Mollusc Fossils

Victorian Marine Fossils series

What are molluscs?

The most important groups of molluscs are the gastropods (snails, slugs and limpets), the bivalves (including clams, mussels, oysters and scallops) and the cephalopods (including octopuses, squids and cuttlefish). Other smaller groups are the amphineurans (chitons) and scaphopods (tusk shells).

Fossil gastropod, Euomphalus northi

Fossil gastropod, Euomphalus northi; Early Devonian (c. 400 my old), Lilydale, Victoria
Source: Museum Victoria

Molluscs thus include a great variety of creatures, some of them so different in appearance and lifestyle that it might be difficult to see that they are all closely related. Most have an external shell but in others the shell is internal (e.g. squids, cuttlefish) or has been lost (e.g. slugs, octopuses). They usually have a strong muscular foot, used to move around, to cling to rocks or other surfaces, or to burrow into sediments. In cephalopods, however, the foot has become joined with the head and is highly modified to form tentacles surrounding the mouth. Many molluscs are slow moving and some, such as oysters and mussels, are permanently attached to the surface on which they live. In contrast, cephalopods are amongst the fastest swimmers in the sea. Most molluscs are marine, but there are also many freshwater gastropods and bivalves, and some gastropods have adapted to life on land.


Fossil bivalve, Panenka gippslandica; Early Devonian (c. 390 my old), Matlock district, Victoria
Source: Museum Victoria

The fossil history of molluscs

The oldest known molluscs are small coiled gastropods and tiny bivalves that appeared during the earliest part of the Cambrian Period, 545-528 million years ago (mya), at the beginning of the Palaeozoic Era. The first cephalopods appeared somewhat later, towards the end of the Cambrian, about 495 mya. In the following Ordovician Period (490-434 mya) gastropods and bivalves became more common, but throughout the Palaeozoic Era neither of them was as abundant or diverse as they are today.

The dominant cephalopods during the Palaeozoic were the nautiloids, a group represented at the present day by the pearly nautilus which lives mostly in warm tropical waters of the western Pacific and Indian oceans at depths of 150-300 m. Nautiloids have an external shell which is divided on the inside into a number of separate chambers. The soft body of the nautiloid occupies the last chamber of the shell, and the other chambers are filled with a gas that gives the animal buoyancy in seawater. In the living Nautilus the shell is coiled into a spiral shape, but in most nautiloids of the Palaeozoic Era the shell was straight or only slightly curved. Like their distant modern relatives, the squids, Palaeozoic nautiloids sometimes grew to a very large size, shell lengths of up to 5.5 m having been recorded.


Reconstruction of a straight nautiloid (cephalopod) of the Palaeozoic Era
Illustration modified from Fenton, C. L. & Fenton, M. A. 1958 The fossil book. Doubleday, New York. Copyright: Tom Rich, Museum Victoria.

During the Mesozoic Era (251-65 mya) gastropods and bivalves increased greatly in numbers and variety, achieving the dominance of shallow marine environments that they have retained to the present day.

The most important cephalopods of the Mesozoic were the ammonoids, which had evolved from the nautiloids in the middle Palaeozoic. Like the nautiloids, ammonoids had an external protective shell that was divided internally into gas-filled chambers. Most ammonoids had shells that were coiled in a single plane, and many were highly sculptured with ribs and knob-like swellings. However, especially in the later part of the Cretaceous Period (141-65 mya), some groups of ammonoids developed shells of different and in some cases bizarre shapes that were variously coiled like a screw, partially or fully uncoiled, or irregularly curved into a tangle. At about the same time, ammonoids began to decrease in abundance and diversity, and at the end of the Cretaceous they became extinct.

Another group of cephalopods present in the Mesozoic were the belemnites. Related to squids, these had evolved from nautiloids in the late Palaeozoic but achieved their greatest abundance and variety in the Jurassic and Cretaceous periods (205-65 mya). Belemnites were almost extinguished in the massive extinctions at the end of the Cretaceous, but a few forms survived into the Cainozoic Era, finally becoming extinct in the Eocene Epoch (55-34 mya). Belemnites had a thick, bullet-shaped shell enclosed within the soft tissues of the body. These shells are normally the only part fossilized, sometimes occurring crowded together in large accumulations in the rock.

During the Cainozoic Era (less than 65 mya) gastropods and bivalves became more abundant and varied than at any other time. They are the most common fossils in rocks of this age, and the types present are readily recognizable as being similar to familiar, present-day forms. Scaphopods or tusk shells are also reasonably common fossils in Cainozoic rocks, especially in fine-grained muddy sediments deposited in deeper water. Scaphopods have a tubular shell that is usually curved and tapers to one end. Although they have a fossil record extending back to the Ordovician they are rare in rocks older than Cainozoic.


Fossil scaphopod (tusk shell), Dentalium; middle Miocene (c. 15 my old), Mornington, Victoria
Photographer: John Broomfield / Source: Museum Victoria

Nautiloids experienced an increase in diversity at the beginning of the Cainozoic following the extinction of the ammonites, which were probable competitors. However, all except one genus, the living Nautilus, had become extinct by the end of the Miocene (5.3 mya). Nautiloids are not common fossils in the Cainozoic, and other groups of cephalopods, such as cuttlefish, are even rarer.

Where are fossil molluscs found in Victoria?

Gastropods, bivalves and nautiloids are fairly common in rocks of middle Silurian to Early Devonian age (425-385 my old) throughout central Victoria, including the Heathcote, Kilmore, Melbourne, Lilydale and Kinglake districts. Farther east, they occur in Early Devonian siltstones and limestones in the Warburton, Walhalla, Tyers, Tabberabbera and Buchan areas. The Early Devonian limestones at Buchan also contain ammonoids that are amongst the oldest known anywhere in the world.

In much younger sediments of Cainozoic age (less than 65 my old), gastropods, bivalves and scaphopods are abundant and are very similar to present-day forms. The marine clays, sands and limestones containing these fossils occur along almost the entire southern coast of Victoria as far east as Orbost. Nautiloids also occur in these sediments but are rare.


Fossil gastropod, Ternivoluta antiscalaris; middle Miocene (c. 15 my old), Victoria
Source: Museum Victoria

Fossil scallop (bivalve), Mimachlamys asperrima; late Miocene (c.10 my old), Bairnsdale district, Victoria
Photographer: John Broomfield / Source: Museum Victoria

Visitor Information

Common species of fossil molluscs from Victoria are exhibited in the Marine Invertebrate Fossil Drawers in the Melbourne Museum Discovery Centre.

Further Reading

 

The following books provide more comprehensive reviews of Australian fossil molluscs:

Jell, P. A. & Darragh, T. A. 1998. The fossil record. In: Beesley, P. L., Ross, G. J. B. and Wells, A. (eds) Fauna of Australia. Volume 5: Mollusca. The Southern Synthesis. CSIRO Publishing, East Melbourne, pages 88–101.

Ludbrook, N. H. 1984. Quaternary Molluscs of South Australia. Handbook No. 9, South Australian Department of Mines and Energy, Adelaide.

Living molluscs from southern Australia are illustrated in:

Edgar, G. J., 1997. Australian marine life. Reed Books, Kew, Victoria.

Comments (7)

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kelly 3 June, 2009 06:42
i am learning about fossisls in school and came across this site which looks very interesting it has really helped me thankyou.
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Theresa 27 August, 2009 03:50
My kids foun dsome fossils in a creek - where woul dbe the best site to find out what fossil type we found?
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Discovery Centre 27 August, 2009 10:26

Hi Theresa and thanks for your query. It is exciting to hear that your kids made such a discovery! The Discovery Centre offers a free identification service for specimens and objects relating to Museum Victoria's collections and research areas. For more information about how to access this identification service please see the identification guidleines.  

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Somzi 25 October, 2010 14:53
I love this site it's great. Can you tell us more about common extinct fossilised molluscs?
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Scott 23 January, 2011 02:54
A few yeas ago I found a Treptoceras Crebriseptum fossil on the high banks of the credit river. It was identified for me by a friend at the Royal Ontario Museum. My son is now becoming interested in dinosaurs and the like, and I appreciate you having a website with which I can investigate these ancient species with my boy. Keep up the good work.
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Nestor Sander 20 June, 2011 08:25
I have 13 slide shows on YouTube about geology and paleontology. I am looking for pictures od verified Cambrian gastropods for inclusion in my show about snails I have D Sc from the University of Paris (Sorbonne)If you can send me UEL's I can dowoad the photos.I am especialy interested in Chippewaella patellitheca for it is generally accepted while several others are not.
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Danika 19 November, 2014 18:31
Where were molluscs found and when
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