Caught in the act: priapulid burrowers in early Cambrian substrates

Abstract

The fossilized traces of burrowing worms have taken on a considerable importance in studies of the Cambrian explosion, partly because of their use in defining the base of the Cambrian. Foremost among these are the treptichnids, a group of relatively large open probing burrows that have sometimes been assigned to the activities of priapulid scalidophoran worms. Nevertheless, most Cambrian burrows have an uncertain progenitor. Here we report a suite of exceptionally preserved trace and body fossils from sandstones of the lower Cambrian (Stage 4) File Haidar Formation of southern Sweden that can unequivocally be assigned to a scalidophoran producer. We further present the first burrow casts produced via actualistic experiments on living priapulids, and demonstrate the remarkable morphological parallels between these modern and Cambrian fossil equivalents. In addition, co-occurrence of scalidophoran-derived cuticular remains permits a unique synthesis of evidence from trace fossil, body and organic remains. Comparative analysis of these exceptionally preserved fossils supports a scalidophoran producer for treptichnids and by extension suggests a latest Ediacaran origin of the ecdysozoan clade.

Survival and selection biases in early animal evolution and a source of systematic overestimation in molecular clocks

“…Recent work suggesting that scalidophorans may have been responsible for Cambrian treptichnids (e.g. [54]) implies that these early treptichnids may be made by organisms close to or within the bilaterian crown group. They lie below an ash bed dated to 545 Ma, and above an unconformity above rocks dated to 548 Ma.…”

Cochleatina: an enigmatic Ediacaran–Cambrian survivor among small carbonaceous fossils (SCFs)

“…Jensen ; Mángano & Buatois ; Kesidis et al . ). Before a precise description of the magnitude, timing and nature of this transition can reasonably be achieved, there is a pressing need for an improved accounting of non‐biomineralizing taxa in order to discriminate genuine macroevolutionary patterns from localized signals or taphonomic shortfalls.…”

The asynchronous growth and movement reconstruction of the early molting animals

“…The growth patterns of the living organism were reconstructed being through the mechanical analysis of the biological forms [1]. The movement type -peristalsis, is widely employed by the worms living near sediment -water (air) interface, which makes their epithelial system under tension -compression rhythm [2] with relatively high potential energy. Conversely, in the steady state, the cell clusters of the living epithelial cell clusters minimize their potential energy [3].…”