6
Journal of Paleontology 91(1):1–11
width; L = main axis length; d1 = width of first-order lateral segment (cylindrical portion); l1 = total length of first-order lateral segment; NodeD = width of node at widest point; d2 = width of second-order lateral segment, measured at the mid-point of length; l2 = length of second-order lateral segment.
Specimen
ROM 63792 ROM 63795 ROM 63800 ROM 63804 ROM 63796.1 ROM 63803
5.8 4.1 4.9 3.5 3.6
W H 6.5
D
27.0 22.0 26.5 23.9 17.0 18.4
0.33 0.30 0.31 0.30 0.30 0.35
L
25.6 20.1 23.4 21.7 17.0 17.0
d1
0.069 (0.007) 0.065 (0.005) 0.060 (0.009) 0.061 (0.007) 0.075 (0.008) 0.065 (0.007)
l1
1.4 (0.09) 1.2 (0.08) 1.1 (0.08) 1.0 (0.07) 1.5 (0.05) 1.7 (0.09)
NodeD
0.12 (0.004) –– 0.11 (0.005) ––
0.11 (0.004) 0.11 (0.005) 0.10 (0.003) 0.10 (0.004)
0.042 (0.004) 0.050 (0.005) 0.041 (0.005) 0.042 (0.004)
d2
Table 1. Morphometric data (in millimeters) for select specimens of Wiartonella nodifera. Values for d1,l1, NodeD, d2, and l2 are averages of measurements taken from 10 different laterals on the same specimen; standard deviations in parentheses.W = thallus width at widest point; H = thallus height; D = main axis
l2
3.4 (0.22) 1.9 (0.21) 2.1 (0.11) 2.2 (0.12)
The rhizoid of Wiartonella is unusual in relation to that of
other Silurian dasycladalean taxa, such as Medusaegraptus and Chaetocladus, in being stout and somewhat wider than the main axis, rather than a simple tapered prone extension of the main axis (LoDuca, 1990, 1997). Its form, however, is not unlike that known for some living Dasycladales (e.g., Berger and Kaever, 1992, fig. 4.2c). Rhizoid form of W. nodifera may reflect attachment to hard substrates, as opposed to anchoring in soft mud, an inference supported by rhizoids observed in direct contact with brachiopod shells (Fig. 2.1, 2.2). The distinct morphological variants included within the species and the reproductive significance of the laterals are considered in detail in the sections below.
Ontogeny
Morphologic differences between the various specimens assigned to Wiartonella nodifera are interpreted herein as reflecting a complex ontogeny involving distinct stages. During the initial stage (stage 1) whorls of unbranched hairlike laterals were produced as the main axis commenced elongation (Figs. 2.2, 5.1). After at least five such whorls were produced, the thallus entered a markedly different growth stage (stage 2), during which numerous whorls of branched laterals were pro- duced in succession, the hairlike second-order lateral segments emerging from distinctly expanded terminations of the first- order lateral segments (Figs. 3.1–3.3, 5.2). Finally, late in the ontogenetic sequence (late stage 2), the second-order lateral segments were shed, resulting in thalli largely or entirely devoid of these structures (Figs. 3.7–3.10, 5.3). Size data for the material is consistent with interpretation of
stage 1 and stage 2 thalli as ontogenetic stages of a single species rather than distinct species. In particular, in terms of main axis length, the largest specimens interpreted as stage 2 thalli are larger than the largest of the stage 1 thalli by nearly a factor of two. That the second-order lateral segments of W. nodifera were shed as a natural part of the life cycle, as opposed to simply detached as a consequence of transport prior to burial, is indicated by two factors. First, thalli are known that retain all of the first-order segments but none of the second-order (e.g., Fig. 3.7–3.10). Second, some thalli are surrounded by haloes of detached second- order lateral segments (Fig. 1.1, 1.2), a pattern consistent with shedding followed by in-situ burial. Notably, shedding of lateral segments during the ontogenetic sequence is common for living Dasycladales, the cytoplasm from these deciduous segments being retracted into the thallus before abscission commences (Berger and Kaever, 1992).
Reproductive functional morphology
In comparing Wiartonella with living dasycladalean algae, it is immediately obvious that the expanded terminations of the first-order lateral segments, referred to in this discussion
definitively confirm or reject a gametophore identity is lacking. In particular, reproductive cysts are not evident anywhere within the thallus, even in scanning electron microscopy images taken in backscattered-electron mode, so that it cannot be determined that gametes occupied the nodes to the exclusion of other parts of the thallus. This is a key point becausemany extinct dasycladalean taxa lacked gametophores and, of these, some are known with certainty to have sequestered gametes within the main axis, such as Diplopora phanerospora (see Pia, 1920), or within the first-order laterals, such as Triploporella remesi and other triploporellids (reviewed in Barattolo et al., 2013), categorized, respectively, as endospore and cladospore by Pia (1920). In addition, although living dasycladalean algae are not known to produce laterals directly from the gametophores, an observationinkeepingwith the view that gametophores have amorphogenetic identity independent of the laterals (so that they are not simplymodified lateral segments) (see Dumais and Harrison, 2000), current knowledge of dasycla- dalean morphogenesis is sufficiently incomplete that production of secondary lateral segments from the nodes of Wiartonella does not unequivocally eliminate the nodes-as-gametophores hypothesis. Finally, because thallus morphology for Wiartonella differs markedly from that of all well-established choristospore dasycladalean genera, it is not possible to establish a gametophore identity for the nodes on the basis of phylogenetic arguments.
generically as nodes to avoid unwarranted functional connota- tions, bear a resemblance to gametophores, at least superficially. In particular, the gametophores of Neomeris compare well in terms of size (diameter of 0.075–0.195mm) and position along the first-order lateral segment (terminal). The earliest dasycla- dalean with structures described as gametophores, and therefore proposed as choristospore according to the terminology of Pia (1920), is Kalania from the Llandoverian of Estonia (lower Silurian; Tinn et al., 2015). A gametophore identity for the illustrated structures, however, is equivocal. A stronger argument can be made for Uncatoella from the Lower Devonian of China (Kenrick and Li, 1998). Here, structures similar in size, shape, and position to the gametophores of the extant dasycladalean Batophora encapsulate unambiguous clusters of spherical objects comparable to the reproductive cysts of extant dasycladaleans, including the presence of an operculum. For the nodes of Wiartonella, evidence of the sort necessary to
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