Sea urchins such as Paracentrotus lividus are marine organisms tolerant to rapid variation in abiotic conditions such as salinity, partial pressure of oxygen and temperature due to a tidal cycle, but without resistance to desiccation. The marine occupation of these organisms is observed through the alveoli or housing cavities registered in rocks, associating this attribute to its infralittoral biotope, between the low-tide midline and the level of -3 meters, it is possible to identify marine levels. In this Natural Monument of Pedras Ruivas the ichnofossils of Paracentrotus lividus occur in flattened tops of rocks, at an altitude of 5 m (platform IV), certifying the marine occupation of these levels, 125 thousand years ago.