Exceptionally preserved crustaceans from western Canada reveal a cryptic Cambrian radiation

Abstract

The early history of crustaceans is obscured by strong biases in fossil preservation, but a previously overlooked taphonomic mode yields important complementary insights. Here we describe diverse crustacean appendages of Middle and Late Cambrian age from shallow-marine mudstones of the Deadwood Formation in western Canada. The fossils occur as flattened and fragmentary carbonaceous cuticles but provide a suite of phylogenetic and ecological data by virtue of their detailed preservation. In addition to an unprecedented range of complex, largely articulated filtering limbs, we identify at least four distinct types of mandible. Together, these fossils provide the earliest evidence for crown-group branchiopods and total-group copepods and ostracods, extending the respective ranges of these clades back from the Devonian, Pennsylvanian, and Ordovician. Detailed similarities with living forms demonstrate the early origins and subsequent conservation of various complex food-handling adaptations, including a directional mandibular asymmetry that has persisted through half a billion years of evolution. At the same time, the Deadwood fossils indicate profound secular changes in crustacean ecology in terms of body size and environmental distribution. The earliest radiation of crustaceans is largely cryptic in the fossil record, but “small carbonaceous fossils” reveal organisms of surprisingly modern aspect operating in an unfamiliar biosphere.

Phylogenomic analysis of Copepoda (Arthropoda, Crustacea) reveals unexpected similarities with earlier proposed morphological phylogenies

“…This implies that the origin of copepods may be earlier (probably Cambrian age) than this period [82, 83]. It is because all copepod taxa in this study belong to the Infraclass Neocopepoda, and Platycopioida (the other infraclass Progymnoplea) is known to be the most primitive group of copepods and possibly closer to the ancestral form [5, 12].…”

Phylogenomic Insights into Animal Evolution

“…Doushantou Formation from South China), yet none of these can be confidently assigned to bilaterians [96]. In contrast, the emerging field of studies of microscopic organic fragments in the Cambrian reveals many such fragments that are clearly bilaterian in origin [97]. Even if bilaterians were tiny in the Precambrian, they would be capable of being preserved in the microfossil record, suggesting that their absence is real.…”

Waptia revisited: Intimations of behaviors

“…Shallow reef-like habitats might have been commonplace. Lower and middle Cambrian ecologies hosted abundant and diverse species (Vannier, 2009), and supported complex food webs (Dunne et al, 2008) and middle Cambrian sediments indicative of shallow-water environments have given up numerous remains of arthropods, such as mouthparts, podomeres, filter-feeding plates and setae, all indicative of shallow water crustaceans, including branchiopods (Harvey and Butterfield, 2011;Harvey et al, 2012;Butterfield and Harvey, 2012). There is also evidence that the water column hosted both benthic and epibenthic species such as Isoxys (Vannier and Chen, 2000), a taxon that has been well studied with respect to its vision and likely predatory behaviors (Schoenemann and Clarkson, 2011;Vannier et al, 2009), in addition to iconic radiodontan predators (Whittington and Briggs, 1985).…”