Thursday 3 January 2019

Hylaecullulus fordi: A new species of Rangeomorph Ediacaran from Charnwood Forest, England.

The Ediacaran Fauna is the first complex macroscopic ecosystem known in the fossil record. It is found in rocks from the Late Ediacaran Period, between about 571 and 541 million years ago, and id dominated by a group of organisms called the Rangeomorphs, which formed fronds or branching structures attached to the substrate by a holdfast. How exactly these organisms lived is unclear, it is thought that their branching structure gave them a large surface area for exchange with the surrounding water, though there is no indication whether they gained nutrients or oxygen in this way. Nevertheless, it is now widely accepted that these organisms were among the first true Metazoan Animals. 

In a paper published in the journal Current Biology on 22 October 2018, Charlotte Kenchington of the Department of Earth Sciences at the University of Cambridge, Frances Dunn of the British Geological Survey and the School of Earth Sciences at the University of Bristol, and Philip Wilby, also of the British Geological Survey, describe a new species of Rangeomorph Ediacaran from Charnwood Forest, England.

The new species is named Hylaecullulus fordi, where ‘Hylaecullulus’ means ‘forest-goblet’, in reference to the shape of the specimens, and ‘fordi’ honours palaeontologist Trevor Ford (1925-2017) of the University of Leicester, for his work on the Ediacaran Fauna. The species is described from six specimens collected from the Ediacaran Bradgate Formation at the Charnwood Forest locality in Leicestershire, England, each of which has a holdfast surmounted by a long stalk then a cluster of branching fronds forming a goblet shape. At least four orders of branching can be seen, with some indication that a fifth was present. Importantly, in Kenchington et al.’s opinion, the specimens all show what they describe as eccentric branches, branches are arranged at random on the specimens, which are oversized compared to neighbouring branches on the same parent branch, and seem to expand to cover a gap in the basket structure.

(A) Specimen GSM105875 (mold), the plastotype (mold of the type, the specimen which is used to define the species; any specimen determined to belong to the same species as the type will be assigned to this species) and largest known example. (B) Interpretive overlay (up to folium level detail) of specimen GSM105875. The dark blue area is the holdfast disc, with dark blue lines outlining its internal rings. Medium blue is its stem, with red lines defining the ‘lineations’ and ‘triangle’. Bright blue outlines the folia. (C) Plastoparatype specimen GSM106040 (mold of a second specimen). (D) GSM105959 (cast). (E) Plastoparatype GSM106112 (cast). (F) GSM105957 (cast), the smallest well-preserved example. (G) GSM 105958 (cast). Scale bars, 2 cm. All molds and casts are held at the British Geological Survey, Keyworth. The interpretative overlay was digitized from a camera lucida interpretation. Kenchington et al. (2018).

Kenchington et al. interpret the eccentric branches as a sign of recovery from injury in Hylaecullulus fordi, something which has not previously been seen in any Ediacaran. The ability to respond to injury, and to grow in a way not preprogramed from the outset, is an important adaptation in a branching modular organism (such as modern Plants or Corals), and provides a way of recovering from damage that is not present in organisms that cannot do this. Such an ability would have made the Rangiomorphs more resilient to predation than has generally assumed to have been the case; the argument has often previously been made that this group became extinct early in the Cambrian because they were unable to cope with grazing by other emerging animal groups.

Increasingly higher-magnification views of the outlined boxed areas; the final image is an interpretative overlay (digitized from camera-lucida drawings) of the penultimate image. GSM106040 (cast) (A), GSM106112 (cast) (B), and GSM105875 (cast) (C) are shown. Scale bars, 2 cm. All casts are housed at the British Geological Survey, Keyworth. Kenchington et al. (2018).

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