Pseudhesperosuchus fossil photos

Earlier I used
Greg Paul and José Bonaparte drawings of the basal bipedal croc Pseudhesperosuchus Bonaparted 1969) for data on this taxon. The specimen has some traits that lead toward the secondarily quadrupedal Trialestes. Together they are part of a clade that is closer to basal dinosaurs than traditional taxa paleontologists have been working with.

The drawings were great,
but I wondered what the real material looked like…and more importantly, what was real and what was not.

A recent request to
the curators at Miguel Lillo in Argentina was honored with a set of emailed jpegs from their museum drawers (Fig.1), for which I am very grateful. These were traced in line and color and reassembled with just a few unidentified parts left over (Fig. 2).

Figure 1. GIF movie of the skull of Pseudhesperosuchus showing the original drawing, the fossil and DGS tracings of the bones.

Figure 1. GIF movie of the skull of Pseudhesperosuchus showing the original drawing, the fossil and DGS tracings of the bones.

Pseudhesperosuchus jachaleri (Bonaparte 1969 Norian, Late Triassic ~210mya, ~1 m in length, was derived from a sister to Junggarsuchus and  Lewisuchus and was at the base of a clade that included Trialestes on one branch and the Dinosauria on the other branch.

Figue 1. A new reconstruction of the basal bipedal croc, Pseudhesperosuchus based on fossil tracings. Some original drawings pepper this image. Note the interclavicle, missing in dinosaurs and the very small ilium, only wide enough for two sacrals. The posterior dorsals are deeper than the anterior ones.

Figue 2. A new reconstruction of the basal bipedal croc, Click to enlarge. Pseudhesperosuchus based on fossil tracings. Some original drawings pepper this image. Note the interclavicle, missing in dinosaurs and the very small ilium, only wide enough for two sacrals. The posterior dorsals are deeper than the anterior ones.

Much larger and distinct from Lewisuchus,
the skull of Pseudhesperosuchus had a smaller antorbital fenestra, an arched lateral temporal fenestra, a deeper maxilla and a large mandibular fenestra. The seven cervicals were attended by robust ribs.

The scapula and coracoid were each rather slender and elongated. An straight interclavicle was present. The forelimbs were long and slender. The radiale and ulnare were elongated, a croc trait. Only three metacarpals and no digits are known.

The ilium was relatively small, but probably longer than tall and not perforated. The femur remained longer than the tibia. The tarsus, if that astragalus is identified correctly, included a simple hinge ankle joint. Only two conjoined partial metatarsals are known.

There is a small box
full of little sometimes interconnected squares among the Pseudhesperosuchus material (Fig. 2, aqua colored). I’m guessing that those are osteoderms, and if so, were probably located along the back. These would have helped keep that elevated backbone from sagging in this new biped.

The improvements in the Pseudhesperosuchus data
changed a few scores, but did no change the tree topology. The large reptile tree (LRT) can be seen here.

It’s good to see what Pseudhesperosuchyus really looked like,
— or at least get a little closer to that distant ideal. Size-wise and morphologically, this largely complete specimen is closer to the basal dinosaur outgroup than any other currently included in the LRT. And yet it is also distinctly different as it shares several traits with Trialestes unknown in any dinosaur. As a denizen of the Late Triassic, Pseudhesperosuchus represents a radiation that occurred tens of millions of years earlier, probably in the Middle Triassic. None of this clade survived into the Jurassic, as far as we know.

References
Bonaparte JF 1969. Dos nuevos “faunas” de reptiles triásicos de Argentina. Gondwana Stratigraphy. Paris: UNESCO. pp. 283–306.

Origin of bipedalism in dinosaurs: Overlooking Carrier’s Constraint

Persons and Currie 2017 debunk on old theory
on bipedalism in dinosaurs and introduce a new one that suffers from taxon exclusion while overlooking a very popular theory from the last thirty years: Carrier’s Constraint (Carrier 1987).

From the abstract:
“Bipedalism is a trait basal to, and widespread among, dinosaurs. It has been previously argued that bipedalism arose in the ancestors of dinosaurs for the function of freeing the forelimbs to serve as predatory weapons.”

I never heard of this reason before. Predatory weapons only happen as a result and much later phylogenetically and only sometimes.

“However, this argument does not explain why bipedalism was retained among numerous herbivorous groups of dinosaurs. We argue that bipedalism arose in the dinosaur line for the purpose of enhanced cursoriality.”

The term ‘enhanced’ is pretty vague. Does it mean ‘better’? But can that be proven? The fastest animals on land now are quadrupedal cheetahs. Bipedal Chlamydosaurus does not have greater speed or endurance. Persons and Currie bring up the “tripping on one’s own forefeet” hypothesis and that, IMHO, has some validity.

“Modern facultatively bipedal lizards offer an analog for the first stages in the evolution of dinosaurian bipedalism. Many extant lizards assume a bipedal stance while attempting to flee predators at maximum speed.”

But quadrupedal lizards are just as fast as bipedal ones. Lizards gain no speed when switching to bipedal locomotion as Persons and Currie also note.

Bipedal lizard video marker

Figure 1. Click to play video. Just how fast can quadrupedal/bipedal lizards run? This video documents 11 meters/second in a Callisaurus at the Bruce Jayne lab.

“Bipedalism, when combined with a caudofemoralis musculature, has cursorial advantages because the caudofemoralis provides a greater source of propulsion to the hindlimbs than is generally available to the forelimbs.”

Yes, at first, especially when the forelimbs are lifted from the ground! Persons and Currie stay clear of the bipedal ability of fenestrasaurs including pterosaurs. There, in taxa like Cosesaurus, the driving force switches to the hips.

“That cursorial advantage explains the relative abundance of cursorial facultative bipeds and obligate bipeds among fossil diapsids and the relative scarcity of either among mammals.”

Actually there is no abundance of bipeds anywhere among diapsids, except in the Fenestrasauria (not related to archosaur-line diapsids) and Archosauria + Poposauria. Persons and Currie also stay clear of the inverted bipeds among mammals, the bats, and they are numerous.

None of the so-called ‘reasons’ why are pertinent
without the random evolution of longer hind limbs than forelimbs and the ability to balance over the hind limbs, whether running or standing still. It also helps to have even a small anterior addition to the ilium, according to Shine and Lambeck 1989. The pubic foot of theropods and the prepubis of pterosaurs also provide femoral muscle anchors.

Unfortunately

  1. Persons and Currie do not indicate the node at which bipedalism arose in the last common ancestor of bipedal crocs and dinosaurs: Gracilisuchus and Turfanosuchus at the base of the Poposauria. In the large reptile tree (LRT)  Gracilisuchus (Fig. is the last common ancestor of bipedal crocs, like Scleromochlus, and bipedal pro-dinosaurs, like Lewisuchus.
  2. Persons and Currie subscribe to the outdated hypothesis of “Avemetatarsalia” in which former members, like pterosaurs now nest with lepidosaurs and Lagerpeton now nests with chanaresuchids.
  3. Persons and Currie also avoid the likely bipeds, Arizonasaurus and Postosuchus.
  4. Persons and Currie discuss the the likely biped, Eudibamus, but incorrectly ascribe it to the bolosaurs.
  5. Persons and Currie overlooked Carrier’s Constraint, which holds that,“air-breathing vertebrates which have two lungs and flex their bodies sideways during locomotion find it very difficult to move and breathe at the same time, because the sideways flexing expands one lung and compresses the other, shunting stale air from lung to lung instead of expelling it completely to make room for fresh air.” — but is that the reason to go bipedal? or just the first and biggest advantage narrow-gauge bipedal reptiles enjoy?
Click to enlarge. Squamates, tritosaurs and fenestrasaurs in the phylogenetic lineage preceding the origin of the Pterosauria.

Figure 2. Tritosaurs and fenestrasaurs in the phylogenetic lineage preceding the origin of the Pterosauria.

What fenestrasaurs gain by a bipedal configuration

  1. height dominance over conspecific rivals for mating privileges. This is emphasized in Langobardisaurus with its long neck. This is emphasized by Cosesaurus by flapping and leaping, both working to increase height.
  2. Ability to breathe while running for added endurance
Chlamydosaurus, the Austrlian frill-neck lizard

Fig. 3. Chlamydosaurus, the Austrlian frill-neck lizard with an erect spine and elevated tail. At one time some paleontologists did not believe what you can see here, that this lizard can stand bipedally. Such was their bias.

What the lizard, Chlamydosaurus, gains by bipedal configuration

  1. combined with their frightfully opening frill neck, dominance over rivals and interlopers, which they charge bipedally.
  2. better ability to survey the local area for rivals (principally) and predators while on the ground, — but Chlamydosaurus is primarily (90%) arboreal for the same reason and 90% bipedal while on the ground, not just while running, which some paleontologists are not aware of or did not believe (Hone and Benton 2007, 2009).
Figure 1. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Herrerasaurus, Tawa and Eoraptor.

Figure 4. The origin of dinosaurs to scale. Gray arrows show the direction of evolution. This image includes Decuriasuchus, Turfanosuchus, Gracilisuchus, Lewisuchus, Pseudhesperosuchus, Herrerasaurus, Tawa and Eoraptor. Note the phylogenetic miniaturization.

What Gracilisuchus gained by a bipedal configuration

  1. Gracilisuchus is not much taller bipedally. Remember, archosaurs had no scales at this point. Feather quills would appear on dino backs. Osteoderms appeared along croc backs to support their longer spinal columns. So, standing erect might have just been sexy at first.
  2. Overcoming Carrier’s Constraint: greater endurance by not having to undulate while breathing and so continue breathing while running.

What do bipedal reptiles have in common?

  1. Other than sauropods and other reptiles that adopt a tripodal pose bipedal reptiles are generally small, having experienced phylogenetic miniaturization.
  2. Other than Tanystropheus, bipeds are terrestrial and/or arboreal
  3. Longer hind limbs than forelimbs
  4. Anterior process of the illiim, no matter how small
  5. Typically stronger or more sacral connections to the ilium
  6. Typically a long neck and short torso (but Longisquama (Fig. 2), as a lemur analog, and lemurs themselves break that rule).
Figure 1. The ancestry of Scleromochlus going back to Lewisuchus, Saltoposuchus, Terrestrisuchus, SMNS 12591 and Gracilisuchus.

Figure 1. The ancestry of Scleromochlus going back to Lewisuchus, Saltoposuchus, Terrestrisuchus, SMNS 12591 and Gracilisuchus.

It’s easy to overlook the most obvious.
I have a feeling that this will not be the first time Persons and Currie are going to be reminded of Carrier 1987.

References
Carrier DR 1987. The evolution of locomotor stamina in tetrapods: circumventing a mechanical constraint. Paleobiology (13): 326–341.
Clemente CJ, Withers PC, Thompson G, Lloyd D 2008. Why Go Bipedal? Locomotion and Morphology in Australian Agamid Lizards.J. Exp. Bio. 211: 2058-2065
Peters D 2000b. A reexamination of four prolacertiforms with implications for pterosaur phylogenesis. Rivista Italiana di Paleontologia e Stratigrafia 106: 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.
Persons WS and Currie PJ 2017. The functional origin of dinosaur bipedalism: Cumulative evidence from bipedally inclined reptiles and disinclined mammals. Journal of Theoretical Biology, 2017; 420: 1 DOI: 10.1016/j.jtbi.2017.02.032
Shine R and Lambeck R 1989. Ecology of Frillneck Lizards, Chlamydosaurus kingii (Agamidae), in Tropical Australia. Aust. Wildl. res. Vol. 16: 491-500.
Snyder RC 1954. The anatomy and function of the pelvic girdle and hind limb in lizard locomotion. American Journal of Anatomy 95:1-46

Hesperornis walking GIF

Figure 1. Hesperornis compared to a king penguin, Atenodytes. Hesperornis has larger feet and a longer tibia. Since penguins swim with their forelimbs, they have large pectoral muscle anchors. That is not the case with Hesperornis.

Figure 1. Hesperornis compared to a king penguin, Atenodytes. The patella is blue. Hesperornis has larger feet and a longer tibia. Since penguins swim with their forelimbs, they have large pectoral muscle anchors. That is not the case with Hesperornis. Click to enlarge. Marsh 1872 thought Hesperornis could stand upright. I do too. That makes only two of us.

Hesperornis regalis
(Figs. 1,2, Late Cretaceous, Campanian, Marsh 1872, 1.8m long) was a toothed, flightless marine bird with vestigial wings and asymmetrical feet. Although not related to living loons, Hesperornis is often compared to loons, which have no teeth and retain the ability to fly. Both swim with powerful hind limbs. Hesperornis can also be compared to another flightless bird clade, the penguins, with the proviso that penguins swim with powerful forelimbs and their skeletons (Fig. 1) reflect this.

Figure 2. Click to enlarge. Hesperornis walking GIF movie. In this hypothetical scenario Hesperornis walks bipedally.

Figure 2. Click to enlarge. Hesperornis walking GIF movie. In this hypothetical scenario Hesperornis walks bipedally. Like penguins and ducks, Hesperornis does not flex its toes while walking. Nor does it take very big steps.

Wikipedia reports,
“In terms of limb length, shape of the hip bones, and position of the hip socket, Hesperornis is particularly similar to the common loon (Gavia immer), probably exhibiting a very similar manner of locomotion on land and in water. Like loons, Hesperornis were probably excellent foot-propelled divers, but ungainly on land. Like loons, the legs were probably encased inside the body wall up to the ankle, causing the feet to jut out to the sides near the tail. This would have prevented them from bringing the legs underneath the body to stand, or under the center of gravity to walk (Reynaud 2006). Instead, they likely moved on land by pushing themselves along on their bellies, like modern [loons].”

It was not difficult
to animate a bipedal Hesperornis (Fig. 2). It appears fully capable of doing so penguin-style. But the comparison to loons is indeed compelling.

Loons are ungainly
on the beach. See a YouTube video here. Yes, it does look wounded, unable to walk like a normal bird. It would probably fly if it was in a hurry. Hesperornis shares many traits by convergence with loons, but, if anything, loon hind limbs are more extreme in their proportions, including a proportionately larger projecting patella (Figs. 3, 4).

Just added after publication: The axis of the acetabulum is further foreword in Hesperornis, at the 51% mark on the torso (measured from the posterior pelvis) versus the 43% mark on the loon. That big butt makes Hesperornis less top heavy.

Figure 3. Loon skeleton with femur (yellow) and tibia/patella (green) highlighted. In this mount the center of gravity is in front of the toes, which makes this an untenable mount, unless the loon is floating on water.

Figure 3. Loon skeleton with femur (yellow) and tibia/patella (green) highlighted. In this mount the center of gravity is in front of the toes, which makes this an untenable mount, unless the loon is floating on water.

The loon femur is a little shorter and the patella is a little larger
(Figs. 3, 4) than on Hesperornis (Figs. 1,2). It’s up to our imaginations whether or not that would enable a more penguin-like locomotion in Hesperornis. Note that penguins do have a patella (knee bone) but it does not extend above the femur as it does in Hesperornis and loons.

Figure 4. Loon femur and tibia/patella. These proportions are more extreme than those found in Hesperornis.

Figure 4. Loon femur and tibia/patella. These proportions are more extreme than those found in Hesperornis. Note the right angle femoral head, as in most birds, but then look at the skeleton (Fig. 3) in which the femora are held laterally, unlike more birds and dinosaurs.

Nat Geo
and Andy Farke report on a bone growth and possible migration study (Wilson and Chin 2014) of Hesperornis here.

According to Marsh:
“The clavicles are separate, but meet on the median line, as in some very young existing birds.The coracoids are short, and much expanded where they join the sternum. The latter has no distinct manubrium, and is entirely without a keel. The wings were represented by the humerus only, which is long and slender, and without any trace of articulation at its distal end.”  

Various authors
believe the humerus would have been hidden beneath the skin and appressed to the ribs. As is typical for Kansas fossils, Hesperornis specimens are typically crushed flat. In the large reptile tree Hesperornis nests with its volant contemporary, Ichthyornis.

References
Marsh OC 1872. Discovery of a remarkable fossil bird. American Journal of Science, Series 3, 3(13): 56-57.
Marsh OC 1872. Preliminary description of Hesperornis regalis, with notices of four other new species of Cretaceous birds. American Journal of Science 3(17):360-365.
Marsh, OC 1880. Odontornithes, a Monograph on the Extinct Toothed Birds of North America. Government Printing Office, Washington DC.
Reynaud F 2006. Hind limb and pelvis proportions of Hesperornis regalis: A comparison with extant diving birds. Journal of Vertebrate Paleontology 26 (3): 115A. doi:10.1080/02724634.2006.10010069.
Wilson L. and Chin K 2014. Comparative osteohistology of Hesperornis with reference to pygoscelid penguins: the effects of climate and behaviour on avian bone microstructure. Royal Society Open Science. 1: 140245. doi: 10.1098/rsos.140245

OceansofKansas/Hesperornis

wiki/Hesperornis

Basal hominid, fenestrasaur and archosaur analogies

When you look at the transition
from quadrupedal locomotion to bipedal locomotion in early hominids (Fig. 1), among many other details, you can’t help but be impressed by the increase in the relative length of the hind limbs.

Figure 1. When hominids became bipedal, their hind limbs became longer.

Figure 1. When hominids became bipedal, their hind limbs became much longer.

The same can be said
for the transition from semi-bipedal Cosesaurus (based on matching Rotodactylus tracks) to the fully bipedal Sharovipteryx (Fig. 2).

Figure 2. Cosesaurus was experimenting with a bipedal configuration according to matching Rotodactylus tracks and a coracoid shape similar to those of flapping tetrapods. Long-legged Sharovipteryx was fully committed to a bipedal configuration.

Figure 2. Cosesaurus was experimenting with a bipedal configuration according to matching Rotodactylus tracks and a coracoid shape similar to those of flapping tetrapods. Long-legged Sharovipteryx was fully committed to a bipedal configuration, analogous to hominids.

As in hominids,
freeing the fore limbs from terrestrial locomotion enabled fenestrasaurs to do something else, like flapping for secondary sexual displays, adding motion to their morphological ornaments. While the forelimbs were relatively smaller in Sharovipteryx, they were relatively larger in Bergamodactylus (Fig. 3) a long-legged basal pterosaur. There were no constraints on forelimb evolution in fenestrasaurs, analogous to theropod dinosaurs that ultimately became birds. Some theropods and birds grew larger forelimbs, while others reduced their forelimbs.

Figure 2. Updated reconstruction of Bergamodactylus to scale with an outgroup, Cosesaurus.

Figure 3. Updated reconstruction of Bergamodactylus to scale with an outgroup, Cosesaurus.

Lest we not forget
in the basal archosaurs (crocs + dinos) early attempts at bipedal locomotion (Fig. 3) also corresponded to a longer hind limb length in bipedal Scleromochlus and Pseudhesperosuchus as opposed to their common ancestor, a sister to short-legged Gracilisuchus.

Figure 3. Short-legged Gracilisuchus, along with sisters, long-legged bipedal Pseudhesperosuchus and Scleromochlus.

Figure 3. Short-legged Gracilisuchus, along with sisters, long-legged bipedal Pseudhesperosuchus and Scleromochlus.

Based on those tiny hands,
the forelimbs of Scleromochlus were becoming vestiges. Based on the long proximal carpals of Pseudhesperosuchus, the manus was occasionally lowered to the ground, perhaps while feeding. The origin of bipedal locomotion in basal crocs is the same as in pre-dinosaurs.

It took much longer and proceeded more indirectly
for bipedal archosaurs to start flapping their forelimbs, giving them a new use that ultimately produced thrust and lift as bird forelimbs continued to evolve and become larger.

See videos produced by ReptileEvolution.com
on the origin of dinosaurs here, on the origin of humans here, and on the origin of pterosaurs here.

A sign of beauty and/or Olympic potential
is a long-legged model or athlete.

Pterosaur worker puts on blinders

Sorry to have to report this, but… 
Witton (2015) decided that certain published literature (data and hypotheses listed below) germane to his plantigrade, quadrupedal, basal pterosaur conclusions, should be omitted from consideration and omitted from his references.

Everyone knows, iI’s always good practice
to consider all the pertinent literature. And if a particular observation or hypothesis runs counter to your argument, as it does in this case, your job is to man up and chop it down with facts and data. That could have been done, but wasn’t. Instead, Witton put on his blinders and pretended competing literature did not exist. Unfortunately that’s a solution that is condoned by several pterosaur workers of Witon’s generation.

Not the first time inconvenient data
has been omitted from a pterosaur paper. Hone and Benton (2007, 2008) did the same for their look into pterosaur origins after their own typos cleared their way to delete from their second paper one of the two competing candidate hypotheses.

Witton (2013) and Unwin (2005) did the much the same by omitting published papers from their reference lists that they didn’t like.

Publication
is a great time to show colleagues that you have repeated all competing observations and experiments and either you support or refute them. To pretend competing theories don’t exist just increases controversy and reduces respect.

So, what’s this new Witton paper all about?

From the Witton abstract: Pterodactyloid pterosaurs are widely interpreted as terrestrially competent, erect-limbed quadrupeds, but the terrestrial capabilities of non-pterodactyloids are largely thought to have been poor (false). This is commonly justified by the absence of a non-pterodactyloid footprint record (false according to Peters 2011), suggestions that the expansive uropatagia common to early pterosaurs would restrict hindlimb motion in walking or running (false), and the presence of sprawling forelimbs in some species (not pertinent if bipedal).

“Here, these arguments are re-visited and mostly found problematic. Restriction of limb mobility is not a problem faced by extant animals with extensive fight membranes, including species which routinely utilize terrestrial locomotion. The absence of non-pterodactyloid footprints is not necessarily tied to functional or biomechanical constraints. As with other fully terrestrial clades with poor ichnological records, biases in behaviour, preservation, sampling and interpretation likely contribute to the deficit of early pterosaur ichnites. Suggestions that non-pterodactyloids have slender, mechanically weak limbs are demonstrably countered by the proportionally long and robust limbs of many Triassic and Jurassic species. Novel assessments of pterosaur forelimb anatomies conflict with notions that all non-pterodactyloids were obligated to sprawling forelimb postures. Sprawling forelimbs seem appropriate for species with ventrally-restricted glenoid articulations (seemingly occurring in rhamphorhynchines and campylognathoidids). However, some early pterosaurs, such as Dimorphodon macronyx and wukongopterids, have glenoid arthrologies which are not ventrally restricted, and their distal humeri resemble those of pterodactyloids. It seems fully erect forelimb stances were possible in these pterosaurs, and may be probable given proposed correlation between pterodactyloid-like distal humeral morphology and forces incurred through erect forelimb postures. Further indications of terrestrial habits include antungual sesamoids, which occur in the manus and pes anatomy of many early pterosaur species, and only occur elsewhere in terrestrial reptiles, possibly developing through frequent interactions of large claws with firm substrates. It is argued that characteristics possibly associated with terrestrially are deeply nested within Pterosauria and not restricted to Pterodactyloidea as previously thought, and that pterodactyloid-like levels of terrestrial competency may have been possible in at least some early pterosaurs.”

Bottom Line: Unfortunately Witton paid little attention to
the literature on non-pterodactyloid ichnites and feet. And he ignored certain basic tenets.

Witton writes: “Given that likely pterosaur outgroups such as dinosauromorphs and Scleromochlus bore strong, erect limbs (e.g.,Sereno, 1991; Benton, 1999), it is possible that these early pterosaurs retained characteristics of efficient terrestriality from immediate pterosaur ancestors.”

Wrong as this ‘given’ supposition is, both of the above taxa (dinos and scleros) are bipedal, yet Witton refuses to consider this configuration in basal pterosaurs (for which he claims have no ichnite record).

Figure 1. Witton's errors with a quadrupedal Preondactylus. For a study on terrestrially, there is little effort devoted to the feet of pterosaurs here.

Figure 1. Witton’s errors with a quadrupedal Preondactylus. For a study on terrestrially, there is little effort devoted to the feet of pterosaurs here. Click to enlarge.

Digitigrady vs. plantigrady
Pterosaur feet come in many shape and sizes. Some have appressed metatarsals. Others spread the metacarpals. These differences were omitted by Witton. Some have a very long pedal digit 5. Others have a short digit 5. These differences were also omitted. Some pterosaurs were quadrupeds (but not like Witton imagines them), others were bipeds (Figs. 1-6). Basal pterosaurs had a butt-joint metatarsi-phalangeal joint, but that just elevates the proximal phalanges, as confirmed in matching ichnites.

Figure 2. Witton's quadrupedal Dimorphodon.

Figure 2. Witton’s quadrupedal Dimorphodon. Click to enlarge.

The quadrupedal hypothesis is a good one,
but it really only works in short-clawed plantigrade clades that made quadrupedal tracks on a horizontal substrate. Otherwise a quadrupedal configuration works only on vertical surfaces, like tree trunks, where the trenchant manual claws can dig into the bark. This was omitted by Witton.

Figure 3. Dimorphdon toes and fingers. Here, in color, I added the keratinous sheath over the claws that show how ridiculous it would be for Dimorphodon to  grind these into the ground. Better to use those on a vertical tree trunk.

Figure 3. Dimorphdon toes and fingers. Here, in color, I added the keratinous sheath over the claws that show how ridiculous it would be for Dimorphodon to grind these into the ground. Better to use those on a vertical tree trunk (figure 2). Click to enlarge.

Quadrupedal pterosaurs can’t perch
on narrow branches. Peters (2000) showed how a long pedal digit 5 acted like a universal wrench for perching.

Figure 1. Anurognathus  by Witton along with an Anurognathus pes and various anurognathid ichnites.

Figure 4. Anurognathus by Witton along with an Anurognathus pes and various digitigrade anurognathid ichnites, all ignored by Witton. Digit 5 behind the others is the dead giveaway.

Quadrupedal pterosaurs can’t open their wings
whenever they want to, for display or flapping. Witton favors the forelimb launch hypothesis for pterosaurs of all sizes, forgetting that size matters.

Figure 5. Quadrupedal Rhamphorhynchus by Witton (below) with errors noted and compared to bipedal alternative.

Figure 5. Quadrupedal Rhamphorhynchus by Witton (below) with errors noted and compared to bipedal alternative.

Pterosaurs were built for speed
whether on the ground or in the air. They were never ‘awkward.’ Remember basal forms have appressed metatarsals, they have more than five sacrals, their ichnites are digitigrade, the tibia is longer than the femur, the bones are hollow, when bipedal the feet plant below the center of balance at the wing root, and some pterodactyloid tracks are bipedal.

Figure 6. Quadrupedal Campylognathoides by Witton (center) with errors noted and compared to bipedal alternatives.

Figure 6. Quadrupedal Campylognathoides by Witton (center) with errors noted and compared to bipedal alternatives. The lack of accuracy in Witton’s work borders on cartoonish.

Accuracy trumped by imagination
By the present evidence, Witton has not put in the effort to create accurate and precise pterosaur reconstructions. Rather his work borders on the cartoonish and I suspect the reconstructions have been free-handed with missing or enigmatic parts replaced with parts from other pterosaurs. That should be unacceptable, but currently such shortcuts are considered acceptable by Witton’s generation of pterosaur workers.

The Sordes uropatagium false paradigm gets a free pass
and no critical assessment from Witton. (So far this uropatagium has been observed only in one specimen, Sordes (in which a single uropatagium Witton believes was stretched between the two hind limbs), was shown to be an illusion caused by bone and membrane dislocation during taphonomy. All other pterosaurs and their predecessors have twin uropatagia that do not encumber the hind limbs. The dark-wing Rhamphorhynchus (Fig. 5) is an example of a basal pterosaur with twin uropatagia.

References
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 2008. Contrasting supertree and total evidence methods: the origin of the pterosaurs. Zitteliana B28:35–60.
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
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The genesis of feathers tied to the genesis of bipedalism in dinosaurs

Earlier we looked at the origin of feathers and the evolution of epidermal structures in dinosaurs, noting that embryo birds first develop primal buds (primordia) in the middle of their otherwise naked back. As we learned earlier, feathers are not elaborate scales, but develop from naked skin. We see this every time we pluck a chicken. We also learned that leg scales on birds are derived from feathers. Remember those 4-winged Mesozoic birds?

Today some further thoughts on the genesis of feathers.

Figure 1. Sinosauropteryx in lateral view on a primitive conifer.

Figure 1. Sinosauropteryx in lateral view on a primitive conifer. Despite the complete preservation of several specimens attributed to Sinosauropteryx, very few reconstructions (Fig. 1) have been made of it. Clinging to trees ultimately led to clinging to dinosaurs in dromaeosaurids. Like Limusaurus, Sinsauropteryx is off the main line of bird evolution.

Feathers are rarely preserved on dinosaur fossils.
One of the most primitive dinosaurs to preserve (admittedly very primitive) feathers is Sinosauropteryx (Figs. 1-3; Ji and Ji 1996) from the late Jurassic (with origins earlier in the Jurassic). It has short filamentous feathers running down its spine and around its throat and apparently nowhere else. This ‘mohawk haircut’- pattern could be due to the process of fossilization. Perhaps only those feathers on the parasagittal plane got preserved. However, from available evidence if the feathers were not restricted to the back, they did not stray very far from the spine at this stage. You don’t see feathers around the belly or legs in Sinosauropteryx (Fig. 2).

Figure 2. Sinosauropteryx fossil.

Figure 2. Sinosauropteryx fossil. As everyone knows, those are primitive feathers lining the spinal column and below the throat. Analysis indicates this is not the most primitive feathered theropod. Note the on/off appearance of the tail feathers indicating a decorative device: stripes!

 

Adding Sinosauropteryx to the large reptile tree
nests it with Limusaurus and both were basal to the much larger Sinocalliopteryx, which also had primitive feathers (Fig. 3). So Sinosauropteryx is not the most basal dinosaur with feathers or proto-feathers (contra Ji and Ji 1996). Unfortunately, more primitive theropods do not preserve feathers or scales. Scales do appear on later, larger dinosaurs of all sorts, not so much on the smaller, earlier dinos. Based on birds we can’t assume that small, early dinos had scales (contra Barrett et al. 2015). Rather, based on the appearance of primordia and feather-like structures on a wide variety of dinosaurs, feather primordia appears to precede scales, and perhaps many of these primordia ultimately became scales on larger dinos.

Figure 2. Sinocalliopteryx along with Limusaurus, Aurornis and Archaeopteryx to scale.

Figure 3. Sinocalliopteryx along with Limusaurus, Aurornis and Archaeopteryx to scale. Similar to Sinopteryx, but includes leg feathers here. Sinopteryx and Limusaurus are off the main line of bird evolution, which includes Haploceheirus and dromaeosaurs. Note the depth of the pelvis here compared to Scleromochlus (fig. 5).

 

Figure 1. Scales on the back of Scleromochlus, a basal bipedal croc and thus a distant sister to basal bipedal dinosaurs.

Figure 4. Scales on the back of Scleromochlus forming a lumbar girdle for support during bipedal excursions. This taxon nests as a basal bipedal croc and thus a distant sister to basal bipedal dinosaurs.

The genesis of feather primordia appears to be correlated to bipedal locomotion and a long torso. Before a feather was a feather, or even a quill, it was something else more primitive.

When one looks
at the pattern of dorsal scalation in Scleromochlus (Figs. 4, 5), a basal archosaur, one gets the impression that it was wearing a kind of lumbar girdle to support the long lower back. Indeed, as a newbie biped, Scleromochlus would have used such support near the fulcrum of the large leverage arm created by its stance, its long dorsal region and short ilium. Nothing appears to be sticking out above the dermal layer here. All of the scales (or whatever they were) appear to in lines, like a weave.

Unlike ancestral rauisuchians and the more closely related and larger Erpetosuchus and Gracilisuchus, there were no dorsal parasagittal scutes on Scleromochlus. It was a small animal that lost these structures as it evolved to depend on speed, not armor, to defend itself from predators.

 

Scleromochlus, a basal crocodylomorph

Figure 5. Scleromochlus, a basal crocodylomorph and an early biped in the archosaur line. Scleromochlus reinforced its long lower back with a dermal lumbar support or girdle. This is same area on a chicken embryo that first develops feathers. Compare torso length here to figure 3.

Primordia evolved into feathers only on the short torso basal dinos
Pre-dinosaurs are distinct from pre-crocs in many ways, but pre-dinos all have a shorter torso and a deeper pelvis (Fig. 3) reducing the leverage arm and the need for a reinforcing lumbar girdle. After the pelvis deepened and the torso shortened in early dinosaurs, the individual primordia of that old girdle were free to evolve into something else, in this case, something decorative.

Sinosauropteryx, with its dorsal line of feathery filaments extending from head to tail is one such example. When more feathers began to wrap around the body, that added insulation as a use. When wing feathers lengthened, the forelimbs began to flap to bring attention to those decorations. Later, wing feathers were co-opted for thrust and lift to enable flight.

But the genesis of feathers
still appears to be in the middle of the back, where primordia first appear on embryo chicks, replaying the old lumbar girdle innovation of Scleromochlus. The ornithischians, Tianyulong and Psittacosaurus had elongated primordia along their backs and tails indicating that this trait probably goes back to Herrerasaurus and Trialestes, no doubt in a smaller, more primitive state. With that small field of primordial  scales on the lower back of an otherwise naked Scleromochlus (Fig. 5), the genesis of extradermal structures appears to extend to basal archosaurs.

Figure 6. Feathers, scales and scutes in the Archosauria.

Figure 6. Feathers, scales and scutes in the Archosauria.

If anyone can provide evidence for scales or any other dermal preservation in any Triassic or Early Jurassic dinosaur, please let us know of them.

If anyone has other thoughts on the origin of feathers, please share them. If the above scenario does not make sense, please tell us your thoughts.

References
Barrett PM, Evans DC, Campione NE 2015. Evolution of dinosaur epidermal structures. Biol. Lett. 11: 20150229. online
Ji Q and Ji S-A 1996. On the Discovery of the earliest fossil bird in China (Sinosauropteryx gen. nov.) and the origin of birds. Chinese Geology 233:30-33.

Some thoughts on Shuvuuia, Mononykus and Sharovipteryx

Modified June 1, 20-15 with new data on Mononykus (Perle et al. 1994). Thanks to M. Mortimer for the reference.

Figure 1. Shuvuuia and Mononykus to scale in various poses. The odd digit 1 forelimb claws appear to be retained for clasping medial cylinders, like tree trunks. The forelimb is very strong. Perhaps these taxa rest vertically and run horizontally. Click to enlarge.

Figure 1. Shuvuuia and Mononykus to scale in various poses. The odd digit 1 forelimb claws appear to be retained for clasping medial cylinders, like tree trunks. The forelimb is very strong. Perhaps these taxa rest vertically and run horizontally. Click to enlarge.

Mononykus and Shuvuuia
(Fig. 1) are two odd bird/dinosaurs from the Late Cretaceous of Mongolia. Their forelimbs are reduced to a single digit (#1) with digits 2 and 3 vestiges in Shuvuuia GI 100/975 and other specimens (Chiappe, Norell and Clark 1998) or absent in Mononykus  IGM N107/6 (Perle et al. 1993), the larger and more derived of the two.

The question is what are those odd forelimbs used for?
They can’t be traditional vestiges because the olecranon process (elbow) is hyper-developed. The forelimbs look to be very strong. The radius and ulna are essentially fused (but not quite) proximally. The digit 1 ungual is a grappling hook.

In modern birds,
extending the elbow unfolds the tucked wing. In Mononykus and kin the hand (wing) can never be tucked or even rotated. Everything appears to be locked in place except the elbow and shoulder.

Senter (2005)
suggested the odd forelimbs of Mononykus were used to rip open termite mounds. Unfortunately for this hypothesis these dinosaurs would have to belly up to each mound they ripped open, making them vulnerable to a counterattack by termites under their feathers. Current anteaters are lumbering creatures with long snouts that keep them well away from termite defenders. Mononykids were built for bipedal speed. Anteating is not a good match no matter how it is considered.

Whatever those forelimbs were used for,
they were not used full time.

Anything those birds touched with their tiny forelimbs
they would have to belly up to. So let’s consider the safest substrate available, a tree trunk. Neither of these mononykids has a perching foot for tree branches. If these birds spent half their lives resting/sleeping, then why not do it within the relative safety of elevation above the ground, clinging to a tree trunk (Fig. 1)? The sternum on these creatures was sturdy, larger than in Archaeopteryx, ideally built for strong adduction (clinging). If Mononykus was too-large for tree clinging, then the forelimbs could have been used as props for maintaining balance while resting horizontally. After all, nest building and egg-laying were requirements.

Sisters had big claws and some were clingers
Mononykids descend from basal alvarezsaurids, like Haplocheirus (Early Late Jurassic, Choinere et al. 2010), a theropod dinosaur nesting between ornithomimosaurs and more bird-like dinosaurs like Archaeopteryx, oviraptosaurs and therizinosaurs. So it is within their phylogenetic bracket, and well within their abilities for mononykids to cling to trees and other suitable substrates.

The Sharovipteryx analogy
Another unrelated, but speedy biped with tiny forelimbs is Sharovipteryx (Fig. 2, Late Triassic), a fenestrasaur also capable of clinging to tree trunks, especially in preparation for a glide. Longisquama had a similar morphology.

Figure 1. Sharovipteryx in various perching attitudes.

Figure 2 Sharovipteryx in various perching attitudes. Similar in overall build to mononykids, Sharovipteryx was unrelated but developed several traits by convergence, including, perhaps, the ability to belly up to a tree trunk to spend the night clinging to it.

The odd forelimbs of mononykids
evolved from the prey-catching forelimbs of basal alvarezsauroids, like Hapolcheirus, to enable mononykids to rest vertically on tree trunks in the present hypothesis. I haven’t read all the literature. Has this idea been put forth earlier? Any other ideas out there?

References
Chiappe LM, Norell MA and Clark JM 1998. The skull of a relative of the stem-group bird Mononykus. Nature, 392(6673): 275-278.
Chiappe LM, Norrell MA and Clark JM 2002. The Cretaceous, Short-Armed Alvarezsauridae: Mononykus and its Kin pp. 87-120 in Chiappe LM and Witmer LM eds, Mesozoic birds: Above the Heads of Dinosaurs. University of California Press. 536 pp.
Choiniere JN, Xu X, Clark JM, Forster CA, Guo Y and Han F 2010. A basal alvarezsauroid theropod from the Early Late Jurassic of Xinjiang, China. Science 327 (5965): 571–574.
Perle A, Norell MA, Chiappe LM and Clark JM 1993. Flightless bird from the Cretaceous of Mongolia. Nature 362:623-626.
Perle A, Chiappe LM, Rinchen B, Clark JM and Norell 1994. Skeletal Morphology of Mononykus olecranus (Theropoda: Avialae) from the Late Cretaceous of Mongolia. American Museum Novitates 3105:1-29.
Senter P 2005. Function in the stunted forelimbs of Mononykus olecranus (Theropoda), a dinosaurian anteater. Paleobiology 31(3):373–381.
Suzuki S, Chiappe L, Dyke G, Watabe M, Barsbold R and Tsogtbaatar K 2002. A new specimen of Shuvuuia deserti Chiappe et al., 1998, from the Mongolian Late Cretaceous with a discussion of the relationships of alvarezsaurids to other theropod dinosaurs. Contributions in Science (Los Angeles), 494: 1-18.