BRANDON, MB – The BU Science Seminar Series continues this Friday after a two-week hiatus with guest speaker Dr. Graham Young. He is the curator of geology and paleontology at The Manitoba Museum, in Winnipeg. His talk is entitled: “Getting Jelly Out of Stone: The Fossil Record and Evolution of Cnidarian Medusae.”
The abstract of his talk is as follows:
“Cnidarian medusae, or jellyfish, are widespread and important in modern oceans. They include three groups: scyphozoan medusae (“true jellyfish”), hydrozoan medusae (aka hydromedusae), and cubozoans (“box jellyfish”).
Sedimentary deposits containing fossil medusae are rare, although reports of fossil “medusoids” (most of which do not represent medusae) are quite common. We have been working to develop criteria for the recognition of bona fide fossil medusae, considering the distinct sedimentologic and preservational features that result from the transport, stranding, and burial of their gelatinous bodies. Recognized fossil jellyfish have distinct body structures comparable to those of the living animals, and tend to occur in large numbers, most often in coastal paleoenvironments (such as tidal flats or lagoons). They are rarely preserved in oxygen-poor deeper water sediments.
Within the 542 million-year record of Phanerozoic life, there are currently understood to be only nine definite medusan fossil deposits (some of these deposits occur at multiple localities within an area). There are, however, several other deposits currently under review, and these may augment the record substantially. Our preliminary analysis of the fossil record of medusae affects our understanding of the evolutionary history of this important group.
Scyphozoa are known from the Cambrian, but more derived scyphomedusae were not demonstrably present until the Carboniferous; Mesozoic scyphozoans are quite diverse. Hydromedusae are known from the Ordovician but may extend back to the Cambrian. The record of cubozoans is shorter and sparser; the oldest definite cubozoan is Carboniferous in age.
This general pattern of diversification establishes an earliest known date for each group and provides a logical starting point for calibrating the timing of divergences predicted by recent work on molecular phylogeny.
The abundance of jellyfish in Cambrian and Ordovician coastal deposits probably reflects the occurrence of jellyfish blooms similar to those seen in modern oceans. It is notable that other groups that have since evolved as potential competitors, such as fishes, were not yet present at that time. The variation in jellyfish occurrence through the fossil record is almost certainly due to changes in preservational conditions, rather than changes in jellyfish abundance along shorelines.
By the Late Paleozoic and Mesozoic, coastal sediments were much more intensively and deeply burrowed than in the Cambrian. In addition to mechanical destruction of carcasses, burrowing enhanced oxygenation of the sediment. Shoreline scavengers had evolved by the mid Paleozoic; this escalated in the Mesozoic with the appearance of birds and crabs. The net result of these changes is that fossil jellyfish are unlikely to be found in Mesozoic and Cenozoic shoreline deposits, but many examples could yet be discovered in the Early Paleozoic.”
Dr. Young will give his talk from 3 to 4 p.m., Friday, February 11, in Room 3-42, in the Brodie Science Building. Admission is free and everyone is welcome.
For more information, please contact:
Joanne F. Villeneuve
P: (204) 727-9762