This is an old paper, but I don't recall that it's been discussed here
before. The point is that you can estimate genome size based on cell
size, and you can estimate cell size from certain fossils, which leads
the authors to infer that the unusually small genomes of birds arose not
in birds but in much earlier dinosaurs.
https://www.researchgate.net/publication/6461735_Origin_of_avian_genome_size_and_structure_in_non-avian_dinosaurs#fullTextFileContent
Organ CL, Shedlock AM, Meade A, Pagel M, Edwards SV. Origin of avian
genome size and structure in non-avian dinosaurs. Nature. 2007 Mar
8;446(7132):180-4. doi: 10.1038/nature05621. PMID: 17344851.
I would also like to plug a similar, earlier paper that followed the
same logic to estimate fossil plant genome sizes. When it was published,
Jane Masterson was a grad student a couple of years ahead of me at U.
Chicago.
Masterson, Jane. (1994). Stomatal Size in Fossil Plants: Evidence for
Polyploidy in Majority of Angiosperms. Science (New York, N.Y.). 264.
421-4. 10.1126/science.264.5157.421.