Demecology in the Cambrian: synchronized molting in arthropods from the Burgess Shale

Abstract

BackgroundThe Burgess Shale is well known for its preservation of a diverse soft-bodied biota dating from the Cambrian period (Series 3, Stage 5). While previous paleoecological studies have focused on particular species (autecology) or entire paleocommunities (synecology), studies on the ecology of populations (demecology) of Burgess Shale organisms have remained mainly anecdotal.ResultsHere, we present evidence for mass molting events in two unrelated arthropods from the Burgess Shale Walcott Quarry, Canadaspis perfecta and a megacheiran referred to as Alalcomenaeus sp.ConclusionsThese findings suggest that the triggers for such supposed synchronized molting appeared early on during the Cambrian radiation, and synchronized molting in the Cambrian may have had similar functions in the past as it does today. In addition, the finding of numerous juvenile Alalcomenaeus sp. molts associated with the putative alga Dictyophycus suggests a possible nursery habitat. In this nursery habitat a population of this animal might have found a more protected environment in which to spend critical developmental phases, as do many modern species today.

The fossil record of ecdysis, and trends in the moulting behaviour of trilobites

“…8A, B, F). This synchronized moulting is similar to that seen in a wide variety of modern crustaceans, insects and arachnids (Haug et al, 2013). The reasons for synchronized moulting are discussed in Section 2.7.…”

“…These are variously triggered abiotically (for example circadian rhythms in aphids; Johnson et al, 1957) or through active communication (pheromones) (Haug et al, 2013). These mass moultings were succinctly reviewed in Haug et al (2013) for extinct arthropod groups. Mass moulting assemblages include trilobites from the Cambrian (Fig.…”

Taphonomy and Depositional Setting of the Burgess Shale Tulip Beds, Mount Stephen, British Columbia

“…These horizons may represent a combination of limited time averaging combined with an original lowdiversity biotic assemblage. In the case of arthropods, these accumulations might also represent a mass molt event, as at the Walcott Quarry (Haug et al 2013). However, mass molting events followed by rapid burial do not provide a plausible explanation for large slabs covered with Anomalocaris claws, as those tend not to be paired (see paired-claw example from the Raymond Quarry, Fig.…”

Recognising moulting behaviour in trilobites by examining morphology, development and preservation: Comment on Błażejowski et al. 2015

“…Direct communication between individuals can also trigger synchronised moulting, either through pheromones, as seen in some colonial spiders and Hexapoda such as beetles and springtails , or through visual communication, as seen in krill . Mass moulting reduces predation pressure on individuals during this vulnerable stage of their development , but the benefit is balanced by higher disease transmission rates within the population . Synchronised moulting events are well established in modern arthropods, but identifying this behaviour in the fossil record is more difficult.…”

“…The early fossil record of moulting is also rich in examples of exuviae, particularly for Cambrian and Ordovician strongly biomineralised arthropods such as trilobites [5]. Several specimens from this time period show clusters of exuviae preserved in close proximity, indicating synchronous mass moulting events [13,14]. This rich fossil record of moulting has allowed for quantitative analyses investigating moulting behaviour trends, and their impact on trilobite evolution [5,15].…”