Soft-tissue preservation in Cloudinomorphs from the terminal Ediacaran Period of Nevada.
[sciencythoughts.blogspot.com]
Commonly envisaged as a prelude to the Cambrian Explosion, the terminal interval of the Ediacaran Period (roughly 550–539 million years ago) chronicles several monumental events during the evolutionary dawn of Animal life. Among the most significant are the emergences of biomineralisation and active motility, which demarcate this interval from the rest of the Ediacaran Period. Toward the Period’s conclusion, the first Metazoan mass extinction event encompassed the downfall of the archetypal Ediacaran Biota. Their demise, however, was coincident with an ecological shift in which organisms such as Cloudina and other occupants of this novel tube-building morphotype become increasingly populous. Collectively, these 'Cloudinomorphs' (to avoid conflating unresolved phylogenetic relationships with shared morphologies) were small, sessile, and epibenthic, but they appeared with several key adaptations that may have enhanced their chances for ecological success. These attributes include: the advent of macroscopic biomineralization in the form of shelly external tubes, potentially serving as an impediment to predation; (ii) the establishment of gregarious habits that may signal the onset of Metazoan ecosystem engineering behaviors; and (iii) the development of enhanced larval dispersal mechanisms, and presumably both sexual and asexual reproductive habits, versus stolon-like reproductive modes of some members of the enigmatic softbodied 'Ediacara Biota'. These compounded ecological innovations may have helped to place the Cloudinomorphs as central players in ushering in a phase of fundamental ecosystem reform and increased trophic complexity. Although its cause is equivocal at present, the changing of the ecological guard from largely sedentary Ediacara-type communities to much more dynamic syn-'Cambrian Explosion' ecosystems was well underway in the terminal Ediacaran. Indeed, as recently proposed, this interval possibly displays an even larger step-change in organismal and ecological complexity than at the Ediacaran–Cambrian boundary itself. Nonetheless, the most crucial task that remains is to untangle the potential relationships between the organisms of the Ediacaran Period and those well-defined as metazoans in the Cambrian Period. The Cloudinomorphs are one of the few groups known to span the Ediacaran–Cambrian boundary and thus understanding their phylogenetic position is key to unraveling the evolutionary and ecological relationships between the seemingly disparate biomes of the Ediacaran and Cambrian Periods.