Pterosaur parental care

https://blog.everythingdinosaur.co.uk/blog/_archives/2017/12/02/hamipterus-nesting-ground-discovery.html

Pterosaurs like birds, were capable of powered flight.  It seems that command of the skies is not the only thing that these two types of vertebrate had in common.  Thanks to a remarkable series of discoveries from the remote Turpan-Hami Basin located in the Xinjiang Uygur Autonomous Region (north-western China), palaeontologists have learned that Pterosaurs, like many living birds nested in colonies, that they had preferred nesting sites and when young, Pterosaurs needed a degree of parental care, just like many species of birds today.

Significantly, the number of eggs discovered are far too many to have been laid by a single female.  This suggests that these flying reptiles nested in colonies and furthermore, the overlaying of multiple clutches of eggs indicates that Pterosaurs, like many birds today, returned to the same nesting sites each year.  As the authors conclude, “the similarity between these groups goes beyond wings”.

The study:

https://www.science.org/doi/10.1126/science.aan2329

The scientific paper: “Egg Accumulation with 3D Embryos Provides Insight into the Life History of a Pterosaur” 

A summary of the basic ideas of this site

 Here is a summary of the basic ideas of this site:

Birds developed in a lineage from short bony tailed pterosaurs to short bony tailed scansoriopterygidae that developed into basal pygostylia birds that developed into modern birds. A side lineage earlier developed from the long tailed pterosaurs to long bony tailed  scansoriopterygidae. That linage went extinct.

And

The category called coelurosaur dinosaurs includes only actual dinosaurs (eg. tyrannosaurs). It does not include birds.

And

The fossil record supports the pterosaur-to-bird theory. The fossil record does not support the dinosaur-to-bird theory.

Barbules in Paraves

https://biblio.ugent.be/publication/8752596/file/8752597.pdf

Some feather morphologies are shared (that is, monofilaments, brush-like and tufted feathers and feathers with along-rachis branching), but others are not—for example, feathers with midpoint branching in pterosaurs and all feathers with barbules in theropods. Barbules are thus a unique innovation of theropod feathers.

This would be more accurate to say:

all feathers with barbules in theropods [Paraves]. Barbules are thus a unique innovation of theropod [Paraves] feathers.

 

Paraves* did not develop from any dinosaurs.

" including secondarily flightless oviraptorids and secondarily flightless ornithomimosaurs 

Similar skull openings

Similar skull openings of pterodactylid and scansoriopterygid:

https://en.wikipedia.org/wiki/Tupandactylus#/media/File:Cast_of_Tupandactylus_imperator_-_Pterosaurs_Flight_in_the_Age_of_Dinosaurs.jpg

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

High level similarities

High level similarities:

Pterosaur pterodactylid (eg tupandactylus):

Membrane wing 

Longest 4th finger

Stage IIIa feathers

Short bony tail 

Paraves scansoriopterygid (eg epidexipteryx):

Membrane wing 

Longest 4th finger

Stage IIIa feathers

Short bony tail 

Significant evidence that short bony tailed pterodactylid developed into short bony tailed scansoriopterygid.

Pterosaur stage IIIa feathers

https://biblio.ugent.be/publication/8752596/file/8752597.pdf

To our knowledge, stage IIIa feathers have not previously been reported in pterosaurs. The Tupandactylus branched structures resemble those in the dromaeosaurid dinosaur Sinornithosaurus millenii27, which are considered homologous to avian feathers28, and differ from the three types of branched feathers described in anurognathid pterosaurs2 . 

This mode of branching is directly comparable to that in stage IIIa feathers19,20 of extant birds, that is, with barbs branching from a central rachis. This is strong evidence that the fossil branched structures are feathers comprising a rachis and barbs.

In other words the pterosaur Tupandactylus stage IIIa feathers resembled those in Sinornithosaurus (paraves/dromaeosauridae) which are considered homologous to avian feathers.

This is exceptional support for the pterosaur to bird theory.

https://media.springernature.com/lw849/springer-static/esm/art%3A10.1038%2Fs41586-022-04622-3/MediaObjects/41586_2022_4622_Tab1_ESM.jpg

Most parsimonious scenario

  https://biblio.ugent.be/publication/8752596/file/8752597.pdf (2022) 

The genotypic and phenotypic characters could both be ancestral to avemetatarsalians; alternatively, both evolved independently in theropods and pterosaurs, or the genes are ancestral and the phenotypic expression occurred independently in the two groups. Our ancestral-state estimations (Extended Data Fig. 9e) reveal that the most parsimonious scenario is that feathers in the avemetatarsalian ancestor had melanosomes with different geometries. This is consistent with a single, deep evolutionary origin for this feature, whereby critical shifts in the genetic machinery facilitating plasticity in melanosome shape occurred in the common ancestor of pterosaurs and birds. Key genomic controls on melanin-based colouration that define the plumage colours of theropods and fossil and extant birds were therefore already in place in early-diverging avemetatarsalians in the Middle to Late Triassic.

Alternatively, I am suggesting that the most parsimonious scenario is that the stage IIIa feathers of short bony tailed pterosaurs (eg  Tupandactylus pterodactylid) developed in their own pterosaur lineage and later  developed into the feathers of short bony tailed scansoriopterygids (eg epidexipteryx). 

There is no need to imagine feathers in an avemetatarsalian common ancestor (and corresponding ghost lineage) since dinosaurs did not have feathers. 

Data availability 

Additional data on melanosome geometry and the character matrix used in the phylogenetic analyses are available in the Zenodo.org data repository at https://doi.org/10.5281/zenodo.6122213. SEM images and samples are available from the corresponding authors on request.

Data:

https://zenodo.org/record/6122213 (Tab 4)

This link brings up a very helpful spreadsheet which shows that stage 5 feathers were only on the pterosaur, ornithomimids, oviraptors and paraves. 

But ornithomimids and oviraptors are flying or secondarily flightless members of Paraves. 

In summary, stage 5 feathers are only on pterosaurs and within Paraves. 

Data:
Cincottaetal. 2022. SData.xlsx

Pterosaur feathers

 https://biblio.ugent.be/publication/8752596/file/8752597.pdf (2022)

Remarkably well-preserved soft tissues in Mesozoic fossils have yielded substantial insights into the evolution of feathers(1). New evidence of branched feathers in pterosaurs suggests that feathers originated in the avemetatarsalian ancestor of pterosaurs and dinosaurs in the Early Triassic(2), but the homology of these pterosaur structures with feathers is controversial(3,4). Reports of pterosaur feathers with homogeneous ovoid melanosome geometries(2,5) suggest that they exhibited limited variation in colour, supporting hypotheses that early feathers functioned primarily in thermoregulation(6). Here we report the presence of diverse melanosome geometries in the skin and simple and branched feathers of a tapejarid pterosaur from the Early Cretaceous found in Brazil. The melanosomes form distinct populations in different feather types and the skin, a feature previously known only in theropod dinosaurs, including birds. These tissue-specific melanosome geometries in pterosaurs indicate that manipulation of feather colour-and thus functions of feathers in visual communication-has deep evolutionary origins. These features show that genetic regulation of melanosome chemistry and shape(7-9) was active early in feather evolution.  

Extended Data Table 1 is also very helpful showing stage IIIa feathers and morphotype 5 feathers in the tapejarid pterosaur Tupandactylus cf. imperator:

https://media.springernature.com/lw849/springer-static/esm/art%3A10.1038%2Fs41586-022-04622-3/MediaObjects/41586_2022_4622_Tab1_ESM.jpg

For reference:

http://prumlab.yale.edu/sites/default/files/prum_n_brush_2002.pdf

Developmental Model of the Origin and Diversification of Feathers 

A predicted transition series of feather follicles based on the hypothesized series of evolutionary novelties in feather developmental mechanisms (Figure 4) from Prum (1999). Stage I—Origin of an undifferentiated tubular collar yields the first feather, a hollow cylinder. Stage II—Origin of a collar with differentiated barb ridges results in a mature feather with a tuft of unbranched barbs and a basal calamus emerging from a superficial sheath. Stage IIIa—Origin of helical displacement of barb ridges and the new barb locus results in a pinnate feather with an indeterminate number of unbranched barbs fused to a central rachis. Stage IIIb—Origin of peripheral barbule plates within barb ridges yields a feather with numerous branched barbs attached to a basal calamus. Stages IIIaIIIb—Origin of a feather with both a rachis and barbs with barbules creates a bipinnate, open pennaceous structure.  

https://www.researchgate.net/publication/272171464_The_origin_and_early_evolution_of_feathers_insights_from_recent_paleontological_and_neontological_data
Xu and Guo

Morphotype 5 is only known in Epidexipteryx. It consists of parallel barbs arising from the edge of a membrane structure (Zhang et al., 2008b). 

Video from Maria McNamara:

 https://www.youtube.com/watch?v=Bmm3IyyjcS0

Difference between feather (with no barbules) and down:

https://www.youtube.com/watch?v=5rcwajqH0Dc&t=1s

Scansoriopteryx

 http://link.springer.com/article/10.1007/s10336-014-1098-9/fulltext.html (2014)

Jurassic archosaur is a non-dinosaurian bird

Stephen A. Czerkas, Alan Feduccia

Re-examination utilizing Keyence 3D digital microscopy and low angled illumination of the fossil Scansoriopteryx, a problematic sparrow-size pre-Archaeopteryx specimen from the Jurassic Daohugou Biotas, provides new evidence which challenges the widely accepted hypothesis that birds are derived from dinosaurs in which avian flight originated from cursorial forms. Contrary to previous interpretations in which Scansoriopteryx was considered to be a coelurosaurian theropod dinosaur, the absence of fundamental dinosaurian characteristics demonstrates that it was not derived from a dinosaurian ancestry and should not be considered as a theropod dinosaur.

Summary

I am suggesting the following:

Long bony tailed rhamphorhynchid --> Long bony tailed scansoriopterygid (eg scansoriopteryx) --> Derived long bony tailed Paraves (eg Anchiornithids) --> Extinction

Long bony tailed rhamphorhynchid --> Short bony tailed pterodactylid -->  Short bony tailed scansoriopterygid (eg epidexipteryx) --> Pygostylia --> Modern birds.

Adding to the picture

To this point I have focussed on the development from long bony tailed pterosaurs (rhamphorhynchids) to long bony tailed scansoriopterygids (eg scansoriopteryx). 

I have not followed on from there to whether long bony tailed Paraves became short bony tailed pygostylia. I suggest they did not.

And I have not talked about the short tailed pterodactylids. 

I suggest they are related. Short tailed pterosaurs (eg pterodactylids) developed into short bony tailed scansoriopterygids (eg epidexipteryx). Which then developed into pygostylia.

Nature experimented with various forms of flight. A big challenge was transitioning from long bony tail to short bony tail. This was accomplished WITHIN pterosaur lineage. 

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

"Pterodactyloidea is traditionally considered to be the group of short-tailed pterosaurs with long wrists (metacarpus), compared with the relatively long tails and short wrist bones of basal pterosaurs ("rhamphorhynchoids")."

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

"Epidexipteryx had a short tail (70% the length of the torso), anchoring long tail feathers, while Scansoriopteryx had a very long tail (over three times as long as the torso) with a short spray of feathers at the tip."

Digging deeper

Some long bony tailed pterosaurs (eg rhamphorhynchids) developed into long bony tailed scansoriopterygids. And some of those long bony tailed scansoriopterygids developed into other more derived long bony tailed members of Paraves. Then they went extinct.

But there are two paths. One from long bony tailed pterosaurs and the other from (later) short bony tailed pterosaurs (eg pterodactylids)

Some short bony tailed pterosaurs (eg pterodactylids) developed into short bony tailed scansoriopterygids. And some of the short bony tailed scansoriopterygids developed into pygostylia. 

This means that later stage feathers (after stage IIIa) developed in both lines.

https://www.researchgate.net/publication/343498354_Potential_for_Powered_Flight_Neared_by_Most_Close_Avialan_Relatives_but_Few_Crossed_Its_Thresholds

This suggests there was greater experimentation with wing-assisted locomotion before theropod flight evolved than previously appreciated. This study adds invaluable support for multiple origins of powered flight potential in theropods (≥3 times), which we now know was from ancestors already nearing associated thresholds, and provides a framework for its further study

A problem with the word "dinosaur"

Take note of the following problem:

http://www.ucmp.berkeley.edu/diapsids/dinomm.html

One important dinosaurian synapomorphy is the perforate [completely open] acetabulum, simply a "hip bone" (actually three connected bones, together called the pelvis) with a hole in the center where the head of the femur ("thigh bone") sits. This construction of the hip joint makes an erect stance (hindlimbs located directly beneath the body) necessary — like most mammals, but unlike other reptiles which have a less erect and more sprawling posture. Dinosaurs are unique among all tetrapods in having this perforate [completely open] acetabulum.

Notice that I have added the words "completely open" within square brackets.  I add that because as the quote says, it is:
"actually three connected bones, together called the pelvis with a hole in the center where the head of the femur ("thigh bone") sits". 

In other words, it is completely open. 

But birds do not have a completely open acetabulum. So it is misleading to say they both have a "perforate" acetabulum.

One needs to distinguish between actual dinosaurs (eg. tyrannosaurids) on the one hand and flying (and secondarily flightless) feathered Paraves with long bony tail (eg. long tailed Scansoriopterygids) on the other. 

Cup-like Acetabulum

Basal paraves had a partially closed (cup-like) acetabulum that allowed them to abduct (splay, sprawl) their legs.
Pterosaurs had a completely closed (cup-like) acetabulum that allowed them to abduct (splay, sprawl) their legs.
Dinosaurs had a completely open acetabulum that did not allow them to abduct (splay, sprawl) their legs.

Dinosaur to bird theorists focus on whether the acetabulum had a hole in it or not. From that point of view birds and dinosaurs were somewhat similar in that they both had a hole in the acetabulum. (Although one had a partially closed acetabulum, the other a completely open acetabulum).

But that overlooks the important issue which is whether the acetabulum was cup-like or not. From that point of view dinosaurs are not like birds. While pterosaurs in that respect are like birds. 

This is significant because dinosaurs could not splay their legs, but basal paraves and pterosaurs could

For more details see:

https://pterosaurnet.blogspot.com/2014/10/pelvic-girdle.html

Ambopteryx

https://www.nature.com/articles/s41586-019-1137-z (2019)

Yi qi, which has membranous wings—a flight apparatus that was previously unknown among theropods but that is used by both the pterosaur and bat lineages6. This observation was not universally accepted7. Here we describe a newly identified scansoriopterygid —which we name Ambopteryx longibrachium, gen. et sp. nov.—from the Upper Jurassic period. This specimen provides support for the widespread existence of membranous wings and the styliform element in the Scansoriopterygidae

The scansoriopterygids (including Ambopteryx) are members of basal Paraves. With their wing skin membrane and longest outermost finger, they are a candidate transitional between pterosaur and later Paraves. They fit right into the pterosaur to bird theory.

On the other hand, there is zero evidence of membrane wings in any claimed dinosaur ancestor. And also, scansoriopterygids have the outermost digit as the longest, which is not found in any claimed dinosaur ancestor. 

For details about Yi qi see here:
http://pterosaurnet.blogspot.com/2015/05/another-scansoriopterygid.html

For details about Scansoriopteryx see here:
https://pterosaurnet.blogspot.com/2014/07/scansoriopteryx.html

https://m.phys.org/news/2019-05-jurassic-non-avian-theropod-dinosaur-flight.html

Convergence

It is sometimes said that coelurosaur dinosaurs and the ancestors of pterosaurs convergently evolved characteristics related to flying.
It is said that pterosaur characteristics related to flying were analogous (convergent) to that of birds, rather than homologous (ancestral).

Here is an example:
https://en.wikipedia.org/wiki/Pterosaur

A 2009 study showed that pterosaurs had a lung-air sac system and a precisely controlled skeletal breathing pump, which supports a flow-through pulmonary ventilation model in pterosaurs, analogous to that of birds.

But the parsimonious conclusion is that basal Paraves evolved FROM pterosaurs.

Pterosaurs had feathers

https://www.nature.com/articles/s41559-018-0728-7.epdf?author_access_token=g0SJk_S0UlYkd9ChFUOfsdRgN0jAjWel9jnR3ZoTv0OhuDOmy4G-1OHvOkIfGGVa9hjNadq-O6la97WKFFA-U_CzDuLx5hrx52MRXIaHCxe0wHtP1JSprPoeDcECtIaKGps55q9OlM6xBJamyA2RWA%3D%3D

Pterosaur integumentary structures with complex feather-like branching

Pterosaurs were the first vertebrates to achieve true flapping flight, but in the absence of living representatives, many questions concerning their biology and lifestyle remain unresolved. Pycnofibres—the integumentary coverings of pterosaurs—are particularly enigmatic: although many reconstructions depict fur-like coverings composed of pycnofibres, their affinities and function are not fully understood. Here, we report the preservation in two anurognathid pterosaur specimens of morphologically diverse pycnofibres that show diagnostic features of feathers, including non-vaned grouped filaments and bilaterally branched filaments, hitherto considered unique to maniraptoran dinosaurs, and preserved melanosomes with diverse geometries. These findings could imply that feathers had deep evolutionary origins in ancestral archosaurs, or that these structures arose independently in pterosaurs. The presence of feather-like structures suggests that anurognathids, and potentially other pterosaurs, possessed a dense filamentous covering that probably functioned in thermoregulation, tactile sensing, signalling and aerodynamics. 

But there is a disagreement on this:

http://www.sci-news.com/paleontology/pterosaur-pycnofibers-08896.html#:~:text=Dr.%20Unwin%20and%20Professor%20Martill%20propose%20that%20the,the%20result%20of%20these%20fibers%20decaying%20and%20unraveling

Dr. Unwin and Professor Martill propose that the branched pycnofibers in pterosaurs are not protofeathers at all, but tough fibers which form part of the internal structure of the pterosaur’s wing membrane, and that the ‘branching’ effect may simply be the result of these fibers decaying and unraveling.

Reply:

https://www.researchgate.net/publication/344413424_Reply_to_No_protofeathers_on_pterosaurs

In our paper1, we explored the morphology, ultrastructure and chemistry of the dermal structures of pterosaurs and showed that they probably had a common evolutionary origin with the integu-mentary structures seen widely in dinosaurs (including birds), their close relatives. Our study of two Middle Jurassic anurognathid pterosaurs from China showed that the whisker-like pycnofibres of the pterosaurs include at least four distinct morphologies, rather than one as had been assumed, and that three of these show branch-ing, a key characteristic of feathers. Further, all four pycnofibre types are morphologically identical to structures already described in birds and non-avialan dinosaurs, not only in terms of gross mor-phology but also in their ultrastructure and chemistry, including melanosomes and chemical evidence for keratin; collectively, these features are consistent with feathers.

Beginning at the same time

Paravians and the type of dinosaur they are purported to have evolved from basal Tyrannoraptora, appear in the fossil record beginning at the same time.
The purported dinosaur ancestor is dated at the same time as the basal paraves. 

Quote

Scansoriopteryx (Paraves)

Temporal range: Callovian to Kimmeridgian, 165-156 Ma

Quote

Serikornis (Paraves)

Temporal range: Middle Jurassic, 165-162 Ma

These Paravians are dated at the same time (165 ma) as the type of dinosaur they are purported to have evolved from. For example with:

Quote

Proceratosaurus (basal Tyrannoraptora)

Temporal range: Middle Jurassic, 165 Ma

Proceratosaurus is a genus of small-sized (~3 metres (9.8 ft) long) carnivorous theropod dinosaur from the Middle Jurassic (Bathonian) of England.[1] It was originally thought to be an ancestor of Ceratosaurus, due to the similar small crest on its snout.[2] Now, however, it is considered a coelurosaur, specifically one of the earliest known members of Tyrannosauroidea,[3] the clade of basal relatives of the tyrannosaurs.[4]

So the actual fossil evidence has paravians and the type of dinosaur they are thought to have evolved from appearing in the fossil record beginning at the same time.

Proceratosaurus

http://www.nhm.ac.uk/resources/nature-online/life/dinosaurs/dinosaur-directory/images//reconstruction/small/Proceratosaurus.jpg

Epidendrosaurus

https://upload.wikimedia.org/wikipedia/commons/thumb/b/b5/Epidendrosauruswiki1.png/260px-Epidendrosauruswiki1.png

For reference:
Notice the imaginary line running from Coleurosaur to Paraves. There is absolutely no fossil evidence for this.

http://science.sciencemag.org/content/345/6196/562.abstract

Feathers

https://books.google.co.uk/books?id=KG86AgWwFEUC&pg=PA454&dq=patagial&hl=en&sa=X&ei=AzRBVendC9DLaOehgMgC&ved=0CCoQ6AEwAg#v=onepage&q=patagial&f=false

This shows that feathers in basal Paraves are not exaptations.

Flight feathers are not variants of thermal insulating material, but structures that have evolved over a long period, enabling the span and aspect ratio of the wings to be progressively increased as they developed.

The structure of a flight feather is dedicated wholly to withstanding the bending and torsional moments caused by aerodynamic forces, and delivering these moments through the follicle to the wing skeleton.

And:

The main features of the hypothetical patagial glider can still be seen, albeit much modified, in the modern bird wing, which has a patagium that joins the side of the body to the elbow joint, and continues as a narrow strip along the posterior side of the ulna, and of the reduced hand skeleton.

The digits transition

I am working with the following hypothesis of the transition of fingers and phalanges:

    I  II  III  IV V

P-2-3-4-5-x Pterosaur
x-x-2-3-4-x Basal Paraves (Scansoriopterygids)

P = pteroid. Numbers represent the number of phalanges. Roman numerals represent fingers (digits).
For example, this shows 5 phalanges in the fourth finger (IV) of pterosaur. 

Summary of Changes:
The first step occurred in pterosaur with the loss of the fifth finger (V).
Then in the transition to basal paraves:
The pteroid was lost. (It continues as a prepollex in bird embryo).
The first finger (I) was lost. (It continues as a vestigial digit in bird embryo).
Each remaining digit lost one phalanx each.

In later Paraves the fourth finger (IV) was shortened. 

Digit II becomes the alula.

This transition from pterosaur digits to the digits of a basal paraves like Scansoriopteryx, is consistent with all the known evidence. It is not contrary to any known evidence.

In regard to the pteroid/prepollex, the change I am proposing is similar to the hexadactyl origin hypothesis (HOH).
In other respects it is like the pyramid reduction hypothesis for reduction in phalanges as well as lost digits.

Hexadactyl origin hypothesis (HOH)
https://vargaslab.files.wordpress.com/2009/03/vargasfallon2005b3.pdf

The early pantetrapod Devonian ancestors of
birds were polydactylous, with up to eight fingers
on both forelimbs and hindlimbs (Clack, 2002). In
the embryos of several modern tetrapods, it is
possible to observe the presence of mesenchymal
condensations other than those of digits 1-5 that
may represent vestiges of the additional digits of
early pan-tetrapods. A mesenchymal condensation
in front of digit 1 is called a prepollex, and a
mesenchymal condensation found posterior to
digit 5 is called a postminimus.

Although only three digits develop in the wing of
the chicken, recent work on the expression of the
Sox9 gene allowing the visualization of mesenchymal
condensations has revealed that six mesenchymal
condensations can be found in the
developing wing that can be compared to digital
condensations (Welten et al., 2005). Remarkably,
it is possible to interpret these condensations as
consistent with the proposal that the anteriormost
condensation may be a prepollex, the following 

three digital condensations are digits 1, 2 and 3,

the next condensation is a vestige of digit 4, and
the posteriormost condensation (named ''element
X'' by Welten et al., 2005) can be assumed to be a
vestige of digit 5

In fact, there is no evidence of phalangeal initiation of any kind [no phalanges] in the
brief  Sox9 expressing region anterior to the first digit in the wing. It remains plausible that this is a prepollex

The bird prepollex has no phalanges, just like the pteroid which also has no phalanges!

Good reference:
http://dml.cmnh.org/2005Dec/msg00213.html

In principle, hands of adult tetrapods where the first four conventional digits plus an ossified prepollex are present while the 5th finger is completely absent have been known for a long time.

ALSO:

SOCS2
https://www.academia.edu/1486957/Transcriptomic_analysis_of_avian_digits_reveals_conserved_and_derived_digit_identities_in_birds

To discover genes that specifically contribute to the second and third wing digit identities, we performed differential expression analysis of the mRNA-seq data between samples LFb and LFc. We found two genes, Tbx3 and Socs2, with high expression in sample LFc (Supplementary Fig. 9 and Fig. 3a). To our knowledge no studies have been published indicating a role for Socs2 in limb development. ISH confirms its strong expression in the third forelimb digit to the exclusion of all other digits in forelimb and hindlimb(Fig.3b-g). Recently it has been shown that the third forelimb digit has a unique mode of development in birds8. This, combined with our gene expression survey,supports the idea that the third wing digit has a unique derived identity in birds.

https://www.ncbi.nlm.nih.gov/pubmed/16419040

Suppressor of cytokine signaling (SOCS)-2 regulates normal postnatal growth and its deficiency in mice causes gigantism with increased bone length and proportional enlargement in skeletal muscles.

http://www.jci.org/articles/view/22710

SOCS2 negatively regulates growth hormone action in vitro and in vivoMice deficient in SOCS2 display an excessive growth phenotype characterized by a 30-50% increase in mature body size. Here we show that the SOCS2-/- phenotype is dependent upon the presence of endogenous growth hormone (GH) and that treatment with exogenous GH induced excessive growth in mice lacking both endogenous GH and SOCS2. This was reflected in terms of overall body weight, body and bone lengths, and the weight of internal organs and tissues.

http://www.tmd.ac.jp/artsci/biol/textbiodiv/17EvoDev7-18.pdf
Welten et al

The bird wing is of special interest to students of homology and avian evolution. Fossil and developmental data give conflicting indications of digit homology if a pentadactyl "archetype" is assumed. Morphological signs of a vestigial digit I are seen in bird embryos, but no digit-like structure develops in wild-type embryos. To examine the developmental mechanisms of digit loss, we studied the expression of the high-mobility group box containing Sox9 gene, and bone morphogenetic protein receptor 1b (bmpR-1b)-markers for precondensation and prechondrogenic cells, respectively. We find an elongated domain of Sox9 expression, but no bmpR-1b expression, anterior to digit II. We interpret this as a digit I domain that reaches precondensation, but not condensation or precartilage stages. It develops late, when the tissue in which it is lodged is being remodeled. We consider these findings in the light of previous Hoxd-11 misexpression studies. Together, they suggest that there is a digit I vestige in the wing that can be rescued and undergo development if posterior patterning cues are enhanced. 

We observed Sox9 expression in the elusive ‘‘element X’’ that is sometimes stated to represent a sixth digit. Indeed, incongruity between digit domains and identities in theropods disappears if birds and other archosaurs are considered primitively polydactyl (more than 5 fingers). Our study provides the first gene expression evidence for at least five digital domains in the chick wing. The failure of the first to develop may be plausibly linked to attenuation of posterior signals. 

Pyramid reduction hypothesis
Good description of pyramid reduction hypothesis even though they do not agree with it:
http://aracnologia.macn.gov.ar/st/biblio/Xu%20and%20Mackem%202013%20Tracing%20the%20Evolution%20of%20Avian%20Wing%20Digits.pdf

One proposed mechanism postulates that an elevation in peripheral BMPs, signaling factors that modulate cell survival and proliferation [60,61], drove bilateral medial and lateral digital reduction [9]. This hypothesis is developmentally plausible, and is also consistent with the phalangeal reduction pattern seen in basal birds [9,23]. However, it predicts that the direct avian ancestor had a five-fingered hand with dominant digits II, III, and IV [9], which is inconsistent with the digital reduction data from basal theropods (e.g., all known basal theropods, including ceratosaurs, have a vestigial digit IV) [5,62–64]. In fact, the pyramid reduction hypothesis implies that either birds are not descended from theropod dinosaurs, or that some as yet to be discovered basal theropods were five-fingered with dominant digits II, III, and IV. 

For more details see:
https://pterosaurnet.blogspot.com/2015/12/pyramid-reduction-hypothesis.html
http://pterosaurnet.blogspot.com/2014/07/pterosaur-fingers.html


https://pterosaurheresies.wordpress.com/2013/12/11/is-the-prepollex-radial-sesamoid-analogous-to-the-pteroid/
Other creatures with a prepollex: