There’s More Than One Way to Make a Sabertooth

Dinosaurs are the celebrities of the paleontological world. From museum halls to Saturday morning cartoons, they have a near-constant presence in the cultural landscape. For scientists and fossil hunters, however, these wonderful beasts have not always been of pressing scientific interest.

To many 19th and early 20th century scientists, dinosaurs were so bizarre that they were not very useful in measuring the ebb and flow of evolutionary change. Their evolution was just as mysterious as their sudden disappearance. Fossil mammals – which were far more abundant – had greater potential to illustrate the means by which evolution worked and the grand patterns it created. Even as museums vied with each other to collect the best dinosaur specimens to bring in the crowds, the behind-the-scenes scientific interests of paleontology departments often focused on extinct mammals.

By the turn of the 20th century, the fossil exposures of the American west spanning the last 65 million years were relatively well-known. Both Othniel Charles Marsh and Edward Drinker Cope had created extensive collections of fossil mammals during their late 19th century competition to become America’s foremost paleontologist, and, while there was still plenty of work to be done, fossil collectors turned their attention to other places. South America was of special interest.

While Cope and Marsh were battling it out in North America the Argentine naturalists Florentino and Carlos Ameghino were beginning to document the strange fauna of prehistoric Patagonia. Carlos was the field man and Florentino was the interpreter of the fossils, though paleontologists were often suspect of Florentino’s conclusions. Among other things, Florentino claimed to have found large strange mammals which lived among the last dinosaurs, evidence that humans originated in South America, and that the fossil exposures of Patagonia marked the area as a major evolutionary center where many mammalian lineages first appeared and diversified. On one point there could be no doubt, however – Patagonia was yielding a slew of strange, hitherto unknown mammals which documented an evolutionary history sharply different from that in North America.

Among those enticed by the bizarre Patagonian fossils was Elmer Riggs. An Indiana-born paleontologist, Riggs learned the craft of fieldwork from the famed fossil hunter and notorious lothario Barnum Brown. While searching the American west for dinosaurs in the 1890’s the two would chat about the bizarre mammals the Ameghinos were describing, and both dreamed of making their own remarkable discoveries in Argentina.

After his work with Brown, Riggs went on to work for Chicago’s magnificent Field Museum. He continued to prospect the fossil outcrops of North America, but it would not be until 1922 – when Riggs was in his mid-50’s – that he would get to realize his dream of traveling to Patagonia. While excavating dinosaurs with paleontologists John Abbott and George Sternberg in the badlands of Alberta, Canada in July of that year Riggs received a telegraph that the long-awaited expedition to Patagonia was imminent thanks to the museum’s eponymous patron Marshall Field. The search for dinosaurs was to cease so that the Patagonian expedition could begin immediately. Together Riggs, Abbott, and Sternberg crated up their finds for shipment and made a beeline for Chicago. From there they headed east to New York, Washington D.C., and Princeton to study collections, get references, arrange permits, and otherwise make preparations for the journey. By November 15th, 1922 everything was finally ready, and Riggs set sail with his team on the Southern Cross for South America.

A little more red tape held up the team when they arrived in Argentina. Concerns over foreign scientists looting the fossil riches of the country had caused tighter restrictions to be placed on fossil collecting, but after meeting with local officials Riggs and his team were assured that they would be able to carry on with their expedition. Finally, on the last day of 1922, they reached the fossil exposures of Río Gallegos, situated near the country’s east coast along the distal tip of Patagonia. The fossil hunting was good. Despite the difficulties with the hired translator and nearly losing their truck to an incoming tide during a visit to the shore, in about a month’s time the team collected 282 specimens. Many of these were skulls – arguably the most prized part of any skeleton – and these fossils were packed up for their long trip to Chicago while the team remained in the field.

From here the expedition almost derailed. While working Río Gallegos Riggs met a fellow named J.G. Wolfe who spoke of fossil human skulls and enchanted cities. If Riggs let him join the team, Wolfe promised, he would lead the scientists to these treasures. This must have sounded too fantastic to be true, but despite his doubts Riggs agreed. (Even the director of the museum, when he heard about these potential finds, cabled Riggs to encourage the team’s adoption of Wolfe.)

The paleontologists chose the fossil human skull for their first quest. Wolfe said that it was kept by an English nurse named of Mrs. Vendrino in El Paso de Santa Cruz, and they set off to find her near the end of April 1923. When Riggs and Wolfe arrived, however, Mrs. Vendrino was gone. She had gone insane, they were told, and had traveled to Buenos Aires for treatment with the skull in tow. (Riggs would later catch up with the skull during a visit to the city. It was nothing more than a vaguely skull-shaped stone. Paleontologists are very familiar with this variety of disappointment. What non-paleontologists recognize as dinosaur eggs, giant turtle shells, and enormous bones often turn out to be concretions or other rocks, but it is always wise to check since many significant fossil discoveries have been made by amateurs.)

With the skull out of reach, the pair decided to investigate Wolfe’s “enchanted city.” It, too, was a disappointment. Located at Lake Cardiel, the “city” was a common and unremarkable lava dike (once-molten rock which solidified in a sheet cutting across other rock layers). By this time Riggs was no doubt becoming frustrated – especially since the southern hemisphere winter was quickly approaching and would put an end to field activities – but he gave Wolfe one more chance. Wolfe said that there was a vast cemetery of fossil mammals that the Ameghinos had only just barely begun to tap before they stopped work, but Wolfe was unable to locate it. He merely took Riggs in a circle back towards Río Gallegos. Exasperated with Wolfe, Riggs parted ways with him and wrote in his journal:

Further inquiries were made in regard to Wolfe. He exhibited as qualifications a long angular personality, a bald head, a bland manner, a mode of speech which never said anything specific but always ended in an unfinished sentence.

Riggs had wasted the remainder of the field season following Wolfe’s false leads. Now, at the end of May, winter was beginning to set in and Riggs dealt with bureaucratic protocol that held up some of the collected fossils while Abbott and Sternberg camped out to do some light fossil hunting. By September the weather was fair enough to begin major operations again, and the team came back together to continue their search for odd fossil mammals.

Riggs, Abbott, and Sternberg continued to experience the highs and lows of fossil hunting over the next season, but after a year and a half in the field they were all thinking of home. At the onset of the winter of 1924 Abbott and Sternberg left for a break from the harsh weather, but they never went back to Patagonia. Riggs stayed on through 1925 before heading back to the United States himself, but he was not at home long. There was much left to discover.

In 1926 Riggs organized a second foray into the field. This time Abbott and Sternberg were unavailable, however, and so he had to choose different assistants. He could hardly have made a more uncomfortable choice. Riggs tapped Robert Thorne, an experienced outdoorsman from Vernal Utah, and Rudolf Stahlecker, a student of Friedrich von Huene at Tübingen (who was also doing research in Patagonia). Both men were veterans World War I, but on opposite sides, and their immediate dislike of each other made camping together constantly contentious. Nevertheless, the fossil beds of Argentina remained fruitful, and the team made a particularly notable discovery from Puerta del Corral Quemado in northwestern Argentina. Among the haul of petrified bones were several partial skulls of a saber-toothed mammal far larger than most of the other predators of its era.

It took several years for Riggs’ sabertooth to make its full debut. Riggs left the field in 1927, and he mentioned the predator in a report about the expedition made to the Paleontological Society of America in 1928. Description of the find took a little while longer. The animal’s initial description was made in the Geological Series of the Field Museum of Natural History in 1933, followed by a more extensive Transactions of the American Philosophical Society monograph the following year. He called it Thylacosmilus – the “pouch saber” – fitting its status as the first saber-toothed marsupial predator ever found.*

To Riggs, Thylacosmilus was the marsupial answer to the better-known sabercats of the Pleistocene (Smilodon being the classic, long-toothed predator). Its peculiar anatomy closely allied it with a peculiar group of dog-like carnivorous mammals endemic to South America called borhyaenids.

Things have become a little more complicated since the time of Riggs’ description. Although traditionally called marsupials, both Thylacosmilus and the boryhaenids were members of a group of carnivorous mammals called sparassodonts which shared a common ancestor with the first true marsupials but were not marsupials themselves. Instead Thylacosmilus belonged to the metatheria, the name for the group of mammals containing marsupials and lineages more closely related to marsupials than placental mammals. This taxonomic tangle aside, the name “pouch saber” remains appropriate – these formidable predators started their lives as tiny, pink babies which had to crawl their way into their mother’s pouches.

Despite the immediate comparison between Thylacosmilus and Smilodon based upon their teeth, however, they had vastly different skull constructions. For one thing the canines of Thylacosmilus were so deeply rooted in its skull that the bones containing them – the maxillae – stretched backwards to the braincase. This arrangement left no almost no room for the animal’s nasal bones and Thylacosmilus probably lacked upper incisor teeth because there was nowhere for them to be rooted. The entire front of its face had been rearranged to accommodate the lengthy, continually growing saber-teeth.

A few other features also distinguished Thylacosmilus from the true sabercats. Its eye was entirely enclosed in a ring of bone rather than sitting in an open cradle, and its teeth formed a straight, low-crowned shearing edge instead of the specialized “carnissal shear” made by the premolars and molars in cats. All of these features were distributed over a comparatively long and wide skull which lacked some of the expanded shelves and ridges for muscle attachments (such as the sagittal crest along the top of the skull). Even when compared to its collateral relatives Thylacosmilus was an oddball, and in his brief 1933 report Riggs wrote, “Not only is Thylacosmilus the most highly specialized of the known family of borhyaenids but the peculiar modifications centering about the development and the use of the great canine tooth mark it as one of the most unique flesh-eating mammals of all times.”

Strangely, however, the unique character of Thylacosmilus caused it to be somewhat marginalized. When it came up in discussions of fossil mammals it was often in context of being a marsupial approximation of a perfected, placental design. Riggs even thought it possible that Thylacosmilus had been displaced by true sabercats like Smilodon when the cats moved south after the recent connection of North and South America about three million years ago, writing “It is quite reasonable to infer that the sharper competition introduced with the appearance of these placental carnivores was responsible for the elimination of the marsupial sabertooth which in its turn had been the most highly specialized, the strongest and no doubt the most destructive of all the long line of South American marsupial carnivores.” This connection has not been conclusively proven – it primarily rests on the assumption of placental superiority over marsupials – but, regardless of why it became extinct, Thylacosmilus has often been cast as a “lower” class of mammal which was trying to reach up the evolutionary ladder by mimicking a very different predator.

As spectacular as Thylacosmilus was, it was just one of a paltry number of rare, poorly-known metatherian predators. Compared to their distant placental cousins, the metatherians just did not seem to enjoy the same kind of evolutionary success – there were fewer species and those species were relatively similar to one another. The way in which they were born was implicated as the reason for their evolutionary sluggishness.

A newborn marsupial has to do two things – crawl and suckle. These necessities of their early existence mean that parts of their skulls and forelimbs transform from cartilage to the actual substance bone early, and therefore it has been proposed that these changes put a constraint on the evolution of metatherians. Natural selection would only be able to adapt the skulls and forelimbs of these animals in a narrowed number of ways so not as to upset their early uses, and this would explain why metatherian predators seemed to be not quite as successful as placental ones.

A 2004 study by K.E. Sears confirmed that the need for marsupials to crawl so early in life had indeed constrained the way their forelimbs could be adapted, but no one had studied whether the same was true of metatherian skulls. To approach this question, scientists Anjali Goswami, Nick Milne, and Stephen Wroe have just published a study in the Proceedings of the Royal Society B in which they compared thirty landmarks on the skulls of carnivorous metatherian mammals and plotted them out into an anatomical map of skull shapes ranging from short and wide to long and narrow. They did the same for placental predators, as well, bumping up the study size to 130 specimens spanning 80 species of living and extinct mammals.

The variety of selected mammals covered different metatherian and placental groups. Among the placental mammals there were the carnivorans (dogs, cats, bears, weasels, etc.) and the group of archaic, dog-like predators known as creodonts. (Mesonychids, a group of hoofed carnivorous placental mammals distantly related to the other groups, were not included in the study.) The mammals of primary interest, however, were the metatherians. These were sorted into several groups. There was the thylacoleonids (carnivorous cousins of wombats), quolls, and the South American sparassodonts.

The point of all this was to measure the disparity among different mammal skulls. The sample of placental carnivores was more diverse – that is, contained a larger number of distinct species – but disparity is the measure of how different those forms are from each other. A lineup of ten different varieties of apples would be diverse, for example, but a collection of fruit from ten different species of tree would be more disparate in addition to being diverse.

As was expected, the mammals analyzed in the study fell across a wide range of different skull shapes. Whereas cats fell towards the extreme of short, wide, and high skulls, the marsupial quolls occupied the anatomical space of long, flat, and narrow skulls. Most of the other mammals – particularly dogs and their close relatives (canids) – fell between these extremes, and the spread of skull shapes across the anatomical map was wide.

While the distribution of skull shapes might look scattershot at first glance, a few patterns are apparent. Despite the idea that their evolution had been constrained, the skull shapes of the metatherian predators were widely distributed and even showed indication of shifts over time. The skulls of opossums, quolls, and thylacoleonids had skulls most similar to living and extinct dogs, and the quolls showed a shift from more dog-like skulls in extinct species to longer, narrower, insectivore-like skulls in modern species. In concern to Thylacosmilus, it again came out as distinct even when compared to other metatherian predators. Its long, broad, and deep skull was closest in shape to that of the prehistoric dog Enhydrocyon. The distribution also showed that – despite their common names “marsupial lion” and “marsupial sabertooth” – the skulls of the metatherians Thylacoleo and Thylacosmilus were far more dog-like than cat-like. If anything, cats were outliers in respect to their skull shapes, most closely approximated by bears and hyenas.

Contrary to what might be expected, diet and ecology may not have been as influential to skull shape as evolutionary history. In general each of the carnivore subgroups included in the study – cats, dogs, bears, hyenas, creodonts, quolls, etc. – clustered closely together even if there was a variety of dietary preferences within the group. The skull shape of the insect-eating hyena called the aardwolf (Proteles), for example, fell out closely to its meat-eating, bone-crunching relative the spotted hyena (Crocuta crocuta) despite their different menu preferences. The predators did not group together based upon their natural history or diet, but according to their actual evolutionary relationships, indicating that shifts in diet did not require major reorganizations of the skull. Then again, Thylacosmilus was the glaring exception to this pattern – the evolution of saber-teeth in this animal drastically reorganized the anatomy of its skull, but in a very different way from the sabercats and other saber-toothed animals of the prehistoric past.

Even if the anatomy of their forelimbs was constrained by their early development, the skulls of metatherians were not so limited. As the authors themselves state, “Specifically, the early ossification of the facial bones and their usage during suckling in the highly altricial marsupial neonate does not appear to have limited the ability of the cranium to evolve morphologies highly specialized for carnivory, including some of most extreme forms encountered in the mammalian record.” That sentence is filled with jargon – as scientific statements are wont to be – but its meaning is very significant. Even though some of the skull bones of marsupials coalesce earlier than in their placental counterparts, this early development has not prevented the skulls of metatherians from being adapted into an array of shapes comparable to – if not actually more disparate than – the variety seen in placental mammals. The evolution of metatherian predators does not represent a lagging evolutionary sideshow, but a rather vibrant branching out of forms.

This is not to say that the evolution of carnivore skulls was entirely unconstrained. As the study itself showed, ancestry had a major influence on the shape of carnivore skulls regardless of diet. In each lineage, the skulls of the predators could only be shaped in a limited number of ways.

The paleontologist Stephen Jay Gould often referred to Charles Darwin’s half-cousin, Francis Galton, on this point. As envisioned by Galton, a species is not like a smooth billiard ball that can move in any direction with the application of evolutionary pressure. Instead there are limits and constraints created by different aspects of an organism’s natural history, and so it is better to envision a species as a multi-sided die which can only move in a limited number of directions from its initial starting point. (This also means that species are relatively stable while at rest and shifts to new positions are relatively abrupt, partially conceptualizing Gould and Niles Eldredge’s theory of punctuated equilibrium.) The natural history of organisms places limits upon what is possible, and the identification of these constraints can help us better understand the nature of broader evolutionary patterns.

Constraints do not act as barriers that prevent evolution from occurring. Instead they are part of the reason why life is so disparate and diverse, and Thylacosmilus is a wonderful example of how constraints alter the form of organisms. Elongated canines evolved multiple times in multiple lineages, but as just one component of different skulls shapes influenced by the ancestry of each group. Clearly there was something different about the ancestors of Thylacosmilus which caused its skull to become modified in such an usual way, yet, when it comes to comparisons between metatherian and placental mammals, old habits are hard to break.

In 2003 the Cambridge paleontologist Simon Conway Morris published Life’s Solution: Inevitable Humans in a Lonely Universe, his paean to convergent evolution. Included within its pages was the traditional introduction to Thylacosmilus as equivalent to Smilodon. To Conway Morris’ credit, he did recognize the litany of differences between the skulls of the predators, but he still used the pair of them to support his thesis that life has a tendency to “navigate” to the same forms over and over again. Thylacosmilus and Smilodon were just two expressions of this same driving trend. He attributed this to constraints so stringent that evolution is perpetually running down exceedingly limited pathways. Somewhere out in the evolutionary aether there are a limited number of adaptive “boxes” which present the only universally-viable forms for organisms to take, in Conway Morris’ view, meaning that evolution is not a messy, contingent process but instead a systematic and law-like whittling down of forms along an established road of increasing perfection.

The new study of Thylacosmilus and other metatherian predators slices through Conway Morris’ implication of fine-tuned convergence among saber-toothed mammals. Yes, Thylacosmilus and Smilodon both wielded elongated canines, but these weapons were housed in strikingly different skull shapes. Even the nimravids – distant cousins of true cats which have often been called “false sabertooths” for their close resemblance to forms like Smilodon – had distinctive skull constructions which caused them to fall outside the cluster of sabercats on the map of skull shapes created by Goswami and her co-authors. Evolution did not intricately reconstruct the same package of traits in three separate mammalian lineages. Contingencies and constraints from the ancestry and natural history of these animals distinguished them all from each other and we cannot take for granted that they all hunted in the same way. The saber fangs of all these forms have acted as red herrings which have prevented us from recognizing the unique characteristics of each group of predators.

Evolution is not infinitely open-ended, nor is it so tightly regulated that organisms are perpetually bound to fill the same vacant roles in what William Diller Matthew once called “life’s splendid drama.” There is no universal niche which requires the evolution of Thylacosmilus just as there is no requirement that a species like us must exist. This is one of the key insights provided by the study of the fossil record. The more we learn about the life of the past, the stranger it becomes. We can’t simply shoehorn all forms into a neat series of boxes representing a limited set of evolutionary ideals. Life on earth has been heavily influenced by contingency, constraint, and quirks of natural history. Each species is unique – a mosaic of the old and the new – and the peculiarities of Thylacosmilus are a beautifully deadly example of evolution’s grand pattern.

*Riggs named two species – Thylacosmilus atrox and Thylacosmilus lentis – but there was nearly no difference between the two other than size. It is likely that T. lentis is a synonym of the much more widely-used name T. atrox, but that taxonomic shuffling doesn’t end there. As Darren Naish pointed out in his review of marsupial predators, it has been argued that fossils given the name Achlysictis in 1891 also belonged to Thylacosmilus. If this is correct then the name Achlysictis has priority over Thylacosmilus and the regulatory body for the scientific names of animals – the ICZN – would have to be petitioned to preserve the name of the “pouch saber” predator. For the sake of aesthetics alone, I certainly hope that Thylacosmilus remains the proper name for this animal!

Top Image: The skull of Thylacosmilus atrox, from Riggs, 1934.

References:

Conway Morris, Simon. 2003. Life’s Solution: Inevitable Humans in a Lonely Universe. New York: Cambridge University Press

Goswami, A., Milne, N., & Wroe, S. (2010). Biting through constraints: cranial morphology, disparity and convergence across living and fossil carnivorous mammals Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2010.2031

Riggs, E. 1933. Preliminary Description of a New Marsupial Sabertooth From the Pliocene of Argentina. Geological Series of the Field Museum of Natural History. Vol VI, 61-66

Riggs, E. 1934. A New Marsupial Saber-Tooth from the Pliocene of Argentina and Its Relationships to Other South American Predacious Marsupials. Transactions of the American Philosophical Society, New Ser., Vol. 24, No. 1., pp. 1-32.

Simpson, G.G. 1984. Discoverers of the Lost World. New Haven: Yale University Press. pp. 164-176

Simpson, G.G. 1980. Splendid Isolation. New Haven: Yale University Press. p. 223