 At least some aetosaurs built nests and protected their eggs . In 1996 , geologist Stephen Hasiotis discovered 220 ‑ million @-@ year @-@ old , fossilized , bowl @-@ like nests in Arizona 's Petrified Forest , in part of the Chinle Formation . The oldest such nests that have been found belonged to phytosaurs and aetosaurs . The nests are compacted and appear very similar to the nests of the modern day crocodiles who guard their nests . The nests were holes dug in the sand in the bank of an ancient river . 
