Letters to Nature

Nature 393, 255-257 (21 May 1998) | doi:10.1038/30473; Received 12 November 1997; Accepted 23 February 1998

Ichthyosaurian relationships illuminated by new primitive skeletons from Japan

Ryosuke Motani1, Nachio Minoura2 & Tatsuro Ando2

  1. University of California Museum of Paleontology, 1101 VLSB, Berkeley, California 94720-4780, USA
  2. Department of Earth and Planetary Science, Hokkaido University, Sapporo 060, Japan

Correspondence to: Ryosuke Motani1 Correspondence and requests for materials should be addressed to R.M. (e-mail: Email: motani@ucmp1.berkeley.edu).

The Ichthyosauria is a group of reptiles with fish-shaped bodies from the Mesozoic (65–250 million years ago)1,2. Their secondary adaptations to aquatic life have obscured their ancestral features3,4, and basal ichthyosaurs, which would be expected to retain these ancestral features (plesiomorphies), are poorly represented in the fossil record1. As a result, their relationships to other amniotes have been controversial for over 180 years5,6. New specimens of Utatsusaurus hataii from the Lower Triassic (240 Myr ago) of Japan are the first basal ichthyosaurs to show detailed features for almost the entire skeleton, including previously unknown parts of the skull and pelvic girdle. Computer-assisted retrodeformation of fossil images7 shows that Utatsusaurus retained features of terrestrial amniotes in both the skull and the postcranial skeleton, such as the connection between the vertebral column and the pelvic girdle. Phylogenetic analyses indicate that ichthyosaurs belong in the Diapsida, but that, unlike the sauropterygians8,9, they are not included with the Sauria (the crown group containing lizards, crocodiles, birds and Sphenodon). Recent studies have reported that the addition of ichthyosaurs to the amniote data altered the relationships among basal saurians10,11, but no major clades were affected by the inclusion of ichthyosaurs in our analyses.