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.

节肢动物（Arthropoda）的典型特征为拥有坚硬且关节联动的外骨骼，该外骨骼依托分节排列的拮抗肌杠杆系统实现运动。这套骨骼肌肉系统起源于作用于静水骨骼（hydrostatic skeleton）的不分节体壁肌肉，其结构与节肢动物的近亲——软体有爪动物（Onychophorans）高度相似。遗憾的是，记录这一演化过渡过程的化石证据极为匮乏。产自格陵兰锡利克帕斯特定点（Sirius Passet）寒武纪特异埋藏化石库（Cambrian Lagerstätte）的特异保存泛节肢动物（panarthropods）化石，包括软体基干节肢动物（stem-arthropod）帕姆德留虫（Pambdelurion whittingtoni）、硬体节肢动物基尔索虫（Kiisortoqia soperi）与坎帕纳穆塔虫（Campanamuta mantonae），其独特之处在于完整保存了大量肌肉组织。本研究证实，帕姆德留虫的肌肉解剖结构（myoanatomy）与现生有爪动物高度吻合：躯干具有不分节的背侧、腹侧及纵行肌肉群，同时拥有控制附肢的外肌（extrinsic muscles）与内肌（intrinsic muscles）。帕姆德留虫还具备斜向肌肉，该结构此前被认为是节肢动物的标志性特征。但此类斜向肌肉仅局限于头部区域（cephalic region）与前数个个体节，并非指向节肢动物式肌肉解剖结构的演化过渡特征。锡利克帕斯特定点的节肢动物基尔索虫与坎帕纳穆塔虫的躯干同样拥有大型纵行肌肉，但与帕姆德留虫不同的是，其纵行肌肉在背板边界（tergal boundaries）处发生了分节分化。由此可见，从叶足动物类（lobopodian）祖先向节肢动物式肌肉解剖结构的演化过渡，大概率涉及外周纵行肌肉分化为分节单元的过程。