Learn Russian: В скелете этого динозавра есть кости толщиной в три сантиметра. - There are bones three centimeters thick in the skeleton of this dinosaur.

In Russian, when you describe the dimension or physical property of an object (like length, width, thickness, or weight), you use the noun for that dimension in the Instrumental case. Here, кости толщиной literally means "bones with a thickness."

What is the preposition в doing in в три сантиметра, and what case follows it?

When specifying an exact measurement after a dimension noun, Russian uses the preposition в followed by the Accusative case. You can think of it as extending "into" a measurement of three centimeters. This is the standard structure for stating measures taught in this lesson.

If в takes the Accusative case, why does сантиметра look like the Genitive case?

This is a classic Russian number rule interacting with a preposition! The preposition в requires the Accusative case for the whole numeral phrase. Because we are counting inanimate objects, the number три in the Accusative looks identical to the Nominative. But the number три itself forces the noun following it to be in the Genitive singular. So, сантиметра is Genitive singular because of the number 3, while the phrase as a whole is Accusative.

Can I drop the preposition в and just say кости толщиной три сантиметра?

Yes, in modern Russian, it is very common and perfectly natural to omit в and simply say кости толщиной три сантиметра. The preposition в makes it sound slightly more formal or precise, but both are grammatically correct and widely used by native speakers.