Baby Penguins in the Fossil Record
Penguin bones are common in the fossil record. Trillions of penguins have lived over course of the Cenozoic, and a tiny portion of these died in environments favorable to fossil preservation. Some fossil penguins are giant and others tiny, some are complete skeletons and others single bones, some pristine and others badly damaged by erosion or chewed on by sharks. However, almost all of them are bones of adult birds. This is actually not too surprising when you think about penguin life cycles. Adults spend most of their time in the water, so it is easier for animals that die and sink to the bottom to be covered by sediments and have a chance at fossilization. Hatchling and juvenile penguins stay on land though, and if they don’t make it to adulthood it often means they were gobbled up by a predator and leave no trace in the fossil record.
Hatchling penguin fossils can be really informative, because they provide unequivacal evidence that a breeding colony existed in the area. Usually, this is very hard to prove in the fossil record, because modern penguins are infamous drifters. Some travel more than a thousand miles over the course of a normal year before returning to their breeding grounds, and a few random individuals always end up on the wrong continent annually (like New Zealand’s stray Emperor Penguin). Because of these penguin proclivities, it is hard to be sure whether adult fossil penguin bones we find at some sites belong to penguins that bred there, or instead belong to a bird that was just passing through.
Unfledged birds are those that haven’t yet left the nest. In dramatic cases, fledging occurs when a baby bird is pushed out of the nest by its parent, and instinctively takes its first flight. For penguins, fledging involves a trip into the ocean instead. That’s why hatchling penguin bones can be so informative to paleontologists. If we find a fossil from an unfledged bird, we know with certainty that bird was hatched in the area because it was too young to have started swimming.
Part of the story in my recent paper with Dr. Daniel Thomas is based on tiny fossils from hatchling penguins. There are quite a few fossils of juvenile animals in the South African records from Langebaanweg. These bones show a very spongy texture because they have not completely ossified yet and parts remain cartilaginous (just like our own bones when we are children). There are also fossil parts of compound bones that remain separate in hatchlings but fuse together in adults. For example, the three small bones on the right side of the photo are parts of the foot that join completely together in fully grown penguins. In order to figure out how old these penguins were when they perished, we looked at modern skeletons of hatchling, juvenile and adult penguins in museums. Based on the patterns we observed, we aged the bones in the photo to a very young bird, which would not yet have started moulting into its adult feathers (and therefor wouldn’t be ready to take its first plunge). It’s sad to think these birds never had a chance to enter the marine realm, but studying their fossil remains pins down the location of their species breeding ground pretty precisely. That’s a rare and valuable peek into a vanished ecosystem.