Abstract
Abstract Reproduction is a key aspect of evolution, but the process is rarely preserved in the fossil record. Organisms fortuitously preserved undergoing reproduction provide an exceptional window illuminating the biology of extinct taxa, especially those with unknown phylogenetic position. Here we report exceptional specimens of chitinozoans (enigmatic Paleozoic organic-walled microfossils) preserved as “test-in-test” morphology, which have previously been interpreted as teratological forms. Application of advanced imaging techniques on newly recovered and reexamined Ordovician materials enabled documentation of critical morphological details of the test’s inner ultrastructure for the first time. The results show that the newly observed spongy material and dendritic structure on or inside the chitinozoan test as well as the test wall itself are all made of clustered rounded spherical particles. Morphological details suggest that those specimens represent key stages of new asexual reproductive strategies, hitherto undescribed, which produce either one or several offspring at a time. This observation challenges the prevailing hypothesis that chitinozoans are eggs of cryptic extinct marine metazoans. Instead, it is more plausible that they represent a new isolated group of protists.
Highlights
Chitinozoans are a group of enigmatic marine microfossils widespread in Ordovician to Devonian rocks
A recent study has, questioned the prevailing egg hypothesis by documenting a large morphological variation in chitinozoan populations that is inconsistent with the variation found in modern and fossil eggs of aquatic invertebrates, and suggested that chitinozoans were independent microorganisms (Liang et al, 2019)
Sixteen (16) of the 20 exceptional specimens are characterized by a complete test that carries
Summary
Chitinozoans are a group of enigmatic marine microfossils widespread in Ordovician to Devonian rocks (ca. 481–359 Ma; the contentious Cambrian data reported by Shen et al [2013] require further study). 481–359 Ma; the contentious Cambrian data reported by Shen et al [2013] require further study). They are bottleshaped radially symmetric organic-walled tests, varying from ∼50 to 2700 μm in length and having an opening sealed with an apertural plug (either operculum or prosome). Analyzed by a combination of novel imaging techniques, important details about their morphology and inner ultrastructure are revealed for the first time. This critical new information provides exciting new insights into the reproductive strategies and biological affinity of this enigmatic group. By demonstrating a protist affinity for chitinozoans, our study provides a greater understanding of the diversity and evolution of skeletonized zooplankton during a critical interval of Earth history, the Great Ordovician Biodiversification Event (ca. 485–443 Ma; Servais and Harper, 2018)
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