Department of Plant Biology and Department of Geological Sciences
Michigan State University
In this detail view of one portion of the famous Carboniferous swamp forest reconstruction at the Field Museum in Chicago, the scene is dominated by the trunks of arborescent lycopods. The trees in the foreground are Sigillaria, while larger Lepidodendrons can be seen in the background. Between the foreground trunks is a true botanical puzzle - fern-like foliage bearing large gymnosperm seeds!
By the beginning of this century, paleobotanists began to suspect that a previously unknown group of plants must have been present in the Carboniferous forests. Although the fossil collections of the time were dominated by what appeared to be fern foliage, large seeds and strange pollen-bearing organs were also present. Although these parts had not yet been found connected, Oliver and Scott did a solid piece of detective work. They noted that some of the seeds, pollen organs, fern-like foliage, and stems seen on coal balls, all had unique little glandular structures on the epidermis. This led the two scientists to propose that a plant group they called the Pteridosperms, or "seed ferns", bore fern-like foliage but had an advanced, seed-type reproductive system. We now know that there were several groups of this type, and the two most common ones will be summarized here.
Lyginopteris oldhamia stem in cross-section (Andrews, 1947). Lyginopteris stem material is particularly common in British coal ball deposits.
The Lyginopterid seed fern complex, the one first recognized by Oliver and Scott, is named for the petrified stems of Lyginopteris, commonly found in British coal balls. It also occurs in North American coal balls but is less common. The stems have a prominant pith and a moderate amount of secondary wood. The most conspicuous feature, which makes them very easy to recognize, is the strong radial bands in the outer cortex, caused by a network of thick-walled fibers.
Sphenopteris, one of the foliage types of the Lyginopterid seed fern complex, from Scotland (Andrews, 1947). The compound fronds were delicate, with a lace-like texture.
Lagenostoma is one of the seed types associated with Lyginopterid
seed ferns (Stewart, 1986, based on Oliver and Scott, 1904). The ovules/seeds,
about the size of a peanut, were surrounded by a cup-like structure, formed
of fused bracts, called the cupule. The seeds were borne directly on the
fern-like frond. This reconstruction shows the small glands which Oliver
and Scott used to connect the various parts of the plant.
This reconstruction of Crossotheca (Jennings, 1976) illustrates one of the several lyginopterid pollen organs, which were produced on highly modified foliage units. Pollen was produced in the finger-like sacs hanging from the margin of highly modified leaves.
Most of the lyginopterid seed ferns appear to have been small plants that formed part of the swamp forest undergrowth.
A whole-plant reconstruction of Medullosa noei
(Stewart and Delavoryas, 1956). Leaves consisted of leathery, multiply-compound
fronds. Much of the stem was covered by the persistent based of previously-shed
fronds. Adventitious roots were produced near the base of the stem. Medullosan
seed ferns ranged from shrub-like to small trees, with larger plants such
as this one forming a dense under-story. Medullosan seed fern remains are
very abundant in North American and European Carboniferous deposits.
A cross-section of a Medullosa noei stem as preserved in a coal ball. The stem was slightly flattened prior to permineralization. A thick periderm (P) surrounded the entire stem. The most obvious feature of medullosan stems is that they were polystelic. This stem has three independent steles of secondary wood, one of which is labeled VS. Ground tissues (GT) occupied the area between the steles.
Linopteris, and Callipteridium are common Medullosan
leaf types recognized in compression floras. Fronds were multiply-compound
with complex anatomical structure. Here is an ultimate Alethopteris
frond as preserved in a concretion from Mazon Creek in Illinois (Stewart,
Very large ovules/sees of the Pachytesta type were borne on the margins of the fronds.
Pollen organs were borne on the fronds and were extremely diverse in form. Here is a Whittleseya-type pollen organ on an Alethopteris frond (Stewart and Delavoryas, 1956). Pollen was produced in a series on long, fused sacs, forming a bell-shaped organ.
The Lyginopterid and Medullosan pteridosperms became extinct in the Permian, although a small number of other seed fern groups persisted into the Mesozoic. The seed ferns are now completely extinct.
Ralph E. Taggart (email@example.com)