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Abstract.-To investigate the phylogenetic affinity of Yuknessia simplex Walcott,

Non-calcified marine macroalgae ("seaweeds") play a variety of key roles in the modern Earth system, and it is likely that they were also important players in the geological past, particularly during critical transitions such as the Cambrian Explosion (CE) and the Great Ordovician Biodiversification Event (GOBE). To investigate the morphology and ecology of seaweeds spanning the time frame from the CE through the GOBE, a carefully vetted database was constructed that includes taxonomic and morphometric information for non-calcified macroalgae from 69 fossil deposits. Analysis of the database shows a pattern of seaweed history that can be explained in terms of two floras: the Cambrian Flora and the Ordovician Flora. The Cambrian Flora was dominated by rather simple morphogroups, whereas the Ordovician Flora, which replaced the Cambrian Flora in the Ordovician and extended through the Silurian, mainly comprised comparatively complex morphogroups. In addition to morphogroup representation, the two floras show marked differences in taxonomic composition, morphospace occupation, functional-form group representation, and life habit, thereby pointing to significant morphological and ecological changes for seaweeds roughly concomitant with the GOBE and the transition from the Cambrian to Paleozoic Evolutionary Faunas. Macroalgal changes of a similar nature and magnitude, however, are not evident in concert with the CE, as the Cambrian Flora consists largely of forms established during the Ediacaran. The cause of such a lag in macroalgal morphological diversification remains unclear, but an intriguing possibility is that it signals a previously unknown difference between the CE and GOBE with regard to the introduction of novel grazing pressures. The consequences of the establishment of the Ordovician Flora for shallow marine ecosystems and Earth system dynamics remain to be explored in detail but could have been multifaceted and potentially include impacts on the global carbon cycle.

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Two species of the enigmatic algaChaetocladus, C. ruedemanni(new species) andC. dubius(previously regarded as a graptolite incertae sedis), are described from the Silurian Lockport Group of New York and Ontario, Canada, respectively. A comprehensive investigation reveals that these and otherChaetocladustaxa occur in distinctive Konservat-Lagerstätten in association with other thallophytic algae, annelid worms, and lightly sclerotized arthropods. The sedimentology, taphonomy, and biotic composition ofChaetocladus-bearing deposits indicate that this alga thrived in shallow, stagnant, occasionally storm-agitated marine environments. In these settings, preservation of thallophytic algae and associated soft-bodied animals apparently was facilitated by a combination of obrution, anoxia, and early diagenesis of the burial muds.The morphology ofChaetocladuscorresponds to that characteristic of the green alga order Dasycladales, and it is herein referred to this long-ranging taxon as a representative of a new subtribe (Chaetocladinae, new subtribe) within the tribe Salpingoporelleae (emended herein), family Triploporellaceae (emended herein). This euspondyl, endosporate genus extends the range of the euspondyl dasyclads significantly, from the Early Devonian back to the Middle Ordovician, and bridges an evolutionary gap between early Paleozoic aspondyl, endosporate forms and middle Paleozoic euspondyl, cladosporate forms.

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Thin beds of silty limestone within a Ludlovian (Ludfordian) section of the Cape Phillips Formation on Cornwallis Island, Arctic Canada, contain numerous specimens of noncalcified macroalgae in association with dendroid and graptoloid graptolites, brachiopods, and trilobites. The algal material, preserved as carbonaceous compressions, represents three new taxa, each characterized by a central axis surrounded by laterals. Laterals ofEocladus xiaoin. gen. n. sp. are thin and branch to the fifth order whereas those ofChaetocladus captitatusn. sp. are undivided and form a distinctive capitulum. Thalli ofPalaeocymopolia nunavutensisn. gen. n. sp. have a branched, serial-segmented form and a corticated structure. On the basis of thallus architecture, all three taxa are assigned to the extant green algal order Dasycladales. Parallels exist between this macroalgal assemblage and a modern macroalgal association in Florida Bay.