Description: Petrified Ferns Plant Fossil 75 mm
Weight of both pieces: 69.01g
Length: 75 mm
Condition: very good for the age
Evolution and classification of ferns
Ferns first appear in the fossil record in the early -
period. By the
, the first evidence of ferns related to several modern families appeared. The "great fern radiation" occurred in the late-
, when many modern families of ferns first appeared.
One problem with fern classification is the problem of cryptic species. A cryptic species is a species that is morphologically similar to another species, but differs genetically in ways that prevent fertile interbreeding. A good example of this is the currently designated species
, the maidenhair spleenwort. This is actually a species complex that includes distinct diploid and tetraploid races. There are minor but unclear morphological differences between the two groups, which prefer distinctly differing habitats. In many cases such as this, the species complexes have been separated into separate species, thus raising the number of overall fern species. Possibly many more cryptic species are yet to be discovered and designated.
Ferns have traditionally been grouped in the Class Filices, but modern classifications assign them their own
or division in the plant kingdom, called Pteridophyta, also known as Filicophyta. The group is also referred to as Polypodiophyta, (or Polypodiopsida when treated as a subdivision of
(vascular plants), although Polypodiopsida sometimes refers to only the
). The term "
" has traditionally been used to describe all seedless
, making it synonymous with "ferns and
". This can be confusing since members of the fern phylum Pteridophyta are also sometimes referred to as pteridophytes.
Traditionally, three discrete groups of plants have been considered ferns: two groups of eusporangiate fernsfamilies
ferns. The Marattiaceae are a primitive group of tropical ferns with a large, fleshy rhizome, and are now thought to be a sibling taxon to the main group of ferns, the leptosporangiate ferns. Several other groups of plants were considered "
, and the
. More recent genetic studies have shown that the Lycopodiophyta are more distantly related to other
, having radiated evolutionarily at the base of the vascular plant
, while both the whisk ferns and horsetails are as much "true" ferns as are the Ophioglossoids and Marattiaceae. In fact, the whisk ferns and Ophioglossoids are demonstrably a clade, and the horsetails and Marattiaceae are arguably another clade. Molecular datawhich remain poorly constrained for many parts of the plants' phylogeny have been supplemented by recent morphological observations supporting the inclusion of
within the ferns, notably relating to the construction of their sperm, and peculiarities of their roots. However, there are still differences of opinion about the placement of the Equisetum species (see
for further discussion). One possible means of treating this situation is to consider only the leptosporangiate ferns as "true" ferns, while considering the other three groups as "fern allies". In practice, numerous classification schemes have been proposed for ferns and fern allies, and there has been little consensus among them.
A 2006 classification by Smith
is based on recent molecular systematic studies, in addition to morphological data. Their phylogeny is a consensus of a number of studies. This phylogeny has been refined in more recent years, and is shown below (to the level of orders).
Low Estimate: 400;
High Estimate: 800;