Abstract
BackgroundThe origin of the body plan of modern velvet worms (Onychophora) lies in the extinct lobopodians of the Palaeozoic. Helenodora inopinata, from the Mazon Creek Lagerstätte of Illinois (Francis Creek Shale, Carbondale Formation, Middle Pennsylvanian), has been proposed as an intermediate between the “weird wonders” of the Cambrian seas and modern terrestrial predatory onychophorans. The type material of H. inopinata, however, leaves much of the crucial anatomy unknown.ResultsHere we present a redescription of this taxon based on more complete material, including new details of the head and posterior portion of the trunk, informed by the results of experimental decay of extant onychophorans. H. inopinata is indeed best resolved as a stem-onychophoran, but lacks several key features of modern velvet worms including, crucially, those that would suggest a terrestrial mode of life.ConclusionsThe presence of H. inopinata in the Carboniferous demonstrates the survival of a Cambrian marine morphotype, and a likely post-Carboniferous origin of crown-Onychophora. Our analysis also demonstrates that taphonomically informed tests of character interpretations have the potential to improve phylogenetic resolution.Electronic supplementary materialThe online version of this article (doi:10.1186/s12862-016-0582-7) contains supplementary material, which is available to authorized users.
Highlights
The origin of the body plan of modern velvet worms (Onychophora) lies in the extinct lobopodians of the Palaeozoic
By incorporating the results of recent work on experimental decay of onychophorans into taphonomic analysis of character preservation we demonstrate that failure to include taphonomic evidence can lead to widespread loss of resolution in the results of phylogenetic analysis
Ilyodes was first described as belonging to the Myriapoda [12], and some 90 years later Helenodora inopinata was described as an onychophoran-like animal [8]
Summary
The origin of the body plan of modern velvet worms (Onychophora) lies in the extinct lobopodians of the Palaeozoic. Helenodora inopinata, from the Mazon Creek Lagerstätte of Illinois (Francis Creek Shale, Carbondale Formation, Middle Pennsylvanian), has been proposed as an intermediate between the “weird wonders” of the Cambrian seas and modern terrestrial predatory onychophorans. Known principally from Lower Palaeozoic Lagerstätten, some lobopodian taxa have been proposed to have affinities with extant Onychophora, but their precise relationships are unclear; recent analyses have recovered lobopodian taxa as stem-Euarthropoda, stem-Panarthropoda and stemOnychophora [1,2,3,4]. Helenodora inopinata Thompson and Jones [8], from the Mazon Creek Lagerstätte of Illinois (Francis Creek Shale, Carbondale Formation, Middle Pennsylvanian), is claimed as the oldest terrestrial lobopodian [8] and is generally regarded as a stem-onychophoran more closely related to extant onychophorans than to Cambrian lobopodians [9].
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