Ontogenies of trilobites from the lower Ordovician Garden City Formation of Idaho and their implications for the phylogeny of the Cheirurina

Abstract

Ontogenies of three trilobites from the Garden City Formation (Tremadocian to early Arenigian; equivalent to Ibexian) are described: Tesselacauda depressa (Pilekiidae), Rossaspis pliomeris (Protopliomeropsinae), and Protopliomerella contracta (Cybelopsinae).Comparison of both larval and adult features of the described trilobites with those of cheirurids and encrinurids suggests that three lineages of the Cheirurina existed during the Early and Middle Ordovician. Members of the first lineage show the following hierarchical relationships; (Tesselacauda depressa (Cheirurinae (Acanthoparyphinae (two separate Sphaerexochinae groups)))). Within the second lineage, the following internested relationships of its members are recognized; (Protopliomeropsinae (“early” cybelines (“advanced” cybelines and Encrinurinae))). The Cybelopsinae constitute the third lineage that is considered as an immediate outgroup to the above two lineages. Pattern of nested distribution of synapomorphies within all these lineages reveals that the two pliomerid subfamilies (Protopliomeropsinae and Cybelopsinae) do not share a recent common ancestor, suggesting that the Pliomeridae is a paraphyletic group.