Why did the ichthyornis go extinct?

Answer 1

There are many theories to why the non avian dinosaurs including Tyrannosaurus rex, became extinct. So far, this is the most widely accepted one by paleontologists.

65.5 million years ago, an asteroid with a diameter of 6 miles crashed into what is now the Yucatan Peninsula, creating the Chixculub crater. It threw so much dust into the atmosphere, that sunlight was blocked for months. Plants died without sunlight, and the herbivorous dinosaurs died without plants to eat. The carnivores soon died out, too, because their prey was gone.

There is evidence that the Chixculub impact preceded the dinosaur extinction by hundreds of thousands of years, until another, possibly larger asteroid struck and eliminated the non avian dinosaurs once and for all.

On top of it all, even without the asteroids, dinosaurs were already dealing with the negative affects of climate change.

Answer 2

Icthyosaurus first evolved in the middle of the Jurassic period, not too long after the giant sauropods evolved. Icthyosaurus went extinct at the end of the Jurassic period, because of the water temperatures and levels.