Some animals make plant sterols.

Most animals abandoned plant sterols early in evolution, but some still depend on them.

Guts, gut contents, and feeding strategies of Ediacaran animals.

The oldest animals appear in the fossil record among Ediacara biota communities. They prelude animal-dominated ecosystems of the Phanerozoic and may hold clues to the appearance of modern animal phyla in the Cambrian explosion. However, little is known about the phylogeny of the Ediacaran organisms and even less about their diet and feeding behavior.1,2,3 An exception is mollusc-like Kimberella, for which a fossilized gut, feeding traces, and even potential coprolites have been found.4,5 By contrast, Ediacaran organic-walled tubes, such as Sabellidites and Calyptrina, are thought to belong to tube worms comparable with modern Siboglinidae that have no gut but gain their nutrition from symbiotic bacteria.6,7 Here, we examine the gut contents of Ediacaran animals using biomarker molecules. We show that 558-million-year (Ma)-old tube worm-like Calyptrina and mollusc-like Kimberella possessed a gut and shared a diet of green algae and bacteria. Despite their ancient age, sterol metabolism within the gut of both organisms was already comparable to extant invertebrates.8Dickinsonia, one of the key Ediacaran animals, show no traces of dietary molecules, indicating a different feeding mode and possible external digestion analogous to modern Placozoa. Lipid biomarkers uncover a range of feeding strategies in Ediacaran communities, highlighting true eumetazoan physiology of some Ediacaran animals.

Algal origin of sponge sterane biomarkers negates the oldest evidence for animals in the rock record.

The earliest fossils of animal-like organisms occur in Ediacaran rocks that are approximately 571 million years old. Yet 24-isopropylcholestanes and other C30 fossil sterol molecules have been suggested to reflect an important ecological role of demosponges as the first abundant animals by the end of the Cryogenian period (>635 million years ago). Here, we demonstrate that C30 24-isopropylcholestane is not diagnostic for sponges and probably formed in Neoproterozoic sediments through the geological methylation of C29 sterols of chlorophyte algae, the dominant eukaryotes at that time. These findings reconcile biomarker evidence with the geological record and revert the oldest evidence for animals back into the latest Ediacaran.

Food sources for the Ediacara biota communities.

The Ediacara biota represents the first complex macroscopic organisms in the geological record, foreshadowing the radiation of eumetazoan animals in the Cambrian explosion. However, little is known about the contingencies that lead to their emergence, including the possible roles of nutrient availability and the quality of food sources. Here we present information on primary producers in the Ediacaran based on biomarker molecules that were extracted from sediments hosting Ediacaran macrofossils. High relative abundances of algal steranes over bacterial hopanes suggest that the Ediacara biota inhabited nutrient replete environments with an abundance of algal food sources comparable to Phanerozoic ecosystems. Thus, organisms of the Ediacara biota inhabited nutrient-rich environments akin to those that later fuelled the Cambrian explosion.

Simple sediment rheology explains the Ediacara biota preservation.

The soft-bodied Ediacara biota (571-541 million years ago) represents the oldest complex large organisms in the fossil record, providing a bridge between largely microbial ecosystems of the Precambrian and the animal-dominated world of the Phanerozoic, potentially holding clues about the early evolution of Metazoa. However, the nature of most Ediacaran organisms remains unresolved, partly due to their enigmatic non-actualistic preservation. Here, we show that Flinders-style fossilization of Ediacaran organisms was promoted by unusually prolonged conservation of organic matter, coupled with differences in rheological behaviour of the over- and underlying sediments. In contrast with accepted models, cementation of overlying sand was not critical for fossil preservation, which is supported by the absence of cement in unweathered White Sea specimens and observations of soft sediment deformation in South Australian specimens. The rheological model, confirmed by laboratory simulations, implies that Ediacaran fossils do not necessarily reflect the external shape of the organism, but rather the morphology of a soft external or internal organic 'skeleton'. The rheological mechanism provides new constraints on biological interpretations of the Ediacara biota.

Putative sponge biomarkers in unicellular Rhizaria question an early rise of animals.

The dawn of animals remains one of the most mysterious milestones in the evolution of life. The fossil lipids 24-isopropylcholestane and 26-methylstigmastane are considered diagnostic for demosponges-arguably the oldest group of living animals. The widespread occurrence and high relative abundance of these biomarkers in Ediacaran sediments from 635-541 million years (Myr) ago have been viewed as evidence for the rise of animals to ecological importance approximately 100 Myr before their rapid Cambrian radiation. Here we show that the biosynthesis of 24-isopropylcholestane and 26-methylstigmastane precursors is common among early-branching unicellular Rhizaria-heterotrophic protists that play an important role in trophic cycling and carbon export in the modern ocean. Negating these hydrocarbons as sponge biomarkers, our study places the oldest evidence for animals closer to the Cambrian Explosion. Cambrian silica hexactine spicules that are approximately 535 Myr old now represent the oldest diagnostic sponge remains, whereas approximately 558-Myr-old Dickinsonia and Kimberella (Ediacara biota) provide the most reliable evidence for the emergence of animals. The proliferation of predatory protists may have been responsible for much of the ecological changes during the late Neoproterozoic, including the rise of algae, the establishment of complex trophic relationships and the oxygenation of shallow-water habitats required for the subsequent ascent of macroscopic animals.

Molecular fossils from organically preserved Ediacara biota reveal cyanobacterial origin for Beltanelliformis.

The Ediacara biota (~575-541 million years ago) mark the emergence of large, complex organisms in the palaeontological record, preluding the radiation of modern animal phyla. However, their phylogenetic relationships, even at the domain level, remain controversial. We report the discovery of molecular fossils from organically preserved specimens of Beltanelliformis, demonstrating that they represent large spherical colonies of cyanobacteria. The conservation of molecular remains in organically preserved Ediacaran organisms opens a new path for unravelling the natures of the Ediacara biota.

Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals.

The enigmatic Ediacara biota (571 million to 541 million years ago) represents the first macroscopic complex organisms in the geological record and may hold the key to our understanding of the origin of animals. Ediacaran macrofossils are as "strange as life on another planet" and have evaded taxonomic classification, with interpretations ranging from marine animals or giant single-celled protists to terrestrial lichens. Here, we show that lipid biomarkers extracted from organically preserved Ediacaran macrofossils unambiguously clarify their phylogeny. Dickinsonia and its relatives solely produced cholesteroids, a hallmark of animals. Our results make these iconic members of the Ediacara biota the oldest confirmed macroscopic animals in the rock record, indicating that the appearance of the Ediacara biota was indeed a prelude to the Cambrian explosion of animal life.