Tag Archives: Palaeontology

Obama vs Attenborough – showdown in the Ediacaran

The Ediacaran period (635-541 Ma) is perplexing for palaeontologists. From the Cambrian onwards (541 Ma – present), fossils are often quite easily recognisable as the remains of ancient life and can mostly be recognised and classified. The Ediacaran, on the other hand, requires a lot of head-scratching; even simply demonstrating that what is being observed is a body fossil can be fraught with difficulty. For the most part, Ediacaran organisms were soft-bodied and, when found as fossils, they are often preserved as casts. When simple body plans abound, it is easy to muddle up trace fossils, body fossils, and the products of abiogenic processes. Additionally, preservation bias can occur within a single organism composed of multiple parts – sometimes only a small part of the organism is preserved. It is sometimes necessary to demonstrate that the fossils in question are not simply a small part of a larger, more complex organism, but are much simpler.

Mary Droser’s research team from the University of California – Riverside have done that for two new organisms, one which has been described and the other which is due to be published. Droser described them as a new, unique body plan and it took them a while to verify that they were individual organisms (in the press release quotation, Droser mistakenly refers to them as animals, despite that not being verified). These new organisms are from a newly described fossil bed in the famous Flinders Ranges of Australia, which they have named the “Alice’s Restaurant Bed” after the Arlo Guthrie song. It’s quite a long song:

The paper describing the bed is titled You can get anything you want from Alice’s Restaurant Bed: exceptional preservation and an unusual fossil assemblage from a newly excavated bed (Ediacara Member, Nilpena, South Australia). Droser and her colleagues have a knack for memorable paper titles, a personal favourite of mine is When the worm turned: Concordance of Early Cambrian ichnofabric and trace-fossil record in siliciclastic rocks of South Australia. 

As already mentioned, they have named two new species, one is awaiting publication, the other has been named already in another memorably titled paperStuck in the mat: Obamus coronatus, a new benthic organism from the Ediacara Member, Rawnsley Quartzite, South Australia. (“Stuck in the mat” is a reference to the organism’s way of life, partially embedded in microbial mats.) This organism has been named after former US President Barack Obama to honour his passion for science, whilst the forthcoming organism has been named Attenborites janeae after naturalist and broadcaster David Attenborough for his advocacy of science and support of palaeontology (Droser and Gehling both worked on Attenborough’s First Life documentary as well).

Obamus coronatus (left) and Attenborites janeae (right). Credit: University of California – Riverside

It seemed to me that Attenborough has a lot of organisms named after him and although I am a big fan of his, and I do think he deserves to have some organisms named after him, it seems to be overkill. So I had a bit of a look into how many organisms have been named after Attenborough, which also led me to find that Obama has a lot named after him too. So who has the most?

I haven’t excluded names which are no longer valid as the naming still happened in the first place.

Starting with Sir David Frederick Attenborough OM CH CVO CBE FRS FLS FZS FSA FRSGS:

Source: EPA

The first organism named after the UK’s adopted grandfather was Sirdavidia, a plant, for which there is only one species (as far as I can tell). He has two other plants named after him, Blakea attenboroughi and Nepenthes attenboroughi, as well. The rest are all animals and include two spiders, Prethopalpus attenboroughi and Spintharus davidattenboroughi, the beetle Trigonopterus attenboroughi, the dragonfly Acisoma attenboroughi which was named to commemorate his 90th birthday, the crustacean Ctenocheloides attenboroughi, the fish Materpiscis attenboroughi, the echidna Zaglossus attenboroughi, and finally the plesiosaur Attenborosaurus. Add on Attenborites and it is quite the haul.

How does everyone’s most loved/hated former US President, Barack Hussein Obama II, compare?

Obama has no plants but he does have the lichen Caloplaca obamae. He instead boasts more animals, one of which belongs to the same genus as one of Attenborough’s: the spider Spintharus barackobamai. He also has the spider Aptostichus barackobamai to his name. There is also the blood fluke Baracktrema obamai, the beetle Desmopachria barackobamai, the bee Lasioglossum obamai, the horsehair worm Paragordius obamai, the fishes Tosanoides obama and Etheostoma obama, the bird Nystalus obamai, and finally the extinct reptile Obamadon. Again, once Obamus coronatus is added, that is quite the haul.

If you decided to tally up these organisms, you will have noticed that they are neck and neck on twelve apiece. Is there a way to declare a winner? Both have two genera each named after them but as far as I can tell they are monospecific, so we can’t toss a load of extra species onto one pile. Is there another way to decide?

There actually is as I held one back. The fish Teleogramma obamaorum is named for both Barack and Michelle Obama so it does add to his list. Astonishingly, Barack Obama has more organisms named after him than the arguably more appropriate David Attenborough. Michelle Obama isn’t limited to sharing with her husband; the spider Spintharus doesn’t just include the species S. davidattenboroughi and S. barackobamai, as there’s also S. michelleobamaae. 

Whilst I’m mentioning family members, Sir David’s late brother Richard Attenborough got in on the act as well. The Jurassic ankylosaur Tianchisaurus nedegoapeferima gets its specific name from a combination of Sam Neill, Laura Dern, Jeff Goldblum, Richard Attenborough, Bob Peck, Martin Ferrero, Ariana Richards, and Joseph Mazzello.

One person who might envy Obama’s impressive list of namesakes is, of course, Donald Trump, who so far has the moth Neopalpa donaldtrumpi and the fossil sea urchin Tetragramma donaldtrumpi. The latter was named simply because its discoverer is a fan of Trump, whereas the moth bears a striking resemblance due to its “hair” and apparently diminutive genitals:

By Close up photograph of the Head of a Male Neopalpa donaldtrumpi.jpg: Dr. Vazrick Nazariderivative work: Kmhkmh – This file was derived from: Close up photograph of the Head of a Male Neopalpa donaldtrumpi.jpg:, CC BY 4.0, https://commons.wikimedia.org/w/index.php?curid=55126737

 

I’m hoping he stops at two.

 

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Science haiku

Haiku is one of the quickest and most fun forms of poetry to write. It’s also perfect for Twitter (which I recently joined). At some point, I intend to do some longer palaeontological poetry but for now, I have written a few haiku (mostly in the shower). I decided to tweet them all and am going to attempt to embed the first post here (I’ve never embedded a tweet before). They are mostly about the Ediacaran biota, with a few about palaeontology in general. Also birds. One of the challenges of writing a haiku about science, especially palaeontology, is the structure which limits the number of syllables one can use; a word like palaeontologist, for example, is too long for the first or last line and takes up a lot of allocated syllables when used in the second line. I had fun with that.

 

This first tweet was about Charnia, the important and puzzling Ediacaran fossil. The next one was about another Ediacaran organism, Dickinsonia. They are all on Twitter as well.

Dickinsonia
Your fossils are confusing
Reveal your secrets

For the next organism, I took a more tongue-in-cheek approach to its appearance.

Oh dear, Spriggina
You look like a used condom
Ribbed for her pleasure

And how could I leave out Kimberella?

Look, Kimberella
Maker of fossil scratch marks
Is that the back end?

I took a look at Ediacarans as a whole in these three:

Ediacarans
Are those animals, maybe?
A lively debate

Ediacarans
Where do you fit on life’s tree?
Pesky Rorschach tests

Precambrian life
Always called “enigmatic”
A truthful label

And onto the Cambrian:

Things got complex in
The Cambrian Explosion
But what was the cause?

I wasn’t as keen on that one as it seems to suggest a singular cause. I had written a different final line in my head but it has sadly disappeared. The next three were about palaeontology as a whole:

Wonderful life, dead
Palaeontologists find
Minds put flesh on bone

The rocks hold within
Secrets for us to unveil
Remnants of deep time

Life of distant pasts
Myriad deep connections
Try to uncover

And, finally, I thought I would hammer one particular point home:

Birds are dinosaurs
Birds? Yes, they are dinosaurs
Living dinosaurs

If you enjoyed these, drop me a message or comment on Twitter, I’d be happy to write some more.

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The best headline about Dickinsonia, ever?

Dickinsonia is one of those pesky Ediacaran mysteries which persistently defies classification. Think of an animal group and someone has suggested that Dickinsonia belongs to it. Think of a group outside of Animalia and there’s a decent chance that Dickinsonia has been placed there too. In more recent years, there has been a tendency to see it as a potential placozoan – the simplest of animals – often in a very cautious manner, or at least that it is at the same grade of complexity (for an example of the case for a placozoan affinity, see here, for insight into just how difficult it has been to classify Dickinsonia, see here). A recent study into the development of Dickinsonia claims to provide strong evidence that it is indeed an animal, though without assigning it to any particular group. It effectively states that Dickinsonia is an animal, so let’s put aside the claims that it is not and focus on where it fits on the animal evolutionary tree. See for yourself, here.

This gained a lot of press coverage back in September, though one, in particular, stood out to me. The Week decided to go with the brilliant headline “550-million-year-old thingamajig determined to actually be an animal”. I honestly can’t express how much I love that Dickinsonia has been labelled a “thingamajig” as it is perfect. Dickinsonia is head-scratchingly confusing, it is rightly considered to be a Rorschach Test for palaeontologists, it is, quite simply, a baffling thingamajig. It’s just a shame that the term has no taxonomic value.

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What’s in a title? – Eokinorhynchus the spiky, armoured worm

If this were a popular blog and not in its nascent stages I would run this as a competition. Titles for science news articles are tough to write and are not often the choice of the author of the article. They need to be attractive, yet risk being misleading click-bait, and they need to be informative which risks being dull. Here is a recent example that has me stumped and wondering what others might come up with.

The news in question is the discovery and description of Eokinorhynchus rarus, along with other unnamed kinorhynchs, providing the first evidence for phylum Kinorhyncha in the fossil record. It is from the Cambrian period and may prove crucial in understanding the evolution of related groups, not least its own. The scientific paper naturally went for the informative title: Armored kinorhynch-like scalidophoran animals from the early Cambrian.

Image credit: Dinghua Yang

Here are a few examples of science news sites and the headlines they went with. First, I Fucking Love Science:

IFLS kinorhynch

Naturally I did click it when I saw it, but that’s because I am already interested in anything which lived “half a billion years ago”. Calling it a spiky, armoured worm is not helpful at all, especially given the attention span of many of their readers – they often think two different pieces of news are about the same thing, even when the articles clearly state otherwise (I dare you to read the comments). They have published a few articles about spiky, armoured worms from the Cambrian, from many different phyla, so whilst it might be attempting to sound interesting it will inevitably create confusion. livescience went with the following:

Livescience kinorhynch

They have obviously gone for attention grabbing with no hint at the importance of the beast. Possibly my favourite comes from ZME Science:

zme kinorhynch

Yep, you read that correctly. The great news about Eokinorhynchus is that it is “ridiculously” armoured (as armoured as it gets!). The article itself isn’t much better. The South China Morning Post went for the most optimistic and patriotic option:

scmp kinorhynch

It’s the most sensational of the lot and, though it does mention it in the headline, its focus is not the armour of old E. rarus. And finally, forgive me for this, but the Daily Mail seemed to really want to put emphasis on how old it is.

daily mail kinorhynch

Half a BILLION indeed. I can mock the headlines offered by other sites, but I honestly don’t know what would be better. Emphasising its potential importance might not be appealing to the casual reader without risking exaggeration (or capital letters). Trying to sound exciting has focussed on the armour of E. rarus, which doesn’t have wide appeal (and may make some think they know it already). What’s the secret to a good headline for palaeo-news?

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Ediacaran ecosystem engineers – the Savannah hypothesis and our Skynet-type origins

Image from Wikipedia.

Out on the savannah, it is easy to find certain resources as they tend to be concentrated in limited areas. Trees, termite mounds, changes in terrain, all contribute to this concentration of resources. It is this sort of environment which has been hypothesised as resulting in our own bipedality which enabled human ancestors to move efficiently between resource hotspots. In a recent review, Budd and Jensen have proposed a ‘savannah’ hypothesis as an explanation for the evolution of bilaterian animals and consequently the Cambrian explosion.

Bilaterian animals are widely thought to have driven the Cambrian explosion, particularly as they function as ecosystem engineers altering the environment around them. The burrowing activities of bilateral organisms altered ocean chemistry and the nature of the sediment, opening up more resources to be exploited and resulting in a cascade of diversification. But why did bilaterians evolve in the first place? Ecological causes for the Cambrian explosion tend to presuppose features they should be explaining, such as the ability to burrow or the presence of predation (both likely contributed enormously to the diversification, but were also caused by it). Environmental causes tend to suggest limiting factors such as a lack of oxygen, which may actually be mistaking cause for effect.

The savannah hypothesis suggests that the Ediacaran biota also functioned as ecosystem engineers, causing carbon hotspots in the sediment and water around the organisms which were exploited by bilaterian animals which went on to diversify, eventually displacing their Ediacaran providers. Dissolved organic carbon in Ediacaran seas would not likely have clumped together, instead being spread out through the water column – not an economical resource for active organisms. Burrowing is highly energetic and would require dissolved carbon to be concentrated; without the Ediacaran organisms it would have been too diluted and sequestered away by the abundant microbial mats. Just like trees on the savannah, the Ediacaran biota concentrated dissolved organic carbon, providing sufficient resources for active burrowing and the need for motility.

In their thorough review, Budd and Jensen challenge the view that Ediacaran organisms went extinct by the start of the Cambrian period having been outcompeted and devoured by bilateral organisms. Instead, they survey putative evidence that shows that bilaterians first appeared towards the end of the Ediacaran period and that Ediacaran-type organisms persisted well into the Cambrian (and perhaps longer). At first, bilaterians would have been dependent on the Ediacaran ecosystem engineering, but went on to evolve their own sessile forms, such as crown-group sponges, and predatory habits which made them a threat to the Ediacaran biota – comparable perhaps to humans in the Terminator franchise creating Skynet and setting up their own demise.

They also reviewed the phylogeny of basal animals and take the view that sponges form a single clade which is the sister group to all other animals. They coined the term “Apoikozoa” which encompasses all animals and their sister group the choanoflagellates. And they made a case for Ediacaran organisms being early animals, albeit hugely problematic, whilst being highly critical of some of the optimistic interpretations. It is a paper which has provided a lot to mull over.

References 

Budd, GE. and Jensen, S. 2015. The origin of the animals and a ‘Savannah’ hypothesis for early bilaterian evolution. Biological Reviews. doi: 10.1111/brv.12239

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Tribrachidium – three arms and a lot of mystery

If you were to have been wading out into the Ediacaran seas, waves lapping against your legs, the slime of the microbial mats squidging under your feet, you might have noticed groups of a unique little organism called Tribrachidium. About the size of a coin, Tribrachidium had three ‘arms’ spiralling gently away from its centre, resembling a Celtic pattern. It rested on the sea floor in a variety of settings, mostly shallow with wave action, not moving, just letting the waves wash over it. It’s one of those organisms which is difficult to classify, but it has been possible to make inferences about its ecology as a recent study has done.

The Ediacaran period has been perceived as being ecologically quite simple, followed by the Cambrian explosion which saw a sharp rise in the number of ecological habits of animals. Bush, Bambach, and Daley (2007) developed a classification of life modes based on the area of marine ecospace inhabited relative to the seafloor, the level of movement of the organism, and feeding mechanisms. Theoretically, there are 216 distinct combinations, 92 of which are found in the Phanerozoic, 98 are possibly not viable, and 26 are possible but have not evolved. When applied to the Ediacaran  (Bush, Bambach, and Erwin 2011), there are just two modes of life in the older Avalon assemblage which is dominated by frond-like organisms, there are ten in the younger White Sea assemblage, and five in the younger Nama assemblage which sees the first skeletal forms. Contrasted with this, the first half of the Cambrian saw a rise to around 30 different modes of life (still only a third of modern life-modes).

A new study by Rahman et al. (2015) looked at the possible feeding habits of Tribrachidium and suggested that it had a mode of life previously unknown from the Ediacaran, indicating that Ediacaran ecosystems were more complex than thought (add this to the optimistic approach to the Ediacaran period). They narrowed down the possible feeding habits of Tribrachidium to just two: osmotrophy and suspension feeding. Osmotrophy involves the passive absorption of organic matter dissolved in the water, a common approach in the Ediacaran due to increased amounts of organic matter in the water, found in organisms such as Charnia and Charniodiscus which had large surface areas to absorb nutrients. Suspension feeding involves the trapping of organic matter in specific parts of the organism and requires a method of directing the water towards those traps.

With the two possible modes of feeding in mind, the researchers used computational fluid dynamics (CFD) to observe how water would have flowed over the organism, a method commonly used in engineering. Osmotrophy requires that the water flow over as much of the organism as possible, as has been observed in some of the frondose Ediacarans. By contrast, suspension feeding requires that the flow would be directed and focussed. What their tests found was that the water was directed passively by the arms, funneling it towards three depressions called ‘apical pits’ where it slowed down so that food particles would fall out of suspension. This directed movement fits neatly with a rare ‘gravity settling’ mode of suspension feeding, rather than with osmotrophy. They explained it as follows:

In summary, our CFD analyses demonstrate that the external surface morphology of Tribrachidium altered ambient water flow to produce low-velocity circulation above extremely localized areas around the organism, which is consistent with the interpretation of Tribrachidium as a suspension feeder rather than as an osmotroph. Specifically, we find that the three primary branches act to slow water flow and direct it up toward the apex of the organism, where small-scale recirculation develops directly above apical pits. This recirculation occurs at a range of simulated current velocities regardless of the organism’s orientation to the principal direction of flow. We suggest that this low-velocity zone of recirculation allowed larger particles to fall out of suspension, whereupon they were collected in the apical pits and subsequently metabolized (suspension feeding via “gravitational settling”). This hypothesis suggests that Ediacaran organisms used a larger diversity of feeding strategies than is currently appreciated and that they may have played a role as rudimentary ecosystem engineers, albeit in a fashion that became rare in the Phanerozoic with the disappearance of microbial matgrounds.

Computer simulation of water flow around Tribrachidium.

If it is indeed accurate, this study sheds light on the nature of the taxonomically tricky Tribrachidium, whilst expanding our understanding of Ediacaran ecosystems as more complex than previously thought. However, it is worth noting that fossils of Tribrachidium found so far show no signs of a mouth or other feature for ingesting filtered particles, it may simply be part of a larger organism – the holdfast of a frond-like organism, for example.

References

Click here for an interview with one of the authors.

Bush, AM., Bambach, RK., and Daley, GM. 2007. Changes in theoretical ecospace utilization in marine fossil assemblages between the mid-Palaeozoic and Late Cenozoic. Paleobiology 33:76-97.

Bush, AM., Bambach, RK., and Erwin, DH. 2011. Ecospace utilization during the Ediacaran radiation and the Cambrian eco-explosion. In Quantifying the Evolution of Early Life, edited by Laflamme, M., Schiffbauer, JD., and Dornbos, SQ, 111-33. Dordecht, Netherlands: Springer.

Rahman, IA., Darroch, SAF., Racicot, RA., and Laflamme, M. 2015. Suspension feeding in the enigmatic Ediacaran organism Tribrachidium demonstrates complexity of Neoproterozoic ecosystems. Science Advances 1, 10.

Singer, A., Plotnik, R., and Laflamme, M. 2012. Experimental fluid dynamics of an Ediacaran frond. Palaeontologica Electronica 15:1-14.

 

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An optimistic approach to the Ediacaran biota?

There’s a bit of a problem with the Ediacaran fossil record – it’s not what was originally expected and the organisms we do find are problematic. Based on the complex and recognisable fossils of the Cambrian, it was anticipated that more primitive forms would be found in the Precambrian, and, in a sense, they were. When they were first recognised, Precambrian organisms appeared to fit what was predicted; amongst them, palaeontologists recognised possible sponges, jellyfish, assorted worm-like creatures, putative arthropods and echinoderms. The more they were studied, however, the more problems in classifying them arose.

At some point along the line, near enough every Ediacaran fossil which had been linked to modern phyla have been reassessed and their connections found wanting. There is a handful which can still tenuously be linked to modern groups, but there is an apparent dearth of expected animal fossils, especially when the molecular clock data is taken into account. There appears to be an evolutionary gulf between the Ediacaran biota and the Cambrian explosion fauna.

Part of the problem is preservation – comparing the fossils of the Ediacaran and the Cambrian is difficult considering that they are mostly preserved in very different ways; the fossils of the Ediacaran are soft-bodied organisms preserved mostly as moulds, the fossils of the early Cambrian are mostly tiny bits of shell and other hard parts, and then there are the exceptionally preserved organisms from deposits such as Chengjiang.

One approach we can take to link the Ediacaran and the Cambrian is to avoid trying to fit them into recognisable taxonomic groups, and instead focus on the attributes they share with modern animals, particularly their ecology. This was the process adopted by Mary Droser and Jim Gehling in a paper earlier this year, titled The advent of animals: The view from the Ediacaran. We can look at the Ediacaran period and see things which are usually associated with animals, even if we cannot properly classify the fossils in question.

Mobility

One thing which clearly sets animals apart is movement – worms wriggle through sediment, fish swim about, and, of course, us humans find as many different ways to move as possible. Many animals don’t move about for most of their lives, not least sponges and corals, both of which we might expect in the Ediacaran in some form, but movement on or in the sediment would potentially be evidence for bilateral animals milling around. Most of what we see from the Ediacaran are stationary organisms, attached to the sediment by a holdfast or resting on the surface. The earliest animal traces are from 565 Ma and are most similar to traces by the polyps of anemones, providing evidence of muscular contraction, evidence of which also comes in the form of the body fossil Haootia quadriformis which possessed bundles of muscle fibres and is a possible cnidarian. The most common trace fossils in later Ediacaran rocks are in the form of grooves and levees, called Helminthoidichnites, and are interpreted as being caused by an animal too small to be preserved and limited in size by the chemical conditions of the sediment. They appear to have been mining the microbial mats, also showing evidence of avoidance behaviour, and are likely to have been created by bilaterian animals.

A few Ediacaran body fossils are associated with traces as well, lending to their interpretation as bilaterian in nature. Kimberella is a box-shaped body fossil which is often associated with scratch marks (Kimberichnus) that has been commonly seen as bilateral and has even been considered to be a possible mollusc. The associated traces have been interpreted as evidence of mat grazing though there are differences between the grazing habits of Kimberella and those of molluscs. The likely related Dickinsonia and Yorgia have been found associated with faint casts of their bodies, which appear to be resting or feeding traces where they sat ingesting the microbial mat before moving on to another patch. They often also have possible muscular contraction marks, though this interpretation depends somewhat on their phylogenetic affinity. The advent of mobility is therefore not confined to the Cambrian period though it does see an increase in the number of modes of mobility, as it is a behavioural trait of bilateral animals found in the Ediacaran.

Reproduction

Sexual reproduction is another trait associated with modern animals found in the Ediacaran period. The puzzling organism Funisia is a collection of tube-like structures which were previously not even recognised as body fossils. They demonstrate branching patterns which are potential evidence of asexual budding, whilst their distribution appears to be due to the production of spats, a form of reproduction mostly found in sexual organisms. Though their phylogenetic affinity is puzzling, the likely sexual reproduction of Funisia highlights another metazoan trait found in the Ediacaran period.

Skeletonisation 

The Cambrian explosion was first recognised in the fossil record due to the geologically sudden appearance of skeletal parts. The evolution of hard parts appears to have been a key stage in the evolution of Metazoa though it is not restricted to the Cambrian. Droser and Gehling discussed the example of Coronacollina, a cone-shaped organism with long, straight spicules radiating outwards, interpreted as a sponge-grade organism which is important for being the oldest known multi-element organism. Other Ediacaran shelled organisms include Cloudina and Namapoikia which are possibly cnidarian-grade organisms but had their study been released more recently they would likely have included the latest interpretation of Namacalathus as a lophophore. Even if we cannot place them phylogenetically, the appearance of skeletal parts, particularly multi-element organisms, is a key step in metazoan evolution found in the Ediacaran.

Ecosystems 

Ediacaran fossils tend to have been preserved in the places they lived, as opposed to having been transported and dumped elsewhere. This allows them to be studied as communities and permits insight into their ecological nature. The Flinders Ranges of Australia contain a succession of beds which are characterised by a range of organisms in shallow marine settings. The same organisms tend to appear on each bed but with different abundances, suggesting a level of sophistication in communities similar to that in the Phanerozoic despite there being a lack of predation, organisms living in the sediment, and widespread skeletonisation.

Conclusions

Setting aside phylogenetic affinities, traits of modern animals are found in the Ediacaran period. An optimistic approach to the Ediacarans allows us to see signs of mobility and the presence of muscles, skeletonisation, sexual reproduction, and the beginning of complex ecosystems – all possible links to the animals found in the Cambrian, suggesting that poriferans, cnidarians and bilaterians were all found in the late Precambrian.

References

Droser, M.L. and Gehling, J.G. 2015. The advent of animals: The view from the Ediacaran. Proceedings of the National Academy of Sciences 112: 16. [Link]

 

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