To bird or not to bird..

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In 2012, the controversial case over whether or not Archaeopteryx lithographica, perhaps the most iconic dinosaur species of all time, was a bird was settled. Apparently. (free pdf) This was an important analysis for two reasons. Firstly, it countered a previous study showing that Archaeopteryx was more closely related to dinosaurs like Velociraptor and Deinonychus, and secondly used advanced, sort of non-traditional methods in palaeontology, called maximum likelihood and Bayesian analysis, to work out its relationships.

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A new feathered dinosaur – worth getting ruffled for?

Two new feathered dinosaur articles appeared in the latest edition of Nature Communications; one on gender identification in a well-known theropod (the meat noshing ones), and the subject of a forthcoming blog post, and another on a new feathered fiend from, surprise surprise, China.

I normally really don’t like writing about theropods, especially of the feathered variety, as it just seems like I’m jumping on the bandwagon that they were awesome and every aspect of them needs extensive media coverage. Ok, yeah, they can be pretty cool. But only, for me, in the context of the larger evolutionary patterns that they can reveal to us, such as the evolution of feathers and flight. Each new fossil doesn’t exactly transform our knowledge of this, but they do help us to refine our theories to a certain extent; whether or not that’s worthy of excessive media coverage and Nature publications, is not my judgement to make (no, it’s not).

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Flying on the wings of dinosaurs

Archaeopteryx lithogaphica is probably the most iconic dinosaur ever. When it was first discovered, it was heralded as the holy grail of palaeontological findings, as it helped to consolidate the evolutionary continuum between theropod dinosaurs and modern birds. What it also represents though, is an example of the evolution of scientific thought through time. Palaeontologists, mechanists and developmental biologists have long puzzled about where exactly this ancient-winged half-bird half-dinosaur fits into the evolutionary tree of life. Perhaps of equal importance, is what it can tell us about the evolution of one of the most extraordinary biological innovations of all time – the development of wings and powered flight.

Mum! MUM!! Mum, are you watching? Watch me fly! MUUUM!!

A new study by a global team of scientists has added yet another page to the complex evolutionary tale of how Archaeopteryx, and it’s not so distant cousin, Anchiornis huxleyi, used their feathers as an adaptation for aerial motion, or flight – whatever you want to call it. The position was long held that the arrangement of feathers in Mesozoic feathered dinosaurs, including birds, was the same as that in modern birds, the Neornithes; a layer of long asymmetrical flight feathers overlain by short covert feathers (it’s the asymmetry of these feathers that generates lift). New imaging techniques used by the team, however, have uncovered hitherto missed out complexities in the arrangements and structures of these feathers in Anchiornis and Archaeopteryx.

Anchiornis on the nosh. (click for larger)

Archaeopteryx and Anchiornis appear to have represented early evolutionary experiments in feather configuration, differing from Neornithines by having multiple layers of feathers. In Archaeopteryx, primary feathers are overlain by dorsal (above) and ventral (below) coverts, and in Anchiornis the configuration is similar, but slightly more primitive by having short, slender, and symmetrical remiges, with adjacent secondary feathers not strongly overlapping. These layers are arranged into four tiers overlying the primary feathers, which decrease in length systematically with each progressive layer.

Right wing of Archaeopteryx. A: Ventral view, B: Line drawing, C: Closeup shot, D: Line drawing of primaries and coverts (click for larger: Copyright Longrich et al)

Wing of Anchiornis, A: Left wing, B: Line drawing, C: Detail of outer wing. D: Line drawing, E: Detail of inner wing, F: Line drawing (click for larger image. Copyright Longrich et al)

The two species show remarkably different styles of preservation. Anchiornis, from 160 million year old deposits in Liaoning, China, has feathers preserved through the remains of organic compounds known as melanosomes. These structures an actually be used to identify the colour that these feathers once were when the animal was alive! Archaeopteryx is entirely different. It was first thought that the infamous feather marks were impressions left in the fine sediment on a single plane, but now it is considered that they represent collapsed moulds, passing through the surrounding matrix in three dimensions. This revelation means that the split in the matrix passes through different layers, so you actually get a transitional cross-section through the different layers of feathers, and has led to an entirely new interpretation of the internal geometry and structure of the feathers.

There has been much debate regarding the flight capability of Archaeopteryx and other early avians. Part of this debate stems from uncertainty regarding the mechanical properties that are influential for modelling flight, such as muscle strength, mass, and thickness of internal feather structures. The macro-structure of feathers, and their precise function tied in with all of this, makes it quite a complex question to pick away at, due to the natural uncertainty tied into such modelling approaches. It is likely that the slender feather shafts exhibited by both Anchiornis and Archaeopteryx were unable to support the mass of the animals during sustained flight. However, the realisation that the feathers are arranged in layers suggests that this compensated for this lack of strength, and may have been sturdy and thick enough to function as airfoils, similar in function to modern birds, but based on a completely different design. Overall, it suggests that both organisms would not have been capable of taking off or flying at low speeds, and that instead the primary function of their wings was in high speed gliding or flapping, which lends support to the highly polarised debate between a ‘trees-down’ or ‘ground-up’ origin of flight, in favour of the former.

But why is all of this important? Nature continues to astound us with the most remarkable inventions beyond our wildest imaginations. It took millions of years for the dinosaurian forelimb to become modified, in terms of bone and muscle configuration and feather structure, to become a highly efficient airfoil adapted for flight and with the ability rapidly change its span, shape and area to exploit a wide variety of flight kinematics and aerodynamic functions. It is this transformation that allowed dinosaurs to take to the skies and dominate them for some 150 million years, still capable of taking our breath away today. However, if we were just to look at modern birds, which predominantly have the same feather configuration, we would have no clue as to how this transition occurred, and it’s only thanks to the fossil record that we can get tantalising glimpses into this. What we see is a departure of a primitive Archaeopteryx-ian arrangement, experimentation with Anchiornis and a seemingly unspecialised suite of different arrangements in other feathered dinosaurs, and less than 20 million years later, the total achievement of the modern configuration within Enantiornithes (first identified in Eoalulavis hoyasi), the earliest of which is Protopteryx fengningensis (131 million years old).

Hypothetical sequence of  early evolution of the avian wing, with Anchiornis (left), early avialae such as Archaeopteryx (central), and pygostylians such as Confuciusornis (right) (click for larger, copyright Longrich et al)

Surprisingly, this format of relatively rapid experimentation and development is not only seen within birds, but also in pterosaurs and bats, both of which also show a punctuated-stasis model, with the final attainment of an ideal morphological design retained once found. Even more bizarrely, this pattern is replicated within modern manmade flight engineering! In the early 20th century, there were rapid advances in the designs of aircraft, followed by a period of much slower progress. So are the processes governing this largely the same? Both must obey the constraints of mechanics and fluid (aero-) dynamics and this actually imposes quite tight limitations on the number of geometrically feasible configurations that allow for flight capability. In both evolution and engineering, once these ideal configurations are found, the process of refinement is much slower, and any significant deviations will be functionally compromising. Weird how that works eh.

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Did Archaeopteryx Really Have Black Plumage?

Not surprisingly, the latest Archaeopteryx study has kicked up quite a stir within the media and scientific realms, considering the iconic status it has attained since discovery some 150 years past. This latest paper by Carney et al. and published in the journal Nature Communications claims to have resolved the plumage colour of a feather possibly from  Archaeopteryx, a pretty neat little addition to the reconstruction of this critical species. They use methods employed during previous studies, namely the morphological or structural analysis of melanin-bearing organelles within feathers called melanosomes to infer that Archaeopteryx possessed an entirely black plumage.

The notorious black feather (Image Copyright: WitmerLab at Ohio University)

There is no doubt that the structures are in fact melanosomes, and no doubt that melanosomes contribute to plumage colour. But the question is, how much do they contribute to the plumage colour..? Well, I don’t know. In fact, no-one knows, at least in extinct avian theropods. The authors don’t discuss this either. It has however been discussed elsewhere in similar studies, albeit only qualitatively and in passing.

One comment is from Zhang et al. 2010: “Melanosomes are lyosome-related organelles of pigment cells in which melanins are stored and are responsible in part for the colours exhibited by modern birds.”

And Li et al. 2010: “Other molecular pigments such as carotenoids and porphyrins also produce plumage colours but are not preserved morphologically, thus we cannot address their possible effects here.”

That seems like a pretty big caveat. It’s like saying if you mix green, yellow and red paint in unknown quantities, you get red every time. Or something similar, I suck at analogies. The point is, if in modern birds, there are other significant structures that dictate or contribute towards plumage colour, does it make sense to try and predict colour when these are absent?

UPDATE: Ryan has been kind enough to clarify this point in a comment below.

Furthermore, the 95% confidence intervals the authors use are practically useless. Look at the ordination provided in Figure 4 (not sure if I can copy it here, so won’t..).  These 95% confidence ellipses mean nothing in the slightest, or at least nothing meaningful. What they should have shown is an envelope includes 95% of all points within a sample, so that when you insert data ‘blind’, if it falls within a completely discriminated envelope, you can be 95% certain that it belongs to that group (i.e., 95 times out of 100, a blind data point will be correct). The envelopes shown in figure 4 clearly do not show this (if you don’t have access to the paper, ask me for a copy, or take my word for it). As a result, the points calculated for the particular Archaeopteryx feather analysed could really be grey or black, or maybe brown at a push (the number of colour choices is simply overwhelming..).

So was Archaeopteryx lithographica black?

Probably, probably not.

Carney et al. (2012) New evidence on the colour and nature of the isolated Archaeopteryx feather, Nature, DOI: 10.1038/ncomms1642

Li et al. (2010) Plumage patterns of an extinct dinosaur, Science, 327, 1369-1372

Negro et al. (2009) Porphyrins and pheomelanins contribute to the reddish juvenal plumage of black-shouldered kites, Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology, 153(3), 296-299

Zhang et al. (2010) Fossilized melanosomes and the colour of Cretaceous dinosaurs and birds, Nature, 463, 1075-1078