UV light vs. LCA (last common ancestor) approach to flapping flight in birds

Schwarz et al. 2019
employed ultraviolet (UV) light to report, “In contrast to previous studies, we show that most of the vertebral column of the Berlin Archaeopteryx possesses intraosseous pneumaticity, and that pneumatic structures also extend beyond the anterior thoracic vertebrae in other specimens of Archaeopteryx. With a minimum Pneumaticity Index (PI) of 0.39, Archaeopteryx had a much more lightweight skeleton than has been previously reported, comprising an air sac-driven respiratory system with the potential for a bird-like, high-performance metabolism.

“The neural spines of the 16th to 22nd presacral vertebrae in the Berlin Archaeopteryx are bridged by interspinal ossifications, and form a rigid notarium-like structure similar to the condition seen in modern birds. this reinforced vertebral column, combined with the extensive development of air sacs, suggests that Archaeopteryx was capable of flapping its wings for cursorial and/or aerial locomotion.”

Schwarz et al. did not perform a phylogenetic analysis
nor did they mention anything about the elongate locked down coracoid present on this specimen. In the large reptile tree (LRT, 1445 taxa, subset Fig. 1) the Berlin specimen of Archaeopteryx (MB.Av.101) nests at the base of all flapping birds, including the Enantiornithes, the first clade to split off. As in the amniotic egg issue, the last common ancestor is where you find the genesis of traits common to all descendant taxa. So, Schwarz et al. are correct: the Berlin specimen is indeed close to the origin of flapping flight.

Figure 1. Subset of the LRT focusing on basal birds and pre-bird theropods. Note many of the various Solnhofen birds nest apart from one another and the Daiting specimen nests outside the birds (Aves). The Berlin Archaoepteryx is the last common ancestor of all flapping birds.

It’s not the reinforced vertebral column
that determines if a bird (or pterosaur) flaps or not. It’s the elongation of an immobile coracoid these two flapping clades share in common at the genesis of this behavior.

Distinctlively different
due to lacking a coracoid, bats employ the hyper-elongation of the clavicle to do the same thing by convergence.

Figure 2. Generic freehand Archaeopteryx (Berlin specimen) from Schwarz et al. 2019, retraced from Wellnhofer 2008 compared to bone-by-bone tracing for ReptileEvolution.com. Wellnhofer’s drawing appears to be a generic Archaeopteryx. Tracings of all specimens show no two are alike.

One more thing…
if possible, don’t freehand your reconstructions (Fig. 2) and don’t redraw freehand reconstructions from Wellnhofer 2008 ~ especially if you’re going to go through all the trouble of extracting more precise data on a fossil than has been recovered before. Do your own more precise bone tracings and reconstructions!

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References
Schwarz D, Kundrat M, Tischlinger H, Dyke G and Carney RM 2019. Ultraviolet light illuminates the avian nature of the Berlin Archaeopteryx skeleton. Nature.com
Wellnhofer P 2008. Archaeopteryx. Der Urvogel von Solnhofen. (Verlag Dr. Friedrich Pfeil, München), pp. 256.

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When elbows and knees start bending in basal tetrapods

You might remember,
earlier we looked at the non-traditional origin of fingers and toes in the Tetrapoda in the basal dvinosaur/plagiosaur, Trypanognathus (Figs. 1, 7). 

Today
we’ll look at the origin of elbows and knees able to bend (distinct from lobefins and kin).

And then a quick peek
at bendable limbs large enough to sustain/lift the weight of the skull and body off the substrate, an ability chronicled in Middle Devonian tracks.

Backstory
Lobefin fish, like Eusthenopteron through Panderichthys, have one proximal limb bone, two more distally and many tiny bones further out, ending in lepidotrichia (fin filaments). In these taxa the radius is much longer than the ulna. The tibia is much longer than the fibula.

Figure 1. Basal tetrapods to scale. Yellow taxa are temnospondyls. Red taxa are reptilomorphs.

Dvinosaurs (basalmost tetrapods, and by definition, reptiles and humans; Fig. 2) like Trypanognathus (Figs. 1, 2), have the same limb bone arrangements (1 bones, 2 bones, then several bones), but the ulna length catches up to the radius and the fibula resembles the tibia. Notably this occurs on limbs too small to support weight. Only tiny fingers and toes are present. These replace the lepidotrichia. There is little indication that a strongly bendable elbow or knee is present.

Figure 2. Subset of the LRT focusing on the basal tetrapods. Trypanognathus forelimb and hind limb shown at right. Also see figure 7.

In colosteids,
like Colosteus (Fig. 3) and Pholidogaster, the limbs have modern proportions, with bendable elbows and knees, but they remain far too small to support the weight of the head and torso. Another traditionally considered colosteid, Greererpeton (Fig. 1) nests at the base of the next derived clade in the large reptile tree (LRT, 1444 taxa).

Figure 3. Colosteus is covered with dermal skull bones and osteoderms. Those vestigial forelimbs are transitional to the limbless condition in Phlegethontia.

Thereafter
limbs get bigger, as documented in Ossinodus (Fig. 1), able to support weight lifted above the substrate. At this stage and with this innovation basal tetrapods split into three clades: Temnospondyli, Lepospondyli and Reptilomorpha in the LRT. Even so, lateral undulation of the backbone is the main driver for stride length.

Temnospondyli
Limbs are larger in temnospondyls (Fig. 1). Bodies are rounder. Tails are longer. The limbs would have been advanced more by lateral undulation than by extension and flexion.

Acanthostega represents a rare reversal,
among temnospondyls. Phylogenetically it is a smaller, apparently neotonous taxon with extra fingers and toes, a reversal to a longer radius than ulna, smaller limbs, but also a smaller, narrower body and a robust tail. Both girdles were quite large and both the humerus and femur had large processes not seen in more primitive dvinosaurs.

Figure 4. Dendrerpeton without raised orbits from Holmes et al. 1998. This configuration is similar to that of basal lepospondyls and reptilomorphs including microsaurs.

Lepospondyli
Trimerorhachis (Fig. 5) is either a basal taxon retaining a wide torso and relatively small limbs or it is yet another reversal because its sister taxon in the LRT is Dendrerepton (Fig. 4) a small taxon with robust limbs and a small body. This clade gives rise to frogs, like Rana, which hyper-emphasizes the limbs and reduces the torso, along with Cacops, which shortens the torso and emphasizes the girdles.

Figure 5. Trimerorhachis was considered a dvinosaurian temnospondyl. Here both Trimerorhachis and Dvinosaurus nest low on the basal tetrapod tree, close to the fin/finger transition. The limbs in dorsal view would have been advanced more by lateral undulation than by extension and flexion.

Reptilomorpha
This clade includes reptiles, like Silvanerpeton, their proximal ancestors and microsaurs, like Tuditanus, in the LRT. Basal taxa were increasingly terrestrial with robust limbs on smaller bodies. Much later several clades within both Reptilia and Microsauria (e.g. Diplocaulus) returned to a more aquatic niche, typically reducing the limbs. In some reptiles (e.g. Ichthyosaurus, Orcinus) limbs evolved back into fins/flippers and tails evolved fish-like flukes.

Figure 6. Subset of the LRT from an earlier post focusing on basal tetrapods and showing those taxa with lobefins (fins) and those with fingers and toes (feet). Inbetween we have now have data in the form of Trypanognathus (Fig. 7) at the base of the Dvinosauria.

Figure 7. Trypanognathus in situ, colorized to bring out ribs and limbs, too small to support the body. Also see figure 2.

Plagiosuchus: confluent orbital-temporal fenestra, like birds and mammals

Just another case of convergence here
reminding us not to define clades on traits, but only on two select taxa, their last common ancestor and all descendants.

Plagiosuchus pustuliferus (von Huene 1922; Middle Triassic, 240mya; SMNS 57921) nests with the smaller, Gerrothorax in the large reptile tree (LRT, 1444 taxa). Plagiosuchus has a narrower skull and upper temporal fenestrae confluent with the orbits. This is partly due to the loss of the prefrontals, postfrontals and postorbitals.

Both taxa belong to the first clade
to split off from the remainder of basal tetrapods at the transition from fins to fingers and toes. Both taxa retained gills and likely never left the water.

Figure 1. Plagiosuchus skull from Damiani et al. 2009, lateral view and colors added here. Lost or fused are the postorbital, postrfrontal and prefrontal bones on this basal tetrapod not far from fish. Note the relatively short dentary and long post-dentary bones.

The wide skull and laterally oriented dorsal ribs
indicate Plagiosuchus was a full-time bottom dweller with little use of its limbs other than to paddle them around like fins, supporting itself like another sit-and-wait predator, the extant frogfish.

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References
Damiani R, Schoch RR, Hellrung H, Wernburg R and Gastou S 2009. The plagiosaurid temnospondyl Plagiosuchus pustuliferus (Amphibia: Temnospondyli) from the
Middle Triassic of Germany: anatomy and functional morphology of the skull. Zoological Journal of the Linnean Society, 2009, 155, 348–373.
von Huene F 1922. Beiträge zur Kenntnis der Organisation einiger Stegocephalen der schwäbischen Trias. Acta Zoologica3: 395–459.

wiki/Plagiosuchus

Untangling the Sclerothorax chimaera

Tough one today
with many puzzle pieces.

The genus Sclerothorax
was first named by Huene 1932 based on two giant salamander-sized Early Triassic specimens. One was a torso and anterior tail lacking a skull and ventral pectoral girdle (HLD-V 608; Fig. 1). The other was a skull and ventral pectoral girdle (HLD-V 607; Fig. 1). Apparently there were no bones in common. Both are shown here at about one-quarter natural size.

Figure 1. Sclerothorax holotypes (two specimens) described by Huene 1932. Colors added. The NMK specimens (at right) are traced from new specimens added by Shoch et al. (2007, Fig. 3), but newly traced here.

The 608 specimen torso and tail are notable
for their exceedingly tall neural spines topped by spine tables, like those of Eryops, and overlapping ribs, like those of Eryops, Sclerocephalus, Mastodontsaurus (Fig. 2) and Petobatrachus.

The 607 skull is notable
for being shorter than the interclavicle like no other basal tetrapods.. Schoch et al. 2007 report, “At first sight, this (second) specimen seemed so different from the first find that Huene himself was struck. Yet his efforts in further preparing the second specimen revealed the morphology of the dorsal spines which he found similar to the first specimen, albeit affected by compaction and consequently distorted.”

Figure 2. Click to enlarge. The largest amphibians of all time include Mastodonsaurus, Prionosuchus, Koolasuchus, Siderops, Crassigyrinus and the extant Andrias, the giant Chinese salamander.

Unfortunately
those purported high dorsal spines on the 607 skull specimen are not visible with the present data, nor were they illustrated (Fig. 1). No doubt the torso with overlapping ribs also resembles that of the hippo-sized Mastodontsaurus (Fig. 3). To that point, Shoch et al. nested Sclerothorax with Mastodonsaurus in their phylogenetic analysis.

Figure 3. NMK-S118 and 117 specimens assigned to Sclerothorax. Colors added. Snout restored two ways. Trying to identify sutures in such a textured skull with lateral line canals is fraught with difficulties. Note the dorsal ribs on the 117 specimen. They do not appear to overlap and appear to be laterally oriented, creating a broad, flat torso, but we are seeing them in ventral aspect.

Contra earlier studies,
in the large reptile tree (LRT, 1443 taxa) the NMK S-118 posterior skull referred specimen nested with the similarly-sized Early Permian Trimerorhachis (Fig. 6), a flat-head, flat-torso taxon without overlapping dorsal ribs or high dorsal spines. Distinct from most basal tetrapods, the jugal does not contribute to the orbit rim. The HLD-V 608 torso and tail holotype specimen nest at the base of Peltobatrachus + Sclerocephlaus, and between the Ossinodus clade and the Eryops clade. So the two specimens are not congeneric in the LRT and that skull does not belong with that torso and tail (Fig. 4). (As always, I am willing to be convinced otherwise with better data.)

Figure 4. Images from Schoch et al. 2007 combining various specimens to create a Sclerothorax chimaera. Note the small size of the limbs relative to the torso.

When it used skull traits from Schoch et al. (2007)
(Fig. 5) the LRT lost resolution. I also discovered the lateral view of the reconstructed skull  did not match the dorsal view with regard to the placement of nares, orbits and certain sutures. A repair of the lateral view is presented here (Fig. 5).

Figure 5. Sclerothorax skull in 4 views from Schoch et al. 2007, colors added. A second skull is shown in lateral view to match the elements and proportions of the dorsal view.

The change in the skull sutures
presented here (Fig. 3) and the subsequent nesting of the 607 skull specimen with Trimerorhachis (Fig. 6) is supported by the preservation of long, only slightly curved and laterally oriented ribs in the NMK S-117 specimen (Fig. 3), like those of Trimerorhachis (Fig. 6).

Figure 6. Trimerorhachis. Like Sclerothorax and distinct from other basal tetrapods the jugal does not contact the orbit rim. 

Fossils typically come to rest
with their major axis parallel to the bedding plane. In this way taxa with a wide, flat, skull and torso will usually be preserved in dorsal aspect. By contrast, taxa preserved in lateral view are more likely to have a deeper than wide torso, as in the Sclerothorax holotype (Fig. 1).

Eryops also had tall neural spines,
overlapping ribs and a deep pelvis in common with Sclerothorax. Perhaps Sclerothorax had a large skull and strong limbs, like Eryops, suitable for terrestrial locomotion.

By contrast
the 607 skull retains lateral line canals, like those of Trimerorhachis, so we might expect shorter limbs and an aquatic environment for the 607 skull specimen.

Figure 7. Subset of the LRT focusing on basal tetrapods. The two Sclerothorax taxa are highlighted in separate clades.

Phylogenetically separating
the 607 skull from the holotype torso resolves the lateral line canal issue when the skull was joined to the torso as a chimaera.

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References
Huene F v 1932. Ein neuartiger Stegocephalen−Fund aus dem oberessischen Buntsandstein. Palaönontologische Zeitschrift 14: 200–229.
Schoch RR, Fastnacht M, Fichter J and Keller T 2007. Anatomy and relationships of the Triassic temnospondyl Sclerothorax. Acta Palaeontologica Polonica 52 (1): 117–136.

wiki/Sclerothorax

Dvinia enters the TST

Not to be confused with Dvinosaurus (a basal tetrapod)…
Dvinia (Fig. 1) is a rabbit-sized chiniquodontid cynodont with a fang-pierced rostrum and a high cranial crest.

Figure 1. From Amalitskii 1922, Dvinia skull and mandible from various views slightly larger than actual size.

Dvinia prima (Amalitskii 1922; Late Permian, 254mya; 7 cm skull) nests between Chiniquodon and Pachygenelus in the Therapsid Skull Tree (TST, 69 taxa). The lower canine fit into a maxillary opening. The molars had a circle of cusps around a single large cusp. The postorbital is very tiny, a vestige that would be lost in derived taxa, like basal mammals and Pachygenelus. The lateral temporal fenestrae were huge housing strong jaw muscles, divided by a narrow crest in which a smal brain was located.

Ivakhenko 2013 reported:
“The study of the skull of the Late Permian cynodont Dvinia prima Amalitzky, 1922 shows a combination of the general primitive skull design (many incisors, preservation of the precanine and large interpterygoid fenestra, etc) with the development of a number of “advanced” features (expansion of the temporal fenestra, development of the parietal crest, and closed pineal foramen, unusual structure of the premaxilla, complicated postcanines, and reduction of the angular wing). Dvinia prima is treated as a specialized omnivore and assigned to the family Dviniidae Sushkin, 1928 of the superfamily Thrinaxodontoidea Seeley, 1894.”

Double canines
sometimes appear in theriodonts (gorgonpopsids, therocephalians, cynodonts) and other synapsids. The second is a replacement canine, so it is not a trait one can score in phylogenetic analysis.

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Sidenote:
My computer was in the shop for about 48 hours Friday and Saturday after downloading a virus originating from .tk (Tokelau, a territory of New Zealand located in the South Pacific famous for free domain registry and malicious web masters) that I downloaded when I clicked on a Facebook video that was supposed to show a sperm whale rotating underwater along with a diver. Do not click on that video.

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References
Amalitskii VP 1922. Diagnoses of the new forms of vertebrates and plants from the upper Permian of North Dvia: Bulletin de l’Académie des Sciences de l’URSS, Math and Natural Sciences, 1922, p. 329-340. and in Izv. Ross. Akad. Nauk, Ser. 6 25 (1), 1–12.
Ivakhnenko MF 2013. Cranial Morphology of Dvinia prima Amalitzky (Cynodontia, Theromorpha). Paleontological Journal 47( 2): 210–222. © Pleiades Publishing, Ltd., 2013. Original Russian Text published in Paleontologicheskii Zhurnal, 2013, No. 2:81–93.

wiki/Dvinia

Trypanognathus: the most primitive fingers and toes in the LRT

(Schoch and Voigt 2019 report on a new dvinosaur,
Trypanognathus (Fig. 1) preserving both cranial and post-cranial data down to a few tiny fingers and one preserved toe phalanx + four metatarsals). In the large reptile tree (LRT, 1441 taxa, subset Fig. 2).

Figure 1. Trypanognathus in situ, colorized to bring out ribs and limbs. Note the flat long body, flat wide head and small limbs. These are primitive traits shared with Panderichthys and Tiktaalik.

According to Schoch and Voigt 2019:
“The skull closely resembles that of the early Permian dvinosaurian genus Trimerorhachis in outline and suture topology, but the occiput and the palate differ substantially. Derived states are the penetration of vomerine tusks through the splenial and symphyseal
tusks through the premaxilla. Trypanognathus shares with dvinosauroids the lack of a squamosal embayment, an elongated basipterygoid process, a foreshortened palatine ramus exclusively reaching the ectopterygoid, the absence of pterygoid denticles, and enlarged palatal tusks. The body is elongate with well-ossified, but small limbs, the presacral count is circa 28, and the pleurocentra are large and reached ventrally almost as far as the intercentrum.”

Schoch and Voight nested dvinosaurs as derived taxa,
more derived than the basal lepospondyls, Neldasaurus, and Trimerorhachis, just the opposite of the LRT, which nests dvinosaurs as the most basal tetrapods with fingers and toes, instead of fins. Note the flat long body, flat wide head and small limbs. These are primitive traits shared with Panderichthys and Tiktaalik two taxa without fingers and toes.

Figure 2. Subset of the LRT focusing on the basal tetrapods including Trypanognathus.

It is traditional to consider Dvinosauria
as derived taxa within the clade Temnospondyli. Here in the LRT, after testing a wide gamut of basal tetrapods, the Dvinosauria is a basal clade/grade leading to all higher limbed tetrapods, including the clade Temnospondyli and Lepospondyli, and ultimately Reptilomorpha. The retention of robust gill bars along with primitive hands and feet are signs of their primitive status.

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References
Schoch RR and Voigt S 2019. A dvinosaurian temnospondyl from the Carboniferous-Permian boundary of Germany sheds light on dvinosaurian phylogeny and distribution. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2019.1577874.

Therocephalians evolved to smaller size? Large Carnivora did not?

Brocklehurst 2019 reports,
“If these results are reliable, they support the traditional paradigm that therocephalians originated as large predators, and only later evolved small body sizes. The patterns observed in mammals do not appear to apply to therocephalians. Mammalian carnivores, once they have reached large size and a specialized bauplan, are apparently unable to leave this adaptive peak. Therocephalians, on the other hand, retreated from the hypercarnivore niche and evolved small sizes later in the Permian.”

Figure 1. Cladogram from Brocklehurt 2019, colors added. Lycosuchus, listed as a basal therocephalian by Brocklehurst, also nests close to cynodonts in the TST. No gorgonopsids are shown here. Biarmosuchus is the outgroup taxon here, a more distant outgroup taxon in the TST.

Brocklehurst’s cladogram
posits that Therocephalia and Cynodontia arose as sisters from a last common ancestor: Biarmosuchus. In the therapsid skull tree (TST, 67 taxa, Fig. 4), Therocephalia (including Cynodontia) arises from Gorgonopsia (Fig. 2).

Figure 2. Gorgonopsids, therocephalians and cynodonts to scale.

The question arises,
what is a ‘large size’ member of the Carnivora? Certainly big cats and walruses (Fig. 3) fall into this definition and do not give rise to smaller ancestors, as Brocklehurst notes. However, if the basalmost member of the Carnivora, Vulpavus, is considered ‘large’ then it breaks the ‘rule’ because it has smaller descendants in the LRT: Mustela and Procyon (Fig. 3). Talpa, the mole, is the smallest member of the Carnivora in the LRT. Talpa has been traditionally omitted from Carnivora studies while being wrongly lumped with the unrelated shrew, Scutisorex, instead.

Figure 3. Carnivora to scale. Note: one branch does increase in size over time (ignoring toy poodles for the moment), while another branch, the one leading to Talpa the mole, shrinks in size. Brocklehurst is correct: once carnivores achieved large size, few to no examples of phylogenetic miniaturization appear in the fossil record.

I wish Brocklehurst 2019 had added
a few sample reconstructions to scale to help readers visualize the size ranges that he found in his cladogram. After all, the subject was ‘size’. I was unfamiliar with the vast majority of therocephalian taxa in his cladogram (Fig. 1).

Figure 4. TST revised with new data on Patranomodon and sister taxa. Here the therocephalian, Bauria, nests closer to cynodonts than in Brocklehurst 2019 (Fig. 1).

Brocklehurst is correct:
once carnivores achieved large size (Fig. 3), no examples of phylogenetic miniaturization subsequently appear. Brocklehurst contrasted this with therocephalians, presuming that Lycosuchus (Fig. 2) was a basal therocephalian, rather than a basal cynodont by definition.

Remember:
Hopson and Kitching 2001 defined  Cynodontia as the most inclusive group containing Mammalia, but excluding Bauria. In the TST (Fig. 4) Abdalodon and Lycosuchus nest on the cynodont side of Bauria.

In the TST
(Fig. 4), cynodonts show no strong size trends until mammals, like Megazostrodon (Fig. 2), evolved tiny sizes. Therocephalians likewise show no strong size trends either (but then, I have not measured every taxon in the Brocklehurt cladogram, Fig. 1). Those that also appear in the TST are in white boxes, and they appear in several clades within Therocephalia.

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References
Brocklehurst N 2019. Morphological evolution in therocephalians breaks the hyper carnivore ratchet. Proceedings of the Royal Society B 286: 20190590. http://dx.doi.org/10.1098/rspb.2019.0590