Page 19 - Introduction to Paleobiology and The Fossil Record

6 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

better way of describing what most scientists Other soft tissues, such as the heart, liver,
do. Reconstructing the bodily appearance and eyeballs, tongue and so on are rarely pre-
behavior of an extinct animal is identical to served (though surprisingly such tissues
any other normal activity in science, such as are sometimes exceptionally preserved; see
reconstructing the atmosphere of Saturn. The pp. 60–5), but again their size and positions
sequence of observations and conjectures that are predictable from modern relatives. Even
stand between the bones of Brachiosaurus the skin is not entirely guesswork: some mum-
lying in the ground and its reconstructed mifi ed dinosaur specimens show the patterns
moving image in a movie is identical to the of scales set in the skin.
sequence of observations and conjectures that The second step is to work out the basic
lie between biochemical and crystallographic biology of the ancient beast. The teeth hint at
observations on chromosomes and the cre- what the animal ate, and the jaw shape shows
ation of the model of the structure of DNA. how it fed. The limb bones show how the
Both hypotheses (the image of Brachiosaurus dinosaurs moved. You can manipulate the
or the double helix) may be wrong, but in joints and calculate the movements, stresses

both cases the models reflect the best fi t to and strains of the limbs. With care, it is possi-
the facts. The critic has to provide evidence ble to work out the pattern of locomotion in
to refute the hypothesis, and present a replace- great detail. All the images of walking, running,

ment hypothesis that fits the data better. Refu- swimming and flying shown in documentaries
tation and skepticism are the gatekeepers of such as Walking with Dinosaurs (see Box 1.2)
science – ludicrous hypotheses are quickly are generally based on careful calculation and
weeded out, and the remaining hypotheses modeling, and comparison with living animals.
have survived criticism (so far). The movements of the jaws and limbs have to
obey the laws of physics (gravity, lever mechan-
ics, and so on). So these broad-scale indica-
Fact and fantasy – where to draw the line?
tions of paleobiology and biomechanics are
As in any science, there are levels of certainty defensible and realistic.
in paleontology. The fossil skeletons show the The third level of certainty includes the
shape and size of a dinosaur, the rocks show colors and patterns, the breeding habits, the
where and when it lived, and associated fossils noises. However, even these, although entirely
show other plants and animals of the time. unsupported by fossil data, are not fantasy.
These can be termed facts. Should a paleontol- Paleontologists, like any people with common
ogist go further? It is possible to think about a sense, base their speculations here on com-
sequence of procedures a paleontologist uses parisons with living animals. What color was
to go from bones in the ground to a walking, Diplodocus? It was a huge plant eater. Modern
moving reconstruction of an ancient organism. large plant eaters like elephants and rhinos
And this sequence roughly matches a sequence have thick, gray, wrinkly skin. So we give
of decreasing certainty, in three steps. Diplodocus thick, gray, wrinkly skin. There’s
The fi rst step is to reconstruct the skeleton, no evidence for the color in the fossils, but it
to put it back together. Most paleontologists makes biological sense. What about breeding
would accept that this is a valid thing to do, habits? There are many examples of dinosaur
and that there is very little guesswork in iden- nests with eggs, so paleontologists know how
tifying the bones and putting them together many eggs were laid and how they were
in a realistic pose. The next step is to recon- arranged for some species. Some suggested
struct the muscles. This might seem highly that the parents cared for their young, while
speculative, but then all living vertebrates – others said this was nonsense. But the modern
frogs, lizards, crocodiles, birds and mammals relatives of dinosaurs – birds and crocodilians
– have pretty much the same sorts of muscles, – show different levels of parental care. Then,
so it is likely dinosaurs did too. Also, muscles in 1993, a specimen of the fl esh-eating dino-
leave scars on the bones that show where they saur Oviraptor was found in Mongolia sitting
attached. So, the muscles go on to the skele- over a nest of Oviraptor eggs – perhaps this
ton – either on a model, with muscles made was a chance association, but it seems most
from modeling clay, or virtually, within a likely that it really was a parent brooding its
computer – and these provide the body shape. eggs (Box 1.1).