Glossopteris (etymology: from Ancient Greek γλῶσσα, glôssa 'tongue' + πτερίς, pterís 'fern') is the largest and best-known genus of the extinct Permian order of seed plants known as Glossopteridales (also known as Arberiales, Ottokariales, or Dictyopteridiales). The name Glossopteris refers only to leaves, within the framework of form genera used in paleobotany, used for leaves of plants belonging to the glossopterid family Dictyopteridiaceae. Species of Glossopteris were the dominant trees of the middle to high-latitude lowland vegetation, often in swampy environments, across Gondwana (which at this time formed the southern part of Pangaea) during the Permian Period. Glossopteris fossils were critical in recognizing former connections between the various fragments of Gondwana: South America, Africa, India, Australia, New Zealand, and Antarctica. The Glossopteris forest ecosystems became extinct as part of the end-Permian mass extinction event approximately 252 million years ago, which was caused by an abrupt rise in temperatures produced by the eruption of Siberian Traps.
Description
The leaves of Glossopteris are characterized by their distinctive tongue shape that gives them their name, as well as their reticulate venation. The leaves were either widely spaced on long stems, or were densely helically arranged on short shoots. Glossopteris bearing plants grew as woody, seed-bearing trees and shrubs. Their trunks had a maximum diameter of 80 centimetres (2.6 ft), with some likely reaching a height of 30 metres (98 ft). They had a softwood interior resembling Araucariaceae conifers. Seeds were borne on one side of variably branched or fused structures, and microsporangia containing pollen were borne in clusters at the tips of slender filaments. Both the seed- and pollen-bearing organs were partially fused (adnate) to the leaves, or, in some cases, possibly positioned in the axils of leaves. The homologies of the flattened seed-bearing structures have remained particularly controversial with some arguing that the fertile organs represent megasporophylls (fertile leaves) whereas others have interpreted the structures as flattened, seed-bearing, axillary axes (cladodes). It is unclear whether glossopterids were monoecious or dioecious, the fact that only pollen organ bearing leaves and not ovules were found in some layers suggest that at least some species were the latter.
Distribution More than 70 fossil species of this genus have been recognized in India alone, with additional species from South America, Australia, Africa, Madagascar and Antarctica. Essentially, Glossopteris was restricted to the middle- and high-latitude parts of Gondwana during the Permian and was an important contributor to the vast Permian coal deposits of the Southern Hemisphere continents. Most northern parts of South America and Africa lack Glossopteris and its associated organs. However, in recent years a few disparate localities in Morocco, Oman, Anatolia, the western part of the island of New Guinea, Thailand and Laos have yielded fossils that are of possible glossopterid affinity. These peri-gondwanan records commonly occur together with Cathaysian or Euramerican plant species—the assemblages representing a zone of mixing between the strongly provincial floras of the Permian. Apart from those in India and the peri-gondwanan localities, a few other fossils from the Northern Hemisphere have been assigned to this group, but these are not identified with great certainty. For example, specimens assigned to Glossopteris from the far east of Russia in the 1960s are more likely to be misidentifications of other gymnosperms such as Pursongia. Confident assignment of fossil leaves to Glossopteris normally requires their co-preservation with the distinctive segmented roots of this group (called Vertebraria) or with the distinctive fertile organs. In 2018, Glossopteris leaves were reported from mid-Permian (Roadian – early Wordian) deposits in Mongolia, then located at high latitudes in the Northern Hemisphere, but these fossils were not found in association with other typical glossopterid organs, such as chambered roots or reproductive structures, so the phylogenetic affinities of these leaves remain uncertain.
Chronology The Glossopteridales arose in the Southern Hemisphere around the beginning of the Permian Period (298.9 million years ago), but became extinct during the end-Permian (Changhsingian) mass extinction. The putative persistence of Glossopteris into younger strata is commonly invoked on the basis of the distribution of dispersed taeniate bisaccate pollen. However, this category of pollen is known to have been produced by various seed plants, and Triassic examples, in the absence of convincing co-preserved Glossopteris leaves, probably belonged to non-glossopterid groups, such as voltzialean conifers. The distribution of Glossopteris across several, now detached, landmasses led Eduard Suess, amongst others, to propose that the southern continents were once amalgamated into a single supercontinent—Pangea. These plants went on to become the dominant elements of the southern flora through the rest of the Permian but disappeared in almost all places at the end of the Permian (251.902 million years ago). The only potential Triassic records are Glossopteris leaves exposed in the banks of the Gopad River near Nidpur, India, but even these records are stratigraphically ambiguous owing to faulting and complex juxtapositioning of Permian and Triassic strata at Nidpur. Moreover, even if some Glossopteris leaves do persist above the end-Permian extinction horizon, this level pre-dates the Permian-Triassic boundary proper in continental settings of Gondwana by several hundred thousand years and there are no convincing examples of Glossopteris in confidently dated Triassic strata. Although most modern palaeobotany textbooks cite the continuation of glossopterids into later parts of the Triassic and, in some cases into the Jurassic, these ranges are erroneous and are based on misidentification of morphologically similar leaves such as Gontriglossa, Sagenopteris, or Mexiglossa. Glossopterids were, thus, one of the major casualties of the end-Permian mass extinction event.
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