The reproductive structures of the Glossopterids were extremely diverse; there were two types of Glossopterid fruits that were so different that the Glossopterids could be easily split into two sections.
The Glossopterid flora is important in the evolutionary story because the fruit types of this order are believed to have had a heavy influence of the flora that evolved later.
SECTION MEGAFRUCTI - Glossopteridales sect. Megafructi
Many fossils of Megafructi Glossopterids have been found in India and Australia, though palaeobotanists had difficulty in understanding exactly what they looked like owing to the poor quality of many of the fossils.
The fruits of this section (Megafructi: mega=big fructi=fruit) are large to massive, have lots of seeds and are borne on unmodified foliage leaves either being stalked or sessile and attached either to the blade or the petiole of the leaf. (The fruits were not produced on stalks of their own). The position on the leaf where the fructi are attached has formed the basis of a new means of natural classification for this group.
The "fructi" (sporangia) of sect. Megafructi consisted of a core or receptacle, which was surrounded by seeds. A cover leaf (bract) gave the seeds protection. It is thought that the bract fell off when the seeds were mature. As time progressed, the fructi were modified in Glossopterids and there was fusion between the parts of the organ, and a reduction in the number of seeds. A fossil discovery in Australia in the 1970's unearthed a Glossopterid remain displaying seeds which were produced on the underside of a leaf, with that leaf wrapped around them.*
The simple reduction of the number of seeds and the development of a fused cover leaf could have lead to the development of the Angiosperms (flowering plants) and Ranales (Magnolia-like flowers). Furthermore, there is evidence from the structure of the fertile branches of Glossopterids that these may have lead to "flowers" through the reduction of leaves.
SECTION MICROFRUCTI - Glossopteridales sect. Microfructi
The Microfructi are similar to the Megafructi in that the "fruiting structures" are produced from the leaves (and not on their own stems). However they are substantially smaller. There are two types of Microfructi, and it is believed that these led to the angiosperms, cycads and southern conifers.
Male reproductive structures
The male sporangia are attached to small forking branches which are attached to the leaves of various Glossopteris species. Many of these structures were borne on fertile leaves though some were borne on catkins, scales or cones. Less is known about the mal structures of the Glossopterids because there are far fewer fossilised specimens.
Summary: Key features of the Glossopteridales
- Fertile leaves are modified like the precursor to petals.
- The fertile cover leaf and the seed fuse as the Glossopterids evolve.
- The leaves show signs of reduction.
- They have fruits which have (i) a receptacle with seeds attached around it and (ii) a cover leaf.
- The male reproductive structures are smaller than the female and sometimes occur near the female reproductive structures on a branch. (Is this the precursor to the bisexual flower?)
- The two sections of Glossopteridales give rise to the possibility of two distinct evolutionary lines from within the Order.
Current theories suggest that:
- Megafructi led to the Angiosperms, as the fusion of the seeds and the receptacle in the Glossopterids could have led to the carpel - a key component of an angiosperm ovary.
- Microfructi led to catkin-bearing angiosperms such as members of the Fagaceae (Oak family). Evidence includes the observed formation of lax or loose catkins in parts of this Section.
- The catkin producing section of Microfructi seems to have led to the Southern Conifers through the development of an early conifer; Walkomiella.
- The Lidgettonia strain of Microfructi led to the seed ferns of the Mesozoic.