Could you please help me with this?

To begin with, sorry for the (very) long post.

Scientists today are able to accurately date fossils, the rock-bound remains of organisms from past geologic ages.

When fossils are arranged along a timeline, scientists can see gradual changes from simple to more complex life forms. In some cases, evolution through various intermediate forms over millions of years can be detected and compared to the present state of an organism.

For example, the earliest known species of horse lived some 60 million years ago and, according to the fossil record, was shorter than 20 inches (50 centimeters) high at the shoulders. Successive rock layers yield fossils of increasingly larger horse species, culminating in the horses of today.

As size changed, so did other aspects of the horses’ anatomy: teeth became adapted to eating grass, the bones of the lower leg fused, and multiple toes evolved into a single toe surrounded by a hoof.

The last paragraph is fairly easy (or maybe the whole text may not seem that difficult, for that matter.)

What I don't understand is this: How are scientists today able to accurately date fossils? (The first paragraph, indeed)

How do they know the earliest horses lived 60 millions ago? (I can't find the answer in the text.)

What does it mean that fossils are arranged along a timeline, or more importantly how? And how is it connected with dating fossils?

Could someone shed some light on this?

Thank you so much.