More tiny birds and tiny pterosaurs

Earlier we took a peek at a few tiny birds and pterosaurs. Here (Fig. 1) are several more.

Traditional paleontologists
insist that these tiny pterosaurs were babies of larger forms that looked different, (Bennett 1991, 1992, 1994, 1995, 1996, 2001, 2006, 2007, 2012, 2014) ignoring or not aware of the fact that we know pterosaur embryos and juveniles were virtually identical to their adult counterparts (Fig. 2). Bennett (2006) matched two tiny short-snouted pterosaurs (JME SoS 4593 and SoS 4006 (formerly  PTHE No. 1957 52) to Germanodactylus, but they don’t nest together in the large pterosaur tree.

Figure 1. Tiny pterosaurs and tiny birds to scale showing that tiny pterosaurs were generally about the size of the tiny Early Cretaceous bird.

Figure 1. Tiny pterosaurs and tiny birds to scale showing that tiny pterosaurs were generally about the size of the tiny Early Cretaceous bird. I have, for over a decade, promoted the fact that these tiny pterosaurs were adults, the size of modern hummingbirds and wrens.

One of the most disappointing aspects of modern paleontology
is the refusal of modern pterosaur workers to include in their analyses the small and tiny pterosaurs. They were all the size of living hummingbirds and wrens. Many were similar in size to extinct Early Cretaceous birds (Fig. 1). Those workers don’t want to add these taxa to their lists on the false supposition that the tiny pterosaurs are babies of, so far unknown adults. Note Bennett’s long body of work (see below) indicated otherwise, but never with phylogenetic analysis.

Phylogenetic analysis (Peters 2007) reveals these tiny pterosaurs are adults or can be scored as adults. They are surrounded by adults and they often form transitional taxa in the evolutionary process of phylogenetic miniaturization between larger long-tailed pterosaurs and larger short-tailed pterosaurs.

Figure 1. Click to enlarge. There are several specimens of Zhejiangopterus. The two pictured in figure 2 are the two smallest above at left. Also shown is a hypothetical hatchling, 1/8 the size of the largest specimen.

Figure 2. Click to enlarge. There are several specimens of Zhejiangopterus. The two pictured in figure 2 are the two smallest above at left. Also shown is a hypothetical hatchling, 1/8 the size of the largest specimen. This is evidence that juveniles were virtually identical to adults, except in size.

More importantly,
earlier we discussed several examples of juvenile pterosaurs morphologically matching adults here, here and here. So young pterosaurs have been shown to match their adult counterparts. They don’t transform like young mammals and dinosaurs do. They were ready to fly upon hatching IF they were the minimum size to avoid desiccation, as discussed earlier here.

The most interesting aspect
to the whole tiny pterosaur story is how small their smallest hatchlings would be. We looked at that earlier here.

References
Bennett SC 1991. Morphology of the Late Cretaceous Pterosaur Pteranodon and Systematics of the Pterodactyloidea. [Volumes I & II]. Ph.D. thesis, University of Kansas, University Microfilms International/ProQuest.
Bennett SC 1992. 
Sexual dimorphism of Pteranodon and other pterosaurs, with comments on cranial crests. Journal of Vertebrate Paleontology 12: 422–434.
Bennett SC 1994. 
Taxonomy and systematics of the Late Cretaceous pterosaur Pteranodon (Pterosauria, Pterodactyloidea). Occassional Papers of the Natural History Museum University of Kansas 169: 1–70.
Bennett SC 1995. A statistical study of Rhamphorhynchus from the Solnhofen limestone of Germany: year classes of a single large species. Journal of Paleontology 69, 569–580.
Bennett SC 1996. 
Year-classes of pterosaurs from the Solnhofen limestones of Germany: taxonomic and systematic implications. Journal of Vertebrate Paleontology 16:432–444.
Bennett SC 2001.
 
The osteology and functional morphology of the Late Cretaceous pterosaur Pteranodon. Part I. General description of osteology. Palaeontographica, Abteilung A, 260: 1–112. Part II. Functional morphology. Palaeontographica, Abteilung A, 260: 113–153
Bennett SC 2006. Juvenile specimens of the pterosaur Germanodactylus cristatus, with a revision of the genus. Journal of Vertebrate Paleontology 26(4): 872–878.
Bennett SC 2007. A second specimen of the pterosaur Anurognathus ammoni. Paläontologische Zeitschrift 81(4):376-398.
Bennett  SC (2012) [2013] 
New information on body size and cranial display structures of Pterodactylus antiquus, with a revision of the genus. Paläontologische Zeitschrift (advance online publication) doi: 10.1007/s12542-012-0159-8
http://link.springer.com/article/10.1007/s12542-012-0159-8
Bennett SC 2014. A new specimen of the pterosaur Scaphognathus crassirostris, with comments on constraint of cervical vertebrae number in pterosaurs. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen, 271(3): 327-348.
Peters D 2007. The origin and radiation of the Pterosauria. Flugsaurier. The Wellnhofer Pterosaur Meeting, Munich 27

 

Bird origins: trees encourage phylogenetic miniaturization

Figure 1. The evolution of birds as a consequence of miniaturization. Artist: Davide-Bonnadonna

Figure 1. The evolution of birds as a consequence of miniaturization. Artist: Davide-Bonnadonna. Unfortunately this horizontal image, while correct, ignores the influence of tree clinging.

Earlier a paper (Lee et al. 2014) demonstrated the well understood concept of phylogenetic miniaturization in birds (Fig. 1). We’ve seen this pattern often in the origin of major clades. Perhaps overlooked in birds, the behavior of tree clinging is key to their reduction in overall size, the increase in forelimb length and the evolution of flight feathers.

During this time some pre-bird dinosaurs became arboreal quadrupeds 
while remaining terrestrial bipeds. Smaller lighter taxa with longer forelimbs find it easier to climb trees. The smallest taxa can perch bipedally on slender branches (Fig. 2), eliminating the need to use the forelimbs for clinging. As a consequence, forelimbs can be modified for flight.

Figure 2. Bird origins should be shown in a vertical format as big tree clingers evolved through phylogenetic miniaturization through Aurornis to become perching taxa, like Archaeopteryx.  Black images are to scale. Gray images are enlarged to show detail.

Figure 2. Bird origins should be shown in a vertical format as big tree clingers evolved through phylogenetic miniaturization through Aurornis to become perching taxa, like Archaeopteryx. Black images are to scale. Gray images are enlarged to show detail.

Archaeopteryx was not the smallest of basal birds.
As early birds continued to evolve, becoming ever more bird-like, taxa continued to shrink in size (Fig. 3). Some were as small as hummingbirds and the smallest adult pterosaurs.

Figure 3. The Eichstätt specimen of Archaeopteryx together with a selection of more derived birds, all smaller.

Figure 3. The Eichstätt specimen of Archaeopteryx together with a selection of more derived birds, all smaller.

The act of tree clinging
builds up those all important pectoral muscles over several hundred generations and finds a likely analogous behavior (based on a similar morphology) in the arboreal non-flying fenestrasaur ancestors of pterosaurs, like Longisquama (Fig. 4).

Figure 1. Longisquama on a tree trunk.

Figure 4. Longisquama on a tree trunk.

The perching ability of birds
finds a convergent ability in basal pterosaurs, with the exception that pterosaurs use pedal digit 5 rather than pedal digit 1 to serve as a universal wrench. (Fig. 5, Peters 2000, 2002, 2010). Even so, most pterosaurs (ctenochasmatids and nyctosaurs not included) continued to retain large, tree-clinging fore limb claws.

Figure 1. The pterosaur Dorygnathus perching on a branch. Above the pes of Dorygnathus demonstrating the use of pedal digit 5 as a universal wrench (left), extending while the other four toes flexed around a branch of any diameter and (right) flexing with the other four toes. As in birds, perching requires bipedal balancing because the medially directed fingers have nothing to grasp.

Figure 5. The pterosaur Dorygnathus perching on a branch. Above the pes of Dorygnathus demonstrating the use of pedal digit 5 as a universal wrench (left), extending while the other four toes flexed around a branch of any diameter and (right) flexing with the other four toes. As in birds, perching requires bipedal balancing because the medially directed fingers have nothing to grasp. Note that most pterosaurs do not lose their tree grappling fingers, but quadrupedal beach combing forms, like ctenochasmatids, generally do.

References
Lee MSY, Cau A, Naish D and Dyke GJ 2014. Sustained miniaturization and anatomical innovation in the dinosaurian ancestors of birds.
Peters, D. 2000. Description and Interpretation of Interphalangeal Lines in Tetrapods. Ichnos, 7: 11-41
Peters D 2002. A New Model for the Evolution of the Pterosaur Wing – with a twist. – Historical Biology 15: 277–301.
Peters D 2010. In defence of parallel interphalangeal lines. Historical Biology iFirst article, 2010, 1–6 DOI: 10.1080/08912961003663500

Aurornis (pre-bird) skull, traced using DGS

Updated July 07, 2015 with new images of Aurornis.

Aurornis xui (Godefroit et al. 2013, Late Jurassic, 50cm in length, 160 or 125mya) is one of the many outgroup taxa known for Archaeopteryx and the birds, but it nests here as the closest of the tested ones.

Auronis is a small, gracile dromaeosaur
without a large elevated pedal digit 2. The skull is complete, but slightly disarticulated (Fig. 1). A little DGS colorizes the bones. These can then be reassembled to form a skull in lateral view.

Fig. 1 Aurornis skull in situ, various elements segregated from the in situ fossil and reassembled into a complete and articulated skull. The hole in the surangular is an artifact. The little lavender ovals are displaced sclerotic bones. Below is the original published image of the Aurornis skull.

Fig. 1 Aurornis skull in situ, various elements segregated from the in situ fossil and reassembled into a complete and articulated skull. The hole in the surangular is an artifact. The little lavender ovals are displaced sclerotic bones. Below is the original published image of the Aurornis skull.

Like many other small theropods,
Aurornis was feathered, agile and fast, a descendant of basal dromaeosaurids, like Halplocheirus. In palatal view, the internal nares are located on the anterior palatines and the anterior palate is narrow but solid. The premaxilla is still relatively short and toothed. The pterygoids are narrow and have lost their primitive triangular shape. As a result of taphonomy, tracings for the anterior dentary teeth are distinct from one another. The wider, more typical, pointed teeth are the correct morphology.

Figure 2. Aurornis in several views alongside Archaeoperyx to scale.

Figure 2. Aurornis in several views alongside Archaeoperyx to scale.

On a side note:
Pappochelys (‘grandfather turtle’) has been getting a lot of press, none critical. Take a fresh look at all the PR here.

On another side note:
Chilesaurus, which the large reptile tree nested as the long sought and current most basal member of the Ornithischia, and we looked at earlier here, was given a good look over at the TheropodDatabase blog here.  Evidently others also think the original Chilesaurus report has issues.

Added July 09, 2015
Dr. Andrea Cau’s note and the paper she sent, along with the SuppData downloaded served to increase the accuracy of these Aurornis images.

Figure 3. The manus of Aurornis as originally interpreted (above). As reinterpreted by comparison to Archaeopteryx below. Digit 3 was damaged and difficult to interpret. Digit 0 was originally overlooked. No only was Archaeopteryx smaller, it was more fully feathered and its bones were more gracile, all adaptations for flight.

Figure 3. The manus of Aurornis as originally interpreted (above). As reinterpreted by comparison to Archaeopteryx below. Digit 3 was damaged and difficult to interpret. Digit 0 was originally overlooked. No only was Archaeopteryx smaller, it was more fully feathered and its bones were more gracile, all adaptations for flight.

I have not seen the fossil itself,
but a DGS tracing of this image of the hand (Fig. 3) suggests ungual 2.3 was buried beneath the m2.2, not absent as originally indicated. Digit 3 of Aurornis was badly damaged, but by comparing it to Archaeopteryx a more accurate interpretation can be rendered with the proper number and length of phalanges.

Godefroit et al. reported the frontal was fused medially, but the fossil shows a medial split. They interpreted the pes with a fused metatarsal 3+4. That is probably not true as metatarsal 4 is likely buried in the matrix.

Figure 5. Aurornis hind limbs with bones colored. Here metatarsal 4 is distinct from mt3 and the fibula is identified. Click to enlarge.

Figure 5. Aurornis hind limbs with bones colored. Here metatarsal 4 is distinct from mt3 and the fibula is identified. Original interpretation of fused mt3+4 in gray. Mt5 is a tiny vestige close to the ankle. Click to enlarge.

References
Godefroit P, Cau A, Hu D-Y, Escuillié, Wu, W-H and Dyke G 201. A Jurassic avialan dinosaur from China resolves the early phylogenetic history of birds. Nature 498 (7454): 359–362.

wiki/Aurornis

The Origin and Evolution of Bird Wings

Earlier we looked at
the evolution of the wing in pterosaurs and in bats. Today we’ll look at the evolution of wings in birds. Other than falsifying/modifying the ‘phase shift’ hypothesis (Wagner and Gauthier 1999), there’s nothing heretical about what you’re going to see and read here. Everyone agrees on the taxon list, phylogenic order and bone identification.

Figure 1. The ancestry of birds illustrated by Haplocheirus, Velociraptor, Aurornis, Archaeopteryx and Gallus.

Figure 1. The ancestry of birds illustrated by Haplocheirus, Velociraptor, Aurornis, Archaeopteryx and Gallus to scale. Click to enlarge. Thanks to Scott Hartman for his Velociraptor, manus flesh outline oddly omitted.

The origin of feathers and wings
in birds has been well documented in hundreds of publications. Here (Figs. 1, 2) all those accounts have been simplified into just two graphics and a little text.

Figure 2. A selection of pre-bird and bird hands/wings including Haplocheirus, Limusaurus, Velociraptor, Archaeopteryx, Anser, Passer and two versions of the Hoatzin , Opisthocomus, adult and juvenile. Click to enlarge. Not to scale. Note the medial digit of the outlier, Limusaurus, which is a product of neotony, retained from embryonic tissue recapitulating the seven-finger manus of basal tetrapods (figure 3). Note the return of digit 0 fused to the anterior rim of Anser, Passer and the adult Opisthocomus.

Figure 2. A selection of pre-bird and bird hands/wings including Haplocheirus, Limusaurus, Velociraptor, Archaeopteryx, Anser, Passer and two versions of the Hoatzin , Opisthocomus, adult and juvenile. Click to enlarge. Not to scale. Note the medial digit of the outlier, Limusaurus, which is a product of neotony, retained from embryonic tissue recapitulating the seven-finger manus of basal tetrapods (figure 3). Note the return of digit 0 fused to the anterior rim of Anser, Passer and the adult Opisthocomus.

Haplocheirus
had grasping hands and trenchant unguals. The fingers were relatively short. Digit 1 was the most robust. Unlike the more basal theropods, Tawa and Herrerasaurus (Fig. 4, 5), manual digits 4 and 5 were absent in Haplocheirus and kin. Digit 3 was also reduced compared to those basal theropods.

Limusaurus
is very much an outlier, not a transitional taxon, different than other related taxa due to its vestigial size and embryonic development. As noted earlier, the Limusaurus manus retains a vestigial embryonic bud of digit ‘0’ which appears in basalmost tetrapods and many embryos, but not otherwise — unless you accept the hypothesis that the anterior process of metacarpal 1 in many extant birds (Fig. 2) is the return of this digit.

Velociraptor
was smaller overall and had longer fingers and longer metacarpals. Note metacarpal 3 is now subequal to metacarpal 2, but metacarpal 1 remains the most robust. One gets the impression that the fingers in Velociraptor had to be stiffer when they supported feathers. At some point they lost or were losing their ability to flex. At the same time the wrist better able to fold the manus in the plane of the forearm, as birds do.

Aurornis
was smaller overall and also had longer more gracile fingers. There is no bow to metacarpal 3 in Aurornis. This manus can be called a wing here.

Archaeopteryx
was overall smaller, but otherwise quite similar to Aurornis. This manus/wing of Archaeopteryx bore large primary feathers.

Anser
is an extant goose. Metacarpal 1 develops an anterior process where digit ‘0’ appeared on Limusaurus. Metacarpal 3 was bowed. The unguals are much smaller. The proximal metacarpals are fused.

Passer
is an extant sparrow. The phalanges are fused to one another.

Opisthocomus
is the extant hoatzin, which goes through a metamorphosis during growth. Juveniles have claws and adults absorb those while fusing the fingers together.

Wagner and Gauthier (1999)
noted the primitive phalangeal formula for tetrapods goes back to Tulerpeton, (Fig. 3) which they considered, “a synapomorphy that arose in the late Devonian, before the origin of Tetrapoda.” Now paleontologists consider Tulerpeton a tetrapod. The phalangeal formula, of course, has roots in Acanthostega, which has three extra digits, one medially and two laterally. Note: it is the reappearance of the medial digit, digit ‘0’, that is key to the present controversy. Note that Tulerpeton has lost one medial and one lateral digit.

Figure 3. Manus of a bird embryo, and two basal tetrapods, Acanthostega and Tulerpeton, the latter with digits 1-3 colorized like the birds in figure 2. Note the extra medial digit in Acanthostega.

Figure 3. Manus of a bird embryo, and two basal tetrapods, Acanthostega and Tulerpeton, the latter with digits 1-3 colorized like the birds in figure 2. Note the extra medial digit in Acanthostega. This is key to the present controversy. The metapterygial axis runs through the longset finger in basal tetrapods.

Embryology
Wagner and Gauthier (1999) report, “There has long been a dissenting view from the hypothesis that the bird hand is composed of digits DI, DII, and DIII. This position is held chiefly by embryologists who argue that the remaining fingers actually represent DII, DIII, and DIV because the DI and DV were thought to have been lost. Morse (19) observed that, when digital reduction occurs in mammals and lizards, the first digit (DI) is invariably the first to be lost in ontogeny, followed by the fifth (DV), and that a modified version of this pattern applies to the foot of birds as well. Thus, the proposition that ultimately became known as Morse’s Law holds that the three functional fingers remaining in adult birds must be DII, DIII, and DIV.”

That hypothesis assumes that the metapterygial axis continued to produce digit 4. The other option is this:  Evidently there WAS a phase shift, shifting the metapterygial axis from 4 in basal archosaurs to 3 in basal theropods and birds. This is a possibility that was not considered in prior studies. And it makes sense because theropods lose manual digits 3 and 4.

Sometimes paleontology does not occur out in the field,
or in the lab, but between the ears, as a new way of thinking becomes the solution to a vexing problem. (Note: no DGS was involved in this heretical appraisal.)

Figure 3. The source of the phase shift hypothesis, assuming the homology of manual digit 4 as the first digit to appear on the manus of Alligator (above) and Struthio (below).

Figure 3. The source of the phase shift hypothesis, assuming the homology of manual digit 4 as the first digit to appear on the manus of Alligator (above) and Struthio (the Ostrich, below). Clic to enlarge. It is easy to see how the mistake was made. Evidently there WAS a phase shift, shifting the metapterygial axis from 4 in basal archosaurs to 3 in basal theropods and birds. This is a possibility that was not considered in prior studies.

Which manual digit is the longest in in basal theropods?
Distinct from most other theropods, manual digit 3 is the longest in Herrerasaurus (Fig. 4) and Tawa (Fig. 5). So, digit 3 is where the new metapterygial axis is located on theropods and birds. Digits 4 and 5 are tiny and tinier vestiges, completely lost in later theropods and birds. It doesn’t make sense that the metapterygial axis should produce a vestige – or no digit at all! Rather, it is the metapterygial axis that has shifted one digit medially. That’s the new heretical phase shift promoted here.

A new nose for Herrerasaurus

Figure 4. Herrerasaurus. The manus has three functional fingers. The two lateral fingers are vestiges.

 

Figure x. The basal theropod, Tawa, with its long manual digit 3. This is where the metapterygial axis has shifted.

Figure 5. The basal theropod, Tawa, with its long manual digit 3. This is where the metapterygial axis has shifted.

Wagner and Gauthier (1999)
also point to the example of the Kiwi manus, some of which have only one finger and two metacarpals. One of these examples had one less phalanx than the other. IMHO you should use fully functioning examples, real wings and real hands, not tiny useless vestiges that are taking various fast tracks toward reduction and disappearance. Wagner and Gauthier also placed the phase shift between Torvosaurus and Allosaurus on their cladogram. That’s an odd place to put a major transition: between two giants. I put the new phase shift at the very base of the Dinosauria, just prior to Herrerasaurus and the basal phytodinosaur, Eoraptor, which also has vestigial lateral fingers.

Wagner and Gauthier (1999) also report,
“We are not aware of any other case in which such a conflict between a developmental and a functional constraint in digit reduction existed.” That’s true. And there is no such conflict in birds if one accepts the novel hypothesis that the metapterygial axis shifted medially as the lateral digits became useless vestiges.

The deeper you get into evolution, the more it all comes together…

References
Müller GB and Alberch P 1990. Journal of Morphology 203, 151–164.
Wagner GP and Gauthier JA 1999. 1,2,3 = 2,3,4: A solution to the problem of the homology of the digits in the avian hand. Proceedings of the National Academy of Science 96:5111-5116.

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.

One key fact overturns a bizarre interpretation steadily gaining steam

Figure 1. Jim Clark putting his best interpretive spin on the weird vestigial hand of Limusaurus. Click to play.

Figure 1. Jim Clark putting his best interpretive spin on the weird vestigial hand of Limusaurus. Click to play.

I ran across this YouTube video featuring Dr. Jim Clark talking about his then new ceratosaur dinosaur, Limusaurus, which we looked at earlier here. Clark addresses the importance of the hand of Limusaurus, which he claims with admirable confidence that this was a ‘transitional’ theropod that demonstrates how digit 1 was lost, digit 2 took on the appearance of digit 1, digit 3 took on the appearance of digit 2 and so on. That’s called a ‘phase shift’ as one digit takes on the identify of the one next to it and it is gaining wide acceptance, despite its bizarre premise.

That same hypothesis
is echoed here online at the Varagas Lab and it is becoming the standard paradigm on theropod hands. As an example, the recent paper on Haplocheirus labeled the three manual digits 2, 3 and 4.

This all started
with a report on chicken embryo hands by Thulborn and Hamely (1984), Thulburn (1993) and Burke and Feduccia (1997). Before then everyone labeled the three digits of theropod hands, 1, 2 and 3, which was eminently  logical. Shortly after the Burke and Feduccia study workers struggled against the new hypothesis, but recently (following the publication of Limusaurus) have rallied to support this hypothesis in papers appearing online here and here and here.

Let’s remind ourselves
Alan Feduccia has a vested interest in separating birds from dinosaurs.

This isn’t the first time Occam’s razor was ignored.
We’ve already seen several very odd paradigms arise in paleontology. A forelimb launch for giant pterosaurs is one such boondoggle. The extra bone in the wing of Yi qi is another. The nesting of Vancleavea and pterosaurs with archosauriformes are yet other examples. There are dozens of others. The theropod finger ‘phase shift’ is one more false paradigm that keeps spreading and needs to be stopped.

As discussed earlier, chicken embryos develop an extra medial finger as embryos. This finger bud ultimately disappears by the time of hatching. It is a relic from our basal tetrapod ancestry, from a time when our ancestors had six or more fingers. This has nothing to do with the normal count of five fingers in all post-Devonian tetrapods. We all develop through an embryonic stage when our hands are webbed mittens. In only one adult animal that we know of, Limusaurus, did this extra medial finger appear in an adult — and only because the hand of Limusaurus is a tiny vestige that stopped developing normally. Essentially it’s an embryo hand that retained the medial bud.

This one key fact
has been overlooked by other workers who have flocked to the ‘phase shift’ hypothesis. Which is the simpler explanation: 1) all the fingers suddenly appear like their neighbor finger, changing phalanx counts, or 2) an embryonic bud appears then disappears before hatching.

There is always a simple explanation
for every seemingly magical event or paleontological problem. The six-fingered ancestry of chickens is a key fact overlooked by modern theropod paleontologists who apparently are content to just count the fingers. Sometimes it’s not what you see that counts, but what experience you bring to what you see that trumps logic-busting arguments.

In theropods, what you see is what you get.
Fingers 1, 2 and 3 are indeed fingers 1, 2 and 3. In embryos you may see another medial digit, but it’s not homologous with the medial digit in any other tetrapod, except Limusaurus, as noted above. There is no phase shift of theropod fingers.

References
Burke AC and Feduccia A 1997. Developmental patterns and the identification of homologies in the avian hand. Science 278: 666–668.
Thulborn RA and Hamley TL 1984. ON the hand of Archaeopteryx. Nature 311:218.
Thulborn RA 1993. A tale of three fingers: Ichnological evidence revealing the homologies of manual digits in theropod dinosaurs. New Mexico Museum of of Natural History and Science Bulletion 3:461-463.

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.