Australia’s Polar Cretaceous mammals

Dr Thomas H Rich (Australia)

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.

Dinosaur Cove

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. 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.

Flat Rocks

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.

The critical ‘discovery specimen’ – named Ausktribosphenos nyktos, the “southern tribosphenic mammal of the night” – has a further twist. The person who discovered it was from the British lass, Nicola Barton – so the nyktos really took on a duel meaning. This discovery specimen was soon joined by another genus and species, clearly distinct but closely related and belonging in the same family, Bishops whitmorei. This was named after two long-time supporters of my search for polar Cretaceous tetrapods in south-eastern Australia, Drs. Barry Bishop and Frank Whitmore, long-time members of the Committee for Research and Exploration of the National Geographic Society.

When they were first described, it was thought that both species might be placental mammals, that is, mammals that bear their young at an advanced stage of development, which constitute about 93% of all living mammalian species (humans included). If this was in fact the case, it would mean that placental mammals were in Australia close to the time they first appeared in the fossil record.

If correct, it would contradict the long held view that placentals originated in the northern hemisphere and dispersed subsequently to the southern. In the case of Australia, except for bats that were there by the Eocene, terrestrial placental mammals do not otherwise appear in the Australian fossil record until the Pliocene when rodents dispersed from Asia. Among other things, an Early Cretaceous record of placentals in Australia would mean that, along with Antarctica, those two would be the only continents where terrestrial placentals were once present and subsequently became extinct.

Not long after their discovery, an alternative interpretation of Ausktribosphenos and Bishops was offered in the literature. In many ways, this was a more radical suggestion – namely that Ausktribosphenos and Bishops were NOT really placental mammals and that there was a southern hemisphere radiation of mammals quite distinct from placentals, together with marsupials, and equal in rank to them. Marsupials include the kangaroo and the American opossum. This southern group was named the Australosphenida – “the southern ones with grinding and shearing molars”.

The alternative interpretations of anatomical features invoked to support this different view include a few subtle aspects of the mandibles and teeth, the only parts thus far known of the animals central to the discussion. One of the objections to Ausktribosphenos and Bishops being placentals is that they seem too advanced for their geological age. The relatively wide aspect of the rear part of their lower molars is not seen in true placental mammals with rather similar teeth until the Oligocene. However, rejecting an hypothesis for the affinities of a fossil on the basis of its geologic age rather than its morphology, as is customary in the identification of a fossil, does not seem to be a justifiable mode of classification.

After these two views – placental or non-placental analogue – were propounded at the turn of the Millennium, no progress has been made to resolve the two interpretations of the evidence. This is because the australosphenidias, with the exception of the monotremes, are only known from mandibles and the lower dentitions. Even if the concept of a major radiation of southern hemisphere mammals distinct from placentals and marsupials proves to be correct, whether or not monotremes will be found to be part of the Australosphenidia will remain unresolved until more material, especially upper dentitions and cranial material, is discovered.

A third mammal recovered from the Flat Rocks locality is another monotreme, Teinolophos trusleri, the generic name meaning “extended ridge”, a feature of its molars. The species name is in honour of the artist, Peter Trusler, who has worked with Patricia Vickers-Rich and me for nearly four decades as a palaeo-reconstruction artist, now with his PhD.

The book, The Artist and the Scientists, by the three of us recounts how together we have carried out a number of palaeoart projects over that time.

As more material of T. trusleri has been found since its original discovery, interpretation of this tiny monotreme has changed time and time again – truly the way of science. At first, it was not even thought to be a monotreme – but rather a member of an exclusively Mesozoic mammalian group, a eupantothere. Later, after being recognised as a monotreme, it was placed in the same family as the living platypus. Finally, with yet more material in hand, it now appears to be the oldest known monotreme, as well as the most primitive and likely, but not certainly, remains in the same family as the living platypus.

It is worth noting that the ancestors of mammals have a number of bones in their lower jaw, while living mammals have only one – the dentary. Two of the bones that were present in the lower jaw of mammalian ancestors evolved into auditory ossicles linking the ear drum with the sound sensory nerve tissue deeper in the skull in all living mammals, including the monotremes. In the most recent interpretation of T. trusleri, it appears to have had an intermediate condition. The bones that eventually evolved into the auditory ossicles were neither intimately joined to other bones in the lower jaw, nor linked the ear drum to sensory tissue. Rather, they were suspended in between with a flange of Meckels cartilage projecting from the lower jaw. This interpretation has been possible because of the discovery of more complete material of T. trusleri, together with comparison to recently discovered and described, exquisitely preserved fossil mammals from the Cretaceous of China.

The latest reconstruction of T. trusleri, incorporating all of the up-to-now known material of this species presented here, required Peter Trusler and colleague, Pamela Gill, of Bristol University in the UK, working together at long distance, five weeks to consolidate their ideas in this illustration. Even with this material in hand, there are features that, after much pondering, cannot yet be resolved owing to the crushed and distorted nature of the most complete specimen.

This is not surprising because fossils at the Flat Rocks locality have been buried to a depth of two kilometres and subsequently brought back up to the surface as the Strezelecki Ranges rose after the Cretaceous. If you look closely at the reconstruction of the lower jaw of T. trusleri, you will observe that there are two hypotheses illustrated as to the nature of the anterior part of the lower jaw. Which is correct is yet to be resolved – again dependent on more and better preserved material.

What we now know of T. trusleri does demonstrate that the condition of the mandible being a single bone as in modern mammals was acquired at least twice in evolutionary history, once in the monotremes and separately in the common ancestor of the placentals and marsupials.

Researchers using molecular techniques on tissue samples from living animals have estimated that the time of separation of the ancestors of the echidna and platypus took place somewhere between 17 and 35Ma. Going a step further, it has been suggested, based on molecular studies, that the terrestrial echidnas arose from an aquatic ancestral platypus.

When Kryoryctes was described, the title of the paper characterised it as “tachyglossid-like” or “echidna-like”, based on the morphology of the elbow joint, suggesting it might not properly be called an echidna because of such morphology. Likewise, Teinolophos was more platypus-like than echidna-like, but not necessarily a member of the family that includes the living platypus. Furthermore, Steorppodon from Lightning Ridge, along with another as yet unnamed monotreme from there, may also possibly (but not necessarily) be members of the family including the living platypus.

There are possible candidates that may be representative members of both sides of the division between the lineages leading to the platypus and echidnas in the fossil record that are much older than the 17 to 35myr age projected by molecular data for the division between these groups advocated by molecular biologists. However, the uncertainty of the allocation of relevant fossils means that the molecular hypothesis cannot be as yet ruled out on the basis of the fossil evidence.

Another interesting and surprising find late in the Early Cretaceous of Southern Australia is the single tooth of a multituberculate. In the northern hemisphere, multituberculates are the most common fossils of Mesozoic mammals. Now extinct, these rodent-like mammals appeared in the Jurassic and made their last appearance during the Eocene in North America, the longest lived order of mammals.

A few specimens of multituberculates are known from South America, but Australia has exactly one – a single, exquisitely preserved most probably posterior lower, blade-like premolar, so characteristic of many members of the group.

The tooth is preserved in a tiny fragment of lower jaw. Named Corriebaatar marywaltersae, the generic name is in honour of Corrie Williams. Baatar means “heroine or hero” in Mongolian and is a suffix given to the names of many multituberculates, including at least one previously found on a continent other than Asia, namely North America. The specific name honours Mary Walters, who found the specimen.

This single discovery is a critical specimen. It demonstrates that this major group was present in Australia. Given the meagre nature of the Mesozoic mammal fossil record on the continent, with only a few sites yielding a few fossils, just knowing they were there at all is critical, and there is no reason to suppose they were not as diverse and abundant as on the land masses of the northern hemisphere.

Eric the Red West

The location known as Eric the Red West, in the Otway Range to the west of Melbourne and Flat Rocks and 17km southeast of Dinosaur Cove, has yielded four specimens of fossil mammals during a dozen years of excavating there.

Although mammalian fossils are far fewer in number there than at the Flat Rocks locality, all excavation effort is now centred there because of one fossil. That critical specimen, battered and worn as it is, demonstrates that, at this locality, another part of a mammalian skeleton can be found in addition to lower jaws.

The critical discovery consists of two upper molars. The individual that had them lived to a ripe old age for its species, because the teeth are worn almost flat.

In addition, the specimen was badly broken in the process of being discovered, owing to the only way that fossils can be found in the brick-hard sandstone where they occur, namely by physically breaking the rock into progressively smaller pieces and examining each break to see if a fossil is present. Much effort has gone into first physically reassembling the pieces, then making images of them both by scanning the fragments and moving images of the pieces about.

Next, Peter Trusler, applying his extraordinary talents of interpretation to the reconstructed images based on both careful examination of the fossil under a microscope and studying scans of it, painted what amounts to an hypothesis of the fossil’s original appearance.

Finally, the resulting scans and Peter’s reconstruction were studied at length and compared to actual specimens of primitive marsupials and placentals. On its own, a very provisional identification of this fossil would be that of a marsupial rather than a placental. A lower jaw referred to Bishops also occurs at Eric the Red West site. Attempting to occlude images of Bishops with the upper molars, given all the damage to the latter, gives quite ambiguous results as to whether or not they could plausibly fit together.

The greatest value of this fossil from Eric the Red West is that it demonstrates beyond a doubt that this locality, for whatever reason, does not suffer from the Samson Effect. This is the name given to the bias towards the preservation only of lower jaws in the sample of mammals from the Flat Rocks locality. The name derives from the biblical story that Samson slew the thousand Philistines with a fresh jaw bone of an ass, choosing that element because it is the most durable bone in the mammalian skeleton (Judges 15:15-16, King James version of the Bible).

Because of that difference in preservation potential, digging will go on as long as I can keep it going at Eric the Red West. This because that locality seems to be the one where, if anywhere now known, a resolution to australosphenidian versus placental hypotheses will be found.

There are two objectives for this Mesozoic mammal project at present. First is to find that critical specimen or specimens that will clarify the competing hypotheses of mammal phylogeny. Second is to make it possible for someone else to carry on when it is no longer possible for me to do so. At age 75, having dealt with the dead all my professional life, I am under no illusion that I shall necessarily be able to persist long enough to answer the fundamental question about the relationships of these early mammals. Twenty-two years passed between when I was hired at Museums Victoria and the discovery of Kryoryctes. It seems highly unlikely that I shall still be able to continue in my current position at Museums Victoria at age 97, if a similar time is needed to find those critical specimens to resolve this dilemma. But, if in the meantime, the financial wherewithal to fund a replacement for my position at Museums Victoria to carry on this work cannot be found, I shall try.

The few hints now about the history of mammals during the Mesozoic Era in Australia are intriguing. These are based on less than one hundred specimens known from the entire continent, none more than a lower jaw, a few teeth or maxilla fragment. Further efforts at the known sites will likely, yet slowly, yield a more complete picture. Despite all that effort, those few places cannot be reasonably expected to give a broad picture of what happened on the continent as a whole. Where else to look and how else to explore to expand both the geographic knowledge and sample times in the Mesozoic as yet totally blank (for example, the entire Jurassic and Triassic as well as all but the middle of the Cretaceous) is not obvious. Mammals were undoubtedly living across the entire continent all through those times. However, only with much concerted dedication and persistence can as yet unknown sites be found, if indeed they exist.

Incidentally, specimens of dinosaurs are more abundant than those of mammals at the three sites where the latter have been found in south-eastern Australia. Perhaps size and robustness matter. But that is another story.

About the author

Dr Thomas Rich is senior curator at the Museums Victoria Australia.

While his research program is focused on the Cretaceous mammals of south-eastern Australia, in the process of carrying out that, he has dealt with nearly as many different kinds of dinosaurs associated with the mammals, as well as other tetrapods in reconstructing the Cretaceous polar habitat of this region.

Further reading

Polar Dinosaurs of Australia by Thomas H Rick, published by Museum Victoria, (2007). Softback. ASBN 978-0975837023.

The Artist and the Scientists: Bringing Prehistory to Life by Peter Trusler, Patricia Vickers-Rich and Thomas H Rich, published by Cambridge University Press, (2010). Softback, ISBN 978-0521676359.

Dinosaurs of Darkness by Thomas H Rich and Patricia Vickers-Rich, published by Indiana University Press (2000). A second edition of this book is in an advanced stage of preparation.

Rich, T.H. and Vickers-Rich, P. 2012. Australia’s Polar Early Cretaceous Dinosaurs. Chapter 27, pp. 488-503 in Godefroit, P. & Lambert, O. (eds.) Bernissart Dinosaurs and Early Cretaceous Terrestrial Ecosystems. Indiana University Press.

Vickers-Rich, P. and Rich, T.H. 2014.  Dinosaurs of Polar Australia. Pp. 46-53 in Dinosaurs!. Scientific American Books. New York.