ORIGIN AND AGE OF HETEROTROPHIC SKELETAL MICROBIAL EUKARYOTES

Heterotrophic protists must have a long evolutionary record, but most of that is unavailable directly because they did not make preservable skeletons. Molecular phylogenetic analyses indicate that many of these had origination times deep in the Proterozoic. Of the skeletal heterotrophs, only lobose thecameobians,marine foraminifera, radiolaria, filose thecameobians, and tintinnids have fossil records. The lobose thecameobians, normally fresh water today, have a clear fossil record at about 750my in the Chuar Group and other rock units around the world but are only sparsely known in younger deposits. The foraminifera, radiolaria, filose thecameobians, and tintinnids have putative fossil records in the Precambrian but these are in error or questionable. Their phylogenetic first common ancestors date molecularly even earlier and for foraminifera, at least, indicate a diverse Neoproterozoic assemblage. The first foraminifera and radiolarian fossils appear with skeletons near the Precambrian/Cambrian boundary, tintinnids in the Jurassic along with other fossil pelagic forms, and filose thecameobians in the Holocene. Their last common ancestors, determined molecularly, place the origins of each group much deeper in time, but these times most certainly do not represent the origin of these specific groups, because evolution along the trajectory to each of them from their LCA formed the groups as we know them, much later. The only way to test where in time these events happened is in the fossil record. Thus we assert that the origin of these groups is close to the time they appear as fossils, the later Neoproterozoic at the earliest for foraminifera and radiolaria, and in the late Triassic or Jurassic for tintinnids. Filose thecameobians certainly had a long evolutionary history before their first positive identification in the Holocene.