Data from: Onychophoran-like myoanatomy of the Cambrian gilled lobopodian Pambdelurion whittingtoni

Arthropods are characterized by a rigid, articulating, exoskeleton
operated by a lever-like system of segmentally arranged, antagonistic
muscles. This skeletomuscular system evolved from an unsegmented body wall
musculature acting on a hydrostatic skeleton, similar to that of the
arthropods’ close relatives, the soft-bodied onychophorans. Unfortunately,
fossil evidence documenting this transition is scarce.
Exceptionally-preserved panarthropods from the Cambrian Lagerstätte of
Sirius Passet, Greenland, including the soft-bodied stem-arthropod
Pambdelurion whittingtoni and the hard-bodied arthropods Kiisortoqia
soperi and Campanamuta mantonae, are unique in preserving extensive
musculature. Here we show that Pambdelurion's myoanatomy conforms
closely to that of extant onychophorans, with unsegmented dorsal, ventral
and longitudinal muscle groups in the trunk, and extrinsic and intrinsic
muscles controlling the legs. Pambdelurion also possesses oblique
musculature, which has previously been interpreted as an arthropodan
characteristic. However, this oblique musculature appears to be confined
to the cephalic region and first few body segments, and does not represent
a shift towards arthropodan myoanatomy. The Sirius Passet arthropods,
Kiisortoqia and Campanamuta, also possess large longitudinal muscles in
the trunk, although, unlike Pambdelurion, they are segmentally divided at
the tergal boundaries. Thus, the transition towards an arthropodan
myoanatomy from a lobopodian ancestor probably involved the division of
the peripheral longitudinal muscle into segmented units.