The Cenozoic Era is commonly referred to as the ‘Age of Mammals’. That is certainly the time in the history of life when their fossils are most abundant and diverse. However, two-thirds of mammalian history was during the Mesozoic Era – and they appeared about the same time as the dinosaurs. All continents except Antarctica have some record of the early, Mesozoic mammals. Of those that do, Australia has the most meagre record of all. Despite this, with this landmass that today has the most distinctive terrestrial mammals on the planet, their Mesozoic origins are so enigmatic that it has motivated a major effort since 1984 to search for fossils of those mammals that lived alongside the dinosaurs on this now isolated continent.
The famous Lightning Ridge opal field has provided some of the answers – two different early Late Cretaceous egg-laying mammals (the monotremes), as well as a third mammal that may be a monotreme, have been discovered there.
One thousand, three hundred kilometres to the south-southwest along shore platforms pounded by the waves of the Southern Ocean, which expose those rocks on south coast of the continent, are three sites that have yielded Cretaceous mammals. Late in the Early Cretaceous, when these fossil-bearing riverine sediments were laid down, south-eastern Australia lay within the Antarctic Circle, forming the northern side of a rift valley separating it from then nearby Antarctica.
More than seven hundred people, lured by the word “dinosaur”, have worked these localities over 32 years, some coming only once to participate and one stalwart, 34 times. Their combined efforts have resulted in the recovery of less than 60 Mesozoic mammal specimens, the vast majority of which are mandibles or lower jaws. However, these few specimens, for the most part tiny fossils which could all together be held in one person’s outstretched open hand, have provided significant insights, some quite unexpected, into the early history of Australian mammals.
After a decade of excavating at the first of the three sites to be worked, Dinosaur Cove, only two mammal specimens turned up. One was a fragment of tooth, the other, a humerus (an upper arm bone) of a monotreme. [Figure 3 here] The humerus is very similar to that of the spine-covered living echidna, Tachyglossus, but its structure differs somewhat from that element in the living echidna, especially the form of the elbow joint. This region of the bone is not like that of a sprawling animal, but is indicative of a mammal with a more upright stance.
The name given to this fossil monotreme is Kryoryctes cadburyi. The generic name translates as the “cold digger”. Interestingly, the specific name is in honour of the Australian chocolate manufacturer, Cadbury. That organisation earned my heartfelt thanks, and thus such an honour, by enabling me to keep a promise I made quite foolishly. A volunteer who I knew was extremely fond of chocolate asked me one day after the work at Dinosaur Cove had gone on for four years with no trace of a mammal fossil what I would give her if she found one. Flippantly, I replied: “a cubic metre of chocolate”. Through an incredible set of circumstances, Cadbury graciously honoured my pledge. Moral – never make a promise you think at the time you will never have to keep.
After Dinosaur Cove had been worked for a decade, the only known fossiliferous rock was three metres below sea level and the bed was dipping downwards into the cove. So operations were moved 190km east to another fossil site called Flat Rocks, which had been discovered a few years earlier.
There, in the fourth year of systematic excavations of the site, the first mammal finally turned up. Taking into account the context in which it was preserved, the procedure for locating fossils at that site was altered. Instead of manually breaking down the fossiliferous rock to fist-sized pieces to try and discover fossils, the size was further reduced to that of a sugar cube. As a result, another 51 mammal specimens were to follow over the next 16 field seasons of intense work at the site.
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Dr Thomas H Rich (Australia)