Sentence:  It can be assumed that as the earlier kite shield transformed into the heater shape, there must have been versions in between the two styles. Indeed, in artwork of the Medieval period, shields are rarely present after 1450 and even then are quite small. The development of flattop <WSD>heaters</WSD> from larger kite shields is likely the result of increased fighting on horseback and the use of improved armor. 
Meaning of the word marked with <WSD> tags:  A medieval European shield having a rounded triangle shape like a clothes iron.