TL;DR Sturgeon fins grow bone similarly to tree rings - in annual layers. By studying the outermost layer of many samples found on the K-Pg boundary they were a

TL;DR Sturgeon fins grow bone similarly to tree rings - in annual layers. By studying the outermost layer of many samples found on the K-Pg boundary they were able to deduce that they all died in spring. Corroborating evidence is found from some other fish exhibit seasonal variation in their C13/C12 ratios due to food availability.