Model of evolution

Model of evolution

This is a model of evolution in which birds evolved from pterosaurs, not theropod dinosaurs. It follows a clear A → B → C... sequence, where each family spreads out into genera, and a derived genus evolves into a basal genus of the next family. Here's how the model plays out:

1. Family A: Scleromochlus
Starting with Scleromochlus, this early archosaur takes to the trees, developing rudimentary membranous wings for gliding.

This family diversifies into genera exploring arboreal and aerodynamic niches.

2. Family B: Rhamphorhynchoids (Early Pterosaurs)
From a derived genus of Scleromochlus, long-tailed gliding forms like rhamphorhynchoids emerge. Their wings, supported by elongated fingers, refine powered flight while retaining arboreal adaptations.

Genera in this family explore various ecological niches using membranous wings.

3. Family C: Pterodactyloids (Advanced Pterosaurs)
A derived genus within rhamphorhynchoids gives rise to Pterodactyloids, which shorten their tails, optimize wing membranes, and improve aerodynamics for more efficient flight.

This family spreads out into genera with specialized flight adaptations.

4. Family D: Scansoriopterygids (Feather-Membrane Transition)
From a derived genus of Pterodactyloids, scansoriopterygids arise. These small, arboreal creatures retain elements of membranous wings while developing feathers, marking a transitional phase.

This family experiments with combining membranes and feathered flight, blending traits of pterosaurs and birds.

5. Family E: Birds
A derived genus of scansoriopterygids gives rise to true birds, where feathers fully replace wing membranes, enabling flapping flight. This family diversifies into the vast array of modern bird species.

Key Features of Proposal:
It emphasizes a logical stepwise progression between families, with transitional forms bridging gaps.

Each family radiates into genera, but only a derived genus "steps up" to the next higher family.

It respects monophyly in the broader groupings (e.g., each family contains all members from its shared ancestor) while allowing for paraphyly at the step-up points.

This model ties together pterosaurs, scansoriopterygids, and birds as part of a coherent evolutionary trajectory, focusing on structural and ecological transitions. It’s straightforward, parsimonious, and visually compelling.

Cladogram

 

https://blogger.googleusercontent.com/img/a/AVvXsEhBVkFTVbNScJKSP9_UjFrEOxmCT8v41e8zTswKYFGR35ZGI5jbsO2I9mfj__057__K0G66aim7DK13mfaP3yF7hkyxD9AACRt8T8kZFHubEKyWVzFyPwDCEUQ8cW4A8xf-9Nq2n5xETjAN5mioDt4bgWxbDj58LHs7BH7Plsl3cEvsl3cx5k4ujEoHHA

From Epidexipteryx membrane to follicle collar

 Very interesting:

https://scienceblogs.com/tetrapodzoology/2008/10/23/epidexipteryx-at-last

Also worth noting is that the pennaceous feathers of birds begin growth as a tubular structure: from a broad 'follicle collar' that extends around the base, the rachis extends upwards, the barbs emerging both from it and from the edges of the collar. Might the membranous bases of the 'non-ETFs' of Epidexipteryx be homologous with the follicle collar? This is all very speculative, but these are interesting questions that need to be examined.

Indeed, the feather follicle originated even further back, in pterosaurs as pycnofibers.

Pygostyle

We can track the development of the pygostyle.

https://en.wikipedia.org/wiki/Epidexipteryx

The tail of Epidexipteryx also bore unusual vertebrae towards the tip which resembled the feather-anchoring pygostyle of modern birds and some oviraptorosaurs.^[2]

https://en.wikipedia.org/wiki/Ambopteryx

it had a short tail that lacked a transitional point and ended in a pygostyle, a set of fused tail vertebrae.

 

In the last step comes Pygostylia. 

Pterodactylid:

https://en.wikipedia.org/wiki/Pteranodon

Other distinguishing characteristics that set Pteranodon apart from other pterosaurs include narrow neural spines on the vertebrae, plate-like bony ligaments strengthening the vertebrae above the hip, and a relatively short tail in which the last few vertebrae are fused into a long rod.[17] The entire length of the tail was about 3.5% as long as the wingspan, or up to 25 centimeters (9.8 in) in the largest males.[17]

From the treetops

The lineage begins with a creature like Scleromochlus running in the treetops. They develop an expanded patagium that they use to glide. Over time they evolve into Rhamphorhynchids that then evolve into Pterodactylids. 

The Pterodactylids evolve into short tailed scansoriopterygids like Epidexipteryx. Which then evolves into Pygostylia.

The early steps:

High level

Rhamphorhynchid -> shorten tail -> Pterodactylid -> semilunate carpal -> short-tailed Scansoriopterygidae* -> replace membrane with feathers ->  modern pygostyle -> Pygostylia.

* with body feathers

The pterodactylid bent at the knuckle. The scansoriopterygid bent at the wrist with the semilunate carpal. That is a major novelty.

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Hypothesis of development of fingers:

2-3-4-5-3 general archosaur

P-2-3-4-5-x rhamphorhynchid
P-2-3-4-4-x pterodactylid
x-x-2-3-4-x short tailed scansoriopterygid

x-x-2-3-2-x Sapeornis (Pygostylia)

x-x-1-2-1-x modern bird

Rhamphorhynchid: long bony tail
to Pterodactylid: short tail
to Short-tailed Scansoriopterygid: bend at wrist, semilunate carpal, feathers on body
Wing membrane replaced with feathers
to Pygostylia: modern pygostyle

Possibility:
Dorygnathus (rhampy)
to
Ctenochasmatoid (pterodactylid)
to
Short-tailed scansoriopterygid?
to
Modern wader/shorebird.

Possible line:
Scaphognathinae [likely terrestrial predator]
to
Azhdarchid that "may have led a more terrestrial existence similar to modern storks and ground hornbills"
to
Short-tailed scansoriopterygid?
to
Modern birds like storks and ground hornbills


Possible filter-feeding lineage:

Rhamphorhynchus
to
Ctenochasmatidae (like Balaenognathus maeuseri)
to
Epidexipteryx
to
Flamingo


Possible arboreal omnivorous lineage:

Rhamphy?
to
Tapejaridae
to
Ambopteryx
to
Various birds are omnivorous

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