Tetrapod Zoology

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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)].

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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.

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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).

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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].

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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…

And for other stuff on neat and obscure fossil mammals see…

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.

Comments

  1. #1 Brian
    August 24, 2009

    Well, you were right in asserting that these are some very obscure groups. So much for my opinion of myself as being reasonably well-versed in Paleogene mammals…
    Anyways, regardless of the affinities of didymoconids and wyolestids, it appears that many fossorial/myrmecophagous mammals evolved during the Paleogene. From the top of my head, I can think of pangolins, palaeanodonts, anteaters, armadillos, moles, aardvarks, golden moles and didymoconids. It seems reasonable to assume marsupial moles and perhaps Necrolestes go this far back too. When are the fossorial rodents (blesmoles, blind mice, etc.) suspected to have appeared?
    Oh and Darren, could the ptolemaiidans be up next? “I’d really like to find out more about these bizarre critters”, he said egotistically.

  2. #2 Dartian
    August 24, 2009

    Darren:

    didymoconids may have been fossorial animals that lived an underground life

    ‘Fossorial’ is a slightly problematic term in this context. Some workers, e.g., Lacey et al. (2000), make a distinction between ‘fossorial’ and ‘subterranean’. The former is a general term that may be used to describe any animal that is adapted for digging in one way or another, while the latter should be reserved for those that are highly specialised for a near-exclusive underground existence. To illustrate this with an example: both a badger Meles and a mole Talpa are fossorial, but only the mole is truly subterranean. Thus, if Wang et al. really think that Didymoconus was specifically mole-like (rather than, say, badger-like) ecomorphologically, they should preferably call it subterranean rather than fossorial.

    Semantics aside, I’m not convinced that modern moles are particularly good didymoconid analogues. The partial Didymoconus skull described by Wang et al. (2001) is 73.4 mm long. That’s more than twice the skull length of a common European mole Talpa europaea, and about the same length as that of a male American mink Neovison vison or a female pine marten Martes martes. To me, Didymoconus berkeyi seems way too large to be a mole-like subterranean burrower.

    it’s evident that Insectivora sensu Lopatin is polyphyletic

    Not to him, apparently. I took a look at Lopatin’s 201-page paper (I didn’t read all of it – long paper is long!); he makes it quite clear that, in his opinion, morphology trumps molecules. In other words, he dismisses the Afrotheria concept and retains tenrecs and golden moles in the traditional order Insectivora. Interesting.

    Reference:

    Lacey, E.A., Patton, J.L. & Cameron, G.N. 2000. Introduction. In Lacey, E.A., Patton, J.L. & Cameron, G.N. (eds.) Life Underground: The Biology of Subterranean Rodents, The University of Chicago Press (Chicago), pp. 1-14.

  3. #3 J.S. Lopes
    August 24, 2009

    Putative relationships to creodonts, mesonychians or lipotyphlans would point to view didymos as a basal Laurasiatherian group.

  4. #4 Larura Tamara Henson
    August 24, 2009

    Dartian | August 24, 2009 9:56 AM said

    Semantics aside, I’m not convinced that modern moles are particularly good didymoconid analogues. The partial Didymoconus skull described by Wang et al. (2001) is 73.4 mm long. That’s more than twice the skull length of a common European mole Talpa europaea, and about the same length as that of a male American mink Neovison vison or a female pine marten Martes martes. To me, Didymoconus berkeyi seems way too large to be a mole-like subterranean burrower.

    I thought the same thing, could Didymoconus be analogous to the American Badger (Taxidea taxus) and similar burrow dwelling carnivores?

  5. #5 Mickey Mortimer
    August 24, 2009

    I gotta agree with Brian- I thought I had at least heard of most families of mammals, but you’ve managed to find two I was completely ignorant of.

  6. #6 Allen Hazen
    August 25, 2009

    Hmm… “fourth premolar is molariform, the third upper molar is reduced or absent.”

    In the drawing, M3 is absent (and M2 about as far aft as there’s room for) and P4 is very molariform. So, how do we know we aren’t dealing with a critter that had three molars and only two premolars? (There are mammals like that: Homo, for instance!) In this instance I take it there are other helpful specimens: some with tiny M3, and the wording suggests that the lower third molar, m3, is not missing, so the identities of the upper teeth can be worked out from occlusal relationships.

    BUT… Suppose we had a taxon known only from a specimen looking like the upper jaw figured. Would it be clear to a professional paleomammalogist whether it had molariform P4 and two molars or had M1-M3? And if so, what is the giveaway? The suture between the premaxillary and maxillary is useful for distinguishing canines from incisors– is there some analogous osteological landmark for premolars and molars?

    (As a REALLY ANNOYINGLY PERSISTENT poster on another discussion board I read used to say, “Keep asking, keep learning!)

  7. #7 Darren Naish
    August 25, 2009

    Thanks for comments, very interesting. Alan (comment 6): I think that P4 has been identified as such (rather than as M1) because the genuine M1 is much wider transversely than the premolars in most didymoconid species (it’s very evident in Wang et al.’s photos of D. berkeyi); however, I’ve only just appreciated that it isn’t the case in D. colgatei illustrated above. How ironic. I hope my mammalogist friends will forgive me for saying this, but much of the tooth and cusp and cuspule identification that goes on seems to be more art than science (at least, to an outsider), and distinguishing such things as molariform premolars and molars often seems to be down to such superficial details as perceived complexity, degree of emergence, or transverse width.

    On fossorial vs subterranean (comment 2): I didn’t know there was a difference (and I’m not sure other people do either), thanks for clarification. Some of the literature on didymoconids (e.g. Wang et al. 2001) does conclude that they were mole-like burrowers, and hence subterranean. However, I’d agree with you that they may not have been quite so specialised, and hence more toward the fossorial end of the scale.

    And Mickey (comment 5): I live to give!

  8. #8 Jason S.
    August 25, 2009

    Didymoconus berkeyi looks more like a small analogue of a sabre-tooth to me with those long canines…

  9. #9 Dartian
    August 26, 2009

    Darren:

    On fossorial vs subterranean (comment 2): I didn’t know there was a difference (and I’m not sure other people do either)

    There’s a difference for sure, but there’s unfortunately also plenty of confusion regarding the use of proper terminology. For example, no less an authority than John Eisenberg (1981) used the term ‘fossorial’ for moles and other near-exclusively subterranean diggers, while using ‘subfossorial’ for badgers, marmots, armadillos, and other digging mammals that spend much/most of their time above the soil surface. The important point here is, however, that Eisenberg, too, made a clear ecological distinction between these two substrate-use categories. (For the sake of simplicity, I suggest we just call them the ‘mole category’ and the ‘badger category’ for now, to avoid any further confusion.)

    Significantly, Eisenberg emphasised the difference in maximum size between the ‘mole category’ and the ‘badger category'; the former are, on average, much smaller*. Thus, since Didymoconus falls, in terms of body size, well into the ‘badger category’, it seems more likely to me that it was also ecologically more analogous to a badger than to a mole. In other words, as there are no extant mammals in that size range that live near-exclusively below the soil surface, it doesn’t seem very likely that Didymoconus lived that way either. And thus, in my opinion, it’s rather misleading of Wang et al. (2001) to compare it to a mole.

    * Rarely attaining head+body lengths of ca. 20 cm, and usually smaller than that.

    Moral of this somewhat convoluted story: always define your terminology, and choose your analogies well.

    Reference:

    Eisenberg, J.F. 1981. The Mammalian Radiations. The University of Chcago Press, Chicago and London.

  10. #10 John Scanlon, FCD
    August 26, 2009

    I’m less familiar with placentals than marsupials, where this is a useful rule of thumb: premolars have two roots fore-and-aft, upper molars three (ant, post and lingual). There are exceptions (e.g. reduced posterior-most molars lacking a posterior root) but it seems to be pretty good at defining the P-M boundary. Does it work for placentals, and didymoconids in particular?

  11. #11 John Scanlon, FCD
    August 26, 2009

    BTW, the cover of Dinosaurs of the Isle of Wight (immediately left of the box I’m typing in) always reminded me of Bakker’s Dinosaur Heresies (Tyrannosaurus-Triceratops combat by John Gurche) but I never looked at them side by side before. That cannot be coincidence, surely!

  12. #12 Darren Naish
    August 26, 2009

    The artist produced the cover in a real hurry (literally, over-night) and let’s just say that he was heavily inspired. It has become a point of contention: I dislike the cover quite a lot, but was unable to do anything about it.

    On premolar/molar roots – no, the number of roots doesn’t help in placentals: P4 (or P3 in taxa with three premolars) often has three roots like M1.

  13. #13 Darren Naish
    August 26, 2009

    By the way, the Gurche painting isn’t Tyrannosaurus vs Triceratops but Daspletosaurus vs Styracosaurus I think. I’ve seen the painting in real life: like all Gurche works, it’s surprisingly tiny (width less than 30 cm).

  14. #14 John Scanlon, FCD
    August 26, 2009

    Daspletosaurus vs Styracosaurus I think

    I copied what was on the back flap, so that was my mistake.

  15. #15 David Marjanović
    August 31, 2009

    much of the tooth and cusp and cuspule identification that goes on seems to be more art than science (at least, to an outsider), and distinguishing such things as molariform premolars and molars often seems to be down to such superficial details as perceived complexity, degree of emergence, or transverse width.

    For instance, eutherians have five premolars by default, not four, and the first one to be lost (resulting in the four premolars that are plesiomorphic for most, though maybe not all, placentals) is number three. So, our (human) premolars are not P1 and P2, and not P3 and P4, but P4 and P5.

    There was an SVP meeting abstract last year which claimed the first molar of marsupials to be homologous to the last premolar of placentals, but I don’t know how that’s supposed to square with animals like Kielantherium which has five premolars and four molars.

  16. #16 J. S. Zijlstra
    August 31, 2009

    I’d propose that other mammals follow the lead of the muroids and get rid of their premolars altogether. That would certainly rid us of a lot of confusion. In the process, the canines could go too, of course, as all Glires will agree. And Mayermys has some even more exciting proposals.

    On a more serious note, thanks for an interesting post and for the interesting comments. It’s interesting to see how these early shoots of the eutherian radiation developed and like someone above me I’d love to read about other such groups, like ptolemaiidans.

  17. #17 Allen Hazen
    September 1, 2009

    David Marjanovic–
    I think the suggestion that the first molar of marsupials should be identified with the last premolar of placentals has been made before. … Maybe it’s the philosopher in me, but I find myself wondering what this MEANS. In typical placentals the premolar/molar distinction has to do with ontogeny: premolars are diphyodont, molars monophyodont. Underlying mechanism or genetic coding still uncertain. But as long as the underlying mechanism and/or genetic coding is uncertain, I’m not sure what the relevant criteria of homology are supposed to be.

    J.S. Zijlstra–
    Thanks! I hadn’t known of Mayermys before. SERIOUSLY strange dentition!

  18. #18 Dartian
    September 1, 2009

    Allen:

    Thanks! I hadn’t known of Mayermys before.

    Neither had I, and I’m delighted to have learned about it. This weird New Guinean rodent taxon – there are two species known (Helgen, 2005) – has only one molar in each jaw quadrant, i.e., only eight teeth in total (four molars and four incisors). That’s fewer teeth than any other rodent, living or fossil.

    Reference:

    Helgen, K.M. 2005. A new species of murid rodent (genus Mayermys) from south-eastern New Guinea. Mammalian Biology 70, 61-67.

  19. #19 Allen Hazen
    September 2, 2009

    The Helgen paper has photos of skulls, in which the molars are (if not completely invisible) hard to make out: they are apparently very tiny.

    Given what rodent molars can be like– look at a Capybara’s– I had imagined something else: a normal rodent amount of molar crown area, but all concentrated on a single enlarged tooth. Sort of like an elephant’s dentition. That would be a REALLY weird rodent, but the actual Mayermys(*) is surprising enough!

    (*) Apparently it has been synonymized: the genus is now, i.i.r.c., Pseudohydromys or Pseudoneomys or something like that.

  20. #20 J.S. Zijlstra
    September 2, 2009

    Well, Mayermys has certainly rapidly become popular, so I might delve a little deeper on it. As said, it actually has only one molar left per jaw quadrant, and its tiny (about 0.4 mm if I recall correctly). The reason for that is that it is a specialized insectivore which doesn’t really need to chew a lot; it justs eats its insects and digests them. It is part of a larger group of rodents from New Guinea and surrounding islands (such as Australia), most of which lost their third, but not second molars. Some are specialized, shrew-like insectivores, others are aquatic. As Allen points out, Mayermys has been synonymized with Pseudohydromys recently, which is a fairly similar animal except that it didn’t lose its second molars, but we’ll have to see whether that becomes generally accepted.

  21. #21 Allen Hazen
    September 3, 2009

    “specialized, shrew-like insectivores”
    Maybe that has something to do with why they are called “shrew mice”?

    BUT: actual shrews don’t have such extremely reduced dentition. Insects have to be cut up (well, if you’re shrew- or mouse-size they do), their cuticles have to be crushed or pierced. Mayermys and its cousins must be eating particularly soft-bodied, or maybe very small, insects? The article certainly said there wasn’t much natural-history data on Mayermys (at least on the newly discovered species, which is known from one specimen), so I suppose we just don’t know what it eats.
    If you do delve deeper and find out something, please try to report back!

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