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Fossil Friday: Chitinozoa — Enigmatic Microfossils from the Paleozoic Era

October 18, 2024, 6:44 AM
Photo credit: en:User:Verisimilus, CC BY-SA 3.0 , via Wikimedia Commons.

This Fossil Friday features the SEM image of a microscopic fossil from the Silurian of Gotland in Sweden. It belongs to a fascinating but enigmatic group of extinct marine organisms called Chitinozoa, which thrived in 56 known genera from the Early Ordovician to the Latest Devonian period, roughly 485 to 359 million years ago. They were first described by Eisenack (1931), who initially considered them as single-celled organisms related to testate amoebae but just a year later changed his mind.

Their abundance suggests that they played an important role in Palaeozoic marine ecosystems, most likely as planktonic organisms, free-floating in the oceans, or perhaps resting on the seafloor. Despite being found in such abundance and diversity in marine sediments across the world, and thus representing crucial biostratigraphic markers, the biological affinities of Chitinozoa were one of paleontology’s long-standing controversies (Jansonius & Jenkins 1998, Paris et al. 1999, Paris & Verniers 2005). These flask-shaped microfossils, typically ranging from 50 to 2,000 microns in size, were composed of an organic material resembling chitin (despite the name not identical with chitin). Sometimes they are found in spirally coiled chains packed in an organic cocoon.

No Definitive Evidence

Despite extensive research, no definitive evidence existed regarding their exact biological nature or life cycle, so that their origin and function remained a mystery until recently. This had led to multiple hypotheses being proposed, each attempting to explain their ecological role and biological affinity. Scientists have debated whether they represent the eggs or egg cases of some larger marine animals such as polychaete worms or gastropods (Kozlowski 1963, Paris & Nõlvak 1999), or of conodonts or cephalopods (Gabbott et al. 1998). Other suggest that they might represent juvenile stages of graptolites (Jansonius & Jenkins 1998). However, any putative association with such other extinct Paleozoic macro-organisms faced the problem that none of them exhibits a precisely corresponding pattern of stratigraphic distribution. Alternatively a relationship with fungi or protists has been considered (e.g., Obut 1973), possibly as their spores or cysts, or even that they represent remnants of a distinct life form altogether. The lack of transitional fossils, anatomical details, or clear links to modern organisms complicated efforts to place Chitinozoa within the biological system.

No Universal Acceptance

The hypothesis that they represent egg cases of some extinct invertebrate remained one of the most widely accepted (Paris & Verniers 2005), but despite decades of research by numerous experts, no single theory about the origins and biological affinities of Chitinozoa has gained universal acceptance. A few years ago, two new studies by Liang et al. (2019, 2020) finally shed some more light on the nature of chitinozoans and strongly supported the alternative protist hypothesis. The scientists not only found anatomical details that are inconsistent with the egg hypothesis, but, with advanced imaging techniques, could also document asexual reproduction. They concluded that chitinozoans “represent a new isolated group of protists.”

An Ever-Growing List

Because for a long time the oldest known Chitinozoa were from the Early Ordovician ( also see: Ghavidel-Syooki & Piri-Kangarshahi 2024), it was widely thought that this group originated during the so-called Great Ordovician Biodiversification event, which has also been called life’s second Big Bang (after the first Big Bang with the Cambrian Explosion) (see Bechly 2024). However, more recently putative chitinozoans of the genus Eisenackitina have been described from the Cambrian Stage 5 Duyun fauna of China (Shen et al. 2013). Earlier evidence for alleged Late Precambrian chitinozoans (Bloeser et al. 1977) was later refuted as misidentified testate amoebae (Porter & Knoll 2000, Porter et al. 2003), while the phosphatized microfossils from the Cambrian of China show the “typical chitinozoan characteristics, i.e., a spiny surface, a flaring collarette with a sinuate rim, and a copula on the base” (Shen et al. 2013). In a previous article (Bechly 2023) I elaborated that the Cambrian Explosion includes not only the sudden appearance of more than 20 bilaterian animal phyla, but also several distinct new groups of protists. We may now add the mysterious Chitinozoa to this ever-growing list of products of the burst of biological creativity in the Early Cambrian.

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