The fossil record is always surrounded by caveats. Consider, for example, a beautifully-preserved dinosaur skeleton – your pick for species – curled up in a bed of sandstone. As stunning as such an osteological remnant might be, there’s plenty we can’t know. We can say that the dinosaur was buried in this spot, surely, but where the dinosaur lived and even died is an open question. What color that dinosaur was and what it sounded like are also beyond our reach if we only have bones to work with. And yet it would be a mistake to say that we’ll never know these things, at least in some cases. We’re only just starting to realize how detailed the fossil record is, as the eye of a prehistoric bird reminds us.
Up until now, we really had no idea of how prehistoric dinosaurs saw the world around them. There’s been a little research on using the details of sclerotic rings – bones inside the eye of reptiles like dinosaurs – to determine whether some species were active during the day or night, but paleontologists disagree on the interpretation, not to mention that the debate didn’t have much to add about color. But in an analysis of a 120 million-year-old-bird from China, Gengo Tanaka and colleagues propose that they’ve detected the rods and cones that might testify to Cretaceous color vision.
The delicately-preserved fossil, given the label SNHM 6105 and identified as an enantiornithine bird, looks more or less like many of the other exceptional skeletons that have been found in the vicinity of Liaoning, China. Most of the bird’s bones are in articulation, and there’s a smattering of preserved feathers around the body. But the standout feature of this fossil can only been seen with powerful microscopes. It seems that part of the bird’s retina became preserved along with the skeleton, retaining the distribution of oil droplets, rods, and cords – tiny structures related to vision at different light levels.
So what could this Cretaceous bird see? By comparing the details of the fossil bird’s eye to those of modern birds, like the common house sparrow, the researchers conclude that the 120 million-year-old avian likely had color vision. This isn’t a huge surprise as previous analyses of fossil feathers have shown that dinosaurs – both avian and non-avian – often bore flashy color patterns. Such hues would be pointless if they couldn't be seen. Color vision and plumage patterns dovetailed. But this is just the start. Now that paleontologists know such features can be preserved, they can begin to look for other examples and start asking further questions. The fossil record isn’t all about bones and footprints. There’s more held in the stone than anyone ever expected. We’ll see what comes next.