchantiophytes are distinctly primitive in the rudimentary state of their sporophytes, which develop inside the archegonia (visible as star-shaped structures) and lack stomata.
  • Anthocerotophyta - hornworts (K - Rec.). Gametophyte broad and lobate as in liverworts, however incorporates extracellular space for cyanobacterial symbionts, giving blue-green color. The have robust sporophytes. In hornworts, sporangia form inside stems, which split open at maturity.
  • Bryophyta - proper mosses (?Sil. (Kodner and Graham, 2001) (definitely Perm.) - Rec.) In mosses the sporophyte produces a long stem before the sporangium matures. Unlike previous groups, the gametophyte produces short stems.

  • Horneophyton lignieri from Wikipedia
    Horneophytopsida (Sil. - Dev.) Sporophyte branches and supports multiple sporangia. These plants, together with more derived Tracheophyta make up Polysporangiophyta the embryophyte clade characterized by this feature.


    Psilotum nudum

    Tracheophyta:

    (?Sil (definitely Early Dev.) - Rec.) The tracheophytes, or "vascular plants" include the majority of land plants.

    Characteristics:

    Modern ferns give a good idea of the ancestral tracheophyte reproductive system. Their gametophytes are generally only a few millimeters across, whereas their sporophytes can be quite large. By means of this innovation, ferns and other tracheophytes can break their dependence on moist environments for most of their life cycles. Although their gametophytes still require moisture, they are small enough that they can develop from spores quickly when favorable conditions occur.

    The bodies of vascular plant sporophytes are generally differentiated into:

    In basal embryophytes, simple diffusion transmitted nutrients from the soil and the glucose derived from photosynthesis to be delivered to the entire plant. The evolution of specialized vascular tissue in tracheophytes emables the distribution of these things inside a much larger plant body.

    "Seedless vascular plants": The first major radiation of vascular plants occurred in the Silurian and Early Devonian, giving rise to groups that dominated land floras for most of the Paleozoic This radiation consisted primarily of plants that reproduced in the manner described above:

  • Rhyniopsida: (?Sil (definitely Early Dev.) - Dev.) The paraphyletic group containing the earliest and most primitive kown tracheophytes is named after a well-known genus, Rhynia. These plants consisted of rhizomes and leafless stems with small simple roots, were generally less than 10 cm. tall, and bore sporangia at the tips of their stems. The leafless modern Psilotum superficially resembles them, and was once thought to be a surviving member of this early radiation, but it is now thought to be a highly derived fern.


    Lycopodium annotinum
  • Lycophyta: (AKA Lycopodiophyta, Lycopods) (Dev. - Rec.)

    Members of this group possess narrow pointed leaves that are vascularized by a single strand of vascular tissue. The leaves cover the stem. When they fall away, they leave the stem covered by characteristic diamond shaped scars. Modern representatives, the "club mosses," (right) are a sad remnant of a once mighty radiation. During the Carboniferous, their tree-sized relatives, including Lepidodendron and Sigillaria were the largest forest plants. The sporangia of lycophytes are born on the upper surface of specialized leaves which grow in groups, forming cones or strobili at the ends of stems.

    Earliest representatives of lycophyte lineage may be Zosterophyllopsida, (Zosterophyls) resembling rhyniophytes but with alternating sporangia on stem.


    Dickinsonia antarctica
  • Monilophyta (ferns and horsetails): (Late Dev. - Rec.)

    Ferns were the first plants with large vascularized leaves. Most, but not all ferns have very short trunks that support large pinnate leaves. Many ferns have independently evolved the "tree-fern" habitus, growing to the size of small trees. Post-paleozoic tree ferns seem to be monophyletic, but there were paleozoic ferns that separately evolved the tree-fern form. Ferns are plesiomorphic in bearing their sporangia on the bottoms of unmodified leaves.


    Equisetum arvense
    One monilophyte group has only recently been found to group within "ferns" - the Sphenopida or "horse-tails. (Dev. - Rec). Like lycophytes, these plants protect their sporangia in strobili that develop at the end of stems. They are distinguished by the unique morphology of their stems. These consist of cylindrical sections separated by nodes. Whorles of slender leaves radiate from the nodes. During the late Paleozoic and Triassic, some sphenopsids such as Calamites attained tree size, but they were never the primary flora of their environments. Modern sphenopsids are known for rendering themselves inpalatable by incorporating silica phytoliths into their tissues.


    Lignophyta - Seed Plants: (Dev. - Rec.)

    Seeds: The second great radiation of land plants occurred during the Late Paleozoic, and was associated with the evolution of the seed. This occurred in stages.

    Wood: A second major evolutionary novelty was the ability of seed plants to lay down secondary tissue. Ancestrally, land plants could create new tissue only in the apical meristem tissue at the tips of their stems. In seed plants, however, we see the appearance of vascular cambium, a sheath of tissue that generated new vascular tissue around the circumference of the stem. New tissue laid down by vascular cambuim appears as growth rings in cross sectioned stems. The ability to thicken existing stems allowed seed plants to attain greater size than arborescent lycopods.

    "Progymnosperms" Paraphyletic grade group of plants bearing vascular cambium but lacking ovule-like reproductive structures:

    "Seed ferns" Paraphyletic grade group including numerous Late Paleozoic land plants characterized by distinct ovules and pollen organs born on separate leaves. The leaves were typically like those of ferns.

    A rogue's gallery of seed fern groups: