Eosinopteryx, junior synonym of Anchiornis + hesperornithiform feather in theropod coprolite from Hell Creek Formation

[Ben Creisler]

Ben Creisler

bcre...@gmail.com

New papers:

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Fei-Fan Li, Dong-Yu Hu, Cui-Lin Zhao, Zhi-Heng Li & Zhong-He Zhou (2026)
Redescription and taxonomic reassessment of the holotype of anchiornithine Eosinopteryx brevipenna
Palaeoworld 201180
doi: https://doi.org/10.1016/j.palwor.2026.201180
https://www.sciencedirect.com/science/article/abs/pii/S1871174X26001150


Anchiornithinae is a group of the earliest known paravian dinosaurs with feathered wings, represented by the first-named taxon Anchiornis huxleyi. The discovery of this clade has greatly reshaped our understanding of the origins of birds and avian flight. Eosinopteryx brevipenna was subsequently reported, based on a specimen from the same locality and horizon as A. huxleyi. The initial study suggested that E. brevipenna might have evolved a different locomotion mode and ecological habit from A. huxleyi. However, recent studies have questioned the validity of E. brevipenna, and the initial description of its holotype also lacks sufficient detail. Our reevaluation of the holotype of E. brevipenna indicates that it is essentially consistent in osteological features with A. huxleyi, and that most diagnostic features of E. brevipenna should be attributed to taphonomic deformation and damage. Consequently, we regard E. brevipenna as a junior synonym of A. huxleyi, and its holotype likely represents an immature individual of that species.

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Jingmai K. O’Connor, David G. DeMar Jr., Nathan R. Carroll, Karen Chin, Elliott Armour Smith, Michael Holland, Alexander D. Clark, Christian Cooper, Pei-Chen Kuo, Thomas S. Tobin, Aaron J. Celestian, David J. Bottjer, Luis M. Chiappe & Gregory P. Wilson Mantilla (2026)
Bird feathers from a Late Cretaceous coprolite
Current Biology (advance online publication)
doi: https://doi.org/10.1016/j.cub.2026.08.054
https://www.sciencedirect.com/science/article/abs/pii/S096098222601105X


Highlights

Exceptionally preserved feathers found in 66 Mya medium-sized theropod coprolite
Associated bones suggest the feathers are from an aquatic hesperornithiform bird
Feathers show a square rachis and medulla evolved in the Mesozoic
Hesperornithiforms also retained primitive feathers possibly relating to extinction

Summary

Fossilized feathers are rare, yet critical for understanding the evolution of flight in pennaraptoran dinosaurs and the appearance of crown birds in the Late Cretaceous. Feathers associated with skeletal remains are abundant in several Lower Cretaceous Lagerstätten but nearly absent from Upper Cretaceous deposits. As such, the evolution of avian feathers close to the emergence of the crown-group Neornithes remains largely unknown. Here we report on an exceptionally preserved three-dimensional pennaceous feather preserved in a medium-sized theropod dinosaur coprolite found in the uppermost Maastrichtian Hell Creek Formation in North America. Arguably the best-preserved feather ever recovered from Mesozoic lithic deposits, it demonstrates that the anatomically modern lightweight and stiff feather with a square, medulla-filled rachis was in place in Pennaraptora before the end-Cretaceous mass extinction. Skeletal remains from the same coprolite suggest the feather belongs to a hesperornithiform, a lineage of flightless aquatic birds within Ornithurae but outside Neornithes. Consistent with this identification, the feather exhibits a low structural index and high barb density, adaptations for reduced penetrability present in extant aquatic birds. Tiny associated gar scales are interpreted as the bird’s last meal. Microcomputed-tomography data reveal the preservation of additional associated feathers, including extinct morphotypes, within the coprolite, suggesting these morphotypes persisted even in crownward, non-neornithine birds until the end-Cretaceous mass extinction. Differences in plumage, especially with regard to body feathers most responsible for thermal insulation, may have played a role in the extinction of all non-neornithine pennaraptorans during the hypothesized asteroid-induced impact winter.

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News:

https://www.fieldmuseum.org/about/press/fossil-feather-preserved-inside-dinosaur-poop-could-help-explain-why-birds-surv

https://www.newscientist.com/article/2588673-exquisite-66-million-year-old-bird-feather-preserved-in-dinosaur-dropping/

https://phys.org/news/2026-09-fossil-feather-dinosaur-poop-birds.html

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[Tim Williams]

> Fei-Fan Li, Dong-Yu Hu, Cui-Lin Zhao, Zhi-Heng Li & Zhong-He Zhou (2026)
> Redescription and taxonomic reassessment of the holotype of anchiornithine Eosinopteryx brevipenna
> Palaeoworld 201180

I've always been skeptical of the previous claim that "With reduced plumage and short uncurved pedal claws, _Eosinopteryx_ would have been able to run unimpeded (with large foot remiges cursorial locomotion was likely problematic for _Anchiornis_)".

I have no doubt _Anchiornis_ could run just fine on the ground - just like other small pennaraptorans that sported long remiges on their hindlimbs: _Microraptor_, _Changyuraptor_, _Xiaotingia_.  More broadly, I don't accept the idea that these 'four-winged' taxa were forced to live in trees because their long hindlimb feathers made running on the ground cumbersome or impractical.  The original _Eosinopteryx_ description doesn't go this far, but I have seen this facile claim made elsewhere.

[Jaime Headden]

It's the same impractical thinking that one objects to when extended to other extravagant structures. "Male peafowl cannot walk on the ground because their long back feathers would drag on the ground," for instance.

Feathers bend. It's part of their nature. Even flight feathers, stiff as they are by necessity, bend. Moreover, they not being anchored to the bone, or even if they were they might still have a flexible base, cannot preclude their terrestrial locomotion. Imagine having to pose your animals so their legs feathers only hang just behind the branch, never that thick, or to either side walking lengthwise, and never at some peculiar *angle*. Aghast noises abound.

I remain skeptical on the tarsal/metatarsal feathers as a fully function mini wing, alike to the forelimb; and that the obvious postural option of tucking for hindleg and not sprawling it sideways makes for far more biomechanical sense, despite Zhou (along with Xu) being proponents of the spread-legged "four wing" microraptoran model. An aerodynamically useful planform, does not exclude the idea the planform was more relaxed and passive in function, even if it's "not as efficient" as the other model, because we're not talking about a creatively designed animal or some spec-evo thing where everything has to make total sense for this one organism *now*.

Animals can have impractical, imperfect features.

Cheers,

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Jaime A. Headden

The Bite Stuff: http://qilong.wordpress.com/

"Innocent, unbiased observation is a myth" - P. B. Medawar (1969)

[Tim Williams]

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

> I remain skeptical on the tarsal/metatarsal feathers as a fully function mini wing, alike to the forelimb; and that the obvious postural option of tucking
> for hindleg and not sprawling it sideways makes for far more biomechanical sense, despite Zhou (along with Xu) being proponents of the spread-legged "four
> wing" microraptoran model.

Yes, I'm also very skeptical.  Microraptorans were not built like flying squirrels.  The hindlimbs could not stick out sideways - the pelvic anatomy prevented this (Brougham & Brusatte 2010 doi/10.1073/pnas.1004977107; Manafzadeh & Padian 2018 doi.org/10.1098/rspb.2018.0727).  As the latter study makes clear, this inability to 'sprawl' the hindlimb applies to all ornithodirans.  Having a spherical femoral head makes no difference, e.g., apparently _Gigantoraptor_ has one (Hao et al., 2026 doi.org/10.1080/02724634.2026.2716748).

So, as you state, the front wings and hind wings of _Microraptor_ could not be aligned in a coplanar way.  This flawed model was heavily influenced by the 'tetrapteryx' arboreal glider hypothesis for the origin of bird flight.  This 'tetrapteryx' hypothesis is now discredited.  It had a good run.

> An aerodynamically useful planform, does not exclude the idea the planform was more relaxed and passive in function, even if it's "not as efficient" as the
> other model, because we're not talking about a creatively designed animal or some spec-evo thing where everything has to make total sense for this one
> organism *now*.

Because it seems likely that _Microraptor_ was capable of flapping/powered flight (Dececchi et al. 2016 doi: 10.7717/peerj.2159; Pei et al. 2020 doi: 10.1016/j.cub.2020.06.105), these hindwings (and tail feathers?) might have been used for aerial maneuvering, particularly if _Microraptor_ could catch prey in the air with its feet (Pittman et al., 2022 doi: 10.1038/s41467-022-35039-1.).  The remiges and rectrices also undoubtedly had a display function, which might reflect their original function before being exapted for flight.  I find this hypothesis quite compelling.