Exceptionally preserved Cambrian loriciferans and the early animal invasion of the meiobenthos
Thomas H. P. Harvey & Nicholas J. Butterfield
Nature Ecology & Evolution 1, Article number: 0022 (2017)
Received: 25 May 2016 Accepted: 25 October 2016 Published online: 30 January 2017
Figure 1: E. deadwoodensis, a fossil loriciferan from the upper
Cambrian Deadwood Formation of Canada.
Microscopic animals that live among and between sediment grains (meiobenthic metazoans) are key constituents of modern aquatic ecosystems, but are effectively absent from the fossil record. We describe an assemblage of microscopic fossil loriciferans (Ecdysozoa, Loricifera) from the late Cambrian Deadwood Formation of western Canada. The fossils share a characteristic head structure and minute adult body size (~300 μm) with modern loriciferans, indicating the early evolution and subsequent conservation of an obligate, permanently meiobenthic lifestyle. The unsuspected fossilization potential of such small animals in marine mudstones offers a new search image for the earliest ecdysozoans and other animals, although the anatomical complexity of loriciferans points to their evolutionary miniaturization from a larger-bodied ancestor. The invasion of animals into ecospace that was previously monopolized by protists will have contributed considerably to the revolutionary geobiological feedbacks of the Proterozoic/Phanerozoic transition.
Animals with minute body size are among the most abundant and diverse organisms in the marine benthos 1 , but most meiobenthic (or meiofaunal) animal lineages are palaeontologically invisible. Instead, inferences on the origins of microscopic body size rely on molecular phylogenetic analyses 2,3 and many groups remain poorly sampled or difficult to place in the tree of life. As such, there are conflicting predictions of ancestral body size for the major metazoan clades, with important implications for addressing the marked discordance between their first unequivocal fossil appearances in the terminal Ediacaran and Cambrian periods, and predictions from molecular clocks of early (or pre-) Ediacaran divergence dates 4,5 . Certainly, biases in the conventional fossil record leave open the possibility that the Cambrian explosion might artefactually conflate ecological and phylogenetic innovations by recording mostly macroscopic animals and their activities 6 . Here, we describe exceptionally preserved fossils demonstrating that microscopic loriciferans—a key lineage of meiobenthic ecdysozoans—had arisen by at least the late Cambrian. In addition to establishing a unique stratigraphic calibration point for this recently recognized phylum, these fossils provide important constraints on functional and phylogenetic models of early animal evolution.
We thank staff at the Geological Subsurface Laboratory, Regina, Saskatchewan, and M. Vélez, University of Regina, for help with core sampling. We thank geoLOGIC for generous access to subsurface data. This work was supported by Natural Environment Research Council Grant NE/H009914/1.
Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK
Thomas H. P. Harvey
Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
Nicholas J. Butterfield
T.H.P.H. and N.J.B. designed and performed the research and wrote the paper.
The authors declare no competing financial interests.
Correspondence to Thomas H. P. Harvey.