Re: [dinosaur] 2016 in paleontology + Dippy's last days in London + burrows around Omeisaurus + more

[Tim Williams <tijawi@gmail.com>]

Ronald Orenstein <ron.orenstein@rogers.com> wrote:

> Is it possible that gliding in a non-flyer is not a good precursor for 
> flapping flight, and that true gliders (eg
> colugos) are not "evolving towards" flight but are on a different, perhaps 
> incompatible evolutionary path
> (so that powered flyers start out as leaders/flappers from an early stage)?


I've wondered the exact same thing - are all powered fliers derived
from leapers/flappers rather than passive gliders?  For flying
vertebrates anyway: pterosaurs, birds (and microraptorines, if they
evolved flight independently), and bats.  Outside of the vertebrates,
pterygote insects are the subject of their own 'top-down'
(gliding-first) versus 'bottom-up' (flapping-first) debate - also
unresolved.

There is a possibility that gliding is not conducive to the evolution
of powered flight in vertebrates.  A transition from gliding to
flapping flight is theoretically possible, both aerodynamically and
biomechanically.  But there may be ecological factors that make this
transition difficult.  Research on colugos ('flying lemurs') has
inferred that, as a form of locomotion, gliding is not as economical
or energy-saving as conventionally thought (Byrnes et al. 2011;
'Gliding saves time but not energy in Malayan colugos' J. Exp. Biol.
214: 2690-2696).  So on this basis, 'passive' gliding might not be a
suitable precursor to 'active' flapping flight, which is a much more
energetically expensive behavior than parachuting or gliding.

Pterosaurs:- Basal pterosaurs are fascinating beasties, but they're
not especially helpful in reconstructing the habits of pterosaur
ancestors.  The closest known relative of pterosaurs is believed to be
_Scleromochlus_, which is inferred to be a terrestrial leaping biped.
Can _Scleromochlus_ be used to support to a leaper/flapper origin of
pterosaur flight?

Bats:- A gliding ancestry has traditionally been favored for the
origin of flight in bats.  This hypothesis still has widespread
support, even though colugos (Dermoptera) are no longer usually
regarded as the sister taxon to bats (Chiroptera) (united in the group
Volitantia).  For example, the description of the most basal known
chiropteran _Onychonycteris_ favored a gliding origin of flapping
flight on "anatomical and aerodynamic grounds" (Simmons et al. 2008;
Nature 451: 818-822). However, the alternative hypothesis is that
proto-bats leaped into the air (from trees or cliffs), and used their
patagia as control surfaces (such as for aerial hawking).  Padian and
Dial (2013) also questioned a gliding origin for bats, citing
phylogenetic and ontogenetic contexts:


Padian K, Dial KP. Did bat ancestors glide? A phylogenetic approach.
Society for Integrative and Comparative Biology 2013 Annual Meeting
28.1  Friday, Jan. 4

Abstract: "The predominant biological view of the evolution of flight
is that it is preceded phylogenetically by a gliding stage. Support
for this hypothesis has mainly rested on what is presumed to be
“easier” or “necessary” based on models. The hypothesis can be
empirically tested by examining the comparative phylogenetic positions
of gliders and flyers. The three known clades of living and extinct
vertebrate flyers are far removed from the 15 known clades of living
and extinct gliders. The problem is particularly acute with bats,
which are far removed from all eight clades of mammalian gliders, and
are nested within a clade that contains only terrestrial and fossorial
forms. We used phylogenetic analyses of major clades of bats, and the
extinct chiropteran outgroups of crown-group bats, to assess ancestral
states for ecological characters related to locomotion, echolocation,
diet, and habitat. The ancestor of crown-group bats likely was
insectivorous, echolocated as most bats do, could climb quadrupedally,
and had poor terrestrial locomotory skills; the ancestral habitat is
difficult to determine. Inferences about stem-group bats involve
character states of fossil bats. Flight phylogenetically preceded
advanced echolocation; the most basal stem-bats could climb, but their
habitats are difficult to specify. No outgroups to bats are or
apparently were bipedal; thus the forelimbs of bats could only be
freed to evolve powered flight if standard quadrupedal locomotion was
modified. Ontogeny also speaks against gliding in bat precursors. Bat
wings develop by hypertrophy of the manus and chiropatagium (the
thrust-producing part of the wing), not the medial part of the wing
(brachiopatagium) that produces lift in gliding."