| Pleuromeia | |
|---|---|
 | |
| Whole plant reconstruction ofPleuromeia sternbergi | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Lycophytes |
| Class: | Lycopodiopsida |
| Order: | Isoetales |
| Family: | †Pleuromeiaceae |
| Genus: | †Pleuromeia Corda (1852) |
| Type species | |
| Sigillaria sternbergi Corda, 1839 | |
| Species | |
See text | |
| Synonyms | |
Lycomeia | |
Pleuromeia is anextinctgenus oflycophytes related to modernquillworts (Isoetes).Pleuromeia dominated vegetation during theEarly Triassic all over Eurasia and elsewhere, in the aftermath of thePermian–Triassic extinction event. During this period it often occurred in monospecific assemblages. Its sedimentary context in monospecific assemblages on immaturepaleosols, 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]
Pleuromeia consisted of a single unbranched stem of variable thickness, which could grow to a maximum of 2 metres (6.6 ft) high inP. sternbergi, (though they were typically smaller),[3] withP. 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 numerousadventive 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 carriesmicrosporophylls, 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 ovoidsporangium and a tongue-like extension nearer to the tip on the upper/inner side. Thetriletemicrospores 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 toTrileites andDensoisporites respectively.[3]
Dense populations ofPleuromeia, in the aftermath ofPermian–Triassic extinction event, are recorded around the world from habitats ranging from semi-arid to tidal.[6] Analysis suggest that they wereperennial 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 ofdormancy, 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 thatPleuromeia hadCrassulacean acid metabolism (CAM)-typecarbon fixation and/or used the Lycopsid Photosynthetic Pathway, as well intaking carbon dioxide from its roots. It is likely that whilePleuromeia 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 typicalC3 type used by other plants.[3]
When theCathedral of Magdeburg was under repair during the 1830s, a block of sandstone crashed and split open, revealing a fragment of the stem ofPleuromeia sternbergi. This was described byGeorge Graf zu Munster in 1839 as a species ofSigillaria. Corda later assigned the species to the new genusPleuromeya. The sandstone had been mined in a quarry nearBernburg (Saale) where later on numerous specimens ofPleuromeia were found, including cones.P. sternbergi has since been found in other Lower and MiddleBuntsandstein 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 familyPleuromeiaceae within theIsoetales, closely related toIsoetaceae, the family which contains modernIsoetes.[5] Around 20 species ofPleuromeia have been described, predominantly from the Northern Hemisphere, though it is not clear that all of these species are valid.[4]
Table of species after Deng et al. 2023.[4]
| Species | Age |
|
|---|---|---|
| Pleuromeia altinis | Induan | North China |
| Pleuromeia epicharis | Olenekian | North China |
| Pleuromeia hataii | Olenekian | Japan |
| Pleuromeia jiaochengensis | Induan | North China |
| Pleuromeia jokunzhica | Olenekian - Anisian | Darvaz, Kazakhstan |
| Pleuromeia longicaulis (Cylostrobus sydneyensis) | Lower Triassic | Eastern Australia |
| Pleuromeia marginulata | Anisian | South China |
| Pleuromeia obovata | Ladinian | North China |
| Pleuromeia olenekensis | Olenekian | Siberia (Russia) |
| Pleuromeia pateriformis | Induan | North China |
| Pleuromeia reniformis | Olenekian | Eastern Australia |
| Pleuromeia rossica | Olenekian | Upper Volga, Russia |
| Pleuromeia sanxiaensis | Anisian | South China |
| Pleuromeia shaolinii | Anisian | Northeast China |
| Pleuromeia sternbergii | Olenekian to Ladinian? | Across Eurasia (from Western Europe to China) and possibly Argentina |
| Pleuromeia sp. 1[8] | Olenekian | Taimyr, Russia |
| Pleuromeia sp. 2[9] | Olenekian | Nevada, USA |
The placement of the speciesPleuromeia 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]
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 followingOlenekian. During theAnisian, 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 earlyLadinian 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]