LoDuca and Tetreault—Paleobiology of a new Silurian macroalga
9
Table 2. Morphometric data and modeled values of cytoplasm volume (C), reproductive volume (R), and C/R for a single reproductive whorl of three species of extant dasycladalean algae: (1) Dasycladus vermicularis, (2) Neomeris dumetosa, and (3) Cymopolia vanbosseae. Morphometric measurements obtained from images in Berger and Kaever (1992, figs. 3.38, 3.45, 3.7) and Berger (2006, fig. 82), and from table 9 in Maksoud et al. (2014). All morphometric measurements in millimeters. C and C/R values for Cymopolia vanbosseae include C from an associated sterile whorl (see text). l = length of lateral segment; r = radius of lateral segment; N = number of lateral segments per whorl; g = gametophore radius; gN = number of gametophores per whorl.
Species 1
1 2 2 3
Source fig. 3.7
fig. 3.38 0.38 0.025 30 0.25 0.018 60 table 9
fig. 82 fig. 3.45 0.21 0.023 12 0.33 0.017 96 0.36 0.020 28 0.22 0.015 56
(choristospore hypothesis; Fig. 6.2), as a first pass, R was modeled as total node volume within a whorl, with nodes approximated as spheres, and C was modeled as cytoplasm volume exclusively for the first-order lateral segments of the same whorl. Results indicate a C/R value of 3.5 for a whorl with values of lateral length, lateral diameter, and node diameter equivalent to those of the holotype, ROM 63795 (Fig. 3.3) (model 1 in Table 3). For five other specimens modeled in this way, C/R values are similar, ranging from a low of 3.5 to a high of 4.7 (avg. 3.95) (Table 3). Note that although modeling of C and R was conducted using 12 laterals per whorl, the number of laterals per whorl is irrelevant from the perspective of the C/R value in this case, because node number and first-order lateral number per whorl are equivalent. Thus, the fact that C is substantially greater than R for Wiartonella when modeled in this way is entirely a consequence of the node being quite small relative to the preceding cylindrical portion of the lateral, the latter being many times longer and over half as wide as the node in all cases (Table 1). Considered more holistically, the differ- ence between C and R is even greater, because C must also include cytoplasm volume from the associated second-order lateral segments (two for each first-order segment). When this factor is included, the resulting cytoplasm volume calculated for the various specimens is roughly an order of magnitude greater than the associated reproductive volume (model 2 in Table 3). Such a relationship casts considerable doubt on the possibility of homology between gametophores and the nodes of Wiartonella, because it would entail that most of the thallus cytoplasm (i.e., >90%) was not used for reproduction. Interestingly, modeling of the same nature, but treating
To evaluate the nodes of Wiartonella as gametophores l1 r1 N1 l2 r2 N2 l3 r3
0.046 0.050 0.040
N3 ggN
0.80 0.075 12 0.50 0.050 48 0.25 0.025 192 0.14 12 0.85 0.075 10 0.50 0.065 40 0.25 0.033 160 0.15 10 – – –
60 0.06 30 56 0.06 28 96 0.11 12
C mm3 Rmm3 0.170
0.186 0.037 0.033 0.086
0.124 0.128 0.021 0.021 0.050
C/R 1.4
1.5 1.8 1.6 1.7
Table 3. Modeled values of cytoplasm volume (C), reproductive volume (R), and C/R for specimens of Wiartonella nodifera using morphometric data from Table 1. Values pertain to a single fertile whorl with 12 laterals per whorl. Model parameters as follows: (1) R calculated for node only, C excludes second-order lateral segments; (2) R calculated for node only, C includes second-order lateral segments; (3) R calculated for entire first-order lateral segment, including node, C includes second-order lateral segments. See text for additional details.
Specimen
ROM 63792 ROM 63795 ROM 63796.1 ROM 63800 ROM 63803 ROM 63804 ROM 63792 ROM 63795 ROM 63800 ROM 63804 ROM 63792 ROM 63795 ROM 63800 ROM 63804
Model 1
1 1 1 1 1 2 2 2 2 3 3 3 3
C mm3 0.0274
0.0217 0.0324 0.0175 0.0297 0.0160 0.0787 0.0649 0.0511 0.0525 0.0787 0.0649 0.0511 0.0525
Rmm3 0.0063
0.0063 0.0084 0.0046 0.0063 0.0046 0.0063 0.0063 0.0046 0.0046 0.0501 0.0376 0.0281 0.0258
C/R 4.3
3.5 3.9 3.8 4.7 3.5
12.5 10.3 11.1 11.4 1.6 1.7 1.8 2.0
reproduction is 1.8. Thus, results of biophysical modeling provide no clear distinction between cladospore and endospore
the entire first-order lateral segment of Wiartonella as a reproductive structure (cladospore hypothesis; Fig. 6.3), yields C/R values comparable to those calculated herein for extant taxa. For example, when R is modeled in this way for the holotype, the C/R value is 1.7, where C includes cytoplasm from the second-order lateral segments, and for the other specimens, C/R values modeled in the same way fall within a narrow range between 1.6 and 2.0 (model 3 in Table 3). Such an outcome provides strong support for the cladospore hypothesis. Nonetheless, cladospore reproduction for Wiartonella cannot be definitively established. Apart from lacking evidence of reproductive cysts (to mark the location of gamete containment), uncertainty arises because modeling indicates C/R values for endospore reproduction (where the entire main axis interior serves as R and C is determined by total thallus cytoplasm volume) not much different from those for cladospore reproduction. For example, for the holotype, the modeled C/R value for endospore
reproduction for Wiartonella. Beyond the distinctive form of the first-order laterals, however, the ontogeny of Wiartonella provides additional support for the cladospore hypothesis, because late in thallus ontogeny the second-order lateral segments were shed but the first-order laterals remained attached. It is also notable in this regard that a few main axes have been found with few or no first- order laterals (Fig. 2.11, 2.12). This suggests that these, too, were eventually shed, during a separate and later shedding event. Such a fate is also known for the reproductive structures of some living dasycladalean taxa, perhaps to aid in dispersal of the gameteswhile still in a protected state.
Evolutionary significance
Regarded as either endospore or cladospore, Wiartonella is accommodated within the family Triploporellaceae, which comprises euspondyl taxa with either endospore or cladospore reproduction (LoDuca, 1997; Granier et al., 2012). If the euspondyl condition evolved only once in the evolutionary history of Dasycladales, which seems a reasonable proposition given the available evidence, then all euspondyl taxa comprise a single clade and the extinct Triploporellaceae is paraphyletic to extant Dasycladaceae and Polyphysaceae (Kenrick and Li, 1998; LoDuca et al., 2003). Within the Triploporellaceae, the lack of a cortex and the presence of two orders of laterals indicate placement within the tribe Triploporelleae, and the inflated nature of the first-order
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