This theme issue pursues an exploration of the potential of taking into account the environmental sensitivity of development to explaining the evolution of metazoan life cycles, with special focus on complex life cycles and the role of developmental plasticity. The evolution of switches between alternative phenotypes as a response to different environmental cues and the evolution of the control of the temporal expression of alternative phenotypes within an organism's life cycle are here treated together as different dimensions of the complex relationships between genotype and phenotype, fostering the emergence of a more general and comprehensive picture of phenotypic evolution through a quite diverse sample of case studies. This introductory article reviews fundamental facts and concepts about phenotypic plasticity, adopting the most authoritative terminology in use in the current literature. The main topics are types and components of phenotypic variation, the evolution of organismal traits through plasticity, the origin and evolution of phenotypic plasticity and its adaptive value.

Ontogenetic stages of trilobites have traditionally been recognized on the basis of the\ud development of exoskeletal segmentation. The established protaspid, meraspid, and holaspid phases\ud relate specifically to the development of articulated joints between exoskeletal elements. Transitions\ud between these phases were marked by the first and last appearances of new trunk segment\ud articulations. Here we propose an additional and complementary ontogenetic scheme based on the\ud generation of new trunk segments. It includes an anamorphic phase during which new trunk segments\ud appeared, and an epimorphic phase during which the number of segments in the trunk remained\ud constant. In some trilobites an ontogenetic boundary can also be recognized at the first\ud appearance of morphologically distinct posterior trunk segments. Comparison of the phase boundaries\ud of these different aspects of segment ontogeny highlights rich variation in the segmentation\ud process among Trilobita. Cases in which the onset of the holaspid phase preceded onset of the\ud epimorphic phase are here termed protarthrous, synchronous onset of both phases is termed synarthromeric,\ud and onset of the epimorphic phase before onset of the holaspid phase is termed protomeric.\ud Although these conditions varied among close relatives and perhaps even intraspecifically\ud in some cases, particular conditions may have been prevalent within some clades.\ud Trilobites displayed hemianamorphic development that was accomplished over an extended\ud series of juvenile and mature free-living instars. Although developmental schedules varied markedly\ud among species, morphological transitions during trilobite development were generally regular,\ud limited in scope, and extended over a large number of instars when compared with those\ud of many living arthropods. Hemianamorphic, direct development with modest change between\ud instars is also seen among basal members of the Crustacea, basal myriapods, pycnogonids, and\ud in some fossil chelicerates. This mode may represent the ancestral condition of euarthropod development

Trilobites offer the opportunity to explore postembryonic development within the fossil record of arthropod evolution. In contrast to most trilobites, the Silurian proetid Aulacopleura konincki from the Czech Republic exhibits marked variation in the mature number of thoracic segments, with five morphs with 18-22 thoracic segments. The combination of abundant articulated specimens available from a narrow stratigraphic interval and segmental intraspecific variation makes this trilobite singularly useful for studying postembryonic growth and segmentation. Trunk segmentation followed a hemianamorphic pattern, as seen in other arthropods and as characteristic of the Trilobita; during a first anamorphic phase, segments were accreted, while in the subsequent epimorphic phase, segmentation did not proceed further despite continued growth. Size increment during the anamorphic phase was targeted and followed Dyar's rule, a geometric progression typical of many arthropods. We consider alternative hypotheses for the control of the switch from anamorphic to epimorphic phases of development. Our analysis favors a scenario in which the mature number of thoracic segments was determined quite early in development rather than at a late stage in association with a critical size threshold. This study demonstrates that hypotheses concerning developmental pattern and control can be tested in organisms belonging to an extinct clade.

We performed a tree-based analysis of trilobite postembryonic development in a sample of 60 species for which quantitative data on segmentation and growth increments between putative successive instars are available, and that spans much of the temporal,

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In many arthropods, there is a change in relative segment size during post-embryonic development, but how segment differential growth is produced is little known. A new dataset of the highest quality specimens of the 429 Myr old trilobite Aulacopleura koninckii provides an unparalleled opportunity to investigate segment growth dynamics and its control in an early arthropod. Morphometric analysis across nine post-embryonic stages revealed a growth gradient in the trunk of A. koninckii . We contrastively tested different growth models referable to two distinct hypotheses of growth control for the developing trunk: (i) a segment-specific control, with individual segments having differential autonomous growth progression, and (ii) a regional control, with segment growth depending on their relative position along the main axis. We show that the trunk growth pattern of A. koninckii was consistent with a regional growth control producing a continuous growth gradient that was stable across all developmental stages investigated. The specific posterior-to-anterior decaying shape of the growth gradient suggests it deriving from the linear transduction of a graded signal, similar to those commonly provided by morphogens. A growth control depending on a form of positional specification, possibly realized through the linear interpretation of a graded signal, may represent the primitive condition for arthropod differential growth along the main body axis, from which the diverse and generally more complex forms of growth control in subsequent arthropods have evolved.

The traditional framework for the description of arthropod development takes the molt-to-molt interval as the fundamental unit of periodization, which is similar to the morphological picture of the main body axis as a series of segments. Developmental time is described as the subdivision into a few major stages of one or more instars each, which is similar to the subdivision of the main body axis into regions of one to many segments each. Parallel to recent criticisms to the segment as the fundamental building block of arthropod anatomy, we argue that, while a firm subdivision of development in stages is useful for describing arthropod ontogeny, this is limiting as a starting point for studying its evolution. Evolutionary change affects the association between different developmental processes, some of which are continuous in time whereas others are linked to the molting cycle. Events occurring but once in life (hatching; first achieving sexual maturity) are traditionally used to establish boundaries between major units of arthropod developmental time, but these boundaries are quite labile. The presence of embryonic molts, the 'gray zone' of development accompanying hatching (with the frequent delivery of an immature whose qualification as 'free-embryo' or ordinary postembryonic stage is arbitrary), and the frequent decoupling of growth and molting suggest a different view. Beyond the simple comparison of developmental schedules in terms of heterochrony, the flexible canvas we suggest for the analysis of arthropod development opens new vistas into its evolution. Examples are provided as to the origin of holometaboly and hypermetaboly within the insects.