Trilobite Development and Ontogeny

Trilobite Ontogeny

Ontogeny, which is also called ontogenesis or morphogenesis refers to the development of a living organism beginning with fertilization of the egg and extending throughout the organism's life and growth phases, including sexual maturation. In theorizing trilobite development it is necessary to consider evolutionary developmental biology (sometines shortened to evo-devo) which concerns how the developmental processes themselves evolved and are, importantly, conserved in ancestral lines due to evolutionary conservation of genes and gene functions. In many cases, the genes that control development are ancient genes whose origins are in deep geologic time. For example, trilobites were animals that molted their exoskeletons in order to grow, a developmental trait that unite them with arthropods within Superphylum Ecdysozoa, including of course insects, chelicerata, crustaceans, members of Subphylum Myriapoda (millipedes, centipedes), even worms in Phylum Nematoda, and other lessor phyla. Modern evolutionary development of molting can now be studies and phylogenetics of arthropods studied using genes coding for 18S rRNA (18S rDNA). Sequence data from these genes are widely used in molecular analysis to reconstruct the phylogenetic or evolutionary history of organisms. These highly conserved sequences evolve slowly, making rendering them useful to construct ancient evolutionary lineage,s divergences, or splitting. Also see: Ecdysis unites trilobites with arthropods

Early Protaspis and Merapis Stages of Trilobite Development

Protaspid Stage of Trilobite Development

The first stage of trilobite growth is called the protaspid stage (Chatterton, 1997). This protaspis larval form was initially comprised of the beginnings of a cephalon and pygidium that were essentially indistinguishable (upper image to left). In the later anaprotaspis (middle image) phase, a transverse furrow appeared between the cephalon and pygidium (i.e., head and tail). New segments grew from this furrow area which, at this stage, did not laterally traverse all of the trilobite, and segments remained fused.

Meraspid Stage of Trilobite Development

During the meraspid development stage (also called anamorphic phase), new segments appeared posterior to the pygidium (pygidial segments) as well as anterior to the pygidium, with those anterior to the pygidium becoming the thoracic segments. All new segment were articulated and could thus freely flex. This adding of segments is called anamorphic development; workers believe segments were added mostly one per molt, with two possible, or perhaps one every two molts. It is likely that the number of molts equaled the number of eventual thoracic segments added and that, all totaled, made up some 25 to 40% of trilobite growth.

Holaspid Stage of Trilobite Development

The holaspid stage (also called the epimorphic phase) of trilobite development begins after the full complement of thoracic segments of a mature trilobite have developed in the Meraspis. Ecdysis continued during the holaspid stage of development, though with no additional segments added to the thorax. Some workers conjecture that some trilobites are continued molting and growing throughout their lifespan, although more slowly after maturity was reached (Chatterton, 1997). Hughes et al. (2003) called this a "segment invariant" phase (of an arthropod's development ) hemianamorphic development, contending that the vast majority trilobite species grew on in this phase, that is, while all external plates and legs before the last exoskeletal ecdysis. Also see: Trilobite Size and Evolution

Ontogeny and Phylogeny