Morphological variation suggests that chitinozoans may be fossils of individual microorganisms rather than metazoan eggs

Abstract

Chitinozoans are organic-walled microfossils widely recorded in Ordovician to Devonian ( ca 485–359 Mya) marine sediments and extensively used in high-resolution biostratigraphy. Their biological affinity remains unknown, but most commonly, they are interpreted as eggs of marine metazoans. Here, we provide new insights into their palaeobiology from three lines of inquiry. We examine morphological variation of a new, well-preserved Late Ordovician species, Hercochitina violana ; analyse a compiled dataset of measurements on 378 species representing all known chitinozoan genera; and compare these data with the size variation of eggs of both extinct and extant aquatic invertebrates. The results indicate that the magnitude of size variation within chitinozoan species is larger than observed in fossil and modern eggs. Additionally, delicate morphological structures of chitinozoans, such as prosome and complex ornamentation, are inconsistent with the egg hypothesis. Distinct and continuous morphological variation in H . violana is more plausibly interpreted as an ontogenetic series of individual microorganisms, rather than as eggs.

Possible metazoan egg fossils from the Darriwilian (Middle Ordovician) of Baltoscandia

“…Another plausible way to explain the nature of Vikisphaera specimens is to consider them not as shells of small organisms but as eggs or other developmental stages of metazoans, similarly to a common interpretation of the Chitinozoa (Paris & Nõlvak 1999). According to Liang et al (2019), it was unlikely that chitinozoans were eggs because of their large size variation. The size variation of Vikisphaera is also rather wide -with the largest specimens being twice the size of the smallest ones and the coefficient of variation reaching ca 12% -but falls within the range of egg-size variation of modern aquatic metazoans, notably molluscs and echinoderms .…”

“…In several cases the biological affinity of a particular microfossil group has remained unknown or debatable. For instance, a number of hypotheses have been put forward to explain the origin of chitinozoans (see overview in Miller 1996), a common interpretation being that they are egg shells of cryptic marine metazoans (Paris & Nõlvak 1999), or perhaps cases of microscopic protozoans (Liang et al 2019(Liang et al , 2020. The uncertain biological origin of chitinozoans has nevertheless not limited their application in biostratigraphy (e.g.…”

Early and early Middle Ordovician chitinozoans from the Baldone drill core, central Latvia

“…Chitinozoans are a common group of organic-walled marine microfossils in Ordovician to Devonian rocks (Eisenack 1931;Servais et al 2013). They are widely used in biostratigraphy despite the fact that their biological origin has remained controversial (see Miller 1996;Paris & Nõlvak 1999;Liang et al 2019Liang et al , 2020a for a review). In the Baltic region, Ordovician chitinozoans have been studied in the pioneering works of Alfred Eisenack since the 1930s (see Eisenack 1976 for references) and by many subsequent authors (e.g.…”

Carbon cycle dynamics and ecology revealed by the carbon isotopic composition of single organic microfossils during the Late Devonian Biotic Crisis

“…The other group of OWM examined in this study are chitinozoans, which are flask‐shaped closed organic‐walled structures (Figure 1c). Some workers believe they are animal egg cases, but it is more likely that they represent an unknown protist (Liang et al, 2019; Paris & Nõlvak, 1999). OWM are abundant in both the Upper and Lower Kellwasser Events in numerous localities in New York State (Kelly et al, 2019).…”