BIOL 321 Lecture Notes - Lecture 5: Tonian, Ctenophora, Symmetry In Biology
Document Summary
Some paleontologists suggest that animals appeared much earlier than the cambrian explosion, possibly as early as 1 billion years ago. Trace fossils such as tracks and burrows found in the tonian era indicate the presence of triploblastic worms, like metazoans, roughly as large (about 5 mm wide) and complex as earthworms. Ordovician extinction events, and decreased shortly after the grazer populations recovered. However the discovery that tracks very similar to these early trace fossils are produced today by the giant single-celled protist gromia sphaerica casts doubt on their interpretation as evidence of early animal evolution. It has been estimated that 99. 9% of animals that have ever existed are extinct. Phylogenetic analysis suggests that the porifera and ctenophoradiverged before a clade that gave rise to the bilateria, cnidaria andplacozoa. Another study based on the presence/absence ofintrons suggests that cnidaria, porifera and placozoa may be a sister group of bilateria and ctenophora.
