SVP, Day 1

I spent the morning in a symposium on early vertebrates. The first talks were on the Cleveland Shale, a Devonian unit from this area that has produced large numbers of fossil fish; 66 species in fact, according to the first talk by Robert Carr and Gary Jackson. One of the most common Cleveland fossils is the giant placoderm Dunkleosteus (top and below):

Two talks, the first by Philip Anderson and the second by Eric Snively et al., discussed the variation in both shape and function in the placoderms, a group of jawed vertebrates from the Devonian. The second talk in particular looked at possible different feeding strategies in juvenile and adult Dunkleosteus, as well as in the even larger Gorgonichthys.

The next talk, by John Maisey, was about Cladoselache, a Devonian shark known from numerous well-preserved specimens from the Cleveland Shale, like the specimens below at the Cleveland Museum of Natural History:

Here is the American Museum of Natural History’s reconstruction of Cladoselache:

In spite of these beautiful specimens, Maisey showed that there is a great deal that isn’t known about this shark, including basic things like the number of gills and the structure of the dorsal fin.

Henning Blom and Mark Wilson presented papers on jawless vertebrates from the Silurian and Devonian (these groups are sometimes lumped together as “agnathans” or “ostracoderms”); this model of an anaspid (the topic of Blom’s paper) is from the Texas Memorial Museum:

Wilson’s talk was on another jawless group, the thelodonts. These had fantastic forked tails, and Wilson showed that their mouths were not just simple holes but may have in fact had a mobile lip-like structure.

John Long and Kate Trinajstic presented on new placoderm specimens from Australia. Incredibly, two of these specimens included embryos, including a mineralized umbilical cord in one specimen, showing that at least this group of placoderms used in vitrofertilization and gave live birth.

After lunch, I headed to a different room for some mammal talks. Julie Meachen-Samuels presented data on the forelimb modifications in saber–toothed cats and nimravids (like Barbourofelis, below, from the Florida Museum of Natural History) compared to other cats.

Saber-toothed forms have more powerful forelimbs than typical cats of the same size, but (surprisingly to me) the forelimb morphology of the two groups converges for very large body sizes.

Wendy Binder and Julie Meachan-Samuels presented a second paper on saber-tooths, specifically Smilodon, looking at sexual dimorphism. It turns out that Smilodon shows less sexual dimorphism (the differences between the sexes) than do lions.

Lauren Berg and Nick Pyenson gave an interesting talk on the relationship between diving depth and the size of the eye socket in pinnipeds (seals, sea lions, and walruses). It turns out that, for most pinniped groups, the deeper the species dives the larger the size of the eyes (most surprising in that no one had bothered to check before).

In the poster session Jason Schein et al. looked at the preservation of fossils across the Cretaceous-Tertiary boundary in New Jersey, to see if it was a lag or a condensed section. A lag deposit is formed when a rising sea erodes pre-existing sediment. The small particles are carried away, while the large particles (including the fossils) are redeposited as a bonebed in the newly formed sediment layer (these are typically referred to as “reworked”). In a condensed section, instead of the redeposition of fossils, the area is simply starved of new sediment for a long period of time. Any animals that die there lay on the surface for a long time without being buried, and over a long period of time a bonebed accumulates (sometimes called an “attritional deposit”). I was interested in this talk because these are the same issues we face in trying to interpret the origin of the Carmel Church bonebed.

Blaine Schubert present a study of changes in tooth shape and body size over time in the short-faced bear Arctodus (below, from the Oklahoma Museum of Natural History).

Arctodus got much larger over time, and the anterior grinding teeth getting relatively larger, suggesting a change in diet.

Kathleen Smith and Dan Fisher looked at the dentin thickness in the tusks of a female American mastodon (below, from the Cleveland Museum of Natural History).

By looking at the variation in thickness of the dentin layers, they were able to determine when that particular mastodont reached sexual maturity. In tusk year seven (about age nine), the tusk shows an abnormally low rate of growth in the spring for the first time. This is mating season, and apparently female elephants (and presumably mastodonts) are so harassed by males attempting to mate that they have difficulty getting enough to eat. The tusk then shows two years of low growth (while she was pregnant and then nursing), followed by three years of increasing growth (after the baby was weaned).

After the poster session I headed to a reception at the Cleveland Museum of Natural History, which has a great collection of Devonian fish from the Cleveland Shale, as well as many other fossils including the one-of-a-kind Haplocanthosaurus, a primitive sauropod dinosaur (this photo is from a trip to this museum from several years ago).

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24 Responses to SVP, Day 1

  1. Doug says:

    All hail Uber Fish!!!

    Sounds like a lot of good stuff. First off, the pinnipeds. Elephant seals are among the deepest diving mammals alive today, and the male’s eye socket is big enough to cradle my fist (females are much smaller, but their eyes socket proportions might be the same)!

    On the bear, interesting. “Prehistoric Predators” made a compelling case for Arctodus being a scavenger. Perhaps the enlargement of teeth is a response to that shift in diet, where he needs larger teeth to crack open the bones and dismember the carcases of the increasingly larger plant eaters.

    The mastodons remind me of an idea i have for research if I ever get out of college. Interestingly, the biggest mastodons come from the Midwest (the Burning Tree Mastodon from Ohio is, i think, eleven feet at the shoulder). At Rancho La Brea, the mastodons are quite small (a female on display is no taller than me). Fossils show that similar size mastodons were living farther back in time in southernmost California. And yet, in Diamond Valley near Hemet, CA, the mastodons are much larger (when the first bones were found, they were thought to be mammoths, because they were so much bigger than the La Brea specimens). One specimen, Max, is the largest mastodon in the western US. Another specimen, Little Stevie, was immature and was still seven feet tall. What spurred my idea is that the Burning Tree mastodon was found with preserved gut contents, which showed that he was eating things like pond weed, swamp grass, water lilies, stuff like that. The scientist quoted in the article said that it was a very nutritious diet. And that got me thinking: Diamond Valley once hosted a large lake, and Little Stevie was found in a wetland deposit. My research would try to conduct stable isotope analysis on mastodon remains from across North America to see if aquatic vegetation was the cause of this size variation.

    Finally, the Smilodon research. How were they able to tell the difference between sexes, if any? If they were scoial animals, it seems their weren’t much like lions. Sounds more like a wolf pack (wolves aren’t sexually dimorphic, so far as i know).

    Hoo, that was a mouthful. Sound like blast. Keep the post coming.

  2. Alton Dooley says:

    Smilodon apparently does show some sexual dimorphism, but it’s not as pronounced as in lions; I don’t know the details, but there’s a huge sample size from Rancho la Brea (the first requirement for a rigorous dimorphism study).

    I have no doubt that Arctodus scavenged. It seems that most bears can and will eat anything they want, and modern bears do a lot of scavenging. I could be mistaken, but I think Arctodus has longer legs than modern bears, which might suggest better running abilities.

    I think in some localities there are abundant lead and stem fragments in which the fragment lengths are the same as the spacing between mastodont tooth cusps. Mastodonts were so widespread that they may have been feeding generalists.

    New long post coming later tonight.

  3. Anonymous says:

    Wow. Looks as though you had a very busy day. Those are some cool photos; they really help me visualize what you’re saying.

  4. Alton Dooley says:

    I worried about making it too photo-intensive; glad to hear they help.

  5. Doug says:

    Dooley,

    The program did discuss the bears ability to run. That’s one of the main lines of evidence that it was a scavenger. The longer legs meant that it cold run slightly faster than modern bears. But the limbs were relatively slim, lanky if you will. That meant that while it could keep up with an ice age horse or bison, the animal could outmaneuver the bear, because if the horse or bison made a sharp turn, the bear couldn’t follow because turning sharply could have resulted in a broken leg. Instead, they posited that the long legs helped the bear cover vast amounts of territory in a fashion much like a modern camel. Also, they showed that the bear was a scavenger by means of isotope analysis. They showed that predators like Smilodon and dire wolves specialized in a couple of species of prey, mainly horse and bison. In contrast, the bear was eating almost all large mammals on the ice age landscape.

    About the mastodons, they were most likely generalists. But by focus their would be to see if the cause of that size variation was a diet made up chiefly of aquatic plants.

  6. Alton Dooley says:

    I’ve always been dubious of the “obligate scavenger” model for terrestrial carnivores. It’s been suggested for lots of different groups (Tyrannosaurus is the most famous example), but I think there are problems with it.

    First is that functional assumptions that supposedly limit a species to scavenging are usually unconvincing. For instance, with Arctodus (and keep in mind I have not read the paper you mentioned), is there any analysis (like a stress study) to indicate that it couldn’t turn with a running herbivore?

    The “long legs to cover ground looking for carcasses” hypothesis sounds good, but there isn’t any actual evidence to support it. There are no living mammals that are pure scavengers, and the hyaena, which was thought to show pure scavenger adaptations, actually hunts as much or more than it scavenged. And, on the other hand, lions were thought to be pure predators but in fact scavenge a lot (often stealing kills made by hyaena). Among living mammals, almost everyone hunts AND scavenges. The exception is the cheetah, an obligate predator.

    All the clear-cut obligate scavengers are birds, which can cover dramatically more ground than any walking critter can.

    To me, walking obligate scavengers are so rare in the modern world (essentially unknown) that making that claim for a fossil organism requires really extraordinary evidence.

  7. Alton Dooley says:

    Incidentally, I suspect Smiloson and other saber-tooths probably were obligate predators, and likely specialists. Dire wolves are more difficult to assess.

  8. Alton Dooley says:

    That would be “Smilodon”; hard for my big fingers to type on the phone.

  9. Doug says:

    All valid points. I am not really arguing for the obligate scavenger, I’m just laying out what i have learned. Here is an article where it’s laid out a little bit better: http://www.wc.adfg.state.ak.us/index.cfm?adfg=wildlife_news.view_article&articles_id=232&issue_id=41

  10. Markus says:

    I think it is highly unprobably that Arctodus or any other big terrestrial animal was ever a specialized scavenger. For some strange reason it seems very popular among many paleontologist to declare extinct carnivores as scavengers, and often for very strange and often oppositional reasons (jaws are weak, it was not able to kill, it was a scavenger or jaws are strong, must was a bone-crusher, so it was a scavenger). I even read that some people thought that saber-toothed cats were specialized scavengers…
    I am sure really most extinct carnivores did use every opportunity to eat carrion or steal prey of smaller carnivores, but a specialized scavenging way of life is nearly impossible. Even vultures can and do hunt, sometimes even very big animals, because there is often not enough carrion to feed even a gliding animal, which needs nearly no energy to search vast regions for food.
    There are also some special things with Arctodus. It is in general presented as a huge super-carnivore of monstrous proportions, which had no problem to steal prey from any other carnivore. The problem is that Arctodus was not that big at all. The widley cited figure of 1000kg is most probably wrong. First of all there were several different subspecies of Arctodus, and they were different in size, with the biggest subspecies Arctodus simus. They showed massive sexual size dimorphism, so the females were much smaller than the males.

    The estimation of 1 ton was made for the largest known specimen of Arctodus simus. Furthermore this estimation is most probably much too high. This animals had very different proportions to modern bears, and weight estimations based on limb-bones alone are not the best way. Arctodus looked mainly so huge because it had such long legs, but its body was not bigger than those of a very large modern Kodiak or Kamtchatka bear. The biggest modern polar bears even surpass clearly in mass the largest known Arctodus simus specimen, and even those are still under 1000kg.

    Daniel Reed made probably one of the best Arctodus reconstruction, which is directly based on a skeleton, so the proportions are exactly right.

    It shows a size comparison of the largest known specimen, but if you look at the body alone, it is not that big. I know another size comparison Daniel Reed made together with a scaled up polar bear (whose shoulder height was even a bit under the record-sizes) of about 1500 Ibs, and the polar is clearly heavier.

    The weight of the largest Arctodus simus specimen we know was much more probable in the 600 kg range, but even among male members of the Arctodus simus subspecies the average was lesser. So even the largest ones of the largest subspecies were “only” about 600kg (what is still huge), the average was still much smaller, and the other subspecies were even smaller too. And the females were even still smaller.

    Carl Buel made also some great reconstruction drawings of Arctodus in comparison with a man (a cool picture of himself) which show clearly that Arctodus was far away from being as massive as a big bull (with a weight of about 1 ton).

    I have also problems to see true crushing teeth in Arctodus. The teeth a comparably big for a bear, but they don´t really look like bone-crushers. If Arctodus had used them to crush bones, there should be also a lot more of abrasion and wear on them, as seen in hyenas. Things like a big nose are also not really arguments for scavenging. Modern bears have a very keen sense of smell too, as well as wolves, but there is no specializtion for scavenging. In contrast the noses of lions are comparably small but they scavenge comparably often.

  11. Doug says:

    Markus,
    No doubt it wasn’t an obligate scavenger, but the question is what it was doing most of the time. Large animals need large amounts of food to keep going, and that is the main question with this bear. You say many paleontologists love to declare extinct carnivores as scavengers. Very odd statement there. I have only heard Jack Horner declare T. rex a scavenger, Matheus declare Arctodus a scavenger, and only have heard it suggested that Borophagus was a scavenger, and only the second and third of those actually made compelling arguments. However, Xiaoming Wang in his dog book noted that the adaptations in Borophagus (and Borophagines in general) that were regarded as adaptations for a scavenging lifestyle may in fact have been adaptations for communal feeding (very unlikely for Arctodus). And I have never heard saber-tooths presented as anything close to a scavenger, and I have never heard anyone but Horner suggest that theropods were scavengers. Sorry, but that makes me wonder where you are getting your information.

    Now, why do you have problems with size estimates? You seem to throw around the words “probable”, “probably”, and “wrong”. How can you be so certain, without any actual research, that it wasn’t that big? Yeah, its limb proportions are different than other bears, but most figures I have heard put Arctodus between 1500 and 1800 pounds. I have only heard one figure that put males at 770 to 825 pounds, but he was basing that on estimates of its body length (as a side note, I doubt that without sufficient genetic material that we can confidently attribute subspecies). But it seems that estimating the size of an extinct animal can be problematic. The extinct diprotodon was long thought to weigh one, maybe two tons. Then research by Stephen Wroe showed that it was actually bigger than previously thought. They used new methods of measurement on several diprotodon specimens and found that it was actually three tons, more that a ton over previous estimates. He also set the weight of the marsupial lion. Thylacoleo was found to be not much longer than a leopard, but length is a very rug indicator of weight (which brings those above estimates I mentioned into doubt). Using two different methods, he and his team found that Thylacoleo had a weight of 215 pounds. They can say this with confidence because two different methods arrived at the same weight estimate (femur+humerus width and brain volume). I wonder if these methods could be applied to Arctodus? So as you can see, estimating weight is very tricky, and can’t be dismissed out of hand. Because reconstructing a prehistoric animal in life is problematic, I wouldn’t use reconstructions to support your argument. Even if based on a skeleton, there are many skeletons, and was sexual dimorphism and/or the supposed subspecies factored in?

    The way the program presented the scavenger idea was that the inability to run and the diet analysis showed that it was probably a scavenger. Then they went on to explain how the features like a strong sense of smell, bone crushing jaws, and a pacing gate could have been used to help it survive on leftovers, rather than being specializations. As cats and borophagines demonstrate, a shortened skull increases jaw power. Now, define “true” crushing teeth. It seems that all the animals I see who supposedly have the capacity to crunch bone usually have broad flat teeth, something Arctosus possessed. That, combined with the shortened domed skull, could give it the capacity to crush bone. You do have a point as to the wear on teeth. I don’t think anyone has ever looked for such wear, since this idea of a bone-crushing bear has only recently been proposed.

    All this seems rather pointless. Neither you nor I have ever examined Arctodus remains up close, so I think neither of us is qualified to say anything with certainty. Paleontology requires research based on evidence, and never personal incredulity. Just saying there is no evidence for the opposing side’s argument doesn’t move the dialogue along one bit. Conduct your own research; see if they are wrong or right. It seems more comprehensive work needs to be done on Arctodus, to try and determine its size, speed, bite force, etc. And saying that there is no modern carnivore that is a total predator or scavenger will never, ever resolve the issue. Perhaps it was mainly a predator. Perhaps it was mainly a (but not obligate) scavenger. Perhaps he was the perfectly designed opportunist. But we will never know until we just sit down with the bones themselves and give them a thorough looking over.

  12. Alton Dooley says:

    I have heard suggestions that the saber-tooths were scavengers, although not recently (can’t remember the reference right now); the argument was that their sabers were to fragile to bit struggling prey, therefore the prey must be dead and the sabers were just used to rip it open.

    Scavenging has also been put forward for entelodonts, I think mesonychids, Dilophosaurus (but I don’t think that ever got any traction), all the other tyrannosaurs (there are several genera), and borophagines (as Doug mentioned), and azhdarchid pterosaurs that’s what I can think of off the top of my head. Only the pterosaurs have a modern analogue.

    I think observing the hypothesized behavior in a living organism is hugely important. It’s not that you can’t have a behavior that isn’t observed in modern animals; it’s that if you do so you must have extraordinary evidence that clearly rules out alternatives.

    Usually hypotheses about walking scavengers go along two routes. One is that the critter in question could not stand up to the rigors of hunting (teeth are too fragile; legs will break in a turn). The other is that the critter has specific adaptations that suggest scavenging (bone-breaking teeth, good sense of smell).

    Both of these arguments are very difficult to make in the absence of a modern analogue. First, you have all the uncertainties with structural studies (if they’re actually done at all). As Doug’s and Markus’ competing comments above indicate, in Arctodus it’s difficult to even get agreement on something as basic as body mass. How would changing the weight estimate effect estimates of the legs’ relative strength? (I’ve seen the range of estimates mentioned above. I tend to think Arctodus was likely much bigger that living bears, at least A. simus, but what do I know? Maybe it _was_ a relative lightweight.)

    As for adaptations for scavenging (or any behavior), that’s tricky too. Dogs and cats have very different teeth, including bone-crushing teeth in dogs (as I type this my dog is on the couch next to me, tearing through a bone), yet they are equally carnivorous. Dentitions specialized for breaking bones are just as effective if the bone happens to still be in a living animal. Incidentally, they also tend to be good for eating plants. So, to me, having bone-crushing teeth indicates that you were crushing bones, not how you got those bones.

    As Doug mentioned, there’s good evidence that Arctodus was eating mostly meat, so we do at least have good evidence to rule out herbivory or significant omnivory. But can we explain away those long legs using a modern analogue (assuming that they really don’t allow the bear to turn at speed, which I seriously doubt)?

    Actually, we can. Wolves might be a very good model for Arctodus. Wolves and other dogs have to cover vast distances at fairly high speed because of their cursorial, migratory prey. They don’t tend to run their prey down with a burst of speed; that chase their prey over a long distance, waiting for it to get tired. There isn’t a lot of twisting and turning involved, just a long steady chase. By the time a wolf goes for the kill, the prey is too tired to fight back effectively.

    Incidentally, modern wolves don’t typically hunt really big prey like bison (which were more common in the Pleistocene).

    Now, I don’t know if this model really works. Maybe for this hunting technique to work, you have to have a pack; I don’t know. If so, how many does it take to make an effective pack? Maybe 2 or 3 is enough. Coyotes are fairly solitary; do they hunt in a similar fashion to wolves?

    I have know idea if this is right. The point is that this is a behavior we actually know can exist because we’ve seen it, and which is still consistent with the morphological and isotopic data in Arctodus.

  13. Markus says:

    There are actually a lot of assumptions about scavenging ways of life for many extinct carnivores.
    Not only the most famous of all, T-rex and other tyrannosaurids as well as Dilophosaurus (when I looked at its teeth and the jaws I was once speculating that it possibly ate fish, and last year or so it was actually discovered that a close relative of Dilophosaurus did wade into water to hunt fish) as Alton already mentioned. Spinosaurus is very often portrayed as a scavenger, as well as Baryonyx and many of their relatives. Another case would be Carnotaurus. I even read in a book that theropods like Spinosaurus had too weak jaws to hunt, and that they were only able to feed on rotten and therefore softer meat. I really ask me what idiot can come to such conclusions.
    For reptiles there is Megalania.
    For birds all Teratorns (from which some were possibly really scavengers, but there are several good indication that some species also hunted), all phorusrhacids, especially Brontornis (which was perhaps no phorusrhacid at all), Harpagornis.
    For mammals nearly all the “giant” carnivorous mammals like Andrewsarchus, Megistotherium, Sarkastodon, a lot of extinct and sometimes gigantic hyenas like Pachycrocuta, dogs like Canis dirus (which is nearly always declared as a scavenger), marsupials like Thylacoleo, the giant extinct Australian tasmanian devil, and a lot more.
    The main problem for me is that some of these assumptions became extremely popular, for example Dilophosaurus or Spinosaurids as scavengers. I have still a lot of books, which say this theropods had too fragile jaws or vulnerable organs like the crests of Dilphosaurus and the sail (I don´t believe anymore in the idea of a sail, but that´s a different topic) or Spinosaurus.

  14. Doug says:

    Alton,
    I never thought of wolves as a comparable mode of hunting. And yes, observing behavior in modern is important. What I was getting at was that when objections to scavenging are brought up, the argument is usually the only one or the first one. I was just saying that that alone is not a sufficient counterpoint. Also, and a bit interestingly, the ancient bear Agriotherium from Africa is regarded by some as a more active predator because of it’s longer legs, but I don’t think it was as big as Arctodus.

    I have heard of Entelodonts as scavengers, and the main argument I usually hear is the chipped and heavily worn teeth. Although there is apparently evidence that Archaeotherium occasionally hunted (and even stored the surplus meat): http://www.douglasfossils.com/multi_kill.html

    Markus,
    I have many of those and some that I haven’t heard of (in respect to being dubbed scavengers). I must say that quite a few of them I have always heard as predators and nothing less. I have rarely encountered Canis dirus as a scavenger, and according to its program on National Geographic (Arctodus, Dire wolf, and Smilodon were part of a series “Prehistoric Predators”) isotope analysis shows it preyed primarily on horse and bison. You can’t scavenge only two species of animals. I have never heard of Thylacoleo as a scavenger. Richard Owen, who first described it, thought it was the “fellest and most destructive of predatory beasts”. It doubted to have even been able to eat meat, let alone kill something, for mush of the 20th century. When new material was studied, it was discovered that Thylacoleo was the most specialized mammalian carnivore ever. Never once did I hear someone refer to it as a scavenger.
    I have heard of Carnotaurus (and the recently discovered Kryptops) presented as scavengers. I read in a book that Carnotaurus seemed incapable of killing large animals, but then it presented the idea that it very well could have preyed on animals it’s size or smaller. Now, I do have doubts as to whether spinosaurids could attack and kill large prey, which includes bite strength (that seems to be very important to a predator) and its teeth. I am sure that, as we saw with Arctodus, paleontologists have reasons for thinking they might have been scavengers. Hell, I just listed a couple reasons. But I would never call them idiots. The moment you resort to personal name calling because they have a contrary view, you’ve lost. Science is about investigation and debate, and neither of those involves ad hominem attacks. Science requires that when new evidence comes to light, you must be ready to admit that you may have gotten things wrong. Attacking the researcher instead of their research does nothing to move the debate along. I am as little a fan of Jack Horner as they come, but he does offer a few words of wisdom: “Strive to be a good, objective scientist, understanding that if you are indeed more interested in being right than in having the right answer, you should go into politics.”

  15. Markus says:

    Hi Doug!

    I think there is some kind of misunderstandig. I don´t say that there is a general agreement that the named carnivores were scavengers, neither did I call all paleontologists idiots. What I wanted to say is, that there was and partly still is some kind of blindness towards logic and reality among “some” paleontologists, and that a scavenging way of life was proposed for many animals and often for very strange and unlogical reasons. Just take Dilophosaurus again as an example. I have really many books and could even read it at Wikipedia and many other sources that it is thought to be a scavenger, because its jaws are too thin and its crests too fragile to overcome big animals, with the conclusion that it search for carrion and feasted on carcasses. What kind of logic is this? Why not assume that it just ate smaller animals, as many modern carnivores do it? Maned wolves of abessinian foxes are really not small, but still they mainly eat bigger rodents. Aardwolves and sloth bears are even specialized to eat big quantities of insects. There are not only big-game-hunters and scavengers, there is a much wider variance of food-niches.
    Of course I know that Canis dirus was also a hunter, many of the skeletons show even fractures which are identical to those of modern wolves when they tackle with big and dangerous prey like mooses or big elks. But as they had shorter legs, proportionally bigger heads and stronger jaws it was and partly still is very popular to see them as scavengers. Actually I know only a small handfull of sources which not describe them as scavengers.
    Today we know of the massive adaptions Thylacoleonids evolved for hypercarnivory, but this was always the case. Owen was completely right when he wrote about Thylacoleo carnifex, but actually it was even assumed for a long time that it was a herbivore which crushed nuts with its teeth, not to speak about the idea that it used them to crush bones from carrion.
    There are some very popular arguments which are often brougth up when a carnivore is assumed to be a scavenger, for example too weak or very strong jaws. If we look at modern animals, we see many contradictory things. Does a scavenger need strong jaws or big teeth to consume as much as possible of a carcass? The probably best proof against this idea it the komodo dragon. Their jaws are extremely weak, not much stronger than those of a cat (really). But they can disembowel even a water buffalo and gobble big bones, which enables them to use nearly 100% of a carcass. But even given this fact, they are still no specialized scavengers at all, but highly specialized predators which only scavenger if there is an opportunity.
    Spotted hyenas as well as wolves have strong jaws and are among the few big carnivores which actually crush bones. But still they are very good hunters. But of course it is very good to be able to consume even the last remains of a carcass, no matter if it was found, stolen or hunted.
    Big cats in contrast have extremely strong jaws, huge carnassials and practize no bone crushing at all, but feed only on the soft parts of a carcass. Not the best indication for a scavenger, but especially lions practize scavenging and stealing prey from other carnivores very often, probably more than any other carnivore. The tiger which has a highly similar anatomy (you can hardly discriminate between their bones, especially if they are fossil) has again a very different way of life. Nile crocodiles, to take a completely different example, have enormously strong jaws and are also able to consume nearly a whole carcass, except those of unusual big animals like elephants. But again, there is nearly no scavenging, but mainly effective hunting. The wold is not full of carcasses, so that it would be extremely hard to to feed a whole population of big or even huge scavengers. Ideas like those of the Spinosaurus which searched for rotten soft meat because it could not kill and eat fresh meat are really…well, let´s say not very clever.
    BTW, there are two very good reconstructions of Agriotherium by Mauricio Anton. The one is of Agriotherium africanum and can be found in “Evolving Eden” (great book!). It is actually highly similar to modern bears, only the legs were a little big longer and the head of slightly different proportions. It also downsized, as many other prehistoric animals. There are statements that it was larger than any living bear, and weighed up to 1 ton. It really big, but with a shoulder height of about 1,2-1,3m not bigger than big modern brown bears.

  16. Alton Dooley says:

    By the way, guys, this is a good discussion, and it just what I’ve been hoping the blog would promote.

  17. Doug says:

    Komodo dragons are not a very good example of predators with weak jaws. One is that their toxic saliva is what usually does the large prey animals in; much in the same way a snake’s venom does, only slower. Second, they can dismember large carcasses because usually you have several lizards pulling on different parts of the carcass, and the end result is the poor boar or goat getting hind and quartered. Third, they live on a small island with no large mammals to compete with. They simply bite the prey and follow the dying animal until it’s too weak from blood poisoning to fight back. After all, it’s got nowhere to go on that island. Isolation has allowed them to develop a unique hunting style suit for their small island home.

    Saber-tooth cats are another good example. Stephen Wroe has done extensive studies on the bite forces of living and extinct carnivores. In his research, he found that Smilodon had a weak bite for an animal of its size, only a third that of a similarly size lion. And yet evidence shows he was taking down horse, bison, and camel. The reason is that he has strong neck muscles powering two 6 inch teeth. Smilodon used long sharp teeth to slash the throats of its victims. Of course, it had to subdue them first. Smilodon had a powerful build and sharp claws to get the job done. So as you can see, successful predators with week bites are able to hunt because of specializations. One uses a highly specialized set of teeth, the other toxic spit. Their bite strength doesn’t really come into play in prey acquisition.

    “Does a scavenger need strong jaws or big teeth to consume as much as possible of a carcass?” Yes, if the animal does indeed make a large part of it’s living from scavenging. The ability to crack open a bone gives the animal access to the marrow inside. Marrow is full of proteins, lipids, and fats, which all together are full of nutrients. This could help a scavenger get by on leftovers. Our early ancestors used crude stone tools to break open animal bones to get at this resource. Of course, most animals can’t use rocks, so they would have to rely on teeth and jaws to crack bones.

    Last I heard, crocodiles have the strongest bites of any modern animal. But look at how they hunt. They wait around in a body of water and have special adaptations that allow them to sneak up on prey. The muscles that open the mouth are rather weak, but the muscles that close the jaws are extremely powerful. They need those strong jaws to grab and keep hold of large prey so that they can manipulate it into the water to drown it. Crocodiles have been around since the dinosaurs, with little change, so their method of hunting must be working. But again, we see specializations at work. Crocs are specialized to hunt prey in a stealthy manner in water.

    Dilophosaurus is a tricky case. I can see why they would think (instead of blowing it off as illogical) it scavenged because of its crests. Very, very few known carnivores have such elaborate head ornaments (only other one I can think of is Cryolophosaurs). If Dilophosaurus was a hunter, it luckily used teeth and jaws to attack prey. But that would place the crests perilously close to a thrashing victim, and a broken crest would hurt quite a lot. And as I stated before, strong jaws are usually important to a predator. But teeth play just as important a role. Jaws are one reason why spinosaurids don’t seem like capable predators. Another is teeth. Their teeth are conical, round in cross section, and lacking in serrations. When we look at the teeth of large theropods that we think of as predators, we find the utter opposite. They have serrations, are flattened, and curve back. Even T. rexs teeth curve back with serrations. Teeth like those of spinosaurus don’t look like the kind of teeth that can inflict large amounts of damage. Teeth like spinosaurids are good for holding things, like the teeth of a crocodile. Bones of a young iguanodon were found near the skeleton of Baryonyx, but whether or not it had killed it, we will never know. It’s just as likely that it was scavenged, if they were related to the carnivore at all.

    “Why not assume that it just ate smaller animals…” Because just assuming things is bad science, especially with paleontology. We have to make our best guess based on the available evidence. We look to modern animals that may have similar features to try and get a rough idea of how long extinct animals may have lived. Assuming doesn’t require much thought, and it makes science look like a string of wild guesses. “Strange” and “unlogical” almost seem like matters of opinion. Okay, only strange, but define “logical”. You actually have to dissect the other guy’s arguments, get to the center of it, and do exhaustive research. You can’t just blow off their argument because you find it “unlogical”. Raise objections, ask tough questions, and make an earnest effort to refute their claims. You listed a lot of animals that supposedly disprove scavenging arguments, but look what I did with them? Sorry, but this just doesn’t seem to be going anywhere. Not all large dinosaurs could have been scavengers, but you know what, they all couldn’t have been top predators either.

  18. Alton Dooley says:

    The thing is though, Doug, that to demonstrate a behavior in an extinct animal you must 1) show that the behavior is feasible and 2) show that other possible behaviors can be excluded. That is much easier to do if you have modern examples of all the behaviors in question.

    In the specific case of walking scavengers, there are no modern examples. That makes the task of demonstrating this behavior much harder, because you have to postulate a behavior that has never been observed, while out the same time ruling out behaviors that have been observed. That’s a tough case to make.

    While I don’t work on most of the animals we’ve been discussing (so I haven’t gone through the primary literature with a fine-toothed comb), I’ve not seen a convincing argument for the walking scavenger. While lots have people have made good arguments that a particular critter’s morphology is _compatible_ with scavenging, what I don’t think has ever been shown adequately is that alternative feeding styles (that are known to exist in modern groups) were impossible. The authors of the Arctodus paper were aware of the need to do this; that’s why they make the arguments about the turning abilities based on leg proportions and the isotope studies, to try to rule out the alternatives. They’re making the right arguments to try to prove their point; it’s just that I don’t find their arguments convincing (ie, I still think the data is consistent with the alternatives). If the data is consistent with two different behaviors (and in my opinion it is, although they would obviously disagree), and only one of those behaviors is definitely known to exist, that’s the one to go with.

  19. Markus says:

    I used komodo dragons as an example to show how even an animal with very weak jaws can be able to consume nearly 100% of a carcass, because it is unusual for many reason. In this case it doesn´t matter how it hunts, because it makes no difference if they eat a self-hunted deer or a dead deer they found by chance. Even a single komodo dragon has no problem to open and eat from a big carcass, because their teeth are very sharp and their neck is so extremely strong. They are not even able to crack at least a soft bone like a rib, but they have a very good alternative: They just consume whole bones and use their acids in the stomach to make this work. I have never ever read that somebody wrote about this possibility for theropods or other extinct reptilian carnivores. It is just not always neccesary to be a bone-crusher to consume bones. Modern birds of prey can also gobble very big chuncks of meat or even bones, so I would not be surprised, if theropods could do this too. We should not look at mammals when we try to reconstruct the biology of extinct reptiles.
    The skulls of theropods are very different from those of modern mammals, and to make comparisons with those of crocodiles, monitors and even birds seems to make more sense.
    To come again to Dilophosaurus: Let´s assume it did not hunt to prevent its crest from damage, why did it developed this crests at all? And there is an even bigger problem. The jaws and teeth of Dilophosaurus seems really very unsuited for a “typical” scavenger. In the case of T-rex we have huge jaws and bone-crushing teeth, but not in Dilophosaurus. But if Dilophosaurus was actually a hunter of smaller animals as well as fish, this could explain a lot of things, even the strange shape of the maxilla, which can be also found in several other piscivorous reptiles. The comparably long and slender teeth would fit with this too.
    In the case of spinosurids there is even much more evidence of piscivory. The long slender jaws with the long unserrated teeth have many similarities to those of fish-eating crocodiles, the strong arms and the huge claw on each hand would be very good to catch big fish, and even more we have those relics of fish in the stomach of Baryonyx. They have even this strange “angle” in the upper jaw, which can be also found in crocodiles (and Dilophosaurus). You say yourself not all big carnivores can be apex predators.
    That´s the thing, there are much more possibilities than only being a big-game hunter or a scavenger. Actually there are nearly no scavengers today. Only brown and striped hyenas life mainly from carrion, but they consume also small animals, eggs and even fruits, and striped hyenas even sometimes hunt. Tasmanian devils are also often seen mainly as scavengers, but they hunt too and are even able to kill a sheep. Nearly every carnivore would prefer it to eat from carrion alone, but this doesn´t work. The world is not plastered with carrion which could feed whole populations of big or even giant scavengers. Spinosaurus reached weights of more than 10tons, i.e. as much as several prides of lions together. I can hardly imagine that there was enough carrion to feed such an animal. It would have to walk huge distances, and even then, it is very probable that other smaller carnivores would have already consumed most parts of a carcass.

  20. Doug says:

    Alright, that’s it. You win. I was way in over my head. In fact, I was never a good debater to begin with. But I’m done, it’s over, you win. Clearly thinking anything was a scavenger is illogical. Never mind that they wouldn’t be scavenging all the time; that they could still occasionally hunt. No, everything has to hunt something for a living. You can never call something a scavenger until you have ruled out all alternatives.

    Why did Dilophosaurus evolve that huge crest? Very good question. Mating display or species recognition perhaps. Why are its jaws and teeth unsuited to scavenging? You have to be specific.

    Spinosaurids do show adaptations for eating fish. But these large theropods, especially the 10 ton (a figure which I highly dispute) Spinosaurus couldn’t have lived on fish alone, even the big ones that lived during the Mesozoic, so that’s where supplemental scavenging comes into play.

    You know why a scavenging Arctodus might also make sense? Ecology! Ice age North America had saber-tooth cats, scimitar cats, lions, cheetahs, cougars, jaguars, dire wolves, and regular wolves. Isotopes show that smilodon and dire wolves were eating mainly horse and bison, so those two are already in competition with each other so the same prey. Arctodus would have to be competing with all the predators above. Yeah, the place wouldn’t have been littered with carcasses, but what if Arctodus routinely stole carcasses from its neighbors, something we see grizzlies doing in Yellowstone today.

    Don’t bother responding, because as I said earlier, I concede to you. But know this: you have to be specific with your criticisms. And you can’t just disprove a theory: you actually have to provide a viable alternative. Poking holes in the other guys argument doesn’t work. You must provide positive evidence for your side. Brown and striped hyenas are scavengers. That means they get a large part of their food through scavenging. They occasionally hunt, but they are still scavengers. I never argued for obligate scavengers, because there is no such thing. But animals that do more scavenging than hunting are not unheard off. Perhaps many of these animals were in fact scavengers, but we just can’t deduce that as of late. But Dr. Matheus has laid out his case for Arctodus being a scavenger. It is now the task of other scientists to prove him wrong. That’s what happened with T. rex. Horner presented his case for a scavenging lifestyle, and scientists demonstrated, with evidence and detail, how T. rex could have hunted. That is how sciences works: you make arguments based on the evidence, and theories are constantly tested to see if they are right or wrong, regardless of what you may think. What if their research confirms the idea that Arctodus was mainly a scavenger? Will you still think they are illogical?

  21. Markus says:

    Hi Doug!
    I really don´t want to have an argument with you, the whole thing became a too hot debate. One problem is, that we have really too much different topics, we would really need an own discussion for every sub-topic.
    I once wrote a really long blog entry on my blog about the scavenger-issue of T-rex, as well as a shorter one about Dilophosaurus, but as they are written in german, this won´t probably help much for this debatte:
    http://bestiarium.kryptozoologie.net/artikel/angebliche-aasfresser-teil-2-tyrannosaurus-rex/
    http://bestiarium.kryptozoologie.net/artikel/angebliche-aasfresser-teil-1-dilophosaurus/
    As you can see that I wrote at least in the case of T-rex really very much, not only things like “Horner´s ideas are illogical”. I discussed every point he made for a scavenging behavior, wrote why his arguments don´t work (including really many comparisons with the biology of living and extinct carnivores), and gave the arguments why are very good indications for a hunting way of life. As this discussion had way too much different issues, I could not write in every case as much as I would like to do, not only because this is would need really very much time.
    I really don´t wanna end the discussion here, as it was really interesting. You made many really good points.
    To come at first at Dilophosaurus again. Why would its skull be not very good for a scavenger? If I would assume that a theropod actually evolved to become a full-time-scavenger, which nearly never hunted for his own, I would suppose it should have at least some adaptions for this behavior. Its teeth and jaws should be able to open even a thick-skinned carcass, and able to ripp of even dry and hard scratches of meat from big bones.It should be able to slice with ease threw meat. But if we look at Dilophosaurus´ jaws and teeth, they hardly fit into this. The jaws in the whole are not very strong compared to many other theropods of its size. But as I already wrote, strong jaws are not alway a need to deal with carcasses. But its teeth are comparably long and thin, especially in the lower jaw. Those of the komodo dragon are highly curved, laterally flatened and comparably short. What do you think what would happen, if a Dilophosaurus would bite in a big chunk of of a carcass and would draw its head back? Very probably at least the lower teeth would break.
    There is an additional thing we could assume for a scavenging theropod. Should it not be able to disembowel a carcass as vultures and komodo dragons do it? Well, at this point we have again a very big problems with the crests…
    In the whole there seems to be at least as much problems with a behavior as a big-game hunter, as well as scavenger. The idea that it possibly ate fish, would make a lot of sense here. It would explain the long and narrow teeth with their weak curvature, the incissure at the top of the upper jaw, the relation with a similar species which turned out to be a fish-eater, and even the fragile crests would not make much problems if it used to life from such comparably harmless prey. It would not even too big for this.
    I have to agree with you, the huge size of Spinosaurus is really problematic. Actually there are really good indications that it was even much heavier than only 10 tons. But interestingly, among all living terrestrial carnivores not the big-game-hunters like lions or tigers are the biggest, but those which gain a lot of their food in any form out of the water. The polar bear and the huge kodiak bears surpass every big cat. I know this is possibly not the very best comparison, but an approach. Wet ecosystems can produce a huge amount of biomass. And it can enable even a very big carnivore to live from much smaller animals with a small amount of energy.
    Spinosaurids show a lot of very weird anatomical things, and I think no matter what you or I think about their actually diet, you will probably agree that they were surely different from all the other big theropods. They most probably were no typical big-game hunters. Especially forms like Suchomimus, with their extremely slender and elongated jaws hardly hunted other big dinosaurs. I suppose they had a very opportunistic feeding behavior. I can very well imagine that big fish (which were even found as remains in the belly of Baryonyx) made out a big amount of their diet, but as nearly all carnivores, no matter if marabous or crocodiles, they might have eaten everything they could get, including small and possibly even medium-sized reptiles and dinosaurs, and if possible surely also carrion. The skull of Spinosaurus was still not that fragile, and its arms were also very powerfull. Bears also use their extremely powerfull claws to bring down even cattle, and they don´t have a half-metre long claw as Spinosaurus had. They were surely able to kill at least medium-sized animals.
    It´s already late, and I would prefer to discuss this topics at first, before we come back to Arctodus again (if that´s okay for you).

    Best regards

    Markus

  22. Alton Dooley says:

    Just one quick comment on Dilophosaurus: thin, weak teeth (assuming they really are weak without a stress analysis!), my not be as limiting in an animal with continuous tooth replacement as it would be in a mammal.

  23. Markus says:

    Yes, that´s true, but would it make sense for a carnivore to evolve to a scavenger without having good “tools” to do it? Even in reptiles it needs some time untill a tooth regrows. If we look at sharks, we see that those which eat mainly fish and squid have in general very long and thin teeth, but those which eat mainly big and sometimes much thougher animals (like turtles) have much shorter and broader teeth, what decrease also the risk of a tooth-fracture. Modern crocodiles which often deal with big animals have strong teeth too, despite the ability to regrow (to a distinct degree, old crocs have sometimes no more teeth at all) teeth. And among them, those with the long and thin teeth feed in general nearly never on big animals and even bigger carcasses are a problem for them, even for species like the indian or the false gharial which can reach respectable sizes.

  24. Alton Dooley says:

    Although see my comments on the October 23 post about killer whale teeth.

    A constant worry with fossils is the persistent tendency of animals to do things that their morphology says they shouldn’t be doing. There have been sperm whales with completely deformed, non-functional lower jaws that were otherwise perfectly healthy and eating. If sperm whales were known only as fossils, we would go on at length about what diet their teeth are suited for.

    Likewise, there have been modern baleen whales with baleen infections that caused all the plates to fall out, yet they were still eating.

    Not that we shouldn’t try to interpret features; but animals have a way of doing whatever they want, no matter what we might think about it.

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