The earliest fossil fragments which have been positively identified as vertebrate come from the Upper Ordovician. They are fragments, the best known of which come from the Harding sandstone near Canon City, Colorado. Though fragmentary, study of the histological structure of the plates indicates that they were bony, therefore vertebrate, and that the bone structure agrees with that in the shields of the Heterostraci, one of the orders of Ostracoderms. (1) These are intriguing fragments, hints of the presence of vertebrates but inadequate to give us any idea of the body form or of any details of structure. · Following this early occurrence the fossil record is a blank through-out the long period between this and Upper Silurian. When the next record available thus far is examined it is found to contain a wealth of Ostracoderm forms. It is this next occurrence which I wish to discuss. I could only wish that I might be able to sketch the history of the group between Upper Ordovician and Upper Silurian, for somewhere in the rocks of that interval must lie the records of a very important chapter in vertebrate evolution.
Proceedings of the Iowa Academy of Science
©1950 Iowa Academy of Science, Inc.
Robertson, George M.
"An Upper Silurian Vertebrate Horizon,"
Proceedings of the Iowa Academy of Science, 57(1), 271-275.
Available at: https://scholarworks.uni.edu/pias/vol57/iss1/33