Early fossil record of Euarthropoda and the Cambrian Explosion

Abstract

Euarthropoda is one of the best-preserved fossil animal groups and has been the most diverse animal phylum for over 500 million years. Fossil Konservat-Lagerstätten, such as Burgess Shale-type deposits (BSTs), show the evolution of the euarthropod stem lineage during the Cambrian from 518 million years ago (Ma). The stem lineage includes nonbiomineralized groups, such as Radiodonta (e.g., Anomalocaris) that provide insight into the step-by-step construction of euarthropod morphology, including the exoskeleton, biramous limbs, segmentation, and cephalic structures. Trilobites are crown group euarthropods that appear in the fossil record at 521 Ma, before the stem lineage fossils, implying a ghost lineage that needs to be constrained. These constraints come from the trace fossil record, which show the first evidence for total group Euarthropoda (e.g., Cruziana, Rusophycus) at around 537 Ma. A deep Precambrian root to the euarthropod evolutionary lineage is disproven by a comparison of Ediacaran and Cambrian lagerstätten. BSTs from the latest Ediacaran Period (e.g., Miaohe biota, 550 Ma) are abundantly fossiliferous with algae but completely lack animals, which are also missing from other Ediacaran windows, such as phosphate deposits (e.g., Doushantuo, 560 Ma). This constrains the appearance of the euarthropod stem lineage to no older than 550 Ma. While each of the major types of fossil evidence (BSTs, trace fossils, and biomineralized preservation) have their limitations and are incomplete in different ways, when taken together they allow a coherent picture to emerge of the origin and subsequent radiation of total group Euarthropoda during the Cambrian.

Keywords: Arthropoda; Cambrian explosion; Paleozoic; evolution; paleontology.

Fig. 1.

Timescale of the Cambrian fossil evidence of euarthropods. Age of important localities shown in columns for trace fossils, BSTs, body fossils, and phosphatic microfossils. Localities in green show evidence of euarthropods, localities in blue do not. Orange stars indicate the FAD of total group Euarthropoda (FAD Trace) and crown group euarthropods (FAD Crown). Time in millions of years (red writing). Data from refs. and .

Fig. 2.

Panarthropod fossils from the Burgess Shale. (A) Lobopodian Aysheaia pedunculata, USNM 83942. (B–E) Lower stem group euarthropods. (B) Opabinia regalis, USNM 155600. (C–E) Radiodont fossils. (C) Hurdia mouthpart, USNM 368583. (D) Anomalocaris canadensis body fossil, GSC 75535A. (E) Anomalocaris canadensis from D with musculature (white arrows) and gut glands (black arrows). (F and G) Upper stem group euarthropods. (F) Megacheiran Leanchoilia superlata, USNM 250299. (G) Bivalved arthropod Perspicaris dictynna, USNM 189245. (H–J) Crown euarthropods, artiopodans. (H) Trilobitomorph Helmetia expansa, USNM 83952. (I and J) Vicissicaudates. (I) Sidneyia inexpectans, USNM 250208. (J) Emeraldella brocki, USNM 57702. (Scale bars: 5 mm in A; 10 mm in B–F and H–J; 3 mm in G.)

Fig. 3.

A generalized phylogeny of panarthropod relationships, distinguishing the crown group Euarthropoda from the lower and upper stem lineage euarthropod taxa. Dashed lines indicates the uncertain phylogenetic placements of Megacheira and Artiopoda. Data from refs. , , , , , and .

Fig. 4.

Earliest fossil evidence of total group Euarthropoda, and extinct and extant crown group Euarthropoda. (A–D) SCFs of early crustaceans. Images courtesy of T. Harvey (University of Leicester, Leicester, United Kingdom) and N. Butterfield (University of Cambridge, Cambridge, United Kingdom). (A and B) Comparison of branchiopod filter plates from early Cambrian Mount Cap (A) and middle Cambrian Deadwood Formations (B, GSC 135392). (Scale bars, 50 μm.) (C) Appendage setae from Mount Cap, GSC 34928. (Scale bar, 70 μm.) (D) Branchiopod mandible from Mount Cap, GSC 34931. (Scale bar, 100 μm.) (E) Oldest crustacean Yicaris dianensis, YKLP 10840. (Scale bar, 150 μm.) (F) Earliest total group euarthropod evidence, Rusophycus trace fossil, GSC 85983. E and F reprinted with permission from ref. . (Scale bar, 20 mm.) (G) Fallotaspidid trilobite from Morocco, OUMNH AX.27. (Scale bar, 10 mm.)