The Biology of Sclerobionts and their Hosts: Comparing Biomechanical Experiments on Brachiopod Hosts to Sclerobiont Distribution Maps

  • Author / Creator
    Barclay, Kristina M
  • Sclerobionts and their host organisms provide a great deal of information for palaeoecological and palaeoenvironmental studies. However, any interpretation of sclerobiont – host relationships depends on an understanding of the host’s orientation. In the past, distinction of host life and post-mortem orientations has depended on the positions of sclerobionts on hosts, but this logic is circular and has lead to multiple, opposing interpretations for the same host organism. The goals of the following thesis are to (1) biomechanically test the life orientation of a common, dorsibiconvex brachiopod host (Order Atrypida), and (2) interpret the position of sclerobionts on brachiopod hosts based on the resulting life orientation of the brachiopod, to create an independent, biologically meaningful test of the timing of encrustation that can be used for palaeoecological studies. Two dorsibiconvex atrypides from the Waterways Formation (Givetian – Frasnian, Alberta, Canada) were modeled and biomechanically tested by placing the models in a recirculating flume in one of three initial orientations: (1) anterior commissure upstream, (2) umbo upstream, and (3) lateral (specimen perpendicular to flow). Each test was conducted with the dorsal and ventral valve topmost, and repeated on both a plexiglass and sand substrate. All scenarios were repeated five times for a total of sixty trials per specimen (120 total). Flume trials indicate that neither brachiopod had a true hydrodynamically stable orientation. Reorientations occurred at low velocities (~0.2 m/s), with transport occurring soon after (~0.3 m/s). Assuming that a juvenile, pedunculate brachiopod would have initially been oriented with its ventral valve topmost, the brachiopods were likely attached via pedicles throughout their lives. Given the results from the biomechanics experiments, the brachiopods would have rested with the umbo of the ventral valve (surrounding the pedicle foramen) and the posterior portion of the dorsal valve against the substrate. This area was designated the “dead zone,” as any encrustation of that portion of the brachiopod would have only occurred after death and reorientation of the brachiopod. Assemblages of brachiopod hosts with little or no encrustation of the dead zone likely represent assemblages that were alive at the time of burial, an important distinction for palaeoecological studies. Two other areas, the shaded zone (the remaining portion of the dorsal valve), and the exposed zone (ventral valve) were also designated. To capture the distribution of sclerobionts within the three zones, the 25 “best” encrusted atrypides were selected from each of six beds of the Waterways Formation. Sclerobionts were mapped directly onto photographs of the dorsal and ventral valves of each brachiopod. A stacked image of all of the brachiopods from each bed was created, and the amount of encrustation in each zone was noted and compared for each stratigraphic unit. The number of brachiopods encrusted post-mortem was immediately apparent, with some assemblages having no post-mortem encrustation and therefore being potentially useful for palaeoenvironmental studies. Additionally, there was no consistent preference for shaded or exposed zones. The life orientation of hosts is therefore critical to interpret the timing and significance of encrustation in fossil assemblages.

  • Subjects / Keywords
  • Graduation date
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.
  • Language
  • Institution
    University of Alberta
  • Degree level
  • Department
    • Department of Earth and Atmospheric Sciences
  • Supervisor / co-supervisor and their department(s)
    • Leighton, Lindsey (Earth and Atmospheric Sciences)
  • Examining committee members and their departments
    • Leighton, Lindsey (Earth and Atmospheric Sciences)
    • Schneider, Chris (Earth and Atmospheric Sciences)
    • Zonneveld, John-Paul (Earth and Atmospheric Sciences)