Because Andrewsarchus is not the world's only mesonychian (mesonychians part I)

What the hell, I've decided to keep the ball rolling with the Paleogene mammals, and do a whole week series on mesonychians. Yes, let's deal with a group that everyone has at least heard of.

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Mesonychians are an assemblage of Paleocene and Eocene mammals, best characterized (or are they?) by the superficially wolf-like carnivorous or omnivorous forms Andrewsarchus and Mesonyx [life-sized Andrewsarchus model above from here]. Why use the term 'mesonychians' and not 'mesonychids'? Answer: because there is at least some evidence linking a number of non-mesonychid taxa with Mesonychidae. Some authors think that Acreodi Matthew, 1909 should take precedence over Mesonychia Matthew, 1937, while some use Acreodi for a more inclusive group that contains Mesonychia as well as some possibly related taxa. In this and the following few articles, we will look in reasonable detail at various of the mesonychian groups.

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What we know of mesonychians indicates that they were superficially canid-like omnivores or carnivores, and many of the better-known forms had limb bones indicating digitigrady and a degree of cursoriality [adjacent Mesonyx reconstruction from the HúsavÃk Whale Museum]. Their post-canine teeth are tall-crowned, laterally compressed and suited for shearing. Their canines are often large and the third molar is sometimes absent. Some taxa have particularly robust jaws and large, heavily worn teeth; they might have been good at breaking into bones, and may have eaten a lot of carrion. Others had rather slender jaws and thin, slender teeth, and have been interpreted as piscivores. Mesonychian digits were tipped with hooves rather than claws, and the supporting phalanges were narrow and fissured (that is, they appeared to be split along the dorsal surface for most of their length).

Sexual dimorphism is present in at least some mesonychid mesonychians: O'Leary et al. (2000) reported substantial dimorphism in overall size and proportional canine size in the basal mesonychid Ankalagon. Extant mammals that exhibit substantial dimorphism in canine size often exhibit a polygynous social system, so it's likely that this was true of some mesonychians. A reasonable sample of the more derived mesonychid Pachyaena indicates that sexual dimorphism was absent in this taxon, leading to the suggestion that it was solitary (Zhou et al. 1992) [life restoration of Pachyaena below by Carl Buell and used with permission].

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The bloated monster that is Condylarthra

Thanks to their hoofed phalanges and to the general idea that these animals were somehow part of the 'ancestral stock' for artiodactyls, perissodactyls and other living hoofed mammals, mesonychians have traditionally been regarded as 'stem-ungulates' or, more formally, as members of Condylarthra. Over the decades, Condylarthra has become a bloated monster that clearly serves as a waste-basket for placentals that can't be allied with extant clades. For this reason, most experts advocate its abandonment (though an effort to maintain a monophyletic - but restricted - Condylarthra has been advocated by some), and some have argued that we should use the term 'archaic ungulate' if and when we need to refer to these animals as a whole (Archibald 1998). The fact that 'ungulates' as once imagined (see Prothero et al. (1988), where Ungulata includes artiodactyls, perissodactyls, cetaceans, dinoceratans, tethytheres, hyracoids and others) are clearly not monophyletic - instead they're scattered about the placental tree - means that 'archaic ungulates' are surely of diverse affinities too.

At the moment, the job of sorting out this gigantic mess has only just begun. Mesonychians were, until recently, regarded as close kin of cetaceans and part of a clade termed Cete (Mckenna 1975, Prothero et al. 1988, McKenna & Bell 1997), but the fact that cetaceans now seem to be nested within Artiodactyla indicates that the characters shared by mesonychians and cetaceans are convergences [or does it? Be sure to read the comments]. Some studies that include broad character sampling (e.g., O'Leary et al. 2004) find mesonychians to be within the same general region of the placental tree as carnivorans, perissodactyls and artiodactyls. If this is correct, then mesonychians are laurasiatheres. No doubt we'll be coming back to the subject of 'condylarth' affinities and diversity in future but, for now, we have to stop and move on.

More in the next article. We begin with the 'best known' mesonychian of them all: Andrewsarchus from Late Eocene Mongolia (and China?)...

For previous articles on Paleogene 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.

McKenna, M. C. 1975. Toward a phylogenetic classification of the Mammalia. In Luckett, W. P. & Szalay, F. S. (eds) Phylogeny of the Primates: a Multidisciplinary Approach. Plenum Press (New York), pp. 21-46.

- . & Bell, S. K. 1997. Classification of Mammals: Above the Species Level. Columbia University Press (New York).

O'Leary, M. A., Allard, M., Novacek, M. J., Meng, J. & Gatesy, J. 2004. Building the mammalian sector of the tree of life: combining different data and a discussion of divergence times for placental mammals. In Cracraft, J. and Donoghue, M. (eds), Assembling the Tree of Life. Oxford University Press (Oxford), pp. 490-516.

- ., Lucas, S. G. & Williamson, T. E. 2000. A new specimen of Ankalagon (Mammalia, Mesonychia) and evidence of sexual dimorphism in mesonychians. Journal of Vertebrate Paleontology 20, 387-393.

Prothero, D. R., Manning, E. M. & Fischer, M. 1988. The phylogeny of the ungulates. In Benton, M. J. (ed) The Phylogeny and Classification of the Tetrapods, Volume 2: Mammals. Clarendon Press (Oxford), pp. 201-234.

Zhou, X., Sanders, W. J. & Gingerich, P. D. 1992. Functional and behavioral implications of vertebral structure in Pachyaena ossifraga (Mammalia, Mesonychia). Contributions from the Museum of Paleontology, the University of Michigan 28, 289-319.

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After Andrewsarchus, the best known mesonychians are the mesonychids... and, as we saw previously, Andrewsarchus may not be a mesonychian anyway. Mesonychids are a mostly Eocene group that originated in the Paleocene; Mesonyx, from the Middle Eocene of North America, was the first member of the…
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…
We saw in the previous article that Andrewsarchus, most 'famous' of mesonychians (even though it may well not be a member of this group), is not just a scaled-up Eocene wolf, but really something quite unusual. Indeed, it's so unusual that Szalay & Gould (1966) decided that it's worthy of its…
The previous article was a brief, cursory introduction to the mesonychians. Time to look at things in a bit more detail... Andrewsarchus mongoliensis is, of course, 'the' mesonychian for most people, and one might get the impression that it's a typical member of the group. In fact it's most…

Some authors think that Acreodi Matthew, 1909 should take precedence over Mesonychia Matthew, 1937

In the ICZN, priority does not apply above the family group of ranks.

I'm always for using former synonyms for nested clades. There are lots of precedents: Eutheria/Placentalia, Metatheria/Marsupialia, Rhynchocephalia/Sphenodonti(d)a, Ophidia/Serpentes, Testudinata/Testudines...

By David MarjanoviÄ (not verified) on 09 Aug 2009 #permalink

Let's not forget the leucrottadae, the last of a once numerous clade. Often referred to as hoofed hyenas, the leucrotta are hoofed predators inhabiting lone and forlorn places in the deep wilderness. Most often barren wastes and arid badlands.

Their vocalizations are most often compared to the crying of a young child or infant. For this reason the myth that leucrotta imitate lost travelers or wandering children in order to lure people into traps has spread widely. In truth, the leucrotta is a shy animal, preferring to stay out of sight or interlopers.

They are cursorial hunters, running on hoofed feet, much like that of the pronghorn of Norh America. Next to the tasmanian wolf of Australia they have possibly the widest gape of any living mammal. Certainly the widest among placentals.

Leucrotta are largely solitary animals, with older animals pair bonding for the purpose of raising young. In rare circumstances short lived packs may form. But such depend on local conditions, and the typical leucrotta territory is too demanding for such to last for any great period of time.

Leucrotta live on the margins, in conditions most other carnivores find impossible to survive in. At present the remaining leucrotta populations are deemed endangered, and the last representatives of the mesonychids may go extinct by the 22nd century.

Now if you want to keep it real world... :)

Funny, last I heard it wasn't even a mesonychian any more.

Yeah, I cover that stuff in part III: 'Andrewsarchus and the triisodontines'. Part II is called 'Andrewsarchus was a hell of a lot weirder than all the books say'. Andrewsarchus may well be allied with mesonychians, but that's all I'll say for the time being.

Acreodi originated in an old subdivision of original Creodonta, into Procreodi (Arctocyonidae), Acreodi (Mesonychidae), Pseudocreodi (Oxyaenidae+Hyaenodontidae) and Eucreodi (Miacidae). The suffix "-creodi" is wrong, because plural of Greek odous (Latinized -odus) is odontes, not odi.

By J. S. Lopes (not verified) on 10 Aug 2009 #permalink

Some cladograms out Andrewsarchus next to whales, but I've never read about any closer relative or direct ancestor. Maybe this "Andrewsarchid" ghost-lineage remained hidden in some obscure Southeast Asian site. It'd be nice to maintain thi suffix -archus for his closest kin... Osbornarchus (after Henry Osborn), Paoarchus (after Pao, genus' namer), Simpsonarchus (after George G. Simpson, not Homer, lol).

By J. S. Lopes (not verified) on 10 Aug 2009 #permalink

Has anyone find more pieces of this majestic moloch than "just" the skull ?

Wow what a time. An Andrewsarchus in a one-on-one with a enraged Sarkastodon over a carcass of a titanic Brontotherium while a pack of 'puny' Hyaenodons watch the battle commence. In the thickets a bear-sized Pachyaena sneaks away.

Dokter Who bring me your Telephone Booth !

By Wilbert Friesen (not verified) on 10 Aug 2009 #permalink

If you want more on Andrewsarchus (and you evidently do) all I will say is pleeeeease just wait until parts II and III get published some time over the next few days. Yes, there is more Andrewsarchus material than just the holotype skull, but don't get your hopes up...

Expect me to go silent soon. I'm going away (conference # 2), but the posts are scheduled to appear in my absence.

Hell yes!

Thank you for this series, even if I'm not an anatomist or paleontologist and I sometimes have to think a little when you mention some structures, I like your posts and those beasts.

Very confused here, you speak of hoofed animals with 4 individual hoofs per foot (at least that what it looks like in the skeleton and the reconstruction)? Guess the non-biologist's definition of hoof being the reduction of physical digits into less-than-four horny contact points is not correct (are there any with three left currently?).

George G. Simpson, not Homer

Last year in Cleveland, the idea was floated that the SVP should give out not just a Romer-Simpson Medal but also a Homer Simpson Medal. I immediately agreed. Unfortunately the leadership has not been informed yet...

Guess the non-biologist's definition of hoof being the reduction of physical digits into less-than-four horny contact points is not correct

"Hoof", "claw" and "nail" are different shapes of the same thing. Rather than having "cleft hooves", cows have 2 hooves per limb, and pigs 4...

By David MarjanoviÄ (not verified) on 10 Aug 2009 #permalink

Mesonychians were, until recently, regarded as close kin of cetaceans and part of a clade termed Cete (Mckenna 1975, Prothero et al. 1988, McKenna & Bell 1997), but the fact that cetaceans now seem to be nested within Artiodactyla indicates that the characters shared by mesonychians and cetaceans are convergences. Some studies that include broad character sampling (e.g., O'Leary et al. 2004) find mesonychians to be within the same general region of the placental tree as carnivorans, perissodactyls and artiodactyls.

O'Leary's and Gatesy's studies tend to have mesonychids as close kin to cetaceans and nested within Artiodactyla. See this, for example. Interestingly, Andrewsarchus here clusters with the hippos and anthracotheres.

By Anton Mates (not verified) on 10 Aug 2009 #permalink

"Sarkastodons"? Really? Wikipedia thinks so. My first reaction was that they had to be notional, like snarkopuses (pause; yes, snarkopuses), puckishidae (pause again), and blogtrollaria (no pause!) and then I was thrown off because I knew Brontotherium and Hyaenodon aren't.

Iranotheriums are not notional, but ironytheriums are.

By Nathan Myers (not verified) on 10 Aug 2009 #permalink

Delighted to see this here...was on my wish list for Tet Zoo topics to tackle. But why convergence and not plesiomorphy for the similarities between archaeocetes and mesonychids?

And I agree, Andrewsarchus was one distinct looking beast, few illustrations do it complete justice I feel, but what do I know

Despite it's inaccuracies, the "Walking With" series no doubt made more people aware of Andrewsarchus, I just wish more remains other than the enormous skull had been found. A terrifying beast though, it would probably make a Kodiak bear look like a labrador puppy! In time, maybe more information will come out about this formidable animal and the family it belonged to.

By Raymond Minton (not verified) on 10 Aug 2009 #permalink

David,

Chris Brochu, I, and others suggested a "Homer Simpson" medal back into the 1990s. We even had a line up for the first batch of recipients: let's just say they all shared a particular view on the origin of Aves which we did not share...

I've heard that mesonychians are somewhere within the Eparctocyonia, a group that includes whales+artiodactyls, the south american ungulates, mesonychids, and a good chunk of the bloated monster that was once Condylarthra, albeit almost entirely as basal members of the clade. Of course, the true affinities of mesonychians would be a lot less confusing if we could genetically test them, but seeing as we don't have any living mesonychid hanging around, that is pretty much impossible.

In a humorous twist, I heard one source say (most likely inaccurately) that Andrewsarchus did not even exist, and that its skull was just a smashed up Paraceratherium/Indricotherium that someone found and put together wrong. Though given the fact that other mesonychians/triisodontines have been found, this seems looney. Or maybe it was vice versa. Anyway, I am almost certain that they deserved a "Homer Simpson Award".

By Anonymous (not verified) on 10 Aug 2009 #permalink

The very last mesonychian was Mongolonyx of (duh) Mongolia: it first appeared during the Middle Eocene, and died out some time during the Oligocene.

Yes, Sarkastodon. I bet it means something like "sharp tooth".

Chris Brochu, I, and others suggested a "Homer Simpson" medal back into the 1990s.

Ah. Not surprising.

By David MarjanoviÄ (not verified) on 10 Aug 2009 #permalink

Anton Mates (comment 12) writes...

O'Leary's and Gatesy's studies tend to have mesonychids as close kin to cetaceans and nested within Artiodactyla. See this, for example. Interestingly, Andrewsarchus here clusters with the hippos and anthracotheres.

You're right; I'd made the classic mistake of looking at some MPTs but not all of them. Some MPTs have a clade that includes Mesonychia, Andrewsarchus and Arctocyon forming the sister-taxon to an Artiodactyla that includes Whippomorpha, while others find mesonychians to be within Artiodactyla and closer to Cetacea than to Hippopotamidae. Ruminants, suiforms and tylopods are successively more distant. It seems that combined evidence finds hippos and whales to be most closely related among extant taxa, but that mesonychians are - within Whippomorpha - closer to cetaceans than are hippos.

This is pretty exciting as, if correct, it means that both of these supposedly competing views are correct: whales are close to mesonychians, but - among living taxa - they're closest to hippos. However, aren't their still researchers who argue that mesonychians are not as close to cetaceans as hippos are?

Regarding a 'Homer Simpson medal'... A little more respect for H. Simpson is in order, people. He is, after all, really a genius: he came up with the theory of a doughnut-shaped universe (although said theory was, apparently, later stolen by Stephen Hawking).

However, aren't their still researchers who argue that mesonychians are not as close to cetaceans as hippos are?

Yep. Here's Geisler and Theodor's Nature paper, which places mesonychians basal to the (other) artiodactyls, with cetaceans and hippos nested together well inside the latter group. Thewissen et al. take this as supporting their claim that raoellids are actually the cetacean sister group, with hippos the sisters to raoellids+cetaceans. Both teams agree that Andrewsarchus is really, really hard to pin down--but in the Gatesy & O'Leary trees, Andrewsarchus was actually quite stable.

So, tons of disagreement still. I have no expertise in the area, so I couldn't begin to guess who will turn out to be right.

By Anton Mates (not verified) on 11 Aug 2009 #permalink

Iranotheriums are not notional

No, but they are national.

He is, after all, really a genius:

But then, so is Baldrick, who after all built a functioning time machine...

By David MarjanoviÄ (not verified) on 11 Aug 2009 #permalink

Oh the convergence!

By Tim Morris (not verified) on 12 Aug 2009 #permalink

But then, so is Baldrick, who after all built a functioning time machine...

*lalala I can't hear you* He didn't. That episode of Blackadder Does. Not. Exist.

Dave:

*lalala I can't hear you* He didn't. That episode of Blackadder Does. Not. Exist.

Let's not forget that Baldrick once had a plan that was as cunning as a fox who's just been appointed Professor of Cunning at Oxford University.