Modularity of a Cambrian ptychoparioid trilobite cranidium

Abstract

Modularity of the cranidium of Crassifimbra? metalaspis, a Cambrian ptychoparioid trilobite, is investigated using landmark-based geometric morphometric methods to gain insight into the integration among morphogenetic processes responsible for shaping the head of an ancient arthropod. Of particular interest is the extent to which the structure of phenotypic integration was governed by direct interactions among developmental pathways, because these interactions may generate long-term constraints on evolutionary innovation. A modified two-way ANOVA decomposes cranidial shape variation into components representing symmetric variation among individuals and fluctuating asymmetry (FA). The structure of integration of each of these components is inferred from correlated deviations in shape among nine partitions of the cranidium. Significant correlation among partitions in FA indicates direct interactions among their respective developmental pathways. An a priori hypothesis that modularity was determined by functional association among partitions is not well supported by the among-partition correlation structure for either component of variation. Instead, exploratory analyses reveal that phenotypic integration was strongly influenced by spatially localized morphogenetic controls. Comparison of the structures of the Individuals and FA components of variation reveals that the two share relatively few commonalities: the structure of phenotypic integration was only weakly influenced by direct interactions. The large contribution of parallel variation to phenotypic integration suggests that modularity was unlikely to have imposed a long-term constraint on evolutionary innovation in these early trilobites.

Analyzing Fluctuating Asymmetry with Geometric Morphometrics: Concepts, Methods, and Applications

“…Accordingly, many studies have used matrix correlation and the associated matrix permutation tests [11,13] or similar statistical approaches. Many such studies have found related patterns for fluctuating asymmetry and individual variation [11,16,44,49,55,56,67,72,79,82,97,106,119,139,[167][168][169][172][173][174]177,358]. Such results support the idea that direct developmental interactions make a substantial contribution to the patterns of integration among individuals.…”

“…Many studies have found matrix correlations between covariance matrices for fluctuating asymmetry and individual variation that are statistically significant, but of rather variable strengths [11,16,44,49,55,56,67,72,79,82,97,106,119,139,[167][168][169]172,173,177,358]. A few studies have reported a mix of significant and non-significant results for analyses of multiple samples or structures [54,75,120,174], and some other studies found only non-significant test results [13,42,73,78].…”

“…Analyses of covariation in the asymmetries of landmark configurations thus can be used to infer whether such developmental interactions exist (Section 6). This approach has been applied in a growing number of studies [11,16,44,49,51,56,59,63,67,72,74,75,78,79,82,139,[167][168][169][170][171][172][173][174].…”

The fossil record of phenotypic integration and modularity: A deep-time perspective on developmental and evolutionary dynamics

“…Studies of FA hold great promise for revealing developmental and evolutionary dynamics in long-extinct taxa (16)(17)(18)(19). A recent analysis of three species of trilobites from early in their Cambrian diversification used comparisons of FA and overall variation to estimate how developmental interactions shaped variation between species.…”

“…Much recent work has focused on characterizing large-scale patterns of trait relationships through comparative studies of extant and fossil taxa (11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23), in some cases demonstrating shifts in phenotypic integration related to changes in development (24)(25)(26), function (27)(28)(29), and environment (30)(31)(32)(33). Although focused overwhelmingly on model organisms, these studies provide a foundation for understanding how phenotypic integration changes through ontogenetic and evolutionary time and how it relates to myriad factors shaping morphological evolution.…”