Completely unlikely in 500My. Ichthyosaur Need long lineages for very different creatures. Smaller organisms overproduce more. Variation and its heritability. No offspring identical and differences inherited.
Some variations compete more successfully.
Change through time. Isolated groups develop separately until they are unable to interbreed. Hawaiian fruit flies. Nucleotide sequences in mammals. Punctuated equilibrium theory - speciation occurs over thousands to tens thousands years.