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.