SciBlogs

Archive January 2011

Bird Evolution- From Archaeopteryx to Modern Birds (Part II) Brendan Moyle Jan 30

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The significance of Archaeopteryx is that it is the first fossil in this sequence that is capable of basic flight. With its laterally facing shoulder joint and split propulsion lift wing with asymmetric feathers, basic flight was now possible. The hallux (the 4th digit) also appears reversed which would give a basic perching function [1].

The postcranial skeletal pneumaticity also shows that Archaeopetryx had the modern bird air-sac system. This is not unique to Archaeopteryx and is found in other non-avian theropods (e.g. Majungatholus) [3]

Nonetheless, the long bony tail of Archaeopteryx, the simple shape of its sternum, the fact it still had bony jaws with teeth, lacked a furcula (wishbone) all showed its transitional status [1].

The evolution of birds then refines an improves these flight ability in a more rapid radiation. The thorax deepens (Confuciusornis, Sinornis) until the deep sternal keel appears in the Euornithes birds in the early Cretaceous [1]. This deep sternal keel would make the attachment of large flight muscles possible.

The hallux lengthens so that advanced perching becomes possible (the hallux in Archaeopteryx is too short). The long bony tail evolves into a truncated, stiff pygostyle (Confuciousornis, Sinornis). More specialised feathers appear (the alular) and the clavicles form an elastic furcula (wishbone).

Birds then undergo an explosive radiation in the late Cretaceous giving rise to the Neornithes (or new birds). In effect, bird evolution from the theropods in the late Triassic took 170my to reach the modern birds we see today. Many gradual transitions can be mapped out by following single traits, such as feathers, the digits, the sternum and the tail.

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References

[1] Sereno, P.C. (1999) The Evolution of Dinosaurs, Science 284:5423, pp2137-2147


[2] Hu, D., Hou, L., Zhang, L. and Xu, X. (2009). A pre-Archaeopteryx troodontid theropod from China Nature 461, 640-643

[3] O’Connor, P.M., Claessens, L.P.A.M. (2005). Basic avian pulmonary design and flow-through ventilation of non-avian theropod dinosaurs, Nature, 436, 253-256.

Doubling or Trebling Tiger Numbers? Brendan Moyle Jan 27

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WWF has recently publicized the a paper that argued a trebling of wild tiger numbers was feasible [1]. This reflects disagreement with another (WCS-derived) paper that argued for the 6% solution [2].

The argument from the Walston et al. “6% paper” was that it is extremely difficult and challenging to save wild tigers. This is kind of obvious as for the last 30 years, tiger numbers have steadily and relentlessly declined. Walston et al. proposed that 6% of the former critical habitat of tigers was feasible and defendable perspective. This would necessarily mean abandoning some reserves or areas of potential habitat.

The Wirkramanayake et al. paper [1] is far more ambitious in scope. Nonetheless, in some ways, it differs little from the IUCN intent to double tiger numbers from the early 2000s. At that point we thought there were around 5000 wild tiger sin Asia. There’s now around 3200. Trebling 3200, is close to doubling 5000.

The paper reflects some of the familiar issues in this debate. This is a very traditional approach based on optimising reserve design. Often reserves are not planned according to the best conservation principles. Rather they are typically placed subject to political and economic factors. As a consequence, they’re often too isolated or too small to meet conservation goals.

Nonetheless, much as I’d like to see tiger numbers (and other large cat species) increasing, the paper neglects the threats posed by human-tiger conflicts and poaching. Low densities in some reserves (e.g. Cambodia) aren’t a function of insufficient reserve size, but poaching. The loss of all tigers through poaching in the Indian reserves of Sariska and Panna reflect this.

Sadly, within the whole paper, nothing concrete is offered as a means to combat poaching. Statements such as “tigers must be worth more to local communities alive than dead” are clear on outcome, but not means. Yes, there has been widespread agreement for years that locals obtain too little benefit from tigers while bearing many of the risks and costs. Reserve mismanagement, corruption etc have all contributed to the dissatisfaction locals have towards tiger reserves.

There is a fundamental cost-benefit analysis that needs to be confronted. A poacher in India can earn say, $US1500 for supplying a dead-tiger to smugglers. Earn a few dollars a day with say wildlife tourism, may not be sufficient compensation.

In short, yes, more tigers would be a great conservation outcome. But the approach through traditional reserve design, seems to be turning a blind eye to threats posed from poaching.

[1] Wikramanayake, E., Dinerstein, E., Seidensticker, J., Lumpkin, S., Pandav, B., Shrestha, M., Mishra, H., Ballou, J., Johnsingh, A., Chestin, I., Sunarto, S., Thinley, P., Thapa, K., Jiang, G., Elagupillay, S., Kafley, H., Pradhan, N. M. B., Jigme, K., Teak, S., Cutter, P., Aziz, M. A. and Than, U. , A landscape-based conservation strategy to double the wild tiger population. Conservation Letters, no. doi: 10.1111/j.1755-263X.2010.00162.x

[2] Walston, J., Robinson, J.G. and Bennett, E.L. (2010). Bringing the tiger back from the brink- the 6% solution. PLoS Biol, 8. e1000485. doi: 10.1371/journal.pbio.1000485.

Bird Evolution- From Ceratosaurus to Archaeopteryx (Part I) Brendan Moyle Jan 26

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One of the most fascinating examples of macroevolution is the path by which birds evolved from reptiles. Birds have many specialised adaptations that make flight possible. These include features like feathers (obviously), hollow bones (for reduced weight), a keel on the sternum (to attach big flight muscles to), and a shortened trunk.

Since the discovery of Archaeopteryx in 1861, more and more transitional fossils have been found. We’ve found a lot more different species with discoveries in Western China and Spain. There are now thousands of specimens available mapping out this transition through the geological column.[1]

But while Archaeopteryx is from the late Jurassic we have to go all the way back to the late Triassic almost 85 million years (my) earlier to start this evolution. It begins with a group of bipedal dinosaurs called the Theropods [1].

The first significant changes are that all long bones became hollow and one digit on the foot was lost (birds have 4 digits on their feet, not 5). Ceratosaurus (L Triassic) is an example of this [1].


This Jacana shows the 4 digits on the feet common to modern birds- an adaptation originating from the Late Triassic
The next was the evolution of a rotary wrist. While for theropods this led to a large grasping hand, the rotary wrist would later become crucially important for wing strokes in birds. Allosaurus (L Triassic) is an example of this [1].

By the early Jurassic the Coelurosaurs had evolved two more features would later be useful for flight.

First, the coracoid and sternum (in the chest) became bigger. This gradual enlargement would be necessary to attach the larger and stronger wing-muscles of birds. Second, body feathers make an appearance. Sinosauropteryx is a representative of these changes. These had no flight function but appeared millions of years before Archaeopteryx. Feathers would precede rather than accompany the evolution of flight [1].Sinosauropteryx Fossil
Figure based on image from Wiki Commons

By the middle of the the Jurassic, vaned feathers with differentiation between primaries, secondaries and rectrices were making an appearance. Again, the theropods concerned still had no ability to fly or glide. The maniraptor Caudipteryx is a transition species showing these accumulated adaptations [1].

These animals were still unmistakeably reptilian in appearance with their long bony tails, lengthy trunks and toothed bony jaws.

At this point in the Jurassic we have hollow bones, differentiated feathers and a broadening sternum, but not any ability to glide. None of these adaptations occurred to make flight possible, but rather produced traits that could be co-opted later.Body length is far too long to permit flight, so the next transition fossils show more fundamental changes to the body.

The next theropod fossils in the transitions start to depict both a shortened trunk and a stiffened long tail. The famous Velociraptor shows this transition [1]. The recently discovered Anchiornis huxleyi (Late Jurassic 151-161mya) adds to this transition by including elongated forelimbs and of course, feathers [2].

Anchiornis fossil
Link from National Geographic

It is only then after all of these changes have accumulated, that flight first makes its appearance in the fossil record. This is with Archaeopteryx in the late Jurassic, about 150mya.\

Archaeopteryx Fossil
(Figure based on image from Wiki Commons)

(Part II tomorrow)


References

[1] Sereno, P.C. (1999) The Evolution of Dinosaurs, Science 284:5423, pp2137-2147


[2] Hu, D., Hou, L., Zhang, L. and Xu, X. (2009). A pre-Archaeopteryx troodontid theropod from China Nature 461, 640-643

Back at work… & a tuatara photograph Brendan Moyle Jan 19

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It always seems a bit frantic getting back to work. There’s the working down the list of emails. Dealing to applications and forms that have suddenly got urgent. Hopefully the research will kickoff soon.

In the meantime, my second favourite reptile (crocodiles appeal the most). The tuatara is that lovely survivor of the sphenodonts. And it only survives in New Zealand. One of the cooler thing about it is the perfect diapsid skull. All reptiles and birds have this diapsid skull (2 holes on each side) while mammals have 1 (they’re synapsid). In most modern diapsids the temporal bar has disappeared. Tuataras are so ancient that they still have an intact temporal bar and two completely divided ‘holes’.

Such holes lighten the skull and provide better attachments for muscles.

Sadly, I’ve yet to make it to an offshore island to see tuatara in the wild. That would be rather cool. But in the interim, I got this shot (through glass) at the National Aquarium in Napier.


I used the newish Sony 70-200/2.8 G for this shot and applied a slight ‘haze’ filter effect (which actually reduces haze) in post-processing.

Email test Brendan Moyle Jan 08

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Just testing to see if email from phone works for blog