The Cretaceous is a word Enjolras has heard: some kind of era, full of ancient monsters Combeferre is extremely enthusiastic about. This explanation seems to be all he's likely to get, but it's enough for the gist.

Enjolras waves away Joly's apology with a small gesture. A good portion of the tension of a few moments ago has bled away now into affectionate resignation; he steps back a little from the table, crosses his arms, waits to see how the beetle will be disposed of.