Mesonychians part II: Andrewsarchus was a hell of a lot weirder than all the books say

there are, however, indications that Andrewsarchus was freakishly weird, and doing something very special [...] Its snout is strikingly long and narrow [...] These features all suggest that the jaws were relatively weak.

A huge mammal with jaws that are long and narrow, but relatively weak? And with (presumably) unusual dietary habits? Hmm. The only extant mammal that fits that description at all is - the sperm whale!

I'll be the first to admit that drawing an analogy between Andrewsarchus and Physter sounds utterly preposterous. But could there be something to it? Could there possibly be some uniquely shared ecological/behavioural similarity between these two taxa? (I have absolutely no idea what that could be, though.)

/Reckless speculation off.

slightly gharial-ish shaped jaws for a riverside fish/frogcatcher he offers speculating wildly?

they would have to be big fish and very big frogs in hindsight.

In some way, Entelodonts could represent a continuation of "Andrewsarchid" morphotype, a kind of omnivore, bone-crushing, carrion-eater that could represent the original morphotype of hypothetical mesonychian-andrewsarchid-cetartiodactyl common ancestor. Suiforms and basalmost proto-cetaceans preserved the omnivory, with tylopodans and ruminants increasing herbivory, and mesonychians (and more derived proto-cetaceans) increasing the carnivory.

Darren: good discussion of a truly enigmatic animal.
Last year, a colleague and I spent some time examining the type skull of Andrewsarchus at the AMNH. As you summarized, there are numerous odd things about the skull. In short, the various features of the skull don't seem to 'add up'. The observation that the occipital condyles are relatively tiny is perhaps significant: such small occipital condyles vs. condylobasal length/breadth is more often seen in secondarily aquatic mammals...
Lastly, a question: does the type specimen in reality represent ONE individual from only ONE species of mammal?...

Ah, for years all I knew of Andrewsarchus was that it was on the right-hand side of the two pages that featured it and Barylambda in that "Prehistoric Animals" book by Peter Zallinger. (And, it would seem, there's not much known beyond that even...)

Is the skull all there is? Not even a scrap of associated postcranial material that would give some clue as to how it made a living? So strictly, we can't even say whether it had the dog-like quadrupedal body form always shown in reconstructions, or a protocetid-like form suited for a semiaquatic existence. Deeply frustrating! The parallel that comes to mind is Gigantopithecus, where the Chinese have found many hundreds (I think?) of isolated teeth, and several mandibles in cave deposits, but I've never seen any mention of skull or postcranial bones at all. Was it a knuckle-walker or a biped? There must be a fossil skeleton out there somewhere.....someone please find it!

Relatively weak jaws. So it couldn't have a devil/hyaena-like lifestyle. Could it be it roamed the beaches and edges of rivers and lakes to feed on large fish? A kodiak bear with a gharial 'look'. Maybe even feeding on rotten carrion which doesn't aquire strength of jaws.

Mongolia in that time was dry or wet? A crossing between a gharial and a kodiak ways of live sounds interesting. An aquatic lifestyle would make its relationship to proto-whales sounds tempting. Would it be a "Gavialarctos" creature... An analogous lifestyle to Baryonyx?

Weak-jawed mammals with narrow snouts, hmmm... More preposterous dietary analogies: Aardvark! Sloth bear!

@JS Lopes- Mongolia had to be wetter than it is now, being in the rain shadow of the Himalayas; in fact I think Eurasia's interior was one big forest then no?

I'm sorry, but how could the jaws be weak with such a giant zygomatic arch, presumably to accommodate a large lower jaw and large jaw muscles? and also how complete is the skull- could there be more to the occipital and sagittal regions that was not recovered?

"Clearly, Andrewsarchus is just crying out for some neat functional study on cranial kinematics and function. Come on: surely someone, by now, must have thought of doing FEA or something on its skull?"

Agreed. Also, interesting parallels you draw to a spinosaurid theropod. And at least some spinosaurs were specialized for feeding on the abundant giant freshwater fish prevalent in that warmer, wetter period.

Such an interesting radiation dentition-wise (the whales/mesonychids/hippos), considering also the superficial similarities between toothed whale dentition and theropods, which is perhaps less superficial when considering the flesh-eaters like the false killer and killer whales. On that topic, why don't whales have canines? What is the evolutionary history of whale canines and possible paleoecological reasons for their reduction? A long evolutionary period of specialization in fish-catching?

Andrewsarchus was a a gophereater. Like an anteater, but with the mechanics adapted to eating gophers. Andy would use a long, sticky tongue to snag gophers in their den, haul them out, and dine on the squirming catch. Andrewsarchus as a genus had not perfected this hunting technique, which is why the animal still had teeth. The Zygotic arch is an archaic relict of an ancestor who did have strong jaws.

So be on the lookout for a more advanced giant gopher eater subsequent to andrewsarchus. Though, if andrewsarchus was efficient at eating gophers, it may explain why the Mongolian area does not have evidence of the vast numbers of gophers necessary to support an animal Andy's size.

a clarification: obviously everyone here knows I was referring to Baryonyx as the fish-eating specialist, but how different was it from other spinosaurs..if it was a fish specialist I'm assuming the others could be too? a minor point though.

@ Alan- as appealing as the image is of Andrewsarchus licking fossorial rodents from their burrows, we would have to wait on the appearance of xenarthran like claws no?
Man, so much even a lower jaw would tell.

Comment 12...

I'm sorry, but how could the jaws be weak with such a giant zygomatic arch, presumably to accommodate a large lower jaw and large jaw muscles? and also how complete is the skull- could there be more to the occipital and sagittal regions that was not recovered?

'Giant zygomatic arch' does not mean powerful bite! In fact the gracile shape of the arch indicates small masseter, and tells little about size of temporalis. Sagittal crest and glenoid process are probably more important, and these are poorly developed in Andrewsarchus.

Ad: Baryonyx/Suchomimus (Baryonyx) had different jaw structures, particularly in the dentition, from Irritator/Spinosaurus (Spinosaurus). The premaxillary teeth of the latter are large and conical, while those of the maxilla are smaller but also conical and unevenly set. In Baryonyx, the teeth are basically homodont: small, very numerous, and slightly recurved.

The shape of the skulls are also different. Baryonyx' snout is longer, proportionately, and fairly uniform in height along its dorsal surface. The diastema between the premaxilla and maxilla is kinked as in coelophysoids, and the nasal is somewhat retracted.

In Spinosaurus, the snout is not quite as long, proportionately, and the skull height begins to rise once you hit the antorbital fenestrae, sloping gradually toward the orbits. The diastema is more pronounced, too, in that the ventral edge of the premaxilla is higher than the ventral edge of the maxilla, so it looks like the premaxilla is "stepped up" from the maxilla.

Heck, just see this post for skull restorations and lifestyle discussions.

Ad, #13

What of aardwolves, who lack such claws? Andrewsarchus as an ungulate aardwolf, now that creates pictures in one's mind. :)

@Zach- interesting figure of the differences between Baryonyx and Spinosaurus jaw structure (halfway down). Actually, it seems like you wrote that post in response to my question. The similarity between Andrewsarchus and esp. Spinosaurus is remarkable- much more so than between Spinosaurus and e.g. sarcosuchus or the gharials. I like your points about the suitability of the kinked, high-nostriled Spinosaurus skull to probing carcasses, and like Spinosaurus, Andrewsarchus would have had the mass to scare off just about anyone else from large carcasses. But I am resistant to the idea of land-based specialized scavengers in general, although the dinosaurs were so different in many ways from the animals we know today that rules might have been different then (perhaps the sail as an adaptation to a slower metabolic rate between carcass finds?). I am especially resistant to dedicated mammalian scavengers, since we know of no morphologically specialized mammalian scavengers today, and I'm sure someone else could provide much better arguments about the (unfavorable) energetics of a high-resting-metabolism mammal wandering large spaces in search of dead animals, as compared to say birds. Note also that the kinked skull of Spinosaurs is also found in the dilophosaurs (are they related???) and i remember hearing that mobile, semiarticulated snout postulated as an adaptation for snatching small animals (I think that was Predatory Dinosaurs of the World). I suppose it could be an adaptation for both approaches- in both Spinosaurs and Andrewsarchus- snatching and pouncing supplementing scavenging. There is some size cutoff below which the snatching/pouncing on small animals strategy no longer is energetically feasible- somewhere around the size of the aardwolf, I think, according to Carbone et al. 2007. Perhaps cobbling the two (dexterous snatching and large-size-based scavenging) together, Andrewsarchus carved out a niche as one of the largest mammalian carnivores ever?
(Or maybe add the fishing lifestyle that Darren hints at?)

But agreed, Andrewsarchus seems to have been doing something special and must have been a most unusual looking beast.

@Daniella- Forgive my lack of knowledge about anatomy, but why the large zygomatic arch, if it's not containing muscle? Also do we know that the back ridges (occipital regions, right?) of the Andrewsarchus skull are complete- perhaps it was rather like a gorilla, with the largest jaw muscle attachment crests at the back of the skull?

@ AK- haha, didn't see your post about aardwolves till after I just posted. Perhaps we are brainstorming toward a consensus theory? :)

also, "size cutoff ABOVE which", re: 17

AND I want to say I was just posing for arguments sake the mesonychid/hippo/whale clade (based off one of the papers cited) in comment 13- seeing the Thewissen Nature paper and the Geisler and Theodor response clarifies this whole debate

Are those skulls in the figure to scale? I had always heard that Andrewsarchus was one of the largest mammalian carnivores known, but I could never really picture the size...until now.

"Relatively weak jaws. So it couldn't have a devil/hyaena-like lifestyle. Could it be it roamed the beaches and edges of rivers and lakes to feed on large fish? A kodiak bear with a gharial 'look'. Maybe even feeding on rotten carrion which doesn't aquire strength of jaws."

That's why the Andrewsarchus in Walking With Beasts was portrayed as a beachcomber.

Anyway, the idea of Andrewsarchus as an animal similar to an entelodont seems intriguing. They may have shared a similar, lifestyle as big, dominant carnivores in the region. Maybe the stronger, bone-crushing jaws of the entelodont gave them an advantage as a scavenger, giving it the overall edge against the mesonychians. However, I doubt entelodonts are triisodont or mesonychian derived. Eocene entelodonts are known, and they have a rather strange, short, pig-like snout.

In addition, it is doubtful that spinosaurs and dilophosaurs are in a sister group relationship. Spinosaurs are tetanurans, for one, while dilophosaurs and the other coelophysoids are not. It appears that both adaptations were due to the environment, spinosaurs for catching fish, and coelophysoids perhaps for use as a generalist predator via snagging small animals.

There is some size cutoff below which the snatching/pouncing on small animals strategy no longer is energetically feasible- somewhere around the size of the aardwolf, I think,

Aardwolves don't pounce! They lick the termites off the ground.

An earlier paper by Carbone et al was criticized here for ignoring the different ways for ant- and termite-eating mammals to achieve larger sizes than small insectivores. (But probably not THAT large...)

In the paper I was referring to
http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0050022
they talk about large animals eating small prey, that's the sense I meant. I bet aardwolves probably eat their share of things other than termites, and they are about the size (<20 kg) that my example required, but I could have picked a better example I suppose.

Anyway sort of not the main point of what I was talking about.

Not sure what happend there, but to complete my sentence:
"
(<20 kg) that my example required, but I could have picked a better example I suppose.

Anyway sort of not the main point of what I was talking about."

Darn it, must be some HTML tag thing.

"
<20 kg) that my example required, but I could have picked a better example I suppose.

Anyway sort of not the main point of what I was talking about."

AAArGH!

less than 20 kg endparentheses that my example required, but I could have picked a better example I suppose.

Now that that's over with, @anonymous, thanks for clearing that up bout spinosaurs/dilophosaurs, couldn't remember

Mesonychians have always fascinated me-- especially this particular creature.

I saw a depiction of this animal alongside a Kodiak brown bear, and it captured my imagination that there was once a predator that was larger than either polar or Kodiak brown bears.

Of course, it may not have been a predator at all. It may have been a scavenger that used its bulk to steal kills that other predators brought down.

Me again!

@windy, I think McNab 2000's point about large insectivores is probably consistent with the Carbone et al. 2007 energetics paper no?

perhaps the sail as an adaptation to a slower metabolic rate between carcass finds?

Raising your body temperature from an external source can't raise your metabolic rate.

mobile, semiarticulated snout

Not either. It was completely immobile and firmly braced by the palate bones.

Darn it, must be some HTML tag thing.

Of course. As soon as you type "<", the ScienceBorg software interprets everything behind it, all the way to the end of the paragraph, as an HTML tag; and invalid HTML tags are deleted.

To get < and > visible, you must type &lt; and &gt;.

Of course, it may not have been a predator at all. It may have been a scavenger that used its bulk to steal kills that other predators brought down.

This does not work! There simply aren't enough carcasses around for a terrestrial scavenger. Andrewsarchus had to kill. Something.

Gigantopithecus ... Was it a knuckle-walker or a biped? There must be a fossil skeleton out there somewhere.....someone please find it!

I would think the null hypothesis would be for it to have been a fist-walker, like its closest living relative, Pongo. (Although there is an interesting idea that hominoids are ancestrally bipedal, at least to a certain extent -- note that gibbons [hylobatids] are primarily bipedal on land.)

In addition, it is doubtful that spinosaurs and dilophosaurs are in a sister group relationship. Spinosaurs are tetanurans, for one, while dilophosaurs and the other coelophysoids are not.

It's worth noting that Paul (1988) posited a relationship for them, although I think he has since acknowledged the newer positions.

Dimetrodon and other early sphenacodonts has a similar dental arrangement as well.

If Andrewsarchus turn to be closer to arctocyonians, could it be related to Eocene Indo-Pakistan-endemics Quettacyonidae (Karakia longidens, Obashtakaia aeruginis, Quetacyon parachai and Sororocyon usmanii)???

they talk about large animals eating small prey, that's the sense I meant. I bet aardwolves probably eat their share of things other than termites

Not very much though, apparently they are highly specialized termite eaters, of one species or genus even. Weirdly enough.

OK enough about aardwolves, but apparently even significantly larger animals could potentially manage on small things as long as they don't have to catch them one by one, but the prey is concentrated someplace where they can be 'hoovered' up. Bears eat a lot of small prey. Apparently a significant part of their diet in some places is... moths.

" perhaps the sail as an adaptation to a slower metabolic rate between carcass finds?

Raising your body temperature from an external source can't raise your metabolic rate."

I meant it could help lower metabolic rates by allowing a sort of active ectothermy, which required less energy than pure endothermy. Just an idea.

@windy- good point about the bears and aardwolves. Actually, the bears approach requires the predator be quite large to flip over the rocks repeatedly. BUt it seems this is getting away from relevance to Andrewsarchus; it doesn't seem to explain anything about the Andy skull morphology, unless that is dominated by adaptation to scavenging. Anyway, from my original post, it's not clear to me how combining two marginal approaches adds up to one successful approach, but perhaps they are complementary in some way.

Kinked snouts are probably an adapation to grabbing small, struggling prey. Fish for spinosaurs, small vertebrates for coelophysoids and Dilophosaurus (isn't it outside the coelophysoids now?).

@ Zach- OK, cool. Don't mean to be dominating the postings, I guess one needs to be more careful in the particulars so the attention isn't focussed on nonessential points.

I think this is an interesting line of argument. We would know so much more if someone could find more bones. C'mon, if they can find transitional archaeocetes in various stages of aquatic adaptation, they can find bones from this animal! Especially if we are publishing Nature articles on the same old skull. Paul Sereno, perhaps we can shift attention for a moment from giant crocs and dinos to giant Paleogene mammals from a pre-Himalayan world?

No, dilophosaurs were semiaquatic fish-eaters, just like spinosaurs. My cast of a Dilophosaurus wetherilli tooth shows longitudinal grooves, which are found only in reptiles that fish. You can clearly see them on gavial, mosasaur, ceratosaur, and spinosaur teeth. Also:

http://geology.utah.gov/surveynotes/articles/pdf/fishing_dinos_39-3.pdf

There is apparently a single species of dilophosaur relative found to be feeding on fish, the one from the St. George site, but remember that a lot of other animals will take advantage of local aquatic prey. Jaguars will feed on caimans, for example.

The fact the coelophysoids are found everywhere, and they almost all have that kinked snout, suggests that they were the "swiss army knives" of dinosaurs. The kinked snout was effective in snagging sphenodontians, mammals, sphenosuchians, and maybe even small theropods and ornithischians, while the claws on their hands (Dilophosaurus has a particularly wicked set) were used to take on larger fare. Maybe in the case of a Dilophosaurus, the occasional prosauropod.

Dilophosaurus is sometimes put outside Coelophysoidea, but this is usually when Coelophysoidea is regarded as a paraphyletic grade. The current train of thought is that Dilophosaurus and Coelophysis are representative of two different families in Coelophysoidea, one larger and generally charictarized by big crests, the other smaller and mostly crestless. Note that I say mostly, as this may be more muddled in the future.

The problem is that there is no "press agent" for Paleogene/Early Neogene mammals. People know all about T.rex and Triceratops, but nothing about hard-headed chalicotheres, giant running rhinos, killer hogs, mesonychians, etc.

The more I think about it, the more likely it seems Andrewsarchus was either an entelodont analogue, or else it was just a big nasty carnivore that ate anything; stuff that washed up on the beah, big mammals, carrion. Hey, if someone is that big, then you don't get between him and what he wants to eat.

Ah Andrewsarchus my favorite carnivorous mammal. The entelodont-like nature of the skull always seems to be ignored in restorations, all of which show it as a sort of giant hyena with a huge head. As an artist who draws animals, this huge head has always really bothered me. Even more annoyingly restorations of the supposedly related mesonychids were never so ill-proportioned. On the other hand I have seen several illustrations of Hyenadon with the same overgrown skull proportions so I assumed the artist were trying to say that Andrewsarchus was some sort of overgrown Hyenadon. Personally I would not be surprised if the real Andrewsarchus had a more entelodont-like shape in life. Of course since the skull is the only part of Andrewsarchus we have found we simply do not know what proportions it had, still the drawings just look odd.

A few years ago the local county fair had life sized mechanical models of various extinct mammals along with some living relatives. One was an entelodont that was in a sort of side niche and, as I was looking as a Basilosaurus coiling itself through a coral formation, I did not see it until it uttered a loud squeal. Turning around I came face to face with what looked at first glance like a hippo the size of a white rhino with the snout of a crocodile. No doubt the fact that it startled me made it seem larger but still I must say it was the scariest looking thing I have ever seen. It looked way more menacing than any mechanical Tyrannosaur. I was suddenly very glad they were extinct!

On the subject of Dilophosaurus Holtz website (and newest Genus list) places the dilophosaurids outside of Coelophysoidea. This scheme was apparently proposed by Nathan Smith and it claims that Coelophysis, Megapnosaurus, Procompsognathus, Segisaurus, are separate from a monophyletic group composed of Dilophosaurus, Dracovenator and Cryolophosaurus. In other words dilophosaurids were more closely related to the ceratosaurs and tetanurines than they were to the coelophysoids.

The top picture: the head reminds me of a beluga with its missing canines. Salmon migrate to Taiwan, Japan and the Amur river, so maybe that beast dined on salmonids seasonally like kodiak bears do.

The pinched snout, flaring zygomatic arches, and narrow braincase (as well as generally wolf-like carnivorous dentition) remind me of a thylacine. If you compare small thylacinids with big ones - e.g. Nimbacinus or Mutpuracinus with Thylacinus cynocephalus or (at the extreme) T. potens - and imagine extending the skull allometry to an animal several times larger, I estimate you'd get something very like Andrewsarchus. Whether this geometric similarity would correspond to functional similarity, over such a size range, is another matter though.

See e.g.
P.F. Murray and D. Megirian. 2006. Cranial morphology of the Miocene thylacinid Mutpuracinus archibaldi (Thylacinidae, Marsupialia) and relationships within the Dasyuromorphia. Alcheringa Special Issue 1, 229-276.

So, I've been reading American Museum of Natural History Novitates #146, of November 11, 1924: Osborn's 5-page description of the Andrewsarchus skull, available at
http://digitallibrary.amnh.org/dspace/

Comments/questions:
(1) Easier for a non-specialist to read than more modern Novitates: the technicality level has risen since 1924.
(2) "P4 enlarged, molariform, with prominent tritocone, three fangs" -- the illustration is pretty small, but the shape (in occlusal view: no pic of tooth, just ventral view of skull: it's the illustration Darren reproduced above) in occlusal view isn't exactly like the molars behind it, so I'd say "semi-molariform." "Tritocone" isn't a term I'm familiar with in more recent writings: is it obsolete for ??? ?
(3) "Canines, superior, not preserved, of enormous size" Neat the way you can comment on a tooth that isn't there! But, from the size of the empty socket, it seems to have been at least double the diameter of the already impressive second incisor.

Note, by the way, that the skull when photographed (Darren's second illustration above) had the missing third incisor and canine restored. Their length is, obviously, conjectural. As reconstructed, it's big in absolute terms (maybe 120 mm exposed length?), but not into sabertooth territory!

I meant it could help lower metabolic rates by allowing a sort of active ectothermy, which required less energy than pure endothermy. Just an idea.

But that's what I mean: when an ectotherm reaches its optimal temperature, it doesn't become able to do things like endurance running as well as an endotherm of the same size and body shape.

The current train of thought is that Dilophosaurus and Coelophysis are representative of two different families in Coelophysoidea

No, you've missed the latest few phylogenetic analyses, which find Dilophosauridae (including Cryolophosaurus and Dracovenator) closer to Ceratosauria and Tetanurae than to Coelophysidae.

Are any aquatic organisms - fish, turtles, crocs or choristoderes? - large or abundant enough to feed a mammalian spinosaur equivalent known from Eocene central Asia?

If John Scanlon is right about the Thylacine our enigmatic friend Andrew muts have had the widest gape in the terrestrial mammalian kingdom.
Big enough to swallow most cars and their passengers.

David, #45

What of the goanna of Australia, which have a metabolism more like that of mammals than reptiles?

The real difference between the typical reptile and the typical mammal is how their exertions are powered. Even mammals use two different methods, the anaerobic one for short term power, the aerobic metabolism for endurance. Goannas use the aerobic metabolism to go long distances, a facility other reptiles apparently do not have access to.

How about an anadromous fish specialist, feasting on eels and salmonids seasonally? Sideswiping or biting at midsection large fish in current, a narrow jaw would be faster than a wide jaw in biting fast through water without much water displacement (however the lower canines couldn't be very long). But retaining a typical terrestrial smallish ground-game predator dentition and carrion feeder without the bone crushing (no need for marrow due to whole fish eating? That's why cats don't crack bones?). Fascinating beast.

How about an anadromous fish specialist, feasting on eels and salmonids seasonally?

Salmonids are anadromous (adults in the sea, breeding in fresh water), eels are catadromous (adults in fresh water, breeding in the sea).

The main problem I can see is that fish runs are only available for very short periods of time. It's still got to find something to eat for the rest of the year.

What of the goanna of Australia, which have a metabolism more like that of mammals than reptiles?

No, it's not all that much higher than in other squamates.

What they do is that they have (like mammals, crocodiles, and ornithodirans) found a way to keep breathing while running. Squamates other than varanids can only run for even shorter periods than their metabolic capacity would allow.

@David, the idea is that a sail could allow better passive temperature regulation, which would have allowed a lower basal metabolic rate whilst perhaps maintaining an endothermic-like body temperature and hence activity level. Basically, the animal wouldn't have had to expend as much energy maintaining body temperature within optimum levels, like a house with lots of south facing windows etc and good insulation (passive heat). This might have allowed a lower overall energy requirement for the same effect (heating/cooling home/dinosaur), perhaps allowing a terrestrial scavenging lifestyle.
'Sides, we don't know what the prey landscape was like back then, if there was tons of slow-metabolism prey biomass maybe it could have supported a significant terrestrial scavenging component to a diet, at least part time.

Whether this fits what the bones are telling us is the real topic of debate here. I don't know if the optimal adaptation to scavenging is being able to reach inside carcasses for a mouthful of half-digested plant matter and intestines :) (for the sake of argument)

@ 50 - Yes, thanks, perhaps it ate fish regularly along with other ground game, and massively fished the migratory runs. I hadn't meant its aquatic diet would be limited to eel and salmon (and smelt?) , just that this may have been a seasonal priority when the fish were so thick that one could almost walk across on them.