Long-snouted marsupial martens and false thylacines

Distractions distractions distractions. Mayfly chameleons. Sea monster carcasses. Avian supertrees. Fake tiger photos. New pterosaurs. Frogs. But... must... complete... borhyaenoid... articles... For the intro, go here. Time now to crack on with hathlyacynids and prothylacinids. If you don't care, let me note that this is (to my knowledge) the biggest amount of information yet made available on these animals outside of the technical literature. Yes yes, I feel your love, thank you...

Hathlyacynidae is the biggest and longest-lived borhyaenoid clade, with members that range in age from Late Palaeocene (Patene) to Early Pliocene (Notocynus and Borhyaenidium). Notocynus might even have made it to the Late Pliocene, but this is debatable. Incidentally, there are two ways of spelling the name of this group, with some authors spelling it Hathliacynidae. Most of the 17 or so hathlyacynid genera are known only from jaw fragments, and these show that members of the group were conservative over their long history: in fact Marshall (1981) described the hathlyacynid genera as "monotonously alike" [incidentally, the animal shown at the top is not a hathlyacynid, but the prothylacinid Prothylacinus. Image © C. Argot].

While often characterised as dog-like, those taxa known from good remains show that at least some hathlyacynids were marten-like predators with good climbing abilities. So far as we know they were all long snouted, and with short gaps in the toothrow between the premolars, and between the first premolar and the canine. Their teeth suggest that they were carnivorous or even omnivorous generalists, though there is a trend for later members of the group to have more specialised carnassials (Marshall 1981). Two hathlyacynids in particular, Sipalocyon gracilis and Cladosictis patagonica (both from the Santacrucian beds of Early Miocene Patagonia), have been subjected to detailed functional analyses [skull of Cladosictis shown here].

Based on its divergent hallux and semi-opposable pollex, Sinclair (1906) thought that Sipalocyon was plantigrade and probably arboreal, and comparison of its bones with those of extant mammals suggest that it was probably scansorial (Argot 2003a, 2004a). Its mobile thumb suggests that it was good at grabbing and manipulating prey. Cladosictis has been repeatedly depicted in the popular and semi-technical literature as an amphibious, otter-like predator. This idea appears to come from Savage's (1977) statement that the forelimbs of Cladosictis recall those of an otter in their proportions. However, otter-like forelimb proportions do not mean an otter-like lifestyle, and in fact the forelimb proportions of otters aren't really reliably different from those of other (non-swimming) mustelids.

Cladosictis, 20-25 cm tall at the shoulder and weighing 4-8 kg, was relatively short-legged with limb details suggesting a good climbing ability [adjacent skeletal reconstruction from Argot 2004a. Image © C. Argot. Scale bar = 10 cm]. It also had a semi-opposable thumb, and hence was able to grasp branches and manipulate prey. The closest analogue is perhaps the Tayra Eira barbata, a tropical American mustelid that forages terrestrially but is also a good climber (Argot 2003a, 2004a), and based on contemporary fossils and morphology, Cladosictis probably predated on small mammals, birds, reptiles and frogs. As is the case in a lot of borhyaenoids, large ventral processes on the neck vertebrae demonstrate that the neck musculature was powerful, and the neck was also long (proportionally, twice as long as that of any living canid, for example).

All the other hathlyacynids

Do we know if other hathlyacynids were doing the same thing? Unfortunately we don't know enough about them to be sure. The forelimb bones of little Pseudonotictis pusillus (also from the Santacrucian) indicate climbing abilities, but this taxon had particularly slender bones, suggesting that it was doing something unusual... but just what we're not sure (Argot 2003a). The lower jaw of P. pusillus is about 4 cm long, suggesting a weasel-like size of less than 40 cm. It's the smallest hathlyacynid known, but its probable close relative Notictis ortizi from the Huayquerian (= Late Miocene) of Argentina was nearly as small (Marshall 1981). Marshall (1981) and Villarroel & Marshall (1983) further proposed that Santacrucian Perathereutes pungens shared an ancestor with Pseudonotictis, and that Perathereutes was close to the ancestry of Borhyaenidium from the Huayquerian. In both Perathereutes and Sipalocyon the first lower premolar is set at an angle relative to the other teeth, leading Marshall (1981) to suggest that both shared an ancestor. Marshall (1981) also regarded Notocynus hermosicus from the Montehermosan (= late Late Miocene) as a close relative or descendant of Sipalocyon as the two share tooth cusp characters.

However, Sipalocyon was regarded as closely related to Notogale by Forasiepi et al. (2006), as both possess a transverse canal in the auditory region. This character is also present in Cladosictis however (Muizon 1999). Santacrucian and Laventan (= Middle Miocene) Acyon - resurrected from the synonymy of Anatherium by Forasiepi et al. (2006)* - shares a single tooth character with Cladosictis and might be closely related to it, and Chasiocostylus castroi is also similar and probably close to Cladosictis [adjacent image shows the lower dentitions of various hathlyacinids, demonstrating relative sizes and proportions of teeth (all drawn to scale). From Marshall (1981)].

* The type species of Anatherium, A. defossus, was referred to Cladosictis sp. by Forasiepi et al. (2006).

Sallacyon hoffstetteri has been regarded as a basal hathlyacynid (Muizon 1999, Forasiepi et al. 2006), as has Patene (Marshall 1981). Procladosictis anomala from the Mustersan (= middle Eocene) resembles Patene. Several other genera, including Agustylus, Ictioborus, and Amphithereutes, continue to be listed by some authors (e.g., McKenna & Bell, 1997), but are regarded as synonymous with other taxa by others (e.g,. Marshall 1981). Palaeocladosictis mosesi, sometimes listed as a hathlyacinid, was based on an ungulate tooth according to Marshall (1978).

Prothylacinids: climbing thylacine-hyaena-binturong hybrids

As we saw in the first borhyaenoid article, character evidence indicates that hathlyacynids are the sister-group to a borhyaenoid clade that includes all the remaining taxa. Among these 'remaining taxa', we begin with the prothylacinids (or prothylacynines): an Oligocene-early Pliocene group regarded by some as part of Borhyaenidae as both share characters not seen in other borhyaenoids (Muizon 1999). However, prothylacinids and borhyaenids are more usually imagined as sister-groups, which if true means that prothylacynids must have a ghost-lineage going back to the Early Eocene at least (as there are apparently borhyaenids this old, like Argyrolestes and Angelocabrerus).

Lycopsis from the Santacrucian and Friasian (= late Early Miocene) has conventionally been regarded as one of the most basal prothylacinids (Marshall 1979). However, more recent studies have failed to recover this position, and have instead found it to be the sister-taxon of, or basal member of, a ((prothylacinid + borhyaenid) + (proborhyaenid + thylacosmilid)) clade (Muizon 1999, Babot et al. 2002, Forasiepi et al. 2006). Again, the characters involved are all trivial details of the back of the skull and if I explain them I can see myself adding 1000 words to this article.

Thanks to a wonderfully near-complete skeleton of L. longirostrus* from the Friasian of Colombia [shown in adjacent image] we have a reasonably good idea of the morphology and functional anatomy of this animal (Marshall 1977, Argot 2004b). About 35 cm tall at the shoulder and weighing about 15 kg (Argot 2004a), it presents an unusual combination of features. The relatively elongate, straight-boned forelimb and semi-digitigrade manus indicate a trend towards cursoriality, but the semi-opposable thumb, short metatarsals and prominent hallux are at odds with this. Lycopsis doesn't appear to have been a particularly good climber, but it could probably clamber in trees if need be, and Argot (2004b) noted that this might have been necessary given the densely forested environment it inhabited and the presence of a diverse crocodilian assemblage. Perhaps it was an ambush predator of small and mid-sized prey, able to engage in some running and some climbing, but not specialised for either. While previously imagined as thylacine-like in lifestyle, it probably mostly preyed on small rodents, and indeed we know it did at least sometimes, as rodent bones and a tooth were preserved adjacent to the pelvis of one specimen (Marshall 1977).

* Argot (2004b) mistakenly spelt it L. longirostris [sic].

Moving on to prothylacinids proper, Argot (2003b) showed that Prothylacinus patagonicus from the Santacrucian had a powerful neck, short, muscular limbs and a flexible body. Its manual phalanges were proportionally long, its manual unguals were deep, sharply curved and with large flexor tubercles, and it probably had a fairly large, pseudo-opposable pollex. What we know of its tail indicates that the organ was long (incorporating 20-30 vertebrae) and gradually tapering. Its metapodials were proportionally short and its feet were plantigrade, with a long first metatarsal probably supporting a long plantar pad that would have enabled the foot to grip curved surfaces.

Combined, these features strongly suggest that Prothylacinus was another scansorial borhyaenoid: a powerful, agile climber that used flexible hands in gripping branches and a muscular neck and large skull to support the weight of large prey items [see skeletal reconstruction and life restoration at very top of article, from Argot 2003b]. No living carnivorous mammal is exactly like this, but the strongest similarity is with binturongs Arctictis (Argot 2003b) [adjacent binturong from wikipedia]. These are large, slow but agile climbers, with robust, muscular limbs and long tails. They're predominantly frugivorous, so (so far as we know) are unlike Prothylacinus in this respect. Argot (2004a) speculated that the prey of Prothylacinus might have included rodents, caenolestoids and some of the smaller sloths. Incidentally, Prothylacinus was relatively large, with an estimated weight of 27-37 kg. However, such a size is not at all incompatible with a scansorial lifestyle, as demonstrated by living wolverines, sun bears, clouded leopards and leopards. Prothylacinus was from the Santacrucian fauna, so did it complete with the also scansorial Cladosictis and Sipalocyon? The difference in size between the hathlyacynids and prothylacinid indicate that they occupied different niches.

The crowded cheek teeth, shortened premolars and molars and tightly fused mandibular symphysis indicate that Prothylacinus was a specialised carnivore less suited for omnivory than other prothylacinids. Indeed, scars indicating large masseter muscles, combined with evidence from tooth cusp morphology, have led other prothylacinids (like Montehermosan Stylocynus paranensis: shown here, from Marshall 1979) to be regarded as omnivores with a bear-like diet (Marshall 1979).

The largest prothylacynid is Dukecynus magnus from La Venta in Colombia. Charactised by a particularly narrow, elongate rostrum (Goin 1997), its teeth suggest that it was more carnivorous than some of the other taxa, and its molars suggest close affinities with Pseudolycopsis cabrerai from the Chasicoan. Pseudothylacinus from the Colhuehuapian was thought by Marshall (1979) to be close to the ancestry of Pseudolycopsis.

And that's where we end for now. Still borhyaenids, proborhyaenids and thylacosmilids to come, and dare I say that I've left the best to last. Incidentally, wikipedia'a entries on borhyaenoids are woefully inadequate. Wouldn't it be great if someone took the information presented here (and in the other articles) and repackaged it for wider dissemination. Hint hint.

Refs - -

Argot, C. 2003a. Postcranial functional adaptations in the South American Miocene borhyaenoids (Mammalia, Metatheria): Cladosictis, Pseudonotictis and Sipalocyon. Alcheringa 27, 303-356.

- . 2003b. 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 analysis of the postcranial skeleton of a Laventan borhyaenoid, Lycopsis longirostris (Marsupialia, Mammalia). Journal of Vertebrate Paleontology 24, 689-708.

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.

Forasiepi, A. M., Sánchez-Villagra, M. R., Goin, F. J., Takai, M., Shigehara, N. & Kay, R. F. 2006. A new species of Hathliacynidae (Metatheria, Sparassodonta) from the middle Miocene of Quebrada Honda, Bolivia. Journal of Vertebrate Paleontology 26, 670-684.

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.

Marshall, L. G. 1977. A new species of Lycopsis (Borhyaenidae: Marsupialia) from the La Venta fauna (Late Miocene) of Colombia, South America. Journal of Paleontology 51, 633-642.

- . 1978. Evolution of the Borhyaenidae, extinct South American predaceous marsupials. University of California Publications in Geological Sciences 117, 1-89.

- . 1979. Review of the Prothylacininae, an extinct subfamily of South American "dog-like" marsupials. Fieldiana Geology New Series 3, 1-50.

- . 1981. Review of the Hathlyacyninae, an extinct subfamily of South American "dog-like" marsupials. Fieldiana Geology New Series 7, 1-120.

McKenna, M. C. & Bell, S. K. 1997. Classification of Mammals: Above the Species Level. Columbia University Press.

Muizon, C. 1999. Marsupial skulls from the Deseadan (late Oligocene) of Bolivia and phylogenetic analysis of the Borhyaenoidea (Marsupialia, Mammalia). Geobios 32, 483-509.

Savage, R. J. G. 1977. Evolution in carnivorous mammals. Palaeontology 20, 237-271.

Sinclair, W. J. 1906. Marsupialia of the Santa Cruz beds. In Scott, W. B. (ed) Reports of the Princeton University Expedition to Patagonia, 1896-1899. Princeton University, pp. 333-460.

Villarroel, C. & Marshall, L. G. 1983. Two new late Tertiary marsupials (Hathlyacyninae and Sparassocyninae) from the Bolivian Altiplano. Journal of Paleontology 57, 1061-1066.