Journal of Paleontology, 91(1), 2017, p. 1–11 Copyright © 2016, The Paleontological Society 0022-3360/16/0088-0906 doi: 10.1017/jpa.2016.144


Ontogeny and reproductive functional morphology of the macroalga Wiartonella nodifera n. gen. n. sp. (Dasycladales, Chlorophyta) from the Silurian Eramosa Lagerstätte of Ontario, Canada


Steven T. LoDuca,1 and Denis K. Tetreault2


1Department of Geography and Geology, Eastern Michigan University, Ypsilanti, Michigan 48197, USA ⟨sloduca@emich.edu⟩ 2Department of Earth and Environmental Sciences, University of Windsor, Windsor, Ontario N9B 3P4, Canada ⟨deniskt@uwindsor.ca⟩


Abstract.—The thallus of a new noncalcified dasycladalean alga, Wiartonella nodifera n. gen. n. sp., from the mid-Silurian Eramosa Lagerstätte of Ontario, Canada, comprises a narrow main axis with laterals in whorls (euspondyl). Laterals branch to the second order and show a distinct expansion (node) at the termination of first-order lateral segments. Morphologic differences between specimens are interpreted as ontogenetic stages similar to those displayed by extant Dasycladales, including late-stage shedding of higher-order lateral segments. Examination of reproductive functional morphology using biophysical modeling indicates that the expanded terminations of the first-order lateral segments probably are not homologous with gametophores, and that reproduction instead was either endospore or cladospore, with details of the ontogenetic sequence pointing to the latter. The distinctive lateral morphology displayed by this species adds to the list of morphological innovation achieved by dasycladalean algae during a significant burst of evolutionary activity that unfolded between theMiddle Ordovician and late Silurian.


Introduction


The mid-Silurian Eramosa Lagerstätte from the Bruce Peninsula region of Ontario, Canada, has yielded nonbiomineralized arthropods, lobopodians, and worms, and is one of only a few fossil deposits known worldwide to preserve soft parts of the conodont animal (Tetreault, 1995, 2001; von Bitter et al., 2007; Collette and Rudkin, 2010). Here, we describe a new, morpho- logically complex noncalcified macroalgal taxon from this exceptionally preserved biota. Abundant well-preserved material provides the basis for detailed studies of the ontogeny and repro- ductive functional morphology of this new taxon, interpreted herein as a member of the extant green algal order Dasycladales. Considered as such, it adds to a considerable list of noncalcified dasycladalean algae reported from Silurian strata in the last three decades, including material from New York (LoDuca, 1990, 1997),Wisconsin (LoDuca et al., 2003), Ontario (LoDuca, 1995, 1997), Arctic Canada (LoDuca et al., 2011), New Brunswick (LoDuca et al., 2013), Sweden (Kenrick and Vinther, 2006), and Estonia (Tinn et al., 2009, 2015;Mastik and Tinn, 2015).


Geologic setting and age


The Eramosa Lagerstätte is situated within the upper part of the Eramosa Formation, as defined by Brett et al. (1995), in the Bruce Peninsula region of Ontario. Most of the Eramosa Formation in this area is composed of well-bedded dolostone with occasional argillaceous intervals, but stromatoporoid-rich bioherms are locally developed (Armstrong, 1993; Tetreault, 2001). A Wenlockian age for this interval was reported by


1


Bancroft et al. (2016) based on conodont evidence and carbo- nate carbon-isotope stratigraphy. In an overview of the Eramosa Lagerstätte provided by von


Bitter et al. (2007), three biotas were recognized, each known only from a single limited geographic area in the southern part of the Bruce Peninsula. The macroalgal taxon described herein belongs to Biota 3 of that report, located in a quarry operated by Owen SoundLedgerock Ltd. near Wiarton (localityDof von Bitter et al., 2007). All specimens of the new taxon occupy bedding plane surfaces of large blocks removed from the lowermost part of a small pit on the northeasternmost part of the quarry property, north of Oliphant Road. The stratigraphic section at this location and level comprises light gray dolostone beds 15 to 30cm thick that alternate with laminated intervals 5 to 15cm thick. Within the laminated intervals, individual beds range from less than 1 to nearly 10mmin thickness and alternate between buff to light gray dolostone and dark gray to brown, organic-rich calcareous shale. All of the alga material described herein was recovered from the organic-rich calcareous shale layers. The thicker dolostone beds that separate the laminated intervals show graded bedding and include chaotically oriented brachiopods and coral heads (favositids). The entire section at this location has a gentle


southward dip, the dip angle increasing updip and eventually flattening out downdip. This local irregularity likely reflects deposition along the flank of a large bioherm, in which case the alga-bearing sediments accumulated near the base of this structure because the blocks that produced the specimens were quarried from an area close to where the dip begins to level out. The biohermitself at this location is not exposed, but similar flank beds and their associated bioherms are exposed in nearby roadcuts.

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