Although used in biostratigraphy and in studies of early Cambrian trilobite evolution, the olenelloid genus Peachella has received little research attention. The ontogenetic and evolutionary origins of its derived features—an effaced cephalon and grossly inflated genal spines—have remained mysterious. Based on examination of new and existing collections, P. iddingsi and P. brevispina are here described in detail, including aspects of their respective ontogenies and the first description of the thorax of P. iddingsi. A new monotypic genus, Eopeachella, is also described from recently collected material from the Delamar Member of the Pioche Formation, Nevada. Eopeachella angustispina n. gen. n. sp., is less derived and stratigraphically older than both Peachella species and bridges the morphological gap between Peachella and typical olenelloids. The study reveals that cephalic effacement was progressively attained during both ontogeny and phylogeny in the Eopeachella + Peachella clade. Comparative ontogeny with other olenelloids reveals that progressive effacement was a trend superimposed upon and independent of a conserved pattern of ontogenetic shape change in the glabella and did not represent a peramorphic “extension” of glabellar ontogeny. Genal spine inflation was also achieved progressively (in a proximal-to-distal direction) through both ontogeny and phylogeny in the Eopeachella + Peachella clade. Genal spine inflation that convergently arose in later trilobite groups may have been similarly ontogenetically dynamic. Discovery of E. angustispina and P. brevispina in the Delamar Member raises olenelloid diversity in this member to at least 20 species; a higher diversity than in any coeval unit.