As more and more fossil groups were found and recognized in the late 19th and early 20th centuries, palaeontologists worked to understand the history of animals through the ages by linking together known groups.
Karena semakin banyak kelompok fosil ditemukan dan dikenal pada akhir abad 19 dan awal abad ke-20, palaeontolog bekerja untuk memahami sejarah hewan selama berabad-abad dengan menghubungkan bersama kelompok-kelompok yang dikenal.