Adult and juvenile specimens of two species of trilobites, Sahtuia carcajouensis and Mackenzieaspis parallelispinosa, were collected from the Mount Cap Formation of the eastern Mackenzie Mountains. This formation was deposited within a semi-enclosed, epicontinental sea during the early and middle Cambrian (approximately 530–500 million years ago). These two species are endemic to one section at Carcajou Falls in strata of the Glossopleura walcotti Zone, where they also are prolific, suggesting they had some type of affinity to the locality. These species are characterized by their unusually low number of thoracic segments and high number of pygidial segments. We describe and quantify their growth and development here and compare them to those of other species from their families, Dolichometopidae and Zacanthoididae, respectively. The results indicate that S. carcajouensis and M. parallelispinosa underwent relatively normal growth and development of their cranidia and developed a typical number of segments in their trunks (the combined thorax and pygidium). However, both species underwent fewer occurrences of segment release, when segments are transferred from the pygidium to the thorax, than was typical for dolichometopids and zacanthoidids. We hypothesize here that these species are not closely related, and instead that their unique development was brought about by changes to the developmental timing of two probable ancestor taxa by local environmental conditions.

The ontogeny of two species of corynexochid trilobites from the middle Cambrian Mount Cap Formation of the eastern Mackenzie Mountains, northern Canada, is documented. Sahtuia carcajouensis (Dolichometopidae) and Mackenzieaspis parallelispinosa (Zacanthoididae) are both endemic to this formation and only known from one locality. They, along with several other corynexochid taxa, occur in a succession of mudstone with scattered carbonate interbeds, deposited in a weakly storm-agitated setting near the flank of a semi-enclosed basin. The ontogeny of both species is characterized by mainly normal cranidial development, but a unique distribution of segments in their thoraxes and pygidia. The number of trunk segments was typical for their respective families, whereas the final number of segments released into the thorax was reduced. This occurred in both species through timing modifications to segment release, indicating heterochrony. Sahtuia carcajouensis and Mackenzieaspis parallelispinosa are likely derived from two separate clades, and heterochrony probably arose separately but synchronously. The endemicity of both species probably reflects unique paleoecological conditions in this part of the basin. Preliminary results indicate that the fossil-bearing mudstone was deposited under well-oxygenated conditions that underwent high nutrient flux and possibly experienced varying salinity. These factors may have affected the organisms' physiology, or perhaps provoked an adaptation to achieve early maturation.