Trilobites in early Cambrian tidal flats and the landward expansion of the Cambrian explosion: COMMENT

Mángano et al. propose several hypotheses, including reproduction and feeding strategies, to explain the migration of olenellid trilobites into the intertidal zone. They discuss the trilobite ‘nursery hypothesis’ with reference to Rusophycus clusters from tidal flat deposits at other Laurentian sites being indicative of egg laying behavior similar to horseshoe crabs, but note that such clusters occur only in subtidal deposits in the Rome Formation, Tennessee, USA. Mángano et al. do not provide morphologic or taphonomic body fossil evidence for reproduction or spawning of trilobites from either the intertidal or subtidal deposits of the Rome Formation, but they should not necessarily dismiss the nursery hypothesis. A study that they may have overlooked on the emuellid trilobite Balcoracania dailyi from the early Cambrian (Stage 4) of South Australia (Paterson et al., 2007) provides compelling evidence, including size frequency data showing a high percentage of juveniles, from the Warragee Member of the Billy Creek Formation to support the interpretation that B. dailyi migrated into the tidal zone to copulate and spawn.