Cartorhynchus: helping ichthyosaurs crawl back into the mainstream.

Over the last 12 months or so, I’ve been researching ichthyosaurs for my masters project, and since then, I’ve been working on publishing my results. As such, I’ve learnt a fair bit of ichthyosaur palaeobiology and systematics. I’m no expert, but at this early stage in my academic career, I can say that at the moment, ichthyosaurs (specifically ichthyosaur endocranial anatomy) are my ‘specialisation(s)’, perhaps even making me an ichthyo-sir (heh). To this end, Motani et al.’s recent publication in Nature was massive news for ichthyosaur palaeobiologists. And that’s why it’s got its own little blog post.

What’s all the fuss about then? This year (2014) marked the anniversary of the 200 years since the first appearance of ichthyosaurs in scientific literature, and ichthyosaurs are closely associated with palaeontological celebrities, both historical (Mary Anning) and rather more recent (Alfred Romer). In those 200 years, we’ve learnt a lot about ichthyosaurs, and whilst they’re not ‘some kind of fish-lizard’, they are diapsid reptiles that were some of the first tetrapods to evolve a thunniform (fish/tuna-like) bodyplan, which aided them in their marine adventures. We know what colour some of them were thanks to melanosomes (Lindgren et al. 2014), we also know that they gave birth to live young, like mammals and some sharks do. We have exceptional fossils of ichthyosaurs actually giving birth, and others with amazing detailed and undisturbed soft tissue outlines. However despite all these amazing discoveries, we still don’t know a whole lot about two major aspects of ichthyosaur palaeobiology: their precise biomechanical function (we can’t create fancy 3D digital models due to the lack of 3D specimens, most ichthyosaur remains are mostly pancake-flat, even the really awesome ones) and perhaps more importantly, how they place in wider diapsid phylogeny.

The miracle of ichthyosaur birth, over 200 million years-ago. Very cool.

The miracle of ichthyosaur birth, over 200 million years-ago. Very cool.

Now before I go any further, it might be a good idea to explain what I mean by diapsid. Diapsida is a group of organisms (more specifically tetrapods) that have two temporal fenestrae (holes) in each side of their heads. Now this is a pretty large group, including archosaurs (dinosaurs, birds and crocs), lizards, snakes and tuataras. So whilst we have some vague idea where ichthyosaurs lie within this pretty large evolutionary tree, we’re not entirely sure. Why? Well because we don’t have transitionary fossils, ichthyosaurs previously (before Motani et al. 2014) appeared in the fossil record as highly adapted marine reptiles, well suited to the marine environment (remember, they look like fishes). So without any hint of which precise group of terrestrial organisms they evolved from, the topic of where ichthyosaurs came from is highly debated. So much in fact that in 2006, a very prominent ichthyosaur worker, Michael Maisch declared that the placement of ichthyosaurs within Diapsida was “…impossible…” (Maisch et al. 2006) without more basal specimens.

The internal phylogeny of ichthyosaurs isn’t a much better state either, with tree topologies changing at every opportunity of the last 10 years or so, threatening to change again when new systematic methods are applied. This again is largely due to the lack of well preserved three-dimensional specimens. But it’s not all doom and gloom, amazingly preserved specimens like those used in Lindgren et al. 2014 have shown us the colour of these ancient fish-dolphins (joke name, please don’t take it seriously), and Fischer et al. 2013’s discovery of Malawania has helped, to some degree, solve the internal phylogeny of at least neoichthyosaurs and ophthalmosaurs (ichthyosaurs from the Jurassic onwards). And obviously, my work on ichthyosaur endocranial and neuroanatomy from an exceptionally three-dimensionally preserved specimen will be hopefully  well received (more on that in the coming months). So it’s not all doom and gloom. But still, ichthyosaurs aren’t exactly the Brangelina of the palaeontological scene, no sir, those celebrity couples occupying all the headlines are dinosaur discoveries such as Deinocheirus and Dreadnoughtus, and the number of ichthyosaur workers isn’t exactly huge.

To my shocked delight, on the afternoon of the 5th of November, 2014 I stumbled upon a fresh new ichthyosauriform, in a Nature paper. Good heavens! Could it be true? Well, of course, other wise I’d have spent the last hour typing madly about ichthyosaurs for no apparent reason. Catorhynchus lenticarpus (‘shortened snout’, ‘flexible wrist’) is a weird, small beast. At first glance, you’d be forgiven for thinking that Catorhynchus wasn’t even a ichthyosaur at all. Well, technically, it isn’t an ichthyosaur at all, it’s an ichthyosauriform.  Catorhynchus comes from the Lower Triassic, approximately 248 million years-ago, and whilst some would call it an ‘ichthyosaur’, the Ichthyosauria (essentially all of the things your properly allowed to call ‘ichthyosaurs’) didn’t occur until later on in the Triassic. So, what the hell is Catorhynchus? Simple, it’s an ichthyosauriform, an ichthyosaur-looking creature which is more closely related to Ichthyosaurus communis than to a hupesuchian? Wait, what the hell are hupehsuchians, and what do they have to do with anything?

Hupehsuchians, think ichthyosaurs but a little more 'vacant' looking.

Hupehsuchians, think ichthyosaurs but a little more ‘vacant’ looking.

Good question. Hupehsuchians are a weird bunch of marine reptiles, who you’d be very much forgiven for calling ichthyosaurs, because well, they look quite a lot like ichthyosaurs. This simple fact has led many researchers to state that ichthyopterygians and hupehsuchians were related, however, there’s been little evidence to really cement this (just because two organisms look the same/have similar features, doesn’t mean they’re closely related. For example birds and bats both have wings, but they evolved powered flight independently and convergent to each other). As I’ve previously said, this is due to the lack of really primitive fossil ‘ichthyosaurs’ as well as our fairly poor understanding of where ichthyosaurs fit relative to other diapsids. However, Catorhynchus has given us a glimmer of hope, enabling us to, for the first time, really start to understand how ichthyosaurs first came about. Now, thanks to Catorhynchus, we think that ichthyosauromorphs (which now includes hupehsuchians) originated in China in the Earliest Triassic, which was a warm tropical archipelago ‘back in the day’. This is interesting, as we know that other groups of marine reptiles, such as sauropterygians (plesiosaurs, pliosaurs et al.) may have also originated in this area at the same time, so Earliest Triassic China may have provided very good conditions to harbour the evolution of many marine reptiles.

Phylogeny of ichthyosauromorphs, modified from Motani et al. 2014.

Phylogeny of ichthyosauromorphs, modified from Motani et al. 2014.

Don’t worry, I’ll stop teasing you now, I’ll actually talk about the fossil for a bit. With a host of unusual features such as really short snout, large flippers and a short body length (the shortest of all ichthyosauromorphs, estimated at a tiny 40 cm) and a really deep lower jaw, Catorhynchus is a weird beast. Yet, despite all these abnormalities, it looks like an ichthyosaur, I mean look at those big eyes! However, it also looks like a juvenile ichthyosaur. For me, and other ichthyosaur workers I’ve spoken to, this is the main reason why people of sceptical about drawing so many big conclusions from Catorhynchus. However, other people (untrustworthy creator of reptileevolution.com) have said that it certainly can’t be an ichthyosaur, and has to be an ‘ichthyosaur-mimic’, because yeah, if it has loads of scientifically diagnostic features of an ichthyosaur the most obvious answer is that it wants you to think it’s an ichthyosaur, just to troll the scientific community, and then years later scream ‘psyche!’ to everyone, you know, because fossils love to mess with us, the jerks.

Catorhynchus fossil from Motani et al. 2014. E represents a  newborn Chaohusaurus, for comparison.

Catorhynchus fossil from Motani et al. 2014. E represents a newborn Chaohusaurus, for comparison.

ANYWAY. I think it’s worth pointing out that it realistically might turn out to be a juvenile, even though Motani et al. do present some evidence that it’s a fully grown adult, for example the forefins of Catorhynchus are almost as long as its skull, a feature found exclusively in adult individuals. However, it’s also worth mentioning that even despite this, Motani et al. don’t completely dismiss the possibility of this specimen being a juvenile. Essentially, I feel it’s best to take this discovery with a pinch of salt until we find a few more specimens of Catorhynchus. Despite this uncertainty, we can be fairly sure that Catorhynchus may have been amphibious. Yeah, that’s right, amphibious and NOT an amphibian. Other articles have said that Catorhynchus is an amphibian, this is incorrect, as Amphibia form their only little group of organisms, which ichthyosauromorphs aren’t part of! However, Catorhynchus is amphibious, i.e. it shares its time between land and water. How do we know this? Well from the fossil, Motani observed that the carpus may have allowed the flipper to bend in a way much like the flippers can bend in seals, and since seals have flippers for limited terrestrial locomotion, it seems likely that this was also the case for the flippers of Catorhynchus. Motani also presents a case for suction feeding in Catorhynchus, which brings the contentious debate of whether other ichthyosaurs fed via suction feeding back to the table.

To summarise Motani and friends have presented the world with a new ichthyosauromorph which, if verified with further specimens, will help us to really start to understand how ichthyosaurs (and perhaps marine reptiles more widely) first evolved, as well as to understand the place of ichthyosauromorphs within Diapsida. And since it was published in Nature, it might turn a few heads, perhaps persuading more people to join the very small field of ichthyosaur of palaeobiology. As always let us know what you think, comment below or Tweet us (or indeed, even Facebook us).

Not only was it amphibious, Catorhynchus was also the most miserable of all the ichthyosauromorphs.

Not only was it amphibious, Catorhynchus was also the most miserable of all the ichthyosauromorphs.

What’s New(s): 6/01/2014 incl.; Naked Dinosaurs, Saudi Arabian dinosaurs and Ichthyosaur storms.

Christmas is a time of rest, festive cheer, spending time with loved ones and (probably most importantly) food.       This is seemingly not the case for academics. Firstly, Richard and I have been busy revising for our January finals, so whilst we’ve tried to give you a few juicy morsels over to tide you over the festive season, we’ve not really had chance to bring you the latest news. Coupled with this is that over this year’s festive period there’s been a lot of palaeontology going on. Holiday, what holiday?

blog

PhD Comics, get used to laughing (and crying) along to them as a postgrad.

So here is a What’s News(s) bumper edition, with 5 of the biggest news stories in palaeontology over the festive period. This week, we’ve got Saudi Arabian dinosaurs, naked dinosaurs, ichthyosaurs, body-size trends in evolution and some Hungarian palaeoneurobiology! Since we don’t want to spam you, we might start evolving our What’s New(s) sections so they are weekly, rather than as-and-when the news comes out (unless it’s really cool).

The first Saudi Arabian dinosaurs. Like has been previously stated, new dinosaur finds aren’t rare occurrences. They happen roughly every 1.5 weeks. Big deal right. Wrong (again). Benjamin Kear’s team have discovered a few caudal vertebrae and some teeth from Saudi Arabia, from the Maastrichtian (75 Ma, ish), and have confidently identified the vertebrae to be from a titanosaur, and the teeth to be from an abelisaurid. The confidence of these groupings is the first time that fossils the Arabian peninsula have been able to be classified as dinosaurian without contention. It also stretches the palaeogeographical ranges of titanosaurs and abelisaurids to the northern margin of Gondwana, whilst showing us (with just one find) that dinosaur ecology in this area may have been quite diverse in the mid-late Cretaceous. The papers also open access (over here on PloS One).

saudidinos

A-C: vertebra of a titanosaur from Saudi Arabia; D-F: tooth of a abelisaurid (again from Saudi Arabia). From Kear et al. (2013).

Naked dinosaurs a common sight during the Mesozoic. For a pretty ‘young’ blog, we’ve already mentioned naked dinosaurs (ooo err!) twice. That says a lot about Richard and I. Moving swiftly on… Since the discovery of the feathered Sinosauropteryx in 1996 (and a plethora of other feathered Chinese dinosaurs since) has caused a bit of frenzy. So much so, that even the Jurassic Park conceded, and created this monstrosity (they’ve now de-conceded, and have yet again ignored feathered dinosaurs). Since 1996, palaeontologists have endeavoured to find just how far back feathers go in the dinosaur lineage. Up until the early 2000s, we thought we had it covered, and that feathers were ancestral to theropods (with discoveries such as Dilong paradoxus, a feathered tyrannosaur sparking fierce debate over whether good old T. rex  had a majestic feathered coat). Yet, as always, it only takes one discovery to turn everything upside down. Pscittacosaurus was that discovery. Pscittacosaurus is a ceratopsian (basal relative to the frilled dinosaur celebrity Triceratops), but with some proto-feathers. Crazy times.

bakker deino

Richard’s favourite naked dinosaur, Deinonychus (which probably wasn’t naked at all).

Paul Barrett then set about to try and solve just where the feathered dinosaur bus stopped. He and his team looked at all of the dinosaur skin impressions found to date, looking for any sign of feathers (or similar structures) and then considered the data is a evolutionary context. He concluded that despite Pscittacosaurus, most ornithischians (ceratopsians, ornithopods, pachycephalosaurs and thyreophorans) and sauropods would have had scales. With the majority of dinosaurian clades having scales rather than feathers, Barrett tentatively concluded (at SVP 2013, in sunny Los Angeles) that scales were probably the ancestral condition in dinosaurs.  But by now we know that all it takes is one feathered dinosaurs from the Triassic (or even the early Jurassic) to upheave this study.

The I(chthyosaur) of the Storm. Quick bit of local (for British palaeontologists  anyhow) news for everyone. After heavy storms (no, seriously, before any Americans/Canadians/anywhere with ‘proper weather’ complain) a 1.5 m long partial ichthyosaur skeleton has been revealed at the base of a cliff in Dorset, and is being restored by the Jurassic Coast Heritage organisation. Three ichthyosaurs have been revealed in similar ways after storms in the past year along the Jurassic Coast. So remember kids, 80mp/h winds and floods aren’t all bad.

Ichthyosaur_hharder

That’s right, icthyosaurs can fly. And then they become storms. True story (Not actually true).

Growing fields: body-size trends throughout the fossil record. Whilst by no means is the study of body-size trends through evolutionary history a new field, but Mark Bell has just published a brilliant, relatively short and Open Access (whoop!) introduction to body-size trends in the fossil record. The article really does make you feel rather small (literally). It also goes through some long established rules on body-size evolution (e.g Cope’s rule), whilst also noting some nice examples of giganticism and dwarfism in the fossil record. Finally, he also states that new computer simulations/software maybe able to help us to further understand these trends in the future.

Bell_gigantism_Figure-1

Where’s Wally, PhyloPic edition. (From Bell 2013 and PhyloPic).

The very Hung-a-ry dinosaur brain. This gem of palaeontological news really does show how fieldwork and digital analysis can produce fantastic results. A new find of a partial skull of Hungarosaurus (from, you guessed it, Hungary) has enabled Hungarian palaeontologists to made a cast of the endocranial cavity, allowing them to analyse the braincase of this European anklyosaur. Initial results suggest that the cerebellum (area of the brain associated with motor control) is larger in volume than other ankylosaurs. This may well mean that Hungarosaurus was better able to run than other anklyosaurs (well known for not being the fastest of starters…).

hungarybrain

Endocast of Hungarosaurus. cbl=cerebellum (roughly circled, from Osi et al. 2013)

References:

Kear BP, Rich TH, Vickers-Rich P, Ali MA, Al-Mufarreh YA, et al. (2013) First Dinosaurs from Saudi Arabia. PLoS ONE 8(12): e84041. doi:10.1371/journal.pone.0084041

Mayr, G., Peters, D. S., Plodowski, G. & Vogel, O. Naturwissenschaften 89, 361–365 (2002)

Zheng, X.-T., You, H.-L., Xu, X. & Dong, Z.-M. Nature 458, 333–336 (2009).

http://www.nature.com/news/feathers-were-the-exception-rather-than-the-rule-for-dinosaurs-1.14379

http://www.bbc.co.uk/news/uk-england-dorset-25548426

http://www.palaeontologyonline.com/articles/2014/trends-body-size-evolution-fossil-record-growing-field/

Ősi, Attila, Pereda Suberbiola, Xabier, and Földes, Tamás. 2014. Partial skull and endocranial cast of the ankylosaurian dinosaur Hungarosaurus from the Late Cretaceous of Hungary: implications for locomotion, Palaeontologia Electronica Vol. 17, Issue 1; 1A; 18p;
palaeo-electronica.org/content/2014/612-skull-of-hungarosaurus

Animals or antediluvian monstrosities?

The famous painting Duria Antiquior, by the Victorian geologist Henry De la Beche, is acknowledged as being the first piece of palaeoart, ie. depiction of prehistoric life based upon fossil evidence.  Because of this it’s palaeontologically important, but it’s also pretty awesome in itself as a picture, with various marine creatures eating one another as pterosaurs swoop overhead, and even a rare depiction of a pooing plesiosaur.  There is in fact so much awesomeness going on that you’d struggle to find room to swing a cat (or whatever the Mesozoic equivalent is-perhaps Pakasuchus?) anywhere in the crowded landscape.  While a great picture, it doesn’t actually do a very good job of illustrating what a Mesozoic seascape would have looked like, instead depicting various monsters doing battle.

Everything looks so happy

The smiley ichthyosaurs make it all look so jolly.

This brings us to the theme of this blog post: the temptation to ‘mythologise’ prehistoric animals and the world in which they lived.  Duria Antiquior was painted in 1830, and obviously palaeontological understanding has come a long way since then.  Equally, the depiction of overcrowded, overdramatised scenes in palaeoart is fair enough.  No-one would be interested in a Mesozoic seascape if it depicted an empty ocean with something that might or might not be the silhouette of an ichthyosaur in the murky distance.  But this popular view of the prehistoric world as a planet populated by antediluvian monstrosities does still sometimes colour the way that people try to understand it.

One of the fundamental tools available to palaeontologists to help them understand extinct animals is information from animals that are alive today.  To understand how a dinosaur’s moved they would look at the principles that govern movement in modern animals, rather than making up special rules for dinosaurs.  Sometimes, however, palaeontologists give in to the temptation to treat prehistoric life specially.

Azhdarchid pterosaurs were a group of large, long-necked pterosaurs from the Cretaceous, including the famous (for a pterosaur anyway) Quetzalcoatlus.  Their shape has led some to suggest that they fed like modern ground hornbills, hunting on the ground with their enormous beaks (see picture).  One argument (among a number) put forward against this hypothesis is that any azhdarchid that landed on the ground to feed during the Cretaceous would be immediately torn apart by voracious theropods.

Quetz

Admittedly hornbills don’t eat sauropods.

But would this actually be the case? Darren Naish (a proponent of the hornbill-esque feeding idea) points out in a recent blog post that it probably wouldn’t be.  Notwithstanding that the size of these pterosaurs offered protection in itself, there’s no reason to think that every inch of the Cretaceous landscape was being constantly monitored by hungry tyrannosaurs.  Taking the modern African savannah as an example; it’s not like every animal that summons up the courage to peek around the side of a baobab tree is instantly ripped to shreds by lions.  To suggest that azhdarchids could never have been safe seems a bit like mythologising the Cretaceous environment and its predators.

It’s not just predators that have been ascribed ‘special rules’.  Amongst ornithodirans (pterosaurs and dinosaurs) are found a amazing array of crests and weird head ornaments (eg. hadrosaurs in picture below), and a number of suggestions have been put forward for why these evolved.  One of the most prominent has been that of ‘interspecific recognition’, where they helped animals to identify mates of the same species.  This hasn’t been conclusively demonstrated to be the reason for ornaments in any animals alive today, but proponents of this idea claim that dinosaurs represent a special case.

hadrosaur_heads_small

I like to think that this is what the album cover for a hadrosaur boy band would look like.

A counter-explanation put forward has been that of mutual sexual selection, where the crests have been selected for (in both genders) to aid attracting a mate (a more in depth discussion of which is found here).  In modern taxa this often seems to be the explanation for such ornaments, and so seems to me to be the more likely hypothesis for those in dinosaurs:  there is no need to invoke ‘special rules’ for extinct animals.  To do so is just another example of (inadvertently) mythologising them and their ecology.

It is true that there are cases where we can’t treat extinct taxa by the same rules as living ones because we have no living analogues to tell us what the rules are.  Enormous bipedal carnivores and giant fully aquatic reptiles are examples.  However, this doesn’t mean we ought to believe that widely applicable principles that we know from modern ecology wouldn’t apply for no reason other than that the animals in question were extinct.  If palaeontologists were studying the function of these animals’ bones they would prefer modern analogues to ‘special rules’, there’s no reason why the same approach shouldn’t be taken to inferring their ecology.

In the two examples I’ve given here, accusing the palaeontologists in question of viewing extinct animals as ‘antediluvian monstrosities’ is an exaggeration.  I do think however that they serve as examples of people applying ‘special rules’ to the ecology of extinct groups just because they’re, well, extinct.  In depictions such as Duria Antiquior such mythologising is both harmless and useful, and sometimes aspects of prehistoric life appear to have no direct modern analogues.  But to view them as anything more than animals in a world governed by the same ‘natural laws’ as those today just gets in the way of understanding these fascinating creatures.

What’s New(s): Unidirectional airflow, a load of hot air?

Whilst being old (last week’s) news, the discovery of unidirectional airflow in monitor lizards is:

  1. Really cool.
  2. Really important.
  3. Allows TDS to explain some key methods/principals used by palaeontologists.

Before we get bogged down in phylogentic-y goodness, let me introduce unidirectional airflow. As humans (and mammals more generally) we suck at breathing. Compared to birds, our respiratory system is pretty inefficient, but that’s fine, we’re not running around at full speed all day. Our breathing system is called tidal breathing, and involves the mixing of ‘new’ air and ‘old air’ in the lungs. This means that there’s not a huge amount of ‘fresh’ air in our lungs, meaning, compared to birds (and the like, more on that later) less oxygen enters the bloodstream per respiratory cycle. However birds have employ a neat trick called unidirectional airflow. Essentially, they’ve got a few more respiratory chambers which allows oxygen to enter the bloodstream during both inhalation and exhalation (by virtue of being a one-way system). This means a birds respiratory system are relatively efficient, which is great for them, as flying’s seriously hard work.

1.14355_MonitorLizard_Skeleton_edit

Unidirectional air flow, shown off excellently by our friend the Savannah monitor lizard.

Up until 2010, unidirectional airflow was only thought to exist in birds. But it has also been found in crocs (Farmer et al. 2010). So another scaly thing with a more efficienty respiratory system than us, big whoop. Yes actually, because with the help of Extant Phylogenetic Bracketing (EPB) we can learn a lot more about the success of diapsids (fancy name for the group containing all the birds, crocs, lizards, tuataras and oh, dinosaurs) during the Mesozoic.

Extant Phylogenetic Bracketing is essentially using common sense to infer anatomy and behaviour in extinct organisms. For something so widely used today, it’s only really been applied in palaeontology since 1995 (Witmer 1995). At it’s core we use our knowledge of what anatomical features (and possibly behaviour) modern organisms have, and put them into an evolutionary context to infer what features their common ancestors might have had. Birds, crocodiles and lizards are an excellent example of how palaeontologists have mastered EPB. So to use UA as an example, if we know birds have it, as do crocs, we can infer that dinosaurs may have had a UA respiratory system, because the last common ancestor of birds, dinosaurs and crocodiles (point A on the diagram below) may have had UA.

UA_phylotree

Very abridged (i.e no pterosaurs, sorry) of the diapsid tree (excluding Mesozoic marine reptiles). A is the last common ancestor of crocs and birds, B is the origin of birds, crocs, dinosaurs and lizards.

Cool story bro, but what about monitor lizards? Ok, so using EPB we’ve inferred that dinosaurs have UA due to their extant relatives both having UA. So if we now factor monitor lizards into the equation, we can now infer that the last common ancestor of birds, dinosaurs, crocs and lizards (i.e diapsids, point B on the diagram above) probably used unidirectional airflow as a respiratory strategy. This means that UA (originally thought to be unique to birds) originated 100 million years before birds evolved. But that’s not all.

1.14355_Image-1_5_SavannahMonitorLizard_CherylErtelt

Just another scaly thing with a more efficient respiratory system than us. All hail the savannah monitor lizard.

Lizards and Birds (and to a certain degree, crocs) are hugely successful organisms. A more efficient respiratory system has certainly aided in their almost global (niche) domination. This may explain why diapsids (dinosaurs in particular) were so diverse and successful during the Mesozoic (and also perhaps why they were so bloody big). So it looks like savannah monitor lizards are a…breath of fresh air.

References

Farmer, C. G. and Sanders, K. (2010). Unidirectional Airflow in the Lungs of Alligators. Science 327, 338–340.

Witmer, L. M. (1995). “The extant phylogenetic bracket and the importance of reconstructing soft tissues in fossils”, in Functional morphology in vertebrate paleontology (ed. J. J. Thomason), pp. 19–33. Cambridge University Press

Schachner, E. R. et al. (2013). Unidirectional pulmonary airflow patters in the savannah monitor lizard. Nature http://dx.doi.org/10.1038/nature12871