Embedded Ivanovia: From CT Scans to

THE FOSSIL GENUS Ivanovia was a  marine green alga that lived in warm, relatively shallow waters.  The genus is known from the Late Paleozoic (Pennsylvanian and Permian; about 320 to 250 million years ago) and from the  Mesozoic Triassic (about 250  to 213 million years ago).  Thus Ivanovia was among the relatively few survivors of the massive Permian-Triassic extinction.

The genus has traditionally been placed in the family Codiaceae although arguments can be made for placement in the Udoteaceae because of its utricular structure.  A remarkable and unifying characteristic of the species in these genera was that the entire thallus or body of the centimeter-sized plants was coenocytic, a single cell.  In addition, many of the cup-shaped thalli were fused to others forming an apparent massive clone reminiscent of living Caulerpa  in the Mediterranean Sea at the present time.

Beautifully preserved fossils were collected from southern Tunisia (J. Paleontology, 69:381-387, 1995) by Donald Toomey .  The numerous specimens are imbedded in a polished limestone cube about 7.5 cm on a side.  The cube was  CT scanned  to avoid destroying the hand specimen by making serial sections to study the three dimensional structure of the algae (J. Paleontology, 73:3154-158, 1999).

That the 2 to 3 cm algal thallus was cup-shaped or cyathiform was inferred from viewing fossil sections in the hand specimen that were U-shaped and ring-shaped.  The most likely three-dimensional form that would yield sections of these types is a cup shape as illustrated in the figure below.

 
 

Drawing showing the results of sectioning a cup-shaped (1) structure in three different planes.  A medial longitudinal section produces a U-shaped section (2); an oblique section yields a smaller U-shaped section (3);  a cross section results in a circular or ring-shaped section (4);  the internal structure of the membrane is shown in 5.
 
 
 
 
 
 
 

The next step was to make actual physical 3-D models of the thalli using a relatively new technique called rapid prototyping (RP), three dimensional printing or layer manufacturing.  The kind of RP used in this study is called stereolithography which produces hard, plastic, semitransparent models.  The most common uses for RP in general are in industry and medicine.

To make the models required the participation of enterprises that had the equipment, expertise, and that were willing to collaborate in the project.  Two were found:  Medical Modeling and TMJ Implants, both in Golden, Colorado.  In addition, a set of high resolution CT scans was produced by the the University of Texas High-Resolution X-ray CT facility.  This second set of CT scans was used to provide a comparison of models made from the original medical CT scans with those using the high resolution scans.

Three models were made:  The first model was of a thallus that had been studied earlier (fig. 6. in J. Paleontology, 73:3154-158, 1999).  The second was of a cup with a small bud attached.  The third model was of about 10 percent of the entire cube made to visualize the complex network of interconnected thalli.  These are shown below.
 

 
 

Model of an asexual reproductive structure called a bud.  It is attached to an assumed remnant of the parental cup-shaped thallus  which could not be modeled because it was at the edge of the cube.  The greatest top to bottom distance of the bud is 46 mm. but all models were made four times actual size hence the fossil bud was actually about 1 cm in height.
 
 
 
 
 
 
 
 

 
 

This model provides an early hint of the great complexity of the mass of thalli formed by the numerous connections between them producing what may be a colony of organisms.  At the left edge of the upper cup is a piece of yet another cup fused to it.  Thus it appears three cups are fused together and a bud is forming off of Cup #1.   The distance from the upper tip of the model to the lowest is 14.5 cm.
 
 
 
 
 
 
 

 
 
 

The complexity noted above becomes even more obvious when a model is made of about 10 percent of the entire hand specimen--about 42 cubic centimeters.  The letters C mark the edges of cups and the arrows mark some of the numerous cavities formed by the insides of cups or spaces between thalli.  The second arrow from the top, for example, is an opening the goes all the way through the model.
 
 
 
 
 

In summary, this project illustrates how newish technologies (CT scanning and rapid prototyping) can be used to study rather oldish plants embedded in rocks without destroying either the fossils or the rocks.  The full version of this effort, complete with materials and methods, will appear in the journal Palaeontology.  The work would not have been possible without the close and generous collaboration of Robert W. Christensen with TMJ Implants, and Richard Ketcham with the UT High Resolution CT facility.  We are deeply indebted to both.

For a brief author's CV,  click here.

This page was created on 15 Apr 03.

Address comments or questions to: andym@ku.edu

Copyright © 2003.  The author's permission is required for the use any of this material.