One records which fossil is younger and which is older.
The discovery of a different means, one for which absolute dating is possible occurred in the early 20th century. Certain naturally occurring elements are radioactive, and they break down or decay at well-known predictable rates.
Besides the order of fossils in the rocks, another method is the use phylogenetic trees.
Phylogenetic trees are used to show how all the species of particular groups of plants or animals relate to each other.
It took a canal surveyor circa 1800, William Smith in England, who noticed that he could map out great tracts of rocks on the basis of their contained fossils.
The sequences he saw in one part of the country could be matched precisely with the sequences in another.
The accuracy of the fossil record using the stratigraphy method has been well documented.
Then by comparing the two proportions of parent to daughter elements in the rock sample, and knowing the half-life, the absolute age can be calculated.
Where are the dates coming from and how is the measurement occurring?
How does the fossil record work with the geologic time scale.
It was all clear when in 1859 Charles Darwin published his "On the Origin of Species".
The 'progress' shown by the fossils was a documentation of the grand pattern of evolution through long spans of time.