Guildayichthys carnegiei
Name: Guildayichthys carnegiei [Carnegiei's fish honoring John Guilday]
When: Early Carboniferous Period, 324 million years ago
Where: Bear Gulch Limestone, Montana
Claim to fame: Guildayichthys represents the first great evolutionary flowering of ray-finned fish, the creatures that we think of today when we hear the word "fish." By the time Guildayichthys lived, ray-finned fish had been around for at least 100 million years, but had lived in the shadows of unfamiliar creatures like jawless vertebrates and placoderms. The mass extinction at the end of the Devonian Period 362 million years ago. Following this event, ray-fins diversified, filling the ecological space left from this catastrophe. Guildayichthys was among them - one of the first ray-finned fish to adopt the deep-bodied laterally compressed form of a reef fish. Indeed, its long pointed snout resembles those of living butterflyfish, suggesting a similar life style.
Guildayichthys' position in the tree of evolution has recently been reassessed by Stack, 2025. It seems that it is a very basal member of Neopterygii, the huge group that includes most living ray-finned fish, and that it shared a more distant common ancestor with Chondrostei - sturgeons and paddlefish.