Filamentous Connections between Ediacaran Fronds.

Fossils of the Ediacaran macrobiota (∼571-539 mya) record phylogenetically diverse marine palaeocommunities, including early animals, which pre-date the "Cambrian Explosion" [1-4]. Benthic forms with a frondose gross morphology, assigned to the morphogroups Rangeomorpha [5] and Frondomorpha (see also Arboreomorpha) [6-8], are among the most temporally wide-ranging and environmentally tolerant members of the Ediacaran macrobiota [6] and dominated deep-marine ecosystems ∼571-560 mya [9-11]. Investigations into the morphology [12-14], palaeoecology [10, 15, 16], reproductive strategies [17, 18], feeding methods [9, 19], and morphogenesis of frondose taxa together constrain their phylogenetic position to the metazoan (for Rangeomorpha) or eumetazoan (e.g., Arborea) total groups [14, 20], but tighter constraint is currently lacking. Here, we describe fossils of abundant filamentous organic structures preserved among frond-dominated fossil assemblages in Newfoundland (Canada). The filaments constitute a prominent component of the ecosystems, and exhibit clear physical associations with at least seven frondose taxa. Individual specimens of one uniterminal rangeomorph taxon appear to be directly connected by filaments across distances of centimeters to meters. Such physical linkages are interpreted to reflect evidence for stolonic connections: a conclusion with potential implications for the phylogenetic placement and palaeoecology of frondose organisms. Consideration of extant stoloniferous organisms suggests that Ediacaran frondose taxa were likely clonal and resurrects the possibility that they may have been colonial (e.g., [21, 22]). VIDEO ABSTRACT.NERC National Geographic Geological Society of London Cambridge Philosophical Societ

Filamentous Connections between Ediacaran Fronds

Fossils of the Ediacaran macrobiota (∼571–539 mya) record phylogenetically diverse marine palaeocommunities, including early animals, which pre-date the “Cambrian Explosion” [1, 2, 3, 4]. Benthic forms with a frondose gross morphology, assigned to the morphogroups Rangeomorpha [5] and Frondomorpha (see also Arboreomorpha) [6, 7, 8], are among the most temporally wide-ranging and environmentally tolerant members of the Ediacaran macrobiota [6] and dominated deep-marine ecosystems ∼571–560 mya [9, 10, 11]. Investigations into the morphology [12, 13, 14], palaeoecology [10, 15, 16], reproductive strategies [17, 18], feeding methods [9, 19], and morphogenesis of frondose taxa together constrain their phylogenetic position to the metazoan (for Rangeomorpha) or eumetazoan (e.g., Arborea) total groups [14, 20], but tighter constraint is currently lacking. Here, we describe fossils of abundant filamentous organic structures preserved among frond-dominated fossil assemblages in Newfoundland (Canada). The filaments constitute a prominent component of the ecosystems, and exhibit clear physical associations with at least seven frondose taxa. Individual specimens of one uniterminal rangeomorph taxon appear to be directly connected by filaments across distances of centimeters to meters. Such physical linkages are interpreted to reflect evidence for stolonic connections: a conclusion with potential implications for the phylogenetic placement and palaeoecology of frondose organisms. Consideration of extant stoloniferous organisms suggests that Ediacaran frondose taxa were likely clonal and resurrects the possibility that they may have been colonial (e.g., [21, 22])

Viewing the Ediacaran biota as a failed experiment is unhelpful

Macroscopic organisms from the late Ediacaran period have often been described as failed experiments in the history of life. We argue that the field of Ediacaran palaeobiology should dispense with unhelpful historical classification schemes and embrace phylogenetic systematics if we are to establish the evolutionary relevance of these fossils. The Ediacaran macrobiota — an assortment of macroscopic, largely soft-bodied organisms that lived during the ~30 Myr interval prior to the Cambrian period — have long been considered a palaeontological conundrum. Many fossils of these organisms exhibit unusual body plans that are unlike anything seen among living taxa (Fig. 1a–d) and it has proved difficult to resolve their relationships to extant groups. Individually and collectively, members of the Ediacaran macrobiota have been both allied with extant clades and deliberately set apart from them by suggestions that they were either ‘failed experiments’ in the history of life, or members of long-extinct higher-order clades1. Despite the role they have played in stimulating debate around these taxa, we argue that these latter viewpoints have hampered Ediacaran research. They have also created confusion within the wider community as to the placement of the Ediacaran macrobiota in the tree of life, forming a barrier to their incorporation within biological and developmental discussions. We advocate abandoning the failed experiment perspective and embracing phylogenetic thinking in order to make progress in determining the phylogenetic positions of these organisms and realizing their evolutionary significance

Modularity and overcompensatory growth in Ediacaran rangeomorphs demonstrate early adaptations for coping with environmental pressures

The first known diverse, complex, macroscopic benthic marine ecosystems (late Ediacaran, ca. 571-541 Ma) were dominated by the Rangeomorpha, an enigmatic group of extinct frondose eukaryotes that are candidate early metazoans[1,2]. The group is characterised by a self-similar branching architecture that was likely optimised for exchange, but nearly every other aspect of their biology is contentious[2-4]. We report locally-enhanced, aberrant growth ("eccentric branching") in a stalked, multifoliate rangeomorph - Hylaecullulus fordi n. gen., n. sp. - from Charnwood Forest (UK), confirming the presence of true biological modularity within the group. Random branches achieve unusually large proportions and mimic the architecture of their parent branch, rather than that of their neighbours (the norm). Their locations indicate exceptional growth at existing loci, rather than insertion at new sites. Analogous over- compensatory branching in extant modular organisms requires the capacity to orchestrate growth at specific sites, and occurs most frequently in response to damage or environmental stress, allowing regeneration towards optimum morphology[e.g. 5-7]. Its presence in rangeomorphs indicates a hitherto unappreciated level of control to their growth plan, a previously unrecognised form of morphological plasticity within the group, and an ability to actively respond to external physical stimuli. The trait would have afforded rangeomorphs resilience to fouling and abrasion, partially accounting for their wide environmental tolerance, and may have pre-adapted them to withstand predation, weakening this argument for their extinction. Our findings highlight that multiple, phylogenetically disparate, clades first achieved large size through modularity

Anatomical and ontogenetic reassessment of the Ediacaran frond Arborea arborea and its placement within total group Eumetazoa

Organisms in possession of a frondose body plan are amongst the oldest and most enigmatic members of the soft‐bodied Ediacaran macrobiota. Appraisal of specimens from the late Ediacaran Ediacara Member of South Australia reveals that the frondose taxon Arborea arborea probably possessed a fluid‐filled holdfast disc, the size and form of which could vary within populations. Mouldic preservation of internal anatomical features provides evidence for tissue differentiation, and for bundles of tubular structures within the stalk of the organism. These structures connect in a fascicled arrangement to individual lateral branches, before dividing further into individual units housed on those branches. The observed fascicled branching arrangement, which seemingly connects individual units to the main body of the organism, is consistent with a biologically modular construction for Arborea, and raises the possibility of a colonial organization. In conjunction with morphological characters previously recognized by other authors, including apical‐basal and front‐back differentiation, we propose that to the exclusion of all alternative known possibilities, Arborea can be resolved as a total group eumetazoan

Anatomy of the Ediacaran rangeomorph Charnia masoni.

The Ediacaran macrofossil Charnia masoni Ford is perhaps the most iconic member of the Rangeomorpha: a group of seemingly sessile, frondose organisms that dominates late Ediacaran benthic, deep-marine fossil assemblages. Despite C. masoni exhibiting broad palaeogeographical and stratigraphical ranges, there have been few morphological studies that consider the variation observed among populations of specimens derived from multiple global localities. We present an analysis of C. masoni that evaluates specimens from the UK, Canada and Russia, representing the largest morphological study of this taxon to date. We describe substantial morphological variation within C. masoni and present a new morphological model for this species that has significant implications both for interpretation of rangeomorph architecture, and potentially for existing taxonomic schemes. Previous reconstructions of Charnia include assumptions regarding the presence of structures seen in other rangeomorphs (e.g. an internal stalk) and of homogeneity in higher order branch morphology; observations that are not borne out by our investigations. We describe variation in the morphology of third and fourth order branches, as well as variation in gross structure near the base of the frond. The diagnosis of Charnia masoni is emended to take account of these new features. These findings highlight the need for large-scale analyses of rangeomorph morphology in order to better understand the biology of this long-enigmatic group.NER

A new interpretation of Pikaia reveals the origins of the chordate body plan.

Our understanding of the evolutionary origin of Chordata, one of the most disparate and ecologically significant animal phyla, is hindered by a lack of unambiguous stem-group relatives. Problematic Cambrian fossils that have been considered as candidate chordates include vetulicolians, Yunnanozoon, and the iconic Pikaia. However, their phylogenetic placement has remained poorly constrained, impeding reconstructions of character evolution along the chordate stem lineage. Here we reinterpret the morphology of Pikaia, providing evidence for a gut canal and, crucially, a dorsal nerve cord-a robust chordate synapomorphy. The identification of these structures underpins a new anatomical model of Pikaia that shows that this fossil was previously interpreted upside down. We reveal a myomere configuration intermediate between amphioxus and vertebrates and establish morphological links between Yunnanozoon, Pikaia, and uncontroversial chordates. In this light, we perform a new phylogenetic analysis, using a revised, comprehensive deuterostome dataset, and establish a chordate stem lineage. We resolve vetulicolians as a paraphyletic group comprising the earliest diverging stem chordates, subtending a grade of more derived stem-group chordates comprising Yunnanozoon and Pikaia. Our phylogenetic results reveal the stepwise acquisition of characters diagnostic of the chordate crown group. In addition, they chart a phase in early chordate evolution defined by the gradual integration of the pharyngeal region with a segmented axial musculature, supporting classical evolutionary-developmental hypotheses of chordate origins and revealing a "lost chapter" in the history of the phylum. [Abstract copyright: Crown Copyright © 2024. Published by Elsevier Inc. All rights reserved.

Orientations of Mistaken Point Fronds Indicate Morphology Impacted Ability to Survive Turbulence

The Ediacaran fossils of the Mistaken Point E surface have provided crucial insight into early animal communities, including how they reproduced, the importance of Ediacaran height and what the most important factors were to their community dynamics. Here, we use this iconic community to investigate how morphological variation between eight taxa affected their ability to withstand different flow conditions. For each of Beothukis, Bradgatia, Charniodiscus procerus, Charniodiscus spinosus, Plumeropriscum, Primocandelabrum, Thectardis and Fractofusus we measured the orientation and length of their stems (if present) and their fronds. We statistically tested each taxon’s stem and frond orientation distributions to see whether they displayed a uniform or multimodal distribution. Where multimodal distributions were identified, the stem/frond length of each cohort was tested to identify if there were differences in size between different orientation groups. We find that Bradgatia and Thectardis show a bimodal felling direction, and infer that they were felled by the turbulent head of the felling flow. In contrast, the frondose rangeomorphs including Beothukis, Plumeropriscum, Primocandelabrum, and the arboreomorphs were felled in a single direction, indicating that they were upright in the water column, and were likely felled by the laminar tail of the felling flow. These differences in directionality suggests that an elongate habit, and particularly possession of a stem, lent greater resilience to frondose taxa against turbulent flows, suggesting that such taxa would have had improved survivability in conditions with higher background turbulence than taxa like Bradgatia and Thectardis, that lacked a stem and had a higher centre of mass, which may have fared better in quieter water conditions.</jats:p

Anatomical and ontogenetic reassessment of the Ediacaran frond Arborea arborea and its placement within total group Eumetazoa

Organisms in possession of a frondose body plan are amongst the oldest and most enigmatic members of the soft‐bodied Ediacaran macrobiota. Appraisal of specimens from the late Ediacaran Ediacara Member of South Australia reveals that the frondose taxon Arborea arborea probably possessed a fluid‐filled holdfast disc, the size and form of which could vary within populations. Mouldic preservation of internal anatomical features provides evidence for tissue differentiation, and for bundles of tubular structures within the stalk of the organism. These structures connect in a fascicled arrangement to individual lateral branches, before dividing further into individual units housed on those branches. The observed fascicled branching arrangement, which seemingly connects individual units to the main body of the organism, is consistent with a biologically modular construction for Arborea, and raises the possibility of a colonial organization. In conjunction with morphological characters previously recognized by other authors, including apical‐basal and front‐back differentiation, we propose that to the exclusion of all alternative known possibilities, Arborea can be resolved as a total group eumetazoan

Anatomy of the Ediacaran rangeomorph Charnia masoni

The Ediacaran macrofossil Charnia masoni Ford is perhaps the most iconic member of the Rangeomorpha: a group of seemingly sessile, frondose organisms that dominates late Ediacaran benthic, deep‐marine fossil assemblages. Despite C. masoni exhibiting broad palaeogeographical and stratigraphical ranges, there have been few morphological studies that consider the variation observed among populations of specimens derived from multiple global localities. We present an analysis of C. masoni that evaluates specimens from the UK, Canada and Russia, representing the largest morphological study of this taxon to date. We describe substantial morphological variation within C. masoni and present a new morphological model for this species that has significant implications both for interpretation of rangeomorph architecture, and potentially for existing taxonomic schemes. Previous reconstructions of Charnia include assumptions regarding the presence of structures seen in other rangeomorphs (e.g. an internal stalk) and of homogeneity in higher order branch morphology; observations that are not borne out by our investigations. We describe variation in the morphology of third and fourth order branches, as well as variation in gross structure near the base of the frond. The diagnosis of Charnia masoni is emended to take account of these new features. These findings highlight the need for large‐scale analyses of rangeomorph morphology in order to better understand the biology of this long‐enigmatic group