Laser stimulated fluorescence literally sheds new light on early bird soft tissue

Wednesday, November 30, 2022 13:17

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But
not much else.

From the Pittman et al 2022 abstract:
“Anatomy of the first flying feathered dinosaurs, modern birds and crocodylians, proposes an ancestral flight system divided between shoulder and chest muscles, before the upstroke muscles migrated beneath the body. This ancestral flight system featured the dorsally positioned deltoids and supracoracoideus controlling the upstroke and the chest-bound pectoralis controlling the downstroke.

This study focuses on superficial soft tissue outlines and ignores the engineering of the elongated locked down coracoids (used in simultaneous flapping, rather than the disc-shaped coracoids in striding primitive tetrapods. The transitional phase takes the forelimbs out of the locomotion cycle in bipedal basal theropods only to give them a new role with feathers and flapping.

The Pittman et al study also lacks a wide gamut phylogenetic context. Instead it generalizes with cherry-picked examples in an attempt to describe the transition to flapping flight skipping both the radiation and essential ladder of taxa in the LRT that document the transition to crown birds.

Bottom line: what is the reason for this paper? To document the origin of flapping muscles in birds? Or to show off some novel color photos? One was the driver. The other the follower.

“Preserved soft anatomy is needed to contextualize the origin of the modern flight system, but this has remained elusive.

Needed? Not necessarily. Especially when the soft tissue outline provides so little data, and the bones and cladogram provide so much.

“Here we reveal the soft anatomy of the earliest theropod flyers preserved as residual skin chemistry covering the body and delimiting its margins. These data provide preserved soft anatomy that independently validate the ancestral theropod flight system.

Figure 1. The Eichstätt specimen, Jurapteryx recurva. The propatagium prevents overextension of the elbow, keeping the flight feathers neat and tidy AND able to fold away as birds do today.

Details (real data) are lacking in the soft tissue.
Bones are essentially ignored and overlooked in lieu of UV stains. Look at figure 1 in Pittman et al. where Early Cretaceous Confuciusornis (Fig 3 below) is the last step before extant Cardinalis. Here’s what the bones alone tell us:

1) torso much shorter (due to fewer ribs, most provided with uncinate processes.
2) That shorter torso makes the elbows overlap the knees;
3) The clavicles are greater than half the humerus length and fused medially.
4) The antebrachium is much longer than the humerus;
5) Carpals are larger and fused;
6) Coracoid longer, immobile, lifting the shoulder joint above the cervicals.
7) Deep sternum that extends below the pelvis, concurrent loss of gastralia

From the Pittman et al 2022 abstract:
“The heavily constructed shoulder and more weakly constructed chest in the early pygostylian Confuciusornis indicated by a preserved body profile, proposes the first upstroke-enhanced flight stroke.

“first upstroke-enhanced flight stroke“ needs a more precise definition or understanding. It also needs a proximal outgroup (not cherry-picked) that lacks this trait.

Figure 2. Early Cretaceous Gansus has extant bird proportions. Look at those locked-down elongatee coracoids.

From the Pittman et al 2022 abstract:
“Slender ventral body profiles in the early-diverging birds Archaeopteryx and Anchiornis suggest habitual use of the pectoralis could not maintain the sternum through bone functional adaptations.

Is this another way of saying, the sternum disappears in early birds, just when they needed it most?

Figure 3. Early Cretaceous Confuciusornithiformes to scale with Late Jurassic Wellnhoferia. Note the lack of a pygostyle in the majority of taxa. This clade left no descendants.

From the Pittman et al 2022 abstract:
“Increased wing-assisted terrestrial locomotion potentially accelerated sternum loss through higher breathing requirements.

Wing-assisted terrestrial locomotion is how pterosaurs began and what happened when certain pterosaurs became flightless. The logic is lost, though, because whether volant or not, flapping benefits from large pectoralis mucles anchored on large ventral bones, including the sternal plates and fused clavicles.

Figure 4. Quetzalcoatlus running like a lizard prior to takeoff. Click to animate.

From the Pittman et al 2022 abstract:
“Lower expected downstroke requirements in the early thermal soarer Sapeornis could have driven sternum loss through bone functional adaption, possibly encouraged by the higher breathing demands of a Confuciusornis-like upstroke.

Why do the authors give Sapeornis (Fig 5) the ability to soar? That’s a highly derived flight mode evolved from flapping relatives. Sapeornis has no sternum and short coracoids. Close relatives were reversing, heading back to more theropod-like morphologies.

Both factors are supported by a slender ventral body profile.

Translation: lacks a sternum. Why not just say that?

These data validate the ancestral shoulder/chest flight system and provide insights into novel upstroke-enhanced flight strokes and early sternum loss, filling important gaps in our understanding of the appearance of modern flight.”

Rather than cherry-picking taxa, I wish the authors had put their hypothesis into a phylogenetic context, noting exxactly which lineages produced flight, and which did not, and which taxa were in the transitional spectrum.

Figure 4. Sapeornis specimen STM 15-15 reconstructed from DGS tracing, figure 1 compared to a more robust specimen with larger feet but an identical humerus.

Curious what workers think about
the strong presence of paired sternae in flightless dromaeosaurids followed by their disappearance in Solnhofen birds and Sulcavis, followed by the appearance of wider than deep fused sternae in Early Cretaceous birds, followed by the deep narrow fused sternae of Crypturellus, Megapodius and other extant crown birds.

I think it’s odd. But evidently every evolutionary novelty or its absence was necessary at the time. I’m looking for nature’s logic if you have it.

Figure 4. Taxa between Solnhofen birds and extant birds in the LRT. Click to enlarge. Here Iteravis nests as the last common ancestor of paleognath and neognath birds (= crown birds). Only crown birds, like Megapodus, and the Late Cretaceous toothed bird and terminal taxon, Ichthyornis, by convergence, develop a deep sternal keel. All others either lack a sternum or have a flat sternum.

It’s of more than passing interest
that only crown birds and Ichthyornis (by convergence) developed a deep narrow sternum. No shallow sternum birds survived the Cretaceous. Was this shape of sternum the key to the survival of megapodes that gave rise to all crown birds? Or did geographic luck after the asteroid impact play the trump card? Or both, at separate times?

One final note:
Crown birds had their origin in the Early Cretacous, with Megapodius sisters like Archeorhynchus. Of course, this was shortly after the Late Jurassic Solnhofen birds. Which means other Cretaceous birds radiated widely, but only the corown bird lineage passed all the tests to re-radiate from a fortuitous (lucky geographic refugium?) bottleneck after the asteroid impact.

References
Pittman M, Kaye TG, Wang X, Hartman SA 2022. Preserved soft anatomy confirms shoulder-powered upstroke of early theropod flyers, reveals enhanced early pygostylian upstroke, and explains early sternum loss. PNAS 119 (47) e2205476119. https://doi.org/10.1073/pnas.2205476119

http://reptileevolution.com/toothed_birds2.htm

Source: https://pterosaurheresies.wordpress.com/2022/12/01/laser-stimulated-fluorescence-literally-sheds-new-light-on-early-bird-soft-tissue/