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Mesozoic evolution of avian features + avian telencephalon and cerebellum volume estimation (free pdfs)

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Ben Creisler

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Jan 21, 2025, 9:17:32 PMJan 21
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Ben Creisler

New papers:

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Free pdf:

Daniel J. Field, M. Grace Burton, Juan Benito, Olivia Plateau and Guillermo Navalón (2025)
Whence the birds: 200 years of dinosaurs, avian antecedents
Biology Letters 21(1): 20240500
doi: https://doi.org/10.1098/rsbl.2024.0500
https://royalsocietypublishing.org/doi/10.1098/rsbl.2024.0500

Free pdf:
https://royalsocietypublishing.org/doi/epdf/10.1098/rsbl.2024.0500


Among the most revolutionary insights emerging from 200 years of research on dinosaurs is that the clade Dinosauria is represented by approximately 11,000 living species of birds. Although the origin of birds among dinosaurs has been reviewed extensively, recent years have witnessed tremendous progress in our understanding of the deep evolutionary origins of numerous distinctive avian anatomical systems. These advances have been enabled by exciting new fossil discoveries, leading to an ever-expanding phylogenetic framework with which to pinpoint the origins of characteristic avian features. The present review focuses on four notable avian systems whose Mesozoic evolutionary history has been greatly clarified by recent discoveries: brain, kinetic palate, pectoral girdle and postcranial skeletal pneumaticity. 

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Free pdf:

Aubrey R. Keirnan, Felipe Cunha, Sara Citron, Gavin Prideaux, Andrew N. Iwaniuk and Vera Weisbecker (2025)
Avian telencephalon and cerebellum volumes can be accurately estimated from digital brain endocasts
Biology Letters 21(1): 20240596
doi: https://doi.org/10.1098/rsbl.2024.0596
https://royalsocietypublishing.org/doi/10.1098/rsbl.2024.0596

Free pdf:
https://royalsocietypublishing.org/doi/epdf/10.1098/rsbl.2024.0596


For studies of the evolution of vertebrate brain anatomy and potentially associated behaviours, reconstructions of digital brain endocasts from computed tomography scans have revolutionized our capacity to collect neuroanatomical data. However, measurements from digital endocasts must be validated as reflecting actual brain anatomy, which is difficult because the collection of soft tissue information through histology is laborious and time-consuming. In birds, the reliability of digital endocast measurements as volume proxies for the two largest brain regions—the telencephalon and cerebellum—remains to be validated despite their use as proxies, e.g. of cognitive performance or flight ability. We here use the largest dataset of histology and digital endocasts to date, including 136 species from 25 avian orders, to compare digital endocast surface area measurements with actual brain volumes of the telencephalon, cerebellum and whole-brain endocast. Using linear and phylogenetically informed regression analyses, we demonstrate that endocast surfaces are strongly correlated with their brain volume counterparts for both absolute and relative size. This provides empirical support for using endocast-derived cerebellar and telencephalic surface areas in existing and future studies of living and extinct birds, with potential to expand to the dinosaur—bird transition in the future.

Tim Williams

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Jan 24, 2025, 8:59:32 PMJan 24
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Daniel J. Field, M. Grace Burton, Juan Benito, Olivia Plateau and Guillermo Navalón (2025)
Whence the birds: 200 years of dinosaurs, avian antecedents
Biology Letters 21(1): 20240500
doi: https://doi.org/10.1098/rsbl.2024.0500


Neat review, but I'd query some of the inferences regarding theropod anatomy. 

Firstly, there's the transformation of the forelimbs into wings.  The authors make the point that cranial kinesis in birds "enhances dexterity and precision of the bill - faculties that became all the more important as the ancestors of birds sacrificed hands capable of fine manipulation in favour of wings as avian flight arose".  This is a rather old view, that theropods gave up their grasping abilities as the forelimb became a wing.  However, the evidence suggests that theropods didn't have much in the way of grasping abilities in the first place.  In general, given their constrained mobility and limited reach (even in long-armed theropods), it's no surprise that in certain lineages the forelimbs became dedicated to functions such as display or flight (or both).  (It's also no surprise that in many theropod lineages the forelimbs were drastically reduced and almost disappeared altogether.)

Secondly, the size of the sternal keel and its proposed role in take-off from the ground.  According to Field &c, the more extensive sternal keel of Euornithes is related to an increased need for ground-level take-offs.  Thus, the well-developed keel is associated here with the more terrestrial or semiaquatic lifestyles of euornithine birds; this contrasts with enantiornithines, which tended to be more arboreal (though not all, e.g., _Elsornis_).  But in modern birds the take-off is initiated by the hindlimbs, regardless of whether the bird is in a tree or on the ground.  The ability to take-off from the ground likely goes back to flying terrestrial theropods like _Microraptor_ and _Archaeopteryx_, with the launch similarly hindlimb-driven.  

Also, the Avisauridae (a Late Cretaceous enantiornithine clade), evolved a well-developed sternal keel convergent with euornithines (as noted by Field &c); but avisaurids have a highly specialized perching pes, which suggests a highly arboreal lifestyle.  So the proposed link between a well-developed sternal keel and a more terrestrial lifestyle doesn't hold.  The well-developed keel has probably more to do with more advanced flight abilities rather than a terrestrial versus arboreal ecology.

Andreas Johansson

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Jan 27, 2025, 8:45:32 AMJan 27
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What's perhaps surprising is that apparently no non-avian dinosaurs lost their forearms entirely. For all that multiple theropod lines reduced their arms to apparent vestigiality, as far as I know the only ones to go the whole way are the moas. Apparently the final step to armlessness is evolutionary "difficult", perhaps for developmental reasons?

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Andreas Johansson

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Andreas Johansson

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Jan 27, 2025, 8:46:17 AMJan 27
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Richard W. Travsky

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Jan 27, 2025, 1:59:32 PMJan 27
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https://explainxkcd.com/wiki/index.php/3042:_T._Rex_Evolution

 

 

From: dinosaurma...@googlegroups.com <dinosaurma...@googlegroups.com> On Behalf Of Andreas Johansson
Sent: Monday, January 27, 2025 6:46 AM
To: DinosaurMa...@googlegroups.com
Subject: Re: [DMG] Mesozoic evolution of avian features + avian telencephalon and cerebellum volume estimation (free pdfs)

 

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Dawid Mazurek

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Jan 27, 2025, 2:10:04 PMJan 27
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Or maybe the so called vestigal arms were each time more important than we assume and there was never a good reason for their disappearance...

Thomas Yazbek

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Jan 27, 2025, 2:27:36 PMJan 27
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What's known is that the nature of forelimb reduction varies between theropod groups. It seems likely that alvarezsaurids were using their arms for unique behaviors, but the abelisaurids & tyrannosaurids seem to have less functional forelimbs. Perhaps it's a stepwise loss of multiple functions for the original limb, and the 3 taxa mentioned may have whittled it down to just one function, or zero.

Thomas Yazbeck

Dawid Mazurek

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Jan 27, 2025, 2:55:46 PMJan 27
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Miss Tyrannosaurus rex could fall for some kind of feathery pom poms...  but that's pure speculation out of the blue, of course ;-)


Tim Williams

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Jan 29, 2025, 2:11:15 AMJan 29
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Thomas Yazbek <yazbe...@gmail.com> wrote:

> What's known is that the nature of forelimb reduction varies between theropod groups. It seems likely that alvarezsaurids were using their arms for unique behaviors, 
> but the abelisaurids & tyrannosaurids seem to have less functional forelimbs. 

Yes, as you say alvarezsaurid forelimb reduction was linked to specialization (hook-and-pull digging).  But in most theropods with reduced forelimbs there's no sign that it's part of any specialization for feeding.

The odd thing about abelisaurids and tyrannosaurids is that despite their extreme reduction in forelimb size, the pectoral girdle is not reduced.  Either the pectoral elements served some function (muscle attachment? defense during intraspecific combat?) or it's some quirk of development.  

> Perhaps it's a stepwise loss of multiple functions for the original limb, and the 3 taxa mentioned may have whittled it down to just one function, or zero.

Similarly in many modern birds the wings have more than one function.  In flightless birds the wings can retain non-flight functions and/or acquire new ones: display, gliding, combat, underwater propulsion, etc.  But in other flightless birds the wings have no function at all (like kiwis, in which the vestigial wings even retain wing-claws as in most ratites); but as Andreas Johansson mentioned the moa are the only birds that lost their wings completely (afaik).

Dawid Mazurek <dawidma...@gmail.com> wrote:

> Or maybe the so called vestigal arms were each time more important than we assume and there was never a good reason for their disappearance..

It's been proposed that _Carnotaurus_ used its forelimbs for display.  (Though this wouldn't explain the unreduced pectoral girdle, which is likely unrelated.)

One hypothesis (proposed over 20 years ago) is that _Tyrannosaurus_ used its forearms to help secure struggling prey (such as a subadult hadrosaur) by clutching the prey against its body while the jaws and teeth dispatched it.  (This hypothesis is at least consistent with well-muscled coracoids.)

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