BIODIVERSITY OF PRECAMBRIAN AND PHANEROZOIC MICROBIAL EUKARYOTES

Algal eukaryotes appear in the Precambrian fossil record ~2.2 billion years ago but heterotrophes are mostly absent from the Precambrian. Tintinnids, radiolaria and foraminifera have molecular origins well back into the Precambrian yet no representatives of these groups are known with certainty in that time. These data infer times of the last common ancestors, not the appearance of true representatives of these groups which may well have diversified considerably since those splits. Reported tintinnids at 1600mya from China are metamorphic shards or mineral artifacts, the many specimens from 635-715mya in Mongolia may be eukaryotes but they are not tintinnids, and the putative tintinnids at 580mya in China are diagenetic alterations of well-known acritarchs. Foraminifera are unknown in the Precambrian with the possible exception of Platysolenites, an agglutinated tube identical to modern Bathysiphon, in the latest Neoproterozoic. Titanotheca from 550 to 565mya rocks in South America and Africa was declared the oldest foraminifera based on the occurrence of rutile in the tests and in a few modern agglutinated foraminifera, as well as the agglutinated tests. Neither of these nor the morphology are characteristic of foraminifera; hence these fossils remain as indeterminate microfossils. Some of the larger fossils occurring in the typical Ediacaran assemblages were said to be xenophyophorids (very large foraminifera), but the comparison is disputed. Radiolaria, on occasion, have been said to occur in the Precambrian, but the earliest known clearly identifiable ones are in the Cambrian. The only certain heterotrophic eukaryotes occur in fresh-water rocks 750mya. Skeletonized radiolaria and foraminifera appear sparsely in the Cambrian, and radiate in the Ordovician. Tintinnids first appear in the Mesozoic, like other modern planktic groups. Algal eukaryotes show similar patterns. Thus microbial eukaryotes evolve similarly to metazoans.