ONTOGENIES OF TRILOBITES FROM THE LOWER ORDOVICIAN GARDEN-CITY FORMATION OF IDAHO AND THEIR IMPLICATIONS FOR THE PHYLOGENY OF THE CHEIRURINA

Ontogenies of three trilobites from the Garden City Formation (Tremado cian to early Arenigian; equivalent to Ibexian) are described: Tessela cauda depressa (Pilekiidae), Rossaspis pliomeris (Protopliomeropsinae) , and Protopliomerella contracta (Cybelopsinae). Comparison of both la rval and adult features of the described trilobites with those of chei rurids and encrinurids suggests that three lineages of the Cheirurina existed during the Early and Middle Ordovician. Members of the first l ineage show the following hierarchical relationships; (Tesselacauda de pressa (Cheirurinae (Acanthoparyphinae (two separate Sphaerexochinae g roups)))). Within the second lineage, the following internested relati onships of its members are recognized; (Protopliomeropsinae (''early'' cybelines (''advanced'' cybelines and Encrinurinae))). The Cybelopsin ae constitute the third lineage that is considered as an immediate out group to the above two lineages. Pattern of nested distribution of syn apomorphies within all these lineages reveals that the two pliomerid s ubfamilies (Protopliomeropsinae and Cybelopsinae) do not share a recen t common ancestor, suggesting that the Pliomeridae is a paraphyletic g roup.