Early Seed Plants (lyginopterids)

fossil cupulesSeed plant cupules (about 8 mm wide). Photo courtesy of Walt Cressler.

The Start of Something Big

Three (and possibly four) early seed plant species have been found at Red Hill. They are uncommon at Red Hill; they account for less than 1% of the total identifiable plant fossils and are found in only a few bedding planes. On the other hand, their diversity exceeds that found in most other Late Devonian localities. They are also among the earliest reports of seed plants in the fossil record.

At least two of the Red Hill species are cupulate seed plants. It’s unclear whether a third cupulate form, which is represented by only a single specimen, is a separate species. (Cupulate seed plants have a cupule, a cup-like structure that partially encloses the ovule. It may have served to enhance wind pollination or help to protect the ovule.) These cupulate species are contemporaneous with and morphologically similar to cupulate taxa found in West Virginia (Elkinsia polymorpha) and Belgium (Moresnetia zalesskyi and Dorinnotheca streelii). However, the state of preservation in the Red Hill specimens makes identification elusive.The other Red Hill seed plant species is aculpate (i.e., lacks cupules). It compares favorably (c.f.) with Aglosperma quadripartita, a slightly younger seed plant from the Late Devonian of Wales.

The fossils of Late Devonian seed plants from Red Hill and elsewhere are typically very fragmentary. They usually consisted of isolated ovules or cupule fragments, which are sometimes either attached to or associated with short stem fragments. Several foliage fossils assigned to the form genus Sphenopteris have also been associated with the seed plant fossils. (Sphenopteris is a rather indistinct foliage genus that has been variously attributed to zygopterid "ferns", progymnosperns and early seed plants.) Probable stem fragments with attached roots have also been found in association with the Red Hill seed plants.

aglosperma ovulec.f. Aglosperma ovule and attached branch. Photo courtesy of Walt Cressler

The Seed Habit

The most significant feature of seed plants is that they reproduce via seeds. Heterospory (the production of haploid female-like megaspores and male-like microspores), which is found in a wide variety of vascular plants (e.g., Lepidodendron, Barinophyton, Gillespeia, Archaeopteris and aquatic ferns), and is widely regarded as a crucial intermediate step in the evolution of seed production. Seed plants also produce megaspores and microspores, but the fates of the resulting megagametophyes and microgametophytes differ dramatically from those in other heterosporous plants.

Microgametophytes in heterosporous plants produce sperm that must swim to the megagametophyte before fertilization can occur. In contrast, the entire microgametophye of seed plants is reduced to only a few cells, encased in a protective shell and then transported to the megagametophyte. This modified microgametophyte is best known by its more familiar name: pollen.

The megagametophyte of seed plants is retained and nourished by the parent plant. It’s also enclosed in a protective integument. Together they form the ovule. Once the pollen grain settles into an intimate proximity to the ovule (i.e., pollination), it must deliver its sperm. The mechanism for delivering the sperm differs between the earliest seed plants and their later relatives. In later seed plants, the ovule is completely encased in a protective integument. Following pollination, the pollen grain grows a pollen tube that transports the sperm through a specialized opening in the integument called the micropyle. Once the pollen tube reaches the megagametophyte, the tube bursts and fertilization occurs.

The earliest seed plants employed what’s called hydrasperman reproduction. In this case, the megagametophye was incompletely enclosed by its integument. Pollination occurred when wind-blown pollen (called pre-pollen by some authorities) was directed into a semi-enclosed pollen chamber. As the megagametophyte matures the opening to the pollen chamber is sealed and the floor is ruptured, allowing fertilization to proceed.

cupules with charcoalTwo seed plant cupules (arrows) and charcoal fragments (black). Photo courtesy of Walt Cressler

Evolutionary Success Story

The salient characteristic of seed reproduction is that fertilization occurs within the confines of the ovule. It doesn’t require standing water as a medium for free-swimming sperm. The evolutionary and ecological consequence is that seed plants could extend farther from water’s edge. This ability would facilitate the diversification of seed plants from the Carboniferous through to the present day.

However, these ultimately successful seed plants apparently had rather humble origins. The early seed plants at Red Hill and elsewhere were relatively modest plants that may not have exceeded 50 cm in height. Moreover, they seemed to have achieved their first successes as pioneer species. Seed plant fossils at Red Hill were most commonly found in layers with abundant charcoal fragments. Layers above and below the charcoal layers had few if any seed plants but were typically rich with the ancient "fern" Rhacophyton. Similarly, deltaic deposits in West Virginia contained dense and monotypic mats of seed plants in one layer and dense, monotypic mats of Rhacophyton in the next. In other words, the seed plants rapidly colonized disturbed habitats (recently burned sites at Red Hill and the leading edges of postgrading deltaic lobes in West Virginia), but they were soon succeeded by Rhacophyton.

The first indication of seed plant dominance occurs in a latest Devonian site in southwest Ireland. The site was probably a well-drained floodplain with relatively little sedimentation. Seed plants become more diverse, abundant and widespread during the Lower Carboniferous, and come to dominate levees, floodplains and drier upland habitats by the Late Carboniferous.

The earliest seed plants (e.g., Aglosperma, Archaeosperma, Elksinia, Lagenostoma, Lyginopteris and Moresnetia) are sometimes collectively known as lyginopterids. Most of these are known only from their ovules and associated organs (i.e., cupules, adjacent stems and possibly Sphenopteris-like foliage). Nonetheless, they exhibited a wide variety of ovule forms. Their record extends from the (Famennian) Late Devonian through the Early Pennsylvanian (Middle Carboniferous). Most are believed to have been either small bushes or vines.

Other important groups of seed plants appeared during the Carboniferous. The Pteridosperms, or seed ferns, is a polyphyletic group of seed plants with fern-like foliage. Different authorities have included a variety of plants within the seed ferns, including the lyginopterids and some Mesozoic forms. One clade of seed ferns, the Medullosaceae (e.g., Medullosa), were abundant trees in Carboniferous floodplains and extend well into the Permian. The Cordiatales (e.g., Cordiates) were also important floodplain trees and shrubs from the Late Carboniferous and Permian. Glossopterids (e.g., Glossopteris) were abundant in the Permian forests of Gondwanaland; the biogeographic distribution of Glossopteris provided crucial early evidence of continental drift.

Conifers (e.g., pine, spruce, redwood) and cycads (e.g., Cycas and Leptocycas) also first appeared in the Late Carboniferous. Cycads were abundant and diverse from the Permian through much of the Mesozoic, but have since declined. They are represented today by only about 100 tropical to subtropical species. Conifers came to dominate many tropical to boreal forest ecosystems during the Permian and Mesozoic. Despite being largely displaced by flowering plants in the Cretaceous and Cenozoic, conifers continue to dominate a variety of temperate and boreal ecosystems.

Ginkgoes appeared in the Triassic, reached their maximum diversity in the Jurassic and are represented today by a single species (Ginkgo biloba) surviving primarily as a valued ornamental. Angiosperms (flowering plants) appeared in the Lower Cretaceous and came to dominate most terrestrial ecosystems. Most of the vascular plants living today are angiosperms.

seed plant diversity over time

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Web:
Hans Kerp's web page on Pteridosperms:
www.uni-muenster.de/GeoPalaeontologie/Palaeo/Palbot/seite9.html
M. & H. Hieb's Plant Fossils of West Virginia:
www.geocraft.com/geocraft/WVFossils/TreeFerns.html
Tree of Life's (tolweb.org) web page on seed plants:
tolweb.org/tree?group=Spermatopsida&contgroup=Embryophyte
U.C. Museum of Paleontology’s web pages on lignopterids:
www.ucmp.berkeley.edu/seedplants/seedplants.html
www.ucmp.berkeley.edu/seedplants/lyginos/lyginoslh.html
U.C. Museum of Paleontology Virtual Lab web pages on early seed plants:
www.ucmp.berkeley.edu/IB181/VPL/Osp/Osp1.html
www.ucmp.berkeley.edu/IB181/VPL/Osp/Osp3.html
Books:
Niklas, K. 1997. The Evolutionary Biology of Plants. Chicago and London: Univ. Chicago Press.
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:
Cressler, W.L., 1999. "Site–analysis and floristics of the Late Devonian Red Hill locality, Pennsylvania, an Archaeopteris-dominated plant community and early tetrapod site." Unpublished Ph.D. Dissertation, Univ. Pennyslvania, Philadelphia, 156 p.
Image Credits:
The spindle diagram is copyrighted © 2005, Dennis C. Murphy. (See Terms of Use.)

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