Hello Apostoulos,
“Horn coral. Horn coral, any coral of the order Rugosa, which first appeared in the geologic record during the Ordovician Period, which began <b>488 million years ago</b>; the Rugosa persisted through the Permian Period, which ended <b>251 million years ago</b>. (Britannica Encyclopedia)” I know virtually next to nothing about Crete geology. If I read this 1996 reference correctly ( http://www.uqac.ca/mhiggins/chapters.html ) It seems like the geology of this area is much younger except for maybe the Permian Period. Let me know what you find out about the Vatos unit. I live in Maine, USA. Perhaps the Natural History Museum of Crete can help. http://www.nhmc.uoc.gr/ or http://www.nhmc.uoc.gr/en
Also see: https://en.wikipedia.org/wiki/Rugosa
Jim