Living fossils are evolving

Charles Darwin coined the term living fossil in On the Origin of Species. He didn’t use it the same way that it has come to be used. He suggested that living fossils are modern species that can be used to link to groups in the same way that fossils can. One of the examples he gave was the platypus, which lactates and lays eggs, which is evidence that mammals and reptiles share a common ancestor. I don't think he meant it to mean an unchanged relict, as some people interpret his words.

Today, a living fossil is a species that retains many features of their fossil ancestor so that it is recognisably closely related. There are some stunning examples of this, such as orb-weaving spiders. In 2011 a 165 million year old spider fossil was described by Seldon et al., which shared so many features with modern Nephila spiders that it was placed within the same genus. Interestingly, I have never heard of web building spiders being referred to as living fossils despite there being amazing conservation of traits in many groups.

The orb-weaving spiders Nephila clavipes (left) and N. jurassica (right) are separated by 165 million years, but placed within the same genus (image of N. clavipes from Wikipedia and N. jurassica from Seldon et al. 2011).

Unfortunately, living fossil has become synonymous with a species, or group of species, displaying no evolutionary change or very slow change. This is completely wrong. Although the conservation of morphology in Nephila is remarkable, there are more than 150 known species in the genus. Clearly there has been evolutionary diversification within the group. Indeed, whenever living fossils are examined in more than superficial detail it becomes difficult to see them as organisms that evolution forgot.

Horseshoe crabs are one of the most iconic living fossils. There are four living species in three genera. They are placed within the subphylum chelicerata, which makes them more closely related to spiders and scorpions than they are to true crabs, which are placed within the subphylum crustacea. Although there are fewer species of horseshoe crabs than Nephila, that fact that there are four species that are all different from fossil species is a strong indication that evolution hasn't stopped for them.

The Atlantic horseshoe crab, Limulus polyphemus, mating (photo Wikipedia)

The general shape of modern horseshoe crabs is strikingly similar to the fossils that date from about 450 million years ago. Close examination, though, shows that parts of their shape, their legs in particular, have changed over time. Briggs et al. 2012 looked at a fossil horseshoe crab from 425 million years ago, which is relatively early in their evolution. They found that modern horseshoe crabs are missing an entire set of limbs that were present in their ancestors.

All modern chelicerates, including living horseshoe crabs, have unbranched limbs; each limb is a single series of segments. Most crustaceans have limbs that branch at the base into two series of segments. Branched limbs, like those in crustaceans, are the ancestral condition and unbranched limbs are thought to have evolved several times among the arthropods. Indeed, Briggs et al. found that the fossil horseshoe crab had branched limbs, which have been lost in their descendents. 

Like horseshoe crabs, tadpole shrimp have a broad semi-circular carapace protecting their heads and are considered living fossils. There are 11 recognised species in two genera, Lepidurus and Triops. The two genera probably diverged about 180 million years ago, but there are fossil tadpole shrimp dating from about 250 million years ago. That's not as long as the really iconic living fossils, like horseshoe crabs and the coelacanths, but it is still an impressive amount of time to retain enough features to be easily recognised as related.

The tadpole shrimp, Lepidurus apus (photo Wikipedia)

The problem with relying on features that preserve in the fossil record is that it underestimates the actual amount of evolutionary change because generally only hard parts are preserved. A recent study of tadpole shrimp highlights this point. Mathers et al. 2013 used genetic analyses to construct the evolutionary relationships among the 11 species of tadpole shrimp. They found that there are actually 38 species and that these species arose relatively recently. This shows that rather than evolutionary stasis, there is likely to be high species turnover in the group.

There are many reasons why some features may be conserved over long periods of time. None of these have to do with natural selection taking a break. In fact, if natural selection did cease we should expect to see features wander under random genetic drift, as has been hypothesised for eyes in cave dwelling animals. Conserved features are much more likely to be the result of developmental constraints or stabilising selection.

References:

Briggs, D., Siveter, D., Siveter, D., Sutton, M., Garwood, R., & Legg, D. (2012). Silurian horseshoe crab illuminates the evolution of arthropod limbs Proceedings of the National Academy of Sciences, 109 (39), 15702-15705 DOI: 10.1073/pnas.1205875109 

Mathers, T., Hammond, R., Jenner, R., Hänfling, B., & Gómez, A. (2013). Multiple global radiations in tadpole shrimps challenge the concept of ‘living fossils’ PeerJ, 1 DOI: 10.7717/peerj.62

Selden, P., Shih, C., & Ren, D. (2011). A golden orb-weaver spider (Araneae: Nephilidae: Nephila) from the Middle Jurassic of China Biology Letters, 7 (5), 775-778 DOI: 10.1098/rsbl.2011.0228

New feathered dinosaur

I've said in the past that the lion's share of the really amazing fossils, in terms of preservation and importance, seem to be coming out of China in the last 10 years or so. But, now an exceptionally well preserved theropod dinosaur has been unearthed in Germany. And it's important too. It's a feathered theropod dinosaur that is only distantly related to the group of theropods that gave rise to the birds.

A photograph of the new feathered dinosaur fossil, Sciurumimus albersdoerferi. The scale bar shows 5 cm, so, as it's name suggests, it's about the size of a squirrel (image taken from the paper).

The researchers gave it the name Sciurumimus albersdoerferi. The genus name means squirrel mimic because the feathers on its tail make it bushy like a squirrel's. The species name is given in honour of Raimund Albersdörfer, who made the specimen available to the researchers for their study.

The fossil is important for a couple of reasons. The first is that it is distantly related to other theropod dinosaurs that we know for sure had feathers. Which pushes the evolution of feathers back closer to the common ancestor of all theropods. Combine this with the evidence that some heterodontosaurs had feathers and the hypothesis that the common ancestor of all dinosaurs was feathered gains some weight. But, the independent evolution of feathers in the theropods and heterodontosaurs is far from refuted.

A phylogenetic tree of theropod dinosaurs. We know feathers are present in group 4 (at the bottom) and now group 8 (in the middle), which takes the evolution of feathers back to at least group 2. This group, known as the Tetanurae ("stiff tails"), includes most theropods (tree taken from the paper).

The other reason this fossil is important is that it is the best preserved fossil megalosauroid thus far discovered. Indeed, it provides the only complete megalosauroid skeleton. Therefore it provides evidence that helps resolve several questions about the evolution of traits in theropods, such as the evolution of bones in the hand.

An interesting side note is that this fossil is of a very young individual, which probably died soon after hatching. Therefore, the bushy tail is almost certainly not a sexually selected character. This pours some cold water on the hypothesis that feathers evolved in dinosaurs primarily as a means to signal mate quality, which was claimed in another recent paper.

Rauhut, O. W. M., Foth, C., Tischlinger, H., & Norell, M. A. (2012). Exceptionally preserved juvenile megalosauroid theropod dinosaur with filamentous integument from the Late Jurassic of Germany Proceedings of the National Academy of Sciences, (Early Edition) DOI: 10.1073/pnas.1203238109

Flatfish eyes: The twice solved mystery

You know it's particularly mysterious when a puzzle stumps Charles Darwin and all the egghead evolutionary biologists that have come since. But, how both eyes of flatfish came to be on the same side of the head was such a bafflingly mysterious puzzle that it needed to be solved twice. By the same person.

The turbot, Psetta maxima (image Wikipedia)

In 2008, Matt Friedman was able to show that the transition to both eyes on the same side of the head was gradual. Now, in 2012, Matt Friedman has done it again and solved the mystery of the flatfish head by demonstrating that the transition to both eyes on the same side of the head was gradual. Or perhaps, in both instances the journalists overcooked the story and tried to make an interesting incremental step in our understanding of the evolution head asymmetry in flatfish into a revolution in understanding.

But, behind every popular science article beat-up of stumped boffins and puzzling riddles, there's usually some interesting science. And that's the case here. 

Flatfish are fascinating creatures. Adults live on the bottom, lying on one side, with both eyes gazing up from the same side of their head. At hatching, though, their larvae look unremarkable in comparison to other fish larvae. Their eyes are on opposite sides of their head and they swim vertically. But, late in their larval development one eye begins to migrate upwards and over the top of the head until it sits near the other eye.

Larval stages of the summer flounder Paralichthys dentatus. Each letter denotes a stage in development and 'early' and 'late' indicate the position withing the stage. The migrating eye is in grey. The migration begins during stage F, with the eye crossing the midline in stage H (image from Martinez & Bolker 2003).

Far from being stumped, several scientists put forward their explanations, including Darwin. Saltationists, such as Goldschmidt, saw it as evidence that some speciation events were the result of large mutations that revolutionised morphology. While others thought that the eye must have gradually migrated, as it does at the end of the larval period.

The evidence seems to have been more strongly in the gradualist camp. And not only because the new synthesis largely killed off the idea of saltation in evolution. It was already known that the more ancestral flatfish groups, such as spiny turbot and flounder, were less asymmetrical and less strongly associated with the bottom than the more derived groups, such as sole. The only thing that was lacking was truly smoking gun evidence.

Three species of flatfish. From top to bottom,  the spiny turbot, Psettodes belcheri, the flounder, Citharus linguatula, and the sole, Achirus klunzingeri. As you move top to bottom, the wandering eye moves further down the head (Pictures from FAO, via FishBase).

Enter Matt Friedman. He found several examples of fossilised flatfish species from two genra that were about 50 million years old. One genus, Amphistium, had been previously described, but had not been placed within the flatfish group. The other genus, Heteronectes, was previously undescribed. They were in the collections of European museums that, like most museums, had a heap of fossils that nobody had really looked at before. 

The two sides of the fossil fish Heteronectes chaneti. Note the eye on the left side (right hand image) is higher than the eye on the right (from Friedman 2012).

The reason that Amphistium had not been placed within the flatfish was that, although the eyes were not in symmetrical positions, the asymmetry was put down to distortion during fosilisation. Friedman was able to show in his 2008 Nature paper that the eye asymmetry was not as a result of distortion that that, therefore, Amphistium and Heteronectes were transitional between the symetrical ancestors and modern flatfish. 

A simplified phylogeny of flatfish showing the progression of eye migration over history. Next to each fish is a diagram of their skull from the left (top), top (middle) and right (bottom). The two rightmost fish are the modern genera Psettodes and Citharus, examples of which are shown above (image modified from Friedman 2008).

Interestingly, Amphistium and Heteronectes were alive at them same time as flatfish with the modern asymmetrical morphology. Which indicates that they aren't the direct ancestors of the modern flatfish and that the origins of flatfish are much older. This, in turn, suggests that the transitional morphology provided some advantages, since it persisted for so long in the presence of more modern eye arrangements.

The fossil flatfish Eobothus that was alive at about the same time as Amphistium and Heteronectes, but, like modern flatfish, had both eyes on the same side of its head (image the Fossil Forum).

How Heteronectes and Amphistium were so successful with one eye pointing at the bottom is not clear. However, extant species provide some clues. The less asymmetrical species spend more time hunting prey away from the bottom, where a downward pointing eye would be more useful. In addition, Friedman speculates in his 2008 paper that like many modern flatfish, Heteronectes and Amphistium may have used their dorsal an anal fins to lift their downwards facing eye into a position where it could be used. But, of course, all this assumes that lying on one side came before eye migration, which is not clear.

The European plaice, Pleuronectes platessa, using its dorsal and anal fins to lift itself off the bottom (image EOL).

Friedman's 2012 paper in the Journal of Vertebrate Paleontology, provides a much more detailed description of the morphology of Heteronectes. Because Heteronectes represents a transitional form, it may also share more characters with the common ancestor. The aim of the paper was, therefore, to use the described characters of Heteronectes to clarify the relationships between the flatfish and other groups of fish.

The analysis suggested that the Latids are the most closely related family of fish. But, Friedman cautions that his analysis was necessarily coarse. Some of the characters identified as uniquely shared by the Latids and Heteronectes may actually be general to a larger group of fish. And, because Friedman didn't examine other flatfish in the study (he must have another paper in the works), the characters identified in Heteronectes may not be shared with other flatfish.


So, two interesting papers. But, although we now know that evolution of the asymmetrical flatfish eye was gradual and, therefore, that transitional flatfish morphologies clearly were not useless, a lot of questions remain. For instance, we can only speculate about the selective pressures that drove eye migration and we don't yet know what the flatfish common ancestor looked like. 


References

Friedman, M. (2008). The evolutionary origin of flatfish asymmetry Nature, 454 (7201), 209-212 DOI: 10.1038/nature07108

Friedman, M. (2012). Osteology of †Heteronectes chaneti (Acanthomorpha, Pleuronectiformes), an Eocene stem flatfish, with a discussion of flatfish sister-group relationships The Journal of Vetebrate Paleontology, 32 (4), 735-756 DOI: 10.1080/02724634.2012.661352

Martinez, G. M. and Bolker, J. A. (2003). Embryonic and Larval Staging of Summer Flounder (Paralichthys dentatus) Journal of Morphology, 255, 162-176

The evolution of living fossils

The term 'living fossil' is a problematic one because its meaning is so frequently misunderstood. The greatest misunderstanding is that a living fossil species has not evolved for tens or even hundreds of millions of years (e.g. anatomically modern coelacanths are know from 409 mya). But, this is completely and utterly wrong. Living fossils are species that are related to, and superficially resemble, other species from the fossil record. But, they can be morphologically distinguished from the fossil species and have almost certainly evolved in ways that don't preserve in stone (e.g. behaviourally, physiologically and immunologically).

Three species of coelacanth. The top two are the fossil species Coelacanthus and Macropoma, while the bottom is the extant specie Latimeria. Note that they are all similar, but easily distinguished.

So, it's a little disappointing when a usually very good science news website perpetuates this misunderstanding by starting a popular science article like this: 

The morphology of coelacanths has not fundamentally changed since the Devonian age, that is, for about 400 million years. Nevertheless, these animals known as living fossils are able to genetically adapt to their environment.

There is nothing at all surprising about populations of living fossils containing enough genetic diversity to adapt to the environment. And there is simply no good reason to assume that their genetic diversity will be any different to any other extant species. 

The paper itself does not make this mistake. The interesting thing about studying the genetic diversity of coelacanths is not because they are living fossils, but because they are considered rare and endangered. A good understanding of genetic diversity within populations and an understanding of gene flow among populations can be very informative for the development of conservation management strategies. And this is the aim of the paper.

The authors obtained genetic material from 71 adult coelacanths from 6 locations across the entire known range of Latimeria chalumnae, the East African coelacanth (L. menadoensis is a second species found off Indonesia). The genetic diversity among the coelacanths was low, as would be expected from their small population sizes. The largest population of 300 - 400 individuals occurs off the Comoros Islands and all other populations appear to derive from it. The greatest genetic diversity, however, was found in populations from Tanzania.

The sites that genetic samples were collected from. The size of the circles indicates the sample size (key at bottom) and the colours indicate the genetic types found in a population (taken from the paper).

The genetic differentiation between the Comoros population and populations in other locations shows that adaptation is still occurring, but that there is unlikely to be much gene flow among populations. Interestingly, and completely unexpectedly, there appears to be two genetically distinct populations occurring at the same locations within the Comoros Islands. It's unclear what factors are driving the differentiation in the two Comoran subpopulations.

The genetic differentiation between the populations along the African coast isn't strong. This suggests that either the populations diverged relatively recently, or that they're evolving slowly. Curiously, the authors argue that their results confirm that the coelacanths are evolving slowly. But, their data can't separate these two hypotheses. Other studies show that the genes which control morphology are evolving slowly (surprise!), but other gene regions are within the evolutionary rates for vertebrates and consistent with rates in sharks, which have similar life histories. It seems more likely, therefore, that the populations have diverged relatively recently, suggesting that populations could be being recolonised from the Comoros Islands after local extinctions. 

So, coelacanths, like other living fossils, are evolving just fine. Their populations off East Africa look to have reasonably good genetic diversity for their population numbers. But, populations will probably need to be managed separately because there is little gene flow among them.

Reference:

Lampert KP, Fricke H, Hissmann K, Schauer J, Blassmann K, Ngatunga BP, & Schartl M (2012). Population divergence in East African coelacanths. Current biology, 22 (11) DOI: 10.1016/j.cub.2012.04.053

The colour of dinosaurs

When I was young, like many young people, I was obsessed with dinosaurs. I felt gyped when I found out that the drawings of dinosaurs in my books weren't based on any real knowledge of dinosaur coloration. The colours used in the dinosaur artwork were just guesses that were extrapolated from the colours of modern reptiles.

Recently, some of the speculation about the colour of dinosaurs has been resolved. In the last decade or so there has been an explosion in the number of dinosaurs that palaeontologists have identified as having feathers (one spectacular example was revealed just last week). In 2008, a paper¹ was published providing details of a technique that could help to determine the colours of fossil bird feathers. Then, in early 2010, another paper² in Nature used the technique to determine what colour dinosaur feathers might have been.

The structures that give mammalian fur and avian feathers some of their colour are called melanosomes. And, it’s the shape and the arrangement of the melanosomes that help determine the colour. Excitingly, the 2008 and 2010 papers showed that these could be preserved in ancient fossil feathers. So, by examining the shape and distribution of the melanosomes in the fossilised feathers palaeontologists could get and idea of what colour the feathers were.

It didn’t take long for people to start speculating about the possible function of coloured feathers. Indeed, one of the authors of the 2010 Nature paper was prepared to speculate when interviewed, that early feathers were for display and only later were they selected for insulation and flight. It was, however, only a minor point in their paper. I am skeptical about their claim that display came first, although I agree that selection for flight came much later.

One of the elements that their 'display-first' hypothesis rests on, is that the protofeathers of Sinosauropteryx (the dinosaur they examined melanosomes in) are only present in the tail and in a crest along the dorsal surface of the body. They argue that such limited coverage of proto-feathers suggests that they had a limited role in thermoregulation. But, neither of these points is strictly true. 

A Sinosauropteryx fossil from the Jehol region of China showing the distribution of well preserved protofeathers along the back and tail (figure taken from Chen, P., Dong, Z., and Zhen, S. (1998)³).

The paper³ that describes the first two specimens of Sinosauropteryx provides evidence that protofeathers were present over most of the body, but were poorly preserved on the animal's sides. Although the protofeathers are most prominent along the back and tail, several small patches were also present elsewhere suggesting a much broader distribution. The paper goes on to argue that the length, density and likely distribution of the protofeathers suggests they would be were most likely for insulation, not display.

Last month a new paper⁴ provided further evidence of colour in fossil feathers, and speculation about the structure of feathers for flight, display or insulation was given new life. This time there was evidence that the feathers of Microraptor, the stunning four-winged dinosaur, were iridescent. Not strongly iridescent like the throat feathers of hummingbirds or the tail feathers of peacocks, but weakly iridescent like the glossy plumage of crows and ravens.

An artists reconstruction of Microraptor showing a possible four-winged flight pose for gliding, and it's long midline tail feathers (taken from Li et al. 2012⁴).

I had been taught that iridescence was all down to the physical structure of the feather, which isn't preserved in fossils, and had nothing to do with melanosome pigmentation, which can be preserved. But, a little reading later and I found that science had moved on! Iridescence requires both the physical structure of the feather and particular arrangements of melanosomes. 

As iridescence increases, black melanosomes become arranged in a more orderly way. But, even a well ordered melanosome layer would appear matte black if it wasn't for the structure of a thin keratin layer over the top. Once an ordered layer of melanosomes forms to a thickness of 150 nanometers, no further changes in the melanosomes affect the strength of iridescence, the rest is down to the keratin layer. So, the physical structure of the feather plays the most important role, but an orderly layering of melanosomes is also required⁵.

The authors of the Microraptor paper were able to infer an iridescent feather colour through two lines of evidence. The shape and the layering of the melanosomes. As far as I am aware, the role that the shape of the melanosome plays in iridescence is unknown. But, in some modern birds the melansomes associated with iridescence have unusual dimensions, and this can be used to distinguish them from other black melanosomes. The authors found that the preserved melanosomes were of the iridescent type and were arranged in a relatively orderly way, strongly suggesting that Microraptor had iridescent plumage.

The authors didn't stop there. The specimen that they had was newly unearthed and provided some interesting additional details about the shape of the tail. It had previously been thought that the tail was quite broad and assisted in flight. But, the new specimen and a re-examination of other specimens, suggested that the tail of Microraptor had two long feathers, or streamers, in the midline right at the back of the tail fan. It also suggested that the tail fan was narrower than previous interpretations.

Two constructions of Microraptor showing different possible gliding positions. Note the size and arrangement of the tail feathers in comparison to the one above (right image is taken from Xu et al  2003⁶, left image is taken from Chaterjee & Templin  2007⁷).

The authors argue that the iridescent feathers and their new interpretation of the tail shape strongly suggested that the tail was more likely to be for display than to assist with aerodynamics. It's true that in many modern birds iridescent feathers and long tail streamers are important in sexual selection. And it would not be surprising if the tail feathers of Microraptor were used for the same purpose. But, I'm not convinced yet.


When traits are sexually selected, the traits are usually much more exaggerated in one of the sexes. Think the tails of peacocks or the bright colours of male guppies. The other sex is more drab or doesn't have the traits at all. In the Microraptor study, the authors examined three different specimens where the tail feathers were well preserved. All three specimens displayed the elongated tail feathers at the end of the tail. 

Closeups of three fossilised Microraptor tails. A and B are previously described fossils, while C is from the newly described fossil. The arrows in A and B point to the elongated midline tail feathers (taken from the supporting online material of Li et al. 2012⁴).

Without a clear demonstration of sexual dimorphism for tail feather length, it's harder to buy the argument that the shape of Microraptor's tail is sexually selected. And unfeathered tails have recently been shown to be important for aerodynamics, so I'm not confident that narrowness of the tail fan rules out an aerodynamic function. The new paper certainly does the sexual selection argument no harm though. And it shows us another amazing fossil from China.

References

1 Vinther, J., Brigs, D. E. G., Prum, R. O., and Saranathan, V. (2008) The colour of fossil feathers. Biology Letters 4, 522 - 525.

2 Zhang, F., Kearns, S. L., Orr, P. J., Benton, M. J., Zhou, Z., Johnson, D., Xu, X., and Wang X. (2010) Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds. Nature 463, 1075 - 1078.

3 Chen, P., Dong, Z., and Zhen, S. (1998) An exceptionally well-preserved theropod dinosaur from the Yixian Formation of China. Nature 391, 147 - 152.

4 Li, Q., Gao, K., Meng, Q., Clarke, J., Shawkey, M., D'Alba, L., Pei, R., Ellison, M., Norell, M., & Vinther, J. (2012). Reconstruction of Microraptor and the Evolution of Iridescent Plumage Science, 335, 1215-1219

5 Maia, R., D'Alba, L., and Shawkey, M. D. (2011) What makes a feather shine? A nanostructural basis for glossy black colours in feathers. Proceedings of the Royal Society B 278, 1973 - 1980.

6 Xu, X., Zhou, Z., Wang, X., Kuang, X., Zhang,. F. and Du, X. (2003) Four-winged dinosaurs from China. Nature 421, 335 - 340.

7 Chatterjee, S. and Templin, R. J. (2007) Biplane wing planform and flight performance of the feathered dinosaur Microraptor gui. Proceeding of the National Academy of Sciences 104(5), 1576 - 1580.

Li, Q., Gao, K., Meng, Q., Clarke, J., Shawkey, M., D'Alba, L., Pei, R., Ellison, M., Norell, M., & Vinther, J. (2012). Reconstruction of Microraptor and the Evolution of Iridescent Plumage Science, 335 (6073), 1215-1219 DOI: 10.1126/science.1213780

Older fourlegs

China seems to have had the lion's share of the cool fossils unearthed in the last 10 years or so. A new   paper¹ on a coelacanth fossil from south China is another example of their phenomenal treasure trove of fossils. It pushes back the origin of anatomically modern coelacanths by 17 million years to 409 million years ago. The previous oldest coelacanth was known from a jaw found in Australia. 

Coelacanths are interesting for many reasons, not least because they are more closely related to us than they are to other fish. The group was though to have gone extinct around the same time as the dinosaurs, 65 million years ago. But, in 1938 an extant representative was found in South Africa (although it had been known by the local fishermen for a while before that). A book about the discovery dubbed the fish 'old fourlegs'. There was more excitement in 1997, when a second extant species of coelacanth was found off Indonesia.

A preserved specimen of the extant coelacanth, Latimeria chalumnae, or old fourlegs.

The modern coelacanths look almost the same as their ancestors did a few hundred million years ago. So, much like the sharks, they represent 'living fossils'. To put their ancient history in a little perspective, about the same time as the anatomically modern coelacanth body plan emerged, our ancestors were probably taking their first steps on land². 


References:
1 Zhu, M., Yu, X., Lu, J., Qiao, T., Zhao, W., and Jia, L. (2012) Earliest known coelacanth skull extends the range of anatomically modern coelacanths to the Early Devonian. Nature Communications 3, doi: 10.1038/ncomms1764


2 Niedz´wiedzki, G., Szrek, P., Narkiewicz, K., Narkiewicz, M., and Ahlberg, P. E. (2010) Tetrapod trackways from the early Middle Devonian period of Poland. Nature 463, 43 - 48.

Anomalocaris had excellent vision

Anomalocaris is a strange and interesting genus. So strange, in fact, that three separate parts of their fossils were once identified as three separate creatures. It took nearly a century from the discovery of the first piece, for Anomalocaris canadensis to be properly identified and unified into single animal¹.

Anomalocaridids, the group in which Anomalocaris belongs, grew to over a meter in length. Although small in comparison to marine animals today, at the time they swam the oceans (see an animation), between 540 and 472 million years ago, they were the biggest predators in the sea by far^1,2.

Anomalocaridids had two segmented tentacle-like appendages that were probably used in hunting, perhaps by stabbing or by grasping prey. Their mouth was circular and had ‘teeth’ that closed like a camera shutter. Some of them may have eaten hard-bodied animals like trilobites, but it is thought that soft-bodied animals were their primary prey.