Field of Science

"Creodonts": Carnivores by Association


"Karianne's Pet" by Carl Buell. The large animal in the painting is the hyaenodontid Megistotherium osteothlastes, a contender for the title of biggest terrestrial carnivorous mammal ever.


As explained in an earlier post (which you may be interested in reading as a bit of background to this one), the earlier part of the Caenozoic (the current era of the earth's history) was home to a number of mammalian lineages of very mysterious relationships. Very few of the familiar orders around us today had yet put in an appearance, and instead the world was home to such oddities as pantodonts, tillodonts and dinocerates. Among the prominent carnivorous mammals of the time were a group known as the creodonts. Creodonts ranged in size from that of a small cat to lion- or bear-size species, and often converged in appearance with those animals. But what were creodonts?

Current authors regard the Creodonta as including two families, the vaguely cat-like Oxyaenidae and the largely dog- or hyaena-like Hyaenodontidae. Oxyaenids were found in North America and Europe during the late Palaeocene and Eocene, while hyaenodontids were found in Africa, Eurasia and North America from the Late Palaeocene to near the end of the Miocene, though they disappeared from North America not long after the end of the Eocene (Gheerbrant et al., 2006). Many authors have suggested a relationship with modern carnivorans (cats, dogs, weasels, bears, etc.), and they have been included with the latter in a superorder Ferae. Popular as this arrangement has been, however, there's just one small problem - there's not a shred of evidence to support it.


The oxyaenid Patriofelis ferox, reconstructed by Dmitry Bogdanov.


Part of the problem is that creodonts are a good example of what might be called "taxonomic drift". Imagine that an author establishes a taxon, and presents a list of organisms that he thinks belong to that taxon. A few years pass by, and the taxon is revised by another author, who excludes some of the originally-included species that he thinks belong elsewhere, and substitutes a few more species that he believes to be related to the remainder. Carry this on through a few subsequent revisions, with species being taken out and put in, and you may end up with a situation where nearly all of the original members of the taxon have been taken out, and the taxon name has become associated with a very different concept from its original intent. This can be horrendously confusing for later readers, because if they don't realise that this taxonomic drift has taken place, they may read things into older publications that their authors never intended.

Creodonta was originally established by Edward Drinker Cope in 1875 as a suborder of the Insectivora*. In his new suborder, Cope included three families - Oxyaenidae, Ambloctonidae (now included in Oxyaenidae - Gunnell, 1998) and Arctocyonidae (another contemporary family of carnivorous placentals, within which Cope also included what are now regarded as the Miacidae). The Hyaenodontidae were not part of the original Creodonta - at the time, Hyaenodon was regarded as a genuine carnivoran. Cope distinguished creodonts from carnivorans by the former's lack of a fused scapholunar bone in the wrist, their ungrooved astragalus, and their less-developed and smoother cerebral hemispheres (Cope, 1884). These features, it should be noted, are all primitive for placentals, but to Cope indicated the creodonts' position in the insectivoran grade. He nevertheless regarded creodonts as ancestral to carnivorans, with cats descended from Oxyaenidae and dogs from Miacidae (Cope, 1880). Later, Cope (1883) included Insectivora and Creodonta as separate suborders of his order Bunotheria, which also included the tillodonts, taeniodonts and prosimians**. Cope (1883) also redefined creodonts to include mammals without continuously-growing incisors and with trituberculate molars, which meant that in addition to the Oxyaenidae and Miacidae, Creodonta now included Mesonychidae, Leptictidae, moles and tenrecs (Arctocyonidae were transferred to the Insectivora). The Hyaenodontidae wormed their way in a year later (Cope, 1884).

*This does not necessarily mean that he thought they were specifically related to modern insectivorans such as shrews and hedgehogs. Cope and most of his contemporaries would have regarded the "Insectivora" as representing the generalised basal form from which all other placental mammals were derived, and recent insectivorans would have been the remnants of that original grade.

**It is also notable that Cope regarded the aye-aye as forming a separate suborder from other prosimians, due to its rodent-like incisors. Cope (1884) held that the tillodonts were "intimately allied to the living Chiromys [aye-aye] of Madagascar, which is itself almost a lemur, by general consent" (emphasis mine).


Skull of the sabre-toothed creodont Machaeroides eothen. Gunnell (1998) places Machaeroides in Oxyaenidae. Photo by Ghedoghedo.


So right from the beginning, the question of what was a creodont was convoluted. Over the years, various families of "creodonts" were reassigned as their relationships became clearer. The Miacidae became regarded as true Carnivora. Arctocyonidae and Mesonychidae became included among the primitive ungulates (another confused mess, but that's a story for another year) and may be related to artiodactyls. Moles and tenrecs, of course, were reunited with their fellow modern insectivorans (though the tenrecs have recently had another falling-out). Eventually, the creodonts were whittled down to their modern content of oxyaenids and hyaenodontids, but, as pointed out by Polly (1996), "Hyaenodontidae and Oxyaenidae are currently grouped together in Creodonta because they are the only taxa that have not been removed from the group, not because there has been specific positive evidence proposed for their grouping". Those few characters the two families do share are also found in other, non-creodont mammals. As for their association with Carnivora, the two orders have been associated because they both possess shearing carnassial teeth. However, while the carnassials in Carnivora are formed by the last upper premolars and the first lower molars, those of Oxyaenidae are derived from the first upper and second lower molars, while hyaenodontids have two sets of carnassials formed by the first upper/second lower and second upper/third lower molars. Carnassials have also developed in other groups of mammals - notably the borhyaenoids, which are metatherians if not marsupials and so definitely not related to carnivorans. The only real reason creodonts have been associated with Carnivora for so long seems to be their prior inclusion of the genuinely carnivoran (or stem-carnivoran) miacids. It's a bit like when one of your friends brings an acquaintance of theirs to a party who just hangs around for hours with everybody being too polite to ask them to leave.

So, if they weren't related to Carnivora, can we say what creodonts were related to? Particularly in the case of Oxyaenidae, the answer is brief, simple and to the point: we really have not got a sodding clue. Whatever their ancestry might have been, oxyaenids were horribly derived little (or not so little) beggars - for instance, they had completely lost the third molars. Van Valen (1969) derived both oxyaenids and hyaenodontids from the Palaeoryctidae, particularly from the Cretaceous-Palaeocene Cimolestes, and other authors seem to have regarded the idea favourably, at least for the hyaenodontids (Polly, 1996; Gheerbrant et al., 2006). The main problem with this scenario, however, is that the Palaeoryctidae of Van Valen and other authors is itself polyphyletic. For instance, the phylogenetic analysis of Wible et al. (2007) included two "palaeoryctids", Cimolestes and Eoryctes (Eoryctes is more likely to represent the Palaeoryctidae proper),and while Cimolestes appeared outside the placental crown group, Eoryctes was placed among the insectivorans as the sister to Potamogale (Tenrecidae). If creodonts (either or both families) are closer to Cimolestes, they may be stem-eutherians. If they are closer to Palaeoryctidae proper, they may even be afrotheres (Wible et al. did not support placement of tenrecs among afrotheres, but it is notable that the earliest hyaenodontids are African). Placement of either the Oxyaenidae or the Hyaenodontidae still awaits proper analysis.

REFERENCES

Cope, E. D. 1880. On the genera of the Creodonta. Proceedings of the American Philosophical Society 19 (107): 76-82.

Cope, E. D. 1883. On the mutual relations of the bunotherian Mammalia. Proceedings of the Academy of Natural Sciences of Philadelphia 35: 77-83.

Cope, E. D. 1884. The Creodonta. American Naturalist 18 (3): 255-267.

Gheerbrant, E., M. Iarochene, M. Amaghzaz & B. Bouya. 2006. Early African hyaenodontid mammals and their bearing on the origin of the Creodonta. Geological Magazine 143 (4): 475-489.

Gunnell, G. F. 1998. Creodonta. In Evolution of Tertiary Mammals of North America vol. 1. Terrestrial Carnivores, Ungulates, and Ungulatelike Mammals (C. M. Janis, K. M. Scott & L. L. Jacobs, eds) pp. 91-109. Cambridge University Press.

Polly, P. D. 1996. The skeleton of Gazinocyon vulpeculus gen. et comb. nov. and the cladistic relationships of Hyaenodontidae (Eutheria, Mammalia). Journal of Vertebrate Paleontology 16 (2): 303-319.

Van Valen, L. 1969. The multiple origins of the placental carnivores. Evolution 23 (1): 118-130.

Wible, J. R., G. W. Rougier, M. J. Novacek & R. J. Asher. 2007. Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary. Nature 447: 1003-1006.

18 comments:

  1. Creodonts and Carnivores are discussed in a chapter of Rose & Archibald, eds, "Rise of Placental Mammals". The authors of the chapter, as I recall, seem to have thought there was some grounds for keeping them together, though they admitted the evidence was thin. Carnivores (and Pangolins) apparently have one odd morphological feature with no obvious functional explanation: the "osseous tentorium". (I doubt I'd recognize this if you split a skull open to sho me, but from descriptions it's apparently a sheet of bone separating the cerebrum and cerebellum.) There was one sentence that I read as implying that this had been found in Creodonts as well: if this is so, there would be a bit of evidence that Creodonts (in the sense of Oxyaenids + Hyaenodontids) might, as traditionally believed, be sister group (or two sister groups) of the Carnivora.
    --
    Rose's more recent "Begining of the Age of Mammals" left me with an even more depressing sense that the evidence for any phylogenetic hypothesis about these animals is pretty thin. Even the integrity of the two "families" (O and H) seems weakly supported: as I recall there is a bunch (taxon name begins, I think, with "L") of pseudo-sabertooth forms often assigned to one or another of these families... but without it being entirely clear which!

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  2. Limnocyoninae. Which (it's been a few months since I last looked at either book) may or may not include the "pseudo-sabertooth" Machaerodontinae (?). Anyway, either Rose or the authors of the C&C chapter of Rose and Archibald left the impression that (though they plumped for one) they though the Limnocyoninae could end up on either Creodont team.

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  3. According to Fox & Youzwyshyn (1994), there's also an osseus tentorium in horses. They also point out that a number of carnivorans lack an osseus tentorium, but I don't think that that's necessarily a problem - it could easily be a secondary loss. Certainly, there's no reason at present why creodonts couldn't be related to Carnivora, but there's no reason it should be favoured over any alternative.

    The big problem, as far as I can see, is that most recent studies linking the two have effectively done so via Cimolestes, and if Wible et al. (2007) are correct, that's no longer a viable option. Mind you, Cimolestes is fairly speciose for a genus of fossil vertebrates (eight species or thereabouts). I wonder if it would be worth testing whether Cimolestes itself is monophyletic?

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  4. Oops, sorry, forgot the reference:

    Fox, R. C., & G. P. Youzwyshyn. 1994. New primitive carnivorans (Mammalia) from the Paleocene of western Canada, and their bearing on relationships of the order. Journal of Vertebrate Paleontology 14 (3): 382-404.

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  5. Hyenodonts were absent in Holarctica before Eocene, and were present in African Upper Paleocene, what would be a good support for being Afrotherians, like Proboscideans, Macroscelideans and other ones. If they are not, they could be immigrants from Southern Asia using the same route as Primates. Oxyaenids were present since Middle Paleocene, in Europe and North America. There are no good and clear ancestors in Lower Paleocene in N.America, which might point to an Asian origin, or maybe, European (Lower Paleocene faunas in Europe is very few known.

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  6. "Ex-creodonts" Arctocyonidae and Mesonychidae, two groups found in Lower Paleocene in both Asia and N.America, but no Oxyaenidae, that seems not to be present in oldest Asian sites. I think it would reinforce an origin for Oxyaenidae in Paleocene Europe, or tropical Southern North America-Central America, places still poorly explored.

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  7. Christopher--
    Thanks for reference: I will try to get hold of the Fox & Youzwyshyn article.

    As for monophyly of Cimolestes, my recollection (this is probably from the chapter in Rose & Archibald) is that different species were dramatically different: differentt numbers of molar teeth. And that at least one writer thought Carnivora and the two families of Creodonta might have originated from different species in the "genus" Cimolestes!

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  8. Largest carnivorous mammal? They were larger than blue whales? or sperm whales, if filter feeders don't count?

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  9. Thanks for the reference! The Fox and Youzwyshyn paper is GREAT!!!!


    bibliographical comment here, methodological comment in separate reply.

    Apparently (F&Y, p. 395, right column) the suggestion that Carnivorans and Creodonts might derive from different species of Cimolestes goes back to Lillegraven in a 1969 "University of Kansas Paleontological Contributions." (I assume it is not worth even LOOKING to see if an Australian university library has it.) With a number of later authors repeating the suggestion. (Hmm. Does it seem wildly... optimistic? ... to think two order-level groups (Ca & Cr) might be traceable to already identified species of the same genus?)

    As for your doubts about the monophyly of Cimolestes, F & Y seem to share them: "the genus is composite: it is a grade of organization rather than a single phylogenetic unit, and is probably both polyphyletic and paraphyletic" (p. 396, left column). On the other hand (lower in same column) they seem willing to accept Palaeoryctidae, including the species of Cimolestes and a few other genera, as monophyletic. Hmm... They seem to leave open the possibility that Creodonta, though NOT Carnivora, might derive from Palaeoryctidae (p. 396, bottom of left and top of right column): that would make P. paraphyletic.

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  10. Methodological comment.
    P&Y's criticisms of Flynn et al.'s alleged synapomorphies of Carnivora and Creodonta (p. 395, left column) seem pretty devastating, BUT:
    --->A trait found in only a few groups can still have SOME value as evidence for a close relationship of two subgroups EVEN IF it is shared by a few, scattered, outsiders. So the argument from the "osseus tentorium" (that should be "osseous," shouldn't it? an English adjective?) might still give SOME support for a Ca+Cr clade even if a few other mammals also have it. Paticularly since Pangolins, on molecular grounds, are thought to be close to Carnivora. (In one of the chapters of Szalay, Novacek & McKenna, eds., "Mammal Phylogeny, v. 2" [Springer-Verlag, 1993], after the surprising suggestion of a Carnivora/Pholidota link is introduced on molecular grounds, the o.t. is mentioned as a possible bit of morphological confirmation.)
    ... Equus would be a convergence.

    On the other hand, I found the discussion of premolar and molar morphology (pp. 397ff) VERY impressive, with clear logic. [Breve: Resemblances between Cimolestes cerberoides and other Viverravids/Miacids are NOT shared by the newly discovered plesiomorphic V and V?M? taxa, and are so likely to be convergent.]

    Until now I think I thought a Carnivora/Creodonta/Cimolestes clade maybe more likely than not: I don't any more!

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  11. Drhoz: You're quite right. I've corrected the error.

    Allen: No, it's "osseus" (Latin for "bone", and in Latin the adjective or possessive follows the noun). And you're right - convergence (or convergent loss) is entirely possible. And even the presence of an osseus tentorium in horses may not be a problem, when you consider that at least some molecular trees have indicated the sister taxon of carnivores + pangolins to be - wait for it - perissodactyls.

    But of course, it is exactly because of this chance of convergence that single-character phylogenetics is a mug's game. At the end of the day, we need the proper analysis!

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  12. How about tha putative Cimolestes-like status in Indian Cretaceous Deccanolestes and an unnamed Bolivian Tiupampan tooth? It would make Cimolestes-like mammals as a worldspread group, present in Gondwana.

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  13. Do other perissodactyls have the osseus tentorium?

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  14. Aaargh! WAY too long since I studied Latin! ... So, after consulting Lewis & Short's Latin dictionary...

    "Tentorium" is a noun: something stretched out, a tent. "Osseus" is bone as an adjective (made out of bone, as in "bone flute" or "bone handles on knives"): bone as a noun is "os" (plural "ossa"). So the phrase means roughly "bony tent," or at least "a stretched out sheet made of bone". But "tentorium" is neuter, so if the modifying adjective was Latin it ought to be something like "tentorium osseum."

    (Thanks for your replies to my often ignorant questions, b.t.w.)

    So I go with my previous interpretation, that the adjective is English and should be osseous.

    It's still a great paper!

    (And yes, it had occurred to me that Equus belongs, like Carnivorans and Pangolins, to the supposed Pegasoferae: I hope that clade is confirmed by further research, because it is a marvelous name.)

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  15. I don't know if there's an osseus tentorium (or osseous tentorium, or tentorium osseum :-) ) in other perissodactyls, though I think Fox & Youzwyshyn were making the implication that a lot of animals hadn't actually been checked for one. Of course, it could still be a synapomorphy for Perissodactyla + Ferae but lost in one of the branches of perissodactyls.

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  16. And yes, it had occurred to me that Equus belongs, like Carnivorans and Pangolins, to the supposed Pegasoferae: I hope that clade is confirmed by further research, because it is a marvelous name.

    Allen, how do you feel about 'Zooamata', the name proposed for this clade by Waddell et al. (1999)?

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  17. Not answering for Allen, of course, but I think "Zooamata" sucks. It's a Greco-Latin hybrid (lots of them in nomenclature, I know, but that doesn't mean I have to like them!), it's got an unnecessary connecting vowel, and it just sounds generally dopey.

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  18. Pegasoferae is a hybrid, too, pegasos (Greek) and ferae (Latin, pl. of fera). It should be Pegasiferae (considering a Latinized Pegasus), or Pegasotheria.

    In fact Zooamata could be Zoamata, and his meanings is "loved by animals", should be Philozoa "friend animals" or Amatizoa (to keep the hybridism).

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