Glossopteris

Glossopteris is the largest and best-known genus of the extinct Permian order of seed ferns known as Glossopteridales (also known as Arberiales, Ottokariales, or Dictyopteridiales). The genus "Glossopteris" refers only to leaves, within a framework of form genera used in paleobotany. (For likely reproductive organs see Glossopteridaceae.) Species of "Glossopteris" were widespread over the supercontinent of Gondwana during the Permian epoch, where they formed the dominant component of high latitude polar forests. "Glossopteris" fossils were critical in recognizing former connections between the various fragments of Gondwana: South America, Africa, India, Australia, New Zealand, and Antarctica.
History.
The Glossopteridales arose in the Southern Hemisphere around the beginning of the Permian Period (), 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 (). 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.
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 misdentifications 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.
Taxonomy.
Long considered a fern after its discovery in the 1820s, it was later assigned to the gymnosperms. The genus is placed in the division Pteridospermatophyta. In reality, many of the plant groups included within this division are only distantly related to one another. Glossopterids' relationships with other groups remain obscure. Most recent phylogenetic analyses favour placement of glossopterids as sister to a large group including Corystospermales, Caytoniales, Bennettitales, Pentoxylales, Gnetales (in some analyses), and angiosperms. A few analyses favour alternative links with Ginkgoales, Cordaitales and Pinales.
"Glossopteris" should strictly be used to refer to the distinctive spathulate fossil leaves with reticulate venation, however, the term has also been used to refer to the parent plant as a whole.
The name comes from Ancient Greek ( 'tongue'), because the leaves were tongue-shaped, and ( 'fern, feathery').
Description.
"Glossopteris" was a woody, seed-bearing shrub or tree, some apparently reaching 30 meters tall. They had a softwood interior that resembles conifers of the family Araucariaceae. 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 least some species were the latter.
Paleoecology.
They are interpreted to have grown in very wet soil conditions, similar to the modern Bald Cypress. The leaves ranged from about 2 cm to over 30 cm in length.
The profile of glossopterid trees is largely speculative as complete trees have not been preserved. However, based on analogies with modern high-latitude plants "Glossopteris" trees probably tapered upwards like a Christmas tree and were relatively widely spaced to take advantage of the low-angle sunlight at high latitudes, instead of needles, they had large, broad lance- or tongue-shaped leaves commonly with well differentiated palisade and spongy mesophyll layers
. "Glossopteris" trees are assumed to be have been deciduous, as fossil leaves are commonly found as dense accumulations representing autumnal leaf banks. The broad fossilized growth rings in many "Glossopteris" woods reveal that the plants experienced strong growth spurts each spring-summer but underwent the abrupt cessation of growth before each following winter. The idea that all "Glossopteris" species are deciduous has been challenged, with an isotopic study finding that Antarctic "Glossopteris" forests were mixed evergreen-deciduous.
"Glossopteris" leaves are morphologically simple so there are few characters that can be used to differentiate species. Consequently, many past researchers have considered the Permian "Glossopteris" flora to be rather homogeneous with the same species distributed throughout the Southern Hemisphere. However, more recent studies of the more morphologically diverse fertile organs have shown that taxa had more restricted regional distributions and several intra-gondwanan floristic provinces are recognizable. Seeds, much too large to be wind-borne, could not have blown across thousands of miles of open sea, nor is it likely they have floated across vast oceans. Observations such as these led the Austrian geologist Eduard Suess to deduce that there had once been a land bridge between these areas. He named this large land mass Gondwanaland (named after the district in India where the plant "Glossopteris" was found). These same observations would also lend support to Alfred Wegener's Continental drift theory.
The first Antarctic specimens of "Glossopteris" were discovered by members of Robert Scott's doomed Terra Nova Expedition. The expedition members abandoned much of their gear in an effort to reduce their load, but kept 35 pounds of "Glossopteris" fossils; these were found alongside their bodies.
Outcrops in Brazil.
The first investigation of a "Glossopteris" flora associated with coal seams within a paleogeographic and palaeoclimatic context, in the Paraná Basin, southern Brazil, was that by geologist Israel Charles White in 1908. This allowed correlation between Gondwanan coal deposits in southern Brazil and those documented in South Africa, Australia, India and Antarctica, and showed that this flora flourished in latitudes near the south pole.
In Rio Grande do Sul, "Glossopteris" leaves were found in paleorrota at "Mina Faxinal", in Arroio dos Ratos at "Mina Morro do Papaléo" in Mariana Pimentel and "Quitéria" in Pantano Grande. Various species were recovered from the Rio Bonito Formation at these sites including "G. angustifolia, G. brasiliensis, G. browniana, G. communis, G. indica and G. occidentalis".