Fossilized lungs

Fossilized soft tissues are rare – even more rare are traces of internal organs (integument like skin and feathers being relatively more common). In an article published in the Proceedings of the National Academy of Sciences U.S.A., I described a specimen of Archaeorhynchus spathula, a primitive ornithuromorph bird, preserving traces of the lung tissue. Scanning electron microscopic images of the lung tissue revealed that the extremely subdivided functional tissue of the lung that allows modern birds to meet the high oxygen demand of powered flight was already in place 128 Ma. 

Read more:
https://www.economist.com/science-and-technology/2018/10/25/a-breathtaking-discovery

https://www.nationalgeographic.com/science/2018/10/news-fossil-lungs-bird-dinosaurs-cretaceous-soft-tissue-paleontology/

https://www.audubon.org/news/this-fossil-probably-first-preserve-ancient-bird-lungs

https://www.iflscience.com/plants-and-animals/firstever-fossilized-lungs-of-dinosaurera-bird-found-hint-at-the-origin-of-flight/

https://www.livescience.com/63880-oldest-fossil-bird-lung.html

https://www.popsci.com/lungs-birds-dinosaur-survival

Link to article:
https://www.pnas.org/content/115/45/11555