I visited an old church today, I asked a tour guide what the oldest thing there was. He told me that the font probably dated back to the crusades because it had carvings of elephants & lions on it (IE animals not many English would have seen in the flesh before then). Does that reasoning make sense?

Foreign animals show up not irregularly in illuminated manuscripts as well, some far more realistic than others. For example, there is an early 9th century manuscript that has a surprisingly accurate image of an Indian elephants head woven into a letter. It makes more sense when you realize that an elephant was presented to Charlemagne by caliph Haroun-al-Rachid in 802. The elephant, Abulabaz, lived until 810. It doesn't take long after this for imagery of that animal to return to a much less realistic style. Chances are that particular book was made during those years that Abulabaz was alive or very shortly thereafter.

If the font in the church has realistic elephants, lions, etc. it can help to give it a "no earlier than" date. Especially if it's thought that the something was created around the time of the Crusades, it would make it much more likely that the artist could have seen the animals or better images of them. However, with those buildings parts are created at a variety of times and can even come from other churches at a later point, so it's difficult to apply the dating from one area or object to another. I know it's not directly about dating churches, but A History of Illuminated Manuscripts has great visual references of how the styles of fonts and illustrations change over the centuries.