SVP abstracts 26: Pterosaur fibers or lack thereof, again

Unwin and Martill 2020
published on this abstract earlier last year and this year (2020).

“Fiber-like structures are frequently preserved in association with fossilized remains of the pterosaur integument. Several fiber types have been recognized. Among the commonest are aktinofibrils, typically 40–100+ μm in breadth and present throughout the flight patagia, exhibiting the same patterns of alignment across Pterosauria.

“Occasionally partially mineralized in distal regions of the patagia, aktinofibrils were composite, helically-wound structures composed of much finer filaments a few microns in diameter.

“Comparable in size to aktinofibrils, but less common, are single-stranded, hair-like pycnofibers, seemingly branched in two specimens of the anurognathid Jeholopterus, that supposedly adorned parts of the cranium, neck, and body. Fiber-like structures have also been reported in cranial crests, foot webs, and tail flaps. The identity, homology, composition, and function of integumentary fibers is fiercely disputed.”

‘Fiercely’? Hyperbole. This issue was just raised by Unwin and Martill and I have yet to see their evidence. Here’s the evidence for pycnofibers on the fluffiest pterosaur of all, the owl-like holotype of Jeholopterus (Fig. 1) and a reconstruction of same (Fig. 2).

Figure 2. Wing and other extra dermal membranes surrounding Jeholopterus.

Figure 1. Wing and other extra dermal membranes surrounding Jeholopterus.

Figure 4. Jeholopterus in dorsal view. Here the robust hind limbs, broad belly and small skull stand out as distinct from other anurognathids. Click to enlarge.

Figure 2. Jeholopterus in dorsal view. Here the robust hind limbs, broad belly and small skull stand out as distinct from other anurognathids. Click to enlarge.

Unwin and Martill 2020 abstract continues:
“This study aimed to resolve these issues through analysis of 150+ specimens where the integument is preserved, representing >25% of known pterosaur species, 15 of the 20 principal lineages, and almost the entire temporal range of the clade. Details of the macro- and microstructure of fibers was obtained using light, UV and laser-UV photography, and binocular and scanning electron microscopy.”

Missing from their taxon list are any outgroups of the Pterosauria (Cosesaurus, Sharovipteryx and Longisquama, Fig. 3), all of which also have extradermal membranes and fibers, some of which form precursor wing fibers (Peters 2009).

Longisquama in situ. See if you can find the sternal complex, scapula and coracoid before looking at figure 2 where they are highlighted.

Figure 3. Longisquama in situ. See if you can find the sternal complex, scapula and coracoid before looking at figure 2 where they are highlighted.

Unwin and Martill 2020 abstract continues:
“Results of this study provide broad support for a new model in which pterosaur integumentary fibers of all types had a single common origin: dermal collagen. This idea is consistent with:

  1. exceptionally preserved examples of cranial crests, wing membranes, and integument associated with the neck and body, which demonstrate that fibers were embedded within the integument, and formed part of the dermis;
  2. calcification of fibers in the cranial crest and, occasionally, in distal parts of the flight patagia;
  3. the composite construction of fibers, which were composed of much finer, helically-wound fibrils.

There’s no argument there. Nothing fiercely disputed. Everyone agrees.

“Multiple specimens with soft tissues preserved in four different preservational modes, show that the integument had a glabrous, fine granular, or even polygonal external texture. Aktinofibrils and other collagenous dermal fibres (e.g., in cranial crests and skin associated with the neck and body) exposed by decay of the remarkably thin epidermis have frequently been misinterpreted as pycnofibers.”

The word ‘misinterpreted’ here should have been the leading sentence followed by evidence. Not the penultimate one followed by no evidence. Unwin and Martill should have taken the strongest evidence against their hypothesis and knocked it down with evidence. They had the opportunity, and they were paid to do this, but failed to do their job.

Figure 2. Here is the Vienna specimen of Pterodactylus in situ and with matrix removed. Now compare this figure with figure 3, which shows the wings and uropatagia unfolding. There is no way to turn this into a deep chord wing membrane. And it decouples the forelimbs from the hind limbs.

Figure 4. Here is the Vienna specimen of Pterodactylus in situ and with matrix removed. Where are the pycnofibers here? I see skin, but no fibers. Then again, the fluffiness of Jeholopterus gave it owl-like silent flight characteristics not needed in a beach combing wader.

Unwin and Martill 2020 abstract continues:
“External fibers fringing the jaws of anurognathids may be an exception, although branching, reported in one specimen, is likely an artifact of preservation.”

Only this one extremely minor exception? Let’s talk about the other major exceptions (Figs. 1, 2). And let’s talk about the lack of similar fibers on wading pterosaurs like Pterodactylus (Fig. 4). The fact that Unwin and Martill got the wing membranes wrong and continue to deny the lepidosaur ancestry of pterosaurs lead one to distrust and discredit everything else they say (= invalid phylogenetic context). And that’s something that should never happen to a couple of pterosaur experts.


References
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29: 1327-1330
Unwin D and Martill D 2019. When the Mesozoic got ugly – naked, hairless, (and featherless) pterosaurs. SVPCA abstracts.
Unwin D and Martill D 2020. Identity, homology, and composition of fiber-like structures associated with the pterosaur integument. SVP abstracts 2020.

https://pterosaurheresies.wordpress.com/2019/10/02/unwin-and-martill-2019-find-pterosaurs-naked-and-ugly/

https://pterosaurheresies.wordpress.com/2020/09/30/naked-pterosaurs-or-feathered-phds-clash/

Naked pterosaurs–or feathered? PhDs clash

Earlier Yang et al. 2019
argued that pterosaurs, like the disc-headed unnamed anurognathids, CAGS-Z070 (Fig. 1) and NJU-57003 (Fig. 2), had protofeathers and thus they were related to dinosaurs with feathers.

Figure 2. CAGS Z020 anurognathid reconstructed in lateral view. As in other disc-head anurognathids the frog-like eyeballs likely rose above the flat skull.

Figure 1. CAGS Z020 anurognathid reconstructed in lateral view. As in other disc-head anurognathids the frog-like eyeballs likely rose above the flat skull.

Figure 2. NJU-57003 insitu. Even though the photo is fuzzy, so is this pterosaur apart from the wing membranes.

Figure 2. NJU-57003 insitu. Even though the photo is fuzzy, so is this pterosaur apart from the wing membranes.

Yesterday Unwin and Martill 2020 
argued that pterosaurs did not have protofeathers. They said, any feathery-looking remains are decomposing fibers shed from the wings. They note that bristle-like integumentary structures do fringe the jaws of CAGS-Z070, but they do not concede any sort of homology other than to call the bristles ‘bristles’.

Yesterday Yang et al. 2020
replied to Unwin and Martill 2020, defending their hypothesis. “In our [2019] paper, 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 integumentary structures seen widely in dinosaurs (including birds), their close relatives.” 

Their first sentence is wrong. As long-time readers are tired of hearing by now, Peters 2000, 2007 tested the pterosaur – dinosaur relationship by adding taxa. The added taxa attracted pterosaurs away from dinosaurs and nested them in a new and overlooked third clade of lepidosaurs, the Tritosauria, of which late surviving Huehuecuetzpalli is a basal member.

Yang et al. 2020 remind us,
“all four pycnofibre types are morphologically identical to structures already described in birds and non-avialan dinosaurs, not only in terms of gross morphology but also in their ultrastructure and chemistry, including melanosomes and chemical evidence for keratin; collectively, thesefeatures are consistent with feathers.”

Or hair. Or scales. Does anyone else see Yang et al. “Pulliing a Larry Martin“? The first thing Yang et al. should do is establish the relationship of pterosaurs with more parsimonious outgroups. They should know convergence is rampant within the Vertebrata and pterosaurs have never nested with dinosaurs whenever other candidates have been offered.

“Mapping these data onto a phylogeny yields a single evolutionary origin for feathers minimally in the avemetatarsalian ancestor of both pterosaurs and dinosaurs.”

That’s what happens when you omit data and citations. Professor Michael Benton is on the list of authors. This is not the first time Benton has omitted data and citations. You might remember when Hone and Benton 2008, 2009 were going to test competing hypotheses of relationship of pterosaurs. They reported they would test the archosaur hypothesis of Bennett 1996 versus the non-archosaur hypothesis of Peters 2000. Peters tested four prior hypotheses (including Bennett 1996) by simply adding Longisquama (Fig. 5), Cosesaurus, Sharovipteryx, and Langobardisaurus (Fig. 6), all of which attracted pterosaurs to their clade. Several of these added taxa have pterosaur-like fibers on their bodies (Fig. 5). When the anticipated results did not go their way, Hone and Benton 2009 deleted all reference to Peters 2000 and wrote that Bennett 1996 had come up with both competing hypotheses.

Getting back to the Reply from Yang et al. 2020:
“In their comment, Unwin and Martill [2020] assert that the branched integumentary structures that we identified are not feathers or even pycnofibres. They make five arguments in favour of their point of
view:”

  1. “superposition or decomposition of composite fibre-like structures or aktinofibrils yields branched structures similar to those in the anurognathids;
  2. the anatomy and anatomical distribution of the anurognathid integumentary structures are consistent with aktinofibrils, but not pycnofibres;
  3. evidence for keratin and melanosomes is not indicative of pycnofibres but rather reflects contamination from epidermal tissue;
  4. the branching we reported is not consistent with exclusively monofilamentous coverings in other anurognathids; and
  5. homology of the branched integumentary structures with feathers cannot be demonstrated conclusively owing to the simple morphology of the former.”

“We refute all five of their arguments.”

The view from ReptileEvolution.com:
Apparently no one has noticed that anurognathids, like Jeholopterus (Fig. 3), are decidedly different than other pterosaurs in terms of the length and quantity of their feathery fluff. In this way, and many others, anurognathids resemble modern owls, predator birds capable of silent flight due to the fluffiness of the pelage.

It is also worth noting
that anurognathids leave no descendants after the Early Cretaceous. In any case, pterosaurs are not related to birds or dinosaurs or archosaurs or archosauriformes or archosauromorphs, as demonstrated in the large reptile tree (LRT, 1740+ taxa) which tests all candidates for dinosaur, bird and pterosaur ancestry back to headless Cambrian chordates.

Figure 4. Jeholopterus in dorsal view. Here the robust hind limbs, broad belly and small skull stand out as distinct from other anurognathids. Click to enlarge.

Figure 3. Jeholopterus in dorsal view. Here the robust hind limbs, broad belly and small skull stand out as distinct from other anurognathids. Click to enlarge.

A figure caption from Unwin and Martill 2020
(Fig. 4) reports, “The inner region of the cheiropatagium adjacent to the body anterior to the pelvis. The dark, slightly granular epidermal surface of the integument (et) covering the torso (t) contrasts with the remarkably thin epidermal surface (ep) of the integument forming the proximal region of the cheiropatagium (c). Much of the epidermis covering the cheiropatagium has been lost, exposing closely packed and aligned aktinofibrils (ak) now slightly decayed. On the far left, much of the cheiropatagium has been pulled away, leaving a few incomplete aktinofibrils and numerous fine fibrils (fb) from which they were composed.”

This is the PIN–2585/36 specimen of Sordes pilosus, which Unwin has not shown in its entirety—ever. The proximal membrane (yellow) is the left fuselage fillet (see Fig. 3), which disproves the batwing hypothesis championed by Unwin and other PhDs. Unwin and Martill say the fine fibrils at left have been ‘pulled away’. I know of no fossil processes that ‘pull’ fine fibrils away from their original insertion points.

Other Sordes specimens have been misinterpreted by Unwin since Unwin & Bakhurina 1994. Peters 1995 argued against the bat-wing interpretation offered by Unwin & Bakhurina 1994 further described nine years ago here.

Figure 4. From Unwin and Martill 2020, colors, arrows and inset added. This is all that has ever been published of PIN-2585/36, a purported Sordes specimen. Given the few clues this appears to be the left fuselage fillet (see Fig. 3), which means this is why Unwin is only showing part of it because, if so, this disproves the batwing hypothesis championed by Unwin and other PhDs.

Figure 4. From Unwin and Martill 2020, colors, arrows and inset added. This is all that has ever been published of PIN-2585/36, a purported Sordes specimen. Given the few clues this appears to be the left fuselage fillet (see Fig. 3), which means this is why Unwin is only showing part of it because, if so, this disproves the batwing hypothesis championed by Unwin and other PhDs.

Yang et al. 2020 conclude:
“In light of this, the most parsimonious interpretation of the simple and branched integumentary appendages in the anurognathid pterosaurs remains our original conclusion that they are feathers.”

This conclusion is not supported by the LRT. Taxon inclusion would have helped Yang et al. 2020. Unwin and Martill 2020 are likewise not correct. They should have shown the same evidence that Yang et al. presented was incorrect, rather than showing their own evidence, which does not support their position.

An online article posted on Phys.org
cites U of Leicester and U of Portsmouth (England) workers. David Unwin and David Martill who claim pterosaurswere in fact bald.”

The article reports, “Feathered pterosaurs would mean that the very earliest feathers first appeared on an ancestor shared by both pterosaurs and dinosaurs, since it is unlikely that something so complex developed separately in two different groups of animals.”

Unlikely ≠ impossible. Just cite the LRT where convergence is rampant. Adding taxa is something paleontologists have been loathe to do for the last twenty years since Peters 2000 moved pterosaurs away from dinosaurs and 13 years since Peters 2007 moved pterosaurs into lepidosaurs. But let’s move on…

The article then states, 
“It would also suggest that all dinosaurs started out with feathers, or protofeathers but some groups, such as sauropods, subsequently lost them again—the complete opposite of currently accepted theory.” 

Once again, add taxa to determine where feathers, or protofeathers, first appeared in tetrapods and if there was a second genesis within the clade.

The article then states,
“The evidence rests on tiny, hair-like filaments, less than one tenth of a millimeter in diameter, which have been identified in about 30 pterosaur fossils. Among these, Yang and colleagues were only able to find just three specimens on which these filaments seem to exhibit a ‘branching structure’ typical of protofeathers.”

Evidence from 30 or just 3 pterosaurs is considerable. Nevertheless, all prior authors omit the pre-pterosaurs (Cosesaurus, Oculudentavis, Sharovipteryx, Longisquama, Fig. 5) most of which have epidermal membranes and fibers. These taxa (Fig. 6) nest between pterosaurs and the lepidosaur Huehuecuetzpalli.

Longisquama in situ. See if you can find the sternal complex, scapula and coracoid before looking at figure 2 where they are highlighted.

Figure 5. Longisquama in situ. See if you can find the sternal complex, scapula and coracoid before looking at figure 2 where they are highlighted.

The Phys.org article then states,
“Unwin and Martill propose that these 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.”

Professor Unwin said:
“The idea of feathered goes back to the nineteenth century but the fossil evidence was then, and still is, very weak. Exceptional claims require exceptional evidence—we have the former, but not the latter.”

The evolution of the pterosaur tail beginning with a basal lizard, Huehuecuetzpalli.

Figure 6. The evolution of the pterosaur tail beginning with a basal lizard, Huehuecuetzpalli.

Professor Martill noted:
that either way, palaeontologists will have to carefully reappraise ideas about the ecology of these ancient flying reptiles. Martill said, “If they really did have feathers, how did that make them look, and did they exhibit the same fantastic variety of colors exhibited by birds. And if they didn’t have feathers, then how did they keep warm at night, what limits did this have on their geographic range, did they stay away from colder northern climes as most reptiles do today. And how did they thermoregulate? The clues are so cryptic, that we are still a long way from working out just how these amazing animals worked.”

As a final note, let’s remember, that when it comes to pterosaur origins, 
workers have been keeping their blinders on for decades. Not sure why, but what results is the current misunderstanding expressed by Yang et al. 2020 AND Unwin and Maritill 2020.


References
Peters D 1995. Wing shape in pterosaurs. Nature 374, 315-316.
Peters D 2000a. Description and Interpretation of Interphalangeal Lines in Tetrapods.  Ichnos 7:11-41.
Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Peters D 2009. A reinterpretation of pteroid articulation in pterosaurs. Journal of Vertebrate Paleontology 29: 1327-1330
Unwin DM and Bakhurina NN 1994. Sordes pilosus and the nature of the pterosaur flight apparatus. Nature 371, 62–64.
Unwin DM and Martill DM 2020. No protofeathers on pterosaurs. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-020-01308-9
Yang Z et al. 2019. Pterosaur integumentary structures with complex feather-like branching. Nature Ecology & Evolution 4, 24–30 (2019).
Yang Z et al. 2020. Reply to: No protofeathers on pterosaurs. Nature Ecology & Evolution. https://doi.org/10.1038/s41559-020-01308-9

https://phys.org/news/2020-09-naked-prehistoric-monsters-evidence-reptiles.html?fbclid=IwAR0YSjtIfZRBUiD6W3b1jwnhzWPFe_eXBE4ABFa2D8QXRCI8GNIfxNQEEs4

From today’s dml.cmnh.org:

“If you can’t cope with the idea of naked pterosaurs, don’t watch my SVP presentation…”

––––––––––––––––––––––––––––––––––––––––––––––
Dr David M Unwin
Associate Professor (Museum Studies)
School of Museum Studies, University of Leicester
t: +44 116 252 3947   e: dmu1@le.ac.uk

Unwin and Martill 2019 find pterosaurs ‘naked’ and ‘ugly’

Unwin and Martill 2019 report:
“With key roles in flight, thermoregulation and protection of the body, the integument was of fundamental importance to pterosaurs. Determination of the basic anatomy of this structure could provide a range of new insights into the palaeobiology of these enigmatic volant reptiles. Presently, however, there are several conflicting hypotheses regarding the construction of the integument, all founded on limited numbers of specimens, and not one of which is fully consistent with the available fossil evidence.

As mentioned yesterday, pterosaurs are not enigmatic. Unwin and Martill have chosen to avoid the scaly lepidosaurian ancestors of pterosaurs cited by Peters (2000, 2007). The integument found on pterosaurs has similar precursor integument on sister fenestrasaurs like Sharovipteryx (Fig. 1) and Longisquama, adding two taxa to their short list of pterosaurs preserving scaly integument and pycnofibers exclusive of the extradermal membranes (wings and uropatagia).

Figure 1. Note the neck skin (integument) of Sharovipteryx, a pterosaur sister.

Figure 1. Note the neck skin (integument) of Sharovipteryx, a pterosaur sister.

Unwin and Martill continue:
“We have developed a new 
model based on investigations of more than 100 specimens all of which show some form of exceptional preservation. This data set spans the entire temporal and systematic ranges of pterosaurs and a wide variety of preservational modes.”

So… “a limited number of specimens” (see above) just turned into “more than 100 specimens.” Did they just want to see if anyone was paying attention?

“The model has three principal components:
(1) A thin epidermal layer. The external surface of the integument was glabrous [= free from hair or down, smooth] with a smooth, slightly granular, or polygonal texture.

Attenuate ‘bristles’ fringed the jaws in two anurognathids and small tracts of filaments may have adorned the posterior cranium in some pterosaurs.

Perhaps these jaw and skull filaments should have been separately numbered because they are different than glabrous tissue.

(2) A layer of reticular and filamentous collagen and of variable thickness and complexity, formed much of the dermis.

Helically wound bundles of collagen fibres (aktinofibrils), were present throughout all flight patagia. Variation of aktinofibrils in terms of their dimensions, packing, orientation and stiffness permitted localized variation in the mechanical properties and behaviour of the flight patagia whichvaried from relatively stiff distally to more extensible and flexible proximally.

‘Feather-like’ structures reported in Jeholopterus appear to be partially unraveled or decayed aktinofibrils.

Again, these are all distinct tissues worthy of their own numbers.

Unwin and Martill have no idea that Jeholopterus was a vampire bat analog (Peters 2008) covered like no other pterosaur with fluffy, silent, owl-like extradermal integument. Neither Unwin nor Martill seem to make reconstructions, so neither has any idea what Jeholopterus looked like, unless they looked here (Fig. 2).

Finally, Unwin and Martill are mixing in flight membranes here. Perhaps THAT is where they get so many examples because otherwise dermal material is exceedingly rare. Integument generally means ‘covering’, so their inclusion of wing membranes is a little misleading, especially considering the ‘naked and hairless’ portion of their abstract headline.

Figure 2. Reconstruction of Jeholopterus. This owl-like bloodslurper was covered with super soft pycnofibers to make it a silent flyer.

Figure 2. Reconstruction of Jeholopterus. This owl-like bloodslurper was covered with super soft pycnofibers to make it a silent flyer.

Collagen fibre bundles were also present in footwebs, and in the integument of the neck and body. These structures have often been mis-identified as ‘hair’ (pycnofibres).

Again, this variety of tissues should have been numbered separately because they are different than tissue forming much of the dermis.

(3) A deep dermal layer with muscles fibres, blood vessels and nerves.

This variety of demal tissues were already described for the flight membranes, but it could also apply to normal tetrapod skin, like our own.

The pterosaur integument was profoundly different from that of birds and bats, further emphasizing the sharp disparity between these volant tetrapods.”

Why didn’t Unwin and Martill compare pterosaur integument to lepidosaur integument, specifically that of Sphenodon and Iguana (Fig. 3)? These are the two closest living relatives of pterosaurs in the large reptile tree. According to the LRT, Unwin and Martill are looking in the wrong places.

The spines of Iguana.

Figure 3. The dorsal and gular spines of Iguana are homologous with those in Sphenodon.

Not sure where Unwin and Martill
are getting data for pterosaur skin exclusive of the extradermal membranes. They don’t say. The dark wing Rhamphorhychus (Fig. 4) has the most incredible preservation of extradermal membranes, but the skull, neck and torso were prepared down to the bone.

Figure 1. The darkwing specimen of Rhamphorhynchus. Top: in situ. Middle: Soft tissues highlighted. Bottom: Neck and forelimb restored.

Figure 4. The darkwing specimen of Rhamphorhynchus. Top: in situ. Middle: Soft tissues highlighted. Bottom: Neck and forelimb restored.

So, why do Unwin and Martill think the Mesozoic got ugly?
Their abstract does not seem to answer their click-bait headline, which describes naked, hairless and featherless pterosaurs without giving one example of same based on evidence. On the contrary, employing phylogenetic bracketing, between Sharovipteryx (Fig. 1), Scaphognathus and Sordes (the hairy devil, Fig. 5), basal pterosaurs were not naked. Their fibers were not the same as hair or feathers, but unique to fenestrasaurs.

The hind limbs and soft tissues of Sordes.

Figure 5. The hind limbs and soft tissues of Sordes. Above, color-coded areas. Below the insitu fossil.

Finally…
Why were pterosaurs considered naked by Unwin and Martill when hairy Sordes (Fig. 5) was studied by Unwin, known to Martill, and not mentioned in the abstract? Very strange, indeed coming from these two.


References
Peters D 2000b. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. – Historical Biology 15: 277–301.
Peters D 2007. The origin and radiation of the Pterosauria. In D. Hone ed. Flugsaurier. The Wellnhofer pterosaur meeting, 2007, Munich, Germany. p. 27.
Unwin D and Martill D 2019. When the Mesozoic got ugly – naked, hairless, (and featherless) pterosaurs. SVPCA abstracts.

Pterosaur pycnofibres revisited: Yang et al. 2018

Yang et al. 2018 bring us a closer look
at pterosaur integumentary structures (= pycnofibres, pycnofibers) courtesy of Tom Kaye and his fluorescence technique,

From the abstract
These findings could imply that feathers had deep evolutionary origins in ancestral archosaurs, or that these structures arose independently in pterosaurs.”

The latter is true and has been known for years. 
Filament structures arose in the lepidosaur fenestrasaur ancestors of pterosaurs, including Cosesaurus, Sharovipteryx (Fig. 1) and Longisquama (Fig. 2). None of these are archosaurs. The archosaur hypothesis for pterosaur origins has failed to produce even one taxon with pterosaur synapomorphies that is not trumped by taxa first specified in Peters 2000 or more recently improved in the large reptile tree at ReptileEvolution.com, which includes pterosaur ancestors extending back to basal lepidosaurs, basal reptiles and Devonian tetrapods.

The problem is co-author Professor Michael Benton
doesn’t want pterosaurs to be derived from fenestrasaurs. The Yang et al. paper insisted that pterosaurs are archosaurs and members of the invalid Benton invented clade, Avemetatarsalia.

You might remember,
Professor Benton and Professor David Hone wrote a two-part set of papers (Hone and Benton 2007, 2009) that declared they would test two competing hypotheses of pterosaur origins Peters 2000 (fenestrasaurs) vs. Bennett 1996 (archosaurs). The second paper (2009) dropped all references to Peters 2000, deleting the taxa therein and falsely ascribed the now gutted hypothesis to Bennett 1996. Ultimately they were unable to find any ancestors for pterosaurs. That’s because they omitted them on purpose.

Figure 1. Sharovipteryx cervicals surrounded by filaments.

Figure 1. Sharovipteryx cervicals surrounded by filaments.

Why?
Benton (1999) declared tiny-fingered Scleromochlus was the nonviolent sister to pterosaurs and evidently Benton wanted to maintain that charade. That’s where he erected the invalid clade, Avemetatarsalia, which makes several appearances in Yang et al. 2018. Peters 2000 is not cited in Yang et al. 2018.

Longisquama in situ. See if you can find the sternal complex, scapula and coracoid before looking at figure 2 where they are highlighted.

Figure 2. Longisquama in situ. The bones are hard to see here due to filaments and skin, especially visible in the throat area.

True to S. Christopher Bennett’s curse,
“You will not get published and if you do get published you won’t be cited.” And that’s why I publish here, online, where I can respond immediately when something gets published that includes taxon exclusion. This is the dark underbelly of paleontology. Sorry that I had to show you this.

References
Bennett SC 1996. The phylogenetic position of the Pterosauria within the Archosauromorpha. Zoological Journal of the Linnean Society 118:261-308.
Hone DWE and Benton MJ 2007. An evaluation of the phylogenetic relationships of the pterosaurs to the archosauromorph reptiles. Journal of Systematic Palaeontology 5:465–469.
Hone DWE and Benton MJ 2009. Contrasting supertree and total evidence methods: the origin of the pterosaurs. Zitteliana B28:35–60.
Peters D 2000. A Redescription of Four Prolacertiform Genera and Implications for Pterosaur Phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106 (3): 293–336.
Yang et al. (8 co-authors including Benton MJ) 2018. Pterosaur integumentary structures with complefeather-like branching. Nature ecology & evolution

Several traits indicate pterosaurs were aerobic and endothermic (warm-blooded)

Pterosaurs, by all accounts, were not your ordinary saurians.
Pterosaurs arose from a previously unreported clade of extinct lepidosaurs, the Tritosauria, not from any living squamates. They could fly and some were fantastically adorned with crests and soft tissues that enabled flight. Moreover, many, if not all, had hair/fibers/fur. The origin of these fibers appears to be in non-volant Middle Triassic fenestrasaurs at the level of Sharovipteryx and Cosesaurus, long before dinosaurs and birds developed protofeathers.

Living lizards are ectothermic (cold-blooded). Pterosaurs are widely considered to be endothermic (warm-blooded) due to their fur-covering, but that’s not the complete story. There’s more:

Jeholopterus2013-588

Figure 1. Jeholopterus in lateral view. Note the extreme length of the dermal fibers, unmatched by other pterosaurs, likely to keep biting insects away from its sensitive skin as it exploited and made wounds on dinosaurs.

1. Ptero fur — aka: pycnofibers, covered pterosaur bodies according to several well-preserved fossils of small pterosaurs. Most of our evidence for a pelage comes from small German and Chinese pterosaurs, but at least some specimens of Zhejiangopterus, a large azhdarchid, had ptero-fur. Preservation of hair apparently depends on subtle differences in substrate geochemistry. Like feathers, ptero-fur could have had uses other than trapping body heat, like keeping flying insects (mosquitoes and flies) from biting sensitive skin, as in Jeholopterus (Fig. 1) and, or course, could be considered decoration or camouflage if striped, spotted or colored.

2. Tiny adult size and even tinier hatchlings — We’ve seen in phylogenetic analysis that tiny pterosaurs succeeded fading larger clades and preceded expanding larger clades. Thus reducing adult size was a survival mechanism for the gene pool. Since moisture loss and heat loss would have been more stressful for tiny pterosaurs and especially the hatchlings of tiny pterosaurs, a pelage might have been useful to keep the wee ones warm, but mostly moist. “Endothermy originated in smaller, active eurythermal ectotherms living in a cool but variable thermal environment,” according to Clarke and Pörtner 2010. Desiccation is the main problem facing today’s tiniest reptiles, all of whom are restricted to moist leaf litter environs (Hedges and Thomas 2001). Unfortunately, we have no examples of tiny pterosaurs with ptero-fur.

Pterodactylus with hair in life pose, preparing to take off.

Figure 1. Pterodactylus with hair in life pose, preparing to take off.

Conversely large pterosaurs with soft and hard crests and extremely long necks and wings increased their surface-to-volume ratios, expanding these natural passive heat radiators when deploying their wings, evidently reducing the need for insulation and fur. We don’t see the body diameter length ptero-hair on large pterosaurs, like we do on Jeholopterus. Rather large pterosaurs, like Zhejiangopterus, appear to have had a short pelage.

3. Flying — Active muscle rapidly gets warm and steady activity due to flying gets a boost from an endothermic aerobic metabolism. The most widely accepted explanation for the evolution of endothermy has been selection for enhanced aerobic capacity.

On the flip side, flying by its very nature, requires a constant airstream and with it, heat loss by convection — if the ambient air is cooler than the body. This is emphasized in pterosaurs with their long wings laced with blood vessels, perhaps acting like giant gills, if not in oxygenation, then in heat exchange.

4. Short, laterally stiff torso — Most lizards cannot breathe while running quadrupedally. Undulating lizards experience Carrier’s constraint because their lungs cannot fill with air while laterally undulating (one lung compresses as the other expands then vice versa beneath the expanding and contracting ribcage). Short torso pterosaurs (and Sharovipteryx) did not undulate. Like birds, they don’t use their tail muscles to retract their hind limbs. Femora retractors have shifted to the enlarged hips. Pterosaurs breathed like we do and like birds do, by expanding both sides of their ribcage at once. (Not but rotating their prepubes back and forth! Gaak!)

5. Hollow bones –- Like warm-blooded birds, many pterosaurs had hollow bones that probably contained air sacs that inflated and cooled the bones with air from their advanced lungs.

6. Erect hind limbs —  Like warm-blooded birds, pterosaurs walked with more or less erect hind limbs that elevated their bellies far above the substrate. Maintaining this configuration required more energy than belly-floppers typically muster.

Clarke and Pörtner (2010) declared the metabolic status of pterosaurs remains unresolved. They reported, “Endothermy has evolved at least twice, in the therapsid-mammal and theropod-bird lineages. The benefits of endothermy are clear: a high and relatively constant internal body temperature allows a fine tuning of metabolism, high muscular power output, fast growth, and a significant degree of independence from environmental temperature. The costs are also well understood: the high rate of metabolism needed to sustain endothermy requires a great deal of food. Undoubtedly the most successful hypothesis, however, has been the suggestion of Bennett & Ruben (1979) that the key factor in the evolution of endothermy was selection for an enhanced aerobic capacity to allow increasingly sustained locomotor activity. The evolution of a higher body temperature and endothermy followed as secondary events. This proposal, the aerobic scope hypothesis, has withstood two decades of further research, and it remains the most widely accepted theory for the evolution of endothermy.”

Benefits of a warmer body

  1. Processing of food proceeds more rapidly
  2. Speed of nervous conduction is temperature dependent
  3. Higher growth rates in the young
  4. Improved food-gathering capability by adults for provisioning developing young. [This is likely not important for pterosaurs, who were likely independent from the moment of hatching because they could fly.]

Embryo development
Like other lizards, pterosaur mothers held eggs within their bodies until just before hatching. This warmth likely decreased the in-utero period by accelerating the embryo’s development, enabling flying shortly after hatching. Since pterosaurs likely laid only one egg at a time, (none have been found in clutches), accelerating embryo development would have increased the reproductive rate, especially among tiny pterosaur adults, which reached adulthood more rapidly.

References
Clarke A and Pörtner H-O 2010. Temperature, metabolic power and the evolution of endothermy. Biological Reviews online.
Hedges SB and Thomas R 2001. At the Lower Size Limit in Amniote Vertebrates: A New Diminutive Lizard from the West Indies. Caribbean Journal of Science 37:168–173.