Heterochrony is considered to be an important and ubiquitous mechanism of evolutionary change. Three components are necessary to describe heterochrony: phylogenetic relationships, size and shape change, and timing of developmental events. Patterns and processes of heterochrony are all too often invoked before all three components have been investigated. Phylogenetic hypotheses affect the interpretation of heterochrony in three ways: rooting of a clade, topology of a clade, and character polarity. To study these effects we examined the distribution of shell microstructure, lophophore support structures, and body size in four different phylogenetic hypotheses of thecideide brachiopods (Triassic to Recent), a group of minute, cryptic, benthic marine invertebrates.
Thecideides are consistently monophyletic in experiments using terebratulide, strophomenate, and spire-bearing outgroups together and separately, varying ingroup membership, and experimentally withholding certain character complexes. Thecideide monophyly is also supported by bootstrap analysis. Hypotheses of heterochrony in thecideide origins and evolution are therefore not merely artifacts of classification and can be pursued further. Using either strophomenate or spire-bearing outgroups, Triassic Thecospira is the most primitive thecideide. Trees constructed using terebratulide outgroups are rooted instead at Eudesella, a taxon derived in every other phylogenetic reconstruction, and the Triassic thecideides occupy derived rather than primitive positions.
Our phylogenetic results support the traditional interpretation of the reduction or loss of the secondary fibrous shell layer as a paedomorphic pattern, whereas the evolution of lophophore support structures suggests a peramorphic pattern. Reduction in thecideide adult body size is gradual, phylogenetically, and results in an overall paedomorphic pattern. Heterochrony in these three character suites may play a role in the subsequent evolution of the clade, but apparently not in the origin of the clade, as is commonly thought. Heterotopy, rather than—or in addition to—heterochrony, may account for both the origin and evolution of the lophophore support structures and in the reduction and loss of the secondary shell layer. These phylogenetic hypotheses suggest that heterochrony can result from a complex mosaic of processes and provide specific, testable predictions about the processes responsible for producing the patterns, whether heterochronic or not. Categorizing an entire clade (such as thecideides), rather than individual characters, as globally paedomorphic may allow interesting peramorphic patterns in individual characters to be overlooked.