LoDuca and Tetreault—Paleobiology of a new Silurian macroalga


central main axis with laterals arranged in whorls and inclined relative to themain axis at an angle of ~45°.Main axis of uniaxial construction, cylindrical, ~0.3mm wide, slightly expanded at whorl junctions, apex rounded (Figs. 3.13, 4.1). Holotype (ROM 63795) bears two distinct types of laterals. Laterals along lower part of main axis unbranched with tapered ends, 0.05mm in diameter at mid-point of length and reaching a length of 1.3mm (Fig. 3.3, 3.4). Laterals along upper part of thallus branched to the second order; first-order lateral segment cylindrical, 0.065mm in diameter and 1.1mm in length, with an expanded termination 0.11mm in maximum diameter bearing from subtly faceted distal surfaces two slender elongate second-order segments 0.05mm in diameter and ~2mm in length, these diverging fromeach other at an angle of 20° to 30° and tapering distally (Fig. 3.4, 3.14; Table 1). Spacing between adjacent whorls ~0.5mm; lateral number perwhorl ~12, this determined fromfirst-order lateral tips visible in upper part of uppermost whorl (Fig. 3.14), the precise number uncertain because of overlapping laterals adjacent tomain axis. Basal part of thallus unknown (beyond slab margin). Specimens ROM 63793, 63794, 63800, and 63804 similar to holotype, but details for branched laterals are more apparent in parts of 63800 and 63804 (Figs. 3.1, 3.2, 4.3, 4.4).Morphological variants include small thalli bearing only unbranched laterals (Fig. 2.2) and thalli lackingmost or all second-order segments for the branched laterals (Figs. 1.2 [left specimen], 3.7–3.10, 4.2). Specimens ROM 63793, 63794, and 63802 show a short, stout rhizoid extending over a circular area roughly twice the diameter of themain axis(Figs.2.1,2.2,3.5,3.6).


Etymology.—The trivial name is in reference to the expanded terminations of the first-order segments of the branched laterals.


Materials.—ROM 63792–63804.


Remarks.—Wiartonella differs from all extant dasycladalean genera except Neomeris and Bornetella in that the laterals branch only to the second order. Bornetella develops gameto- phores along the sides of the first-order laterals, but in Neomeris the gametophores are borne along the tips of the first-order lat- eral segments, a position analogous to that occupied by the expanded terminations, or nodes, of the primary laterals in Wiartonella.Akey difference, however, is that the second-order lateral segments of Wiartonella emerge directly from the tips of


the nodes, whereas in Neomeris they clearly diverge from the first-order lateral segment and not the gametophore. Both of these genera differ significantly from Wiartonella in that the second-order laterals develop expanded portions that coalesce to form a cortex, and both develop calcification. It may be significant from a morphogenetic standpoint, however, that both genera include species that develop deciduous hairs from the


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cortex facets in a fashion roughly analogous to how the nodes of Wiartonella bear second-order laterals. Among fossil genera, Wiartonella is similar inmany respects


to the noncalcified dasycladalean Kalania Tinn,Mastik, Ainsaar, and Meidla, 2015, known only from the type species, K. pusilla from the Llandoverian of Estonia. Both taxa are characterized by euspondyl lateral arrangement, primary laterals with a complex form(i.e., they are not simple cylinders), and elongate secondaries produced in pairs fromthe tips of the primaries.Akey distinction, however, concerns the form of the primaries. Specifically, according to the diagnosis provided by Tinn et al. (2015, p. 209), those of Kalania are pyriform, gradually and continuously expanding distally, whereas the primaries of the new Canadian material are cylindrical alongmost of their length, expanding only at the end to form a bulbous terminal node. The distinctive form characteristic of the latter is sufficient to preclude assignment of the Canadian material to Kalania and comprises an essential element in the diagnosis of Wiartonella. A further key distinction according to the diagnosis provided by Tinn et al. (2015) is that gametophores are developed along the sides of the primaries of Kalania. Nothing comparable in this position, however, has been observed among the Canadian material. Rhabdoporella Stolley, 1893, a calcified taxon known from Ordovician and Silurian


strata, is characterized by laterals with a form comparable to the first-order segments of the branched laterals of Wiartonella,but key differences here are that the laterals have an aspondyl as opposed to euspondyl arrangement and do not branch. The same distinctions apply to the Mesozoic calcified taxon Gyroporella Gümbel, 1872 (see Pia, 1920). Cyclocrinitids, a group of calcified algae well known from the Ordovician and Silurian, also share some key characteristics with Wiartonella, including general lateral form (both have laterals with distinctly inflated termina- tions) and, in this case, euspondyl lateral arrangement. All cyclocrinitid genera, however, develop a cortex. In addition, no cyclocrinitid genera are known to have produced secondaries with an elongate form, although it is possible that these were present in some species and not preserved because they extended beyond thezoneofcalcification. The same may apply to some Mesozoic calcified euspondyl taxa that lacked a cortex and are characterized by inflated primary laterals somewhat similar in form to those of Wiartonella, such as Uragiella Pia, 1925, Holosporella Pia, 1930 and Neogyroporella Yabe and Toyama, 1949 (see Barattolo et al., 2008). The Mesozoic calcified euspondyl taxa Triploporella Steinmann, 1880 and Petrascula Gümbel, 1873 are also characterized by inflated primary laterals, and here these bear secondaries as well. Both taxa differ from Wiartonella in details of the form of the primaries. In addition, the primaries of Triploporella typically bear three or four secondaries, as opposed to two in Wiartonella, and the laterals of Petrascula branch beyond the second order (see Bernier, 1979).


Figure 3. Wiartonella nodifera n. gen. n. sp. from the Silurian Eramosa Formation, Wiarton, Ontario. (1, 2) Nearly complete thalli with most second-order lateral segments attached to primary laterals, branching details for laterals marked by arrows are shown in 4.3 and 4.4, respectively (ROM 63804, 63800). (3, 4) Holotype, nearly complete thallus with most second-order lateral segments attached to primary laterals and detached pair of second-order lateral segments near top right of thallus (ROM 63795). (5) Complete thallus with full complement of attached second-order lateral segments and holdfast at base (ROM 63794). (6) Enlargement of holdfast for specimen shown in (5). (7–10) Thalli lacking all or most second-order lateral segments, areas marked by arrows in (9) are shown in detail in 2.3 and 4.2, respectively (ROM 63792, 63798, 63803, 63796.1). (11, 12) Thalli lacking most laterals (ROM 63799, 63796.2). (13) Enlargement of middle part of thallus shown in (10). (14) Enlargement of upper part of holotype showing uppermost whorl with two second-order lateral segments for each primary lateral segment and expanded terminations for the latter. All specimens photographed dry in direct light. Scale bar is (1–3, 5, 7–12) 5mm; (4) 1.33mm; (6) 1.2mm; (13) 1.25mm; (14) 0.715mm.


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