Introduction to the Glossopteridales

The Glossopteridales are an extinct group of seed plants that arose during the Permian on the great southern continent of Gondwana. These plants went on to become a dominant part of the southern flora through the rest of the Permian, though they dwindled to extinction by the end of the Triassic Period.

Glossopteris, the genus from which the group gets its name, is also the largest and best-known member of the Glossopteridales. More than 70 species of this genus have been recognized in India alone, with additional species from South America, Australia, Africa, and Antarctica. Only a few fossils from the northern hemisphere have been considered as members of this group, but these are not identified with great certainty.

The rapid appearance, expansion, and relatively quick extinction of this group, as well as the large number of species, has made the group very important for understanding paleobiogeography, specifically in the recognition of areas that were once connected together, but are now separated through the action of continental drift. As a result, there is a wealth of descriptive literature available on glossopterids. However, most of the available fossils are sterile leaves lying unattached to stems, and so a number of species which have been described are not well differentiated, and few are known from distinctive reproductive structures. This makes it difficult to be certain of the actual number of species.

The large number of leaves found preserved, and the character of the deposits in which these are found, suggest that glossopterids were deciduous, losing their leaves in the autumn, and growing new leaves in each spring. Some specimens show an abscission zone, such as that along which a leaf detaches in living plants. A number of tiny scale-leaves have also been found; these may represent first-growth leaves or bud scales. Mature leaves were often 10 cm long, and some leaves have been found over a meter in length. These mature leaves are very easy to recognize, since they have a strong central vein and a network of smaller veins. This is quite different from the other seed plants of the Late Paleozoic and early Mesozoic, which usually had no midvein and secondary veins which ran parallel to each other. You can see the striking preservation of these vein networks in the picture below.


The pictures above show features of Glossopteris leaves. The image on the left shows the fine preservation of leaf veins, while the one on the right shows smaller leaves which may be first-growth leaves or bud scales. The leaf on the left was collected in Antarctica, and those on the right from South Africa. Click on either image to view an enlargement.

The reproductive structures of glossopterids are as unusual as the foliage leaves. They appear to have been borne on leaves as in other "pteridosperms". Poor preservation has led to much controversy over their structure and their arrangement on living plants from which they came. At least one point has become clear: pollen and seeds were produced in different organs, attached to separate leaves, though the specifics of the organs themselves are not as clearly settled. Pollen organs have been described as anything from a modified leaf bearing stalked pollen sacs to cone-like clusters.

More common as fossils than pollen organs are ovule-bearing organs (those that are responsible for producing seeds). A dazzling variety of these structures has been found, suggesting that this really was a large and diverse group of plants. Seeds appear to have been produced on the underside of the leaf, with the leaf edges rolled over to form an enclosing chamber. This may not have been the case for all species, since at least some fossils have been reconstructed with ovules hanging in clusters from a much smaller, and un-enclosing leaf.

In addition to their distinctive foliage and reproductive structures, glossopterids also have unusual roots. These have been asigned to the form genus Vertebraria, so called because of the regularly spaced partitions which give the appearance of a backbone. The roots are remarkable for the lobed wood at their center, with internal spaces between the lobes -- a highly unusual morphology among plants.


Vertebraria: The roots of glossopterids have regular cross-partitions that give them the appearance of a vertebrate backbone. This specimen was found in Permian deposits from India. Click on the above image for a more detailed view.

The most frustrating aspect of glossopterid paleobiology is that no one is really certain what these plants looked like. Large portions of plants have never been preserved intact, and so reconstructing them has been done from rather tiny pieces. Our best guess is that they were large shrubs or small trees, perhaps a bit like a magnolia or ginkgo. There is at least some evidence that, like Ginkgo, glossopterids had a stem system of both long and short shoots. In Ginkgo, it is the short shoots that produce most of the leaves, and that bear the seeds and pollen structures. This may also have been the case for the glossopterids, but no one is certain.


Arizona State has an article in their on-line magazine which includes information about Glossopteris fossils from Australia.



Source:
Taylor, T. N. & E. L. Taylor. 1993. The Biology and Evolution of Fossil Plants, Prentice Hall, NJ, USA.