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Interesting Birds, er Dinosaurs:

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Caudipteryx and the propatagium

Scanscoriopteryx and the styliform element

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Ambopteryx

Xiaotingia

Epdexipteryx

Archaeopteryx

Auronis

MIcroraptor

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Does Caudipteryx represent an evolutionary step towards flight from flightless dinosaurs, or is it secondarily flightless from an already volant ancestor?

Many paleontologists continue to hold the view that the feathered early Cretaceous basal maniraptoran Cadipteryx, classified as a theropod dinosaur, represents a transition from non-flying dinosaurs to volant birds. As this view is more parsimoniously indicated by their cladograms, they prefer not accept the view that Caudipteryx is secondarily flightless. They instead offer that some early dinosaur became more diminutive in size; developed feathers for display; later evolved wings for catching bugs, high jumping, or flapping while running up a log; and only then learned to fly. No amount of logical proof can convince them that Caudipteryx had flying ancestors.

Here is a list of features tying Caudipteryx to being secondarily flightless I put up in 1985 on the Paeontology Usenet, This thread started as a debate about whether or not Caudipteryx was closely related to oviraptors, and therefore not a secondarily flightless bird. However I have no doubt that both species were secondarily flightless, as well as Microraptor.

My post:

I'll bite. Here are some of Feduccia's arguments that Caudiptteryx is a
flightless bird, and then I have a challenge afterwards:

Caudipteryx and Protoarchaeopteryx are replete with features of
secondarily flightless Mesozoic sauriurine birds (Feduccia, 1999)
including:

A short tail, similar to that of Confuciusornis, with some fusion of
the vertebrae and therefore a quasi-pygostyle.

Secondarily reduced wing feathers that attach to its hands in the same
manner as flighted birds such as Confuciusornis, locking up it’s hand
for any type of predatory function.

Birdlike skull with a posteroventral foramen magnum. In theropods,
foramen magnum is posterior and the neck enters the skull from behind.

No caudofemoralis or pelvic musculature that defines theropods.

The middle and third metatarsal is longer than the outer two, a
distinctive avian feature.

An avian style halux.

Waisted teeth like those of Archaeopteryx.

No furcula, common in flightless birds.

Reduced fibula, opisthopubic foot has no boot, acetubulum is
nontheropod-like.

Eats it’s food with gizzard stones.

Extensive suite of cursorial-bird-like locomotory characters (Jones et
al).

(EDITED FOR CONTEXT:) I propose that although it is not considered advanced as far a Avetheropods go, it has
been flightless for many millions of years. I propose that it descended
from a bird with aprox 30 caudial vertbrae, two more than
Protoarcheopteryx.

src: https://groups.google.com/g/sci.bio.paleontology/c/of0rzHFg8wM/m/cVyzwQopEP8J

Caudipteryx and the Propatagium

Ornithologist Allen Feduccia (2015) has discovered propatagiums on the Caudipterex fossils.

Protopatagium as found on the wings of Archaeopteryx and the modern duck.

Feduccia states: “The importance of a propatagium to the evolution of the avian wing is significant, as it has no apparent function other than contributing to the aerodynamics of the animal... The discovery of a propatagium in members of all clades of core maniraptorans... is additional evidence that flight was basal in Aves….The loss of flight is of such common occurrence within Aves that it should be expected to have occurred any time after flight was initially achieved.”

He concludes: "The revelation that Maniraptora consists of volant and neoflightless types of birds resolves many of the problematic issues… Recognizing that flightless members of Maniraptora are neoflightless may answer the question of where the secondarily flightless birds of the Mesozoic have been hiding".

Source: Testing the Neoflightless Hypothesis: Propatagium Reveals Flying

Ancestry of Oviraptorosaurs; Alan Feduccia, Stephen A. Czerkas; J Ornithol (2015) 156:1067–1074

Scanscoriopteryx and the Styliform Element

The Scansoriopteryx lived during the Late Jurassic Callovian to Oxfordian period, which was 165–156 million years ago

Feathered head of Xi Qi,

A new Scanscoriopteryx fossil was found in China, covered with feathers, possessing a “styliform element”, and said to sport a membrane attached to its arms resembling a bat’s wings. Scanscoriopteryx is a middle Jurassic maniraptor, considered by some to be a basal oviraptor.

A bizarre Jurassic maniraptoran theropod with preserved evidence of membranous wings-- Xing Xu et all

…Because other amniotes that possess a styliform element invariably utilize this structure to support an aerodynamic membrane that contributes to gliding or powered flight, and alternative functions for a long, unjointed rod of bone or cartilage extending from a distal limb joint are difficult to imagine , the occurrence of a styliform element in Yi is a strong indication that membranous aerodynamic surfaces and some degree of aerial capability were present in this taxon.

Most surprisingly, Yi has a long rod-likebone extending from each wrist, and patches of membranous tissue preserved between the rod-like bones and the manual digits. Analogous features are unknown in any dinosaur but occur in various flying and gliding tetrapods

In predicting the exact layout of the bones and wing in life, the authors compare it to a number of extant and extinct flying taxon and mammals, barely mentioning: “similar structures are present in the wrists of all pterosaurs.”

Possible wing configurations and position of styroform element of Xi Qi as suggested by authors:

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