THE FOSSIL GENUS Ivanovia has long been known as a Late Paleozoic, marine green alga that lived in warm, relatively shallow waters.  Near relatives include Eugonophyllum and Calcipatera, and these have traditionally been grouped in the family Codiaceae although arguments have been made for other placements such as Udoteaceae and Halimedaceae.  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.

The genera had been known only from the Late Paleozoic (Pennsylvanian and Permian; about 320 to 250 million years ago),  until Pamela Reid of the Universtiy of Miami described an Ivanovia from the Stikinia terrane of the Yukon Territory of Canada in rocks of Mesozoic Triassic (about 250  to 213 million years ago) age.  Consequently, Ivanovia was among the relatively few survivors of the massive Permian-Triassic extinction.   Reid generously gave me for study a weathered, craggy, dark gray limestone rock about 17 x 14 cm that she had collected from Lime Peak in the Yukon.  This specimen is the basis for this report.

Ivanovia has been collected in many parts of the world, but perhaps the most beautifully preserved fossils are those collected by Donald Toomey from southern Tunisia (J. Paleontology, 69:381-387, 1995).  It was the Toomey material that was CT scanned   (J. Paleontology, 73:3154-158, 1999).  The preservation of the Yukon material is not nearly as good as the Tunisian, but the deterioration is not surprising as terranes are huge blocks of crust that were transported vast distances.

Recent studies have shown that some  species of these algae actually had a cup-like or cyathiform shape similar to living Udotea cyathiformis but with a very different membrane structure--the term membrane referring to the approximately 1 mm thick sheet forming the thallus, the body of the organism. Being cyathiform, the membrane had an inner and outer surface, made up of the cortices surrounding a central medulla.  The cortices were a layer of finger-like protrusions called utricles (from the Latin meaning a bottle or vessel).    When the algae were alive, there was a coating of araganite, a type of calcium carbonate, over the whole thallus, apparently similar to what occurs with living Halimeda.  When the organism died, this coating made a mold that was filled with fine crystals of calcite, another kind of calcium carbonate.  Thus, the fossils we study today are likely casts.  The following figure illustrates the sections that result from cutting a cyathiform structure and shows the terms and relationships of the several parts. It is commonly diffucult for one to accept that the entire organism, some 2 cm wide and 3 cm tall,  was a single cell, a coenocyte.  It no doubt had many chloroplasts, but we have no idea of their form or distribution.
 
 

      This figure shows the appearances of various sections when an embedded cup-shaped structure is cut in different planes.

 

      Also shown is the detail of the membrane structure, two cortices, one inner and one outer, sandwiching a central medulla.  The entire thallus was a single cell; that is, there were no cross walls separating the structure into numerous cells.

 

 
 
 
 
 

The cyathiform shape for the algal thallus is inferred from sections that are U-shaped and ring-shaped.  It seems that the  most likely three-dimensional form that would yield sections of these types is a cup shape.  In the well preserved I. tebagaensis from Tunisia, the utricles of the inner and outer cortices clearly  had very different morphologies.  The inner being like a palisade of tightly packed utricles while those of the outer cortex were more open and spongy.  The utricles of the inner and outer cortices of the Yukon Ivanovia, while not as clear because of the poor preservation, seemed also to have been dimorphic.

There were on the surface of the rock some small protrusions that appeared as embedded algal fragments of the sort that is commonly seen by collectors.  To view the material more closely, it was cut, rather at random, into four pieces that provided six faces or surfaces that were highly polished and viewed with a dissecting microscope using fiber optic illumination.  One face is shown below.

 

 
 
 

     This photo is of one of the six faces made by cutting the rock perpendicular to the longest axis.  The larger arrow indicates the stratigraphic up direction for this face,  and the smaller one points to a cross section of a distorted cup that is a complete circle or ring.  The widest part of the ring is about 8 mm across.

It soon became apparent that there were many specimens in the rock and that some of the structures had never before been reported.  It should be pointed out that in  none of the species of the genus had any reproductive structures been observed although spherical cells seen in Eugonophyllum (J. Paleontology, 71:493-499, 1997) were assumed to have something to do with reproduction. Some of the Yukon structures were best interpreted as a stalked oogonium, others as a dome-shaped male gametangium.  Examples of each are shown below.

 


    On the left is what appears to be a cup-shaped oogonium, a structure that produces eggs.  It is about 1 mm wide.

    On the right is what seems to be a dome-shaped male gametangium.  It is about 1.5 mm high.

 

 

That the oogonium is a differentiated part of the thallus is evidenced by the utricular casts which clearly form the stalk of the oogonium and coming from the lower left- toward the upper right-trending membrane.  The small  cup with an apical pore encloses a single egg.  Indeed, the spherical cell to the left of the stalk may be a zygote or young embryo.  The male gametangium  presumably would release sperm that would seek out an egg in the oogonium.

In summary, the Yukon Ivanovia was a Permian-Triassic mass extinction survivor with sexual reproductive structures, perhaps like those of Eugonophyllum except stalked, was cyathiform like I. tebagaensis and at least one species of Eugonophyllum,  and apparently had dimorphic cortices like I. tebagaensis. With each discovery in this fascinating group of extinct algae, their relationships become clearer.

For a brief CV,  click here.

This page was last updated  on 8 Jun 01.

Address comments or questions to: andym@ku.edu

Copyright © 2001.  The author's permission is required for the use any of this material.