Pleuromeia is an extinctgenus of lycophytes related to modern quillworts (Isoetes). Pleuromeia dominated vegetation during the Early Triassic all over Eurasia and elsewhere, in the aftermath of the Permian–Triassic extinction event. During this period it often occurred in monospecific assemblages. Its sedimentary context in monospecific assemblages on immature paleosols, is evidence that it was an opportunistic pioneer plant that grew on mineral soils with little competition.[1] It spread to high latitudes with greenhouse climatic conditions.[2]
Description
Fossils of Pleuromeia sternbergi (A) Cast of stem (B-C) compression fossil of strobilus with sporophylls, (D-F) compression fossil of stem showing rhomboidal leaf bases (G-H) leaf compression fossils
Pleuromeia consisted of a single unbranched stem of variable thickness, which could grow to a maximum of 2 metres (6.6 ft) high in P. sternbergi, (though they were typically smaller),[3] with P. jiaochengensis only reaching about 30 centimetres (0.98 ft) in height.[4] Around the stem were helically arranged triangular leaves, which became tapered towards their ends. These were attached to rhomboidal-shaped leaf bases on the stem.[3] It had a 2-4 lobed bulbous base to which numerous adventive roots were attached. Pleuromeia produced a single heterosporus large cone (strobilus) at the tip of the stem or in some species many smaller cones. The top of the cone carries microsporophylls, the lower part megasporophylls, and both types may be intercalated midlength. Sporophylls are disposed from the bottom up. Both types are obovate, with a round to ovoid sporangium and a tongue-like extension nearer to the tip on the upper/inner side. The triletemicrospores are hollow, round and 30–40 μm in diameter. Megaspores have a layered outer skin with a small trilete mark, are also hollow, round to ovoid and up to 300–400 μm in diameter.[5] The megaspores and microspores are assigned to Trileites and Densoisporites respectively.[3]
Ecology
Dense populations of Pleuromeia, in the aftermath of Permian–Triassic extinction event, are recorded around the world from habitats ranging from semi-arid to tidal.[6] Analysis suggest that they were perennial plants with relatively slow growth rates. However it is likely that they were also capable of rapid growth shortly prior to reproduction at the end of their lifespan. Their spores were likely able to survive long periods of dormancy, forming spore banks in the soil that were able to germinate long after the parent plants had died. Like modern quillworts, it has been suggested that Pleuromeia had Crassulacean acid metabolism (CAM)-type carbon fixation and/or used the Lycopsid Photosynthetic Pathway, as well intaking carbon dioxide from its roots. It is likely that while Pleuromeia was stress tolerant, it was poorly competitive against other plants under less stressed growing conditions, which may have been due to the much lower efficiency of the CAM-type carbon fixation compared to the typical C3 type used by other plants.[3]
Taxonomy
When the Cathedral of Magdeburg was under repair during the 1830s, a block of sandstone crashed and split open, revealing a fragment of the stem of Pleuromeia sternbergi. This was described by George Graf zu Munster in 1839 as a species of Sigillaria. Corda later assigned the species to the new genus Pleuromeya. The sandstone had been mined in a quarry near Bernburg (Saale) where later on numerous specimens of Pleuromeia were found, including cones. P. sternbergi has since been found in other Lower and Middle Buntsandstein deposits elsewhere in Germany, France and Spain. Other species have been described from several localities in Russia, Australia, South America and Japan.[7]
Pleuromeia is placed in the family Pleuromeiaceae within the Isoetales, closely related to Isoetaceae, the family which contains modern Isoetes.[5] Around 20 species of Pleuromeia have been described, predominantly from the Northern Hemisphere, though it is not clear that all of these species are valid.[4]
The placement of the species Pleuromeia dubia from the Early Triassic of Australia in the genus has been questioned, due to its anatomy strongly differing from the typical species of the genus.[4]
Evolutionary history
The earliest members of the genus are known from the earliest Triassic (Induan) of North China, with the genus becoming abundant and globally distributed during the following Olenekian. During the Anisian, the genus declined following the development of a more warm humid climate and the subsequent diversification of other plant groups, with the youngest records of the genus being from the early Ladinian of North China, around 241.0-241.6 million years ago, with the genus likely being extinct by the beginning of the Late Triassic.[4]
References
^Retallack, Gregory J. (1997). "Earliest Triassic origin of Isoetes and quillwort evolutionary radiation". Journal of Paleontology. 7 (3): 500–521. doi:10.1017/S0022336000039524. S2CID140566050.
