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Popular Science: The Ordovician struggle for a solid base in a sea of soft sediment: focused on the conulariids of the Prague Basin

For a large part of sessile animals, whether current or long-extinct, a hard base is a prerequisite for a full-fledged life. In ancient times, there was a bitter struggle for space to attach to on the seabed. Jana Bruthansová from the Department of Geology and Palaeontology, Faculty of Science, Charles University, has published an article in the prestigious journal Palaeogeography, Palaeoclimatology, Palaeoecology about sessile organisms on conulariids.

Ordovician is the second period of the Proterozoic. It was codified in 1960 at the 21st International Geological Congress in Copenhagen. However, the proposal for its definition dates back to the latter half of the 19th century. At the time, the English geologist Charles Lapworth, who was studying index fossils, proposed the creation of a new geological period that would include rocks younger than Cambrian ones of North Wales and older than Silurian ones of South Wales. Lapworth correctly recognized the significant differences in the fauna of the two periods and included in the Ordovician those organisms that were not typical of either period.

Ordovician was one of the coldest periods of the Phanerozoic, which is defined as beginning in the Cambrian and continuing to this day. Low global temperatures were caused by the position of much of the Gondwana continent around the South Pole. At the end of this period, there was one of the largest waves of mass extinction in the history of life on Earth.

The bottom in many areas of the period ocean was formed primarily by sediments. In this environment, living organisms had to fight for every bit of solid ground to which they could attach. Many groups of animals and plants used other organisms for this purpose. A large number of different organisms known from the fossil record overcame the inhospitable soft environment in this way.

One species of the many that served as a solid foundation were conulariids. These were fourfold animals belonging to the cnidarids group. Conulariids lived throughout the Proterozoic, from the Upper Ediacaran to the Triassic, when they disappeared after the largest known mass extinction wave. They subsisted by filtering seawater, where the tentacles picked up organic detritus from the water column. Their mouths were covered with four triangular flaps that could hide the animal inside a shell-like structure, which was composed of chitinophosphatic and organic microlamellae.

Jana Bruthansová found over 200 individuals carrying sessile organisms. Sometimes only attachment scars have been preserved on the conulariid shell, other times whole epibionts (organisms that live on the surface of another living organism) have. Fossils come from the Ordovician rocks of the Prague Basin. Most of the finds belong to the Letná and Zahořany formations of the Sandbian and Katian stages. This corresponds to the greatest diversity of conulariids in this period. From some findings it is evident that the epibiont only attached after the death of the individual.

Geological map of the Prague Basin with studied localities. Authors of the article.

Sessile organisms belong to several groups of animals. The most common epibionts on the bodies of the conulariids include brachiopods (the Craniidae family), bryozoans, edrioasteroids (echinoderms); less often, there are also monoplacophorans and the taxonomically problematic genus Sphenothallus, which belongs to the conulariids affinity.

An interesting outcome of the research is also the preference of several genera of conulariids instead of others. Epibionts were most often found on the outside of shells of conulariid of the Anaconularia anomala species and the Archaeoconularia genus. Findings inside the shell or sitting on the inside of the closable flaps are very rare and belong to the monoplacophorans. These did not reach the shell until the death of the conulariid.

Of the approximately 5,000 described conulariids of the Ordovician of the Prague Basin, only 4% exhibit at least some indication of a sessile animal. The results were compared with the findings of epibionts on conulariids from the area of ​​today’s Morocco. At that time, today’s Czech Republic was relatively close to the African country. This is evident primarily thanks to the findings of very similar trilobite fauna from both areas of the same age. In both studies, a similar proportion of dominant groups of epibionts was found on conulariid shells.

In addition, a certain targeting of sessile animals, mostly in the larval stage, was found in individuals of the Archaeoconularia genus. For example, the larvae of brachiopods and edrioasteroids would likely seek the most suitable places to attach to the shell. The best one was near the centre of the sides of the shell, which provided the largest space for epibionts to grow. Overall, it appears that the attachment occurred in random places rather than in pre-selected ones.

The study of interactions between animals in a paleontological record is an interesting subfield of this scientific discipline. In particular, the processes observable in the modern animal kingdom are applied. During the Ordovician period, there was an observable increase in the number of species, when individual organisms tried to adapt as best as possible to the then environmental conditions. And some animals skilfully used other organisms on which they perched.

The scientific article is a contribution to the project GAČR (18-05935S):

Z minulosti do přítomnosti: fosilní versus recentní schránky mořských živočichů jako substrát pro kolonizaci a bioerozi.
Co-researcher from the Department of Geology and Palaeontology, Faculty of Science, Charles University:
doc. RNDr. Katarína Holcová, CSc.

Bruthansová, Jana and H. V. Iten. “Invertebrate epibionts on Ordovician conulariids from the Prague Basin (Czech Republic, Bohemia).” Palaeogeography, Palaeoclimatology, Palaeoecology 558 (2020): 109963.

Jan Geist

Published: Feb 09, 2021 04:30 PM

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