Fig. 4
From: Trilobite compound eyes with crystalline cones and rhabdoms show mandibulate affinities
Simplified cladogram of stem-lineage and crown-group of Euarthropoda (Mandibulata + Chelicerata). The evolutionary key characters of eye evolution are shown (combined from several articles38,39,40,41). The ‘other stem-lineage Euarthropoda’ largely comprise the taxa that are sometimes considered as ‘upper-stem Euarthropoda’46. Compound eyes with crystalline cones likely evolved in the lineage leading to Trilobita and Mandibulata comprising Myriapoda, Hexapoda, and paraphyletic crustaceans, rendering trilobites part of the mandibulate stem-lineage. Alternative hypotheses about the phylogenetic position of trilobites (stem lineage of Chelicerata39 or stem-lineage of Euarthropoda38) are indicated by dotted lines. If crystalline cones were already present in the compound eyes of Radiodonta3, then this character would not be informative with respect to the phylogenetic position of Trilobita. However, this changes to the opposite case if some of the other stem-lineage euarthropods that are more closely related to crown-group euarthropods possessed ommatidia with cuticular cones, as is likely the case for megacheirans3
