360 PRECAMBRIAN/Eukaryote Fossils constrained, but possibly 620–780 Ma) of north-western Canada can be compared to chrysophyte skeletons Vase-shaped microfossils (Figure 3F) from the Chuar Group and other Early Neoproterozoic successions (e.g the Visingsoă Formation in Sweden and the Draken Conglomerate Formation in Spitsbergen) have been interpreted as testate amoebae whose mineralized tests are typically preserved in casts and moulds The Chuar testate amoeba fossils add another dimension to our consideration of the Neoproterozoic biosphere, and that is heterotrophy Because testate amoebae are heterotrophic protists, the Chuar vaseshaped microfossils suggest that the Early Neoproterozoic biosphere was ecologically complex Of course, heterotrophic eukaryotes must have evolved earlier In fact, the earliest eukaryotes may be heterotrophic, given that eukaryotic autotrophy evolved through primary and secondary endosymbiotic events However, evidence for heterotrophy in the fossil record is scarce Vase-shaped microfossils and ciliate biomarkers from the Chuar Group are probably the earliest known evidence for heterotrophic eukaryotes The best-known heterotrophic eukaryotes are perhaps the animals There have been many reports of animal fossils from Early Neoproterozoic and Mesoproterozoic successions, but their interpretation has been controversial Sinosabellidites huainanensis, Pararenicola huaiyuanensis, and Protoarenicola baiguashanensis (Figure 3G), from the Huainan Group and Huaibei Group (ca 740–900 Ma) of northern China, are some of the often-cited Early Neoproterozoic animal fossils They are carbonaceous compressions of tubes of millimetric diameter and centimetric length with transverse annulations The transverse annulations superficially resemble animal metameric segmentation A few specimens bear poorly defined terminal structures that have been interpreted as proboscis-like structures However, recent study has shown that these carbonaceous compressions are probably benthic tubular algae Middle Neoproterozoic The Middle Neoproterozoic is characterized by multiple global glaciations, and is unofficially labelled as the Cryogenian Period by some Precambrian geologists On a broad scale, acritarchs and other eukaryotes suffered significant losses of diversity in the Middle Neoproterozoic The documented diversities of several Middle Neoproterozoic assemblages are extremely low, and such assemblages are typically dominated by Sphaerocongregus variabilis (or Bavlinella faveolata) This Cryogenian drop in eukaryote diversity may be a true evolutionary pattern that was related to the glaciation events Despite the loss of diversity, the occurrence of red algae, green algae, photosynthetic stramenopiles, and testate amoebae in Mesoproterozoic and Early Neoproterozoic rocks suggests that some members of these groups must have survived the Middle Neoproterozoic glaciations Late Neoproterozoic Eukaryote diversity rose sharply in the Late Neoproterozoic Both acritarchs and multicellular algae reached unprecedented levels of complexity and diversity in the Late Neoproterozoic Some of the multicellular algae are preserved in anatomical detail, allowing them to be placed within the red algae One of the most important landmarks in Late Neoproterozoic eukaryote evolution is the emergence of animals and animal biomineralization Molecularclock studies suggest that the deepest (protostomes– deuterostomes) divergence within the crown-group bilaterian animals probably occurred in the Mesoproterozoic or Early Neoproterozoic But, as discussed above, there is no convincing palaeontological evidence to support these molecular-clock estimates Some have suggested that perhaps the earliest animals were microscopic in size and would not be well preserved in the fossil record The Doushantuo Formation and Dengying Formation of South China provide several taphonomic windows onto the Late Neoproterozoic biosphere Carbonaceous shales, cherts, and phosphorites of the Doushantuo Formation (ca 600–550 Ma) preserve some of the most extraordinary eukaryote fossils in the Neoproterozoic More than 20 taxa of macroscopic carbonaceous compressions have been reported from the Yangtze Gorges area and elsewhere in South China Most of these compressions can be unambiguously interpreted as multicellular algae (Figure 4A) Some of them show clear evidence of holdfast anchoring, dichotomous branching, apical meristematic growth, and specialized reproductive structures A few of these compressions (for example Calyptrina striata and Sinospongia typica; Figure 4B), however, have been interpreted as sponges or cnidarians, but such interpretations are not unique and an algal interpretation cannot be falsified conclusively In any case, it is safe to conclude that none of the Doushantuo compressions can be interpreted as macroscopic bilaterians (bilaterally symmetrical animals) Multicellular algae also occur in the Doushantuo cherts and phosphorites Cellular features are preserved (Figures 4C–F), so anatomical detail can be deduced from these fossils Many of these silicified or phosphatized algal fossils show pseudoparenchymatous thallus construction, apical meristematic