Pterosaurs : from deep time

352 Pages·2006·28.665 MB·English

by  David Unwin

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PTEROSAURS
C O N T E N TS
1 DRAGONS OF THE AIR, 2
2 PTEROSAUR PLANET, 16
3 CONSIDERING MEDUSA, 30
4 A TREE FOR PTEROSAURS, 56
5 THE HEAD INSIDE OUT, 88
6 THE BODY INSIDE OUT, no
7 BABES ON THE WING, 140
8 HIGH FLIERS, 164
9 GROUNDED, 196
10 THE PTEROSAUR STORY, 224
11 POSTSCRIPT, 266
APPENDIX: LIST OF VALID PTEROSAUR SPECIES, 271
NOTES AND SOURCES, 275
BIBLIOGRAPHY, 311
ACKNOWLEDGMENTS, 329
INDEX, 331

DRAGONS OF THE AIR
The dragon blinked in the fierce light of the sun as it emerged from
the clouds and banked hard, its tremendous wings arching under the
load. Reflected in a massive dark eye, the world below slowly tilted
into view. Vast herds of dinosaurs were strung out across a dusty
yellow-orange plain, occasionally gathered in knots where they had
stopped to feed on patches of stunted vegetation. Then came marshes
and— the dragon focused— a long, still, clear-blue lake. In its glassy
depths, rainbow-hued fish hung almost motionless in the warm water,
fins undulating, gills slowly pulsing. Suddenly, a huge dark shadow
swept across the water's surface and, before it could dive into the safety
of the weeds, a fish was gripped by long, sharp-pointed jaws and
jerked violently into the air. As its consciousness faded, it gazed up
into a limpid eye, set in a reptilian skull framed by fine, straggly hair.
Slender, powerful wings beat smoothly up and down, membranes
tensed and relaxed as the animal rose swiftly. At the top of its climb
the Quetzalcoatlus paused, swallowed, then dived down again toward
the shimmering lake far below.'
FIGURE 1.1 A beautiful new fossil of the Upper Jurassic
(approximately 148 million years old) Solnhofen Limestone
pterosaur Anurognathus, only the second example yet known for
this species. This pterosaur had a wingspan of about 16 inches
(40 centimeters). (Photograph courtesy of Helmut Tischlinger.)
4 THE PTEROSAURS
Around 215 million years ago, at about the same time as dinosaurs first
spread across the continents, an altogether different group of reptiles took
to the air. Reptiles had tried this before, on several occasions, but this time
was different. This time they got beyond gliding, where all previous groups
had given up, and, as bats and birds were to do millions of years later, they
evolved a rare and complex adaptation: true flapping flight. Going boldly
where no reptile had gone before, these intrepid aeronauts entered a new
realm— the open sky— and developed into a riotous multitude of species.
Some, such as the frog-headed beetle-cruncher Anurognathus, shown in Fig
ure 1.1, were as small as a starling, but others, like Quetzalcoatlus, became as
large as an airplane. They thrived for 150 million years, only to disappear
forever in the events that also killed off many of their contemporaries, in
cluding, most famously, the dinosaurs, and brought evolution's finest hour,
the Mesozoic, to a close. This extraordinary group of animals— nature's real
dragons— was the pterosaurs.2
The Trouble with Pterosaurs Pterosaurs, "winged reptiles," as their
Greek name puts it, are familiar to most people as rather fierce-looking,
leather-winged monsters featured in classic novels, such as Arthur Conan-
Doyle's Lost World, or flit across the screen in TV documentaries3 and in
movies from King Kong to Jurassic Park.4 No dinosaur scene is complete with
out them— usually a Pteranodon, the most famous pterosaur of them all, with
large teeth to give it added fierceness, even though this species was actually
completely toothless.5 Trying to see beyond these superficial images and get
a glimpse of the real thing triggers all sorts of questions: What were ptero
saurs really like? How big did they get? Could they fly as well as birds? And
one of the first questions that is always asked and hardest to answer: Why
did they become extinct?
Driven more than most by curiosity, scientists find these strange crea
tures absolutely fascinating (pterosaur talks always fill the hall at scientific
meetings) and some of the sharpest paleontological minds ever to ponder a
fossil were so beguiled by these extraordinary animals that they made them
the centerpiece of their researches. The first thing they discovered was that
pterosaurs are really hard to understand. Even the term embodied in their
name— winged reptile— seems contradictory. The word reptile, from the
French "repere," meaning to creep, is not especially flattering, but describes
living reptiles rather well. Wings, on the other hand, are about the last thing
DRAGONS OF THE AIR 5
one might expect such a creature to have, and yet for pterosaurs, this defined
them and their very existence.
As soon as pterosaurs were discovered, at the end of the 18th century,
naturalists started quarrelling about them. Disagreements came thick and
fast: Pterosaur origins, for instance, were hotly debated on several occa
sions, and, in one particular case detailed in Chapter 4, led to a protracted
and acrimonious feud, while several other disagreements, for example, over
wing shape and walking posture, continue even now. It was a long time be
fore scientists could even agree as to what kind of animal pterosaurs might
be— reptiles, as it turned out, but others argued that they were birds, and
several eminent naturalists became convinced that they were bats. Debates
also raged over the lifestyle of pterosaurs. Were they some kind of water
creature? Or could they fly? And what did they do on the ground— strut
around on their hind limbs like birds, or clump around on all fours? Even
their likely metabolism became a scientific battleground: cool-blooded like
crocodiles and lizards, insisted several authorities, warm-blooded like birds
and mammals, countered others.
Pterosaurs became a cause celebre, one of the great paleontological myster
ies. Each generation of scientists "had a go" at them, and everyone had an
opinion that, almost without exception, differed from that held by everyone
else. The arguments, the confusion and the misunderstandings continued
right up until a decade ago, and a few persist even today, but before we get
into that, we should look a little more closely at why these ancient fliers
caused so much controversy in the first place.
A Fossil Problem Pterosaurs have successfully defied more than two
centuries' worth of scientific probing for several reasons. The most obvious
is the problem of trying to understand animals that are known only from
fossils. Just a tiny proportion of all the pterosaurs that ever existed, prob
ably less than one individual in a million, has actually made it into the fossil
record. The processes by which their cadavers became fossils, normally sur
vived only by the hardest parts of the body— bones and teeth6— mean that
most of the important information about anatomy, movement and behavior,
how pterosaurs were colored, what noises they made, was lost forever.
Compounding the problem, even pterosaurs' hard parts were not well-
suited for the rigors of fossilization. Pterosaurs were creatures of the air,
with a relatively light and delicate skeleton constructed from slender, hollow
6 THE PTEROSAURS
bones whose walls were often little thicker than a credit card. This is not
a good design if you want to become a fossil. To begin with, it meant that
even pterosaurs' skeletons were relatively easily destroyed, so, compared with
other backboned animals, their fossil remains are rare. Worse still, if you
pull open a museum drawer, you find that most of their fossilized remains
consist of isolated, often broken, bones— dumb witnesses that tell us little
more than "here be pterosaurs." To cap it all, most of the decent pterosaur
fossils that we do have, whole skeletons and, very occasionally, fragments of
fossilized soft tissues, come from just a few locations scattered across the
world and are separated by vast, barren, pterosaur-less gaps of thousands of
miles and millions of years.
Thin-walled bones also mean that the complete skeleton of Anurogna-
thus, shown in Figure 1.1, buried at the bottom of a Bavarian lagoon 148 mil
lion years ago, is rather less helpful than one might expect. Like many other
beautiful-looking pterosaurs, several of which are featured in this book, it is
absolutely flat— a "picture" fossil— its hollow-tube bones unable to resist the
inexorable crushing weight applied over countless millennia by the overlying
rock. Without the three-dimensional, sticking-out-here, dimpled-in-there
form of the skull, the pelvis, or any of the 300 or so bones that made up a
pterosaur skeleton, and unable to measure the exact shape, size and position
of the joints, paleontology is robbed of critical data.
Confronted with a row of these "road-kills," it is often hard for an observer
to establish even basic facts, such as: How many species are there? Two? Or
more? Or just one, its representatives flattened in different ways? Trying to
go further and find out, for example, how these pterosaurs might have stood,
walked, or flown, is even harder. Strictly speaking, these are relatively simple
questions (some of the really tough ones, for example, about physiology and
breathing, will pop up later), but, enigmatic as the Mona Lisa, and sometimes
just as smiley, the "picture" pterosaurs rarely give an answer.
More Problems: Analogy and Chauvinism As if fossils themselves
were not difficult enough to interpret, the ways in which scientists have
gone about studying them also have their pitfalls. One trap that pterosaur
researchers seem to have queued up to throw themselves into is a method
much used, and no less often abused, by paleontologists: analogy. Confront
ed with the fossilized remains of an organism from deep time— be it a tiddly
little ammonite, a 10-ton dinosaur or a toothless Pteranodon, all extinct for
half an eternity and tragically bereft of any living descendants— it is terri-