First macromolecular characterization of fossil Choanoflagellates.

Choanoflagellates are microeukaryotes that inhabit freshwater and marine environments and have long been regarded as the closest living relatives of Metazoa. Although molecular clock evidence suggests their appearance by late Neoproterozoic, only recently the first identification of fossil choanoflagellates in sediments from the Cretaceous was made (Fonseca et al., 2023). Here, attenuated total reflection (ATR) micro-Fourier-transform infrared (micro-FTIR) spectroscopy was used to achieve the first macromolecular characterization of single-specimen fossil choanoflagellates, and to assess differences and similarities with dinoflagellate cysts from the same sample. ATR micro-FTIR spectra reveal consistent molecular differences between fossil choanoflagellates and dinoflagellate cysts. Both microfossils are composed of a predominantly hydrogen-cross-linked, aliphatic biomacromolecule, with notable contributions of (probably polycyclic) aromatic rings, aromatic ethers, and phenols. However, choanoflagellates were systematically more aliphatic in nature than dinoflagellate cysts. The generally aliphatic-aromatic compounds in fossil choanoflagellates differ from actin found in modern choanoflagellates (Karpov and Leadbeater, 1998) and a cellulose-like carbohydrate in modern dinoflagellates (Versteegh et al., 2012) and were likely a result of (possibly early diagenetic) molecular alteration due to oxidative polymerization, which is a common taphonomical phenomenon in organic fossils (Gupta and Briggs, 2011). Despite taphonomical effects, a parallel molecular analysis of different fossil organisms from the same sample can reveal chemosystematic signatures and, in this case, highlights a different pre-diagenetic macromolecule present in the analyzed fossil choanoflagellates compared to other microeukaryotes. These results provide a first basis to further decrypt the chemospecific signatures of fossil choanoflagellates using complementary analytical methods, as well as to explore the fossilization and preservation behavior of these primitive metazoans. This is necessary to be able to construct chemotaxonomical arguments which could support established morphological affinities of modem and fossil organisms. [ABSTRACT FROM AUTHOR]