Mesophyllum crassiusculum (Foslie, 1902) Lebednik, 2004

Hrabovský, Juraj, Basso, Daniela & Coletti, Giovanni, 2019, The first identification of fossil Mesophyllum in accordance to the modern taxonomic concepts in coralline algae, Acta Palaeontologica Polonica 64 (4), pp. 897-909 : 900-905

publication ID	https://doi.org/ 10.4202/app.00591.2019
persistent identifier	https://treatment.plazi.org/id/03DB8787-AF5C-BA25-FCAC-C89D1D66FCB9
treatment provided by	Felipe
scientific name	Mesophyllum crassiusculum (Foslie, 1902) Lebednik, 2004
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Figs. 2–4 View Fig View Fig View Fig .

1985 Mesophyllum galettoi Mastrorilli, 1968 ; Schaleková and Čierna 1985: 42.

Material.— Species was identified in the two thin sections NHM B1857/1a and NHM B1857/1b, and their counterparts studied on SEM NHM B1857/2a and NHM B1857/2b. One specimen was detcted in Schaleková collection ( Schaleková and Čierna 1985)—thin section NHM B1858.

Description.—The species develops non-protuberant rhodoliths or encrusts other coralline algae or corals ( Fig. 2A View Fig ). The core of the rhodoliths usually consists of corals but carposporangial-carpogonial plants also encrust other fruticose protuberant coralline algae (e.g., Phymatolithon calcareum ; Fig. 2B View Fig ). Growth form is encrusting without prominent or branched protuberances. Applanate branches locally develop ( Fig. 2C View Fig 1). Two short warty protuberances 0.5× 0.5 mm were also detected. Thickness of the superimposed thalli could reach few mm. Individual lamellae are 0.2–2 mm thick ( Table 1).

Thallus is pseudoparenchymatous with dorsiventral organization, consisting of a single system of branching filaments (monomerous construction). Filaments of the hypothallus run in parallel with the substrate and can be arranged either coaxially or non-coaxially ( Fig. 2C 2 View Fig ). In the asexuate plants the hypothallus is 43–146 μm thick ( Table 1). The hypothallial cells (n = 38) are 11–26 μm long (mean ± SD, 18±4 μm) and 5–11 μm in diameter (mean ± SD, 7±1.2 μm). Hypothallus filaments bend upward toward thallus surface and develop the peripheral portion of the thallus perithallus). Cells (n = 180) are 4–16 μm long (mean ± SD, 9±2.3 μm) and 4–11 μm in diameter (mean ± SD, 7±1.5 μm) Table 1). Cells within single filaments show cyclic variation in length. Sets of 2–5 long cells are alternated with –2 short cells. Such pattern results in irregular growth zones ( Fig. 2C View Fig 1). Cells are laterally joined by cell fusions Fig. 2C View Fig 3 View Fig ). Epithallial cells (n = 7) are rounded or flattened but not flared, measuring 3–4 μm in length (mean ± SD, 3±0.5 μm) and 5–7 μm in diameter (mean ± SD, 6±0.8 μm) Fig. 2C View Fig 4 View Fig ). One or two cells are present above meristematic cells. Meristematic cells (n = 12) are as long or longer than cells immediately subtending them and measure 8–12 μm in length (mean ± SD, 10±1.5 μm) and 6–8 μm in diameter mean ± SD, 7±0.6 μm) ( Fig. 2C View Fig 4 View Fig ). The size of the vegetative characters observed in the sexuate plants are largely overlapping with those of the asexuate plants ( Table 1).

Gametophyte encrusts tetrasporophyte. However, where margins of the two thalli meet, multiple overgrowths may occur. Presumed gametangial male conceptacle is of the type 1 (Johansen 1981) and the mode of projection complies with those of Mesophyllum and Lithothamnion (according to Lebednik 1978) ( Fig. 3A View Fig ). In fact, in Mesophyllum and Lithothamnion the initiation of the male conceptacle development takes place just below the meristematic cells in contrary to Phymatolithon in which initiation takes place deep in the vegetative filaments, thus male conceptacle is not prominent in the latter( Lebednik1978:393, 394, text-figs.8–9, 11).Male conceptacles (n = 7) measure 185–310 μm in diameter (mean ± SD, 254±63.7 μm) and 42–60 μm high (mean ± SD, 51±9.1 μm). Pore canal is 77–108 μm long (mean ± SD, 89±16.4 μm) and up to 54 μm broad ( Table 1). Some conceptacles are filled with dark material of uncertain origin ( Fig. 3B View Fig ).

Carposporangial conceptacles are large and occur on other plant than male conceptacles ( Fig. 4A View Fig ). The presence of male and carposporangial conceptacles on distinct plants suggests that gametophytes are most likely dioecious. Chambers (n = 4) are dumbbell shaped, 519–689 μm in diameter (mean ± SD, 604±119.9 μm) and 155–229 μm in height (mean ± SD, 192±52.3 μm) measured aside of a well-developed central pedestal ( H 2 according Basso et al. 1996) ( Fig. 3A View Fig , Table 1). Pore length is 108–178 μm (mean ± SD, 143±49.6 μm) and its width is 112–143 μm (mean ± SD, 127±22.6 μm).

Multiporate sporangial conceptacles are rounded to oval ( Fig. 4B View Fig 1). Conceptacles protrude above the thallus surface and are overgrown by thallus growth from the sides of the conceptacle. Chambers (n = 7) are 179–418 μm in diameter (mean ± SD, 326±59.2 μm) and 90–199 μm high (mean ± SD, 171±16.3 μm). D/H ratio is 1.6–2.4. The roofs are 42–69 μm thick and are formed by 6–9 cells that tend to be shorter towards the apex ( Fig. 4B View Fig 2 View Fig , C 1; Table 1). Pore canals are mostly lined by cells similar in size and shape to the other roof cells ( Fig. 4B View Fig 2, C 2 View Fig ). Locally specialized thinner cells were observed ( Fig. 4B View Fig 2, C 2 View Fig ). At least 6 cells lining each pore canal were observed under SEM ( Fig. 4B View Fig 2 View Fig ). Pore canal cells thinner than those of the roof were observed also in the thin section ( Fig. 4C View Fig 2 View Fig ). At the top of the pore canals epithallial cells and PCLC appear at the same level, indicating that rosette cells around the pore canal are not sunken ( Fig. 4B View Fig 1). Conceptacles are embedded and locally filled with adventitious cells ( Fig. 4C View Fig 3 View Fig ). Remains of these cells are present in some chambers ( Fig. 4C View Fig 1). The roof is either flat or convex, with no peripheral rim and sunken pore plates. Old conceptacles become buried when covered by successive thallus growth ( Fig. 4C View Fig 3 View Fig ).

Remarks.—We have identified M. crassiusculum from the newly collected middle Miocene material. The asexuate (bi/ tetrasporangial) specimens fully correspond with the type (Athanasiadis et al. 2004; Athanasiadis 2007). During the revision of the historical collection of Anna Schaleková we have identified one specimen of M. crassiusculum formerly described as M. galettoi ( Schaleková and Čierna 1985) . This specimen matches with both, Modrý Majer specimen and M. crassiusculum type ( Fig. 5 View Fig , Table 2).

Stratigraphic and geographic range.—Early Langhian of the central Paratethys (this work) to Recent of the Pacific Ocean (Athanasiadis et al. 2004; Athanasiadis and Ballantine 2014).