Substrate control of benthos in a Middle Cambrian near-shore, epeiric palaeoenvironmental setting

Abstract

Late Middle Cambrian Mail Change Limestone, a thin (< 30 m) lime mudstone unit of the epeiric Middle and Late Cambrian Georgina Basin, Australia is unusually rich in epifaunal linguliform brachiopods, especially acrotretids. It is the only unit of this abundantly fossiliferous Cambrian basinal succession where the representation of this group is more prolific than that of trilobites. The diminutive size of acrotretids, generally less than 3 mm in shell diameter in the case of common taxa from the Mail Change Limestone, raises questions of how an epizoic lifestyle was achieved on muddy substrates. Facies analysis indicates that lime mud substrates associated with deposition of the Mail Change Limestone included microbial firm-ground which developed in upper shoreface settings, and ephemeral evaporate horizons of supratidal or high intertidal association, during the shallow-water phases of repeating parasequence cycles. Lime muds of shoreface association are brachiopod-rich, suggesting that firm-ground substrates were favoured attachment sites for populations of anchoring epizoic acrotretids. The conical shell form typical of this group evolved in part to minimize current drag, reducing stress on pedicle attachment, and in part to induce turbulence at the shell gape to facilitate feeding. The high incidence of undamaged acrotretid valves, representing a spectrum of growth stages, obtained from the Mail Change Limestone suggests that failure of pedicle attachment was a major contributor to mortality in this group. Firm-ground was also characterised by ubiquitous fine-scale, Planolites-like, open burrows which housed substantial populations of wormlike suspension feeders (?polychaetes) which also took advantage of firm-ground substrates. Near-shore substrates were also host to suspension feeding Lingulella and Protospongia and a deposit feeder responsible for Thalassinoides ichnofossils. Substrates associated with deposition of the Mail Change Limestone were unfavorable to colonization by vagile, seafloor feeding ptychopariid trilobites which are poorly represented in the unit. In contrast, agnostid trilobites of nektonic habit were less limited by trophic opportunities and are more commonly represented. Diverse agnostids ranged into very shallow marine, shoreface, environments represented by the Mail Change Limestone. The association of abundant epifaunal linguliforms and infaunal suspension feeders with firm-ground suggests that substrate stability strongly influenced the nature and biomass of Middle Cambrian sessile benthos. Evolutionary strategies among sessile benthos to take full advantage of shallow marine level bottom environments were not realised until the Ordovician biodiversity event.