Abstract
The fossil record of the terminal Ediacaran Period is typified by the iconic index fossil Cloudina and its relatives. These tube-dwellers are presumed to be primitive metazoans, but resolving their phylogenetic identity has remained a point of contention. The root of the problem is a lack of diagnostic features; that is, phylogenetic interpretations have largely centered on the only available source of information—their external tubes. Here, using tomographic analyses of fossils from the Wood Canyon Formation (Nevada, USA), we report evidence of recognizable soft tissues within their external tubes. Although alternative interpretations are plausible, these internal cylindrical structures may be most appropriately interpreted as digestive tracts, which would be, to date, the earliest-known occurrence of such features in the fossil record. If this interpretation is correct, their nature as one-way through-guts not only provides evidence for establishing these fossils as definitive bilaterians but also has implications for the long-debated phylogenetic position of the broader cloudinomorphs.
Highlights
The fossil record of the terminal Ediacaran Period is typified by the iconic index fossil Cloudina and its relatives
The structures reported are the first recognizable soft tissues in cloudinomorphs, and the oldest guts yet described in the fossil record
The Wood Canyon tubular fossil assemblage has provided a unique view into early animal anatomy
Summary
The fossil record of the terminal Ediacaran Period is typified by the iconic index fossil Cloudina and its relatives. Toward the Period’s conclusion, the first metazoan mass extinction event[4,5] encompassed the downfall of the archetypal Ediacaran biota Their demise, was coincident with an ecological shift in which organisms such as Cloudina and other occupants of this novel tube-building morphotype[4] become increasingly populous. These “cloudinomorphs” (to avoid conflating unresolved phylogenetic relationships with shared morphologies6) were small, sessile, and epibenthic, but they appeared with several key adaptations that may have enhanced their chances for ecological success These attributes include: (i) the advent of macroscopic biomineralization in the form of shelly external tubes[2], potentially serving as an impediment to predation[7]; (ii) the establishment of gregarious habits that may signal the onset of metazoan ecosystem engineering behaviors[8] Satisfying the perceived majority of their exterior tube characteristics, most researchers currently fall into either anthozoan cnidarian[15,19] or polychaete annelid[2,14,16,18] camps, but further distinction has been hindered by the absence of preserved soft tissues
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