A marine stem-myriapod from the Silurian Waukesha Lagerstätte, Wisconsin, USA: terrestrial traits pre-date the transition to land

A new aquatic myriapod-like arthropod, Waukartus muscularis, from the Silurian (Llandoverian) Waukesha marine Lagerstätte of Wisconsin with uniramous limbs also preserves details of muscle tissue and endoskeleton. Phylogenetic analyses place it in a position just stemward of crown-group myriapods, sister taxon to a clade comprising euthycarcinoids and Myriapoda, providing evidence of the ancestral morphology of the living clade. A number of key myriapod synapomorphies, including the presence of an intercalary segment with a surmised tritocerebral appendage, and the decoupling of trunk tergite and appendage expression, evolved in euthycarcinoids within the stem lineage but crownward of the Waukesha myriapod. The new taxon is an aquatic member of this stem lineage and demonstrates that the loss of exopods, which results in uniramous limbs, pre-dated the common ancestor of the amphibious euthycarcinoids and terrestrial myriapods and does not represent an adaptation to terrestrial locomotion. The Waukesha Lagerstätte is unusual among Silurian localities in preserving taxa more typical of Cambrian communities alongside those characteristic of the post-Cambrian.