Many ichthyosaurs are known only from fragmentary material and unsuitable
for phylogenetic analyses at the moment.
When choosing 27 better-known taxa, a phylogenetic analysis gave the
following strict consensus tree. See Motani
(1999) for details of the analysis.
There have been other studies of ichthyosaurian phylogeny since Motani
(1999). I will deal with them in the future.
When adding the time scale to the phylogeny, we see a certain pattern in
the evolution of ichthyosaurs. Ichthyosaurs were quite diverse by the end
of the Early Triassic.
By early Late Triassic, there already were forms approaching the typical
body profile seen in truly fish-shaped ichthyosaurs. So, the fish-shaped
body plan, which, persisted for about 140 million years once established
(until the extinction of the group>, evolved somewhat quickly (within
20 million years or less).