COMBINING ONTOGENETIC AND EVOLUTIONARY SCALES OF MORPHOLOGICAL DISPARITY: A STUDY OF EARLY JURASSIC AMMONITES

Two major research themes have been central in Evolutionary Paleobiology for the analysis and interpretation of morphological changes in macroevolution:

- The interplay between ontogeny and phylogeny. The role of development in large-scale evolution, classically analyzed in the general framework of heterochrony, has recently been broadened in scope to include a wider variety of developmental mechanisms explaining form changes through temporal and spatial modifications of inherited ontogenetic processes.

- The dynamics of morphological disparity. Disparity analyses attempt to rigorously characterize secular changes in the amount of morphospace ‘occupation' through time, as well as to assess the potential evolutionary significance of the frequent discrepancies between patterns of disparity and those derived from other biodiversity metrics.

To date, these themes remain rather isolated. While the former has always had a more mechanistic concern (even if still descriptive at the organismal level), the latter has mostly focused on documenting macroevolutionary histories of whole clades. Surprisingly, integrating both approaches has received relatively little attention. Such an integrative view might be exemplified as follows: ontogenetic changes entail shifts in morphospace, which in turn may alter disparity. The variational properties of ontogenetic dynamics may often constrain the frequency and magnitude of types of developmental changes. Thus, clade-wide disparity patterns might reflect a composite of distinct developmental causes underlying lineage evolution.

Concepts such as developmental morphospace help in bridging the gap between disparity and developmental dynamics. The approach is illustrated here with the early Jurassic ammonite family Hildoceratidae. Morphometric analyses of shell shape are used to quantify and contrast juvenile and adult disparity across taxa. Adult disparity appears to be significantly greater than juvenile disparity at the family level, but subclades suggest different patterns. Developmental morphospaces complement other recent approaches and appear as a fruitful starting point in (re)assessing the prevalence of the developmental component relative to other putative factors (e.g., ecological interaction) driving macroevolutionary dynamics.