Origin of jawed vertebrates: the state of the art

Vaskaninova and Ahlberg 2017
studied the placoderm Radotina (similar to Romundina, Fig. 1) and described the state of the art regarding the phylogenetic placement of acanthodians and placoderms (Fig. 1) along with the origin of jaws (clade Gnathostomata, Fig. 2).

Figure 2. A sample of taxa related to Autroptyctodus with homologous skull bones color identified

Figure 2. A sample of taxa related to Autroptyctodus with homologous skull bones color identified

From their introduction:
“The early evolution of vertebrates has recently become a major research topic in vertebrate biology. One of the main areas of interest is the gnathostome (jawed vertebrate) stem group, which is important from both evolutionary and phylogenetic perspectives. In evolutionary terms, the gnathostome stem group encompasses the origin of jaws and associated major changes in facial architecture; in phylogenetic terms, it is a segment of the vertebrate tree whose content and topology has long been the subject of debate.”

Recently the large reptile tree (LRT, 1565 taxa) nested the whale shark (genus: Rhincodon) and catfish (genus: Clarias) +  placoderms (genus: Entelognathus) adding a novel hypothesis (Fig. 2) to that debate.

“A key development in the understanding of this stem group has been the recognition that the placoderms (armoured jawed fishes of Silurian to Devonian age), which until recently were regarded as a clade branching off the gnathostome stem group, probably form a paraphyletic segment of that stem group.”

The LRT recovers jawed placoderms, jawed sharks and other jawed fish at the base of their respective clades. Reduced and ventral jaws, as in sturgeons (genus: Pseudoscaphirhynchus), rays (genus: Rhinobatos) and placoderms (genus: Qilinyu) are derived, not basal taxa).

“Some groups of placoderms appear to be very primitive and close to jawless vertebrates whereas others possess what were previously regarded as osteichthyan autapomorphies (notably a maxilla, premaxilla and dentary) and are probably close to the gnathostome crown-group node.”

As mentioned above, but reordered.

“Associated with this reinterpretation of the placoderms has been the recognition of homologies between the macromeric dermal skeleton of placoderms and osteichthyans, previously regarded by most workers as independently evolved.”

The LRT supports the ‘homologous’ interpretation.

“Another Palaeozoic vertebrate group, the acanthodians (ªspiny sharksº), which were previously seen as stem osteichthyans or as a multiply paraphyletic array of stem gnathostomes, stem osteichthyans and stem chondrichthyans, are in the most recent analyses assigned in their entirety to the chondrichthyan stem group.”

The LRT nests acanthodians (genus: Ischnacanthus) between certain ray fins (genus: Cheirolepis) and other ray fins (genus: Tachinocephalus) and notes that other fins turn into spines in basal tetraodontiforms (genus: Plectocretacicus) and sticklebacks (genus: Gasterosteus).

“This leaves the upper part of the gnathostome stem group occupied entirely by placoderms, and gives rise to the idea of a ªplacoderm-osteichthyan continuumº, where osteichthyans essentially continued the gradual development of the placoderm bauplan whereas chondrichthyans departed more radically from it, inter alia by losing their perichondral and macromeric dermal bones.”

The LRT does not recover this hypothesis of relationships. Chondrichthyans (sharks, rays, chimaera and kin) have a sister group origin parallel to catfish + placoderms originating with a Silurian sister to the whale shark (genus: Rhincodon) arising from the jawless outgroup taxon, Thelodus.

Figure 1. Rhincodon typus, the extant whale shark, shares traits with jawless Thelodus, armored Entelognathus, and the walking catfish, Clarias.

Figure 2. Rhincodon typus, the extant whale shark, shares traits with jawless Thelodus, armored Entelognathus, and the walking catfish, Clarias.

“Detailed studies of primitive placoderms therefore have the potential to provide crucial information about the early evolutionary steps on the path leading to our own body plan.”

Key to understanding the origin of jaws is the inclusion of whale sharks and catfish, two taxa that have been traditionally excluded.


References
VasÏkaninová V, Ahlberg PE 2017. Unique diversity of acanthothoracid placoderms (basal jawed vertebrates) in the Early Devonian of the Prague Basin, Czech Republic: A new look at Radotina and Holopetalichthys. PLoS ONE 12(4): e0174794. https://doi.org/10.1371/journal.pone.0174794

wiki/Gnathostomata

Almaco jack + phylogenetic miniaturization = Plectocretacicus

Plectocretacicus clarae (Sorbini 1996, Fig. 1; early Late Cretaceous; about 3cm) is traditionally known as the earliest known tetraodontiform, the clade that includes queen trigger fish, ocean sunfish and pufferfish. Earlier the large reptile tree (LRT, 1565 taxa) nested the extant amberjack, Seriola rivoliana (Fig. 2) basal to this clade, a novel nesting.

Figure 1. Plectocretacicus skull and overall. This tiny fish is the phylogenetically miniaturized transitional taxon linking amberjacks and tetraodontiformes.

Figure 1. Plectocretacicus skull and overall. This tiny fish is the phylogenetically miniaturized transitional taxon linking amberjacks and tetraodontiformes. The pelvic fins have turned into spines.

Tiny Plectocretacicus is about 3m long,
about 1/30 the length of the 90 cm Almaco jack. We’ve already seen phylogenetic miniaturization at the genesis of many, many vertebrate clades. This appears to be one more case of that.

Figure 3. Seriola rivoliana is the high fin Amberjack is basal to gobies and tetraodontiformes.

Figure 2. Seriola rivoliana is the alamo jack or high fin Amberjack is basal to gobies and tetraodontiformes. Typical length = 90cm.

Distinguishing traits,
such as the spine-like pectoral fins, show that Plectocretacicus had a more ancient sister with more plesiomorphic traits. The radiation of current sisters would also have preceded the Late Cretaceous. Acanthodians (spiny sharks) also reduce their ray fins into spikes by convergence.

Seriola rivoliana (Valenciennes 1833; 90cm) is the extant Almaco jack or high-fin amberjack. This sister to Seriola zonata nests at the base of the Tetraodontiformes. Subtle differences separate these two species. Note the tall dorsal and anal fins, further exagerated in Mola(below). Pelvic fins are lost in derived taxa.


References
Sorbini L 1979. Segnalazione di un plettognato Cretacico Plectocretacicus nov. gen. Bollettino del Museo Civico di Storia Naturale di Verona, 6:1–4.
Tyler JC and Sorbini L1996. New Superfamily and Three New Families of Tetraodontiform Fishes from the Upper Cretaceous: The Earliest and Most Morphologically Primitive Plectognaths. Smithsonian Contributions to Paleobiology. 82: 1–59.

Geographic cladogram of pterosaurs

So many pterosaurs come from so few places.
And those places are spread around the world. So, here (Fig. 1) is the large pterosaur tree (LPT, 239 taxa) with color boxes surrounding Solnhofen, Chinese, North American, South American and other geographic areas where they are found.

Figure 1. LPT with pterosaurs colorized according to geography.

Figure 1. LPT with pterosaurs colorized according to geography.

As before,
the traditional clades ‘Pterodactyloidea’ and ‘Monofenestrata‘ become polyphyletic when traditionally omitted taxa are included. Here (Fig. 1) four clades achieve the pterodactyloid-grade by convergence. Other pterosaur workers (all PhDs) omit or refuse to include most of these taxa, leading to false positives for the tree topologies they recover. Moreover, none recognize, nor cite literature for, the validated outgroup members for the Pterosauria (Fig. 1) preferring to imagine pterosaurs arising from unidentified and/or invalidated archosaurs or archosauriforms. Here we get to peak beneath the curtain.


References
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.

It’s hammerhead time!

One of the smaller hammerhead sharks,
Sphyrna tutus (Figs. 1, 2), enters the LRT alongside Isurus, the mako shark. No surprise. The interesting thing about hammerhead sharks is their heads! What is going on inside that sharkskin?

Figure 1. The small hammerhead shark, Sphyrna tutus, is best appreciated in dorsal or ventral view.

Figure 1. The small hammerhead shark, Sphyrna tutus, is best appreciated in dorsal or ventral view.

In hammerhead sharks of all sorts,
the eyes and nares are widely separated by the lateral expansion of the nasals, prefrontals and postfrontals creating the classic hammerhead cephalofoil (Fig. 2), made entirely of cartilage, not bone. Even so, each piece has a bone analog, despite the fusion of several elements, especially in the jaws (Figs. 1, 2).

Figure 2. Skull of Sphyrna tutus in three views from Digimorph. org and used with permission. Colors added.

Figure 2. Cartilaginous skull of Sphyrna tutus in three views from Digimorph. org and used with permission. Colors added.

Sphyrna tudes (orignally Zygaena tudes Valenciennes 1822; 1.3m in length) is the extant smalleye hammerhead shark. It prefers muddy habitats with poor visibility. As a juvenile Sphyrna prefers shrimp, then grows up to prefer catfish eggs. Gestation is 10 months. Females produce 19 pups each year.


References
Valenciennes A 1822. Sur le sous-genre Marteau, Zygaena. Memoires du Museum National d’Histoire Naturelle. 9: 222–228.

wiki/Sphyrna

 

Heterohyus enters the LRT

Figure 1. Two of several Heterohyus specimens from the Messel Pit of Germany.

Figure 1. Two of several Heterohyus specimens from the Messel Pit of Germany.

Heterohyus nanus (Gervais 1848, late Eocene) from the Messel Pit in Germany nests with Apatemys in the large reptile tree (LRT, 1563 taxa). Relatively few traits differentiate the two. The lumbar region is shorter. The skull is larger. The naris is smaller and higher on the rostrum.

According to Wikipedia members of the Apatemyidae were
“small and presumably insectivorous. Size ranged from that of a dormouse to a large rat. The toes were slender and well clawed, and the family were probably mainly arboreal.[2] The skull was fairly massive compared to the otherwise slender skeleton, and the front teeth were long and hooked, resembling those of the modern aye-aye and marsupial Dactylopsila, both whom make their living by gnawing off bark with their front teeth to get at grubs and maggots beneath.”

The LRT nests apatemyids
as basal members of Glires, not related to other so-called cimolestids, a polyphyletic assembly of placental taxa. according to the LRT. Rather apatemyids are a clade of gnawing tree shrews, now extinct.


References
Gervais P 1848–52. Primates, Microchoeridae? Zoologie et Paléontologie Françaises. Paris, Arthrus Bertrand, text, 2 vols; atlas, 80 pls.

wiki/Heterohyus
wiki/Apatemyidae

 

Estimating weight for Quetzalcoatlus

Short one today.
Imagining the weight of a pterosaur known from bits and pieces requires extrapolation from data provided by smaller, but similar hollow-boned vertebrates. A graph of height vs. weight in large pterosaurs and large birds (Fig. 1) might help, but estimates at the far end still vary greatly. As you can see, hollow bones make a big difference. It may be surprising, that a stork at 1.3 m (5 ft) tall weighs about 5 kilograms (15 lbs). Pterosaurs were similar.

Figure 1. Estimating giant azhdarchid weight from estimated height and comparables with similar smaller taxa.

Figure 1. Estimating giant azhdarchid weight from estimated height and comparables with similar smaller taxa.

Earlier we looked at the short wings with vestigial distal phalanges present in Quetzalcoatlus, removing the big ones from the possibility of flying, as in giant birds.

In conclusion,
does it make sense that the smaller Q. sp. weighed no more than 15kg (33 lbs)? Does it make sense that the larger one, Q. northropi, known from fewer bones, might weigh between an unlikely 20kg (44 lbs) to a more reasonable 125 kg (275 lbs)? Here are some other largest pterosaurs for more comparisons (Fig. 2).

Figure 1. Click to enlarge. The largest flying and non-flying birds and pterosaurs to scale.

Figure 1. Click to enlarge. The largest flying and non-flying birds and pterosaurs to scale.

 

Dinosaurs Rediscovered, new book by Dr. Michael Benton

FIgure 1. Dinosaurs rediscovered by MJ Benton book cover.

FIgure 1. Dinosaurs rediscovered by MJ Benton book cover.

Dr. Stephen Brusatte wrote
on the cover: “If you want to know how we know what we know about dinosaurs, read this book!”

‘Amazon Customer’ wrote
at the book’s website, “Nice production, but highly biased and speculative.” (more below)

Dr. MJ Benton is professor of vertebrate paleontology and head of the Palaeobiology Research Group at the U of Bristol, England. He has written more than fifty books, including the standard textbooks in palaeontology.

From the intro:
“One by one the speculations about evolution, locomotion, feeding, growth, reproduction, physiology, and, finally, color have fallen to the drive of transformation. A new breed of dinosaur palaeobiologist replace the older ones, and they have applied a hard eye to the old speculations. Smart lateral thinking, new fossils, and new methods of computation have stormed the field.”

Funny though,
Scleromochlus (Fig. 2) is not mentioned. Benton 1999 promoted this genus close to the origin of pterosaurs and in the book he maintains that pterosaurs remain close to the origin of dinosaurs with no further explanation. Evidently Scleromochlus is no longer in favor. Nearly 20 years ago Peters 2000 invalidated the pterosaurs-close-to-dinosaurs = ornithodire hypothesis by testing Benton’s cladogram and three others by simply adding taxa overlooked and poorly scored by Benton and other prior authors. But let’s move on…

Figure 3. According to the AMNH, Scleromochlus is "one of the closest early cousins of pterosaurs." Oddly, they gave it the skull of Longisquama. Note the vestigial hands. These cannot elongate to become wings and pedal digit 5 is a vestige that cannot elongate to match basal pterosaurs.

Figure 2. According to the AMNH, Scleromochlus is “one of the closest early cousins of pterosaurs.” Oddly, they gave it the skull of Longisquama. Note the vestigial hands. These cannot elongate to become wings and pedal digit 5 is a vestige that cannot elongate to match basal pterosaurs.

Chapter 1 — Origin of the Dinosaurs
Even in 2019, Benton writes, “One thing is known for sure: the dinosaurs originated during the Triassic period, between 252 and 201 million years ago. Nearly everything else is uncertain.” This is not exactly a teaser, because it does not jive with what Benton writes earlier (Benton 1999) and later (see below).

Benton reports
that he raised traditional eyebrows back in 1983 when he suggested the old standard model of one group/clade giving way to another should be replaced with a scenario in which new clades only appeared and/or radiated after an extinction event. This view makes great sense and is supported by strong evidence. Ironically, Benton reports, “This new idea of mine was probably quite annoying for the established paleontologists.” Now that he’s older, the tables have turned and it’s Benton’s turn to be annoyed. Philosophically he has taken the place of his 1983 opponent and mentor, Dr. Alan Charig in that Benton now refuses to consider, test or replace invalid scenarios with new ones.

Let’s not forget…
in his unbiased youth, Benton 1985 used an early form of phylogenetic analysis to show that pterosaurs were sister taxa to lepidosaurs, closer to lizards than to dinosaurs by a long shot. Now that this hypothesis has become heterodox, he and others have avoided it ever since by selective deletion of pertinent taxa.

Figure 2. Cladogram from Benton 1985 in which he nests pterosaurs closer to lepidosaurs than to dinosaurs and other archosaurs.

Figure 2. Cladogram from Benton 1985 in which he nests pterosaurs closer to lepidosaurs than to dinosaurs and other archosaurs. Lots of confusion here due to taxon exclusion going back to the advent of Reptilia (= Amniota).

A subchapter follows
on the lepidosaur rhynchosaur, Hyperodapedon (Benton 1983), where Benton first published on taxa he was given to worth with, but made the phylogenetic mistake of lumping rhynchosaurs with archosauromorphs. This was due to taxon exclusion.

The next subchapter, “What was the first dinosaur?”
Benton correctly identifies one of the first dinosaurs as Herrerasaurus. That agrees with the large reptile tree (LRT, 1562 taxa) which uses the last common ancestor method for determining clade member inclusion.

Basal bipedal dinosauriformes, from Lagerpeton through Marasuchus, Lewisuchus, Asilisaurus, Sacisaurus and Silesaurus.

Figure 3. Basal bipedal ‘dinosauriformes’, from Lagerpeton through Marasuchus, Lewisuchus, Asilisaurus, Sacisaurus and Silesaurus, according to Nesbitt (2011). The LRT does not support this listing or sequence.

Then Benton reports on the poposaur
dinosaur-mimic, Silesaurus (Fig. 3), the Early Triassic ichnite Prorotodactylus, and another poposaur dinosaur-mimic, Asilisaurus (Fig. 3). Benton reports, “The discovery of Asilisaurus unequivocally re-dated the origin of dinosaurs back from 230 to 245 million years ago, or older.” There is little to differentiate Asilisaurus from Silesaurus. Both remain poposaurs and dinosaur-mimics, unrelated to the dinosaurs, except through basal bipedal crocodylomorphs, which Benton avoids. So, taxon exclusion strikes Benton, once again.

Quote here, an anonymous, but well-educated, review from Amazon.com:
“Dinosaurs Rediscovered is an engagingly written and highly personalized account of dinosaurs, generally covering the field’s perceived advances from 1980 to the present. The publisher Thames & Hudson did an outstanding job in producing the book, formatting, and in the selection of paper.

“The author notes that the field transformed from 1984 onwards by cladistic methods, and the resulting phylogenetic trees or cladograms have thus become the “basis” for evaluating evolutionary models and all things dinosaurian, including anatomical reconstructions, physiology, behaviour, etc. The work described is rather restricted, with most emphasis given to the University of Bristol’s vertebrate palaeontologists, often ignoring important discoveries from other groups, and regrettably ignoring most conflicting evidence. The most egregious is the complete omission of any discussion of the persisting problem of dinosaur/avian digital homology.

“Benton begins with the discovery (in his laboratory) of microsomes known as melanosomes from SEMs of fibers from the back of the small theropod Sinosauropteryx, that were described as “proto-feathers” back in 1998. However, there was never any evidence that the fibers had any feather affinity, and many who studied the specimens found an external coating of small tubercular scales above the layer of fibers —- since prepared away and lost! It is clear, however, that the fibers called proto-feathers or “dino-fuzz” were beneath the skin and therefore not feathers. Too, as South African palaeontologist Lingham-Soliar showed in several important papers (not cited) the structures called melanosomes cannot be interpreted from the scanning electron micrograph (p. 8) as being within the fibers. Speculation!

“Plate V shows a fuzzy Sinosauropteryx with a ring tail like that of a civit or ring-tailed cat!! Then there is an outlandish image of a reconstruction of the Jurassic urvogel Anchiornis (incorrectly called a troodontid, see Pei ref below), as a terrestrial animal; but the feathers emanating from the legs and feet would have been a hindrance in ground locomotion. New fossil images (Pei et al., 2017 AMNH Bull 411, 66 pp) show claws consistent with tree-trunk climbing, similar to those of other urvogels. Plate VI shows photos of a “dinosaur tail” in amber, but there is NO evidence it is from a dinosaur and is most likely an enantiornithine bird.

“The section on dinosaur evolution is straight forward, but laden with speculation, and given the massive convergence among various archosaur lineages during the Triassic it is difficult to have full faith in the interpretations; and authors from Cambridge and the British Museum have questioned the time-honored phylogeny (pp. 82-84).

“Most of the remainder of the book is a romp through the various dinosaurian groups, with comments on everything from brains and internal organs to behaviour. Archaeopteryx is depicted as an earth-bound runner (p. 112), with open wings (like no living avian cursor – e.g. capercallie, chicken, etc.), despite the fact that Manchester’s Derek Yalden showed conclusively that the urvogel’s claws were those of a trunk-climber, quite similar in structure to those of woodpeckers and climbing mammals.

“Benton notes (122) without reservation that Sinosauropteryx “was the first dinosaur to have its feather colour determined”—-and on page 123 he shows a feathered Caudipteryx with avian wing feathers and notes “it is clearly a theropod and not a bird” in contrast to numerous papers arguing that it is a secondarily flightless bird. If not, flight feathers, a perfection of aerodynamic engineering, would have to evolve in a non-flight context, a real stretch of biological thought!

“In chapter 5 “Jurassic Park” he seems ambivalent about reconstructing dinosaurs from ancient DNA, although most would agree that it is impossible. Certainly Mary Schweitzer’s supposed discovery of T. rex blood vessels and proteins has been firmly refuted. He comments on small genome size in birds and dinosaurs, but the studies conflated the two groups, and small genome size is to be found in flying animals: bats, pterosaurs and birds. Growth studies on dinosaurs are discussed but much of that has recently been brought into question. Allosaurus (188) and Tyrannosaurus, with no evidence, are shown with a feathered coat! Diplodocus (210) is shown with neck high in the air, a posture disputed by computer-generated imaging. Benton appears to favor the model of flight origin of Dial and Heers, but such a model requires a fully developed flight apparatus, and both putative dinosaurian ancestors of birds, urvogels, and even archosaurian antecedents, all lacked the pectoral architecture to enact this model. It just will not work. Much speculation!

“Finally, although there is no citation in the text, the monolithic bibliographic listing in the section on ‘Further Reading’ is alarming; it appears highly selected to bolster the Bristolian view of dinosaurs, while ignoring any contrary views, many of which are supported by solid scientific data. Most disturbingly, the discoveries by Chinese palaeontologists, especially those at Beijing’s Institute of Paleontology and Paleoanthropology, which in reality propelled the recent revolution in our knowledge of dinosaur/bird evolution is largely ignored.”

Conclusion:
Dr. Benton’s new book gave us old, misguided and too often invalid information. In 2019 we know better how taxa are related to one another and Benton should have known better, too. Taxon exclusion (= phylogenetic context) seems to be his number one problem because his descriptions and illustrations of specimens are typically excellent. After messing up on his first paper (removing rhynchosaurs from rhynchocephalians), Benton’s reputation and output continue to be tarnished with his latest book and many of his recent papers all due to taxon exclusion. On the other hand, and in the present climate, Dr. Benton understands there is no consequence for ignoring new hypotheses. If only he could recall what it was like for him back in 1983, trying to promote his own new scenario to the establishment.

Those paleo professionals who wrote glowing reports
for this book should also have known better, but allegiance can sometimes trump good science. Author and paleontologist, Stephen Brusatte (quoted above) was a student at Bristol, where Benton teaches.

A wide gamut phylogenetic analysis based on specimens
is a necessary ingredient before, during and after any specimen description. It remains the one and only way to minimize taxon exclusion.


References
Benton MJ 1983. The Triassic reptile Hyperodapedon from Elgin, functional morphology and relationships. Philosophical Transactions of the Royal Society of London, Series B, 302, 605-717.
Benton MJ 1985. Classification and phylogeny of diapsid reptiles. Zoological Journal of the Linnean Society 84: 97-164.
Benton MJ 1999. Scleromochlus taylori and the origin of the pterosaurs. Philosophical Transactions of the Royal Society London, Series B 354 1423-1446. Online pdf
Benton MJ 2019. Dinosaurs rediscovered. Thames & Hudson.
Nesbitt SJ, Sidor CA, Irmis RB, Angielczyk KD, Smith RMH and Tsuji LMA 2010. Ecologically distinct dinosaurian sister group shows early diversification of Ornithodira. Nature 464 (7285): 95–98. doi:10.1038/nature08718. PMID 20203608.
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.

The snakebird lacks external nares, breathes through its mouth

Figure 1. Skull of Anhinga rufa, an Old World relative of the New World Anhinga anhinga. Note the expansion of the maxilla (or overlying horny tissue) nearly obscuring the naris and antorbital fenestra. Compare to the loon in figure 3.

Figure 1. Skull of Anhinga rufa, an Old World relative of the New World Anhinga anhinga. Note the expansion of the maxilla (or overlying horny tissue) nearly obscuring the naris and antorbital fenestra. Compare to the loon in figure 2.

Anhinga anhinga (Linneaus 1766; 89cm) is the extant snakebird, which swims underwater and stabs its fish prey with its sharp beak, striking like a snake. It breathes only through the mouth as the bones and other hard tissues around the nostrils are overgrown. The feathers do not shed water, so some time is spent drying the feathers prior to flying. Snakebirds are related to grebes (genus: Aechmophorus) and loons (genus: Gavia, Fig. 2).

Figure 2. Skull of the common loon (Gavia stellata) showing the primitive state, with large external nares and antorbital fenestra.

Figure 2. Skull of the common loon (Gavia stellata) showing the primitive state, with large external nares and antorbital fenestra.

The large number and length of cervical vertebrae
in snakebirds (Fig. 3) is more or less matched only by flamingoes (genus: Phoenicopterus) by convergence.

Figure 3. Anhinga anhinga skeleton. Note the large number of cervical vertebrae. These enable the snake-like darting of the sharp skull while attacking prey underwater.

Figure 3. Anhinga anhinga skeleton. Note the large number of cervical vertebrae. These enable the snake-like darting of the sharp skull while attacking prey underwater.

Hackett et al. 2008 nested loons with penguins.
While close, the large reptile tree (LRT, 1562 taxa) nests loons + grebes derived from terns (genus: Thalasseus) and sisters to kingfishers (genus: Megaceryle) + jabirus (genus: Jabiru) and murres (genus: Uria) + penguins (genus: Aptenodytes). Among these taxa, only Jabiru experiences a reversal in having such long, stork-like legs, a primitive trait for extant birds.

Figure 1. The rostrum of Spinosaurus. Note the maxilla rising to close off the elongate naris into a reduced anterior and posterior opening.

Figure 4. The rostrum of Spinosaurus. Note the maxilla rising to close off the elongate naris into a reduced anterior and posterior opening.

Footnote:
Another aquatic dinosaur taxon that expanded its maxilla to shut off its nostrils was Spinosaurus (Fig. 4) as we learned earlier here.


References
Hackett S et al. 2008. A phylogenetic study of birds reveals their evolutionary history. Science 320:1763–1768.
Kennedy M et al. 2019. Sorting out the Snakebirds: The species status, phylogeny, and biogeography of the Darters (Aves: Anhingidae). Journal of Zoological Systematics and Evolutionary Research (advance online publication)
doi: https://doi.org/10.1111/jzs.12299 https://onlinelibrary.wiley.com/doi/10.1111/jzs.12299

The tiny croc that can’t keep its trap shut: Orthosuchus

It’s tiny and well known from 3D skeletal material.
And yet, for a long time Orthosuchus (Fig. 1) was a bit of an enigma in traditional paleontology.

Figure 1. The well-known skull of tiny Orthosuchus. Note the concave maxilla and dentary, resulting in a large gap.

Figure 1. The well-known skull of tiny Orthosuchus SAM-PK-K409. Note the concave maxilla and dentary, resulting in a large gap. Was the maxilla deformed? Maybe. Note the mid-skull position of the internal nares (choanae). Skull shown more than twice natural size. Dentary based on BP/1/7979,

Orthosuchus stormbergi (Nash 1968; Early Jurassic 196mya; 60cm in length; SAM-PK-K409, Fig. 1) is a tiny crocodiliform close to Protosuchus, but closer to Sichuanosuchus (Fig. 2) in the large reptile tree (LRT, 1562 taxa). The low flat skull is covered in shallow pits. The upper and lower jaws attempt to close but with a large gap between them. One wonders if long, filter-feeding teeth bridged that gap. The choanae are midway betwen the nose and throat.

Figure 2. Sichuanosuchus is another small protosuchid croc.

Figure 2. Sichuanosuchus is another small protosuchid croc.

Dollman et al. 2017 recently reported,
“The phylogenetic position of Orthosuchus is not solidified, it has previously been recovered as either a basal ‘protosuchian’ (Pol et al. 2004) or derived ‘protosuchian’ (Pol et al. 2014) just outside of Protosuchidae.” No mention was made of Sichuanosuchus in the text.

Later, Dollman et al. 2018
did mention and nest Sichuanosuchus more or less close to Protosuchus and Orthosuchus, and also close to Fruitachampsa.

Figure 3. Subset of the LRT with the addition of Lagosuchus next to Saltopus among the basal bipedal Crocodylomorpha. The nesting of skull-only Yonghesuchus near the skull-less taxa provides clues to the morphology of the skulls in the headless taxa.

Figure 3. Subset of the LRT with the addition of Lagosuchus next to Saltopus among the basal bipedal Crocodylomorpha. The nesting of skull-only Yonghesuchus near the skull-less taxa provides clues to the morphology of the skulls in the headless taxa.

References
Dollman KN, Vigliett PA and Choiniere JN 2017. A new specimen of Orthosuchus stormbergi (Nash 1968) and a review of the distribution of Southern African Lower Jurassic crocodylomorphs. Historical Biology 31(5):653–654.
Dollman KN, ClarkJM, Norell MA, Xu X and Choiniere JN 2018. Convergent evolution of a eusuchian-type secondary palate within Shartegosuchidae. American Museum Novitates. 3901: 1–23.
Nash D 1968. A crocodile from the Upper Triassic of Lesotho. Journal of Zoology. London 156:163–179.
Nash DS 1975. The Morphology and Relationships of a Crocodilian, Orthosuchus stormbergi, from the Upper Triassic of Lesotho. Annals of the South African Museum 67: 227-329.

wiki/Protosuchus
wiki/Orthosuchus
wiki/Sichuanosuchus

Quetzalcoatlus wingspan compared to other azhdarchids

There are those who think
the giant azhdarchid pterosaur, Quetzalcoatlus (Fig. 1), was flightless. Almost all others think Quetzalcoatlus was the largest flying animal of all time. The question is: were the wings of Quetzalcoatlus large enough to initiate and sustain flight?

Sometimes it just helps to compare
azhdarchids to azhdarchids to azhdarchids. In this case we’ll compare Quetzalcoatlus in dorsal view to two azhdarchids so small that traditional paleontologists don’t even consider them to be azhdarchids. BSPG 1911 I 31, (Figs. 2, 3) is a traditional, small volant pterosaur with a long neck and a standard pterosaur wingspan. JME-Sos 2428 (Fig. 2) is an odd sort of flightless pterosaur with a very much reduced wingspan. Neither of these taxa seems to ever make it to the cladograms of other workers.

Figure 1. Quetzalcoatlus in dorsal view compared to two much smaller azhdarchids from the Solnhofen formation, JME-Sos 2428, a flightless pterosaur, and BDPG 1911 I 31, a volant pterosaur. The wingspan of Quetzalcoatlus does not match that of the much smaller azhdarchid, so perhaps the giant was unable to fly. At least, this is the evidence for flightlessness.

Figure 1. Quetzalcoatlus in dorsal view compared to two much smaller azhdarchids from the Solnhofen formation, JME-Sos 2428, a flightless pterosaur, and BDPG 1911 I 31, a volant pterosaur. The wingspan of Quetzalcoatlus does not match that of the much smaller azhdarchid, so perhaps the giant was unable to fly. At least, this is the evidence for flightlessness.

When you compare azhdarchids to azhdarchids to azhdarchids
you get the overwhelming impression that IF Quetzalcoatlus was volant, it would not have reduced the distal wing phalanges so much. And yet it did, just like other flightless pterosaurs did. Since weight increases by the cube as size in dorsal view increases by the square, the wings of the giant should actually be larger than those of the smaller azhdarchid to handle the relatively larger mass.

So what did Quetzalcoatlus use its flightless wings for?
Thrust (Fig. 2).

Quetzalcoatlus running like a lizard prior to takeoff.

Figure 2. Quetzalcoatlus running like a lizard prior to takeoff. Click to animate.

Quetzalcoatlus and its ancestor, no 42, note scale bars.

Fig. 3. Quetzalcoatlus and its ancestor, BSPG 1911 I 31, note scale bars. At 72dpi, the pterosaur on the left is nearly full scale on a monitor. The one on the right is as tall as a tall human, with giant relatives more than doubling that height. 

Contra tradition, the azhdarchid bauplan
was initiated with Late Jurassic small pterosaurs like BSPG 1911 I 31, so misbegotten  that traditional paleontologists have forgotten to give it its own generic and specific name distinct from the wastebasket taxon Pterodactylus, with which it is not related, as we learned earlier here.


References
Kellner AWA and Langston W 1996. Cranial remains of Quetzalcoatlus (Pterosauria, Azhdarchidae) from late Cretaceous sediments of Big Bend National Park, Texas. – Journal of Vertebrate Paleontology 16: 222–231.
Lawson DA 1975. Pterosaur from the latest Cretaceous of West Texas: discovery of the largest flying creature. Science 187: 947-948.
Witton MP and Habib MB 2010. On the size and flight diversity of giant pterosaurs, the use of birds as pterosaur analogues and comments on pterosaur flightlessness. PloS one, 5(11), e13982.

More data here: why-we-think-giant-pterosaurs-could-fly-not/

wiki/Quetzalcoatlus