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.

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

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

Two new Fossil Cowries

A small paper has just been published on two new fossil cowries from the Miocene of South Australia (Yates 2008). Although it is unlikely to set the palaeontological world on fire it is a personally satisfying paper as it represents my first published foray into a subject area that has actually been close to me for most of my life. As I have mentioned before growing up in South Australia provided next to nothing in the way of actual dinosaur digs or even museum displays of dinosaur bones. If you wanted to get out and dig for your own fossils then the marine limestones and marls of the River Murray cliffs was about the only game in town. Most of these sediments are rather coarse grained calcarenites that unfortunately offered no protection ravages of groundwater which dissolves shells made from aragonite (the form of calcium carbonate that the majority of molluscs use). As a consequence nearly all mollusc fossils are present only as moulds surrounding the void where the shell once was. There is a gleaming exception: a silty marl unit called the Cadell Formation (formerly the Cadell Marl Lens of the Morgan Limestone).


The Cadell Formation in outcrop (note the house boat on the river channel in the background).



The Cadell Formation in context. The creamy coloured beds (largely grassed over) are the Cadell Formation while the strongly banded orange limestones above it belong to the Bryant Creek Formation.

This formation is packed with aragonitic mollusc shells, sometimes so well preserved they look as if they have freshly come off the beach. The extent of the shelly facies of the Cadell Formation is quite limited, the main exposure stretches for just over 10 km between the towns of Murbko and Morgan, however for most of this length the exposures form sheer cliffs that plunge straight into the river. Only one decent access point exists, at the type locality for the formation, about 6km south of Morgan. This site is well-known and collectors have visited it for over a century. The first thorough documentation of the fossils of the Cadell Formation were published by Ralph Tate, a British born geologist, palaeontologist and botanist who emigrated to South Australia, and became the head of the Department of Geology at the University of Adelaide. Incidentally the Tate medal is still awarded each year to the best honours research project in the department for that year. In 1994, yours truly was the recipient of this award, definitely one of the proudest moments of my life, not least because Tate with his extremely broad knowledge of natural history was a personal hero of mine. I was very surprised to learn, while googling around for details of the man’s life I found that he and I share the same birthday.
Anyway back to the Cadell Formation, one would think that with such a long and venerable history of study and collection, there would be little left to discover. Not so; the mollusc fauna has not received a comprehensive survey since Tate’s pioneering work and the paleoenvironment of the Formation remains an enigma. I first visited this site when I was just 13 years old and fell in love with the site. I visited the site several times a year until I finally left Australia when I was 28. Over the years I’ve amassed a collection that includes more than 200 species of mollusc. Many of these are new records for the formation, and several represent new species. However as my academic career took me into vertebrate paleontology and dinosaur research, I left my interest in these mollusc fossils lie dormant but not forgotten. Late last year I finally got my chance to produce my first publication in this field. I hope many more of greater significance will follow. The paper outlines two new species of cowry from the Cadell Formation that were formerly thought to belong to middle Miocene species from the mollusc-rich basins to the east in Victoria. The first of these is Umbilia caepa, an extraordinarily fragile member of the basal cowire genus Umbilia.


Umbilia caepa

Umbilia was featured on this blog here and here. U. caepa is quite similar to the Victorian contemporary species U. leptorhyncha but consistently differs from it in a number of respects including the weaker apertural dentition, the development of a plate-like posterior columellar callus bordering the posterior canal and broad plate-like flanges on each side of the anterior rostrum. It also has a more strongly developed pyriform shape which resembles the bulb of an onion (hence the name). Of course with palaeontological samples it would have been impossible to demonstrate that U. caepa was a reproductively isolated from the eastern U. leptorhyncha or was simply the western end of a clinally variable species. However much to my surprise that when sorting through the various fragments from the Cadell Formation I found a small thin piece of Umbilia that does indeed have strong apertural dentition and weak lateral ridges on each side of the anterior rostrum (as opposed to broad flanges) amongst other features that indicate it was actually a true U. leptorhyncha. No intermediate specimens could be found indicating that the two species were sympatric in the Murray Basin but only U. leptorhyncha extended east into Victoria.
The next species I described belongs to the endemic southern Australian clade Austrocypraea, which is now regarded as cold-water adapted subgenus of the large tropical Indo West Pacific genus Lyncina (this includes the famous ‘golden cowry’ Lyncina aurantium) based on molecular evidence (Meyer 2003).


Lyncina aurantium image from en.wikipedia.org/wiki/Cypraea_aurantium

The species, which I called L. (A.) cadella, is abundant at the site and many specimens had been found and examined by previous researchers but had not received its own name due to a particularly bad tangle of taxonomic confusion surrounding the species.
Tate had found this species but had regarded as a mere variant of the Victorian species L. (A.) contusa. In a similar case to U. caepa, L. (A.) cadella is close to L. (A.) contusa but consistently differs from it in a number of respects relating to size, dentition and shape of the fossula.

Lyncina (Austrocypraea) cadella

As there are consistent differences between the two populations I think that the South Australian population is deserving of separate species status. Frank Schilder thought so too, when he revised the Australian fossil cowries in 1935. Schilder was a dedicated cowry researcher, and it is a testimony to his deep knowledge of the group that much of his generic classification of these extremely conservative and homoplastic shells was upheld by recent molecular phylogenic work. Sadly his work on Australian fossil cowries was not among his better efforts. The main problem was that he was working from collections held in Europe that were rife with poor locality data, leading to all sorts of confusion. To cut a very long story short Schilder described L. (A.) cadella -twice! – using two different names, neither of which are available. In the first instance he confused his own specimen of L. (A.) cadella with an Eocene species named by Tate – ‘Cypraea’ ovulatella and referred it that species using the combination ‘Austrocypraea ovulatella’. But ‘C’ ovulatella (now Willungia ovulatella) clearly isn’t the same thing as L. (A.) cadella, it isn’t even a cowry! (the confusion was the result of relying only on illustrations and an icorrect locality label). Secondly he described a second sample of L. (A.) cadella that was obtained directly from Tate himself by the French malacologist Alexandre Cossmann as a new species ‘Austrocypraea subcontusa’. So the species from the Cadell Formation should be called L. (A.) subcontusa right? Wrong. In an inexplicable move after describing the Cossmann’s sample Schilder selected an aberrant dwarfed Victorian specimen as the holotype of his new species. After looking at the Victorian specimens I’m convinced that the holotype of Austrocypraea subcontusa is just an extreme variant of true L. (A.) contusa. It still differs from L. (A.) cadella in a number of respects and can be connected to typical L. (A.) contusa by a number of intermediates. The upshot of all this is that the common species of Lyncina (Austrocypraea) from the Cadell Formation has never received a valid scientific name despite being known for well over a century.
So what is the significance of all this arcane taxonomy? The main significance is that these species are more evidence of faunal differentiation between the various Miocene epicontinental basins. This is in contrast to the modern molluscsn faunas of southern Australia where most species have broad ranges stretching across the entire southern Australian seaboard. Although there certainly were many widespread middle Miocene species in southern Australia there does appear to have been higher levels of endemicity, perhaps fueled by the presence of restricted epicontinental basins and the propensity for many southern Australian molluscs to abandon the planktonic dispersal stage of their development.

Adam Yates (2008). Two new cowries (Gastropoda: Cypraeidae) from the middle Miocene of South Australia Alcheringa: An Australasian Journal of Palaeontology, 32 (4), 353-364 DOI: 10.1080/03115510802417927

C.P. Meyer (2003) Molecular systematics of cowries (Gastropoda: Cypraeidae) and diversification patterns in the tropics. Biological Journal of the Linnean Society 79: 401-459.

From the galleries of the BPI: The Cape Giant Zebra

This specimen is a partial set of jaws of Equus capensis, the so called cape giant zebra, from Makapansgat, the most northerly australopithecine site in South Africa. Actually, although robust these equids are not so giant, being about the size of a big modern horse. Fossils of this robust equuid are widespread throughout South Africa, with the type coming from close to Cape Town, way down in the southwest.
It is often said that Africa escaped the megafaunal extinctions of the late Pleistocene but there is a definite set of large African mammal species that clearly did not make it through to the present. These include the giant buffalo Pelorovis, other bovids like Megalotragus, and supposedly Equus capensis. But if Charles Churcher is right reports of E. capensis' demise are greatly exaggerated. It s apparently alive and well in the form of.... Grevy's Zebra.


Image from wikimedia commons

Apparently the teeth (which are what most extinct Equus taxonomy is based on)of E. capensis do not differ in significant ways from those of E. grevyi and a bunch of east and northern african fossil equiids (e.g. E. oldowayensis).
Grevy's zebra is now restricted to East Africa and cannot be found anywhere near Suth Africa. So if it doesn't represent an actual extinction it does represent a drammatic range contraction.

reference

Churcher CS (2006)Distribution and history of the Cape zebra (Equus capensis) in the Quaternary of Africa. Transactions of the Royal Society of South Africa 61:89-95

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.

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.

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