1. About me:
I’m a third year PhD student at
the School of Biological
Sciences, University of Bristol. I
work as part of the
Palaeobiology Research Group.
My PhD project focuses on
reconstructing early steps in
eumetazoan evolution by
integrating palaeontology,
phylogenetics, comparative
morphology, and
developmental biology.
I’m broadly interested in the
evolution of complex
organisation and I view my PhD
project as a case study of this
more general phenomenon!
The Ediacaran Dickinsonia is a stem-eumetazoan
Email: an16227@bristol.ac.uk
2. The fossil record: an irreplaceable window into the past
The origin and early evolution of eumetazoans has been an open question for
more than 150 years. Molecular clocks place their origin and early radiation deep
in the Neoproterozoic (Cunningham, Liu et al. 2017), where fossils are scarce or
not particularly well-understood. Reconstructing early eumetazoans has
therefore traditionally mainly relied on comparative studies of development and
morphology of extant taxa, resulting in an evidential gap that can only be filled by
the fossil record: the only direct window into evolutionary history.
Reconstructing early
eumetazoans
Phanerozoic
Neoproterozoic
Cenozoic
MesozoicPalaeozoic
Ediacaran
Cryogenian
Tonian
Mesoproterozoic
Proterozoic
Cambrian
Ordovician
Silurian
Devonian
Carboniferous
Permian
Triassic
Jurassic
Cretaceous
Arthropoda
Onychophora
Nematoda
Priapulida
Chaetognatha
Rotifera
Mollusca
Annelida
Ambulacraria
Chordata
Cnidaria
Placozoa
Porifera
300mya
200mya
100mya
400mya
500mya
600mya
700mya
800mya
900mya
1000mya
50mya
150mya
250mya
350mya
450mya
550mya
650mya
750mya
850mya
950mya
Metazoa
Eumetazoa
Bilateria
Deuterostomia
SpiraliaEcdysozoa
Dickinsonia from the Ediacaran
Sandstone beds from the Ediacaran period (635-
541mya) contain a variety of macroscopic fossils,
many long thought to be of metazoan origin.
Dickinsonia (Sprigg 1947) has been a focus of
attention due to its metazoan-like features
(macroscopic, modular, active) and its temporal
range (657-550mya). Recent geochemical
evidence supports its placement within Metazoa
(Bobrovskiy, Hope et al. 2018), potentially
informing the evolution of key early eumetazoan
characters once it is more accurately placed. To
this end, we present here a new phylogeny of
Metazoa incorporating Dickinsonia (pictured) and
the closely related Yorgia.
Protostomia
3. Body fossils of Dickinsonia (and the closely related Yorgia) are normally preserved as shallow
negative impressions in the overlying sandstone (i.e. negative hyporelief). They provide us with
a wealth of information about their general morphology (Sprigg 1947; Evans, Droser et al.
2017), growth/development (Dunn, Liu et al. 2018; Hoekzema, Brasier et al. 2017), and even
chemical composition (Bobrovskiy, Hope et al. 2018).
Besides body fossils, Dickinsonia and Yorgia have also left large numbers of trace fossils
consisting of comparatively faint positive impressions in the overlying sandstone (i.e. positive
hyporelief). These often occur in series and are highly informative regarding the life habits of
these organisms (Ivantsov, Nagovitsyn et al. 2019), much like early hominid fossilised footprints
pointing to their bipedality!
The Ediacaran seafloor is characterised by the widespread occurrence of thick microbial mats
comprised by autotrophic as well as heterotrophic, and eukaryotic as well as prokaryotic
organisms. This ecosystem disappeared during the Ediacaran/Cambrian transition (Gehling
1999).
There are several features of Dickinsonia and Yorgia traces that enable us to infer some key
characters. Firstly, their preservation in positive hyporelief and their increasing depth correlated
with proximity to the body fossils indicates formation via the removal of the underlying mat by
the organisms, most likely as a result of active feeding indicating that they used their lower
surface to digest and absorb the mat, much like modern-day placozoans (Sperling and Vinther
2010). Secondly, their orientation and distribution on the mat in relation to body fossils and
each other strongly suggests active locomotion with a preferred direction of movement, with
their “deltoidal” region tending to be at the front (Evans, Gehling et al. 2019).
Body and trace fossil evidence together shed light on several characters possessed by
Dickinsonia and Yorgia. We used primary evidence and published literature to infer some key
characters, many of which are comparable to characters possessed by extant metazoans (see
table in next slide). We added these characters to a modified version of an existing matrix
consisting mainly of morphological characters (Zhao, Vinther et al. 2019) using Mesquite, and
performed a Bayesian Inference phylogenetic analysis using MrBayes to infer the position of
Dickinsonia and Yorgia within the broader metazoan tree (see next slide).
Inferring characters: Dickinsonia & Yorgia fossils
4. Character Dickinsonia Yorgia
Epithelia Present Present
Extracellular digestion Present Present
Sensory cells Present ?
Muscle cells Present ?
Symmetry Bilateral & Glide-plane Glide-plane
Diploblasty Present Present
Digestive sole Present Present
Gastric cavity Absent Absent
Triploblasty ? ?
Basal laminae Present Present
Nerve cells ? ?
Frontal region Deltoidal Crescentoidal
Protonephridia ? ?
Choanoflagellata
Porifera
Phylogenetic analysis and results
Placozoa
Yorgia
Dickinsonia
Xenacoelomorpha
Deuterostomia
Ecdysozoa
Spiralia
Ctenophora
Anthozoa
Medusozoa
Key characters inferred for Dickinsonia and Yorgia
Our results place Dickinsonia as sister to Eumetazoa, Yorgia as
sister to Dickinsonia + Eumetazoa, and Placozoa as sister to
Yorgia + Dickinsonia + Eumetazoa. Notably, our results leave
some uncertainty about whether Yorgia + Dickinsonia is
paraphyletic or monophyletic.
Results
5. KimberellaAspidella
SprigginaDickinsoniaYorgia
Charnia
SpiraliaCnidaria Deuterostomia EcdysozoaPlacozoaPorifera
Metazoan multicellularity
Benthic feeding,
“bilateral” symmetry
Gastric cavity
Sessile lifestyle
Dickinsonia and Yorgia are stem-group eumetazoans
Discussion
The Dickinsonia and Yorgia fossil photographs were provided by J. Vinther. Their icons in Slide 4
were downloaded from the public domain, alongside fossil photographs of Kimberella,
Aspidella, Spriggina, and Charnia. All silhouettes were downloaded from PhyloPic.
Furthermore, I would like to thank Dr Luke Parry (Oxford) and Mr Edmund Moody (Bristol) for
invaluable help with running the analyses, as well as Dr Amir Pandi (Max Planck) for feedback
on the slides.
Image references and acknowledgements
Implications for early eumetazoan character evolution
The placement of Dickinsonia and Yorgia has far-reaching potential implications
for the evolution of key early eumetazoan characters such as bilateral symmetry,
gastric cavities, muscular locomotion, basal laminae, nervous systems,
diploblasty, modularity, predation, and perhaps metazoan-bacterial coevolution.
Specifically, this challenges the commonly-held view that radial symmetry
evolved as a result of the evolution of gastric cavities and bilateral symmetry
evolved later as a result of directional locomotion (the Gastraea hypothesis). We
hope to further investigate these possibilities in future work.
Our results support the hypothesis that Dickinsonia and Yorgia are stem-
eumetazoans—i.e. more closely related to eumetazoans than to non-
eumetazoans, but excluded from the eumetazoan crown-group—by integrating
palaeontological, morphological, and developmental evidence. Nonetheless,
whether they form a clade or a paraphyletic group on the eumetazoan stem-
lineage remains unresolved. We intend to address this issue in further work by
including more Ediacaran as well as early Cambrian metazoans such as
Kimberella, Aspidella, Spriggina, and possibly Charnia, as well as others not
depicted here such as Parvancorina and Andiva.
