MACROEVOLUTIONARY TRENDS IN THE DEGREE AND NATURE OF VARIABILITY IN TRILOBITE SPECIES

Morphological variability is a raw material upon which natural selection operates. Documentation of temporal and phylogenetic changes in the degree and nature of morphological variability within species therefore offers potential insight regarding processes underlying macroevolutionary trends in the magnitude and rate of evolutionary innovation within clades.

The present study utilizes published cladistic analyses to explore the extent and nature of intraspecific morphological variability (coded as polymorphisms in the character-taxon matrices) in trilobite clades through time. Over 50 analyses meet the stringent criteria for inclusion in the present study; the final data set includes more than 1,000 trilobite species representing all trilobite orders. Each positively coded character (of which there are more than 40,000) represents an “opportunity to detect polymorphism” in a phylogenetically relevant (i.e., evolving) morphological feature within a clade.

More than 70% of Early and Middle Cambrian trilobite species exhibit some degree of variability in phylogenetically relevant features; this value is significantly lower for post-Middle Cambrian trilobites (less than 40%). Variability is documented significantly more frequently in agnostid and basal polymerid clades/grades (redlichiids and “ptychopariids”) than in more derived polymerid clades. The highest degrees of intraspecific polymorphism in evolving features are also found in Early and Middle Cambrian species, again particularly in agnostid and basal polymerid clades/grades. A trend towards declining intraspecific variability through time can also be detected within some, but not all, long-ranging orders (e.g., Proetida).

Restricting analyses to characters relating to a particular anatomical feature or a particular type of state change allows exploration of temporal trends within evolving features presumably sharing common developmental underpinnings within and across clades. Combined with ontogenetic data, this offers insight into trends in the prevalence of particular evolutionary modes, such as heterochrony or heterotopy. Although caveats remain, such meta-analyses can usefully highlight potentially informative areas for more detailed future research.