Now is the winter of our fish content

once again the dreaded lurgy has struck our family this winter with Anwen needing a stay in hospital. So this post and the next few to follow were supposed to be posted a month ago.

Teleost fish outnumber all other modern vertebrates two to one. Despite this staggering diversity it is accurate to call fish palaeontology the poor cousin of amniote palaeontology, particularly when it comes to grabbing publicity. Nevertheless with such a huge diversity it is not surprising that the clade has thrown up more than a few subgroups that do grab public attention. What is surprising is that the austral winter’s edition of Journal of Vertebrate Paleontology (volume 29, number 2) carries articles on no less than three of these attention grabbing teleost groups.
One is the giant ocean-going sunfish (Molidae). These giant jellyfish-suckers include the largest living bony fish but due to their poorly developed skeletons and pelagic habits have left a scrappy fossil record. Thus the finding of three articulated skeletons that exceed modern sunfish in size (one skeleton reaches 4 metres from fin tip to fin tip) is a big deal.
Another curious fossil fish reported in this issue is a deep-sea anglerfish (Ceratoidea). These bizarre fish are well known for their disproportionately large mouths lined with needle-like fangs and the ability attract prey with a luminescent lure. Finding a fossil of one of these is also quite unusual for the ceratoid clade is not all that old by geological standards. There simply aren’t that many places where such recent sediments have been laid down at great depth but have since been brought to a position above sea-level where someone might find any fossils that they might contain.
Both of these discoveries are blogworthy finds, however it is the fossil piranha that I want to highlight in this post (Cione et al. 2009). Piranhas are, of course, the fabled freshwater fish of South America that are said to be able to skeletonise a cow in a matter of minutes when a school is whipped up into a feeding frenzy.
The fossil piranha was found late Miocene (about 5 to 11 million years old) river sediments in northeastern Argentina. It has a concave dorsal margin of the premaxilla and tall, sharp triangular teeth that indicate that its affinities lie with the piranhas among the serrasalmids. Aptly named Megapiranha the fish is immediately striking for its great size. Known from a single jaw bone (the premaxilla) and some isolated teeth it is about two and a half times larger than the premaxilla of an average modern piranha. Assuming similar proportions to a modern piranha may have approached a meter in length.

A modern piranha against a silhouette scaled up to fit the size of the Megapiranha premaxilla.

Now the thought of a school of those getting into a feeding frenzy is worthy of any Hollywood B-grade creature feature. However it is far from certain that Megapiranha would have indulged in such hypercarnivorous behavior. It should be noted that piranhas form a clade of closely related species amongst a broader family of fish known as Serrasalmidae. Most serrasalmids (for example pacus) and even some piranhas are vegetarian, indicating that the herbivory is the ancestral diet of serrasalmids. Given the primitive position of Megapiranha (the sister group of all other known piranhas) it is quite likely that Megapiranha was at least partly vegetarian.
What makes Megapiranha interesting, other than its size, is that it gives us some idea how the piranhas evolved their famous dentition. Primitive herbivorous serrasalmids have seven rounded, flat-topped teeth arranged in two rows. In contrast piranhas have a single row of six double-cusped, blade-like teeth. One would expect that the single rowed condition evolved from the double rowed condition by simple suppression of one of the rows, most likely the inner row which contains just two teeth. An alternative, proposed by Gosline (1951) is that the two rows integrated to become one. Megapiranha provides evidence that supports Gosline’s hypothesis, for it shows just a single tooth row but with the teeth placed in a staggered arrangement as if two rows were merging.

The premaxillae of a pacu (top), Megapiranha (middle), and a modern piranha (bottom)in lateral (left) and ventral (right) views. Scale bars equal 1 cm. Images from Cione et al. 2009.

The teeth themselves are intermediate as well for although they bear tall, sharp-edged triangular cusps like modern piranhas, there is no secondary cusp and the bases are broad, perhaps supporting the idea that Megapiranha was not a hypercarnivore. It is a pity really, I find the idea of a giant ground sloth or an astrapothere being stripped to its bones by a school of meter-long piranhas somehow appealing.

References

Cione, A.L., Dahdul, W.M., Lundberg, J.G. & Machado-Allison, A.(2009) Megapiranha paranensis, a new genus and species of Serrasalmidae (Characiformes, Teleostei) from the upper Miocene of Argentina. Journal of Vertebrate Paleontology 29: 350-358.

Gosline, W. (1951) Notes on the Characid fishes of the subfamily Serrasalminae.
Proceedings of the California Academy of Sciences 27: 17–64.

Three New Dinosaurs - at long last, some dinosaury goodness from Australia

Three new Australian dinosaurs, Australovenator wintonensis at the top, Wintonotitan wattsi in the middle and Diamantinasaurus matildae below. Scale bar equals 1 metre. From Hocknull et al. 2009.

What a year for dinosaur research it has been. We’ve had: the publication of a Cretaceous heterodontosaurid with filamentous integument; a slew of new taxa including a member of the perennially popular tyrannosauroids; a toothless, herbivorous ceratosaur !!! with a bizarre hand (which may or may not shed light on the homology of bird fingers). Now that dinosaur depauperate continent , Australia, that produced naught but a handful of decent dinosaur fossils in all the years I lived there, has thrown up three new taxa. The open access paper is here.
Aussie geo-folklore would lay the blame on its apparent dinosaurlessness on the geological quiescence Australia has experienced since the Mesozoic. With no major mountain building events to thrust up strata of the right age there is very little exposure to search and much of that has just been sitting around since it was deposited getting ever more deeply weathered.
While much of this is true it does not preclude the preservation and excavation of decent dinosaur fossils as this new paper shows. The fossils come from the cattle country of central Queensland. Where the land is as flat as a tack, and almost completely grassed over. Not exactly promising territory for palaeontological exploration. Nevertheless the area is unlerlain by the Winton Formation an sedimentary unit laid down on a floodplain fringing a great epicontinental (‘inland’) sea during the middle part of the Creataceous Period.
So why not dig down to the sediments? That is exactly what the team reporting (Hocknull et al.) these new dinosaurs has done. According to the pHocknull et al. the subcrop of the Winton Formation lay under just 1 m of overlying soil. We’ve cut through thicker piles of overburden in our Elliot Formation, so it is not that the Winton Formation is out of reach. Of course, on such flat soil covered land you are lacking the usual clues like fragments of weathered bone falling downslope to lead you to productive sites, nonetheless chunks of weathered bone in the soil can signal something worthwhile lies below. Furthermore the excellent state of preservation of some of these bones (particularly those of the theropod, Australovenator) show that deep weathering profiles may not be quite the problem they’ve been made out to be.
So what did Hocknull et al. find? Two new sauropods (Wintonotitan wattsi and Diamantinasaurus matildae, both titanosauriforms, and an allosaurid theropod (Australovenator wintonensis). Actually to say that two new sauropods were found is not strictly accurate. The holotype specimen of Wintonotitan had been found decades earlier, described as Austrosaurus sp. (Coombs and Molnar 1981) and even incorporated into a phylogenetic analysis of sauropod relationships (as Austrosaurus, Upchurch et al 2004). The type of Austrosaurus is just a series of beat-up dorsal vertebrae, consisting mostly of the centra alone. It comes from the slightly older Allaru Mudstone and differs slightly from the dorsal vertebra of Wintonotitan, indicating that the two are not synonyms. In anycase the name Austrosaurus is best dropped as a nomen dubium based on inadequate remains. A couple of rather preliminary phylogenetic analysis suggest that both sauropods are somphospondyls (titanosauriforms more closely related to ‘classic’ titanosaurs (exemplified by the armoured Saltasaurus from Argentina) than to Brachiosaurus. This is an unsurprising result as almost all sauropods of this age belong to this group. Wintonotitan was found to be a relatively basal member of the group, outside the Titanosauria proper but certain features such as the plate-like ischium with elongate iliac peduncle, medially shifted deltopectoral crest on the humerus and the eye-shaped pleurocoels in the dorsal vertebrae indicate that Wintonotitan may actually hold more derived position within Titanosauria, like its compatriot Diamantinasaurus.
Anyway it is the carnivore, Australovenator that I want to discuss for the rest of this post. As Hocknull et al. point out the non-avian theropod record from Australia is abysmal so there is little to compare it too. One recently described Australian theropod is NMV P186076, an isolated ulna from the Cretaceous of Dinosaur Cove, Victoria that Smith et al. (2008) found to be closely related to the large tetanuran Megaraptor from Argentina. While Hocknull et al. point out some differences between the ulna of Australovenator and NMV P186076 they don’t make much of the strong similarities that these ulnae display in comparison with other basal tetanuran theropods. These similarities include an hypertrophied, mediolaterally compressed olecranon process and an enlarged, proxiodistally elongated lateral tuberosity, defining a cranial fossa. You can see the similarity in the composite figure I whipped up below.



Ulnae of Australovenator and cf. Megaraptor from Dinosaur Cove. Lateral view on left anterior vie in the middle and proximal (top) view on the right. The larger pale brown bone is Australovenator the smaller dark grey bone is cf. Megaraptor.

A ‘megaraptorid’ identification for Australovenator also gels with its rather large wicked looking thumb claw that is about three times longer than its proximal height. More interesting is the astragalus (main ankle bone) of Australovenator which bears a striking resemblance to another isolated Victorian bone, this time from the Cretaceous deposits near Inverloch, which has been touted as everything from Allosaurus to an abelisaurid. Thus despite the small time gap between the two isolated Victorian theropod bones it is quite possible they came from closely related animals. That three separate occurrences from the middle of the Cretaceous of Australia seem referable to this clade indicates to me that these ‘megaraptorids’ were the dominant large carnivores in the middle Cretaceous of Australia.
‘Megaraptorids’ (if indeed they are a clade) are still rather fragmentarily represented, so the addition of Australovenator is most welcome. It helps pull in some other poorly known theropods into this newly recognized fold. One of these is Fukuiraptor kitadanensis a small possible allosauroid from Japan, that has an astragalus that closely resembles the astragalus from Inverloch and that of Australovenator. Although damaged the ulna also appears to bear an unusually large mediolaterally compressed olecranon process. Chilantaisaurus tashuikouensis is a second possible ‘megaraptorid’ from the Cretaceous of Asia. Last year when Smith et al. published the Dinosaur Cove ulna I suggested that this giant asian theropod was a megaraptorid based on: 1, the enlarged manual ungual 2, an apparent close relationship to spinosaurids found by Rauhut (2003)and 3, a similar position for ‘megaraptorids’ found by Smith et al. (2008). Although the spinosauroid position for ‘megaraptorids’ is looking weaker than their position as a basal radiation of carcharodontosaurid allosauroids, I still think there may be a possibility that Chilantaisaurus is a megaraptorid. Supporting this is the presence of a distal craniomedial ridge of the tibia in Australovenator much like the ridge seen in Chilantaisaurus (admittedly this ridge is also found in Suchomimus and Coelurus, so it is not unique to ‘megaraptorids’). Adding a little strength to this idea is the rather hatchet-like deltopectoral crest of Fukuiraptor which looks like a partially developed version of the strongly hatchet-shaped deltopectoral crest of Chilantaisaurus.
In conclusion the ‘megaraptorids’ might be a cosmopolitan clade of Cretaceous allosauroids that share a hatchet-shaped deltopectoral crest, an unusual ulna morphology with an enlarged, blade-shaped olecranon process, an enlarged thumb claw, a femoral head that is not as strongly elevated as other known carcharodontosaurids, a medial ridge on the distal tibia and a distinctive astragalus with a square shaped ascending process. If this clade really exists we can expect earlier representatives to extend back into the Jurassic to allow them to have achieved their near cosmospolitan distribution. Time, new fossils and further analysis may tell.

References.

Coombs WP Jr., Molnar RE (1981) Sauropoda (Reptilia, Saurischia) from the Cretaceous of Queensland. Memoirs of the Queensland Museum 20: 351-373.

Hocknull, S., White, M., Tischler, T., Cook, A., Calleja, N., Sloan, T., & Elliott, D. (2009). New Mid-Cretaceous (Latest Albian) Dinosaurs from Winton, Queensland, Australia PLoS ONE, 4 (7) DOI: 10.1371/journal.pone.0006190

Rauhut OWM (2003) The interrelationships and evolution of basal theropod dinosaurs. Special Papers in Palaeontology 69: 1-213.

Smith ND, Makovicky PJ, Agnolin FL, Ezcurra MD, Pais DF and Salisbury SW (2008) A Megaraptor -like theropod (Dinosauria: Tetanurae) in Australia: support for faunal exchange across eastern and western Gondwana in the Mid-Cretaceous. Proceedings of the Royal Society B: doi:10.1098/rspb.2008.0504

Upchurch P, Barrett PM and Dodson P (2004) Sauropoda.Pp. 259-322. In Weishampel DB, Dodson P and Osmolska H (eds) The Dinosauria. Second Edition.University of California Press: Berkeley.

stegopod!

Photo (Octavio Mateus) and reconstruction (from the paper) of the new stegosaur Miragaia longicollum.

Once again I'm late to the party. Miragaia longicollum is the newly published, long-necked sauropod mimicing stegosaur from the Late Jurassic of Portugal that was featured in loads of blogs yesterday. Attendees of the SVP annual meeting may have actually caught the reconstruction of Miragaia way back in 2007, in Austin. Even though it was only up on screen for a short time, the crazy long neck was immediately obvious, and I've been waiting for it to be published ever since.

Ayway neck elongation in this stegosaur was covered very well by Matt Wedel here. So I won't spend anymore words on it here. Instead I'll delve into the systematics of Miragaia. At the outset let me say that I think the authors have done a great job in this paper and they are thoroughly deserve the publicity they are getting. However this one lttle niggly aspect of the paper has not convinced me and I want to raise the issue here because the fault seems to lie with the whole culture of publication in palaeontology, rather than with these specific authors.

First off Miragaia shares a bunch of characters with the roughly co-eval English stegosaur, Dacenturus. These include fusion of the cervical vertebra to their respective centra, dorsal vertebral centra that are wider than long and olecranon process of the ulna developed into a horn-like prong (you can see this last feature clearly in the photo above). Indeed when the two are included in a cladistic analysis the two form a well supported clade which the Mateus et al. call the Dacenturinae. Another interesting tidbit is that the new data from Miragaia shifts Dacenturus from its usual basal position amongst stegosaurids to a derived position next to Stegosaurus itself. Interestingly though when Mateus showcased this skeleton in 2007 he had identified it AS Dacenturus.
So what makes Miragaia distinct? Obviously its unusually high number of neck vertebrae is a wierd feature (and there are several others listed in the diagnosis) but this cannot be determined in Dacenturus because it is mostly known from the backend of the animal while Miragaia is largely known from the front.

Indeed due to this non-overlapping parts problem just about all of the autapomorphies of Miragaia are not determinable in Dacenturus. So is there a justification for erecting a new taxon? I went through the character taxon matrix in the supplemental material in order to find out if there was any observable differenceand found a few points of difference. All but one f these concerned the continuously variable characters (e.g. ratio of distal width of the humerus to its length)and in most cases the difference was slight so that in more traditional discrete character state coding these features might be given the same state. The one discrete character difference in the matrix concerned the robustness of the dorsal plates but once again the known plates of Miragaia are from the front of the animal while those known from Dacenturus come from further back so we may not be compareing the same thing.

In all there seems justification for at most a new species of Dacenturus (especially since its sister taxon Stegosaurus, houses three species which display more disparity than these two genera). Maybe I'm missing something but f so the authors really could have made the distinction between these two genera clearer.

Does erecting a new taxon make publishing a new fossil easier? It shouldn't, the Miragaia fossils are fantastic enough to deserve publication in the Proceedings. But nonetheless I have heard talk that editors of these high-impact broad science journals are less keen to publish palaeontology if it doesn't involve a new taxon. If this is so, and I hope it isn't, then it is a practice that has to stop, an important fossil that throws new light on evolution or palaeobiology deserves recognition wether or not it represents a new taxon.

Mateus, O., Maidment, S. C. R., Christiansen, N. A. (2009). A new long-necked ‘sauropod-mimic’ stegosaur
and the evolution of the plated dinosaurs Proceedings of the Royal Society B DOI: 10.1098/rspb.2008.1909

Fossil Human Hair

200 000 year old human hair from a hyaena coprolite. Image from Backwell et al. 2009

Over a week ago Lucinda Backwell, who also works at the BPI at Wits, announced the discovery of fossilised human hair that exceeds the previous oldest known hair (from a 9000 year old mummy) by about 200 000 years. Indeed it is so old it might not even belong to our own species but might instead belong to H. heidelbergensis.
The story was picked up by some of our local papers but doesn't appear to have generated much interest in the blogosphere, so I thought I'd timidly foray into the world of palaeoanthropology and discuss Backwell et al.'s paper here.
What adds some iterest to the story is where the fossil hair was found: inside a hyaena coprolite from Gladysvale Cave in the 'Cradle of Humankind' , South Africa (practically next door to such famous sites as Sterkfontein and Swartkrans). Coprolites are, of course fossilised faeces.
Does this mean a hyaena attacked and killed a Homo species in the Late Pleistocene of South Africa? Well I'm sure our ancestors and relatives may have on occasion fallen prey to spotted hyaenas and some of the larger extinct forms. However this fossil does not record such an event. The coprolite was part of a latrine buried in situ in Gladysvale Cave,and the details of this latrine, such as its location inside a cave, small size and well circumscribed boundaries, all indicate that it was made by a brown hyaena (Parahyaena brunnea). Brown hyaenas are rather smallish and are not known to kill humans. Far more likely is that this represents scavenging on an already deceased member of our genus.



A brown hyaena


It was the cave setting that allowed the latrine to be dated. The latrine is sandwiched between two flowstones which contain enough Uranium to be used for Uranium-Thorium dating. This dating was done as part of a larger project by Robyn Pickering, one of the brightest students to come through the BPI in recent years.
Sadly the hairs are preserved as casts in carbonate. No trace of organics are left so we won't be getting any molecular data for 200 000 year old hominids just yet.
By itself that is about all that the paper can tell us. Perhaps if more such latrines could be found we could then survey more scats for fossil hair and discover how frequent such scavenging events occured. Discovery of even older hair, might start to reveal systematic variation and we might even be able to hazard some guesses as to what kind of hairs our more remote relatives bore. For instance we might be able to get a handle on when modern style short fine body hair evolved. The Gladysvale deposits cetainly go back much further in time so the potential for finding australopithecine hair is there.
This also serves to remind us that coprolites are unique microenvironments that have unusual preservation potentials. Ever snce the oldest known mammalian hair was found in Paleocene coprolites, I've thought that coprolites offer us the best chance to find out just when our unique mammalian pelage evolved. I have looked through coprolites from a Middle Triassic synapsid bearing site in the hopes of finding non-mammalian hair but so far no luck.

UPDATE: Randy rightly asked how the ID was made. Mammalian hair is not completely diagnostic to low taxonomic categories. It helped that several specimens were found in the coprolite. Any one hyaena scat usually contains the hair from just one sitting so is not likely to be mixed with other species. Thus there was a sample of several hairs to work from. Only human hairs were found to match the range of variation seen in the fossil hairs (using characteristics such as scale margins, scale spacing, hair width etc.). Other primates came close but most non-human primates produce finer hair. So the ID is a probabilistic one, hence the equivocation in the title of the paper.

L BACKWELL, R PICKERING, D BROTHWELL, L BERGER, M WITCOMB, D MARTILL, K PENKMAN, A WILSON (2009). Probable human hair found in a fossil hyaena coprolite from Gladysvale cave, South Africa. Journal of Archaeological Science DOI: 10.1016/j.jas.2009.01.023

Before they were giants, a new fossil from the dawn of the age of dinosaurs

I was shut off from the internet all this morning. When I got back online I find the wonderfull news that a brand new dinosaur from Argentina has been described by Ricardo Martinez and Oscar Alcober. And not just any dinosaur, a basal sauropodomorph, indeed THE basal sauropodomorph. How could I not blog about it?
Called Panphagia protos (meaning 'the first that eats all' - a reference to its basal position and probable omnivory) it hails from the Ischigualasto Formation. The authors give its age as the earliest Carnian but this is actually not likely given the shake up that Triassic dating and stratigraphy has been getting over the last few years. The revised stratigraphy puts the Ischigualasto right at the very end of the Carnian and probably extending into the earliest Norian.
In anycase Panphagia is a much more primitive sauropodomorph than anything else described so far including the equivalent aged Saturnalia from Brazil. It doesn't have reduced skull and the neck is barely elongated relative to the neck of basal theropods (this depends on wether or not you include herrerasaurids and Eoraptor among the theropods).
However it does show a handfull of rather subtle sauropodomorph features including an enlarged external naris (also in Eoraptor), basally constricted tooth crowns (also in some teeth of Eoraptor) and a non-articulating gap between the transverse processes of the first sacral vertebra and the ilium in the pelvis. The authors cite a few other characters but none are overwhelmingly convincing. And this is the point. We are dealing with the very roots of the saurischian dinosaur radiation and the different lineages had not yet changed enough to accrue distictive characters to convincingly diagnose them. Indeed the similarity between Panphagia and the contemporary dinosaur Eoraptor (which some claim is the basalmost theropod) is very strong, a point not lost on the authors. If I read the subtext of the article correctly I think the authors are hinting at the possibility that Eoraptor is also a basal sauropodomorph. I have certainly entertained the idea in the past but unfortunately I've never seen the Eoraptor fossils and published details are frustratingly sparse so I cannot claim to have an informed opinion.
How did Panphagia make a living? It was certainly not a full time herbivore, nonetheless its somewhat leaf-shaped, imbricated teeth are not quite the slashing deadly blades weilded by its herrerasaurid and rauisuchian contemporaries. Martinez and Alcober are clearly in favour of an omnivorous diet and I concur. Looking at the jaw I can easily imagine such a set of teeth catch and slicing up small time prey while also shredding soft nutritious vegetable matter (e.g. new shoots and fleshy reproductive structures).


All purpose teeth of Panphagia from Martinez and Alcober 2009

To sum up, the morphology of Panphagia actually presents no real surprises, its pretty much exactly what I would have suspected the earliest of sauropodomorphs to have looked like (is this a sign that we are on the right track and our phylogenies are a pretty accurate reflection of dinosaur evolution?). The bigger surprise s its age. Since there is a more advanced sauropodomorph of similar age (Saturnalia) I wonder wether the survival of the less specialised Panphagia alongside it might just be hinting that the initial radiation wasn't that much older. Put more simply the initial divergence of theropods from sauropodomorphs, and indeed saurischian dinosaurs from ornithischian dinosaurs may have only occured during the Carnian Stage of the Late Triassic, rather than extending back into the Middle Triassic as is often postulated.

Ricardo N. Martinez, Oscar A. Alcober (2009). A Basal Sauropodomorph (Dinosauria: Saurischia) from the Ischigualasto Formation (Triassic, Carnian) and the Early Evolution of Sauropodomorpha PLoS ONE, 4 (2) DOI: 10.1371/journal.pone.0004397

Another giant from the tropics: Superlucina

As the blogosphere buzzes about Titanoboa I’m going to review another recent paper that hasn’t received the same degree of publicity but describes another tropical giant that is as equally interesting to me. I’m talking about Superlucina: a new generic named coined for an old species ‘Lucina’ megameris named in 1901. Superlucina megameris is a giant bivalve from the Eocene of Jamaica that was revised and interpreted by Taylor and Glover in the latest issue of Palaeontology. Yes that’s right, a paper about the proverbial Eocene clams. This one is for you Mike ;-)



That’s a big cockle. The giant bivalve Superlucina megameris. From Taylor and Glover 2009.

All joking aside, bivalve molluscs have a reputation for being simple, dull filterfeeders with little interest anyone except perhaps those few crazy taxonomists that specialize on them. Even Chris Taylor, who appears to have a boundless love for the systematic s of all biota confessed that it was hard to get excited about bivalves.
It is a tribute Taylor and Glover that they have produced such a fascinating and readable paper on these maligned creatures. However some of this credit has to go to the organisms themselves, which are fascinating once you look below their dull, clammish exterior.
Superlucina megameris belongs to the family Lucinidae which lead an unusual lifestyle. They are chemosymbiotic sulfide miners that inhabit the interface between the oxic and anoxic zones of marine sediments. By using an elongate muscular foot they build a tunnel up to the sediment surface in order to bring down oxygenated water. They also push holes down into the anoxic sediment below to bring up water with dissolved sulfides. These sulfides are oxidized by bacteria held symbiotically in the bivalve’s tissues and provide much of the nutrition that the bivalve requires.
To understand some of the unusual adaptations of S. megameris and the lucinids I first need to give a quick primer in bivalve anatomy. Bivalves are shell-bearing molluscs so that surround their body with a skirt-like fringe of tissue known as the mantle. The mantle secretes the shell, which in bivalves is divided into left and right valves that are joined dorsally along the hinge. The space between the mantle and the body forms a chamber into which the gills (called ctenidia) protrude. To ventilate the gills a water current needs to be drawn into the mantle cavity, passed over the gills and then expelled. To help with these currents many bivalves have evolved inhalant and exhalent siphons (which are modifications of the mantle. The mantle has a series of flap like, medially directed folds that partially enclose the mantle cavity. The inner fold is controlled by pallial retractor muscles which leave a long linear scar on the inner surface of the shell. This line is called the pallial line. At each end of the pallial line are two shell-closing muscles known as adductor muscles (which also leave prominent scars). Ventrally there is a muscular organ known as the foot. That will do for now.



A schematic , grossly simplified, diagram of bivalve anatomy in cross-section. Drawn hurredly by myself last night.

Ok, now to the interesting stuff. First of all lucinids house their symbiotic bacteria in the tissues of the ctenidia which makes breathing a little difficult, especially since the bacteria need to be supplied with anoxic, sulfide-rich water. In order to compensate lucinids use the anterior end of the mantle folds, as respiratory surfaces. In some large lucinids the anterior end of the inner mantle fold is thickened and pleated with complex folds that act as mantle gills. To keep the oxygenated water separate from the anoxic water, the mantle cavity is partially divided. To help with this division the anterior adductor muscle of many lucinids becomes highly elongate and extends posteroventrally, thus creating a channel between it and the mantle gills. Concomitant with this adaptation lucinids lack the posterior inhalant siphon that many bivalves have and take water in at the anterior end of the animal. In S. megameris the elongation of the anterior adductor muscle is more extreme than in other lucinid. A pustulose channel runs between the anterior adductor scar and the pallial line on internal moulds of S. megameris seems to mark the presence of a unique respiratory channel in this species that was longer than in any other lucinid.



An internal mould of S. megameris from Taylor and Glover 2009. Scars from several anatomical features impressed upon the internal surface of the shell are replicated in the mould. I have colourised these: blue – anterior adductor muscle; red – respiratory channel; yellow – pallial line; green posterior adductor muscle.



A reconstruction of the internal anatomy of S. megameris from Taylor and Glover 2009 showing the division of water inflow. Colours follow the figure above.

S. megameris also differs from other lucinids in its great size. It is the largest lucinid known and is one of the largest burrowing bivalves of all time (epifaunal bivalves like giant clams get even larger).
S. megameris inhabited a transitional zone between an open shelf region and a shallow lagoon filled with seagrass meadows. The presence of seagrass is important because it is the decaying grass that provides the sulfide that fuels the bacteria in their bodies. Although the great size of S. megameris is impressive by itself it is all the more impressive when one compares it to modern shallow water lucinids. These rarely reach a height of 10 cm (less than one third the height of S. megameris) while the vast majority range in height from 0.5 cm to 3 cm. Some chemosymbiotic bivalves inhabiting sulfide rich deep-water cold-seeps reach similar impressive sizes (as indeed did some extinct lucinids from cold seep deposits) and it has been suggested that it was the cold-seep environment that allowed for gigantism in chemosymbiotic bivalves. However the paleoenvironment of S. megameris was definitely not a deep-water cold seep. So why did S. megameris get so big? At this stage we do not know.
Lastly it is interesting to consider the relationships of Superlucina. The genus presents no particularly close similarity with any other but seems most closely related to the genera Miltha, Pseudomiltha and Eomiltha. These also tend to have the most elongate anterior adductor muscles and n some cases toothless hinge lines (like Superlucina). This potential clade was diverse in the past but is now reduced to a handful of mostly rare species, apparently being replaced by more advanced lucinids that have broader shorter anterior adductor muscles, instead using a septum to effectively divide the mantle cavity and have highly plicated mantle gills to efficiently extract oxygen. This takeover appears to have occurred rather recently, I have collected sizeable Miltha specimens from the latest Pliocene or earliest Pleistocene of southern Australia where none now live.


JOHN D. TAYLOR and EMILY A. GLOVER (2009). A GIANT LUCINID BIVALVE FROM THE EOCENE OF JAMAICA – SYSTEMATICS, LIFE HABITS AND CHEMOSYMBIOSIS (MOLLUSCA: BIVALVIA: LUCINIDAE) Palaeontology, 52 (1), 95-109 DOI: doi: 10.1111/j.1475-4983.2008.00839.x

Titanoboa and paleophidiothermometry

Yes that's right a new word to describe the measuring of ancient temperatures using fossil snakes. By now most of you will have heard of the Jason Head and colleagues' paper describing Titanoboa, the largest known snake ever. For those that might not have seen it, Titanoboa was a boa that lived 58-60 million years ago in Colombia. Many fossils (mostly vertebrae) deriving from multiple individuals have been found and the largest of these came from a snake close to 13 metres in length and probably weighing in at 1.2 tons. Fossils from eight individuals were found in this size category indicating that we aren't dealing with a single freakily large individual but a species that regularly attained this gigantic size.
What I'm going to discuss however is the second part of the paper, where the authors attempt to estimate the mean annual temperature of Titanoboa's habitat by using maximum snake size. The idea rests on the observation that the size of snakes, as animals that require external sources for their body heat (ectotherms), and cannot maintain a constant internal temperature (poikilotherms), is constrained by the temperature of their environment. Makarieva and colleagues (2005a, b)observed that the largest species of many terrestrial clades of pokilotherms occur in the tropics, while the largest in the temperate realms were smaller and the largest from the polar regions were smaller still. They argued that there is a minimum mass specific metabolic rate below which an organism simply can't function. As mass specific metabolic rate decreases with increasing size there is a maximum size over which the mass-specific metabolic rate is too low. Increasing ambient temperature allows the metabolic rate to be increased and consequently a larger size can be attained.
Head et al. then used the mathematical relationship between temperature change and the change maximum attainable body size to calculate the mean annual palaeotemperature which Titanoboa experienced. Using modern green anacondas as a model for a modern snake that is probably at its maximum acheivable size (which was taken as 7.5 metres) and the temperature data for their habitat, Head et al. calculated that Titanoboa lived in a sweltering tropical forest that averaged somewhere between 31 and 32 degrees Celcius throughout the year (compare that to the 26-27 degrees averaged by modern lowland equatorial forests that anacondas live in).
It is an interesting idea but I see all sorts of problems. Firstly the size of the fossil snake may not be accurately estimated (actually this is the least likely area of inaccuracy - Head et al.'s methods seem pretty solid in this regard). My main problems are that Makariev's formula has not yet been rigorously tested (I'm not even sure HOW you would go about testing it) and most of all I don't know how we can demostrate that modern green anacondas are up against the theoretical maximal size limit for an ambient temperature of 26-27 degrees. The mere existance of exceptional individuals of anacondas and reticulated pythons reaching length close to or even slightly over 10 metres indicates that it is possible for snakes to exceed 7.5 m in the modern tropics without their metabolic rate dropping to fatally low levels. I suspect other ecological pressures (e.g. prey size, predation, availability of suitable cryptic resting places or something else entirely) may be keeping anacondas from reaching their theoretical maximum body size.



Graph from Head et al. 2009 that shows the curve of theoretical maximum size of a boiine snake against mean annual ambient temperature and the predicted temperature derived from the size of Titanoboa.

Despite all these objections, the results are remarkable in that they appear to work. That is a MAT of 32 degrees is entirely plausible, indeed expected for the tropics of the mid Paleocene Epoch. It is well known that CO2 levels were high in the Paleocene, and the world was in a greenhouse phase. During such phases there were no icecaps and we can find fossil evidence for abundant vegetation and animal life at the poles. It used to be thought that the temperature gradients between the equator and the poles were flatter than in modern times and that although the poles were nice and warm the equatorial regions were no hotter than they are now. We now know we were wrong - the tropics were fiercely hot during such times and this fits perfectly with Head's paleophidiothermometry.

REFERENCES

Jason J. Head, Jonathan I. Bloch, Alexander K. Hastings, Jason R. Bourque, Edwin A. Cadena, Fabiany A. Herrera, P. David Polly, Carlos A. Jaramillo (2009). Giant boid snake from the Palaeocene neotropics reveals hotter past equatorial temperatures Nature, 457 (7230), 715-717 DOI: 10.1038/nature07671

Makarieva, A. M., Gorshkov, V. G.&Li, B.-L. (2005)Gigantism, temperature and metabolic rate in terrestrial poikilotherms. Proc. R. Soc. Lond. B 272, 2325–2328.

Makarieva, A. M., Gorshkov, V. G. & Li, B.-L. (2005b)Temperature-associated upper limits to body size in terrestrial poikilotherms. Oikos 111, 425–436.

Wierdo of the Week; Protuberum cabralensis

Image from Reichel et al 2009

Imagine you found this fossil. At first glance you might think it was some kind of ornamented cranial horn but a closer look would reveal that one end bore two articulation facets on a vaguely hockey-stick shaped head. These features identify the bone as the rib of a tetrapod. But the rib of what, exactly? The row of knobs along the dorsal surface is very unusual and does not bear a close resemblance to any other known tetrapod.

These questions were raised when some of these unusual ribs were unearthed at a locality in middle Triassic sediments of southern Brazil by Father Cargnin, a preist with an interest in palaeontology. Since these ribs were isolated in an assemblage of mixed vertebrates, little could be said about their affinities. About all one could surmise is that they belonged to some exceptionally weird tetrapod. The answer came in 1989 when Father Cargnin found a partial articulated skeleton, with a skull, of the beast with the knobbed ribs at a second locality. It turned out to be a traversodontid cynodont, one of a group of plant-eating close mammal relatives that were common across the globe in the Middle and Late Triassic. Ranging in size between something that was roughly rabbit-sized up to something slightly larger but lower slung than a wolf. As advanced cynodonts they had much of the mammalian adaptive toolkit,probably including endothermy and a furry pelt, but lacked the defining dentary-squamosal jaw joint.





A reconstruction of Exaeretodon a typical traversodontid. From Wikipedia

Considering the unusual nature of the ribs, it is actually remarkable how unremarkable the rest of the animal is. The skull is much like that of any other traversodontid (there are some minor differences in skull proportions and the thickness of the skull roof).



The skull of Protuberum, above (taken from Reichel et al. 2009) in comparison to another traversodontid Massetognathus , below (taken from Romer 1967).
Anyway it has taken some time but Father Cargnin’s lumpy traversodontid has finally been published. It was given the name Protuberum cabralensis by Reichel et al. in the most recent issue of Palaeontology. The entire ribcage of Protuberum from the neck to the hips (and the dorsal edges of the hips themselves) was covered in these lumpy excrescences. The lumps are protrusions of the bone itself, not fused on dermal bones as one would expect since no cynodont is known to be armoured with dermal bones. Nonetheless these lumps would have protruded abone the level of the skin during life and were probably covered in tough, leathery or even keratinized skin. It would have made for an unusual looking animal, particularly if these lumps protruded above a layer of fur. The purpose of the knobs seems to have been defensive, perhaps against predators (they shared the earth with big crocodile relatives called rauisuchians ) or perhaps against each other.

Hat tip to Bill at Chinleana for alerting me to this publication.

References

Romer, A.S. (1967) The Chanares (Argentina) Triassic Reptil Fauna. III. Two new gomphodonts, Massetognathus pascuali and M. teruggii. Breviora 264: 1-25.

MÍRIAM REICHEL, CESAR LEANDRO SCHULTZ, MARINA BENTO SOARES (2009). A NEW TRAVERSODONTID CYNODONT (THERAPSIDA, EUCYNODONTIA) FROM THE MIDDLE TRIASSIC SANTA MARIA FORMATION OF RIO GRANDE DO SUL, BRAZIL Palaeontology, 52 (1), 229-250 DOI: 10.1111/j.1475-4983.2008.00824.x

Where do spiny sharks go?

Hi and welcome to fish fortnight on Dracovenator. You may notice a slight change in style here, as a new year's resolution I'm going to try to write for a more general audience. Does this mean I'm dumbing Dracovenator down? No, I'm just going to try to stop assuming a lot of specialist knowledge on a part of my readers, and will throw in some more basic anatomy for the Form and Function students I'll be teaching later this year.
Anyway by pure coincidence a bunch of fish-related items have come up all at once so I'm running a kind of fish-festival over the course of the next two weeks.
The first of these is a new paper in Nature this week by Martin Brazeau, which desribes the braincase and jaws of an acanthodian (popularly called spiny sharks, although they are not sharks, at least not in the conventional sense). Why is this a big deal? I'll get to that, but first some background.
Most modern vertebrate animals have jaws (only lampreys and hagfish do not). Those that have jaws belong to the great clade Gnathostomata (literally 'jaw-mouths'). Modern gnathostomes can be further divided into two clades: The cartilaginous fish (Chondrichthyes) including sharks and rays and the bony vertebrates (Osteichthyes)including everything else from goldfish to humans.
Both groups are monophyletic, that is they include all descendants from a common ancestor, thus neither group was ancestral to the other, both having split from a common jawed ancestor somewhere back in the past (probably in the Late Ordovician or Early Silurian Period).
Now there are some fossil jawed vertebrates that don't fit into either of these two clades. These are traditionally placed into two groups: The placoderms (armoured fish) and the acanthodians (so-called spiny sharks).



Dunkleosteus, a giant placoderm. Painting by Charles R. Knight. From universe-review.ca

How do these groups relate to the surviving Chondrichthyes and Osteichthyes, or in other words, what is the shape of the first part of the evolutionary tree of jawed vertebrates?
That has been a big mystery indeed. Just about every possible arrangement has been suggested. Placoderms are either regarded as the sister group of all other gnathostomes (that is they branched off first and the others all share a more recent common ancestor) or are thought to share a closer relationship with Chondrichthyes than with Osteichthyes. Nevertheless the plates of dermal bone which encase the front end of these fish has been thought to be an evolutionary novelty (synapomorphy in technical parlance) which marks out the placoderms as a monophyletic group.
The acanthodians have proven even more difficult to place, partly because the internal skeletal anatomy of most species has not been preserved (because like hagfish, lampreys and sharks it was cartilaginous). The favoured hypothesis of more recent years was that acanthodians shared a close relationship with Osteichthyes, that is they split from the line leading to bony vertebrates after the placoderms and chondrichthyans had already split and started their own evolutionary journeys. This was the position taken by Phillippe Janvier in his book 'Early Vertebrates' (1996, Oxford University Press)- an excellent book by the way. Once again the group had been thought to form a clade largely on the characteristic of bony spines supporting the leading edge of their paired fins and some details of their scales. Recent discoveries have shown that both of these features are probably general features for early gnathostomes that were lost in the modern groups, rather than evolutionary novelties unique to acanthodians. It should be noted that even back in 1996, Janvier noted that acanthodian monophyly wasn't all that strong and presented the suggestion that some acanthodians might turn out to be more closely related to chondricthyans (what we would call stem group chondrichthyans) while others might be stem group osteichthyans.



A diversity of acanthodians by Stanton Fink. Image from wikipedia-commons.

Enter Brazeau's recent paper. It includes a cladistic analysis of early jawed vertebrate phylogeny (the branching pattern of evolution) that uses individual genera, rather than large monophyletic groups. Lo and Behold, Janvier was right, or at least on the right track. Indeed Brazeau's analysis finds former acanthodians falling out all over the tree. Some are stem-group gnathostomes that branched away before modern gnathostomes split into Chondrichthyes and Osteichthyes, others are stem-group chondrichthyans others are stem-group osteichthyans. In particular the former acanthodian that forms the centrepiece of the paper, Ptomacanthus, is found in some of his trees to be the closest sister group to the gnathostome crown-group (the crown-group is the clade including the extant species and all descendants of their most recent common ancestor - in this case the common ancestor of Chondrichthyes and Osteichthyes). Ptomacanthus is special because unlike most other acanthodians, including all the early, more relevant species, does preserve some of its internal skeleton, namely the braincase and the jaws. The braincase is remarkably primitive, actually looking more like that of a placoderm than a crown-group gnathostome. This unusual combination of characters (externally crown-group like but internally more primitive) helps to break up the acanthodian group and gives this new tree. There are other odd things in this tree, for instance placoderms are not found to be monophyletic either but that is a topic for a later post (and further investigation methinks).

UPDATE: Check out the ever-informative Catalogue of Organisms for a post that slipped past me before for a little bit more about acanthodians.

Martin D. Brazeau (2009). The braincase and jaws of a Devonian ‘acanthodian’ and modern gnathostome origins Nature, 457 (7227), 305-308 DOI: 10.1038/nature07436

A new Beipiaosaurus - beautifull plumage!

We're just under a fortnight into the new year and already the new dino papers are stacking up. The DML brings news of Ceratonykus a newly named alvarezsaur. And PNAS have published a short paper on a stunning new specimen of the therizinosaur Beipiaosaurus. This specimen is much less badly fragmented, than the holotype but sadly still only consists of the front end of the skeleton. It gives us all sorts of new details to mull over. First and foremost are the weird large single filament feathers that line the neck, back and tail (present in the holotype but not the new specimen). This fossil combined with last year's Epidexipteryx is showing us that a diversity of feather forms, now extinct, evolved before the pennaceous feathers that dominate modern birds plumage. In this case the feathers take the form of stiff, single filaments, that are about 2mm in diameter. The authors call them EBFFs (Elongate Broad Filamentous Feathers) but I will simply call them 'quills' and I really wonder whether they had some spiny defensive function. The apparent lack of any modern style pennaceous feather, with a central rachis and rows of barbs on either side, does suggest that therizinosaurs are not so closely related to oviraptorosaurs (which have pennaceous feathers in spades) as once thought. This does back up recent analyses based on skeletal anatomy which have not been returning an oviraptorosaur-therizinosaur clade of late. Furthermore these quills bear more than a pasing resemblance to the structures of the tail of the ceratopsian Psittacosaurus. The authors suggest what I guess a great number of us have wondered: did feathers evolve much earlier in archosaur history than we currently recognise?
Of course the complete skull alone is reason enough to make theropodophiles drool. Intriguingly it appears quite derived (it acyually looks like a little Iguanodon, oops that should be Dollodon skull) which indicates that cranial modification occured earlier than some of the postcranial modifications that therizinosaurs are famous for, e.g. the re-enlarged hallux, or big toe.
Other cool details include the outlines of a throat pouch, a feature that seems to have been reasonably widespread amongst dinosaurs.
2009 is shaping up to be a good year!

X. Xu, X. Zheng, H. You (2009). A new feather type in a nonavian theropod and the early evolution of feathers Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0810055106

Monographs aint dead

A common lament amongst the vertebrate palaeontology community is the trend toward quick, brief publications in high-impact journals with long delayed to non-existant followup with detailed descriptions. The problem is a symptom of today’s ‘publish or perish’ academic climate where the cost of spending a lot of time producing a monograph that will inevitably appear in a low impact journal can actually harm an early career. I am guilty of adding to the problem myself. Five years after my publication of Antetonitrus in the Proceedings of the Royal Society the descriptive osteology is still in preparation (though not because I don’t wish to produce it, it is just that so many other projects have pushed their way to the front of my to do list). Fortunately more organised and focused researchers have not forgotten the value of a thorough descriptive osteology for the good of the science, if not the individual. One of these crossed my desk last week. “Anatomy and phylogenetic relationships of Tazoudasaurus naimi (Dinosauria, Sauropoda) from the late Early Jurassic of Morocco” does exactly what it says on the cover and how sorely was this needed! Tazoudasaurus is a relatively newly named Moroccan sauropod from the Early Jurassic. It is significant because it is rather completely represented non-eusauropod. For those only cursorially interested in dinosaurs eusauropods are the 'classic' sauopods with enormous bodies, long necks, tiny heads with large retracted nostrils, unique tubular, fingerless hands atop elephantine limbs and lightweight vertebrae constructed from thin laminae. In all the construction of the eusauropod 'bauplan' is one of the great transformations in dinosaur, if not vertebrate, evolution equal in my mind to the evolution of birds. Sadly our understanding of this evolution has been severely hampered by the extremely fragmented nature of pre-eusauropod sauropods. The usual standard basal taxon is Vulcanodon which is hopelessly fragmentary as you can see in the diagram below. There is enough of Vulcanodon to show that it is clearly outside Eusauropoda but it tells us nothing about the evolution of dorsal vertebrae (which must really bother these guys), hands or the skull. Help is now at hand: Tazoudasaurus is clearly a close relative of Vulcanodon (the monograph adds new derived character states which shores up the monophyly of Vulcanodontidae) and it preserves some skull bones along with good neck and dorsal vertebrae and a complete articulated hand.
It is apparent that vulcanodontids are actually quite close to Eusauropoda and are distinctly more advanced than the fragmentary rabble of basal sauropods that has been steadily accreting to the base of Sauropoda in recent years (e.g. dinosaurs like Antetonitrus, Gongxianosaurus and Isanosaurus). For instance the dorsal vertebrae have transversely expanded laminar neural spines, unlike the transversely flattened, plate-like affairs seen in the aforementioned trio. The hand is really cool. The palm is not wrapped into the semitubular arrangement seen in eusauropods, and the fifth metacarpal is still quite a bit shorter than the others (as in prosauropods) but the fingers are reduced right down to mere clawless stumps.
Given the high number of synapomorphies linking vulcanodontids and eusauropods over the more 'prosauropody' taxa like Antetonitrus, Allain and Aquesbi go ahead and erect a new higher level taxon: Gravisauria ('the heavy lizards'), an action I support.
The authors also discuss faunal turnover at the end of the Early Jurassic but I want to save that for another post, not least because I had come to identical conclusions and even have the idea in press (where it will probably linger for another year). If you like sauropods get a hold of this paper (no I do not have a pdf).

Allain, R., Aquesbi, N. (2008). Anatomy and phylogenetic relationships of Tazoudasaurus naimi (Dinosauria, Sauropoda) from the late Early Jurassic of Morocco. Geodiversitas, 30(2), 345-424.

Dinosaur supertree- Mark II

Back in the day when I was a postdoc at the University of Bristol I was involved in a project to build the first supertree for non-avian dinosaurs (Pisani et al. 2002). Now our initial efforts have been thoroughly superseded by a new supertree created by a new team, also from Bristol, headed up by Graeme Lloyd. What is a supertree? Basically its a very large phylogenetic tree stitched together from smaller trees made from standard cladistic analyses of a feasible size (known as source trees). This might, at first, sound like a trivial operation but when you have partially overlapping sets of taxa in the source trees it quickly becomes a calculatory nightmare to produce a rigorous consensus of the source trees. About the only feasible solution available then (and probably still now) was Matrix Representation using Parsimony, or MRP for short. In this technique each node in each tree was treated as a character and all of the ingroup members of that node are coded as '1' whereas all outgroup members are coded as '0'. Taxa that don't appear in that particular source tree are given a '?'. In effect the 'winning' signal from each analysis has all competing signals stripped away and then is combined with all the other 'winning' signals from the other source trees. A large matrix is thus compiled and can be analysed using standard parsimony-based techniques. Some of the early practitioners of supertrees got quite carried away with this and thought that by combining all these small sub-signals together a new signal, perhaps more closely approximating the truth than any other, was created. I lampooned this point of view at SVP with the slide: "Supertrees - a marvelous new way of generating new, more inclusive phylogenies without all that tedious mucking about with actual specimens" (Hat tip to the comic genius of Douglas Adams). No, in my view, supertrees should only be used as a consensus technique to form baseline phylogenies for other studies. It is good to see that that is how the supertree is used in the Lloyd et al. paper. So what did they use their supertree for?
The supertree was used to look at the evolution of dinosaur diversity through time. Unsuprisingly they found that raw diversity was strongly affected by sampling bias. Crudely put there are a lot of Late Cretaceous Dinosaurs because there are a lot of late Cretaceous rocks and a lot of man-hours have been spent collecting from them. Perhaps a little more surprising is the find that after a initial burst of adaptive radiation, dinosaur diversification proceeded at a fairly steady pace throughout the Mesozoic and did not appear to change during the KTR (Cretaceous Terrestrial Revolution) where modern ecosystems were forged by the explosive radiation of flowering plants, insects and numerous other modern groups.
David Hone is one of the authors on this new effort and he gives a much more over at archosaur musings

References

Lloyd, G.T., Davis, K.E., Pisani, D., Tarver, J.E., Ruta, M., Sakamoto, M., Hone, D.W., Jennings, R., Benton, M.J. (2008). Dinosaurs and the Cretaceous Terrestrial Revolution. Proceedings of the Royal Society B: Biological Sciences, -1(-1), -1--1. DOI: 10.1098/rspb.2008.0715

Pisani D, Yates AM, Langer M, Benton MJ (2002) A genus-level supertree of the Dinosauria. Proceedings of the Royal Society B 269: 915-921

The most primitive short-tailed bird?

The latest issue of Palaeontology is choc-full of palaeo goodness. I might end up blogging quite a bit of it, then again time may elude me. For now I just want to say a few quick words about the paper by Gao Chunling et al. It describes yet another new bird from the Yixian Formation of Liaoning, China. Called Zhongornis haoae, Gao et al. bestow upon it a relatively high degree of importance because they believe it to be the sister group of all other short-tailed birds, a clade called Pygostylia. The image is from their paper (Gao et al. 2008). As the name would suggest pygostylians are characterised amongst other things by having a reduced number of tail vertebrae where the distal most ones are fused into a single element called the pygostyle. Although Zhongornis has various advanced pygostylian characters such as an elongate strut-like coracoid and a short tail (just 13 caudal vertebrae), it does not have a pygostyle. That is all 13 caudal vertebrae are unfused, separate bones. However, and this is a big however, it is clearly a juvenile specimen (based on small size, lack of fusion of wrist and manual elements and porous, grainy, bone texture). An therein lies the biggest problem with accepting Zhongornis to this privileged position. Other very juvenile early pygostylians also have unfused caudal vertebrae (e.g. ' Liaoxiornis'). Clearly like wrist fusion the formation of the pygostyle did not occur until quite late in ontogeny amongst early pygostylians. Gao et al try to side-step this problem by claiming the specimen was skeletally mature, or close to maturity because it had fledged, ie. it had grown long vaned feathers from its wings (remiges) and tail (retrices). This I think is the biggest fault with the authors' reasoning. Firstly fledging itself is a variable trait with variable timing of onset amongst modern birds (if I am not mistaken the modern mallee fowl, a kind of megapode, are hatched in a fully fledged state). Late fledging can be expected to be tied to altricial lifestyles. Altriciality is a derived trait in modern birds and should not be expected amongst the early pygostylians of the Cretaceous. Indeed the very same Liaxiornis I mentioned above is a very juvenile early pygostylian with well developed remiges. This more or less falsifies the argument of Gao et al.
So what is Zhongornis? Hard to say. It is toothless so there aren't that many contemporary taxa it could be a juvenile of (Bolouchia is a possibility). It has an unique configuration of phalanges in the third finger which could argue for its status as a valid taxon (although I would classify it as Pygostylia incertae sedis, rather than the pygostylian sister group). However I even wonder if it is possible for the digit three to be reduced as the bird matures, much in the same way that the juvenile claws of the hoatzin are lost as it matures.

GAO, C., CHIAPPE, L.M., MENG, Q., OCONNOR, J.K., WANG, X., CHENG, X., LIU, J. (2008). A NEW BASAL LINEAGE OF EARLY CRETACEOUS BIRDS FROM CHINA AND ITS IMPLICATIONS ON THE EVOLUTION OF THE AVIAN TAIL. Palaeontology, 51(4), 775-791. DOI: 10.1111/j.1475-4983.2008.00793.x

There can be only one ........ Diprotodon

Every natural history buff that I know has a set of favourite taxa that captivates a disproportionate amount of their attention. For me there are the dinosaurs (of course), whales, beetles and Banksias amongst others, and the Diprotodontoidea. Diprotodontoids were large quadrupedal browsing marsupials from Australia. First appearing in the Oligocene, they ranged from dog-sized up to white rhino sized and were Australia’s largest herbivores up to their abrupt extinction in the late Pleistocene. Perhaps my interest in these megaherbivores arose because the first mounted skeleton of an extinct vertebrate I ever saw was a diprotodontoid. In fact it was the Diprotodon skeleton that used to stand near the entrance to the galleries of the South Australian Museum (shown on the right). Growing up in what is, after Antarctica, the most dinosaur-poor continent meant that this skeleton was as close to a dinosaur as I was going to get for quite a while. It was sufficiently weird enough for me. With its unusual retracted nose and my poor anatomical knowledge (I was only five or six when I first saw it) I mistakenly imagined the eyes fitting in the narial opening giving it a bizarre otherworldly appearance. Actually it vaguely resembles the giant South American rodents like Hydrochoerus and Josephoartigasia.











Giant Rodent (Josephoartigasia) on the left compared to Diprotodon on the right.

Diprotodon was one of the last diprotodontoids and the largest of all. Its species taxonomy has remained confused due to a flurry of poorly diagnosed species being named during the nineteenth century. Most palaeontologists have recognised a large and a small form, although the species name for these is uncertain (D. optatum is often used for the large form and D. minor is often used for the smaller). If we wish to know how many species perished during the late Pleistocene megafaunal extinctions and what their ecological and geographical distributions were we need to know how many Diprotodon there were and where and when they lived. Enter today’s featured article, a much needed, and long overdue, taxonomic review of Diprotodon by Gilbert Price (2008). Price looked at all type specimens and large collections from all major sites bearing multiple individuals (of these Bacchus Marsh, Darling Downs, Myall Creek and Lake Callabonna are most important). An analysis of adult tooth size indicates a bimodal size distribution at most localities. The exception was Bacchus Marsh where only the small morph was present. A look at the rest of the morphology showed that there was almost no other consistent difference between the two morphs save for the shape of the mandible. The smaller morph tends to have slightly shallower mandibles with a more rounded profile. This is most noticeable in the symphyseal region where the large morph develops a pronounced ‘chin’ with a steep anterior margin (even in juvenile specimens of the large morph). There certainly was other variation, particularly in the upper premolar, but this was inconsistent, with no apparent pattern even within samples from a single horizon at a single locality. Some had well developed parastyles, others not, some of them had a well developed buccal cingula, others not, some of those with parastyles had buccal cingula,while others didn’t and so on. Price concluded that the small premolar was less significant for Diprotodon feeding and so was open to more variation than is usual in other diprotodontian marsupials. The overall conclusion of this study was that since the large and small morphs co-occur at most localities, show no obvious trophic specialisations and are almost identical except for size and mandibular shape then they represent sexual dimorphs of a single widespread species. So why is only one morph represented at Bacchus Marsh? Price speculates that like some other dimorphic mammalian megaherbivores Diprotodon lived in segregated herds. The geology and taphonomy of the Bacchus Marsh site suggests that the Diprotodon (which show an unusual degree of articulation) represent a single mass death assemblage. The fossils of Lake Callabonna represent a chance to test these ideas. At this site numerous animals became mired in the floor of the lake (which obviously dried out intermittently in the Pleistocene – as opposed to its perennially dry state now). It seems that this happened several times over many thousands of years accumulating several spectacular Diprotodon fossils, including a mother preserved with a very young individual between her legs (presumably it was a pouch bound joey). Sadly the field records of the original digs are inadequate to sort out if the large and small morphs were preserved in segregated groups. Even more frustrating the pouch young was separated from its mother and we can no longer associate it with its correct adult skeleton, thus we cannot test the hypothesis that the small morphs were the females. I would say that this provides ample reason to re-open excavations at Lake Callabonna.
Price further notes that if he is correct in his interpretations then Diprotodon was spread virtually continent wide, in all sorts of habitats from woodland to semi-arid saltbush plains. In other words it was an ecological generalist. This poses a little bit of a problem for those who would deny the hand of Homo sapiens in the late Pleistocene megafaunal extinctions of Australia. Severe climatic change may well have caused loss of suitable habitat in the dry center but surely huge swaths of suitable habitat remained around the margins of the continent?
One final note. The holotype of D. optatum (the type species) has the shallow rounded mandibular symphysis of the small morph. If we eventually do decide that the large and small morphs represent two sympatric species, then D. optatum would be the valid name of the small morph (not the large morph it is commonly ascribed to) and the next available name for the large morph would be D. annextans.

References

Price GJ (2008) Taxonomy and palaeobiology of the largest-ever marsupial, Diprotodon Owen, 1838 (Diprotodontidae, Marsupalia). Zoological Journal of the Linnean Society 153: 369-397.