Abstract

The mainstream dinosaur-to-bird thinking is that Paraves is a node on a proposed lineage from dinosaur to bird. The thinking is that basal Paraves was not a power flier. However, the evidence shows that basal Paraves was a power-flying, primitive bird. It had extensive flying bird-like characteristics.

1.0 Materials and methods

An extensive review and analysis was done of the literature concerning the basal Paraves. The mainstream idea is that basal Paraves were non-flying. However the evidence supports the idea that basal Paraves were powered fliers.

To begin, let us look at the characteristics of the basal Paraves as seen in the published material:

1.1 Basal Paraves bird-like characteristics

Basal Paraves includes Anchiornis, Aurornis, Scansoripoterygids, Pedopenna, Xiaotingia and others.

Notice below the extensive set of flying bird-like characteristics attributed to Paraves

(Xu et al 2014). Keep in mind that Paraves also had the extensive list of flying bird-like characteristics attributed to Pennaraptora.

Used with permission of the author.

Attributed to Pennaraptora:

V-shaped furcula; advanced costosternal ventilator pump; initial arm flapping capability; further increased basal metabolic rate; cerebral expansion; increased laterally folding capability; partial pubis posterior orientation; short bony tail; three-fingered hand; symmetrical vaned feathers.

* NOTE: Only highly derived members (Pygostylia) had a short bony tail. Basal Pennaraptora had a long bony tail.

Attributed to Paraves:

Arm elongation and thickening; initial aerial locomotion; extreme miniaturization; partial knee-based locomotion; visually associated brain regions elaboration; asymmetrical vaned feathers.

* Note: Only derived members of Paraves had asymmetrical vaned feathers. Basal Paraves had symmetrical vaned feathers.

Basal paravians had many hallmark features necessary for flight, including extremely small body size; a laterally oriented, long, and robust forelimb; an enlarged forebrain and other derived neurological adaptations; and large flight feathers.

(Xu et al 2014)

1.2 Tetrapteryx (4 winged)

It has been hypothesized that bird flight went through a four-winged “tetrapteryx" stage.
Epidendrosaurus, Epidexipteryx and Eosinopteryx, are basal paravians. Their phylogeny is entirely consistent with the presence of a tetrapterygian condition (= four winged) and elongated rectrices in basal Paraves. (Godefroit et al, 2013a)

Large pennaceous feathers are now known to occur on the lower leg and particularly the metatarsus of at least one basal member of each of the three major paravian groups, namely the basal troodontid Anchiornis, the basal avialan Pedopenna and the basal dromaeosaurid Microraptor. This led to a four-winged condition at the base of the Paraves. (Hu et al 2009)

1.3 Arboreal

Scansoriopterygidae (a basal Paraves) had clear adaptations to an arboreal or semi-arboreal lifestyle–it is likely that they spent much of their time in trees.

Scansoriopteryx is considered to be arboreal based on the elongated nature of the hand and specializations of the foot. The long hand and strongly curved claws were adaptations for climbing and moving around among tree branches. (Zhang et al. 2002).

Scansoriopteryx, presents evidence for an arboreal lifestyle. (Czerkas, Yuan 2002)

1.4 Pennaceous Feathers

Pennaraptora is defined as having “remiges and rectrices, that is, enlarged, stiff-shafted, closed-vaned (= barbules bearing hooked distal pennulae), pennaceous feathers arising from the distal forelimbs and tail”. (Gauthier, de Queiroz 2001).

Paraves were members of Pennaraptora and thus had this characteristic.

1.5 Long and robust forelimbs

The significant lengthening and thickening of the forelimbs indicates a dramatic shift in forelimb function at the base of the Paraves, which might be related to the appearance of a degree of aerodynamic capability (Xu et al al 2011)

1.6 Miniaturization

Miniaturization and wing expansion, critical anatomical requirements to be a bird, arose among the wider clade Paraves (Puttick et al 2014)

1.7 Pelvic limb

The evolution of enlarged forelimbs is strongly linked, via whole-body centre of mass, to hindlimb function during terrestrial locomotion. The evolution of avian flight is linked to anatomical novelties in the pelvic limb as well as the pectoral. (Allen et al 2013)

1.8 Brain

If Archaeopteryx has a ‘flight-ready’ brain, which is almost certainly the case given its postcranial morphology, then so did other paravians. The hypothesis that dromaeosaurs and troodontids had the neurological capabilities required of powered flight, gliding, or some intermediate condition is congruent with the discovery of the ‘four-winged’ deinonychosaurs, Microraptor zhaoianus and Anchiornis huxleyi (Balanoff et al 2013)

1.9 Uncinate processes

The uncinate processes in non-avian maniraptoran dinosaurs [paravians] are not reduced as in running birds but resemble those of the flying or diving birds. (Codd et al 2008)

What can we conclude about the flying capability of basal Paraves?

2.0 Evidence that Paraves could fly

Basal Paraves includes Anchiornis, Aurornis, Scansoripoterygids, Pedopenna, Xiaotingia and others.

We have seen the extensive set of flying bird-like characteristics they had. Could they fly?

We need to look at the characteristics related to forewings, hindwings, airfoil, sternum, propatagium, semilunate carpal and feathers. And we need to consider possible objections.

2.1. Forewings (Shoulder mechanism)

It is sometimes thought that the primitive birds could not fly because they did not have the needed shoulder mechanism for powered flight. It is true that the lack of a derived supracoracoideus precluded takeoff from the ground. But takeoff from an elevated perch would still be possible.

The dorsal elevators, principally the deltoideus major, can effect the recovery stroke by themselves, as they did in Archaeopteryx. Maxheinz Sy proved this when he cut the tendons of the supracoracoideus in living crows and pigeons (1936). Sy found that pigeons were capable of normal, sustained flight; the only capacity they lost was the ability to take off from level ground. (Feduccia 1999)

Microraptor lacked the necessary adaptations in its shoulder joint to lift its front wings high enough vertically to generate lift from the ground. This leaves only the possibility of launching from an elevated perch and even modern birds do not need to use excess power when launching from trees, but use the downward-swooping technique found in Microraptor. (Chatterjee, Templin 2007).

2.2. Hindwings

It is sometimes thought that the primitive birds lacked sufficient lift. But the hindwings generated additional lift.

In Microraptor the metatarsal feathers are similar in general arrangement (nearly perpendicular to the metatarsus, forming a large flat surface) and in having stiff vanes and curved rachises. These features suggest that the metatarsal feathers were aerodynamic in function, providing lift and thus played a role in flight (Zheng et al 2013)

Microraptor gui preserves evidence of extensive, lift-generating feathers on each manus and forearm, but also preserves evidence of lift-generating feathers associated with the hindlimbs, effectively forming a pair of “hindwings”. ( Hall et al 2012)

2.3. Airfoil

It is sometimes thought that the primitive birds could not fly because they did not have asymmetric flight feathers and thus lacked an airfoil. It is true that they did not have asymmetric flight feathers. But that does not preclude powered flapping flight.

Although the slender feather shafts of Archaeopteryx and Anchiornis make individual feathers weak, layering of the wing feathers may have produced a strong airfoil. (Longrich et al 2012)

The elongated wing feathers of primitive birds exhibit small barb angles in cutting-edge leading vanes that are comparable with those of modern flying birds. This suggests that the leading vanes of these Mesozoic feathers were similarly capable of withstanding aerodynamic forces in airflow. (Feo et al 2015)

2.4. Sternum

It is sometimes thought that the primitive birds could not fly because some did not have an ossified sternum. It is true that some did not have an ossified sternum but that does not preclude flapping flight.

An ossified sternum and uncinate processes are absent as in Anchiornis, Xiaotingia and troodontids. (Xu et al 2011)

But:

The supracoracoideus muscle, and hence an ossified sternum, is not necessary to effect the recovery stroke of the wing. There is nothing in the structure of the pectoral girdle of Archaeopteryx that would preclude its having been a powered flier. (Olson, Feduccia, 1979)

Ossified sternal plates are known in basal dromaeosaurids, oviraptorosaurs, and scansoriopterygids, forming a fully fused sternum in some individuals of the first two groups. (O'Connor, Sullivan 2014)

2.5. Propatagium (Lift)

The lift generating effect of the propatagium must also be considered.

The cambered propatagium is the major lift generating component of the wing proximal to the wrist. (Brown et al 1996)

2.6. Semilunate carpal

Paravians are characterized by long arms and three-fingered hands as well as a "half-moon shaped" (semi-lunate) bone in the wrist (carpus).

Scansoriopterygids had a semilunate carpal (half-moon shaped wrist bone) that allowed for bird-like folding motion in the hand. By folding its wings (decreasing the wingspan) a bird can reduce drag during the upstroke.

Anchiornis exhibits some wrist features indicative of high mobility, presaging the wing-folding mechanisms seen in more derived birds and suggesting rapid evolution of the carpus. (Xu et al 2009)

2.7. Weak Feathers

It is sometimes thought that the primitive birds could not fly because their feathers were too weak. However:

The relatively weak-looking flight feathers of basal birds, do not necessarily suggest that flight capability was poor, let alone entirely absent. Early birds and their close relatives could assemble an effective flying wing using multiple rows of relatively weak feathers. (Longrich et al 2012)

2.8 Uncinate processes

It is sometimes thought that the primitive birds could not fly because some lacked uncinate processes.

Moreover, Anchiornis and the more derived Troodontidae lack the skeletal adaptations for powered flight present in Microraptor and Eudromaeosauria：ossified sternum & sternal ribs & uncinate processes on dorsal ribs（Paul, 2002, 2010； Hu etal., 2009) (Sorkin 2014)

But:

The screamers are a small clade of birds (Anhimidae) The clade is exceptional within the living birds in lacking uncinate processes of ribs.[3] (Fowler ME & Cubas ZS (2001). Biology, medicine, and surgery of South American wild animals. Wiley-Blackwell. p. 103.)

Some basal Paravians had uncinate processes:

The uncinate processes in non-avian maniraptoran dinosaurs [paravians] are not reduced as in running birds but resemble those of the flying or diving birds. (Codd et al 2008)

2.9 Apomorphy of Avialae

Also, we can conclude that basal Paraves were capable of flapping flight because their flight-related characteristics have often caused them to be placed within Avialae:

Avialae is defined as an apomorphy-based clade that “possessed feathered wings used in flapping flight, and the birds that descended from them.”

For example, Pedopenna was originally classified as a paravian, but some scientists have classified it as a true avialan.

3.0 Problems with the idea that basal Paraves were ground dwelling

There are a number of problems with the mainstream idea that basal Paraves were ground dwelling (non-flying).

Problems include:

· flight had to evolve multiple times independently (homoplasy).

· it requires numerous exaptations (they were not using their bird-like characteristics for flying)

· the hindwing feathers would interfere with running

3.1 Extensive Homoplasies Required

Convergent evolution creates analogous structures that have similar form or function, but that were not present in the last common ancestor of those groups. The cladistic term for the same phenomenon is homoplasy.

A polyphyletic group is characterized by one or more homoplasies.

Many biologists aim to avoid homoplasies in grouping species together and therefore it is frequently a goal to eliminate groups that are found to be polyphyletic. This is often the stimulus for major revisions of the classification schemes.

The origin of flight in avialans might have involved several convergent achievements of aerial abilities. (Foth, Tischlinger, Rauhut 2014)

As a result of the high amount of homoplasy that characterizes derived maniraptoran evolution, the identity of the avian sister taxon remains debated despite the rapid accumulation of morphological data. (O’Connor, Sullivan 2014)

Flight capability is likely to have evolved independently on multiple occasions among Archaeopteryx and its kin (Longrich et al 2012)

Xiaotingia zhengi independently evolved some salient features seen in other maniraptoran taxa, which highlights the extensive homoplasy that exists among maniraptorans. ( Xu et al 2011)

Thus, bird-like encephalization indices evolved multiple times, supporting the conclusion that if Archaeopteryx had the neurological capabilities required of flight, so did at least some other non-avian maniraptorans. (Balanoff et al 2013)

3.2 Numerous Exaptations Required

Exaptation and the related term co-option describe a shift in the function of a trait during evolution. For example, a trait can evolve because it served one particular function, but subsequently it may come to serve another.

In the dinosaur to bird theory, there are an extensive number of required exaptations.

Abducted wrists, feathers, enlarged brains and laterally oriented, long and robust forelimbs and forelimb myology and breathing apparatus are claimed to have evolved before they were used for flight.

Cretaceous dromaeosaurs, troodonts, oviraptorosaurs and therizinosaurs had advanced flight-related characters. Obvious flight adaptations (oversized sternal plates, folding arms, pterosaur or bird-like tails) are usually explained away as exaptations, and pennaceous feathers are supposed to have evolved before flight. (Paul)

Abducted wrists (Semilunate carpal)

It is likely that mobility of the wrist was initially associated with other functions, such as predation.
Partial folding of the wing during the upstroke in extant birds, which requires significant abduction of the wrist could then be seen as an exaptation. (Sullivan et al 2010)

Feathers

Researchers have speculated early feathers may have been used for attracting mates or keeping warm. But later on, feathers became essential for modern birds’ flight.

This further supports the hypothesis that "flight feathers" that first evolved for non-aerodynamic functions were later exapted to form lifting surfaces. (Dyke et al 2013)

Enlarged brains

If Archaeopteryx had a ‘flight-ready’ brain , which is almost certainly the case given its postcranial morphology, then so did other paravians. Paraves had the neurological capabilities required of powered flight, gliding, or some intermediate condition. (Balanoff et al 2013)

Laterally oriented, long and robust forelimbs

Basal paravians had many hallmark features necessary for flight, including a laterally oriented, long, and robust forelimb. ( Xu et al 2014)

Forelimb myology and breathing apparatus

The origin and evolution of flight were more complex than previously thought, and forelimb myology and breathing apparatus could draw on structures that evolved in different functional contexts. [exaptation] (Foth et al 2014)

Attributed to Pennaraptora:

advanced costosternal ventilator pump (Xu et al 2014)

Objection

Current exaptational explanations are often not fully formulated and rarely offer a biologically plausible hypothesis to account for their origin. (James, Pourtless 2009)

3.5 The hindwing feathers would interfere with running

With large foot remiges cursorial locomotion was likely problematic for Anchiornis. (Pascal Godefroit et al (2013b)

In both Anchiornis and Microraptor the long metatarsal feathers would have interfered with terrestrial locomotion（Xu et al., 2003, Hu et al., 2009）(Sorkin 2014)

4.0 Possible Objection

It may be objected that exaptations must have occurred, because the flying bird-like characteristics were around before basal Paraves.

The counter to this is that many of the flying bird like characteristics appeared for the first time at the base of the Paraves.

The evidence indicates that the Paraves flying bird-like characteristics appeared at the base of Paraves.

Before the origin of Aves, on the branch leading to Paraves, high rates of evolution led to a smaller body size and a relatively larger forelimb in Paraves. These changes are on a single branch leading to Paraves, representing a shift to a new smaller size and larger forelimb at this point.
Paraves, rather than Aves alone, shifted to a different evolutionary model. On all trees and for both femur and forelimb size, the model with a regime shift at Paraves, rather than Aves, is favored. (Puttick et al 2014)

This suggests that large pennaceous feathers first evolved distally on the hindlimbs, as on the forelimbs and tail. This distal-first development led to a four-winged condition at the base of the Paraves. (Hu et al 2009)

The significant lengthening and thickening of the forelimbs indicates a dramatic shift in forelimb function at the base of the Paraves, which might be related to the appearance of a degree of aerodynamic capability (Xu et al 2011)

Paraves, exclusive of Epidexipteryx hui, is marked by a suite of modifications to the shoulder girdle typically associated with the origin of the ‘‘avian’’ flight stroke (Ostrom, 1976b; Jenkins, 1993). The acromion margin of the scapula has a laterally everted anterior edge (char. 133.1) (fig. 55), the coracoid is inflected medially from the scapula forming an L-shaped scapulocoracoid in lateral view (char. 137.1) and the glenoid fossa faces laterally (char. 138.1) as opposed to the plesiomorphic posterior orientation (fig. 50). Additionally, the furcula is nearly symmetrical in shape as opposed to the asymmetry present in the furcula of more basal taxa (char. 474.1).
(Turner et al. 2012)

For example, the early evolution of paravian theropods features cerebral expansion and elaboration of visually associated brain regions (71), forelimb enlargement (22, 67), acquisition of a crouched, knee-based hindlimb locomotor system (67), and complex pinnate feathers associated with increased melanosome diversity, which implies a key physiological shift (72). Together these features may suggest the appearance of flight capability at the base of the Paraves (22, 67). (Xu et al 2014)

5.0 Conclusion

Altogether the evidence is substantive that Paraves was a power-flying, primitive bird. This hypothesis is supported by the evidence and does not require the extensive amount of exaptation and convergence that the current mainstream hypothesis requires. It is thus the more plausible, parsimonious hypothesis.

The next question is: What was the ancestor of flying Paraves like? That is the direction future research could take.

=============================================================

REFERENCES

Allen, Vivian, Karl T. Bates, & John R. Hutchinson (2013)

Linking the evolution of body shape and locomotor biomechanics in bird-line archosaurs

Nature 497, 104–107 (02 May 2013)

Balanoff , Amy M., Gabe S. Bever, Timothy B. Rowe & Mark A. Norell (2013)

Evolutionary origins of the avian brain

Nature. 2013 Sep 5;501(7465):93-6

Brown, Richard E. and Allen C. Cogley (1996)

Contributions of the propatagium to avian flight

Journal of Experimental Zoology Volume 276, Issue 2, pages 112–124, 1 October 1996

Chatterjee, S., and Templin, R.J. (2007).

Biplane wing planform and flight performance of the feathered dinosaur Microraptor gui. Proceedings of the National Academy of Sciences, 104(5): 1576-1580

Codd, Jonathan R, Phillip L Manning, Mark A Norell, Steven F Perry (2008)

Avian-like breathing mechanics in maniraptoran dinosaurs

Proc Biol Sci. 2008 Jan 22; 275(1631): 157–161.

Czerkas, S.A., and Yuan, C. (2002). "An arboreal maniraptoran from northeast China." Pp. 63-95 in Czerkas, S.J. (Ed.), Feathered Dinosaurs and the Origin of Flight. The Dinosaur Museum Journal

Dyke, Gareth, Roeland de Kat, Colin Palmer, Jacques van der Kindere, Darren Naish & Bharathram Ganapathisubramani (2013).

Aerodynamic performance of the feathered dinosaur Microraptor and the evolution of feathered flight. Nature Communications 4, Article number: 2489doi:10.1038/ncomms3489

Feduccia, Alan (1999)

The Origin and Evolution of Birds

Yale University Press

Feo Teresa J., Daniel J. Field, Richard O. Prum (2015)

Barb geometry of asymmetrical feathers reveals a transitional morphology in the evolution of avian flight Royal Society Publishing

Foth, Christian, Helmut Tischlinger & Oliver W. M. Rauhut (2014)

New specimen of Archaeopteryx provides insights into the evolution of pennaceous feathers Nature 511, 79–82 (03 July 2014)

Gauthier, J. and de Queiroz, K. (2001).

"Feathered dinosaurs, flying dinosaurs, crown dinosaurs, and the name 'Aves'".

Pp. 7-41 in Gauthier, J. and L.F. Gall (eds.), New Perspectives on the Origin and Early Evolution of Birds: Proceedings of the International Symposium in Honor of John H. Ostrom. New Haven: Peabody Museum of Natural History, Yale University.

Godefroit ,Pascal , Andrea Cau , Hu Dong-Yu, François Escuillie´, Wu Wenhao & Gareth Dyke(2013a)

A Jurassic avialan dinosaur from China resolves the early phylogenetic history of birds

Nature 498, 359–362 (20 June 2013)

Godefroit, Pascal, Helena Demuynck, Gareth Dyke, Dongyu Hu, François Escuillié & Philippe Claeys (2013b)

Reduced plumage and flight ability of a new Jurassic paravian theropod from China

Nature Communications 4, Article number: 1394

Hall J, Habib M, Hone D, Chiappe L (2012)

A new model for hindwing funtion in the four-winged theropod dinosaur Microraptor gui Society of Vertebrate Paleontology Annual Meeting, 2012. Raleigh, NC.

Hu Dongyu, Lianhai Hou, Lijun Zhang & Xing Xu (2009)

A pre-Archaeopteryx troodontid theropod from China with long feathers on the metatarsus

Nature 461, 640-643 (1 October 2009)

James, Frances C. and John A. Pourtless IV (2009)