So what’s the deal with thylacosmilids?
So, the idea that saber-toothed cats and similar mammals actually had their canines covered by lips is gaining widespread attention. With enamel patterns similar to those of normal teeth rather than tusks, clear evidence of sheathing and complexity not seen in tusks, it seems clear that most of these mammals followed the pattern of their closest modern analogue, the cloud leopards, and had their long canines protected, rather than visible as in classical depictions.
Or did they?
One lineage of saber-toothed predators does seem to defy these newfound conventions, the thylacosmilid sparassodonts. Already considered highly aberrant mammals by all accounts, thylacosmilids, perhaps unsurprisingly, also have aberrant saber-teeth. Their canines and other aspects of their anatomy offer highly contradictory implications in regards to the soft-tissues in their mouth, aggravating this controversy to levels never imagine.
A carnivore canine’s answer to rodent incisors
For one thing, thylacosmilids have enamel patterns seen more often in tusked mammals than other saber-teeth like machairodontines. As opposed to Smilodon, which has several enamel layers (Feranec 2004, Riviere & Wheeler 2005), thylacosmilid canines are only very shallowly “dressed”.
But, to be fair, this is actually a pattern seen in sparassodonts as whole, with non-saber-toothed species like proborhyeanids showing a progressive decrease in enamel coating. Unless these forms somehow had no lips, it stands to reason that they, like most mammals, had covered canines. Combined with the Hunter-Shreger bands seen in borhyeanids — a rare trait among mammals besides placentals -, this may possibly be simply a form of canine reinforcement.
Thylacosmilid canines do possess another bizarre trait, however: they were ever-growing. Their tooth-roots, extending all the way above the animals’ eye-sockets, essentially functioned like the rodent incisor roots, providing constant fodder for the ever-erupting teeth (Marshall 1978, Maria J. 2005, Mauricio Antón 2013).
In life, this would have made thylacosmilids easily very lucky when compared to other saber-toothed predators, as the frailty of their teeth would be compensated with regeneration. It does, however, pose a problem to animals with sheathed teeth, which would see themselves very likely to puncture their own flesh.
Once again, however, we turn to other sparassodonts. Proborhyeanids, for example, also possess perpetually-growing canines (Mashall 1978), to the point that this has been at times brought up as a possibly common trait between them and thylacosmilids (Maria J. 2005), though not all experts agree that it is a synapomorphy between both groups (Anália Forasiepi 2009).
Once again, these sparassodonts had no saberteeth.
One specimen refered to Proborhyaenidae seems to imply that the canines ceased to grow at some point in sparassodont lifespan (Bond & Pascual 1983), but a proborhyaenid identity for this specimen is controversial, and at any rate does not reflect the general trend in these sparassodonts to have perpetually growing canines (Maria J 2002). Instead, it can be inferred that regular use probably wore these teeth down.
More comparisons between thylacosmilid and proborhyaenid canines are probably needed, especially if both groups developed perpetually-growing canines independently. However, I think that proborhyaenids provide an overall good example as non-saber-toothed predators with the “tusk-like” adaptations seen in thylacosmilids, so the later’s saberteeth do not necessarily imply that they were external.
Lower jaw flanges
Thylacosmilid lower jaws, by contrast, provide the best outright evidence for sheathing among all saber-toothed predators. Most saber-tooths developed lower jaw structures vaguely akin to flanges, extensions that in life would have served as sheaths for the canines. Thylacosmilids are by far the most extreme example, with Thylacosmilus proper possessing extremely long flanges that are almost a third of the whole skull’s depth.
In most restorations, these flanges are depicted as lip-less, giving way to the canines. I personally always found this extremely weird, even before this sheathed-saber-tooth era; virtually all mammals possess lips, and in metatherians suckling is extremely important in early stages of life, as joeys are attached to their mother’s nipples. If marsupials are suspected of less facial diversity than placentals because of this, what chance did the closely-related-but-still-pouched sparassodonts have?
If thylacosmilids did have exposed sabers, we would expect some seriously radical facio-muscular changes in their development, a kind of change not reccorded even among placentals. As it stands, the most logical conclusion is that my suspicions are correct and that their flanges were covered by lips in life.
Which would, by default, imply sheathed sabers after all.
A problem I posited occasionally is how sheathed saber-tooths would clean their flanges. Indeed, an animal like Thylacosmilus would have very long and deep areas where food residue and other waste would gather to extremely detrimental effect. A long tongue should probably be enough, and studies on the thylacosmilid hyoid could definitely be the turning point in this debate.
Conslusion, and Lotheridium
Based on the above, I think that the thylacosmilid “tusk-like” canine anatomy does not imply a different scenario from other saber-tooths. “Tusk-like” canines are found in sparassodonts with proportionally shorter teeth, and the very existence of lower jaw flanges implicates that these fangs were protected.
What does show, however, is that thylacosmilids were basically OP by saber-tooth standards. They had rather robust, perpetually growing canines that were protected by the best mouth covering nature could provide, making these far-less vulnerable than the sabers of Smilodon or any other machairodontine, nimravid or barboroufelid.
To think that these animals were outcompeted by the comparatively less efficient placental predators seems rather laughable, and indeed, the first saber-toothed cats only appeared in South America several million years after the last thylacosmilids became extinct (Francisco et al 2013). I know concepts like “superior” and “inferior” are utterly meaningless in any discussions about evolution, but I wager that thylacosmilids were functionally better saber-teeth than their placental counterparts.
Lastly, no discussion about metatherian saber-teeth is complete without the most recent addition to the roost, the deltatheroidean Lotheridium. Deltatheroideans have long been assumed to be carnivorous mammals specialised to feed on vertebrate prey, so a saber-toothed carnivore, even if weasel-sized, is not terribly surprising.
Comparisons between Lotheridium and thylacosmilids are, as far as I know, nonexistent. We do know that Lotheridium appears to lack lower jaw flanges, but its canines are noted as being laterally compressed (Li et al 2015), a characteristic usually associated with saber-toothed predators. This to me seems to implicate a functional analogy very strongly, even if it was nowhere near as specialised.
In life, therefore, Lotheridium probably also did cover its canines. Though probably lacking the remarkable attributes of thylacosmilids, it served as a nice prototype, being among the first true mammals to develop saber-teeth.
References
Robert S. Feranec, Isotopic evidence of saber-tooth development, growth rate, and diet from the adult canine of Smilodon fatalis from Rancho La Brea, 2004
https://www.researchgate.net/publication/7799339_Cementum_onSmilodon_sabers
Antón, Mauricio (2013). Sabertooth.
Forasiepi, Analía M. (2009). “Osteology of Arctodictis sinclairi (Mammalia, Metatheria, Sparassodonta) and phylogeny of Cenozoic metatherian carnivores from South America”.Monografías del Museo Argentino de Ciencias Naturales 6: 1–174.
http://blogs.ucl.ac.uk/gee-research/2013/06/07/skulls-and-teeth-constraints-on-mammalian-evolution/
Marshall, L. Evolution of the Borhyaenidae, extinct South American predaceous marsupials. Berkeley: University of California Press, 1978.
Prevosti, Francisco J.; Analía Forasiepi; Natalia Zimicz (2013). “The Evolution of the Cenozoic Terrestrial Mammalian Predator Guild in South America: Competition or Replacement?”.Journal of Mammalian Evolution 20: 3–21. doi:10.1007/s10914–011–9175–9.
S. Bi, X. Jin, S. Li and T. Du. 2015. A new Cretaceous metatherian mammal from Henan, China. PeerJ 3:e896
