Walcott's discovery of Middle Ordovician vertebrates

Paleozoic fish remains were identified by T. W. Stanton in 1890 at Canon City, Colorado, but it remained for Walcott to recognize the significance of their great antiquity. After a hurried trip in December 1890, he was able to date them authoritatively as Middle Ordovician (Trentonian) by the presence of associated and overlying marine invertebrates. Fieldwork by S. W. Loper the following year, and by Walcott in 1892, added additional vertebrate specimens and verified Walcott's conclusions as to age. Walcott's 1892 paper, which significantly pushed back the record of antiquity of vertebrate life, was well known to European authorities but was essentially ignored by them because the age of the beds was questioned; the fossils were assumed to be similar to the fish described from the "Old Red" and therefore, Devonian. Further, discovery of these remains in sandstone somehow led to the assumption, common for many years, that vertebrates originated in fresh water. In spite of initial skepticism, Walcott's fossils were eventually accepted as the oldest occurrence of vertebrates and remained so for more than 80 years. Among the many specimens collected by Loper in 1891 are excellent examples of Dictyorhabdus priscus. Of the three genera described by Walcott in 1892, that one was moved by later workers from Vertebrata to Problematica, but it may indeed be a vetebrate.