Auroralumina attenboroughii, Dunn & Kenchington & Parry & Clark & Kendall & Wilby, 2022

Dunn, F. S., Kenchington, C. G., Parry, L. A., Clark, J. W., Kendall, R. S. & Wilby, P. R., 2022, A crown-group cnidarian from the Ediacaran of Charnwood Forest, UK, Nature Ecology & Evolution 6, pp. 1095-1104 : 1-3

Auroralumina attenboroughii gen. et sp. nov.

Etymology. Aurora (latin) dawn, referencing the great age of the fossil; lumina (latin) light, alluding to the torch-like appearance of the organism; attenboroughii , after Sir David Attenborough for his work raising awareness of the Ediacaran fossils of Charnwood Forest.

Holotype. See Figs. 1–3 . The holotype specimen remains in situ in the field; the plastotype is housed at the British Geological Survey, Nottingham ( GSM 106119 ). For the Reflectance Transformation Imaging (RTI) image of the holotype specimen (GSM 106352),

see Data availability. These casts were taken from primary mould GSM 105874 .

Diagnosis. Thecate, medusozoan cnidarian with colonial polypoid phase. Equi-sized, bifurcating polyps are encased in goblet-shaped, organic-walled, periderm with deep-corner sulci imparting a polyhedral outline and a form of radial symmetry but without conspicuous external ornament, excepting a thin concentric band near the aperatural rim ( Fig. 1 ). Periderm divided into two regions: the stalk and the cup. Polyp possesses a dense crown of uniform, unbranched tentacles which extend beyond the aperture of the periderm. Genus diagnosis the same by monotypy.

Description. The holotype ( Fig. 1 ) is ~20 cm in total length and is surrounded by a subtle microbial mat fabric that shows no indication of wrinkling or folding ( Fig. 1 ). It comprises two, well-defined, subparallel, goblet-shaped impressions that bifurcate from a less distinct area partially obscured beneath a thin cover of sediment: no detail is preserved proximal of this point ( Fig. 2 ). The goblet-shaped structures are equi-sized and are each constructed of a stalk (~12cm in length) which abruptly expands into a distinct cup (~6 cm in length). Each goblet has a well-defined linear ridge, running proximodistally, dividing them into two visible faces which, at their maximum, are ~6 cm wide. The left-hand goblet is divided symetrically by the ridge, which runs its entire length up to the apical margin of the cup, whereas the right-hand goblet is asymmetrically bisected.

The apical margin of the cup is defined by a straight rim and by a narrow trench (corresponding to a low ridge in the living organism) that runs parallel to it ~0.8 cm below. No other surface ornament is present. Fringing the apical margin is a dense crown of short (~2.75 cm), apparently uniform and simple projections, each maintaining an approximately constant width and with a blunt termination. These are not contiguous with the apical margins of the cups; instead, they appear to emanate from within them. Aminimum of 30, locally overlapping, projections are distinguishable in the better-preserved (left-hand; Fig. 3a,b ) cup.

Taphonomy and interpretation

The fossil is sharply differentiated from the irregularly textured background substrate and, like all other fossils on the surface, only one side of its lateral exterior is preserved. The left-hand goblet outline is symmetrical across the left and right, suggesting that the other side of the goblet was identical and therefore indicating that the goblet was probably tetraradial ( Fig. 3e ). Preservation of the goblets and the crowns is markedly different ( Fig. 1 ). The goblets are preserved in negative epirelief with raised rims, in common with most other fossils in the assemblage but the rims are notably sharper and higher and the goblet surfaces are smooth ( Fig. 1 ). The central ridges show the greatest relief of any fossilized remains on the surface ( Fig. 1 ). The absence of evidence for deformation, the sharper definition and the higher relief of the fossil relative to other co-occurring taxa (for example, rangeomorphs) all imply that the goblets were constructed of stiffer material. As these are negative epirelief impressions, the relief of structures is in the opposite sense—so in life, the raised structure would have been a trough, separating distinct faces of the goblet as a sulcus. There is no evidence for the former presence of biominerals (for example, brittle fracture or dissolution features), leading us to conclude that the goblets were originally organic-walled. There is no original carbonaceous material remaining in any Ediacaran Charnwood Forest locality. The preservation of two faces separated by a deep sulcus is common in fossil cnidarians, such as conulariids ( Fig. 3e ) and is a consequence of the compression of a three-dimensional organism onto a two-dimensional surface during burial ( Fig. 3e ).

The different bisections of the two goblets imply that they are preserved in different orientations. The occurrence of two symmetrical faces in one goblet (left-hand), and of a similarly sized face and partial face in the other (right-hand), is consistent with each goblet presenting a different view of an originally tetraradially symmetrical structure, much as in fossil conulariids and Carinachites 3, 4 ( Fig. 3e ). However, it could also represent a biradial structure as with hexangulaconulariids 5 or—if the margins of the left-hand goblet do not represent the margins of the periderm faces—triradial symmetry 6. The preservation cannot be reconciled with pentaradial symmetry, which would require unequal face widths, a condition not currently known in cnidarians with those symmetry states (for example, refs. 7, 8). We view tetraradial symmetry as most plausible because we consider that the margins of the left-hand goblet probably reflect the margins of faces and note that the maximum width of the largest face in the right-hand goblet is the same as the maximum width of the equi-sized faces in the left-hand goblet. However, we acknowledge uncertainty that might be resolved by discovery of additional specimens. The basal-most part of the specimen, past our inferred bifurcation point, does not align with the orientation of either of the two goblets individually, which supports our interpretation of the goblets as bifurfcating rather than two separate individuals. The obscured and indistinct nature of the most basal point means that we cannot say how much of the original organism is missing—the specimen we have may have been much larger in life, with additional goblets that are absent from our preserved view of the organism.

Unlike the goblets, the crowns are preserved in positive epirelief, recording the upper surface of the organism. They have poorly defined margins and faint wrinkling, recording the combined impressions of multiple overlapping projections ( Fig. 3f ) as is seen in, for example, multifoliate rangeomorphs 9 wheremultiplebranches overlap. The specimen is preserved in lateral view—as with all other fossils on the surface—so it is not possible to see the arrangement of this crown axially, on the interior of the goblets. The projections in the crown bear greatest similarity to tentacles of living animals, but are preserved as a compound impression rather than as individual tentacles. They lack external ornament and do not appear to taper.

The combined taphonomic expression of the fossil suggests stark differences in tissue toughness between the two parts, implying that these were originally constructed of different materials: one more rigid than rangeomorph fronds and able to deform the underlying sediment surface (the goblets) and the other sufficiently less resilient than rangeomorph fronds to have had its volume cast by sediment ingressed from below (the crown) 10, 11. We therefore interpret Auroralumina as a polypoid cnidarian, with a smooth, resistant, organic-walled periderm encasing a soft polyp that bears unbranched tentacles ( Fig. 4a ). The combination of a polyhedral organic-walled exoskeleton and corner sulci with associated softer tissues emerging from the aperture is compatible with interpretation of this structure as a cnidarian periderm to the exclusion of other potential structures. The body of the polyp would have been inside the cup in life and so only the protruding tentacles are preserved in this lateral view.

Phylogenetic position and morphospace occupation

Our phylogenetic analysis recovers a topology that agrees with most modern molecular studies (for example, ref. 12) and places Auroralumina in the medusozoan stem group ( Fig. 4b and Extended Data Fig. 1 ). We recover olivooids, Pseudooides and conulariids as stem-group medusozoans, which differs from recent analyses that have resolved them as crown-group scyphozoans (for example, ref. 13). Together, these data reconstruct the medusozoan ancestor as being broadly scyphozoan-like, with a polyp-encasing periderm ( Fig. 4b ). Our results are stable when ctenophores are constrained as the sister to all other animals (Extended Data Fig. 2a ) and when the specific inter-relationships of the Cnidaria are fixed to match recent molecular phylogenies (Extended Data Fig. 2b ).

We investigated morphospace occupation of tubular fossils (those with an external tubular skeleton within which an animal resided) across the Ediacaran–Cambrian transition as a mechanism to determine whether Auroralumina is significantly different from other Ediacaran tubular fossils and whether it is more similar to those fossils found in the early Cambrian period. As disparity analyses are phylogenetically independent, we incorporated a large suite of Ediacaran tubular taxa including those that are controversial and may or may not represent ancient cnidarians. The disparity matrix used in our analyses was based on characters published previously (refs. 14, 15 and other publications; see Supplementary Data 2 for a full list) which compared various phenotypic features of tubular, exoskeletal fossils across the Ediacaran and early Cambrian periods.

Calculating the non-metric multidimensional scaling (NMDS) with four axes produced a fair fit (stress value <0.1), so was used as the basis for further analysis. Inclusion of Auroralumina in the Ediacaran tube morphospace increased all aspects of disparity measured here ( Fig. 5 ).

Auroralumina has a major impact on the extent of Ediacaran tube morphospace and brings the Ediacaran tube morphospace closer in position and size to that of the Cambrian. The variance and extent of tubular morphospace occupation is comparatively low in the Ediacaran, indicating that tubular anatomies were not highly distinct, despite an increase in the abundance of tube-forming group(s) at this time 14. Auroralumina ’s location in the morphospace confirms that its anatomy is distinct from all other known Ediacaran tubular fossils and it is nested within Cambrian cnidarians, between presumed anthozoan and medusozoan taxa. Overall, morphospace variance increases into the Cambrian for all metrics we analysed, as tubular body fossils become more distinct and disparate and the distinctive Ediacaran nested tube-in-tube morphology 14 declines. Analysis of variance of disparity by group shows that the morphospace occupied by Ediacaran tubular taxa without Auroralumina is significantly different to the Cambrian morphospace (R 2 Pr(> F) <0.001) but, when Auroralumina is added, the Ediacaran and Cambrian morphospaces are not stastistically distinguishable (R 2 Pr(> F) = 0.586), while the Ediacaran without Auroralumina is significantly different from Ediacaran with Auroralumina (R 2 Pr(> F) = 0.045). This further supports the greater similarity of Auroralumina to Cambrian rather than other Ediacaran taxa.

Figures

Fig. 1 | Holotype specimen of Auroralumina attenboroughii. a, In context alongside rangeomorph fossils preserved in a comparable manner and distinct from the textured background substrate (GSM 105874); imaged under low-angle light. b,c, Plastotype (GSM 106119) (b) and interpretative drawing (c) showing the differentiated stalk and cup of each goblet, well-defined corner sulci (now ridges) and texturally distinct tentacles. The proximal portions of both goblets, including their mutual branching point, are concealed beneath a thin cover of sediment but are nonetheless discernible as topographically and texturally distinct tracts (dashed grey line); see Fig. 2 for more information. RTI file available76.

Fig. 2 | Details of the proximal part of the holotype specimen of Auroralumina (GSM 106119). a, Interpretative drawing of entire specimen, with area shown in b–d outlined. b, Base of the preserved specimen, showing progressive cover of the left-hand goblet towards the bifurcation point and the mostly concealed proximal part of the right-hand goblet. The margins of the fossil in the concealed area are impressed—albeit weakly—through the sediment and the area underlain by the skeleton is defined by a change in sediment texture. Fossil photographed under low-angle light. c, Interpretative overlay, generated by combining observations made under multiple lighting directions. d, Interpretative drawing from c, showing symmetrical bifurcation of the two goblets and probable broken proximal termination of the specimen. Key in d covers all annotations in this figure. Scale bar in b and c, 5 cm.

Fig. 3 | Details of the distal anatomy of Auroralumina attenboroughii (GSM 106119) and the mode of preservation. a, Left-hand goblet, with dense crown of overlapping tentacles and conspicuous corner sulcus (now a ridge) and band (now a trench) near the aperture rim. The margins of the fossil are well-defined and the tentacle crown texturally and topographically distinct from the smooth periderm. b, Interpretative drawing of region in a. c, Right-hand goblet, principally preserving only one face but with a second partially visible where its edge (and intervening corner sulcus) was twisted into the plane of preservation, towards the right-hand side. d, Interpretative drawing of region in c. Specimen photographed under low-angle light and interpretations based on features revealed by varying the lighting direction. Scale bar in a and c, 5 cm. e,f, Preservation of the goblet and tentacles of A. attenboroughii. e, Apical view of the two goblets showing how their different orientations at the time of burial generated different views of the tetraradial structure in the fossil in lateral aspect. Schematic goblets (labelled 1 and 2) are representative of the two goblets in Auroralumina. The interpretative drawing of Auroralumina is also shown, with goblets labelled 1 and 2 next to a conulariid cnidarian (OUMNH DU17), also inferred to have been tetraradial in life, to illustrate analogous preservation of multiple faces in lateral view. f, Hypothetical arrangement of the tentacles in oral view in vivo and probable arrangement of tentacles in lateral view at time of burial along with proposed preservational pathway of the tentacles. 1: Tentacles, mostly overlapping, buried by sediment. 2: Partial retraction and deflation postmortem. 3: Decay and casting of the resultant space by sediment from below.

Fig. 4 | The Phylogenetic position of Auroralumina attenboroughii. a, Artistic reconstruction of Auroralumina. b, Bayesian phylogenetic analysis of animals (348 characters, 108 taxa, mk + gamma model) incorporating Auroralumina attenboroughii. Numbers indicate posterior probabilities and scale bar shows expected number of substitutions per site. Fossils are indicated by dagger symbols. Raw polyp width is shown on the right, with the mean size shown for the extant groups sampled (for logged polyp size graph, see Extended Data Fig. 3). Branch length shown. Maximum polyp width data also shown in Extended Data Fig. 3. NA indicates where ancestral state values were inapplicable because they were derived from characters recovered as absent.

Fig. 5 | Tubular morphospace occupation across the Ediacaran–Cambrian transition. a–c, The sum of variances (a), sum of ranges (b) and the median of centroids (c) for tubular morphospace occupation. The sum of variances examines the evenness of morphospace occupation, the sum of ranges examines the extent of morphospace occupation in all computed dimensions and the median of centroids measures the clustering of taxa around a central point. Adding Auroralumina increases the sum of variances, ranges and (marginally) the median of centroids compared to Ediacaran morphospace excluding Auroralumina. The boxes represent the interquartile range, with black line showing the median. The whiskers indicate minimum (Q1 −1.5× IQR) and maximum (Q3 +1.5 ×IQR), excluding outliers. Outliers are shown in black squares. d, Morphospace occupation with convex hulls showing Ediacaran morphospace occupation with and without Auroralumina and Cambrian morphospace occupation. Black circles represent Ediacaran taxa and white circles represent Cambrian taxa.