Is there any information on the oldest species that we know about?
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3 Answers

The oldest metazoan animals I have read about have been found in the  Precambrian Ediacaran & Vendian fossil assemblages found in sediments (630 MYA – 540 MYA). This epoch was dominated by soft-bodied organisms: jellyfish-like animals, sponge-like animals, bilateral worms amidst bacterial biofilms and bacterial stromatolite mounds. Traces from these early life forms are pretty much limited to imprints, trackways and residual organic signature molecules as evidence since they had no hard skeletal parts to become fossils.

Species with similar basic body plans still exist but little can be determined about the exact physiology and detailed forms of these earliest animals.

There are some groups that are still with us as the basic animal phyla: radially symmetric jellyfish and the bilateral segmented worms (annelids). Small flat wormlike animals roamed the sediments in the seabed then. There is evidence of many simple bilaterally symmetric mollusc-like organisms in the Vendian fauna.

These Kimberella were likely a ur-mollusc genus of filter feeders  showing the bilateral form and univalved rigid shell indicating it was evolved enough to have three tissue types and organ systems of a coelomate. The  Arkarua were echinoderms leaving the oldest five sided symmetric fossil along with some arthropodlike organisms in the latest precambrian (Vendian) sediments.

The stromatolites declined in number so there likely were heterotrophs able to eat them in the new metazoa.

http://ircamera.as.arizona.edu/NatSci102/NatSci102/text/cambrian.htm

http://www.ucmp.berkeley.edu/vendian/critters.html
by Level 2 User (700 points)
I would be guessing, but suggest either sharks or crocodiles.
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All species, currently living, would be royghly the same age.  It stands to reason that they are all related to some prehistoric animal, but to pinpoint when and where its evolution took place would be a near impossibility.  The nearest living animals, related and looking like their forebears, would most likely be crocodiles and turtles. (and or any relation in their respective subspecies)
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