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.