Fossils show us that at the time of the dinosaurs' disappearance,
birds had already been in existence for a long time and had
20 developed into many different species. What happened to them
when the disaster occurred? Experts have different opinions on
this question, and the known fossils are still too few in number
(4)
to allow the question to be resolved. However, it is possible that
a great extinction took place among birds as well and that the
25 group's evolution, when it started again at the beginning of *the
Tertiary period, was based only on a few surviving species.
the