While retaining the shell present in the ancestral Lophostoma (in contrast to the Malacaformes), the various species of Stenostraca have reduced or modified said shell, making them the more mobile group of Diplostomes. Stenostracans lean towards carnivorous lifestyles (especially compared to their herbivorous sister clade Placostraca).
Stenostrac emerged roughly around 4180ma. The first representatives of this clade were generalist omnivores roughly 40cm in length, whose thinner and narrower shells allowed them to partake in climbing and digging. In 4185ma, the Coleostraca lineage (which were the first land predators in the planet's history) evolved, followed by Desmostraca roughly 3 million years later.
Like all Diplostomes, most species of Stenostraca were driven to extinction by the emergence of the Osteopods. The lineage survived by evolving more specialized lifestyles, notably the Opisthoptera clade, although their role as large-bodied creatures has long since been overtaken.