Beyond the Burgess Shale: Cambrian microfossils track the rise and fall of hallucigeniid lobopodians

Abstract

Burgess Shale-type deposits are renowned for their exquisite preservation of soft-bodied organisms, representing a range of animal body plans that evolved during the Cambrian 'explosion'. However, the rarity of these fossil deposits makes it difficult to reconstruct the broader-scale distributions of their constituent organisms. By contrast, microscopic skeletal elements represent an extensive chronicle of early animal evolution-but are difficult to interpret in the absence of corresponding whole-body fossils. Here, we provide new observations on the dorsal spines of the Cambrian lobopodian (panarthropod) worm Hallucigenia sparsa from the Burgess Shale (Cambrian Series 3, Stage 5). These exhibit a distinctive scaly microstructure and layered (cone-in-cone) construction that together identify a hitherto enigmatic suite of carbonaceous and phosphatic Cambrian microfossils-including material attributed to Mongolitubulus, Rushtonites and Rhombocorniculum-as spines of Hallucigenia-type lobopodians. Hallucigeniids are thus revealed as an important and widespread component of disparate Cambrian communities from late in the Terreneuvian (Cambrian Stage 2) through the 'middle' Cambrian (Series 3); their apparent decline in the latest Cambrian may be partly taphonomic. The cone-in-cone construction of hallucigeniid sclerites is shared with the sclerotized cuticular structures ( jaws and claws) in modern onychophorans. More generally, our results emphasize the reciprocal importance and complementary roles of Burgess Shale-type fossils and isolated microfossils in documenting early animal evolution.

A superarmored lobopodian from the Cambrian of China and early disparity in the evolution of Onychophora

“…1 A, D, and G, and 2 A and D). The spines have subcircular bases, a punctate surface ornamentation, and a cone-in-cone construction similar to that described in hallucigeniids (3,14,15) (Figs. 1C and 2G).…”

Hallucigenia’s onychophoran-like claws and the case for Tactopoda

“…The Palaeozoic form-taxon Lobopodia encompasses a diverse range of soft-bodied 'legged worms' known from exceptional fossil deposits [1][2][3][4][5][6][7][8][9] . Although lobopodians occupy a deep phylogenetic position within Panarthropoda, a shortage of derived characters obscures their evolutionary relationships with extant phyla (Onychophora, Tardigrada and Euarthropoda) 2,3,5,[10][11][12][13][14][15] .…”

“…Hallucigenia spines each comprise a stack of one to five constituent elements 6 that are separated by 1-6u along the logarithmic spiral given by W 5 3, T 5 0, D 5 1.07. Spines that have been compressed obliquely to their plane of curvature express a smaller value of D, representing a preservational artefact (Extended Data Fig.…”

A bradoriid and brachiopod dominated shelly fauna from the Furongian (Cambrian) of Västergötland, Sweden

“…Ranges of morphological variability in both form species are, however, broad and overlap, so that it is not possible to associate them with a single or several natural species (Barragán et al 2014a, b). Topper et al 2007), lobopodians (Dzik 2003;Caron et al 2013), or trilobites (Li et al 2012 Mongolitubulus spinosus (Hinz, 1987) Description.-Gently curved hollow calcium phosphatic spines nearly circular in cross-section. The wall consists of two layers.…”