The chalk that stands up as Monument Rocks and the Little Jerusalem badlands is the floor of a shallow sea that covered the middle of the continent in the Late Cretaceous. It is fine and soft, and it preserved delicate bones well. In the 1870s it gave up a pair of birds that helped settle an argument about where birds came from.
The expeditions
Othniel Charles Marsh, Yale's professor of paleontology and the first in the country, came to the Smoky Hill late in 1870 and found the lower end of a bird's shinbone in the chalk about twelve miles east of Fort Wallace. The cold drove his party out. In the summer of 1871 he came back with a larger group of students and a stronger army escort, because the Cheyenne and Arapaho were still fighting for this country. It was 110 to 120 degrees in the shade, and men went down with sunstroke. On the south bank of the Smoky Hill about twenty miles east of the fort, Marsh found most of a skeleton of a big bird with strong legs and almost no wings. He named it Hesperornis regalis, the regal western bird. The skull was missing.
In October 1872, near the same spot, Marsh and a student named Thomas H. Russell found a nearly complete skeleton with its skull and lower jaws. The jaws were lined with teeth.
Fish bird
That same year Benjamin Mudge, a professor at the Kansas State Agricultural College, sent Marsh a slab of chalk from near the Solomon River with the bones of a small bird in it and a pair of toothed jaws. Marsh named the bird Ichthyornis, fish bird, for its vertebrae, which were hollowed on both faces like a fish's. He took the jaws for a small reptile and named that separately. By early 1873, after more of the bones were cleaned from the rock, he saw that the jaws belonged to the bird. In a paper that year he wrote that the discovery did "much to break down the old distinction between Birds and Reptiles."
The two birds lived in the same sea and could hardly have been more different. Hesperornis was a big flightless diver built something like a loon, with teeth set in a groove along the jaw much like those of the mosasaurs it swam among. Ichthyornis was a small, strong flier that looked something like a gull, with teeth in separate sockets. Marsh put them in a new subclass he called Odontornithes, toothed birds. By 1880 the expeditions he sent out had brought more than a hundred individuals to Yale, and he described them in a monograph that year.
Darwin's letter
Darwin had published On the Origin of Species in 1859. In 1880, by the usual account of his letter, he wrote to Marsh that the toothed birds had offered the best support for the theory of evolution since the book appeared.
Neither bird is a direct ancestor of the birds alive now, and Marsh's subclass was abandoned long ago. The chalk that held them is still wearing away along the Smoky Hill and its creeks.