Zavacephale, new pachycephalosaur from Early Cretaceous of Mongolia

[Ben Creisler]

Ben Creisler

bcre...@gmail.com

A new paper:

Zavacephale rinpoche gen. et sp. nov.

Tsogtbaatar Chinzorig, Ryuji Takasaki, Junki Yoshida, Ryan T. Tucker, Batsaikhan Buyantegsh, Buuvei Mainbayar, Khishigjav Tsogtbaatar & Lindsay E. Zanno (2025)
A domed pachycephalosaur from the early Cretaceous of Mongolia
Nature (advance online publication)
doi: https://doi.org/10.1038/s41586-025-09213-6
https://www.nature.com/articles/s41586-025-09213-6


The dome-headed pachycephalosaurians are among the most enigmatic dinosaurs. Bearing a hypertrophied skull roof and elaborate cranial ornamentation, members of the clade are considered to have evolved complex sociosexual systems. Despite their importance in understanding behavioural ecology in Dinosauria, the absence of uncontested early diverging taxa has hindered our ability to reconstruct the origin and early evolution of the clade. Here we describe Zavacephale rinpoche gen. et sp. nov., from the Lower Cretaceous Khuren Dukh Formation of Mongolia, the most skeletally complete and geologically oldest pachycephalosaurian discovered globally. Z. rinpoche exhibits a well-developed frontoparietal dome and preserves the clade’s first record of manual elements and gastroliths. Phylogenetic analysis recovered Z. rinpoche as one of the earliest diverging pachycephalosaurians, pushing back fossil evidence of the frontoparietal dome by at least 14 Myr and clarifying macroevolutionary trends in its assembly. We found that the earliest stage of dome evolution occurred by means of a frontal-first developmental pattern with retention of open supratemporal fenestra, mirroring proposed ontogenetic trajectories in some Late Cretaceous taxa. Finally, intraskeletal osteohistology of the frontoparietal dome and hindlimb demonstrate decoupling of sociosexual and somatic maturity in early pachycephalosaurians, with advanced dome development preceding terminal body size.

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[Tim Williams]

Cool name _Zavacephale rinpoche_

Not even close to being the most remarkable feature, but wow, the forelimbs are puny.  They look next-to-useless.  Even more so than _Psittacosaurus_.  In _Psittacosaurus_ the hands couldn't reach the mouth, but might have been useful to carry things (based on Senter, 2007; doi:10.1111/j.1469-7998.2007.00329.x)  Not sure if the tiny hands of _Zavacephale_ could even do that.  The forelimbs were not drastically reduced in later pachycephalosaurs.  Why did they have them at all?

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[Thomas Richard Holtz]

To be fair, the postcrania of most pachycephalosaurs are poorly known. The few which do have forelimb elements (Stegoceras, Homalocephale, the Sandy Site Pachycephalosaurus/Stygimoloch) show very reduced limbs compared to most other ornithischians.

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[Jaime Headden]

The distal manus being unknown,since it's apparently proximal phalanges all, I doubt we can ascribe function as far as Psittacosaurus and mobility. For P, the manus is completely known and exaxonal, with a plausible grasping function as suggested by Senter, and with an ontogenetic shift from facultative quadrupedalism to bipedalism. It's really hard to see this in the forelimbs of Z. Rather, the humerus of pachycephalosaurids tends to be not merely abbreviated, but slender, with a reduced deltopectoral crest and distal end, lacking robust ent- and ectepicondylar crests, supporting forelimb muscles. It resembles the humerus of some ornithomimosaurs---except being very, very short. This resembles also the condition seen in tyrannosaurs and abelisaurids, for which we now have exemplars showing the complete forelimb, and which demonstrate a similar pattern:

The longest bone is the humerus, equallying for being subequal to the length of the rest of the forelimb, with simplified morphology; a short antebrachium; reduced phalanges and simplified joints.

The contrast here is to alvarezsaurians, which also have extremely shortened forelimbs) in which the humerus is less than half the total arm length, the humerus is far from simple, the joints are well-developed with distinct crests supporting forelimb musculature, and well-developed (and large) phalanges. Pachycephalosaurids, from Z here to Goyocephale/Homalocephale to Stegoceras, and so forth, all show associated robust pectoral girdles similar to tyrannosaurs and abelisaurids, which compares well to the idea that shoulder-joint musculature was coopted towards supporting the trunk and neck, as in those theropods.

This should be studied in better detail, the comparative anatomy is just looking for the smoking dinosaur, the skeleton that gets everyone fired up. Pachys have tiny, tiny arms and it's just so constantly under-reported.

That said, I'm actually surprised how long the hindlimbs are! The thing looks like a kangaroo. Now, there's a plausible function for the arms. If the dome developed prior to somatic maturity, primarily over the frontals, it would hardly have aligned well with the vertebral columb for translation of shock through the skull. Absence of closure of cranial sutures and the supratemporal foramina would have further impaired the domes' use for caprine/ovine-like head-butting. It likely functioned more as a social signaling semaphore than for agonism, which frees the forelimbs for play/territory. Developing the dome for headbutting by incorporation of the parietal and later the supraoccipital and squamosal, would have only supported later-ontogenetic head-butting. That said, even goat kids like to head butt, (but in contradiction, so do cats).

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Jaime A. Headden

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[Jaime Headden]

This is the post where I beg someone for a copy of this paper, please.

[Indy Storvick]

Here you go!

 zavacephale pdf.pdf

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[Jaime Headden]

Thank you!

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[Jacqueline Silviria]

I remember a talk by Carry Woodruff at the SVP Toronto which compared pachycephalosaurs favorably to kangaroos in terms of hindlimb and tail function (esp. in “tripedal" postures). Has that been published yet?

Jacqueline S. Silviria

The Last King of the Jungle

Department of Earth & Space Science
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[Alberta Claw]

Not yet.

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[Cary Woodruff]

Not the individual paper (but we're damn close)....but the University of Toronto published my entire dissertation last year....

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In its adaptation for the application of those mechanical principles which combine maximum strength with minimum weight and increased surface for muscular attachment, the vertebral column of Diplodocus exhibits a remarkable degree of specialization, unsurpassed if not unequaled by other vertebrates. - J. B. Hatcher

[Brad McFeeters]

Are there any ichnites referred to pachycephalosaurs? If their hindlimb function was convergent with kangaroos their tracks should be conspicuously absent or present in the fossil record, otherwise they may be difficult to distinguish from other small bipedal neornithischians.

[Jaime Headden]

Nothing that would be distinguished from a generic small ornithopodan, I fear. Phalangeal pads aside, their variation in birds and non-avian theropods would likely suggest we could hardly confirm they belong to any specific subset of small-bodied bipedal ornithischian. That said, it's not to hopping I'm implying the connection: Saltation in mammals usually involves a well-displaced metatarsal inlever -- the calcaneal tuber, for example -- and a less mobile, shorter femur. Not only are the metatarsals unbound to one another and spread, reducing the ability of the pes to resisting splaying during impact, the ankle is poorly suited for maintaining the right balance of forces during jumping to make a constant hop viable. (Not that they couldn't hop on occasion.)

I'm referring to the tail model primarily due to the length of the hindlimbs, the robusticity and rigidity of the tail, and the function of a distally deep and muscular tail. It's unlikely the whole tail would also have been readily useful as a "springboard," either, as that would require a more flexible, muscular tail. Instead, a rigid tail still provides strain reduction when "kicking" off with the feet, forming a temporary but common tripodal stance. This isn't a rigorous interpretation, but a generalized one. Note I was not witness to Woodruff's talk, nor have I read the abstract. None of this excludes these behaviors being possible.

My baseline model for a stiffened, thick tail is as a support rod for rapid turning and angling during speed, similar to that modeled for rhamphorhynchoid pterosaurs and many cursorial tetanuran theropods. Indeed, dromaeosaurs are the next most plausible model, having a similar deep, albeit less muscular tail, and with a stronger taper. Banking and yaw control with a long spar perhaps ornamented or adorned with some sort of structural "fan" of "dinofuzz" (entirely, utterly hypothetical, modeled upon implied close relation between Psittacosaurus and Tianyulong; but see, Kulindadromeus) would have offered an alternative view of this feature, save that the tail is retained in later taxa for which less cursorial limb proportions are known than in Zavacephale.

Interim conclusion: The style of balancing on the tail as a structural "third leg" for possible agonistic behavior of the feet, a la, kangaroo kicking, is a plausible explanation.

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[Cary Woodruff]

Yup. That's what Ch.5 is all about/concludes as well 👍

Cary Woodruff, PhD, FLS

Curator of Vertebrate Paleontology;

Phillip and Patricia Frost Museum of Science

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[Russell Engelman]

I'm not sure if we'd even notice if there were any small dinosaur species specialized for hopping. I know the smaller passerine birds tend to locomote by hopping a lot but I'm not aware of any features of their skeleton that would suggest frequent hopping versus other forms of locomotion. The vast majority of larger-bodied saltators are mammals and their ankle morphology is so divergent from archosaurs who knows if one would be able to notice similarities beyond the very broadest mechanical constraints. The only non-avian archosaurs I am aware of that have really been proposed to hopping are lagerpetids, and I'm not sure how good the evidence looks for on them.

[Jaime Headden]

I thought about this years ago: why small birds tend not to "walk" as much as hop on branches, even the ground, propelling themselves forward. Was some mechanism binding their legs that they had to move them at the same time? That would, of course, create the "hop" by necessity. Upon observation, one can simply conclude that what they're doing is using the first phase of a powered jump to get into the air, but never engage the next step, which is their wings, resulting in just a rapid upward movement---followed by gravity. It seems it's just more efficient for them, and likely, less problematic for knee-driven locomotion in smaller neoavians like passerines. Due perhaps to other factors, larger birds like ducks and grebes waddle because of their girth, pushing themselves side to side in an exaggerated manner because it's, again, easier for them to do so than an axially parallel forward movement of the knee.

This is all odd thinking.

Most birds do have a strongly developed inlever for the ankle joint, a developed tarsal structure called the hypotarsus, onto which inserts the M. gastrocnemius. In some birds this is simple, and can be shallow; on others its well-developed, guising flexor tendons around the ankle joint, but otherwise there's more of a bony lever in birds for such behaviors as "hopping" (in bursts, at least) than in (most) non-avian therpods, and certainly all other dinosaurs, including this one.

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[Ben Creisler]

News stories and blogs:

Meet Zavacephale rinpoche

https://paleonerdish.wordpress.com/2025/09/18/meet-zavacephale-rinpoche/

https://www.nhm.ac.uk/discover/news/2025/september/new-dome-headed-dinosaur-most-complete-ever-found.html 

https://www.discoverwildlife.com/dinosaurs/zavacephale-rinpoche-gobi-desert-mongolia

https://www.nationalgeographic.com/science/article/dinosaur-dome-head-new-fossil-oldest-mongolia

https://www.scientificamerican.com/article/oldest-known-pachycephalosaur-fossil-discovered-in-mongolia/

https://www.nytimes.com/2025/09/17/science/pachycephalosaurus-fossil-mongolia.html

https://www.sci.news/paleontology/zavacephale-rinpoche-14223.html

https://novataxa.blogspot.com/2025/09/zavacephale.html

[Ronald ORENSTEIN]

In my experience many hopping birds tend to be arboreal while walking is confined to terrestrial birds, even if they have arboreal near relatives. I suspect that hopping may have evolved as a way to navigate in trees and has been largely retained when these birds descend to the ground, as many of them do. 

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[Alberta Claw]

Arboreality as a precursor to hopping in birds was indeed the conclusion reached by Provini and Höfling (2020). On the functional morphology side of things, Zinoviev (2018) briefly noted convergence in hindlimb musculature between hopping birds and hopping rodents.

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[Tim Williams]

Jaime Headden <jaimeh...@gmail.com> wrote:

>  Rather, the humerus of pachycephalosaurids tends to be not merely abbreviated, but slender, with a reduced deltopectoral crest and distal end, lacking robust ent- and ectepicondylar crests,

> supporting forelimb muscles. It resembles the humerus of some ornithomimosaurs---except being very, very short. This resembles also the condition seen in tyrannosaurs and abelisaurids, for

> which we now have exemplars showing the complete forelimb, and which demonstrate a similar pattern:

Yes, exactly.  The forelimbs of _Zacavephale_ are absurdly feeble, and later pachycephalosaurs appear to have stuck with this.  There are obvious parallels with certain large theropods.  I find it odd that the forelimbs were not lost altogether, but there may be entirely plausible explanations.

Russell Engelman <neovena...@gmail.com> wrote:

>  The only non-avian archosaurs I am aware of that have really been proposed to hopping are lagerpetids, and I'm not sure how good the evidence looks for on them.

Yes - _Lagerpeton_ was suggested to be a saltator, back in a 1993 publication; but there is no good evidence for saltatorial/ricochetal locomotion in lagerpetids.  Same for _Scleromochlus_, which one publication in 2020 proposed hopped like a frog.

 

[Jaime Headden]

It should be noted that the specific case in kangaroos is a form of hopping called ricochetal locomotion; this is primarily seen in kangaroos, jerboas, and some lagomorphs, but not all). Most hopping animals don't sustain it for long, and not as a primary means of locomotion. Terrestrial bounding, as seen in some antelopes, but primarily in pronghorns (where I think it was compared to "pronking"?) is a quadrupedal form of this, where the adaptations are in the forelimbs, but without the spring-like compression of the limb as seen in kangaroos.

It was to this particular variation on hopping I was referring to, and was (probably) unlikely, and counter-indicated, as I mentioned in the previous comments.

I wrote:

>>  Rather, the humerus of pachycephalosaurids tends to be not merely abbreviated, but slender, with a reduced deltopectoral crest and distal end, lacking robust ent- and ectepicondylar crests,

>> supporting forelimb muscles. It resembles the humerus of some ornithomimosaurs---except being very, very short. This resembles also the condition seen in tyrannosaurs and abelisaurids, for

>> which we now have exemplars showing the complete forelimb, and which demonstrate a similar pattern:

To which Tim Williams replied:

> Yes, exactly.  The forelimbs of _Zacavephale_ are absurdly feeble, and later pachycephalosaurs appear to have stuck with this.  There are obvious parallels with certain large theropods.  I find it odd that the forelimbs were not lost altogether, but there may be entirely plausible explanations.

Before it inevitably derailed, I mentioned how in tyrannosaurids and the like, the constraint of forelimb retention, especially the humerus, likely has to do with various humeral-inserting muscles that perform roles in stabilizing the scapulocoracoid, which in turn stabilizes the trunk and supports the neck. This is likely why it remains the largest bone in these taxa, and the humeral joint with the scapulocoracoid remains well-developed, especially in Zavacephale, in which the joint surfaces are extremely incrassate an rugose and likely possessed an acetabulum's worth of soft tissue to support the forelimb. But despite this, the humerus and forelimb remain functional, and I doubt were neglected in food manipulation, burrow making, mound-building, sociosexual functions, or territorial agonism, or whatever else. But walking, lifting, carrying? Perhaps not so much.

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[Tim Williams]

Jaime Headden <jaimeh...@gmail.com> wrote:

> Before it inevitably derailed, I mentioned how in tyrannosaurids and the like, the constraint of forelimb retention, especially the humerus, likely has to do with various humeral-inserting muscles that

> perform roles in stabilizing the scapulocoracoid, which in turn stabilizes the trunk and supports the neck. This is likely why it remains the largest bone in these taxa, and the humeral joint with the

> scapulocoracoid remains well-developed, especially in Zavacephale, in which the joint surfaces are extremely incrassate an rugose and likely possessed an acetabulum's worth of soft tissue to

> support the forelimb. But despite this, the humerus and forelimb remain functional, and I doubt were neglected in food manipulation, burrow making, mound-building, sociosexual functions, or

> territorial agonism, or whatever else. But walking, lifting, carrying? Perhaps not so much.

I agree regarding the retention of the humerus.  But what was the purpose of the highly reduced antebrachium?  I'm skeptical of most of the proposed functions, especially food manipulation.  A sociosexual function is possible (though hard to prove, in the absence of preserved display surfaces etc).

[Jaime Headden]

I assume the arm remains useful, not that it developed a specific purpose. It could be as simple as sociosexual functions, like display. In some of my reconstructions, I stuck large fans of dinofuzz or filamentous "quills" on them, similar to that seen in cassowaries but finer. That would seem a general first run at their functional "non-function." That said, there are other sociosexual functions that can operate other than pure display, just as grasping mates.

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[Guy Leahy]

In the National Museum of Natural History's "Journey through Deep Time" exhibit, Stegoceras is depicted as using its forelimbs to scratch an itch... :)

[Brad McFeeters]

Extreme forelimb reduction appears to have happened at least twice in ornithischians, once in pachycephalosaurs and once in the Jakapil lineage. Convergence upon a particular lifestyle? 

[Jura]

The adage of use it or lose it doesn't really apply to vestigial structures. It just frees them up from selection (they become neutral). The forearms may be that reduced just because the developmental genes had shifted to switching off sooner, and having a reduced nub doesn't interfere with the life history of the animal. This may even have been a pleiotropic effect as those genes may have needed to stay on long enough for the humerus to properly develop, leaving the antebrachium as the leftover.

[Tim Williams]

Jura <arch...@gmail.com> wrote:

> The adage of use it or lose it doesn't really apply to vestigial structures. It just frees them up from selection (they become neutral). The forearms may be that reduced just because the

> developmental genes had shifted to switching off sooner, and having a reduced nub doesn't interfere with the life history of the animal. This may even have been a pleiotropic effect as those genes

> may have needed to stay on long enough for the humerus to properly develop, leaving the antebrachium as the leftover.

That makes sense.  Even most flightless birds retain distal elements of the forelimb, including most (but not all) ratites.  (Granted, many extant flightless birds use their wings for non-flight functions.)  

In hesperornithids the forelimb is reduced to just a poorly developed humerus.  But here the extreme wing reduction was probably driven by selection, and associated with their particular kind of aquatic lifestyle.  

Brad McFeeters <bradleyd...@gmail.com> wrote:

> Extreme forelimb reduction appears to have happened at least twice in ornithischians, once in pachycephalosaurs and once in the Jakapil lineage.
> Convergence upon a particular lifestyle?

Or an indication that in both ornithischian lineages the forelimb was not used for very much (if anything at all), and just dwindled away?

There is also an outside chance that _Jakapil_ is a marginocephalian, even a pachycephalosaur.