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.