And since there were several very distinct kinds of Mosasaurs, there is no reason to insist that no evidence for a back fin in any one of the known species or genera precludes the possibility of any of the others from ever having a back fin.
Ancient ocean predators were reptiles that swam like sharks
The first mosasaur fossil was discovered in the 1700s. From their run-on spines, researchers first guessed the animals were related to snakes, and later proposed that the ocean-swimming reptiles swam like fish. Rare soft-tissue preserved on the new prognathodon fossil, one member of the family of mosasaurs, shows a well-defined body plan and the trademark shark-like forked tail, supporting that theory.
"The proportions of its body are amazingly similar to those that we see in pelagic sharks," Lindgren said. He expects that other, later mosasaurs may have been "even more fish-like than this guy."
Remarkably, the mosasaurs, the Cambrian-age ichthyosaurs before them, and today's toothy sharks — all top ocean predators in their time — independently arrived at roughly the same, "drop-shaped" stream-lined body plan and a two pronged tail.
Unlike sharks, the spine of the mosasaurs curved downward into the lower lobe of the tail. This may have been designed to assist the reptilian swimmers come up to the surface for air, Lindgren said.
Johan Lindgren, Hani Kaddumi and Michael Polcyn are authors of "Soft tissue preservation in a fossil marine lizard with a bilobed tail fin" published in Nature Communications.