The Cambrian diversifi cation of animals was long thought to have begun with an explosive phase at the start of the Tommotian Age. Recent stratigraphic discoveries, however, suggest that many taxa appeared in the older Nemakit-Daldynian Age, and that the diversifi cation was more gradual. We map lowest Cambrian (Nemakit-Daldynian through Tommotian) records of δ 13 C CaCO 3 variability from Siberia, Mongolia, and China onto a Moroccan U/Pb-δ 13 C CaCO 3 age model constrained by fi ve U/Pb ages from inter bedded volcanic ashes. The δ 13 C CaCO 3 correlations ignore fossil tie points, so we assume synchro neity in δ 13 C trends rather than synchroneity in fi rst appearances of animal taxa. We pre sent new δ 13 C org , 87 Sr/ 86 Sr, uranium, and vanadium data from the same carbonate samples that defi ne the Moroccan δ 13 C CaCO 3 curve. The result is a new absolute time line for fi rst appearances of skeletal animals and for changes in the carbon, strontium, and redox chemistry of the ocean during the Nemakit-Daldynian and Tommotian ages at the beginning of the Cambrian. The time line suggests that the diversifi cation of skeletal animals began early in the Nemakit-Daldynian, with much of the diversity appearing by the middle of the age. Fossil fi rst appearances occurred in three pulses, with a small pulse in the earliest Nemakit-Daldynian (ca. 540-538 Ma), a larger pulse in the mid-to late Nemakit-Daldynian (ca. 534-530 Ma), and a moderate pulse in the Tommotian (ca. 524-522 Ma). These pulses are associated with rapid reorganizations of the carbon cycle, and are superimposed on long-term increases in sea level and the hydrothermal fl ux of Sr.

Vase-shaped microfossil (VSM) assemblages from early diagenetic carbonate nodules in >742 ± 6 Ma black shales of the Chuar Group, Grand Canyon, provide evidence for affinities with testate amoebae. Not only are VSMs exceptionally preserved in Chuar rocks, they exhibit a much higher degree of morphological diversity than was previously known. Using the taxonomy of modern testate amoebae as a guide, nine new species and eight new genera of VSMs are described, augmenting the eight species and two genera already recognized. Taxa described here areMelanocyrillium hexodiademaBloeser, 1985,Trigonocyrillium horodyskii(Bloeser, 1985) n. comb.,T. fimbriatum(Bloeser, 1985) n. comb.,Cycliocyrillium simplexn. sp.,C. torquatan. sp.,Bonniea dacrucharesn. sp.,B. pytinaian. sp.,Trachycyrillium pudensn. sp.,Palaeoarcella athanatan. sp.,Hemisphaeriella ornatan. sp.,Bombycion micronn. sp., andMelicerion poikilonn. sp. All of the test characters observed in VSM taxa (e.g., collars; indentations; hexagonal symmetry; lobed, triangular or invaginated apertures; curved necks) occur in modern testate amoeban taxa, though not always in the same combinations. Some VSM species have characters found today in diverse extant taxa, making it difficult to assess their relationships. A few species, however, have character combinations that closely approximate those found in specific genera of both lobose and filose testate amoebae, suggesting that at least stem group, and possibly crown group, representatives of these taxa were present ∼742 Ma.These fossils indicate that ecosystems were diverse and complex, that eukaryotic biomineralization had already evolved, and that the last common ancestor of animals+fungi had already appeared by ∼750 Ma.

Vase-shaped microfossils (VSMs) occur globally in Neoproterozoic rocks, but until now their biological relationships have remained problematic. Exceptionally preserved new populations from the uppermost Chuar Group, Grand Canyon, Arizona, display details of morphology and taphonomy that collectively point to affinities with the testate amoebae. The fossils are tear-shaped tests, ∼20–300 μm long and ∼10–200 μm wide, that are circular in transverse section, expand aborally toward a rounded or slightly pointed pole, and taper orally toward a “neck” that ends in a single aperture. Apertures may be circular, hexagonal, triangular, or crenulate, and may be rimmed by a distinct collar. Approximately 25% of the Chuar VSMs are curved, such that the oral and aboral poles do not lie opposite each other. Tests are preserved as mineralized casts and molds, commonly coated with organic debris or iron minerals, but they were originally composed of nonresistant organic matter. Approximately 1% have a “honeycomb-patterned” wall attributable to the original presence of mineralized scales whose bases were arranged regularly in the test wall. Scale-bearing testate amoebae, such as members of the Euglyphidae, are essentially identical to the honeycomb VSMs, and a close relationship between other Grand Canyon VSMs and additional testate amoebae, both lobose and filose, is likely. The VSM population therefore most likely represents a multispecies assemblage whose spatial association reflects a common habitat and/or taphonomic circumstances that favor test preservation. The assignment of these fossils to the testate amoebae strengthens the case for a major diversification of eukaryotic organisms by mid-Neoproterozoic times and, more significantly, provides the earliest morphological evidence for heterotrophic eukaryotes in marine ecosystems.

The first appearances of aragonite and calcite skeletons in 18 animal clades that independently evolved mineralization during the late Ediacaran through the Ordovician (approximately 550 to 444 million years ago) correspond to intervals when seawater chemistry favored aragonite and calcite precipitation, respectively. Skeletal mineralogies rarely changed once skeletons evolved, despite subsequent changes in seawater chemistry. Thus, the selection of carbonate skeletal minerals appears to have been dictated by seawater chemistry at the time a clade first acquired its mineralized skeleton.

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