New findings reveal that the Middle Triassic ichthyosaur Mixosaurus cornalianus is the oldest amniote with a dorsal fin

Two excellently preserved specimens of Mixosaurus cornalianus from the Anisian layers of the Middle Triassic
Formazione di Besano, with soft parts associated with well-articulated skeletal elements, revealed the presence in this
species of a dorsal fin and of a well-developed, triangular dorsal lobe of the caudal fin, both stiffened by an array of
fibre bundles, as in Jurassic fast-swimming ichthyosaurs. This finding testifies that efficient swimming exaptations were
already present in some Middle Triassic ichthyosaurs. Mixosaurus is then the oldest amniote so far known that developed
a dorsal fin. The preservation of the fin shapes, scaleless skin, and three-dimensional dermal fibres is remarkable,
allowing observation of their microstructure, and ruling out any artefactual interpretation. Stomach contents indicate
that both specimens preyed upon cephalopods and small fishes. An internal organ, possibly a tract of the intestine, is also
preserved in one specimen, which represents the first documented case in the ichthyosaurs of the Besano Formation, and
a rarity in the fossil record of the clade.