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

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Description

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.,Fig. S1FIG. S1. Preservation of external anatomy in Pambdelurion whittingtoni: A, MGUH 31546, showing external annulations of legs, frontals appendages and trunk in low relief. B, MGUH 31553 showing frontal appendage and its spines, rounded outline of the head and associated spine and lateral flaps, all preserved as a two-dimensional reflective film. See appendix for abbreviations. Abbreviations: an, annulations; cs, cephalic spine; fa, frontal appendage; fas, frontal appendage spine; fl, flap; lg, leg. All scale bars represent 10 mm.Fig. S2FIG. S2. Pambdelurion whittingtoni: A, MGUH 31547, showing well-preserved longitudinal muscle in the cephalic region, frontal appendages and associated musculature. B, drawing of MGUH 31547. C, enlargement of boxed area in A, showing extrinsic frontal appendage musculature. Abbreviations: an, annulations; fa, frontal appendage. See appendix for colour coding of muscle groups. Scale bars represent: 10 mm (A, B); 1 mm (C).Fig. S3FIG. S3. Putative circular musculature in Pambdelurion whittingtoni: MGUH 31531, showing transverse, widely-spaced fibrous impressions crossing the trunk in a crescent-shape path. Scale bar represents 5 mm.Fig. S4FIG. S4. Kerygmachela kierkegaardi: A, MGUH 31548. B, drawing of MGUH 31548. Boxed area enlarged in Fig. S5A. Abbreviations: lm, longitudinal muscle, annulations; cm, circular muscle; fa, frontal appendage. See appendix for colour coding of muscle groups. All scale bars represent 10 mm.Fig. S5FIG. S5. Kerygmachela kierkegaardi: A, MGUH 31548, enlargement of boxed area in Fig. S4, trunk region showing faint, fibrous transverse impressions, possibly representing circular muscle. B, MGUH 31549, typical preservation showing pharynx and gut, but no musculature. All scale bars represent 10 mm.Fig. S6FIG. S6. Campanamuta mantonae: A, MGUH 31545, showing extensive axial longitudinal musculature, caudal and cephalic shields and thoracic tergites. B, drawing of MGUH 31545. Boxed area enlarged in Fig. 20B. Abbreviations: cas, caudal shield; ces, cephalic shield; gu, gut; lm, longitudinal muscle; tg, tergite. All scale bars represent 10 mm.Fig. S7FIG. S7. Campanamuta mantonae, A, MGUH 31529, showing transverse musculature. B, drawing of MGUH 31529. Abbreviations: oe, oesophagus; tg, tergite; tm, transverse muscle. All scale bars represent 5 mm.Fig. S8FIG. S8. Campanamuta mantonae: A, MGUH 31550, showing axial striated structures representing either gut diverticulae or musculature. B, drawing of MGUH 31550. C, close-up of area enclosed by white box, showing axial, striated structures. Abbreviations: fg, foregut; gu, gut; tg, tergite. Scale bars represent: 10 mm (A, B); 2 mm (C).Fig. S9FIG. S9. Pambdelurion whittingtoni, MGUH 31551 showing frontal appendages and head preserved as a reflective film. The right appendage reveals a high degree of flexibility, stretching over the left appendage. The spines emerging from the distal end of the appendage are narrow and extremely elongated. Abbreviations: an, annulations; cs, cephalic spines; fa, frontal appendage; fas, frontal appendage spines. Scale bar represents 10 mm.Fig. S10FIG. S10. Undescribed Sidneyia-like arthropod from Sirius Passet: A, MGUH 31552, showing segmented longitudinal musculature in the thorax. B, drawing of MGUH 31552. Arrows mark segmental boundaries of muscle. Abbreviations: ces, cephalic shield; gu, gut; lm, longitudinal muscle. All scale bars represent 10 mm.,
Date made available24 Oct 2017
PublisherDryad

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