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.

Finding a fossil and filling a gap: The story of Lyncina onkastoma Yates, 2009

February saw the release of my second paper on the fossil cowry shells of Australia. This one is potentially more interesting for it deals with some of the oldest fossils of this group in Australia and thus sheds some light (admittedly not too much) on the somewhat mysterious origins of the southern Australian endemics.

The modern Australian cowrie fauna is divisible into two great provinces: Those from the north and those from the south. The tropical northern fauna is just a subset of the tropical indopacific fauna and displays little in the way of endemicity. In the south however we have a range of distinctive clades that are endemic to the region. Each of these clades have been given their own genus name: Umbilia, Zoila, Austrocypraea (now a subgenus of Lyncina) and Notocypraea. Notoluponia is a fifth endemic southern Australian cowrie clade but is unfortunately extinct. Phylogenetic analysis has shown that these lineages are not each others closest relatives amongst cowries but each shares relationships with other non-Australian cowrie groups. When did these lineages arrive in Australia and where did they come from? These apparently simple questions are quite difficult to answer. Firstly the fossil record of cowries in Australia is almost entirely restricted to the last half of the Cenozoic. Until recently the oldest cowries did not appear until right at the end of the Oligocene Epoch (about 23 million years ago) whereas the cowrie elsewhere in the world cowries belonging to modern genera can be found back as far as the Eocene and other cowries go back into the Cretaceous. What is more the oldest cowries Australian endemic cowries were clearly members of the endemic lineages and betray little of their origins. Why is this so? Perhaps cowries enterered southern Australia during the early Oligocene. This represents a ‘black hole’ in our record of molluscs in Southern Australia. We have good molluscan faunas from the late Eocene (about 35 million years old) but virtually nothing in the 12 million years or so between these and the late Oligocene appearances. Nor are there any well-preserved molluscan assemblage that fill this gap. Do we just give up at this point?
Of course not. The glaring gap in our knowledge is the result of various workers almost entirely ignoring molluscs preserved as moulds and casts, in favour of those with the original shell preserved. It is true that an original shell is a much easier object to study than a series of moulds and casts (and can be an object of great natural beauty) but if moulds and casts is all you’ve got, shouldn’t we be looking at them?

Enter the Port Willunga Formation. This is a marine unit exposed on the coast of Fleurieu Peninsula, South Australia that dates from right in the middle of that ‘black hole’ in our knowledge of molluscan faunas. It is too porous to preserve mollusc shells but moulds and casts can be found if you look in the right places. This is one of the right places:


Limestone cliffs and shorecut platform just south of the mouth of the Onkaparinga River. Image used with the kind permission of Glenn Alderson. You can see more of Glenn’s pictures here.

Isn’t it beautiful? The Fleurieu coast is full of wonderful little beaches like this one. Apart from fantastic swimming, snorkeling and diving they also have fossils! Wow, who could ask for more? So when spending time with my family in Adelaide I always try to get down to some of the nearby fossil sites.
Late one afternoon when returning from further afield, my father and I stopped off at this beach (precisely for the reason of seeing if mollusc moulds and casts were preserved in the mid Oligocene rocks that crop out there). While wandering around on the rocks I happened to look down and noticed what appeared to be a cowrie internal mould sitting in its external mould. I got pretty excited straight away for I knew this was amongst the oldest known cowrie fossils found in Australia and might belong to a primitive stem-form of one of our endemic lineages. It may not be in the same league as Tiktaalik but it is nice when you set out to find something in palaeontology and you find it exactly where you were predicting it to be. The image below is actually a little volute from the same site – it gives you an idea of how unprepossessing these fossils are in the field.
Nevertheless if you collect the external mould and carefully chip as much of the apertural impression as you can away from the internal mould and glue it to the external mould, you can then take a pretty decent latex peel. This is what I did for my cowrie and this is the result.


I went back to the site two days later and found a further four specimens although none were quite as good as the first which subsequently became the holotype specimen of Lyncina (Austrocypraea) onkastoma.
It is indeed a very early member of one of our endemic lineages: Austrocypraea which I’ve talked about before on this blog. However it is a rather odd Austrocypraea, most noticeably because its fossula (see primer on cowrie shell anatomy here) is smooth and its apertural teeth are short, weak and confined to the anterior end of the shell. Such features are derived among members of Lyncina but are shared to some extent with L. (A.) archeri, the next oldest known member of L. (Austrocypraea). L. (A.) archeri dates to the earliest Miocene Epoch (about 22 million years old) and would appear to be a close relative of L. (A.) onkastoma. If these two early Austrocypraea form a clade diagnosed by specializations not seen in later Austrocypraea, or indeed any other members of the wider Lyncina clade, then it suggests that some diversification had already gone on by the early Oligocene (the age of L. (A.) onkastoma) and that we can expect to find more cowrie species in the Oligocene of South Australia – if only we take the time to look.

Yates, A.M. (2009) The oldest South Australian cowries (Gastropoda: Cypraeidae) from the Paleogene of the St Vincent Basin. Alcheringa 33, 23-31.

Thankyou to everyone - and happy Australia Day!

Thankyou to everyone who wished us well and has shown support. I'm sure the risks, though scary, are not very likely. Anyway I hope you all have a wonderfull Australia Day. A couple of us expats thought about singing the national anthem at the tea table this morning - but couldn't remember the words.

Picture of the Day: a Temnospondyl

This is a drawing I did for a book that never appeared. A pity, since I don't even have the original, just this bromide. It is one of my better works and portrays the Australian Middle Triassic Paracyclotosaurus davidi, a member of the Mastodonsauridae (but thats another story).

Umbilia gazing - part II

We last left off our survey of Umbilia in the middle Miocene where we looked at U. eximia the most abundant and widespread species.
Two of the remaining four described middle Miocene species are some of the weirdest of all crown-group cowries (I say crown group because there were some truly bizarre looking stem-group cowries, e.g. Gisortia).

U. siphonata (above) is one of these. It is a very large cowry, attaining a length of almost 17 cm, which is not far behind the biggest specimens of Macrocypraea cervus, the largest extant cowry, which used to reach sizes of 19 cm in length. However U. siphonata is cheating a little since the anterior and posterior rostra of this species are produced into great upwardly curving ‘horns’. The rudimentary flanges that support the bases of each rostrum of U. eximia are much better developed in this species. Even stranger is U. gastroplax, the ‘flanged cowry’ (specimen on the left is from Darragh 2002). This species also has elongated horn like rostra, although they are not as long as in U. siphonata. However the flanges have expanded outwards, merging together and making a continuous brim that encircles the entire base of the shell. The result looks much like a snow-shoe. Indeed it has been suggested that this is exactly what its function was and that it was an adaptation to living on soft, ‘soupy’ bottoms. One wonders then if U. siphonata was adapted to the same conditions but simply didn’t bother to keep itself on top of the sediment surface, and used its long rostra to carry its siphons up into clear water.
The fourth middle Miocene Umbilia we will look at is little U. leptorhyncha (below). Although common and widespread, good specimens are rare due to the thin-shelled fragility of this species. This species is a departure from the other mid Miocene species in its small size, globose shape and poorly developed rostra. In these respects it most closely resembles U. prosila and may be closely related to it.

All of these species can be found in both South Australia and Victoria (U. gastroplax has not been officially recorded from South Australia but I have personally collected two specimens from the Cadell Formation on the banks of the River Murray)
Darragh recorded the extant U. hesitata as a fifth middle Miocene species, albeit one that only appears at the end of the stage, with little to no time overlap with the previously mentioned species. The taxonomy of these rare later middle Miocene Umbilia is a complicated issue. Two species have been named Umbilia tatei and Umbilia cera. Both are short , with weakly developed beaks and heavily calluses surrounding the basal margins and probably represent the same species whatever they are. Problematically if these really are small specimens of the extant ‘wonder cowry’ (as U. hesitata is sometimes called) then we have the problem that U. tatei would have priority over U. hesitata. I’m sure all the avid cowry collectors would object to replacing the entrenched U. hesitata with U. tatei. Fortunately I don’t think they have to. Although U. hesitata does display a range of adult sizes which overlaps with the small shells of U. tatei, small modern U. hesitata resemble typical large specimens more than they do U. tatei. In particular no modern U. hesitata has such thick marginal calluses as U. tatei, nor do they develop the elongate coarse dentition seen on the holotype of U. cera (these are not present in the types of U. tatei but the dentition of these specimens appears to be underdeveloped due to immaturity).

A late Miocene U. 'hesitata', probably belonging to U. tatei.

From the late Miocene and Pliocene (there is no Pleistocene record of Umbilia at all) there is just a single species, the extant U. hesitata (although some of these are a little different from modern U. hesitata while others probably belong to U. tatei). Then in our modern seas we find five species: U. hesitata, U. armeniaca, U. capricornica, U. orriettae and U. petillirostris.
However unlike the middle Miocene where you can find up to four species at the same locality almost all of the modern species have mutually exclusive ranges (only U. capricornica and U. petillirostris overlap in the deep Capricorn Channel of the Great Barrier Reef. Moving anticlockwise around the Australian coast we find U. armeniaca (Western Australia to Kangaroo Island, South Australia), U. hesitata (south eastern South Australia to Southern Queensland), U. orriettae (Moreton Bay, Queensland) and U. capricornica/U. petillirostris on the Great Barrier Reef. It is interesting to note that this pattern matches the phylogenetic pattern recovered in a comprehensive molecular phylogenetic analysis of modern cowries (Meyer 2004). In this analysis U. armeniaca was the sister group to all other living species and U. hesitata was the sister group to U. capricornica + U. petillirostris (U. orriettae was not included but morphologically it appears to be intermediate between U. hesitata and U. capricornica). Thus the modern forms appear to be the result of a radiation that proceeded from west to east. All of these living species are rather similar to one another and have a rather generalised shell structure compared to the excesses of the middle Miocene.

Umbilia hesitata, the most abundant extant species.

Most specimens display a moderately well-developed posterior rostrum and have highly reduced anterior tubercles that are separated by an oblique sulcus. These characters suggest that the modern taxa are more closely related to U. eximia and U. tatei than any of the other extinct species. However the type specimens of U. petillirostris stand out as something unusual. Unlike other living Umbilia the types of U. petillirostris are globose and thin-shelled with a very short posterior rostrum. In these respects U. petillirostris resembles the smaller middle Miocene species, U. leptorhyncha and the late Oligocene U. prosila. Darragh (2002) suggested that these three species represented a lineage that had been separate since the Oligocene. But using the molecular phylogeny this would suggest that all of the modern species have been separate since at least the late Oligocene, despite sharing a similar hesitata-like morphology that does not show up in the fossil record until the late Miocene. However I strongly doubt that U. petillirostris is closely related to U. prosila and U. leptorhyncha. Despite its globose shape and thin shell it displays characters typical of the modern clade such as large size, a moderately produced posterior beak, and weak anterior tubercles. A greater sample of specimens shows that U. petillirostris and U. capricornica are quite variable and that individuals of each species can be found that approach the other in morphology. Indeed some have suggested that the two species are not distinct at all (Wilson and Clarkson 2004). Nevertheless limited genetic sampling does indicate that U. petillirostris does maintain a distinct haplotype (Meyer 2004). Excellent photographs of all the living forms can be seen here.

Next week: the origin of Umbilia.

References

Darragh, TA (2002) A revision of the Australian genus Umbilia (Gastropoda: Cypraeidae). Memoirs of the Museum of Victoria 59: 355-392.

Meyer CP (2004) Toward comprehensiveness: increased molecular sampling within Cypraeidae and its phylogenetic implications. Malacologia 46: 127-156.

Wilson B, Clarkson P (2004) Australia's Spectacular Cowries: A Review and Field Study of Two Endemic Genera-Zoila and Umbilia. Odyssey: El Cajon, 369 pp.

The last dicynodont

With so much going on I've had little time for blogging. Recently there was some discussion of the supposed Australian Cretaceous dicynodont (maxillary fragment of the specimen is pictured on the left) over at Chinleana. I'll add my two cents here rather than commenting there just to keep something ticking over on my blog. Randy Irmis made a startling comment that the consensus was that it was indeed a dicynodont. This is news to me, I had always thought that the identity of the specimen was always the weak part of the claim. I have to add that I've never seen the specimen myself. Randy goes on to add that because it was surface float it is the provenance of the specimen that is suspect. Here I have to add my voice in support of Thulborn's original assesment, whatever it was there can be little doubt that it came from the Cretaceous. As has been noted Australia is flat and rather geologically quiescent. The nearest Triassic rocks are many hundreds of kilometres away. Nor do these Triassic rocks have much in the way of dicynodonts in them anyway - just one beat-up quadrate from more than 20 years of intensive collecting in the Arcadia formation (the main fossil-bearing Triassic formation of south-eastern Queensland). When you are out prospecting in most parts of the world you almost never find fossils more than a few tens of metres from the formation that bore them (unless there is a transport mechanism such as a river). Australia certainly never had post-Triassic glaciations that can randomly transport objects over large distances. So if the morphology is definately saying dicynodont then hey, I'm prepare to accept this extraordinary claim. Indeed recently another clade thought to have died out before the end of the Triassic has been found to have survived until the Cretaceous (I can say no more) so survival of the Dicynodonts may not be so weird after all.

Umbilia gazing

I'll do the big reveal on the puzzle fossil tomorrow. For now I want do something I've wanted to do since I started this blog. Post on Cenozoic molluscs. Please stick around they are fascinating - and beautiful as well.
The genus Umbilia is an endemic Australian genus of cypraeid (cowry shell).
Umbilia eximia from the Miocene of Victoria and South Australia.

Cowries are marine gastropods distantly related to periwinkles (littorinids). They are generally predators on sessile invertebrates and have a distinctive shell characterised by determinate growth. As maturity approaches the outer lip reflexes, closing the aperture to a narrow slit and causing the cessation of growth. Umbilia take their name from their countersunk spires, that look like a little belly-buttons. Other features of the genus include large size, the anterior and posterior canals produced into well developed ‘beaks’ (rostra) and a poorly developed to non-existent fossula. To those not steeped in the arcana of cypraeid anatomy, the fossula is a broadened, scooped-out area on the inner wall of the aperture (the columella) at its anterior end. The diagram below should help a little.


A typical cowry shell (Trona stercoraria)showing the major parts of the shell.

In terms of life-history, Umbilia is unusual amongst cypraeids in having direct development. That is to say that they forgo the usual planktonic larval stage, instead hatching directly into benthic crawling snails. This of course severely limits dispersal ability, and may be a reason why the genus has not been able to spread beyond the continental shelf of Australia. Members of this genus have produced a number of remarkable morphologies that are very unusual amongst cowries (a terribly conservative group on the whole) although the extant species are rather boring compared to those of the past. First lets survey this diversity.

The species of Umbilia
Umbilia makes its first appearance in the fossil record in the Late Oligocene of Victoria in south-eastern Australia, specifically at one location, the Bird-Rock Cliffs of Jan Juc Beach (right next door to the famous Bell’s Beach). Two quite different species are found here, indicating that the genus has a deeper, hidden history. Umbilia prosila is one of the Bird Rock species and is the smallest member of the genus, only reaching 39 mm long, with a globular shell and weakly produced rostra.
Umbilia prosila, this and all other specimen photos are from Darragh 2002.

Indeed U. prosila is one of the plainest, simplest members of the genus. Although U. prosila may not display any of the trademark weirdness of the genus, its contemporary U. platyryncha certainly does. At 95 mm is a medium-sized species with its anterior rostrum produced out into a broad, flat spatula-like process. Posteriorly some specimens have no rostrum at all, just a pair of heavy calluses on each side of the posterior canal, while others show the the weakest signs of posterior projection. The aperture bears only sparse, weak denticulations.
Umbilia platyrhyncha

The early Miocene contains but one named species, U. angustior, which is more widespread than its predecessors, being found at a number of localities in Victoria and across Bass Strait in Tasmania as well. It is clearly related to U. platyrhyncha but differs in smaller size, a narrower and less flattened anterior rostrum and a weakly developed posterior rostrum. Some specimens also show a vague pair of tubercles on the dorsal surface of the anterior rostrum.
Umbilia angustior

The species may have extended at least as far west as the Murray Basin of South Australia but the appropriate aged rocks (the Mannum Formation) only contain poorly preserved cypraeid moulds and casts that are presently inadequate for diagnosis. This is a common problem for the Cenozoic marine sediments of South Australia. It seems that here the section is dominated by porous bioclastic calcarenites that have allowed groundwater to flush away the original aragonite that made the shells of cowries and indeed most other molluscs. My pet hypothesis is that this is due to the drier climate of South Australia during the Cenozoic compared to Victoria, so that there were far fewer creeks and rivers dumping terrigenous silt and mud into the sea that would eventually settle out and protect the aragonite shells from the ravages of groundwater.
Like many other molluscan clades, Umbilia radiates drammatically in both diversity and disparity in the middle Miocene. This is when the shallow epicontental seas of southern Australia reached their maximum extent. Five species have been recorded from the middle Miocene and a sixth (described by yours truly) is in press. Commonest of these was U. eximia.
Umbilia eximia

It is a moderately large species, similar in size to the extant U. hesitata. It has a shorter anterior rostrum than either U. platyrhyncha and U. angustior but has a well-developed posterior rostrum that is usually bent toward the body wall. The anterior rostrum bears a strong pair of knob-like tubercles on its dorsal surface. These may indicate that the species is related to U. angustior or may even be a direct descendant of it. Another feature of U. eximia is that it often displays is a set of small basal flanges on each side of the rostra. U. eximia has been found in numerous localities across Victoria and into South Australia. It is a somewhat variable species (perhaps just a function of its larger sample size) and a host of synonyms have been named in the past (U. mccoyi, U. frankstonensis, U. sphaerodoma, U. brevis, U. montismarthae). Thomas Darragh (2002) has examined the holotypes of all of these and found that they differ only slightly (by no more than the normal variation seen in a single sample from a rich site), if at all from the holotype of U. eximia. To me the most intriguing feature of U. eximia is the denticulation of the inner lip. Unlike earlier species which have rather simple weak denticulations along the margins of the aperture, the columellar denticulations become strong, close-set ridges with rectangular cross-sections that extend across at least half the width of the base. The reason that this feature is interesting that another cypraeid genus, Zoila, evolved a sympatric species (Zoila platypyga) that displays the same morphology. Earlier species of both genera have normal to weak dentitions, as do all species of both genera after the middle Miocene. Why? The best hypothesis I can think of is that the middle Miocene of south-eastern Australia was home to a predator that specialised on cypraeids in the 90-100 mm size range (there are both larger and smaller sympatric cypraeids that do not show this modified dentition) and that these highly elongate ridges could have been an adaption to stop propagation of cracks when the shell was placed under stress by the predator trying to break in. What was this predator? I don’t know but some kind of teleost fish or starfish seems to be likely candidate (neither have left a good fossil record in the Miocene of south-eastern Australia). Whatever it was it either went extinct or switched prey and/or tactics by the end of the middle Miocene and the elongated columellar teeth disappeared from both Umbilia and Zoila. More to come later....

References

Darragh, TA (2002) A revision of the Australian genus Umbilia (Gastropoda: Cypraeidae). Memoirs of the Museum of Victoria 59: 355-392.

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.

My dark secret

It is time for me to out a dark secret of mine. Dinosaurs are not my only subject of research these days. Just this year I’ve submitted papers on…….Cenozoic Mollusca. Yes fossil seashells are a passion of mine and have been so for most of my life. And why not? As an avid fossil hunter growing up in Adelaide, South Australia, Cenozoic marine invertebrates was about all that could be collected easily.
Why? The south-eastern corner of Australia was inundated with shallow seas several times during the Cenozoic (the two biggest transgressions happened in the late Eocene and the middle Miocene). The sediments left behind from these transgressions contain a rich record of the animals that lived in them. And what a fauna it was! Riotous diversity seems to be the watchword for the middle Miocene mollusc faunas. And not just high species diversity, morphological disparity seems way in excess of modern groups. Take the collectors favourite Cypraeidae, or cowrie shells as they are commonly known as an example. Although there are hundreds of modern species of these beautiful shells nearly all consist of simple ovoid shells with the smallest reaching no more than 7 mm and the largest 190 mm in length. However in the middle Miocene of south eastern Australia we find a size range that exceeds the modern global range, with the smallest adult sizes being 8 mm while the largest reaches a whopping 220 mm. Furthermore there is a species with its anterior and posterior canals produced into great upwardly curving siphons, another with a rectangular shell shape and yet another that is surrounded by a broad but thin, snowshoe-like flange. All in all I count 21 valid cypraeid species in the mid Miocene of south eastern Australia (compare this to the modern 14 or so species from the entire southern half of Australia). Similar extraordinary diversity can be seen most other molluscan families.
I’ll be returning to the lost Cenozoic seas of southern Australia several more times as my research gets published or as the mood strikes me. For now enjoy this picture of one such deposit from South Australia, the famed Mannum Formation of the River Murray cliffs. There are about 200km of almost continuous outcrop along the Murray. Its where I collected my first fossil – an irregular echinoid, Lovenia forbesi.





Lovenia forbesi