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.
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).
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)
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)
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)
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.
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.
=============================================================
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