Ichthyornis fossils have been found in almost all levels of the Niobrara Chalk, from beds dating to the late Coniacian age (about 89 million years ago) to the Campanian age (about 83.5 million years ago).
Even earlier remains attributed to Ichthyornis have been found in the Greenhorn Formation of Kansas, dating to the early Turonian age (about 93 million years ago).
Specimens of Ichthyornis from earlier eras were, on average, smaller than later ones.
The holotype specimen of Ichthyornis dispar, YPM 1450, had a humerus about 58 centimetres (23 in) long.
In many geologically younger specimens like YPM 1742, the same wing bone was 73 centimetres (29 in) long.
Both the older, smaller specimens, and the more recent, larger specimens show signs that they had reached skeletal maturity and were adults, and came from the same geographic area.
It is likely that Ichthyornis dispar as a species increased in size over the several million years it inhabited the Western Interior Seaway ecosystem.
Ichthyornis fossils have been found in almost all levels of the Niobrara Chalk, from beds dating to the late Coniacian age (about 89 million years ago) to the Campanian age (about 83.5 million years ago). Even earlier remains attributed to Ichthyornis have been found in the Greenhorn Formation of Kansas, dating to the early Turonian age (about 93 million years ago). Specimens of Ichthyornis from earlier eras were, on average, smaller than later ones. The holotype specimen of Ichthyornis dispar, YPM 1450, had a humerus about 58 centimetres (23 in) long. In many geologically younger specimens like YPM 1742, the same wing bone was 73 centimetres (29 in) long. Both the older, smaller specimens, and the more recent, larger specimens show signs that they had reached skeletal maturity and were adults, and came from the same geographic area. It is likely that Ichthyornis dispar as a species increased in size over the several million years it inhabited the Western Interior Seaway ecosystem.
Ichthyornis