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Dinogeorge Digest #5
Subj: Re: Back-evolution of limbs.
Date: 98-06-29 15:17:37 EDT
In a message dated 98-06-29 12:36:56 EDT, email@example.com
<< before either reptiles or mammals were terrestrial, their ancestors were
There's no way that the flippers of pinnipeds and cetaceans (and plesiosaurs,
mosasaurs, and ichthyosaurs, too) can be regarded as >reversals< to an
original ichthyoid condition. Tetrapod flipper anatomy differs greatly among
the groups and bears no more than an entirely superficial resemblance to fish
fins, the simple result of convergent evolution to adapt to aquatic
locomotion. If evolution is in fact a random walk through a multidimensional
morphological space, it is >unbelievably< unlikely for the walk to retrace
its path by chance for more than a couple of steps.
Subj: Re: Back-evolution of limbs.
Date: 98-06-29 15:17:29 EDT
In a message dated 98-06-29 12:39:19 EDT, firstname.lastname@example.org
<< Swans and geese use their wings as weapons. >>
I think most birds use their wings as weapons. Several years ago I watched a
mourning dove protect her young from a large jay on my office window sill by
smacking at the jay with her wings. At the time, this suggested to me that
the striking power of the maniraptoran forelimb might just as well have been
derived from a wing beat, rather than the other way round.
Subj: Re: SANDCOLEIFORMES&FLUTTERING
Date: 98-06-29 12:46:25 EDT
To: email@example.com, firstname.lastname@example.org
In a message dated 98-06-29 11:20:58 EDT, email@example.com
<< Aerodynamic stability is found in all sorts of flying. Fluttering for
display still is conjectural and it still needs some sort of control to
evolve into flight. >>
The glider-to-flier step may have been abetted by having two locomotor units:
a gliding unit to keep the animal in the air, and an evolving flying unit for
fine-tuning aerial control. For example, in _Longisquama_ the elongate
pre-feathers are implanted in the back and seem to have been movable (see
paper by Haubold & Buffetaut) to a horizontal position for gliding. If an
animal like _Longisquama_ (or even _L._ itself) also developed feathered
forelimbs, it could flutter with the forelimbs while sailing with the
pre-feathers. Eventually, as the forelimbs became better at flight control,
the dorsal gliding unit would vestigialize, until the forelimbs were doing
all the flying. As I noted in 1991, it seems far-fetched, but if there is
just no way to convert gliding forelimbs to flying forelimbs, something like
this may do.
I've noticed a correlation that might be worthwhile investigating further: in
both pterosaurs and birds, the relative size of the braincase increases at
about the same time as the tail shortens. I suggest this is part of the
conversion from stable flying to unstable flying, when the animal begins to
rely less on the tail and more on its brain power to control its flight.
Unstable flight is possible only if the brain is an active part of the flight
control system (so to speak). If this is so, one might predict the existence
of birds and pterosaurs with somewhat enlarged braincases and long tails, but
never the smaller braincases and short tails. It is mainly the enlargement of
the braincase that results in the rearrangement of the cranial bones from
typically reptilian to typically avian in birds, also enlargement of the
orbits--which could well be part of the same flight-control-improvement
Subj: Re: Natural Enemies
Date: 98-06-28 22:23:54 EDT
To: Raptor RKC
In a message dated 98-06-28 22:03:35 EDT, RaptorRKC@AOL.COM
<< Correct me if I'm wrong, but I think Lufengosaurus and Dilophosaurus were
found together (according to the book _Fossil Animals of China_) sort of
like Velociraptor and Protoceratops. >>
That's Dilophosaurus sinensis and Yunannosaurus sp. (probably robustus), not
Another such association is Aliwalia and Euskelosaurus in South Africa (see
Galton's paper on Aliwalia).
Association of skeletons is proof of contemporaneousness but not necessarily
of a predator/prey relationship. For predator/prey relationship, Tracy Ford
suggests teeth marks on bone matching teeth spacings in jaw of predator, and
shed teeth of predator found imbedded in prey bone. These still do not prove
that the predator actually killed the prey, only that it ate the carcass.
Behavior is notoriously difficult to infer from the fossil record.
Subj: Re: MY THOUGHTS ON THE 'DINO/BIRDS'
Date: 98-06-28 14:37:39 EDT
In a message dated 98-06-28 11:29:03 EDT, firstname.lastname@example.org
<< This highlights one of the bugbears I have argued about repeatedly here -
the assumption that gliding is THE intermediate stage between full powered
flight and flightlessness. There is no reason for this to be so, especially
in a cursorial form - fluttering as an assist to a leap could, in my view, do
just as well. >>
This is not some kind of untested assumption. I've yet to see a published
scenario in which avian ornithoptering flight evolves with any facility from
cursorial flightlessness. The physics is against it, and the change from a
jumping-and-fluttering behavior to a taking-off-and-flying lifestyle is
simply too profound. The evolution of avian flight--or any vertebrate
flight--with all its highly specific anatomical modifications requires
environmental selection pressure that will act on a large group of animals
for tens of millions of years. It is not a simple modification of some quirky
behavioral pattern; if it were, there would be hundreds of different kinds of
flying vertebrates instead of only three.