Buckman's Paradox: variability and constraints on ammonoid ornament and shell shape

Buckman's Law of Covariation states that ammonoid shell shape and ornamentation are typically correlated, such that compressed, involute forms have light ornament while more inflated, evolute forms have heavier ornament. Such covariation has been observed in many ammonoid groups, and implies a link between the morphogenesis of shell shape and ornamentation. However, other evidence suggests that while ornament growth is controlled by the genetic‐developmental program of the ammonoid, shell shape is strongly influenced by environmental factors. These differing viewpoints lead to Buckman's Paradox – are ornamentation and shell shape tightly linked, as implied by Buckman's covariation, or is the morphogenesis of ornament controlled genetically, while shell shape is controlled environmentally? To address this issue, the variability of shell shape and rib morphology has been compared for a group of endemic acanthoceratid ammonites from the Cretaceous Western Interior Seaway of North America. If Buckman's Law holds due to a morphogenetic connection between shell shape and ornamentation, we would expect taxa with more variable shell shapes to also show more variable rib features and growth. Morphometric analysis of seven shell shape and two rib characters for the Western Interior acanthoceratids finds no such correlation, suggesting that shell shape and rib growth are controlled by different processes. Indeed, rib growth appears to be more constrained than shell shape, consistent with the view that ornamentation is more tightly controlled by the developmental‐genetic growth program of the ammonoid. These results emphasize the complexity of ammonoid morphogenesis and highlight our limited understanding of the causes underlying Buckman's Law.