A critical reappraisal of the holotype of Beothukis mistakensis, a unique exceptionally preserved rangeomorph organism from Mistaken Point, Newfoundland, Canada
Reevaluación crítica del holotipo de Beothukis mistakensis, único organismo rangeomorfo excepcionalmente preservado en Mistaken Point, Terranova, Canadá

R.S. Taylor¹, J.B. Hawco¹, R. Nichols², D. McIlroy¹

¹Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, NL, Canada. Email: rodt@mun.ca; ORCID ID: http://orcid.org/0000-0003-3072-6040, http://orcid.org/0000-0001-9162-2013, http://orcid.org/0000-0001-9521-499X

²Palaeocreations, Bristol, UK. ORCID ID: http://orcid.org/0000-0001-8980-7297

IntroductionTOP

The Mistaken Point Ecological Reserve is a UNESCO World Heritage Site situated on the southeast coast of Newfoundland, Canada. It is home to multiple localities preserving complex accumulations of a group of late Ediacaran fossils known as the Avalonian Assemblage (Waggoner, 2003). These fossiliferous sites represent whole communities of deepsea seafloor-dwelling organisms that were preserved as ‘snapshots in time’ through burial by volcanic ash (Clapham et al., 2003; Ichaso et al., 2007) ranging in age from 570.94 ± 0.38 to 566.25 ± 0.35 Ma (Pu et al., 2016). Beothukis mistakensis (Fig. 1A) is one of the most recognizable Ediacaran fossils from Mistaken Point. It has been illustrated in numerous publications, although it represents an exceptionally small component of the total fossil assemblage. It is perhaps best known for its occurrence on the Mistaken Point ‘E surface’ in a small area colloquially known as ‘Seilacher’s Corner’, in close proximity to other well-known Ediacaran taxa (e.g. Fractofusus, Charniodiscus) in a highly photogenic portion of the surface.

Beothukis mistakensis was a uniterminal, frond-shaped organism with an oval to spatulate outline. Like all rangeomorphs, B. mistakensis is composed of a complex modular architecture, with each component unit being made up of similar but smaller units arranged in a fractal-like manner (Narbonne, 2004; Brasier et al., 2012). The taxonomic status of the genus Beothukis is currently uncertain: it has been amended to include specimens with a stem and basal disc (Brasier et al. 2012), although these features are not preserved on the holotype. Liu et al. (2016) reconsidered Beothukis mistakensis in the context of new discoveries of Culmofrons plumosa (LaFlamme et al., 2012; Fig. 2). C. plumosa was considered to possess a frond with similar gross morphology to Beothukis, but also a stem and holdfast. Culmofrons plumosa was thus considered to be a junior synonym of B. mistakensis, thereby creating the new taxon Beothukis (Culmofrons) plumosa. Part of the rationale for this decision was on the basis that continuous characters (i.e., stem length) are unsuitable for genus-level differentiation of taxa (Liu et al., 2016), the authors instead recommended utilizing gross architecture, the presence/absence of morphological features, and overall growth programme. The number of second order rangeomorph branches (8-12 in B. (C.) plumosa versus 5-8 in B. mistakensis) is an important feature used by Liu et al. (2016) to distinguish between the two species. The objectives of this study are to re-assess the morphology of the holotype of B. mistakensis, and to re-examine the relationship between B. mistakensis and B. (C.) plumosa.

Results and discussionTOP

Careful study of the B. mistakensis holotype reveals a considerably more complex morphology than previously recognized. The frond is asymmetrical, with the left side being made up of nine primary order units (‘branches’) while the right side consists of only eight such units (Fig. 1B). Primary order units were added sequentially from the distal tip of the frond towards its base, our interpretation of the primary order units differs from earlier authors but is based on retention of equivalent number of orders of “branching” within all primary order units. Although the overall outline of the organism is approximately symmetrical across the midline, the units making up the left and right sides show dissimilar sizes and orientations. The number of secondary order rangeomorph units within each primary order unit varies across the medial axis, compensating for the variability in size and shape of the primary units to maintain the overall frond shape. The angle of the primary order units relative to the midline increases from being acute at the distal tip of the frond to being approximately perpendicular near the base of the frond. Tertiary order rangeomorph branches are rotated and furled near the central axis but displayed and furled when bound proximally and distally by secondary growth. The tertiary units located at the lateral margin of the frond show variable morphologies.

Secondary growth occurs around the fringes of several of the first order units on the left side of the organism (Fig. 1C). Arcuate units without typical rangeomorph branching are also visible on the left-hand side of the holotype, in places overgrowing the secondary fringe growth. Several first order units without rangeomorph branching are located near the basal region of the organism, each composed of multiple rectangular second-order units and obliquely adjoining the frond’s central axis.

The paratype of B. mistakensis was also examined as a part of this study, revealing that there are 3 vanes of growth within the fossil. We thus determine that it is not congeneric with the holotype and thus needs to be reassigned to a different taxon. Of the many fossils previously described as Beothukis, none are morphologically comparable to the holotype, beyond perhaps having a spatulate outline. The other material is thus in need of reassessment, which is the focus of a separate morphometric study.

Several important morphological differences between B. mistakensis (Fig. 1A) and B. (C.) plumosa (Fig. 2) have been identified as a result of this study:

ConclusionsTOP

The holotype of Beothukis mistakensis is the only currently known specimen that can be definitely placed within the species. Morphological differences between the holotype of B. mistakensis and the well-constrained species plumosa suggest that the two are not congeneric and C. plumosa should be retained as a valid taxon as originally established. We hope that further study of the Avalonian Assemblage in Newfoundland will reveal more material belonging to B. mistakensis, and that morphometric analysis of the spatulate fronds will provide rationale for discrimination of the diversity within that morphospace.

ReferencesTOP