Time for more borhyaenoids. Finally, we get round to the taxa that you might have seen or read about in prehistoric animal books: the sabre-toothed thylacosmilids, the supposedly bear-like borhyaenids, and the gigantic and even more bear-like proborhyaenids. We previously looked at basal borhyaenoids here, and at the mostly scansorial, mustelid-like hathlyacinids and prothylacinids here. Here we go…
We begin with the borhyaenids (yes, borhyaenid borhyaenoids), a group of about ten genera of superficially dog- or thylacine-like borhyaenoids. The oldest (Nemolestes) is from the Early Eocene or possibly Late Palaeocene while the youngest (Eutemnodus and Parahyaenodon) are from the Early Pliocene (Marshall 1978). Easily the best known member of the group is Borhyaena tuberata from the Santacrucian (= Early Miocene), and it’s like its contemporary Cladosictis in being frequently depicted in the popular and semi-technical literature. As was also the case with Cladosictis, its ‘conventional’ image is – so it turns out – pretty far off the mark.
The best known life restoration of this animal (that produced by John Long for Mammal Evolution: An Illustrated Guide) depicts it as something like a long-tailed bear, and I had always thought that Borhyaena was giant, and perhaps similar in size to a brown bear. In fact, weight estimates (Argot 2003) put it at 19-29 kg, which is about equivalent to a small hyaena or wolf, or a large thylacine. However, its robust skull, with its particularly broad zygomatic arches, indicate a disproportionately large amount of skull and neck musculature, and mass estimates for the whole animal based on skull proportions alone (and assuming body proportions resembling those of extant carnivorous mammals) give Borhyaena an inflated mass of 74 kg (vividly illustrating why knowing the overall proportions of an animal are important when estimating its weight: Van Valkenburgh 1985, 1987). The substantial neck musculature indicates an ability to carry heavy loads.
A semi- or fully digitigrade manus, short, blunt claws, and forelimbs that were restricted to parasagittal movement and exhibit reduced distal musculature indicate that Borhyaena was terrestrial and cursorial – and, in fact, the most cursorial of all borhyaenoids – but its limbs weren’t as proportionally elongate as those of extant cursorial predators. However, that might not have been such a problem, because virtually all the potential large-bodied prey (an incredible assortment of xenarthrans, astrapotheres, notoungulates and big rodents) were not cursorial either (in the Santacrucian fauna, only proterotheriid and macraucheniid litopterns can be considered cursorial) (Argot 2004a). While previously depicted as being plantigrade, what is known of ankle morphology and forelimb proportions show that Borhyaena was more likely digitigrade in the hindlimb (Argot 2003: life restoration and skeletal reconstruction above © C. Argot). Whether the other borhyaenids were like ‘new-look Borhyaena‘ remains to be shown.
Proborhyaenids, the ‘giant marsupial bears’, and their kin the sabre-toothed thylacosmilids
Proborhyaenids have usually been thought of as hyaena-like or bear-like, while thylacosmilids are well known for being strongly convergent with sabre-toothed cats. While Marshall (1977) thought that proborhyaenids might be the most basal of borhyaenoids (while at the same time the most specialised), it has more recently been argued that proborhyaenids and thylacosmilids share a number of derived characters and are hence sister-taxa (Muizon 1994, 1999, Babot et al. 2002). One of the most interesting characters present in both proborhyaenids and thylacosmilids is the presence of ever-growing, open-rooted upper canines. Both groups also exhibit strongly projecting occipital condyles, indicating that they were capable of greater rotation and movement at the head-neck joint than other borhyaenoids. Babot et al. (2002) suggested that the presence of fine ridges and grooves on the canine roots might be diagnostic for proborhyaenids, but the fact that thylacosmilids have such strongly modified canines raises the possibility that thylacosmilids had this character ancestrally but later reversed it.
Proborhyaenids are unique to the Eocene and Oligocene: there are only four recognised genera (Callistoe, Arminiheringia*, Paraborhyaena and Proborhyaena), and Proborhyaena in particular has often been referred to as huge and bear-like, with there being estimates here and there of skulls perhaps 60 cm long [P. gigantea holotype lower jaw shown here, from Patterson & Marshall (1978). From top to bottom, the specimen is shown in labial, occlusal and lingual views. Scale bar = 50 mm]. However, as has turned out to be the case for other archaic carnivorous mammals (like the hyaenodontids), big, bear-sized skulls do not necessarily mean big, bear-sized bodies, and an articulated proborhyaenid skeleton (read on) shows that these animals had big heads for their size.
* Arminiheringia was originally given its own ‘family’, Arminiheringiidae Amegino, 1902.
As is the case with most borhyaenoid genera, proborhyaenids are mostly known from fragmentary jaws and other parts of skulls, but in 2002 a near-complete proborhyaenid skeleton [shown above: from Babot et al. (2002). Scale bar = 100 mm], representing the new taxon Callistoe vincei, was described from the Casamayoran (= Early Eocene) Lumbrera Formation of Salta, Argentina (Babot et al. 2002). The generic name comes from Callisto, the Arcadian nymph loved by Zeus and changed by him into a bear to protect her from Hera’s wrath (Jupiter has a moon called Callisto). In contrast to the other proborhyaenids, Callistoe has a gracile, narrow snout and might have superficially resembled a thylacine in facial shape.
Arminiheringia had unusual procumbent lower canines: there has actually been some disagreement as to whether this is natural or not (Bond & Pascual 1983), but it does appear to be according to Babot et al. (2002) (A. auceta shown here, from Simpson 1932). Paraborhyaena also had somewhat procumbent lower canines. After Thylacosmilus, Simpson (1932) regarded Arminiheringia auceta (one of three species in the genus) as the most specialised borhyaenoid, and also one of the largest, ‘about the size of the great Pharsophorus lacerans‘ (Pharsophorus is a borhyaenid). Exactly how big Pharsophorus was, however, I have no idea – and was it supposed to be bigger than, say, Proborhyaena?
The marsupial sabre-tooths
Finally, we come to the superficially cat-like thylacosmilids, the only borhyaenoid group that are at all familiar, and all thanks to the incredible Thylacosmilus atrox Riggs, 1933 of the Late Miocene and Early Pliocene of Catamarca, north-western Argentina. In contrast to all other borhyaenoids, thylacosmilids were short-faced and had a complete bony postorbital bar (although not all did – read on). T. atrox is the only thylacosmilid we ever hear about, but it isn’t the only one.
Achlysictis Ameghino, 1891, known only from mandibular fragments and teeth (three species have been named), was smaller than T. atrox and differed from it in tooth cusp characters. Similar comments can be made about Hyaenodonops Ameghino, 1908, known only from teeth (though postcrania has been referred to it). Notosmilus Kraglievich, 1960, named for a maxilla and canine, was only about half the size of T. atrox. Marshall (1976) regarded all of these taxa as distinct, but Goin & Pascual (1987) and McKenna & Bell (1997) regarded them all as synonyms of Thylacosmilus. Of course, if that’s true, then the generic name Thylacosmilus is predated by both Achlysictis and Notosmilus and the ICZN would have to be petitioned if we want to preserve Thylacosmilus (which we do). Incidentally, Riggs (1933) named a second species of Thylacosmilus, T. lentis, but it was argued by Marshall (1976) to be synonymous with T. atrox. Most recently, Goin (1997) named the Middle Miocene species Anachlysictis gracilis from the La Venta site in Colombia. This animal differed from the other thylacosmilids in smaller size, in possessing a flattened skull roof, and in lacking a postorbital bar.
You might think you know Thylacosmilus, but it’s rather stranger than usually thought. It has no upper incisors at all (though this is not completely certain: see Churcher 1985) and only one pair of lower incisors. The laterally compressed, ever-growing upper canines were not just rooted in maxillary sockets as they are in sabre-toothed cats, but arced up and over the orbits, forming a notably convex skull roof. The premolars and molars were narrow lineal blades, specialised for slicing. Huge, laterally flattened flanges grew downwards from the lower jaw, and the sabre teeth would have rested against their sides when the jaws were closed. Protuberances, rugosities and excrescenses at the back and base of the skull show that a substantial amount of powerful musculature allowed both great power and fine control to be exerted over the head, and large muscle attachment sites on the neck vertebrae also show that the neck was very strong and flexible. Several studies have looked at these features and should be referred to if you want more information. Marshall (1976) discussed skull function and its possible role in behaviour, Turnbull (1976) reconstructed the cranial musculature, and Turner & Antón (1997) and Argot (2004b) analysed postcranial morphology.
How exactly did the thylacosmilids live? Again, Argot’s (2004b) detailed functional analysis gives us a huge amount of information. With its semi-plantigrade hindfeet, stout fibulae and curved tibiae, Thylacosmilus looks poorly suited for fast running, and a massively powerful upper arm and a reinforced and relatively inflexible lumbar region imply that it was an ambush predator that attacked prey after a short dash. It is now widely thought that sabre-toothed predatory mammals used their elongate and delicate weapons for precise attacks inflicted on the ventral surface of the neck, and for this to work the predator has to be able to physically manipulate and restrain the prey, and use coordinated and precise neck and head movements to attack in the right place. A semi-opposable thumb and tremendous forelimb strength suggest that Thylacosmilus could pin down and restrain prey, and its elongate, powerful neck demonstrates the presence of a ‘neck-driven’ precision biting style. In their study of bite forces seen in various mammalian predators, Wroe et al. (2005) found bite forces of Thylacosmilus to be very low. This is also the case for sabre-toothed cats like Smilodon and indicates that these animals were not ‘power biting’ like short-toothed predators, but driving their unusual killing style with their neck musculature [image of Thylacosmilus model below from Link & Pin Hobbies. Most thylacosmilid reconstructions make the animals look far too cat-like, and also give them completely incorrect limb details].
Another thing to consider about thylacosmilids: given that sabre-toothed biting appears to have been so specialised and difficult, it is inferred that sabre-toothed predators had to go through a very dangerous apprenticeship in which they learnt how to successfully inflict a bite without getting smacked in the head or breaking a tooth. But, for this to occur, juveniles and adults must stay together for an extended post-weaning period. In marsupials* in general, this is very rare, with juveniles rarely staying with their parents for more than a few weeks once weaning is finished. Were thylacosmilids a major exception? Unfortunately we just don’t know.
* Remember that borhyaenoids may not actually be marsupials, but members of the more inclusive group Metatheria. This was all explained in the first borhyaenoid article.
What were thylacosmilids preying on anyhow? Argot (2004a) proposed that mesotheriid notoungulates, litopterns and big rodents like capybaras were among the prey of Thylacosmilus. And it’s also interesting to note that thylacosmilids were living alongside other borhyaenoids: in the Huayquerian, Thylacosmilus was sharing its habitat with late-surviving hathlyacinids, the probably omnivorous prothylacinid Stylocynus, and the borhyaenid Eutemnodus. Phorusrhacids were around too: did they compete with borhyaenoids, or did the groups occupy different niches and avoid competition? So many animals, so many questions.
While that’s hardly everything, I think that’ll do on borhyaenoids for now.
Refs – –
Argot, C. 2003. Functional adaptations of the postcranial skeleton of two Miocene borhyaenoids (Mammalia, Metatheria), Borhyaena and Prothylacinus, from South America. Palaeontology 46, 1213-1267.
– . 2004a. Evolution of South American mammalian predators (Borhyaenoidea): anatomical and palaeobiological implications. Zoological Journal of the Linnean Society 140, 487-521.
– . 2004b. Functional-adaptive features and palaeobiologic implications of the postcranial skeleton of the late Miocene sabretooth borhyaenoid Thylacosmilus atrox (Metatheria). Alcheringa 28, 229-266.
Babot, M. J., Powell, J. E. & de Muizon, C. 2002. Callistoe vincei, a new Proborhyaenidae (Borhyaenoidea, Metatheria, Mammalia) from the Early Eocene of Argentina. Geobios 35, 615-629.
Bond, M. & Pascual, R. 1983. Nuevos y elocuentes restos craneanos de Proborhyaena gigantea Ameghino, 1897 (Marsupialia, Borhyaenidae, Proborhyaeninae) de la Edad Deseadense. Un ejemplo de coevolución. Ameghiniana 20, 47-60.
Churcher, C. S. 1985. Dental functional morphology in the marsupial sabre-tooth Thylacosmilus atrox (Thylacosmilidae) compared to that of felid sabre-tooths. Australian Mammalogy 8, 201-220.
Goin, F. J. 1997. New clues for understanding Neogene marsupial radiations. In Kay, R. F., Madden, R. H., Cifelli, R. L. & Flynn, J. J. (eds) Vertebrate Paleontology in the Neotropics: The Miocene fauna of La Venta, Colombia. Smithsonian Institution Press (Washington, D.C.), pp. 187-206.
– . & Pascual, R. 1987. New on the biology and taxonomy of the marsupials Thylacosmilidae (late Tertiary of Argentina). Anales de la Academia Nacional de Ciencias Exactas, Físicas y Naturales 39, 219-246.
Marshall, L. G. 1976. Evolution of the Thylacosmilidae, extinct saber-tooth marsupials of South America. PaleoBios 23, 1-30.
– . 1977. Cladistic analysis of borhyaneoid, dasyuroid, didelphoid, and thylacinid (Marsupialia: Mammalia) affinity. Systematic Zoology 26, 410-425.
– . 1978. Evolution of the Borhyaenidae, extinct South American predaceous marsupials. University of California Publications in Geological Sciences 117, 1-89.
McKenna, M. C. & Bell, S. K. 1997. Classification of Mammals: Above the Species Level. Columbia University Press.
Muizon, C. 1994. A new carnivorous marsupial from the Palaeocene of Bolivia and the problem of marsupial monophyly. Nature 370, 208-211.
– . 1999. Marsupial skulls from the Deseadan (late Oligocene) of Bolivia and phylogenetic analysis of the Borhyaenoidea (Marsupialia, Mammalia). Geobios 32, 483-509.
Patterson, B. & Marshall, L. G. 1978. The Deseadan, Early Oligocene, Marsupialia of South America. Fieldiana Geology 41, 37-100.
Riggs, E. S. 1933. Preliminary description of a new marsupial sabertooth from the Pliocene of Argentina. Geological Series of Field Museum of Natural History 6, 61-66.
Simpson, G. G. 1932. Skulls and brains of some mammals from the Notostylops beds of Patagonia. American Museum Novitates 578, 1-11.
Turnbull, W. D. 1976. Restoration of masticatory musculature of Thylacosmilus. In Churcher, C. S. (ed) Athlon, Essays in Palaeontology in Honour of Loris Shano Russell. Royal Ontario Museum (Toronto), pp. 169-185.
Turner, A. & Antón, M. 1997. The Big Cats and Their Fossil Relatives. Columbia University Press, New York.
Van Valkenburgh, B. 1985. Locomotor diversity within past and present guilds of large predatory mammals. Paleobiology 11, 406-428.
– . 1987. Skeletal indicators of locomotor behaviour in living and extinct carnivores. Journal of Vertebrate Paleontology 7, 162-182.
Wroe, S., McHenry, C. & Thomason, J. 2005. Bite club: comparative bite force in big biting mammals and the prediction of predatory behaviour in fossil taxa. Proceedings of the Royal Society B 272, 619-625.