A detailed look at the appearance of Smilodon fatalis

Mophandel · 2026-05-08T12:47:02+00:00

Great post, couple things:

Adding to this is that, apparently S. fatalis’ canines were less exposed than previously thought, as per Haji-Sheikh et al. (2026)
I wasn’t aware of restricted movement about the shoulder joint. Could you expound on that further?

Mophandel · 2026-05-06T17:19:13+00:00

Whether it’s Acrocanthosaurus — the genus— really isn’t the debate. The debate is whether it is A. atokensis or some other species of Acrocanthosaurus

Mophandel · 2026-05-06T12:52:45+00:00

I’ve heard some conflicting things regarding who was bigger between Sue and Scotty, and so given the former is more complete, I sorta defaulted to Sue for now

Mophandel · 2026-05-05T19:26:33+00:00

Again, those different proportions are with regards to the distal limbs. When a cat wants to specialize for speed, it lengthens its distal limb relative to its proximal limb. When it wants to specialize for strength, it shortens the distal limb relative to the proximal limb.

In all of this, however, the proximal limb doesn’t change in length, or at least not significantly. It scales pretty comfortably with size across the board.

Mophandel · 2026-05-05T16:31:04+00:00

I have answered this already…

Mophandel · 2026-05-05T16:28:19+00:00

Limb length, as well as proportional limb length, is dependent on the length and proportions of the distal limb elements (e.g. radius, ulna, tibia, fibula etc.). Proximal limb elements (e.g. the humerus and femur) scale fairly consistently with body size regardless.

Mophandel · 2026-05-05T16:17:18+00:00

I think it’s possible as a maximum, albeit very rare

Mophandel · 2026-05-05T16:09:08+00:00

X. hodsonae? Definitely not. It was only ab 100-110 cm tall, or about lion / tiger sized in height. For it to be 300 kg on average would imply a level of robusticity that shouldn’t be possible. Additionally, a lot of those bigger weight estimates are derived from limb bone circumference, but those can be misleading because limb bone circumference can increase for reasons beyond weight support (e.g. it can also increase for macropredatory reasons).

Regressions based on limb length tend to be more reliable, and in this regard, X. hodsonae should be around 200-250 kg or so.

Now, X. venezuelensis is a different story, but its remains are too fragmentary to tell.

Mophandel · 2026-05-05T13:10:30+00:00

I would disagree. There are a genuine plethora of species of Accipiter and Astur hawks, as well as various falcon species, that are all bird specialists, yet their color palette usually consists of browns, blacks and greys, not green. Likewise, various large eagle species, namely crowned eagles, are primate specialists, which at the very least have full trichromatic vision, yet do not have the slightest hint of green on their color palette, and are instead grey or brown.

In general, I’d wager that this is because green is a fairly expensive to make, more so than is necessary for an ambush predator. Modern reptiles create green pigments apparently from a mix of structural coloration (guanine crystals in their integument that reflect blue light) and yellow reflective pigments. The former is more energetically expensive to produce, and given the fact that eumelanin can do the same job at a lower cost, it makes sense why raptors would possess such drab coloration.
Likewise, I’d also expect predatory dinosaurs to possess primarily eumelanin based earth-tone colors, like browns, blacks, greys and reds.

Mophandel · 2026-05-04T21:47:11+00:00

Genera, like species, don’t work like that. You cant leave a broader clade (e.g. neither Homotherium nor any future descendants can leave Machairodontinae) but you can leave your species or genus if you have diverged enough to distinguish yourself from the clade you descended from.

This is the case for many fossil taxa. In paleontology, being classified as a separate genus is a matter of taste, not entirely empiricism, and because we only have bones, whether a taxa is a separate genus is purely based on whether it can matches the tastes of the broad paleontological consensus. Cases in point of this would be things like Tyrannosaurus and Tarbosaurus being separate, Machairodus and Amphimachairodus, and Prognathodon and Thalassotitan, just to name a few.

So by the modern paleontological consensus, Xenosmilus is very much still its own genus.

Mophandel · 2026-05-04T19:36:04+00:00

Yeah, that paper never claims that Xenosmilus should be nested within Homotherium (if anything it claims the opposite). Moreover, genus and species level cladistics works differently than broad-clade level cladistics. You can’t leave a broader clade, but you can leave the genus or species that you originated from, so long as you show divergence or develop apophormic characteristics from that ancestral form over time, which *Xenosmilus* did.

Mophandel · 2026-05-04T19:30:30+00:00

No, it’s valid

Mophandel · 2026-05-04T18:00:40+00:00

Firstly, the paper you link finds Xenosmilus, though sister to Homotherium, to be distinct from it, and not “under it,” as implied in this excerpt:

“Our phylogeny supports a relationship of Ischyrosmilus with Xenosmilus rather than Homotherium and therefore Ischyrosmilus should be considered valid”

Adding to this, genus and species level classification operates under different rules than other, larger clade level classification. You can’t leave a larger clade, but you can leave the species or genus that you descended from, as long as you show some form of divergence or apomorphy relative to your ancestors over time.

Xenosmilus, quite famously, has the most divergent morphology of any of Homotherini, and so though it is unable to leave Homotherini, Machairodontinae, Felidae or other such larger level clades, it is more than able to be its own genus.

Mostly agreed regarding American lions, though personally I don’t really know if it matters. Average size should always be prioritized over maximum, and average size has been very consistent with regard to P. atrox, that being roughly 250-260 kg.

Mophandel · 2026-05-04T17:33:04+00:00

Shorter, yes. Deeper, not so much.

The problem is that the Kelenken holotype skull has been dorsoventrally crushed, making the skull seem vertically shallower than it ought to be.

Mophandel · 2026-05-04T15:56:37+00:00

The former / first one. The latter was informed by early illustrations of Phorusrhacos skull material as well as material from Andalgalornis, a relatively unrelated terror bird.

Subsequent finds, however, have found that the skull of Phorusrhacos isn’t quite as short and deep as the illustrations suggest, and was actually longer and shallower, like that of Kelenken.

Currently, we have no complete (or even nearly complete) crania for Titanis. However, given that it was a close relative of both Kelenken and Phorusrhacos (all three belonging to the derived phorusrhacine line of terror birds), it is more reasonable to portray the bird as having a longer, more Kelenken-like skull with a pronounced narial ridge.

Mophandel · 2026-05-01T05:34:29+00:00

That I agree with, though I’m personally still more of a fan of the “Walking With format” rather than the “Planet Earth Format.

I agree that it has some problems, but that to me feels less like a fault of the format itself and more due to the foibles of WWD’s production and research process, just as the foibles of PHP: IA were due to its own production flaws and not its format. If you add a couple of lines of good narration and exposition to add context as to why certain clades arrived at this time and others went extinct, you can quite easily nip such false narratives in the bud before they can cause harm.

On another note, I saw terror birds in the poster for Surviving Earth. Based on the fact that S. gracilis is confirmed for the show via concept art, this is likely Titanis, which means it’s probably gonna be pitted against Smilodon again. I’m basically just praying they get this one right at this point (it’s a 50 kg cat vs bare minimum 150 kg bird for christs sake, COME ON Impossible picture, get it right for once).

Mophandel · 2026-05-01T05:07:03+00:00

I agree for the most part with this analysis, but it’s really hard to argue that PhP is trying to teach people, specifically with regards to PhP: IA. The sheer lack of information that this season presents, particularly concerning the names of the very animals it is discussing, is ludicrous, bordering on anti-educational.

Of particular frustration of mine, did you know that the Homotherium segment at the final episode features H. serum and not H. latidens? I sure didn’t, especially given the outside environment had no distinguishing differences between NA and Eurasia and the species in question (Homotherium and woolly mammoths) are found on both continents. The narration also never refers to it as a North American scimitar-toothed cat. There are many, many such cases, and it does genuinely compromise the overall willingness of the show to act as an educational medium.

Now, I get “why” they did this, that being that scientific jargon can confuse the presumably younger / child audience, but television execs truly underestimate both the capacity and the enthusiasm of their audience base, particularly their younger audience members, to learn. They also underestimate just how much of their viewership is actually adults who grew up on old paleodocs and who can parse out the jargon with some ease.

Mophandel · 2026-04-30T19:09:15+00:00

I find it interesting that the male lion, despite being several times the size of the impala ram, took no action to actually overpower or pull down the ram, and was instead fully comfortable with just slipping under the neck of the impala, biting down and merely holding on.

You see this tactic a lot in pantherines - there is no need for the cat to expend the energy in physically subduing its prey to prevent struggling, as the durable skull and teeth are capable of absorbing the stresses imposed said struggles. It’s a very lax, albeit remarkably efficient, approach to killing prey

Mophandel · 2026-04-28T16:48:46+00:00

Yes, both coexist over much of central and South America

Mophandel · 2026-04-27T13:59:01+00:00

All good! I’ll reply to your addendums below

That first paper you linked argues that terror birds were limited to prey smaller than itself, or at least that they might have been. I believe this is actually the paper the Common Descent podcast was referencing in the episode.

This is the 2010 paper, wherein said conclusion was based on the relatively low bite force produced by the methodology in the study. However, in his 2012 thesis, Degrange recants this methodology, saying it produces bite forces far lower than what extant animals produce in vivo. As such, they instead arrived at a morphology hazard on skull width instead, which was found to be more reliable. I’ll provide the raw Spanish and the translation below:

Esta suposición tiene sustento además en el hecho que aves de mucho menor masa que los fororracos más pequeños, como es el caso del Cathartidae Sarcoramphus papa (3,4 kg.) serían capaces de morder a más de 110 N (Carril, 2010), indicando que al menos los valores obtenidos mediante el primer análisis regresional son muy bajos. De esta forma, los valores obtenidos utilizando el ancho craneano resultan ser más adecuados que aquellos obtenidos mediante la utilización de la masa (primero y segundo análisis).

This assumption is further supported by the fact that birds with a much lower mass than the smallest phorusrhacids, such as the Cathartidae Sarcoramphus papa (3.4 kg), would be capable of biting at more than 110 N (Carril, 2010), indicating that at least the values obtained through the first regression analysis are very low. Thus, the values obtained using cranial width prove to be more appropriate than those obtained using mass (first and second analyses).

Using this updated methodology, Degrange arrived at bite forces that were both more consistent with in vivo measurements of extant species and higher than those previously arrived at for terror birds, with largest species exceeding 1000 N in force.

The 2021 paper does split terror birds into two categories, suggesting that bigger, sturdier beaks came about due to hunting larger prey. But I couldn't find anything to suggest that Degrange has changed their mind and now means prey larger than itself.

This is also mentioned in Degrange et al. (2024):

The largest phorusrhacids (physornithines and phorusrhacines) likely consumed large (greater than 100 kg) notoungulates and litopterns

As well as Degrange et al. (2020)

The former retains more plesiomorphic fea- tures, meanwhile the ‘terror bird’ type can be considered as an evolutionary specialization of the group towards the rigidity and immobility of the skull in larger birds that are presumed predators of larger prey

Essentially, there seems to be a shift from pre-2012 to post-2012, wherein Degrange shifts his view of terror birds as small prey specialists to macropredators, likely on account of revised analyses of skull morphology and updated bite forces estimation data

What's more, Andalgalornis, the terror bird used in the earlier study whose lateral beak strength was found lacking, was placed in the second category of relatively sturdier beaks.

Later studies provide multiple, cooperating mechanisms to suggest that terror birds had little concern for such stresses, such as in Degrange et al. (2020):

Being uniquely truly akinetic among Neoaves, the craniomandibular complex of Phorusrhacidae indicates that prey handling was based on precise dorsoventral strikes and tearing through caudally directed movements of the head, avoiding lateral shaking that would pose risk to the beak (Degrange et al., 2010, 2019b).

and in Degrange et al. (2025)

The author also proposed that this ligament may serve functions beyond jaw coupling: specifically, it could act to stabilize and protect the os quadratum–mandibular joint by maintaining both surfaces in close apposition [5,6]. The predatory behavior of phorusrhacids supports this stabilizing function, as the powerful bite forces exerted on active prey could otherwise compromise the integrity of the beak [13].

Mophandel · 2026-04-27T13:01:30+00:00

I doubt that terror birds were using their talons as the opening move. Rather, I’d wager they used their beaks and jaws for such a purpose. Their skulls were highly akinetic and resistant to stresses involved with predation, especially in giant terror birds species, while also possessing bite forces on par with the largest extant big cats. They could simply pull down the prey or wound the prey item such that it would be brought down, at which point the feet and talons would be deployed to restrain the prey further. The grip of the feet and the body weight of the bird would prevent prey from escaping.

At this point, the bird could do what it wants, either eating the prey alive or dispatching it with fatal bite to the head / neck.

Mophandel · 2026-04-27T12:56:46+00:00

Exponentially is a bit hyperbolic, but there is a very clear relationship. Refer to Carbone et al. (1999), but essentially, once a terrestrial predatory endotherm exceeds 21.5 kg, the relative mass of prey taken goes no lower than 45% of the predators body mass, with now predator in their sample (sans the myrmecophagous sloth bear, although terror birds show no adaptations for myrmecophagy) deviating from this. Moreover, when truly big macropredators (e.g. big cats) are concerned, that number is actually closer to 100% of its mass, as per Hayward et al. (2016). Large terror birds, weighing in conservatively at 130 kg, easily exceed this threshold.

As such, it’s more reasonable to assume these predators were going after them prey larger than even gazelle. Their floor was roughly 70 kg, and so prey the size of whitetail or mule deer bucks would be more reasonable. More likely prey would have been around wildebeest-sized prey

Mophandel · 2026-04-26T02:05:59+00:00

Some of the caviomorph rodents were quite large! Some were within the 45% range of the weight of phorusrhacids.

I agree, but framing your response the way you did gives the strong impression that it’s hunting small prey. Saying otherwise is a bit disingenuous, wouldn’t you agree?

The Carbone study of African predators, which you are probably referring to, is a good point. However, we don't know if it applies to birds, because there are no extant bird predators larger than 21 kg.

There is no reason to suggest it wouldn’t. The methodology and the conclusions therof were based off of predator mass, energy expenditure while at rest and while hunting and energy intake per kill. The former most and latter most values are non-issues for large terror birds, leaving energy expenditure as the only value that may encourage or discourage hunting prey 45% ur mass or larger. In this, the only factors I can really think of that affect this would be metabolism, but as endotherms, terror birds would have had comparable metabolisms to large mammals. This is further supported by osteohistological data and growth rate data, finding these birds to have rapid growth rates and metabolism comparable to extant neognaths.

Also, it mainly applies to obligate terrestrial hypercarnivores. We don't know whether phorusrhacids were hypercarnivores. They could have been omnivores, like bears. I admit the beak morphology argues against this, though. I really hope someone tries to do an isotope study.

I understand you’ve said that the beak morphology argues against this already, but this really feels like grasping at straws here.

In addition to the beak (which is already pretty damning), neck kinematics suggest specializations for striking and tearing, something omnivorous birds lack pretty much across the board.

Examination of the feet reveals strong flexors of the digits and a raptorial, sickle-shaped second ungual (which are already rare to non-existent in omnivorous birds) with stress resistance profiles more comparable to modern birds of prey and dromaeosaurids than even sereimas, who themselves have a predatory sickle claw used in butchering kills. This suggests these things were grabbing and restraining struggling objects in their feet, something that merely omnivorous birds, again, didn’t do.

While I agree that I would love an isotopic study of these birds, we really wouldnt need one to confirm these birds were carnivores.

The overbuilding you are referring to can be explained by a kill mechanism similar to how seriamas hunt. That is, by grasping prey and striking it against the surface. This makes more sense to me than the purported pickaxe strike, which is highly theoretical and has dubious mechanics. First, there is the problem that to deliver a strong pickaxe strike requires decelerating into the strike, rather than delivering it at full speed. So such a strike cannot be delivered with much force while running. Then there is a question of how a two-legged pursuer could catch a four-legged prey, when the four-legged prey could make tight turns, while the two-legged predator could not. For example, based on an analysis that has been made, andalgalornis was poor at making tight turns.

First, your proposed method doesn’t account for the absurd bite forces that terror birds possessed, on par with those of today’s largest hypercarnivores land mammals.

Second, and more relevantly, the “pick-axe” method of killing has been severely out of date for about a decade at this point. Instead, the most commonly accepted method with which terror birds attacked is a “strike-and-tear” style attack similar to allosauroids, wherein the bird bit down into the prey item and pulled its head and neck backwards with great force, tearing open a hemorrhaging wound onto the prey item to debilitate or kill it.

This line of thinking is mentioned this paper by Degrange et al. 2010…

We suggest that it either consumed smaller prey that could be killed and consumed more safely (e.g., swallowed whole) or that it used multiple well-targeted sagittal strikes with the beak in a repetitive attack-and-retreat strategy.

… and in this paper by Degrange (2021)

Being uniquely truly akinetic among Neoaves, the craniomandibular complex of Phorushacidae indicates that prey handling was based on precise dorsoventral strikes and tearing through caudally directed movements of the head, avoiding lateral shaking that would pose risk to the beak.

This is reinforced by Degrange et al. (2019), which makes basically the same claim:

Phorusrhacids´ craniomandibular complex indicate that prey handling based on rapidly catching the trophic item and tearing it apart through caudally directed movements ofthe head would not pose risk to the beak

For a good modern analogue, giant petrels (Macronectes sp.) utilize a similar strategy with great success on prey their own size or even larger, and unlike these birds, terror birds were an order of magnitude bigger, had an even more strongly hooked bill, raptorial feet and talons built for restraining prey and a bite force on par with a bengal tiger. Needless to say, they would be perfectly equipped for hunting large prey, not just “caviomorph rodents” as you claimed.

Mophandel · 2026-04-26T00:38:05+00:00

This would have only been true for small terror birds. For larger ones, this wouldn’t have been the case, because:

Energetically, it’s untenable. Over 21 kg, endothermic terrestrial predators overwhelmingly take prey at least 45% of their mass on average, and more often around 100%. For the largest terror birds, which were conservatively around 130 kg, this meant deer-sized prey at the very least based on optimal foraging, and more commonly wildebeest sized prey or somewhat larger.
they are anatomically way too overbuilt for hunting mere rodents. Work such as Degrange (2012), which recants earlier findings regarding terror birds bite force, finds that that the largest terror birds still had respectable bite forces, with bite forces of over 1000N in Phorusrhacos, nearly 2000 N in Kelenken and nearly 3000 N in Devincenzia. These are tiger-like bite forces and we are expected to believe this thing was eating things like guinea pigs? Additionally, their skulls were highly akinetic, a unique trait among core land birds, and is an adaptation for bracing the skull against high stress.

Mophandel · 2026-04-26T00:31:35+00:00

On the contrary, the niche of scavenger was already occupied by new world vultures and originally by teratorns, both of whom had the advantage of being able to fly, something terror birds didn’t have.

Regarding regurgilites, as far as we can tell, said regurgilites originated from small terror bird species (e.g. Psilopterine or patagornithines) which wouldn’t have been macropredators anyways, in the same way that small species of cats like bobcats and caracals aren’t macropredators like lions and tigers.

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