Review paper on the fossil record of symbiotic organisms in bryozoans has just been published

Olev Vinn, Andrej Ernst, and I have been working for years on various case studies of symbiotic endobionts (organisms that live within the skeletons of others) in the fossil record. This week our data-rich review paper has been published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology. Olev led us through the complex literature to produce this compendium and analysis. It contains some of my favorite concepts, like boring, bioclaustration and the Ordovician Bioerosion Revolution. Here is the abstract:

Trepostome bryozoans, with their thick calcitic skeletons, formed the largest number of symbiotic associations with endobionts in the Phanerozoic. Such associations were also formed by cystoporates, fenestrates, cyclostomes and cheilostomes. Bryozoans formed most of their symbiotic associations with endobiotic cnidarians, and markedly fewer with endobiotic worms and endobiotic lophophorates. The majority of Ordovician endobionts colonized borings in living bryozoans, or bored themselves into living hosts, during the Ordovician Bioerosion Revolution, which created new niches for the evolution of symbiotic relationships. The bryozoans likely became more selective and less symbiont-tolerant over the time. Assumed mutualistic endobionts were more common than likely parasites in Phanerozoic bryozoans. The decrease in diversity of parasitic associations and the increase in the number of mutualistic associations from the Ordovician to Devonian can be explained by the evolution of possible bryozoan defense mechanisms likely in the form of chemical secretions. Paleozoic endobiont faunas were more diverse than their Mesozoic and Cenozoic counterparts because of endobiont-friendly Paleozoic trepostomes, and because of the peak in diversity of bryozoans with massive colonies in the early and middle Paleozoic.

Bryozoans are excellent subjects for the study of symbiosis over time because they usually have thick skeletons of stable calcite that record many of the critters that lived on and in their colonies. This is especially true in the Paleozoic. Within the Paleozoic the Ordovician had by far the most recorded symbiotic relationships, which is not surprising considering the abundance of bryozoans then.

Caption for the top image, which is from Figure 4 of the paper: Ordovician symbiotic endobionts. A, Sanctum laurentiensis in Batostoma? sp. with opening at the typical bifurcation point from the Decorah Formation (Ordovician: Mohawkian) of Minnesota (modified from Erickson, 2020). B, Anoigmaichnus zapalskii in Mesotrypa expressa from Kullaaru ditch, Oandu Stage (Katian) northern Estonia (GIT 770–39). C, Kuckerichnus kirsimaei in Diplotrypa sp. from Kohtal-Järve, Kukruse Stage (Sandbian), northern Estonia (TUG 72–826-2). D, Bioclaustration in Orbiramus sp. from the Fenhsiang Formation (Tremadocian), China (Modified from Ma et al., 2020, Fig. 7B).

This was a very satisfying project because it summarizes two decades of our work and, of course, that of many others. It is also, by the way, a product of the recent pandemic. We all spent far more time than usual writing in our offices and labs during 2020-2022. This is why I’m posting an unusual number of new papers during the first few months of 2023.


Vinn, O., Wilson, M.A. and Ernst, A. 2023. Macroscopic symbiotic endobionts in Phanerozoic bryozoans. Palaeogeography, Palaeoclimatology, Palaeoecology 615: 111453.

[Click the link for a free pdf for the next 50 days; after that you can ask me for a copy.]

About Mark Wilson

Mark Wilson is a Professor of Geology at The College of Wooster. He specializes in invertebrate paleontology, carbonate sedimentology, and stratigraphy. He also is an expert on pseudoscience, especially creationism.
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