Mysterious fossils: Chitinozoans

Scanning electron micrograph of Sphaerochitina sp.(Late Silurian of Sweden). From Wikimedia Commons

Scanning electron micrograph of Sphaerochitina sp.(Late Silurian of Sweden). From Wikimedia Commons

Chitinozoans are marine organic-walled microfossils,  bottle-shaped, consisting of  hollow organic vesicles of uncertain affinity.  The vesicle ranges from 30 to 1500 μm, but most are 150–300 μm long. The outer wall of the vesicle may be smooth, striate, tuberculate, folded into hollow  spines or extended into a tubular sleeve. The group was named by Eisenack in 1931 who was the first to noted that the material of the wall was composed by a pseudochitinous material.

Chitinozoans can be found as single forms or joined together in chains. They first appeared in the Early Ordovician and became an abundant and diverse group, until the Silurian, and they extinct in the Early Carboniferous. They were exclusively marine and lived in a wide range of shelf environments.

Because of their rapid evolution and wide diversity of forms,  are useful  for local and global stratigraphical correlations.

 Lagenochitina sp. (right) Conochitina sp. (left)


Lagenochitina sp. (right) Conochitina sp. (left). From UCL.

Systematic position of the taxon still remains problematic.  The pseudochitin wall suggests animal affinities, but whether they are metazoan or protistan is still uncertain. In 1963, Kozlowski  proposed they were the eggs of annelid worms. The co-occurrence of chitinozoa with scolecodonts is a strong argument to his proposal.

Prosomatifera, longitudinal section.

The Chitinozoans are classified based on the structure of their opening and the overall shape of flask and neck. The Order Operculatifera contains one family, the Desmochitinidae, characterized by an operculum, reduced oral tubes and a relatively small subspherical vesicle. The Order Prosomatifera contains two families the Conochitinidae and the Lagenochitinidae, distinguished by the relationship between the chamber and the neck.

Armoricochitina nigerica, from late Ordovician Hirnantian glaciation.

Armoricochitina nigerica, from late Ordovician Hirnantian glaciation.

Most  of chitinozoan genera appear to be cosmopolitan, others show latitudinal provinciality. A reconstruction of the paleobiogeographic distribution of chitinozoan before and after the Hirnantian glaciation (∼440 Ma) showed a pattern that revealed the position of ancient climate belts. This patterns look “very modern” and suggests that ancient carbon dioxide levels could not have been as high as previously thought, but were more modest, about five times current levels.

 

References:

Armstrong, H. A., Brasier, M. D., 2005. Microfossils (2nd Ed). Blackwell, Oxford.

Vandenbroucke, T.R.A., Armstrong, H.A., Williams, M., Paris, F., Zalasiewicz, J.A., Sabbe, K., Nolvak, J., Challands, T.J., Verniers, J. & Servais, T. 2010. Polar front shift and atmospheric CO2 during the glacial maximum of the Early Paleozoic Icehouse. PNAS doi/10.1073/pnas.1003220107.

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