Development
Development of Trilobites can be divided into 3 parts; protaspid, meraspid, and holaspid (Benton and Harper, 2009). The protaspid period is the larval stage where the larva lacks articulated segments. It is the simplest form with more and more features in each molt before the meraspid period (Gon, 2013).
Figure describing the lifecycle of Isotelus parvirugosus; from protaspid (larval) stage all the way to holaspid (adult) (Gon, 2013).
The next stage of development is the juvenile stage where they gain articulated segments. It is marked by a body with 2 or more articulated segments. It starts with the articulation of a cephalon (head) and trunk of 3 fused segments (Gon, 2013). With each molt, thoracic segments are added released from the pygidium or cephalon. Where they originate from is unclear (Gon, 2013). During this meraspid stage, is when specialization structures form such as spines (Gon, 2013). The final part of development is the holaspid stage when the number of thoracic segments reached adult stage (usually 6 thoracic segments). At this point, it enters the last stage of development where there are no more articulations but increase in size with successive molts (Benton and Harper, 2009).
Figure showing the different stages of development for trilobites. Note features start showing in the meraspid stage along with special features (Benton and Harper, 2009)
Molting (Ecdysis)
Molting, or ecdysis is the process in which trilobites grow via shedding of their exoskeleton (Gon, 2013). In a typical molting situation, the trilobite plants their spine into some sediment, acting as an anchor, creates a bend in the body, breaks open their cephalon via facial sutures and exit through the breakage (Gon, 2013). Facial sutures are breaking points in the cephalon that allow for the exit of the undergoing molting of the trilobite (Gon, 2013). There are three main categories for facial sutures that group the type of trilobite when it molts. These are proparian, gonatoparian, and opisthoparian (Gon, 2013). Proparian facial sutures break forward of the genal angle, as seen in most of the Phacopina, Gonatoparian facial sutures break at the genal angle, as seen in most of the Calymenina, and Opisthoparian facial sutures break behind the genal angle along the hind cephalic margin as seen in most Ptychopariina (Gon, 2013).
Figure showing the proces of ecdysis in trilobites (Gon, 2013).
Modes of Reproduction
Trilobites were assumed to be a sexually reproducing species. Until recently, trilobites were thought to be a species that produced a brood pouch encasing the eggs and/or developing young that rests within the cephalon near the genal portion (Fortey and Hughes, 1998). This is similar to the horseshoe crab that exists today. These brood pouches were found only in natant trilobites and were believed that eggs were released ventrally, anterior to the hypostome(Fortey and Hughes, 1998). Recently, trilobite fossils were discovered with fossilized eggs (Fortey and Hughes, 1998)
"Brood pouch" in a trilobites (Fortey and Hughes, 1998)