Pleuromeia
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''Pleuromeia'' is an
extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...
genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...
of lycophytes related to modern quillworts (''Isoetes''). ''Pleuromeia'' dominated vegetation during the
Early Triassic The Early Triassic is the first of three epochs of the Triassic Period of the geologic timescale. It spans the time between 251.9 Ma and Ma (million years ago). Rocks from this epoch are collectively known as the Lower Triassic Series, which ...
all over Eurasia and elsewhere, in the aftermath of the
Permian–Triassic extinction event The Permian–Triassic extinction event (also known as the P–T extinction event, the Late Permian extinction event, the Latest Permian extinction event, the End-Permian extinction event, and colloquially as the Great Dying,) was an extinction ...
. During this period it often occurred in monospecific assemblages. Its sedimentary context in monospecific assemblages on immature
paleosol In Earth science, geoscience, paleosol (''palaeosol'' in Great Britain and Australia) is an ancient soil that formed in the past. The definition of the term in geology and paleontology is slightly different from its use in soil science. In geo ...
s, is evidence that it was an opportunistic pioneer plant that grew on mineral soils with little competition. It spread to high latitudes with greenhouse climatic conditions.


Description

''Pleuromeia'' consisted of a single unbranched stem of variable thickness, which could grow to a maximum of high in ''P. sternbergi'', (though they were typically smaller), with ''P. jiaochengensis'' only reaching about in height. 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. 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 A sporangium (from Late Latin, ; : sporangia) is an enclosure in which spores are formed. It can be composed of a unicellular organism, single cell or can be multicellular organism, multicellular. Virtually all plants, fungus, fungi, and many ot ...
and a tongue-like extension nearer to the tip on the upper/inner side. The trilete
microspore Microspores are land plant spores that develop into male gametophytes, whereas megaspores develop into female gametophytes. The male gametophyte gives rise to sperm cells, which are used for fertilization of an egg cell to form a zygote. Megaspo ...
s 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. The megaspores and microspores are assigned to ''Trileites'' and ''Densoisporites'' respectively.''''


Ecology

Dense populations of ''Pleuromeia'', in the aftermath of
Permian–Triassic extinction event The Permian–Triassic extinction event (also known as the P–T extinction event, the Late Permian extinction event, the Latest Permian extinction event, the End-Permian extinction event, and colloquially as the Great Dying,) was an extinction ...
, are recorded around the world from habitats ranging from semi-arid to tidal. 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 Dormancy is a period in an organism's Biological life cycle, life cycle when growth, development, and (in animals) physical activity are temporarily stopped. This minimizes metabolism, metabolic activity and therefore helps an organism to conserv ...
, 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 Biological carbon fixation, or сarbon assimilation, is the Biological process, process by which living organisms convert Total inorganic carbon, inorganic carbon (particularly carbon dioxide, ) to Organic compound, organic compounds. These o ...
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.


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 ''Sigillaria'' is a genus of extinct, spore-bearing, Lepidodendrales, arborescent lycophyte, known from the Carboniferous and Permian periods. It is related to the more famous ''Lepidodendron'', and more distantly to modern Isoetes, quillworts. ...
''. Corda later assigned the species to the new genus ''Pleuromeya''. The sandstone had been mined in a quarry near
Bernburg Bernburg (Saale) () is a town in Saxony-Anhalt, Germany, capital of the Salzlandkreis district. The former residence of the Anhalt-Bernburg princes is known for its Renaissance castle. Geography The town centre is situated in the fertile Magdeb ...
(Saale) where later on numerous specimens of ''Pleuromeia'' were found, including cones. ''P. sternbergi'' has since been found in other Lower and Middle
Buntsandstein The Buntsandstein (German for ''coloured'' or ''colourful sandstone'') or Bunter sandstone is a lithostratigraphy, lithostratigraphic and allostratigraphy, allostratigraphic unit (a sequence of rock strata) in the Subsurface (geology), subsurface ...
deposits elsewhere in Germany, France and Spain. Other species have been described from several localities in Russia, Australia, South America and Japan. ''Pleuromeia'' is placed in the family Pleuromeiaceae within the Isoetales, closely related to Isoetaceae, the family which contains modern '' Isoetes''. 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. Table of species after Deng et al. 2023. 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.


Evolutionary history

The earliest members of the genus are known from the earliest Triassic (
Induan The Induan is the first age of the Early Triassic epoch in the geologic timescale, or the lowest stage of the Lower Triassic series in chronostratigraphy. It spans the time between 251.9 Ma and 249.9 Ma (million years ago). The Induan is so ...
) of North China, with the genus becoming abundant and globally distributed during the following
Olenekian In the geologic timescale, the Olenekian is an age (geology), age in the Early Triassic epoch (geology), epoch; in chronostratigraphy, it is a stage (stratigraphy), stage in the Lower Triassic series (stratigraphy), series. It spans the time betw ...
. During the
Anisian In the geologic timescale, the Anisian is the lower stage (stratigraphy), stage or earliest geologic age, age of the Middle Triassic series (stratigraphy), series or geologic epoch, epoch and lasted from million years ago until million years ag ...
, 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.


References


External links


Information and image
{{Taxonbar, from=Q142219 Prehistoric lycophytes Triassic plants Fossil taxa described in 1852 Anisian life Induan genus first appearances Anisian genus extinctions Prehistoric lycophyte genera