Ctenoid fish fossil, from Louis Agassiz, Recherches sur les poissons fossiles, 1833-43.
Placoid fish fossil, from Louis Agassiz,
Recherches sur les poissons fossiles
, 1833-43.

44. Agassiz, Louis (1807-1873).
Recherches sur les poissons fossiles.
Neuchatel: Imprimerie des Petitpierre, 1833-1843.

Although the large fossil vertebrates received much of the attention in the early nineteenth century, some naturalists realized that more information about the history of life on earth could be gained from fish, which were much better represented in the fossil record.  Louis Agassiz was a Swiss naturalist who undertook this task single-handedly.  By studying all the specimens he could locate, Agassiz was able to re-classify the fish into four orders, based on their scale structure. Some fish were found only in older rocks (Secondary rocks, as they were called at the time), while others were found only in more recent rock.  Some of the oldest specimens were quite strange, such as the placoid fish illustrated here, with its bony head shield.

Agassiz therefore recognized the reality of extinction, but, like Cuvier, he rejected the possibility of evolution.  Agassiz would become quite famous when he proposed his glacial theory in 1840, and he would later move to Harvard University, where he became one of the staunchest opponents of Darwinian evolution in the 1860s.

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