Arcellites disciformis ( Miner, 1935 ) Ellis and Tschudy, 1964

Santamarina, Patricio E., Barreda, Viviana D., Iglesias, Ari & Varela, Augusto N., 2018, Salvinialean megaspores in the Late Cretaceous of southern Patagonia, Argentina, Acta Palaeontologica Polonica 63 (3), pp. 607-616 : 610

Arcellites disciformis ( Miner, 1935) Ellis and Tschudy, 1964

Fig. 3 .

1935 Arcellites disciformis sp. nov.; Miner 1935: 600, pl. 20: 61, 64–66. 1964 Arcellites disciformis ( Miner, 1935) ; Ellis and Tschudy 1964: 75,

pl. 1: 1–12, text-fig. 1.

Material.— 16 specimens measured. Sample MPM-Pb- 18907a ( N40 /1); MPM-Pb-18907b ( R48 /2); MPM-Pb-18907 SEM stub 1 (4 specimens), MPM-Pb-18907 SEM stub 2

2 specimens), MPM-Pb-18907 SEM stub 3 (4 specimens), and MPM-Pb-18907 TEM (4 specimens). Cenomanian of Patagonia , Argentina. Cerro Waring locality, Mata Amarilla Formation .

Description.— Trilete megaspore with spherical body and long acrolamella at proximal face, covering the trilete mark Fig. 3A 1, B 1 ). Megaspore body with 25 to 40 short appendages regularly distributed, with reticulated ends ( Fig. 3C 2 ). Body sculpture foveolate ( Fig. 3A 2, B 2 ). Fovea perpendicular to surface ( Fig. 3B 3 ), rounded between appendages, and ovate to slender at their bases ( Fig. 3A 2 ). Acrolamella composed of leaf-like appendages twisted along their length, with fimbriate margins and smooth surface ( Fig. 3C 1 ). In SEM and TEM, the megaspore wall shows a tripartite structure composed of an outer exoexine, an inner exoexine and an intexine

( Fig. 3B 3 , D). In TEM, the outer exoexine presents a coarsely granular aspect ( Fig. 3D 1 ), with granules that range in diameter from 0.3–0.4 μm. Towards the surface, outer exoexine becomes massive, and numerous pits penetrate it perpendicularly giving a palisade-like appearance. The inner exoexine is loosely and finely granulated (granules <0.2 μm in diameter). The intexine presents the most solid aspect of the three wall layers, and at high magnifications ultra-thin and irregular channels (<0.1 μm in diameter) are observed ( Fig. 3D 2 ).

Dimensions. —Total length (body and acrolamella) 367– 378 μm, body diameter 255–278 μm (with appendages), length of appendages 21–51 μm, width of appendages 21.2– 36.3 μm, fovea 0.9–2.5 × 0.9–2.2 μm (1.5 × 2.7 μm at base of appendages), acrolamella length 165.7–196.6 μm, acrolamella width 111–155 μm, exine thickness 10.7–18.3 μm, outer exoexine thickness 6–8 μm, inner exoexine thickness 2.3–5.7 μm, intexine thickness 2.4–4.6 μm.

Remarks.— Arcellites disciformis ( Miner, 1935) Ellis and Tschudy, 1964 and Arcellites hexapartitus (Dijkstra, 1951) Potter, 1963 share a similar general morphology; Batten et al. (1996) compared and contrasted these species and listed a series of characters useful to separate them. Arcellites disciformis is characterized by the presence of: (i) leaf-like segments of the neck tightly twisted against each other, (ii) leaves of acrolamella with well-developed fimbriate margins, (iii) megaspore wall surface profusely pitted, (iv) appendages with reticulate tips and absence of surface swellings of the exoexine. The Patagonian specimens conform to the diagnosis of Arcellites disciformis and present all the morphological features that characterized the species ( Hueber 1982; Batten et al. 1996).

The Argentinean specimens have similar dimensions to those reported for the Barremian–Aptian of Virginia, USA ( Hueber 1982) and for the Albian–Cenomanian of Maryland, USA ( Lupia 2015). However, they are smaller than those described for the Albian–Cenomanian of the Denver Basin, central USA ( Ellis and Tschudy 1964) and for the Cenomanian of Alberta, Canada ( Singh 1983).

Differences in size may be related to dehydration during processing as previously noted by Hueber (1982).

Mays (2011) reported the presence of A. disciformis for the Cenomanian of Chatham Islands, New Zealand, but the illustrated specimens do not show the main morphological features that characterized the species ( Batten et al. 1996) and might be related to A. hexapartitus .

Stratigraphic and geographic range. —Barremian–Cenomanian of USA ( Schemel 1950; Hall 1963; Potter 1963; Ellis and Tschudy 1964; Hall and Peake 1968; Hueber 1982; Kovach and Dilcher 1988; Lupia 2015), Barremian–Cenomanian of Canada ( Singh 1964, 1971, 1983; Hopkins and Sweet 1976; Sweet 1979), Albian–Cenomanian of Greenland ( Miner 1935; Koppelhus and Pedersen 1993; Batten et al. 1996), Aptian–Cenomanian of Sudan ( Kaska 1989), Turonian of France ( Colin 1975), Aptian of Germany ( Schultz and Noll 1987), Cenomanian of Patagonia (this work).

Figures

Fig. 3. Megaspores of the water fern Arcellites disciformis (Miner, 1935) Ellis and Tschudy, 1964; Cenomanian, town of Tres Lagos, Argentina. A. MPM- Pb-18907a (N40/1), general view under light microscope (A1), a detail of the pitted pattern (A2). B. MPM-Pb-18907 SEM stub 1, general view under SEM (B1), a detail of pitted surface under SEM (B2), a detail of wall in cross-section under SEM, showing the palisade structure of outer exoexine (B3). C. MPM-Pb-18907 SEM stub 3, detail of acrolamella under SEM, showing leaves with well-developed fimbriate margins (C1), detail of the reticulated tips of appendages (C2). D. MPM-Pb-18907 TEM, body of the spore showing wall-layers under TEM, with coarsely granulated outer exoexine, loosely and finely granulated inner exoexine and massive intexine (D1); detail showing ultra-thin channels (arrows) of intexine (D2). Abbreviations: Ot. Ex., outer exoexine; In. Ex., inner exoexine; Int., intexine.

Fig. 1. Geographic (A) and geological (B) maps of the studied area (Tres Lagos Town, Santa Cruz Province, Argentina), showing the location of the studied sections (modified from Varela 2011).

Fig. 2. Stratigraphic sections of the Mata Amarilla Formation at Cerro Waring and Estancia Mata Amarilla localities, showing samples location. Abbreviations: c, conglomerate; cs, coarse sandstone; fs, fine sandstone; ht, heterolitic; m, mudstone; ms, medium sandstone.