Tomescu, Alexandru Mihail Florian , Rothwell, Gar W. .
Probing the systematic affinities of early terrestrial organisms in the Passage Creek biota (Lower Silurian, Virginia).
The Passage Creek biota of Virginia comprises the oldest macrofossil assemblages known from a continental setting, providing the earliest direct evidence for a well-developed terrestrial groundcover. As such, it plays a crucial role in uncovering the organisms and processes involved in the early colonization of land. Previous studies at the locality have focused on the microfossil content of the rocks, but the proposed nematophyte affinities are uninformative in view of the equivocal nature and significance of the group. Here we consider the morphology and internal organization of fossils in a discussion of their systematic affinities. Given the techniques used to study the fossils, recent contaminants have to be excluded before systematic relationships are considered.
Except for one instance where cyanobacterial origin could be documented, intense diagenetic effects of tectonics have altered the original anatomy of the fossils, blurring their systematic affinities. Characterized principally by thalloid morphologies, fossils are preserved in abundance as carbonaceous compressions, and some exhibit complex, multilayered internal organization interpreted as reflecting anatomical differentiation within the organismal body. In the absence of cellular detail, two lines of evidence can be used to constrain the systematic relationships of the fossils. A survey of the occurrence of thalloid macroscopic morphology and internal anatomical differentiation among living groups singles out the embryophytes, fungi, and lichens as potential producers of the Passage Creek fossils; algae are less probable, not having extant terrestrial representatives with anatomically differentiated thalli. Trilete spores in the sediments demonstrate the presence of embryophytes within the Passage Creek communities, but evidence for fungi is less certain. The thickness, layering, and structures of some of the fossils match corresponding features of living lichen taxa. A similar combination of microbial and anatomically complex organisms, combining thalloid and crustose morphologies, occurs presently in biological soil crusts.
1 - Ohio University, Department of Environmental and Plant Biology, Porter Hall, Richland Avenue, Athens, Ohio, 45701-2979, USA
Presentation Type: Paper
Location: Maybird (Cliff Lodge)
Date: Wednesday, August 4th, 2004
Time: 10:30 AM