Multibranched rangeomorphs from the Ediacaran Mistaken Point assemblage, Newfoundland, Canada

Abstract

Rangeomorphs are a distinct group of millimeter- to meter-scale soft-bodied macrofossils that are restricted to the latter half of the late Neoproterozoic Ediacaran Period (635Ma- 542Ma). These fossils represent an extinct higher level taxon characterized by a modular construction based on a single architectural unit: the centimeter-scale, chevron-shaped rangeomorph element which displays several orders of self-similar branching. These elements could be arranged in a variety of different ways, constituting the wide array of gross morphologies found within the Group Rangeomorpha. The largest and most diverse assemblage of rangeomorph fossils in the world is found at Mistaken Point, on the Avalon Peninsula of Newfoundland, Canada, where these organisms are preserved within their original, in situ paleocommunities. Multibranched rangeomorphs are typified by bush-, comb- and network-shaped fossils which display multiple rangeomorph-bearing structures attached to an untethered basal stolon or central attachment point.

Multibranched, comb-shaped rangeomorphs are endemic to Mistaken Point, and are represented by fossils displaying multiple parallel struts emerging along one side of an elongate, curved pedicle rod. Morphological and taphonomic evidence suggests that, in life, this organism had two rows of struts, each bearing a rangeomorph frondlet, arranged in an alternating pattern along the curved, tubular pedicle rod. Biometric analyses imply that the struts were added to both ends of the pedicle rod throughout the organism’s lifetime, with later inflation of the rangeomorph frondlets. Each comb-shaped rangeomorph locality likely represents a different age cohort within the organism’s lifecycle, providing rare evidence for spatfall reproduction in Ediacarans, which is similar to that found in modern macrobenthic organisms with pelagic larvae. Network-shaped multibranched rangeomorphs, represented by symmetrical to asymmetrical net-like fossils, are also endemic to Mistaken Point. This genus is reconstructed as having a symmetrical arrangement of flexible, rangeomorph-bearing leaflets that were, in part, neutrally buoyant with respect to the seawater. This flexible leaflet structure is unique, and shared only with a rare, previously undescribed, Ediacaran frond-like organism. It is suggested that the enigmatic leaflet structures shared by these two morphologically distinct taxa represent a new type of rangeomorph branching architecture, and therefore constitute a new type of rangeomorph.