OPEN ISSUES. “Many of the key questions of carpel evolution remain to be answered. For example, it is not known to which organ in gymnosperms the carpel is homologous. The mechanism of carpel closure and the potentially diverse mechanisms of fusion between carpels have yet to be discovered in the more highly evolved syncarpic species.”
https://academic.oup.com/jxb/article/57/10/2143/471376/An-evolutionary-perspective-on-the-regulation-of