Tsidiiyazhi abini, a fossil bird newly described by the Bruce Museum’s science curator Dr. Daniel Ksepka and his co-authors, may be only the size of a house sparrow, but it’s making big news. The paper was published a day ago in the Proceedings of the National Academy of Science, and has since spread from local Greenwich papers to the BBC.
The story of this ancient bird begins with its discovery in the rocks of the Nacimiento Formation in New Mexico. The fossil site was found by a pair of 11-year-old twins, Ryan and Taylor Williamson, out on a fossil collecting trip with their paleontologist father, Dr. Thomas Williamson. The rocks containing the fossils are very precisely dated, at between 62.2 and 62.5 million years old. This time period is of special interest to paleontologists- it is a mere 4 million years younger than mass extinction that killed off the dinosaurs. Dr. Williamson collected a number of fossils from the site over the next few months, mostly small mammals, but also something far rarer: A fossil bird.
When the dinosaurs went extinct 66 million years ago, their loss threw the global ecosystem into flux. They had dominated land ecosystems for millions of years and now their previous niches were left empty. Though tragic for dinosaurs, this opened the door to evolutionary innovation for those that survived. There is a strong record of mammal evolution in the critical first few million years following the mass extinction, but data is much more scant for birds. Tsidiiyazhi abini fills an important gap in this understanding.
Dr. Ksepka’s analysis found that Tsidiiyazhi abini is a form of primitive mousebird. Though mousebirds today live only in Africa, they had a much wider distribution in the past. The six modern species of mousebirds spend most of their time in trees, where they scurry through branches in the rodent-like manner that inspired their name. The foot structure of Tsidiiyazhi abini shows that it was able to flip its fourth toe to face backwards, an adaptation called semi-zygodactyly which is excellent for gripping branches. Tsidiiyazhi abini must have lived in trees too, making it the oldest known arboreal bird.
This discovery shows mousebird lineage to be older than ever before realized. However, the implications don’t end there. If mousebirds evolved that early, it can be assumed that bird groups that evolutionarily split off prior to mousebirds must have evolved even earlier. This means that up to 10 major modern bird groups, from hawks to woodpeckers, evolved in the first four million years following the mass extinction. This is an extraordinary explosion of evolution.
The name of this bird with the zany little foot, Tsidiiyazhi abini, is derived from the Navajo language and means “little morning bird.” Given its small size and its position in the “dawn” of bird diversity, it is a very appropriate moniker for the little fellow.
- Kate Dzikiewicz, Paul Griswold Howes Fellow
For more information: Daniel T. Ksepka et al. Early Paleocene landbird supports rapid phylogenetic and morphological diversification of crown birds after the K–Pg mass extinction, Proceedings of the National Academy of Sciences (2017). DOI: 10.1073/pnas.1700188114