ONTOGENY AND REPRODUCTIVE FUNCTIONAL MORPHOLOGY OF A NEW DASYCLADALEAN ALGA (CHLOROPHYTA) FROM THE SILURIAN ERAMOSA LAGERSTÄTTE OF ONTARIO, CANADA

The thallus of a new genus and species of noncalcified macroalga from the mid-Silurian Eramosa Lagerstätte of Ontario, Canada, comprises a narrow main axis with unbranched laterals in whorls (euspondyl). Laterals of two types, (1) hairlike with tapered ends (trichophore) and (2) cylindrical with expanded, ball-shaped terminations (phloiophore), are segregated by type into alternating whorls distributed along much of the length of the main axis. The combination of thallus architecture and lateral morphology characteristic of this alga is unique to the extant green algal order Dasycladales and, in this context, morphologic differences between specimens can be readily interpreted as ontogenetic stages similar to those displayed by the extant dasycladalean Halicoryne. Biophysical modeling of reproductive functional morphology for living dasycladalean algae, when applied to the new alga indicates that the ball-shaped terminations of the phloiophore laterals are not homologous with gametophores, and that reproduction was instead either endospore or cladospore, details of the ontogenetic sequence pointing to the latter. Production of two types of laterals in alternating whorls (= cyclical heterocladous development) for this alga adds to the list of developmental innovation achieved by dasycladalean algae during a significant burst of evolutionary activity that unfolded between the Middle Ordovician and Late Silurian.