Time to finish with the mesonychians. Previous articles have looked at Andrewsarchus and the triisodontids, the mesonychids, and the hapalodectids. That’s essentially it… though – as mentioned a few times now – Andrewsarchus doesn’t seem to be a mesonychian after all. However, there are a number of other obscure Paleogene mammal groups that have been considered to be allied to (or part of) Mesonychia by some authors, and in the interests of completeness I want to look at them here.
We start with the didymoconids. This is a peculiar group of Asian mammals known from the Paleocene, Eocene and Oligocene. They’re mostly known from teeth and jaw fragments and seem to have been similar in size to skunks, martens and other mid-sized carnivorans. Didymoconid teeth are superficially similar to those of mesonychians in being closely spaced and tall-crowned; they also have a similar cusp arrangement. The fourth premolar is molariform, the third upper molar is reduced or absent, and the upper molars are symmetrical when seen in side view [Didymoconus berkeyi skull shown above, from Wang et al. (2001)].
However, in other respects, didymoconids were decidedly unlike mesonychians. The didymoconid snout was broad, the orbits were relatively small, the back of the skull was wide and flat, and the jaws were deep with large canines [skull pictures above, showing Didymoconus colgatei from the Hsanda Gol Formation, from Matthew & Granger (1924)]. This skull shape looks well suited for burrowing, and others authors have suggested likewise on the basis of the robust arm and hand bones described for Didymoconus [chunky D. berkeyi manus (and other postcranial bones) shown below. From Wang et al. (2001)]. The hand claws of Didymoconus were long and have been compared to those of burrowing mammals like moles. In contrast, the hindlimbs were slender. A peculiar feature is the presence of a complicated system of enlarged sinuses in the middle ear region. These might, suggested Wang et al. (2001), have provided these animals with improved low-frequency hearing. Low-frequency hearing is often seen in fossorial mammals. Because burrowing features (a flattened skull and extensive sinus system) are present in Hunanictis (Meng et al. (2001) – a more basal taxon within Didymoconidae than Didymoconus – it may be that fossorial specialisations were present throughout the group.
So didymoconids may have been fossorial animals that lived an underground life, preying on arthropods and worms: if so, they were obviously pretty different from the large, omnivorous or carnivorous mesonychians.
The best known didymoconid is Didymoconus Matthew & Granger, 1924 from the Late Eocene and Oligocene of Mongolia and China. However, a list of other genera have been allocated to the group: Ardynictis Matthew & Granger, 1925, Kennatherium Mellett & Szalay, 1968, Zeuctherium Tang & Yan, 1976, Archaeoryctes Zheng, 1979, Hunanictis Li et al., 1979, Jiajianictis Tong, 1997, Khaichinula Lopatin, 2006 and Erlikotherium Lopatin, 2006. Wanolestes Huang & Zheng, 2002 – described as a shrew – might also be a didymoconid (Lopatin 2006).
Kennatherium from the Late Eocene of China (named for Malcom McKenna) is one of the smallest known members of the group and – with a skull less than 5 cm long – would probably have been less than 30 cm long in total length [Kennatherium lower jaw shown here, from Mellett & Szalay (1968)]. In their original decription of the taxon, Mellett & Szalay (1968) regarded Kennatherium as a didymoconid (though they also noted that the style of tooth wear and some details of the back of the mandible hinted at a marsupial identification), but they didn’t regard these animals as anything to do with mesonychians; rather, they regarded them as palaeoryctoids and as part of Van Valen’s Deltatheroida. I won’t explain those group names here (another time); they have rather tangled and confusing histories (feel free to elaborate in the comments and save me some work!). Together with Erlikotherium edentatum Lopatin, 2006, Kennatherium has most recently been placed within the didymoconid ‘subfamily’ Kennatheriinae. Lopatin (2006) has made the interesting suggestion that kennatheriines had a reduced dentition, and that this dentition indicates a diet of social insects. I haven’t seen this discussed elsewhere – pretty exciting if true. An Ardynictinae ‘subfamily’ was also recognized by Lopatin (2006) for Ardynictis and Archaeoryctes.
Wyolestines or wyolestids
Finally, Gingerich (1981) argued that his new taxon Wyolestes apheles from the Lower Eocene of Wyoming was a didymoconid and that – together with Mongoloryctes from Upper Eocene Mongolia – it belonged to another new didymoconid ‘subfamily': Wyolestinae (Hsiangolestes from Lower Eocene Hunan has also been regarded as part of this group). This hasn’t been widely accepted and most authors have recognized these animals as representing a distinct group (Wyolestidae) of uncertain affinities within Eutheria. A second Wyolestes species, W. dioctes (also from the Lower Eocene of Wyoming) was named by Gingerich (1982) (the type specimen [USNM 22456] had originally been identified as a hyaenodontid and referred to Sinopa and tentatively to S. viverrina; this taxon was later placed back into Prototomus and USNM 22456 was proposed as the neotype for Prototomus viverrinus (Van Valen 1967). This was never acted upon). A third species, W. iglesius, was described from the Lower Eocene of Baja California (Novacek et al. 1991). Wyolestid anatomy is poorly known, but their lower jaws are long and slender, so they may well not have been as stout-headed as didymoconids [cranial elements of the didymoconid Ardynictis are shown below, mostly because I couldn't find any good wyolestid images].
The hypothesis that didymoconines, kennatheriines, ardynictines and wyolestids really do form a clade requires testing, though it seems generally accepted that didymoconines, kennatheriines and ardynictines are allied within Didymoconidae. These were peculiar, archaic placental mammals with stout forelimbs and short, broad skulls, though their exact appearance remains uncertain and I can’t recall seeing any life restorations. Are they anything to do with mesonychians? Wang (1976) and Gingerich (1981) thought so, and both authors argued that the Paleocene mesonychid Yantanglestes might be intermediate between more typical mesonychids and didymoconids (the latter including wyolestids according to Gingerich (1981)). Gingerich (1998) continued to support this view but now regarded wyolestids as a distinct ‘family’ from didymoconids; furthermore, he now included Yantanglestes within Wyolestidae.
However, it has to be said that the idea of a link between didymoconids and/or wyolestids with mesonychians is very much a minority opinion. While the exact affinities of didymoconids and wyolestids remain uncertain, some authors have suggested that they might have been allied with leptictids (McKenna & Bell 1997), cimolestids (Archibald 1998) or hyaenodontids (Novacek et al. 1991). In earlier decades, didymoconids were included within Carnivora or Insectivora, but only because these names were used for catch-all groups that incorporated huge, motley assemblages of Paleogene placentals. Lopatin (2006) recognized a group called Didymoconida that were included within an Insectivora ‘superorder’, but it’s evident that Insectivora sensu Lopatin is polyphyletic.
Given their weird morphology and probable burrowing habits, it would be neat and surprising if didymoconids really were part of the mesonychian radiation. For now, however, they remain of uncertain phylogenetic position and we await more work.
For previous articles on Paleogene mammals see…
- Homage to The Velvet Claw (part I)
- Giant killer pigs from hell
- Snow White and the six perissodactyls
- Thunder beasts in pictures
- Thunder beasts of New York
- Because we all love Paleogene ‘ungulates’
- What did a dinoceratan do?
- Because Andrewsarchus is not the world’s only mesonychian (mesonychians part I)
- Mesonychians part II: Andrewsarchus was a hell of a lot weirder than all the books say
- Mesonychians part III: Andrewsarchus and the triisodontines
- Mesonyx and the other mesonychid mesonychians (mesonychians part IV)
- Hapalodectids, the once otter-like proto-whales (mesonychians part V)
- DO NOT PANIC: we are not yet done on the mesonychians…
And for other stuff on neat and obscure fossil mammals see…
- Ten things you didn’t know about sloths
- Five things you didn’t know about armadillos
- Dude, where’s my astrapothere?
- Snorki the giant’s friends and relatives
- What was that skull? (glyptodonts)
- Invasion of the marsupial weasels, dogs, cats and bears… or is it?
- Long-snouted marsupial martens and false thylacines
- Marsupial ‘bears’ and marsupial sabre-tooths
- Killer sperm whales
- Because it would be wrong not to mention a sperm whale named like a tyrannosaur
- Dromomerycids: discuss
Refs – –
Archibald, J. D. 1998. Archaic ungulates (“Condylarthra”). In Janis, C. M., Scott, K. M. & Jacobs, L. L. (eds) Evolution of Tertiary Mammals of North America. Volume 1: Terrestrial Carnivores, Ungulates, and Ungulatelike Mammals. Cambridge University Press, pp. 292-331.
Gingerich, P. D. 1981. Radiation of early Cenozoic Didymoconidae (Condylarthra, Mesonychia) in Asia, with a new genus from the early Eocene of western North America. Journal of Mammalogy 62, 526-538.
– . 1982. Second species of Wyolestes (Condylarthra, Mesonychia) from the Early Eocene of western North America. Journal of Mammalogy 63, 706-709.
– . 1998. Paleobiological perspectives on Mesonychia, Archaeoceti, and the origin of whales. In Thewissen, J. G. M. (ed) The Emergence of Whales: Evolutionary Patterns in the Origin of Cetacea. Plenum Press (New York), pp. 423-449.
Lopatin, A. V. 2006. Early Paleogene insectivore mammals of Asia and establishment of the major groups of Insectivora. Paleontological Journal 40, S205-S405.
Matthew, W. D. & Granger, W. 1924. New Carnivora from the Tertiary of Mongolia. American Museum Novitates 104, 1-9.
McKenna, M. C. & Bell, S. K. 1997. Classification of Mammals: Above the Species Level. Columbia University Press (New York).
Mellett, J. S. & Szalay, F. S. 1968. Kennatherium shirensis (Mammalia, Palaeoryctoidea), a new didymoconid from the Eocene of Asia. American Museum Novitates 2342, 1-7.
Meng, J., Ting, S.-Y. & Schiebout, J. A. 1994. The cranial morphology of an early Eocene didymoconid (Mammalia, Insectivora). Journal of Vertebrate Paleontology 14, 534-551.
Novacek, M. J., Ferrusquia-Villafranca, I., Flynn, J. J., Wyss, A. R., Norell, M. A. 1991. Wasatchian (early Eocene) mammals and other vertebrates from Baja California, Mexico: the Lomas Las Tetas de Cabra fauna. Bulletin of the American Museum of Natural History 208, 1-88.
Van Valen, L. 1967. Prototomus viverrinus Cope, 1874 (Mammalia): proposed designation of a neotype under the plenary powers together with a grant of precedence to Palaeonictidae over Ambloctonidae. Bulletin of Zoological Nomenclature 24, 93-94.
Wang, B. 1976. Late Palaeocene mesonychids from Nanxiong Basin, Guangdong. Vertebrata Palasiatica 14, 252-258.
Wang, X., Downs, W., Xie, J. & Xie, G. 2001. Didymoconus (Mammalia: Didymoconidae) from Lanzhou Basin, China and its stratigraphic and ecological significance. Journal of Vertebrate Paleontology 21, 555-564.