Tuesday, January 28, 2014

Wildlife Wednesday: It came from the Tanami

This monstrous beetle was brought in to me from Yuendumu, an aboriginal community out in the Tanami Desert of the Northern Territory, about 350 km NW of Alice Springs.

Dorsal view of the specimen. Those scale divisions are centimetres. Photo by Steve Jackson.

It is a female Haploscapanes barbarossa, a large rhinoceros beetle of the subfamily Dynastinae of the family Scarabaeidae.This one measured 52 mm long and weighed 62 grams. Males bear a horn on their head (hence the common name rhinoceros beetle) which they use for fighting each other, presumably over mates and territory.

Photo by Adam Yates

Normally one would expect such a giant insect to be an inhabitant of moist tropical habitats, if not tropical rainforest and jungles. An indeed H. barbarossa is an inhabitant of the wet tropics of Australia, however in an unusual biogeographic twist the range extends not only across the drier parts of the top end but southwards in the Northern Territory, penetrating the arid country of the Tanami.
H. barbarossa has two other claims to fame: it is, according to some sources, the heaviest Australian beetle (I do wonder about this, there are some pretty enormous cerambycids) and it was the first Australian insect to receive a scientific name (by Fabricius in 1775).

A spiracular new look at the breath of early tetrapods

Yes its a terrible title but its the best I could manage. What's it all about? Well some may not know but an interesting paper was published in Nature Communications last week. It concerns the function of spiracles in bichirs. What's a spiracle? Or for that matter what is a bichir ?
OK so time for a little zoology catchup for those readers not immediately familiar with these terms.
A spiracle is a hole in the head of some groups of jawed vertebrates that opens behind the eye and leads to the chamber behind the mouth (we would normally call this the 'throat' but for us zoologists it is specifically called the pharynx). It is an ancient feature that was almost certainly present in the common ancestor of living jawed vertebrates but has been secondarily closed over in the vast majority of living vertebrates (us included). Sharks are probably the most familiar living vertebrates that retain a spiracle (although some sharks lose theirs as well).

A zebra shark with  a close-up of the eye and spiracle. The spiracle is arrowed.

Bichirs are a group of nine species of really weird freshwater fish from Africa that are classified in the family Polypteridae. They are part of the enormous ray-finned fish group that includes such familiar and diverse fish as salmon, seahorses and sunfish (and many more besides). However they are an extremely early branch and are the sister group of all other living ray-finned fish. Among the oddities that bichirs display is the presence of a pair of lungs, and they are well known for supplementing their oxygen intake from their gills with air breathing.

A bichir (Polypterus weeksii). Image from Wikimedia commons.

As you had probably guessed bichirs also retain a spiracle. In this case they open on the dorsal surface of the head, although they are not immediately obvious in pictures because they are covered by flap-like bony valves. The question is now what do bichirs do with their spiracles? It had been suggested along time ago that they breathed air through them but this suggestion became more-or-less forgotten. It took a team of scientists applying all sorts of techniques, such as CT scanning, behavioural cinematography and air pressure measurements to demonstrate conclusively that they do indeed breathe through their spiracles. Indeed the great majority of all breaths are taken through the spiracles This makes a lot of sense, gulping air for a fish  is energetic and immediately draws attention to your location, whereas quietly protruding just the top of your head to take some quiet lungfulls through your spiracles seems like a really good idea.
Enter stem tetrapods. What is a 'stem' tetrapod. 'Stem' is a term used in cladistics classifications in conjunction with the term 'crown'. A crown is a clade consisting of all living members of the clade, the most recent common ancestor of all of those living members, and all descendants of that common ancestor, whether or not they are extinct. For example crown clade tetrapods include the common ancestor of all living mammals, birds, reptiles (not actually a natural group - unless one includes birds) and amphibians and all descendants of that ancestor. By this definition Tyrannosaurus is definitely a crown clade tetrapod even though it is most definitely extinct. However other more ancient 'tetrapods' like Acanthostega are not crown-clade tetrapods because it diverged away from the tetrapod lineage before the origin of the common ancestor of all living forms (as can be determined from its anatomy which retains some decidedly 'fishy' traits not seen in any living tetrapod). This diagram should clarify these points.

So back to those stem tetrapods. The skulls of these guys show a hole (or open notch in more derived taxa that have lost some of the bones that encase the pharynx region of the head) in exactly the same position as the spiracles of the bichirs. It seems very likely that spiracles were retained in the early tetrapod lineage as well. This is all well and good. It is apparent that the early stem tetrapods like Acanthostega were largely aquatic creatures, and that if it weren't for their relationship to later tetrapods would be just regarded as another unusual group of 'fish'.

Skull and partial skeleton of Tiktaalik roseae a rather 'fishy' stem tetrapod. This is one that retained fins, instead of limbs with digits and so would fall on the non-tetrapod side of an arbitrary divide in more conventional classifications. Note the large obvious spiracles.

A model of the skeleton of Acanthostega gunnari a more 'tetrapody' member of the tetrapod stem group. This guy does have limbs with digits instead of fins and so is usually classified as a tetrapod despite retaining such fishy features as an internal gill chamber in adults, a fishy tail fin and, of course, spiracles. Both images from wikimedia commons.

But how far up the tetrapod lineage did spiracles persist? One could argue that no living tetrapod, be it frog, caecilian, snake or whale has a spiracle and therefore that common ancestor of all of these (i.e. the common ancestor of the crown group) similarly lacked a spiracle.
But not so fast! Convergence is a real feature of evolution, and seems to happen particularly easily when one is looking at the loss of characters, in this case the loss of spiracles. Temnospondyls are a group of early tetrapods that appear to be related to modern amphibians (frogs, salamanders and caecilians), and, if so, would be part of the tetrapod crown. I freely admit that this is by no means settled, and their is also evidence that temnospondyls are stem tetraopods outside the tetrapod crown (indeed this is the position I took when I wrote my big paper on temnospondyl relationships some 15 years ago - oh how time flies). However for now I accept that temnspondyls are indeed stem-amphibians with small dissorophoid temnospondyls like Gerobatrachus being particularly close to modern amphibians, in effect they are transitional forms.
Temnospondyls also have dorsally-facing open notches at the back of the skull between the roof of the braincase and the cheek region, exactly the same spot as they occur in Acanthostega. These notches become partly enclosed by outgrowths of the surrounding bones in several different groups of temnospondyls.

Kupferzellia (or Tatrasuchus) wildi, a Triassic, German temnospondyl with almost completely enclosed spiracular notches at the back of the skull. Image from wikimedia commons.

In the past these notches have been called 'otic notches', with 'otic' referring to the ear. It was envisioned that the notch was the frame across which the ear drum was stretched. The idea received some support from the orientation of the stapes in temnospondyls. The stapes is the ear ossicle that attaches to the ear drum and transmits these vibrations to the middle ear (via another two ossicles in the case of mammals like us humans). In temnospondyls the stapes forms a rod that sweeps up and out from the ear region of the braincase towards the otic notch.
However there are a few observations that would hint that the otic notch was not a frame for an ear drum. Firstly the stapes, although rod-like for most of its length, is usually quite large and chunky and not apparently well suited to transmitting delicate sound vibrations. Secondly the end that fits into the ear region of the braincase does not always maintain a mobile joint. In adult Mastodonsaurus the stapes rigidly sutures to the braincase thus immobilising the stapes. Not a good move when a freely mobile ossicle is optimal for transmitting sound. Thirdly, as noted by Warren and Schroeder (1995) in a particularly well-preserved and uncushed temnspondyl skull the tip of the stapes does not actually protrude precisely into the otic notch as one would expect if it was attached to the inner side of an ear drum. Rather the stapes seems to run along side of the chamber below the otic notch so that in life it would have been embedded in the wall of the chamber. Consequently Warren and Schroeder proposed that the stapes formed structural support for a persistent spiracle. Given the similarity between the so-called otic notches of temnspondyls and the probable spiracles of stem tetrapods, I'm inclined to agree.
I would add one more observation in support of persistant spiracles and spiracular breathing in temnospondyls. There is no doubt that the majority of temnospondyls were semiaquatic to fully aquatic. A look at the heads of  modern aquatic tetrapods will all show dorsally migrated nostrils that are often mounted on low prominences so they breathe without having to lift the head out of the water. A look at the position of the nostrils of temnospondyls does not show this feature. In most stereospondyls (the subgroup of temnspondyls apparently particularly well-adapted to an aquatic existence usually have laterally located nostrils that are low down on the snout, usually separated from the gum-line of the upper jaw by a narrow bar of bone. This doesn't make much sense, unless of course they weren't breathing through their nostrils at all but were relying on their appropriately placed spiracles to get a breath of air.

A comparison between a nostril breathing crocodile (left) and a potentially spiracular breathing temnospondyl (Broomistega putterilli) on the right. Arrows indicate the position of the nostrils. Note the poor position of Broomistega's nostrils for breathing while in the water and the spiracular notches at the back of the skull. As an aside the Broomistega skull was rapid prototyped   from CT data gathered from a scan of a specimen embedded deep in a Thrinaxodon burrow cast, no-one has ever actually seen the fossil itself. To be honest this 3D print is probably far better than any attempt to mechanically prepare the skull out of its burrow cast. Photo by Adam Yates.

The up shot of this is that it is quite likely that the most recent common ancestor of all living tetrapods, the direct ancestor of crown group tetrapods, was infact a spiracular breather. Now that is something I don't think anyone has proposed before.


Graham, J.B., Wegner, N.C., Miller, L.A., Jew, C.J., Lai, N.C., Berquist, R.M., Frank, L.R., Long, J.A. 2014. Spiracular air breathing in polypterid fishes and its implications for aerial respiration in stem tetrapods. Nature Communications 5: (online) http://dx.doi.org/10.1038/ncomms4022

Warren, A.A. & Schroeder, N. 1995. Changes in the capitosaur skull with growth: an extension of the growth series of Parotosuchus aliciae (Amphibia, Temnospondyli) with comments on the otic area of capitosaurs. Alcheringa 19: 41-46.  

Wednesday, January 22, 2014

Wildlife Wednesday: Night Tiger!

Yes its another snake, but this one is not from Central Australia. I snapped this last week while on a short holiday to the top end of the Northern Territory: the little town of Mataranka to be precise. It is a 'night tiger', the wonderfully evocative name given to this particular colour variety of the extremely dull-named brown tree snake (Boiga irregularis). We found this guy sitting in the driveway of the Caravan Park we were staying at. A night drive through the town and surroundings turned up two other individuals so I guess they are pretty common in the area.
The species is venomous, though not dangerously so and rear-fanged, nonetheless I decided against picking up this mature individual. In case you weren't aware, this is same species as the introduced treesnake that has been wreaking environmental havoc on the island of Guam.

Tuesday, January 7, 2014

Wildlife Wednesday: Accidental Endorsement

It is the height of summer and we've had some rain, so these guys have been out in their deafening  hundreds.

The Golden Drummer Cicada (Thopha colorata). Photo by Adam Yates.

No prizes for guessing how the nickname 'MacDonald's Bugs' came about.

Monday, January 6, 2014

How Pythons Got Their Grip On Australia. Part 1

The Woma python (Aspidites ramsayi). Image from http://www.free-desktop-backgrounds.net/Animal-reptiles-wallpapers/Snake-wallpapers/Woma-python-snike--Aspidites-ramsayi-.html.

 Following on from the Stimson’s python I featured in my last post, I thought I’d make pythons the subject of my first lengthy scientific post. Pythons are well-known and much-admired snakes (at least amongst those of us not afflicted with an overwhelming fear of snakes). They are medium-sized to gigantic, non-venomous constrictors that lay eggs in contrast to the similar boas, which give birth to active young. Unlike boas, which have their stronghold in the new world, it is Australia and New Guinea that forms the global hotspot of python diversity. Of the nine recognised genera of python, seven occur in Australo-Papua. We also have some of the most disparate pythons in terms of anatomy including the distinctive terrestrial Aspidites species, the relatively tiny Anthill python, Antaresia perthensis and the green-tree pythons (Morelia viridis and M. azureus) which show remarkable convergence with the emerald tree boa (Corallus caninus) of the neotropics.

So an obvious question is why are the pythons in Australo-Papua so diverse? One hypothesis immediately suggests itself: pythons originated in Australia, presumably descended from primitive snakes that inhabited Gondwana and were isolated on Australia when it broke away from Antarctica in the Early Cenozoic (about 45 million years ago). They would have then undergone their initial radiation here, and once Australia had drifted far enough northwards spawned a relatively recent lineage (the genus Python) that dispersed to Asia and Africa by Island hopping through the archipelagos of south-east Asia. We can label this the ‘Out of Australia’ hypothesis.

There is something intuitively appealing about the idea that pythons were dispersing in the opposite direction while Australia was getting the founders of much of the rest of its lizard and snake fauna (front-fanged snakes, skinks, dragons and monitor lizards) from Asia. The ‘Out of Australia’ hypothesis makes a couple of clear, testable predictions. Firstly the fossil record of pythons in Australia should extend back to a time before Australia had drifted into range of terrestrial animals dispersing from Asia. That date would appear to be somewhere close to the Oligo-Miocene boundary (about 23 million years ago) for that is when skinks and dragon lizards make their first appearance in our fossil record. Secondly the pattern of evolutionary relationships within the python family tree should show a basal radiation of Australian lineages with the non-Australian pythons arising from within that radiation.

Prior to the 1990’s the fossil record of snakes earlier than the Pleistocene in Australia was far too scant to determine whether or not the first prediction had been met. So it fell to the study of the evolutionary relationships of living python species to test the second prediction. The first numerical cladistic analysis of python relationships was published by Kluge in 1993. He used morphology and found a result that strongly supported the ‘Out of Australia’ hypothesis. In broad terms he found that the Australian genus Aspidites branched off first, followed by a large paraphyletic array of Australo-Papuan species that have at some time in the past been placed in the genus Liasis, followed by a clade consisting of the Australo-Papuan genus Morelia and the African Australian genus Python.

Cladogram from Kluge (1993) with illustrations and simplified generic classification added. Specifically the genera Antaresia, Leiopython, Liasis, Bothrochilus and Apodora have been collapsed into the paraphyletic 'Liasis group'. Biogeography added above makes it clear that in this analysis the non-Australian genus python had its origin within an Australo-Papuan radiation.

 One of the robust and central findings of this analysis was Aspidites as the sister taxon to all other pythons. Aspidites includes two species: the black-headed python (A. melanocephalus) and the woma (A. ramsayi, see picture at the top) and these are definitely the oddball pair of the python family. They lack a host of features found in other pythons that Kluge interpreted as primitive characters that indicate they lie outside all other pythons. These include a lack of teeth on the premaxilla bone (at the front of the snout), absence of a constricted neck behind the head and a lack of heat-sensing pits along the lower lip.

Heat sensing pits along the lower lip of a Stimson's Python (Antaresia stimsoni). These are technically called infralabial pits. Image by Adam Yates.

Head of the aptly named Black-Headed Python (Aspidites melanocephalus). Note the lack of infrabial pits (although a single rostral pit under the tip of the snout has recently been demonstrated. Photo by John Tann from http://commons.wikimedia.org/wiki/File:Aspidites_melanocephalus_Head.jpg.

 But are these odd features of Aspidites genuinely primitive or do they represent secondary specialisations to a terrestrial burrow-inhabiting lifestyle? We’ll see what molecular data has to say in part 2.

 Kluge, A. 1993. Aspidites and the Phylogeny of Pythonine Snakes. Records of the Australian Museum. Supplement 19: 1-77.