AFFINITIES OF THE LOWER SILURIAN WAUKESHA ‘MYRIAPOD’

The Waukesha biota occurs within the Brandon Bridge Formation (late Telychian-early Sheinwoodian) of the Milwaukee area, a unit that consists typically of thinly bedded, light-green to pinkish-gray argillaceous dolomite. The marine fauna includes trilobites, phyllocarids, graptolites, conulariids, conodonts, and worms, preserved in a finely laminated mudstone. The Waukesha ‘myriapod’ is known from over 40 specimens and reached up to 40 mm in length. Many specimens preserve muscle tissue in calcium phosphate and in some the cuticle is preserved as a carbonaceous remnant. The head region is undifferentiated and has three pairs of small, segmented, lateral appendages and possibly a pair of anterior appendages. The trunk is composed of 11 segments, each with a pair of uniramous limbs with at least six podomeres and a broad, annular base. Each trunk segment is divided into a short anterior component followed by a longer, limb-bearing component. Preserved rings of cuticle around the leg bases, which are connected to ventral struts, are interpreted as integument strengthened to resist deformational forces generated during locomotion. There is evidence of both dorsal and ventral longitudinal muscles in the trunk, and what appear to be circular muscles. The terminal trunk segment bears a pair of ventral processes.

The affinities of the Waukesha ‘myriapod’ were considered originally to lie with the Myriapoda. It was compared to the late Silurian–early Devonian Kampecarida, with the suggestion that the origin of terrestrial Myriapoda may have been marine. However, it is now evident that myriapods radiated much earlier than previously thought, as derived clades of Diplopoda are represented at several localities in the Wenlock of Scotland. In addition, trace fossil evidence from the English Lake District suggests the presence of millipedes with penicillate or arthropleuridean affinities by the mid-Ordovician. Members of the myriapod stem group are notoriously difficult to identify due to a lack of apomorphies with preservation potential. However, circular muscles do not occur in Myriapoda, or Arthropoda in general, while they are present in extant Onychophora. The affinities of the Waukesha ‘myriapod’ may lie closer to the lobopods or to stem group arthropods.