Phylogenetic relationships, trunk segmentation, and gut metamerism of trilobites and their close relatives.

A. Phylogenetic position trilobites relative to other various groups of trilobite-like arthropods [68]. The trilobite-like articulation is synapomorphic for a distinct clade that includes trilobites, helmetiids and nektaspidids; the presence of this condition in saperiids is uncertain due to preservation and rarity of specimens. Xandarellids do not feature this type of exoskeletal organization, but rather are distinguished by the presence of several pair of walking legs under the posterior tergites (i.e. polypody). B. Inferred dorsoventral expression of engrailed (blue) in the trunk of trilobites and nektaspidids. The ventral expression of engrailed was most likely typical for arthropods in general, restricted to the posterior part of the segment, adjacent to the intersegmental border, and extending into the posterior portion of the limbs. The anteriorly shifted tergites that conform the trilobite-like articulation, coupled with the ancestral correlation between engrailed and the position of the tergites, strongly suggest that the domain of this gene was also anteriorly shifted, and thus expressed dorsally relative to the limbs. In nektaspidids, it is only possible to make accurate inferences on the dorsal expression of engrailed on the cephalothoracic articulation; consequently, the dorsal activity of this gene on the rest of the trunk segments remains uncertain. C. Metamerism of the digestive tract in trilobites, trilobite-like arthropods and remipedes (modified from [72]). The digestive tract of the ptychopariid Meniscopia beebei shows a metameric organization that consists on several paired gut diverticulae that are located exactly under the tergite-to-tergite articulation throughout the trunk. The remipede Speleonectes gironensis also features paired gut diverticulae; however, in this case the diverticulae are confined within each segment, indicating that they follow the plesiomorphic ventral pattern of arthropod segmentation. The diverticulae in the gut of the nektaspidid Naraoia spinosa is consistent with the organization observed in Meniscopia, even though the only functional articulation.