More About Ferns (monilophytes)
The Familiar Non-Seed Plants
Modern ferns are familiar spore-producing plants common in a wide range of tropical to temperate environments. Most grow as small to medium-sized bushes, but some are trees while others are aquatic. With approximately 11,500 living species they are second in diversity only to the flowering plants (angiosperms). Their record extends from the Middle or Late Devonian to the present.
What's a Fern?
Although we’re familiar with many of their modern (extant) representatives, their classification has long proved problematic. Some past authorities had lumped them in with other non-seed bearing vascular plants such as lycopsids (club mosses, spike mosses and quillworts), sphenophytes (horsetails) and psilophytes (whisk ferns); the common phrase "ferns and fern allies" is a legacy of this classification scheme. More recent classifications place lycopsids in a separate group, but the relationships among various groups of modern ferns (e.g., "pterdophytes", "leptosprangiate" and "eusproangiate") were a matter of considerable uncertainty. Moreover, the relationship of these ferns to related extant and fossil forms was even more uncertain.
However, a growing consensus on fern systematics has been developing during the last decade as a result of the cladistic analyses of morphology (including fossil taxa), comparisons of sperm ultrastructure, and DNA sequencing; the DNA analyses have proven particularly compelling. Ferns are now firmly placed within the Moniliformopses (monilophytes). This monophyletic group and its sister group, the spermatophytes (seed plants and progymnosperms) form the two extant (not-extinct) clades of the euphyllophytes; this latter taxon, along with its sister group, the lycophytes (including lycopsids) form the two major lineages of vascular plants. Euphillophytes and lycophytes probably diverged in the late Early Devonian, while the monilophyte and spermatophyte clades probably separated during the Middle Devonian.
The recent studies of phylogenetic relationships among modern or extant (not extinct) ferns has yielded some surprises. The most widely accepted previous schemes placed three groups of extant ferns (Marattiales, Ophioglossales and leptosporangiate ferns) within the "true ferns" while two other groups, sphenophytes (horsetails) and Psilotales (whisk ferns) were regarded as more distantly related "fern allies" that may or may not be "true ferns".
The designation "true fern" is now no longer valid. As it turns out, one order of "true fern", Ophioglossales, and one "fern ally", Psilotales, form one lineage of monilophytes while two other "true ferns", leptosporangiates and Marattiales, and one "fern ally", sphenopsids, form another.
Both whisk ferns and ophiolossoid ferns exhibit simplified or reduced morphologies. Whisk ferns (Psilotales) have no roots, greatly reduced leaves and spore producing organs that are borne on short lateral branches; all other ferns produce spores on their leaves. The reduction is so striking that one genus, Psilotum, resembles Rhinia, one of the earliest known and most primitive fossil trachaeophytes known and. Whisk ferns are presently represented by species belonging to two genera, Psilotum and Tmesipteris. There is no fossil record.
Ophiolossoids (succulent ferns) are less dramatically reduced than the whisk ferns. They have simple unbranched roots that lack root hairs. They also have sterile (non-reproductive) fronds with webbed leaves and homosporous spores on a specialized spike-like frond. There are about 100 species belonging to either four genera (Botrychium, Helminthostachys, Ophioglossum and Rhizoglossum) or one (Botrychium). The fossil record is meager, with the earliest record occurring in the Paleocene of Canada.
Two of the remaining extant fern groups, sphenopsids (horsetails; also known as equisetopsids) marattioid ferns may be more closely related to each other than they are to the third group, leptosporangiate ferns. Sphenopsids, long considered a fern ally rather than a true fern, have a distinctive pattern of nodal growth and branching (including leaves) in regular whorls. Spores are produced in strobili, cone-like organs that contain multiple clusters of sporangia. All living and most fossil sphenopsids were homosporous, but heterospory is known in some arborescent (tree-like) fossils.
Extant sphenophytes are represented by 15 species belonging to a single genus, Equisetum. However, their diverse fossil record extends from the Late Devonian and includes some large arborescent forms (e.g., Calamites/Annularia) that were abundant in the Late Carboniferous swamp forests. Extinctions in the early Permian greatly reduced spehnophyte diversity. This increased somewhat in the Triassic, but declined again in the late Jurassic. Fossils of Equisetum are known from the early Cenozoic.
Marattioid ferns first appeared in the Middle Carboniferous. They diversified considerably during the late Paleozoic and arborescent forms dominated Stephanian (late Late Carboniferous) and Lower Permian swamp forests in Euramerica. The best known of these Paleozoic marattialeans is Psaronius. This Late Carboniferous fern reached 8 m in height and had fronds as long as 1.5 m. Unlike most arborescent plants, these plants did not produce secondary tissues (i.e., tissues that increase axial girth). Instead, the "trunk" was encased by a dense mantle of branching, adventitious roots; leaf scars may have provided additional support. The Marattiales declined during the Mesozoic and are represented today by about 200 tropical species belonging to either four or six genera (e.g., Angiopteris, Danaea and Marattia). They are homosporous.
The leptosporangiates, represented by more than 11,000 species belonging to more than 300 genera, account the vast majority of living ferns. They are distinguished from other ferns by their method of spore development. Sporangia develop from single cell, and the sporangium is a delicate, thin-walled structure with zones specialized for dehiscence.
Leptosporangiate ferns first appear in the fossil record during Lower Carboniferous and rapidly diversified. Most of the Carboniferous forms apparently died out during the Stephanian (late Late Carboniferous) or Early Permian. The most primitive living family, Osmundaceae (e.g., Osmunda), first appears later in the Permian. Several other extant families appeared in the Triassic and Jurassic, including the modern tree ferns (Cyatheaceae and Dicksoniaceae), Dipteridaceae, Gleicheniaceae, Matoniaceae and Schizeaceae. Members of the water ferns (Azollaceae, Salviniaceae, and Marsileaceae) first appeared during the Late Jurassic Unlike the rest of the leptosporangiates these unusual ferns are heterosporous.
A third wave of leptosporangiate diversification occurred during the Late Cretaceous/early Tertiary, and was dominated by a group known as the polypod ferns (e.g., Asplenium, Blechnum, Pteris, Pteridium). The earliest confirmed members of this diverse group appeared during the Lower Cretaceous. Today, they consist of 15-30 families (depending on which authority you choose) account for about 80% all living fern species. One family, the Polypodaceae, accounts for about 60% of all living species. The polypod radiation occurred at about the same time as the ascendence of the flowering plants (angiosperms) which has led a number of scientists (e.g., Schneider et. al., 2004) to suspect the two sets of diversifications may be interrelated.
DNA sequence analyses have been critical in the new understanding of the relationships of extant fern. Unfortunately, they aren't directly applicable to the relationships of extinct groups prominent in Devonian and Carboniferous floras. However, the monophyletic status of monilophytes and the placement of sphenophytes (horsetails) well within the extant monilophyte clade provide insights into the status of various Paleozoic "prefern" and "fern-like" groups. Considerable debate and confusion have surrounded the study of these ancient plants, with some authorities arguing that the ancient and some extant forms were probably polyphyletic. Now, the recent work on extant ferns provide convincing evidence that these ancient plants are also monilophytes; they are also ferns.
One important feature that distinguishes monilophytes from spermatophytes is the development of their xylem (the water-conducting tissues that are frequently preserved in plant fossils). They have a pattern of xylem growth called mesarch protoxylem. The earliest mature xylem (protoxylem) has later-maturing xylem (metaxylem) growing both inward and outward from it relative to the center of the stem. In contrast, spermatophytes have endarch protozylem: metaxylem grows inward from the protoxylem.
The utility of mesarch protoxylem as a diagnostic feature of monilophytes was questioned in part because it's also found in sphenopsids (horsetails) and an enigmatic group of ancient plants called cladoxylopsids. Since the demonstration that sphenopsids are more closely related to leptosporgiate ferns than are some other "true ferns", the presence of mesarch protoxylem is useful in identifying ancient monilophytes.
Although it's now widely accepted that assorted "prefern", "putative fern" and "fern-like" groups can now be classified as ferns or monilophytes, their relationships to each other and to extant ferns are obscure. This is not too surprising considering that these ancient ferns first appear in the fossil record shortly after the monilophyte and spermatophyte clades diverged in the Middle Devonian. Indeed, most of the major groups of extant ferns (leptosporangiates, marattoids, sphenophytes, and ophioglossoids/whisk ferns) apparently diverged from each other during the next 10-20 million years (i.e., during late Middle Devonian and Late Devonian).
The earliest generally accepted fossil monilophyte is Ibyka, a Middle Devonian fossil from New York. Classification schemes vary, but this plant and a number of other Middle Devonian to Early Carboniferous (e.g., Cladoxylon, Hyenia and Pseudosporochnus) taxa have often been placed within the cladoxylopsids, an enigmatic group of ancient ferns that share features with sphenopsids and other extant ferns that are not shared by the extant forms. Cladoxylopsids exhibited a variety of life habits. Most were low-lying, but some arborescent forms such as Pseudosporochnus grew up to 3 m in height and were the giants of the Middle Devonian.
Three other groups of ancient ferns, Stauroteridales, Zygopteridales and Rhacophytales, are restricted to the Paleozoic. The Stauropteridales (e.g., Gillespeia and Stauropteris) extend from the Late Devonian to the Late Carboniferous. They were small bushy plants with non-webbed ultimate branches. This order contains both homosporous (producing one type of bisexual spore) and heterosporous (producing female-like megasores and male-like microspores) taxa. One species, Gillespiea randolphensis, was recorded at Red Hill.
Zygopteridales (e.g., Ellesmeris, Etapteris and Zygopteris) were also small bushy plants that ranged from the early Late Devonian (Frasnian) to the Early Permian. Some zygopterians were common in Late Carboniferous forests. They possessed webbed megaphylls and quadriseriate (four-ranked) branching, and were probably all homosporous.
Rhacophytales (e.g., Protocephalopteris and Rhacophyton) is a group of Middle to Late Devonian plants placed by some authorities within the Zygopteridales. They share several anatomical details of vascular tissues with the zygopterids, but they differ significantly in the structure of their fertile branches. In addition, unlike the zygopterids, the Rhacophytales possess secondary xylem in their axes and roots. One species, Rhacophyton ceratangium, was a dominant plant in several Late Devonian localities, including Red Hill.
- American Soceity of Ferns' introductory web page on ferns:
- The Australian National Herbariums web pages on ferns:
- Hans Kerp's web page on fossil ferns:
- Kathleen Pigg's web pages on ancient and modern ferns:
- Tree of Life's (tolweb.org) web pages on leptosporangiate ferns:
- U.C. Museum of Paleontology's web pages on cladoxylopsids, ferns (pteridopsida), psilophytes, sphenophytes and trimerophytes:
- U.C. Museum of Paleontology Virtual Lab web pages on ferns:
- Niklas, K. 1997. The Evolutionary Biology of Plants. Chicago & London: Univ. Chicago Press., Chicago, London.
- Stewart, W.N and G.W. Rothwell. 1993. Paleobotany and the Evolution of Plants. Cambrige: Cambrige Univ. Press.
- Taylor, T.N and E.L. Taylor. 1993. The Biology and Evolution of Fossil Plants. New York: Prentice Hall.
- Scientific Papers:
- Ash, S.R., R.J. Litwin, and A. Traverse. 1982. "The Upper Triassic fern Phlebopteris smithii." Paleontology 6: 203-219
- Hill, S.A., S.E. Scheckler and J.F. Basinger. 1997. "Ellesmeris sphenopteroides, Gen et. Sp. Nov. A new zygopterid fern from the Upper Devonian (Frasnian) of Ellesmere, NWT, Arctic Canada." Amer. J. Botany 84(1): 85-103.
- Kendrick P. and P.R. Crane. 1997. "The origin and early evolution of plants on land." Nature 389(4): 33-39.
- Pryer, K.M. E. Schuettpelz, P.G.Wolf, H. Schneider, A.R. Smith & R. Cranfill. 2004. "Phylogeny and evolution of ferns (Monilophytes) with a focus on the early Leptosproangiate divergences." Amer. J. Botany 91(10): 1582-1598.
- Schneider, H, E. Schuettpelz, K.M. Pryer, R. Cranfill, S. Magallón and R. Lupia. 2004. "Ferns diversified in the shadow of angiosperms." Nature 428(1): 553-557.
- Schweitzer, H, -J. 1978. Die Rato-Jurassichen Floren des Iran und Afghanistans. 5. Todites principes, Thaumatopteris brauniana und Phlebopteris polpodioides. Palaeontographica B 168: 17-60.
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