The Chi-Rho was also the first two letters of the word "gold" in greek. The symbol can be found on ancient Roman gold coins. (Museum in Trier) Just plain "X" was also used. I've seen it in museums and churches in my travels. The first depiction of the sign of Jonah on an ossuary can be found in a first century tomb in Israel. There is some controversy about the figure of the fish spitting out Jonah. I definitely see a fish. Some see an amphora bottle and some, a Nephesh tower. The word "Yonah" is spelled in Hebrew on the fish's mouth/cheek. Others say it says something similar but different. There is also controversy about the simple stylized fish on the border (faint but there). All that being said, I can see why the earliest Christian symblol on an ossuary would be ignored or attacked. It's in a tomb near the one called the Jesus family tomb.