358 PRECAMBRIAN/Eukaryote Fossils end of the Mesoproterozoic, several algal groups had already diverged Acanthomorphic acritarchs (or acritarchs bearing spines) and acritarchs with patterned vesicles are known in the 1400–1500 Ma Roper Group of northern Australia and the >1000 Ma Ruyang Group of northern China The Roper and Ruyang assemblages include such acanthomorphs as Tappania plana and Shuiyousphaeridium macroreticulatum (Figure 2C), as well as acritarchs with polygonally patterned (Dictyosphaera delicate and Shuiyousphaeridium macroreticulatum; Figure 2B and 2C) or striated (Valeria lophostriata) vesicles Some of these fossils have wide geographical and long stratigraphical ranges For example, Valeria lophostriata and Tappania occur in Mesoproterozoic and Neoproterozoic rocks in Laurentia, Australia, India, and northern China As no prokaryotes are known to have comparable levels of morphological complexity, these acritarchs are probably eukaryotic Some of these acritarchs (for example Dictyosphaera delicate) preserve an organic d13C signature that is consistent with eukaryotic photosynthetic biochemistry, but it is unclear which algal group (e.g chlorophytes, rhodophytes, or stramenopile algae – a group that includes chrysophytes, xanthophytes, diatoms, and brown algae; Figure 1) they belong to One of the earliest eukaryotic fossils that has been confidently attributed to a modern algal group is Bangiomorpha pubescens (Figures 2D and 2E) from the 1200 Ma Hunting Formation in arctic Canada This is a multicellular filamentous fossil that shows evidence of holdfast differentiation and sexual reproduction It is interpreted as a benthic bangiophyte red alga Another phylogenetically resolved eukaryotic fossil is Palaeovaucheria clavata (Figure 2F), interpreted as a xanthophyte alga, from the upper Mesoproterozoic Lakhanda Group in south-eastern Siberia Xanthophyte algae are members of the photosynthetic stramenopiles whose plastids were derived from a secondary endosymbiont (probably a red alga) The occurrence of Bangiomorpha pubescens and Palaeovaucheria clavata in Mesoproterozoic rocks suggests that not only must crown-group eukaryotes such as red algae have diverged but also the secondary endosymbiotic event leading to stramenopile algae must have occurred by the end of the Mesoproterozoic Neoproterozoic (1000–540 Ma) Eukaryotes The Neoproterozoic era includes several major milestones in eukaryote evolution The diversity and morphological complexity of eukaryotes increased appreciably in the Neoproterozoic, and several phylogenetically and ecologically important eukaryotic groups make their first appearance in the Neoproterozoic fossil record These include heterotrophic protists, biomineralizing protists, and, towards the end of this era, animals Molecular-clock estimates also indicate that land plants and fungi may have diverged in the Neoproterozoic, but so far this has not been confirmed by palaeontological evidence Major environmental crises occurred in the middle Neoproterozoic Between about 720 and 600 Ma, the Earth experienced at least two global glaciations (also known as ‘snowball Earth events’), during which glaciers reached the tropical oceans More glaciations may have occurred in the Neoproterozoic, but these were not nearly as extreme It is therefore convenient to divide the Neoproterozoic into three intervals: Early (1000–720 Ma), Middle (720–600 Ma), and Late (600–543 Ma) Neoproterozoic Early Neoproterozoic A quick look at several fossiliferous units of Early Neoproterozoic age gives us a broad picture of eukaryote diversity at that time The Early Neoproterozoic Little Dal Group (850–780 Ma) in northwestern Canada, the Chuar Group (>742 Ma) in the Grand Canyon, and the Huainan Group and Huaibei Group (ca 740–900 Ma) in northern China contain some of the best-preserved carbonaceous compressions in the Early Neoproterozoic Chuaria and Tawuia (Figures 3A and 3B) are abundant in these successions In addition, Longfengshania stipitata – a benthic alga with an ellipsoidal head, a stipe, and a simple holdfast – has been described from the Little Dal Group and from the Changlongshan Formation (ca 800–900 Ma) in northern China Individuals of L stipitata sometimes occur in clusters (Figure 3C), like a bunch of inflated balloons tethered together These carbonaceous compressions are probably multicellular eukaryotes, although Chuaria and Tawuia have been interpreted as colonial cyanobacteria by some palaeontologists Acritarchs, particularly those with more complex morphologies, became more diverse in the Early Neoproterozoic (Figure 3D) More than 20 acritarch species with complex morphologies have been described from Early Neoproterozoic successions Except a few (e.g Tappania and Valeria lophostriata), they are not known in the Mesoproterozoic The Wynniatt Formation (ca 800–900 Ma) of arctic Canada, the Svanberfjellet Formation (ca 700–800 Ma) in Spitsbergen, and the Mirojedikha Formation in Siberia, for example, contain some of the best-preserved acritarchs in the Early Neoproterozoic The Svanberfjellet