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

Yan Liang,Joseph Bernardo,Peng Tang,Wenhui Wang,Daniel Goldman,Olle Hints,Jaak Nõlvak

doi:10.1098/rspb.2019.1270

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.

Highlights

Chitinozoans are an extinct group of cryptic organic-walled microfossils widely distributed in Ordovician to Devonian marine sedimentary rocks
The results show that the range of variation in most chitinozoans is much larger than is typical of eggs of extant marine invertebrates or the hitherto reported undisputed fossil eggs
The range and type of size variation shown in H. violana sp. nov. potentially indicate an ontogenetic series rather than an intraspecific variation

Summary

Introduction

Chitinozoans are an extinct group of cryptic organic-walled microfossils widely distributed in Ordovician to Devonian marine sedimentary rocks (ca 485–359 Myr old). They are characterized by a radially symmetrical shell, known as vesicle, which is commonly jar-, vase- or tube-shaped and has an opening in one end sealed with a plug ( prosome) or lid (operculum). Chitinozoans originated (leaving out the problematic record in Cambrian [2]) and diversified in the Great Ordovician Biodiversification Event [3,4], survived the end Ordovician extinction [5] and disappeared during the Late Devonian extinction [6] Except for their enigmatic life history, they have attracted wide attention as a tool in high-resolution biostratigraphy [7,8,9], providing insights for several key events [10,11,12]. This allowed a reliable comparison of the magnitude of size variation between chitinozoans and undisputed eggs to further assess their possible analogy

Material and methods

Results

Discussion

Findings

Conclusion