Isoetites sp.
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publication ID |
https://doi.org/ 10.4202/app.2012.0010 |
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persistent identifier |
https://treatment.plazi.org/id/03C52B56-FFB0-FFCF-FCB1-6F39FA6F44CD |
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treatment provided by |
Felipe |
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scientific name |
Isoetites sp. |
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Figs. 2A, B View Fig , 3A–D View Fig , 4A–E View Fig .
Material. —MPZ 2010/919, from Estercuel quarry, Teruel, northeastern Spain.
Description
Microsporophyll.—The leaf impression is simple, linear, about 22.1 mm long and 4.4 mm wide ( Fig. 2A, B View Fig 1 View Fig , B 2 View Fig ). The margin is entire, with its base slightly expanded and the apex acute. The leaf shows a large midvein up to 2 mm wide ( Fig. 2A View Fig , indicated as central vein). The lamina is trabeculated, with trabeculae at intervals of about 1 mm ( Fig. 2A, B View Fig 1 View Fig , B 2 View Fig , indicated by arrows in Fig. 2B 2 View Fig ). However, the presence of trabeculae (which would not be confused by horizontal lines appearing on the laminar surface of the sporophyll are related to air channels in extant and fossil Isoetes and related forms) inside the microsporangium is not evident possibly due to taphonomic biases ( Fig. 2B View Fig 4 View Fig ). The impression of an elliptical ligule is preserved with an acute apex ( Fig. 2A, B View Fig 1 View Fig , B 3 View Fig ).
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The base of the microsporophyll is spatulate proximally and oval distally ( Fig. 2A, B View Fig 1 View Fig ). It measures 9.7 mm long and 2.5 mm wide and contains the elongate microsporangium ( Fig. 2A, B View Fig 1 View Fig , B 4 View Fig ). There is no evidence of a velum. The microsporangium contains a mesh of circa 135 polygonal bodies ( Figs. 2A, B View Fig 1 View Fig , B 4 View Fig , 3A View Fig ) of variable sizes (largest axes between 300 μm and 450 μm) and different morphologies with pentagonal or hexagonal shapes.
The masses are elliptical in shape, approximately 400– 580 μm long and 290–320 μm wide, containing hundreds of microspores ( Fig. 3B, D View Fig ). The microspores seems to be grouped in tetrads ( Fig. 3C View Fig , indicated by arrows).
Microspores ( Figs. 3C View Fig , 4A–E View Fig ).—Bilateral, plano-convex, monolete with a perinous layer and the straight laesura is well marked on almost the whole length of the proximal face, with elevated lips 2.6–4.5 μm high ( Fig. 4D, E View Fig ). Microspores are elliptic in polar view 20–34 μm long and 17–32 μm wide. Proximal and distal faces appear flattened due to compaction. A distinct feature of the microspore is a flange-like extension of the equatorial ridge 1.4–3.7 μm wide ( Fig. 4A–E View Fig ). On the proximal and distal face the perispore has a microgranulate micro-ornamentation ( Fig. 4A–E View Fig ). A characteristic tuberculate ornamentation occurs only on the distal face, with tubercles (0.1–2 μm high and 0.4–2 μm wide) spaced 0.2–4.9 μm apart and showing an apical more or less circular pit 0.1–1.6 μm in diameter ( Figs. 3C View Fig , 4A, B View Fig ). A cavate exine is 0.4 μm thick with a smooth inner layer of 0.1 μm and a tuberculate outer layer of 0.3 μm.
Remarks.—During the Triassic there is a change in isoetalean microspore apertures from trilete to monolete, a character related to modern-day Isoetes ( Pigg 2001) . Nonetheless, Pleuromeia Corda in Germar, 1852 ( Glaessner and Rao 1955), Lycostrobus Nathorst, 1908 ( Kempf 1971; Scott and Playford 1985) and Lepacyclothes Emmons, 1856 (formerly Annalepis Fliche, 1910 in Retallack 1997: 507) are other Triassic genera related to Isoetites , which have yielded in situ monolete microspores similar to Aratrisporites ( Dettmann 1961; Retallack 1997; Grauvogel-Stamm and Lugardon 2001), showing this evolution. Aratrisporites microspores have also been found from Tenellisporites trilete megaspores ( Fliche 1910; Grauvogel-Stamm and Duringer 1983). Moreover, Cretaceous genera Nathorstiana Richter, 1909 , and Nathorstianella Glaessner and Rao, 1955 are also related to Isoetites ( Kustatscher et al. 2010) . There is a clear evolution trend from the Triassic Pleuromeia , through Cretaceous Nathorstiana and Nathorstianella , to Isoetes ( Glaessner and Rao 1955) . However, Nathorstiana and Nathorstianella have not yielded any reproductive structures within the fossil record to compare those microspores with the microspores presented herein.
Similarities between Selaginella and Isoetes have been established due to the similarities in wall structure of the microspores and megaspores, suggesting an old connection of these groups ( Tryon and Lugardon 1990). However, the distinctive vegetative characters and the difference in aperture of the microspores (monolete in Isoetes and trilete in Selaginella ) support the association of the studied fossil material with Isoetes rather than Selaginella .
Morphological characters of the here described in situ microspores, correspond most closely to the cavate genera Peromonolites Couper, 1953 and Aratrisporites Leschik, 1955 . The original descriptions of both genera and emendation of the genus Aratrisporites ( Klaus 1960: 145; Playford and Dettmann 1965: 151), are similar in terms of morphological characters. Microspores belonging to Isoetes are also morphologically similar to the fossil spores Aratrisporites (see illustrations in Harris 1955: pl. 2: 1, 2) and Peromonolites (see illustrations in Couper 1953: figs. 31, 32; 1960: pl. 2: 1). Aratrisporites has been used for Triassic and Peromonolites for Cretaceous–Cenozoic spores.
Initially, the genus Peromonolites was delimited for the Cretaceous by Couper (1953) as anisopolar, bilateral monolete (occasionally alete) spores with laesura occasionally indicated only by a weak area in the exine, surrounded by a sculptured perispore. In the diagnosis of this genus this author did not give any indication of the differences in the ornamentation on distal and proximal faces. Moreover, he described the type species Peromonolites bowenii Couper, 1953 as having sub-verrucate ornamentation of the central body in both faces, a hyaline psilate perispore, and as being distinct by its monolete aperture with a laesura extending the whole length of proximal face.
The first description of the genus Aratrisporites given by Leschik (1955) included zonate spinulose spores, however, this first description did not include any reference of the type of aperture, but was considered monolete by Klaus (1960). The latter emended the genus Aratrisporites stating that it was a gondola-shaped monolete spore, with granulate to spinulose sclerine, and zonate exine. Later, Playford and Dettmann (1965) emended the diagnosis of Aratrisporites and transferred it from the Subturma Zonomonoletes Luber, 1935 ( Luber 1935) to Suprasubturma Perinomonolites Erdtman, 1947 ( Erdtman 1947) on the grounds that it is cavate and not zonate. Playford and Dettmann (1965) also considered the genus Saturnisporites Klaus, 1960 a junior synonym of Aratrisporites ( Leschik, 1955) Playford and Dettmann, 1965 . They assumed that the characters of presence of elevated laesurate lips and the absence of typical anchor-shaped laesura in the polar ends, as described by Klaus (1960), were not diagnostic features for differentiating these genera. None of these authors gave any indication of differences in ornamentation of the sclerine between the proximal and distal faces in the description of the genus and type species, and it is difficult to observe in the original figures of Leschik (1955: pl. 5: 2, 4).
In situ microspores of Aratrisporites (probably A. minimus Schulz, 1967 ) have been described from Triassic microsporophylls of Annalepis zeilleri Fliche, 1910 ( Grauvogel-Stamm and Duringer 1983), which are considered isoetalean lycopsids ( Pigg 1992). The sculpture of those microspores are specified in the text as being infragranulate to punctate, however it seems from the photographs that the proximal face is verrucate and distal face baculate ( Grauvogel-Stamm and Duringer 1983: pl. 6), similar to herein described microspores. Moreover, in situ microspores of Isoetites brandneri from the Triassic of the Dolomites (northern Italy) could belong to the genus Aratrisporites ( Kustatscher et al. 2010: pl. 2: 4), however, proximal and distal sculptures of those microspores are not described and they are difficult to observe from the figure. It seems that in Aratrisporites flexibilis and A. paenulatus Playford and Dettman, 1965 the proximal face is verrucate and the distal face baculate.
According to Playford and Dettmann (1965) Peromonolites Couper, 1953 differs from Aratrisporites in having a sculptured inner wall layer. Brenner (1963) described the species Peromonolites allensis as having a smooth inner layer and an extremely wrinkled outer layer. However, this character is very difficult to observe, and hence it is not very useful for systematics. There are also some problematic species included in the genus Peromonolites such as Peromonolites archangelskii Baldoni, 1987 , which lacks a cavate exine ( Baldoni 1987) and Peromonolites problematicus Couper, 1953 , which lacks an evident aperture and has morphological similarities to the trilete Bryosporis anisopolaris Mildenhall, 1990 ( Bussell and Mildenhall 1990). For this reason, a more detailed taxonomic revision of the genera Aratrisporites and Peromonolites seems necessary, in order to better understand those morphotypes. Thus, we have used the first described genus Peromonolites Couper, 1953 , which also coincides with the terminology used for Cretaceous–Cenozoic spores.
Microspores studied herein resemble more closely Peromonolites densus Harris, 1965 characterized by a thick perispore and granulate ornamentation with granules which are more or less uniformly distributed. The presently described in situ microspores are very different from Peromonolites archangelskii Baldoni, 1987 which are not cavate spores and present a fibrous ornamentation on both faces so this species should not be included within this genus. Peromonolites asplenoides Couper, 1958 differs by a greater size (overall 70–90 μm) and thicker perispore (about 25 μm in thickness). Peromonolites bowenii Couper, 1953 differs in having a verrucate ornamentation on both faces. Peromonolites granulatu s Norton, 1969 is distinct in having a baculate to granulate ornamentation and indistinct laesura. Peromonolites pehuenche Volkheimer, 1972 differs by its distinct irregularly-shaped hyaline perispore, a more circular outline of the spore and microgranulate ornamentation. Peromonolites subengelmannii (Elsik, 1968) Jameossanaie, 1987 differs in having a less distinct laesura, thicker perine and microgranulate ornamentation on both faces. Peromonolites vellosus Partridge in Stover and Partridge, 1973 is distinguishable from microspores studied herein by having a thicker perispore and fibrous, mat-like ornamentation on both faces. Peromonolites fragilis Burger, 1966 , without any indication of tubercles in the original description, differs in having a minutely and densely wrinkled, scabrate-reticulate ornamentation on both faces, with the perine more loosely attached. Peromonolites allensis Brenner, 1963 is distinct by its highly wrinkled perine which is more closely attached to the central body, and a smaller camera.
The microspores are typically tuberculate similar to the modern species Isoetes capensis Duthie, 1929 from South Africa ( Tryon and Lugardon 1990: 624, fig. 232.14), which also shows a circular aperture at the end of the tubercles.
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