Taphonomy and palaeoecology of the emuellid trilobite Balcoracania dailyi (early Cambrian, South Australia)

The depositional environment and taphonomy of the Homerian “Aulacopleura shales” fossil assemblage near Loděnice, Czech Republic (Prague Basin, Perunican microcontinent)

“…That assemblage belongs to the Type II lagerstätte of Brett et al (2012). Paterson et al (2007) described the taphonomy of trilobites on a variety of bedding planes of early Cambrian age in the Emu Bay Shale of the Adelaide area of South Australia. These authors contrasted a surface with multiple, complete exoskeletons of Balcoracania dailyi that showed a positively skewed size distribution, which they interpreted to be a life assemblage, with other surfaces with normal sclerite size distributions and in which the degree of disarticulation were notably higher.…”

“…In some cases specimens in different clusters have different mean sizes and, as additional dense clusters have been described (e.g. Gutiérrez-Marco et al 2009, Karim & Westrop 2002, Paterson et al 2007, the idea that such clusters reflect reproductive activity has gained currency, although other reasons for clustering such as feeding (Fortey & Owens 1999), or protection (Hughes & Cooper 1999) have been suggested. Although the timing of onset of sexual maturity remains obscure in the ontogeny of any trilobite , the presence of non-transported meraspid clusters of A. koninckii suggests that clustering behaviour was almost certainly unrelated to collective reproductive activity in this case.…”

The geological context of the Lower Cambrian (Series 2) Emu Bay Shale Lagerstätte and adjacent stratigraphic units, Kangaroo Island, South Australia

“…The argillaceous limestone comprises calcareous mudstone with iron-stained, carbonate filled, burrow galleries and nodules (Figure 6e). The presence of Balcoracania dailyi indicates very shallow marine conditions (Paterson et al 2007a Paterson et al 2007a) should now be considered occurrences within the Rouge Mudstone Member. The Rouge Mudstone Member is overlain by about 40 m of an essentially coarsening and upwards thickening package that in its lower part comprises red to grey, micaceous, feldspathic, moderately sorted, fine sandstone beds that are laminated in places, and bear Cruziana trackways.…”

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

“…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.…”

“…While plausible, this strategy to periodically exploit a rich food resource would seem particularly risky in that tidal flats represent stressful environments with fluctuating water levels and temperatures, in addition to hypersalinity; this would also seem at odds with the view that olenelloids typically occupy deep subtidal environments (Webster et al, 2008). Modern marine arthropods, such as limulids, that visit such settings to reproduce do so in an attempt to avoid predators in deeper waters, but counteract this risk with a high spatfall at nursery sites (Botton and Loveland, 1989), as has been suggested for the trilobite B. dailyi (Paterson et al, 2007). Limulid forays into the intertidal zone are also seasonal, in contrast to the frequency that would be expected for feeding.…”