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THE MORPHOLOGY of a Permian, calcified,
green
alga, Ivanovia tebagaensis, had been studied from a
polished
limestone cube some 7.5 cm on a side (J. Paleontology, 69:381-387,
1995).
The rock had been collected in southern Tunisia. The embedded algae
were
exquisitely preserved and numerous. They appeared as dark curves and
lines
against a rusty colored embedding matrix. Details were revealed of
features
not found in any living green algae. The 3-D form was determined from a
single physical section and from surface views. The algae were highly
variable
in size but apparently were cup-shaped and about 2 cm wide and 3 cm
tall.
It is not known how the cup may have been anchored to the substrate.
This is a single face of
the cube
showing the curved membranes which made up the algae. Ca. 75mm on a
side.
The same cube was CT (CAT) scanned with a
medical General
Electric 9800 Advantage. The scans were 1.5 mm thick and taken each 1.5
mm. The .tif CT image files were copied onto 3.5 inch floppies and
transferred
into an IBM compatible microcomputer with 64 mb of RAM and 4 mb of RAM
on the video card. The CPU was a 233MHz AMD K6. The files were
converted
with JASC's Paint Shop Pro to an 8 bit uncompressed format that could
be
read by Fortner Software's 3-D imaging program, T3D, known formerly as
Slicer.
This figure shows a
typical gray
scale CT scan slice about midway through the rock. The light colored
curves
and swirls are the algal fossil membranes. Note the black hole toward
the
middle of the rock. X1.5
T3D reads the numerical sequence of 46 image files to build a data volume saved in .hdf format of about 3.2 mb in size. The volumes were then modified so that only the algal membranes were visible. Interesting views were saved as .svw files about 12.4 kb in size.
A typical view shows how very numerous the algae
were
in the rock but also raises questions about their form. It is apparent
that the algae were commonly fused together suggesting budding as a
method
of asexual reproduction. Apparent sexual
reproduction in a different species of Ivanovia from
the Yukon has been noted.
A view of the deep
interior of the
specimen that shows the detail revealed by the CT scans. A detailed
study
of many such views shows that these are branches of dichotomously
divided
utricles. Ca. 250 X
A view of utricular casts, as seen below,
shows
cavities that are presumed vacuoles.
A highly magnified view of
a portion
of the utricular casts showing at right center a dichotomous pair of
utricular
casts. Ca. 500 X
This
animation is of 19 (out of 92) Z-axis, high resolution CT sections of
utricles
being stacked one at a time to illustrate and to study utricular
structure.
The frame with the most white background is frame number 1. Ca.
220
X
Although this work has now been published (J. Paleontology, 73:154-158, 1999), it has raised many questions about earlier concepts of the form of the alga. Indeed, features can be studied that would have been obscured by traditional serial sectioning or other destructive methods. The project also demonstrates that CT scanning can be used as a powerful, but nondestructive method for studying fossils and that the data can be processed on readily available desktop computers with reasonably priced software.
The next step in this project was to explore the possibility of making a 3-dimensional model or replica of the algal thallus using rapid prototyping or stereolithography, a process that reconstructs the thallus from the CT slices. A report on that effort is just a click away.
Thanks to Don Toomey who collected the rock in Tunisia, to Linda Lucas who helped with the initial medical CT scans, to Richard A. Ketcham who took the high resolution scans at the University of Texas-Austin, and to Cathrin DeNooyer who helped with the computer imaging.
A related site of interest at the University of Texas:
For a brief CV of the author, click here.
This page was last updated on 27 Oct 06.
Address comments or questions to: andym@ku.edu
Copyright © 2006. The author's permission is required to use any of this material
.