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There is no branch of detective science so important and so much neglected as the art of tracing footprints.A. Conan Doyle 1861
(Study in Scarlet)
This begins two lectures on dinosaurian metabolism, the first of which looks at aspects of dinosaur metabolsim and behavior that can be extracted from the tracks of reptiles.
First however we should consider what aspects of metabolism we are interested in and how we want to define our questions.
Bob Bakker framed the most provocative question on dinsaurian metabolism by asking, "were dinosaurs hot-blooded?". By this he meant, did dinosaurs have a metabolism closer to that of mammals and birds or did they have one much more like living reptiles?
We need to be more specific, however, and need to focus in on what we mean by "hot-blooded". Really Bakker meant "warm blooded", but what does that mean? Many lizards have blood as warm as ours, but are still regarded as "cold blooded". We can clarify this by breaking temperature regulation into two catagories:
1) the source of heat to keep warm:animals can be ectothermic - which means they obtain heat from their environment,2) their temperature variability:
or they can be endothermic - which means they can produce their own heat.animals can be poikilothermic - having a body temperature that fluctuates over a fairly wide range,
or they can be homeothermic - having body temperatures that don't fluctuate much at all.
|We are homeothermic endotherms, that is our body heat is produced by our own metabolism, as are other mammals (to a greater or lesser extent) and birds. Lizards are poikilothermic ectotherms, so their temperature fluctates greatly and they warm up by seeking a warm place, as are crocodilians.
The differing response of the body temperatures of different kinds of animals can be seen on the graph on the left. Mammals (red) change their body temperature little with increasing ambient (surrounding air) temperature. The bodies of reptiles (green), on the other hand, change in proportion to the ambient temperature.
But Bakker was looking at much more than just temperature regulation per se. He was also looking at what we see associated with bird and mammal style thermoregulation - high activity levels and agility.
So first we will look at activity levels and then relate this back to temperature regulation and the general question of dinosaurian metabolism.
Tracks can be preserved in a remarkable variety of ways. One common way is for the tracks to be made in regular sticky mud. These tracks are often poorly preserved because of partial collapse of the mud back into the track and mud adhering to the foot.
|The sorts of tracks made in sticky, new mud rarely show much anatomy, and they are typical of most really large tracks as well as quite a few small ones. On the right is a modern lizard trackway made in mud that had been deposited the night before by a flash flood along a river. Note the lack of detail. This is the style of track most of us make when walking in mud.
Lizard tracks, Red River, Palo Duro Canyon, TX
Trackway of Rhynchosauroides hyperbates.
However, some fossil tracks are exquisitely preserved with almost unbelievable detail. Above left is an impression of a manus of the lizard-like form Rhynchosauroides hyperbates from Late Triassic age strata of Pennsylvania (Newark basin).
This track comes from a very long series (left) in which the animal actually sat down several times (below).
Impression of chest of Rhynchosauroides hyperbates.
The horizontal rows of scales are from the belly and the two patches of scales above that area was made by the animal's pectoral muscles. This pattern is nearly identical to what we see on many modern lizards. However, we don't not know the kind of animal that made these Triassic tracks. It could have been a lizard, but we don't know.
Obviously, we can learn a lot about the trackmaker's anatomy from these kinds of tracks. That is not necessarily true of the tracks from Palo Duro Canyon we looked at above.
Why is it that these tracks are preserved so well, but the ones made in fresh mud are so bad? A clue can be gleamed from looking not at newly deposited mud, but rather at "old" mud.
|On the right are tracks a large number of birds and I made on the shores of a little pond in Texas. Although my tracks are deep (below) the impression is very clear, as are those of the birds.
This is because of a coating of algae and bacteria that not only binds the surface together, but also act as a parting medium, preventing our feet from sticking to the mud.
You can seen the greenish coating on the damp mud. This algal and bacterial coating also slows the drying of the mud.
Any given real track can be thought of as falling somewhere in between three "end members" of track types. These end members are - 1) tracks whose form faithfully reflect the trackmaker's anatomy; 2) tracks whose form is a result of the motion of the foot (or body) through or on the mud, which we call kinematics; and 3) tracks whose form is a result of the nature of the substrate.
|We can imagine these three end-members forming the corners of a triangle - a kind of "space" into which a given track somewhere falls. This kind of diagram is called a ternary diagram and it is a type often used by mineralogists and geochemists.
Here the red dot represents the position of the Rhynchosauroides hyperbates tracks, while the green dot represents the modern lizard tracks from Palo Duro Canyon, TX. The Rhynchosauroides hyperbates tracks reflect mostly the anatomy of the trackmaker, while the modern lizard tracks reflect the interaction between the animal's motions (kinematics) and the substrate.
The latter point illustrates an asymmetry between the levels of interactions of the making of tracks. Obviously, the nature of the substrate (mud) influences the animal's kinematics and the kinematics influence how the substrate yields; however, neither substrate nor the animal's movements can change the actual anatomy of the animal - except through natural selection.
So providing we can get a track that shows anatomy, how do we compare it to fossil animals known only from their skeletons, not the soft tissue of their feet?
|The problem is well illustrated from the trackways of the non-dinosaurian track, Chirotherium, which we saw earlier (and to the right).
Chirotherium means, literally, hand animal and refers to the way the impression of digit V of the hand sticks out like a thumb. This bit of anatomy in the track posed a difficulty for the reconstruction of the skeleton, because no skeleton has a digit V of the pes that sticks out in such a manner. In fact, no skeleton of any modern reptile does that either.
However, Kevin Padian (University of California at Berkeley) and I had a suspicion that it was commonplace for sauropsid feet to interact with the ground in this way. In 1983 we set out to test this hypothesis by means of an experiment.
With the cooperation of the San Diego Zoo, we enlisted the use of a large 13 year old male Komodo Dragon or Ora (Varanus komodoensis), native to the island of Komodo in Indonesia. He was, at that time, 2.54 m (8 ft, 4 in) long and weighed 82.5 kg (~182 lb). We also looked at a smaller female (below) that was 1.8 m and 26.25 kg as well, because she was blind and could be handled! Komodo Dragons are monitor lizards and are the largest fully terrestrial reptiles alive today. However our zoo male was about twice as meaty as wild ones.
The female Komodo Dragon.
The foot has very primitive sauropsid proportions with digit IV being the longest. Digit V points forward, as it does in modern skeletons and in fossil lizard and even archosauromorph skeletons.
One of the keepers enticed the male to walk on the clay bed with a tasty dead rat on the end of a stick (right). Notice the large amount of saliva dripping from the ora's mouth. This saliva is loaded with bacteria that are instrumental in producing massive infections that the wild Komodo Dragons use to bring down larger prey.
|In order to test our hypothesis we had to convince the male ora to make some footprints for us. We made a bed of terra cotta clay and smeared glycerin on it to act as a separator (left). The zoo keepers arranged barriers to keep the ora on the clay bed.
Unfortunately, on this go-round the ora pretty much ignored the ladder barricade and walked on it producing a half-trackway (right). So we had to level the clay. We really did not need the glycerin, because the ora's saliva was all over the clay and made an excellent separator. Fortunately, we had no cuts on our hands!
The Komodo Dragon was kept at bay for a while and then turned around, again using the rat as bait (left).
|This time (above) he made a perfect trackway and was allowed to keep his tidbit.
On the right is a plaster cast of a manus and pes set of tracks. The manus track is in front of the pes. Because this is a plaster cast of a left set of tracks, the tracks are in mirror image and digit V is on the right.
Note the scale impressions and that the claws (and ther drag marks) are longer than they would be for a wild animal.
These tracks reflect the anatomy of the trackmaker very well. Digit V, however, sticks out laterally, much like in Chirotherium. Thus the thumb-like shape of digit V in Chirotherium is a primitive feature (at the level of the Diapsida) and reflects nothing particularly unusual in foot structure.
We then produced an outline drawing of the manus-pes set (below middle) and then reconstructed the skeleton of the hand and foot based on the assumption that the pads underlie the articulations between bones. The result (bottom right) compares well to the skeleton of an ora. This is the process we can go through to compare well-preserved small-to-medium sized dinosaur tracks to the skeletons of potential trackmakers.
|We can also learn something about the gait and even perhaps breathing mode by looking at the trackway (left).
Note that the trackway is very broad, not at all like the trackway of Chirotherium. In addition, there is a tail drag that seems to indicate a sinuous motion to the body and certainly a sprawling posture and gait. This sinuous motion can also be seen in the photograph of the walking ora (below)
Such motion in lizards and other more primitive tetrapods alternately compresses each lung, with the result that lizards cannot breath while walking and running. They must stop to breath or while walking breath between steps. This is not true of birds, and presumably dinosaurs. It is also not true of mammals. Lizards exhibits the so called Carrier's constraint, the inherited fish-like motion of their ancestors that limits breathing while walking or (for tetrapods) swimming.
The maximum sprinting range of Komodo Dragons is thus about 10 m (Cowen, 1997).
Crocodiles in a gallop compress their lungs symmetrically and thus presumably can breathe. But while swimming they undulate their bodies side to side and hence are under "Carrier's constraint" and can only swim fast for very short spurts. The trackways of birds and dinosaurs shows that dinosaurs did not move sinuously while walking or running and thus could breathe while running (see below).
Crude Notes follow.
II. LETS LOOK AT RUNNING - GETTING AT ACTIVITY LEVELS
A. FOOTPRINTS ONLY REAL INDICATION OF HOW DINOSAURS
MOVED AND IT IS REASONABLE TO ASK IF THEY CAN TELL
US ABOUT ACTIVITY
1. POSTURE OBVIOUS FROM TRACKWAYS - BUT ACTIVITY
A. IT IS POSSIBLE TO GET SPEED FROM TRACKS
B. WHEN YOU RUN YOUR PACE AND STRIDE INCREASE
SUGGESTING A WAY
SLIDE OF RUNNING PG 3.7 MCN ALEX
1. SHIP DESIGNERS NEED WAY OF MAKING MODELS
WAY GRAVITY INFLUENCES MOTION IS NOT
PROPORTIONAL TO LENGTH
2. THERE IS A RELATION CALLED DYNAMIC
SIMILARITY WHICH TAKES THIS INTO ACCOUNT
3. THIS VALUE TENDS TO BE VARY IN PROPORTION
FOR DIFFERENT SIZE SHIPS
4. R. McNEILL ALEXANDER - HAS WORKED OUT A
SIMILAR RELATIONSHIP FOR RUNNING TETRAPODS
5. SUGGESTED RELATIONSHIP
RELATIONSHIP BETWEEN STRIDE AND SPEED
NOW WE CAN GRAPH DIMENSIONLESS SPEED AGAINST RELATIVE STRIDE, AS FOLLOWS
6. WE CAN SOLVE FOR SPEED GIVEN THIS RELATIONSHIP
7. BUT WE STILL HAVE TO GET LEG LENGTH FROM A FOOTPRINT
8. WE DO THIS BY OBSERVING THAT LEG LENGTH SCALES
TO FOOT LENGTH FOR DINOSAURS NO MATTER WHAT THE
SIZE OR WHETHER THEY ARE BIPEDAL OR QUADRUPEDAL
SLIDE ALBERTOSAURUS A TYRANNOSAUR
SLIDE DATA ON LIMB VS FOOT
IN FACT LEG = 4 X FOOT (to metatarsals)
9. THUS, FOR A VARIETY OF DINOSAUR TRACKS WE GET
THE FOLLOWING RELATIONSHIPS
SLIDE OF TABLE OF SPEEDS
MOST DINOSAURS TRACKS ARE THUS OF WALKING ANIMALS
BUT SOME COULD RUN FAST, AS FAST AS THE FASTEST MAMMALS
SO THIS SHOWS DINOSAURS COULD MOVE AS FAST AS FASTEST MAMMALS
II. GETTING AT METABOLISM FROM DINOSAURIAN
A. NOW WE CAN SEE THAT DINOSAURS COULD PROBABLY
RUN VERY FAST BUT SO CAN SOME REPTILE FOR
B. STRUCTURALLY, HOWEVER, THEY ARE MAMMAL OR BIRD LIKE
1. STRUTHIOMIMUS - OSTRICH
2. DEINONYCHUS - ARCHAEOPTERYX
3. HADROSAUR - ELK
C. LARGE ANIMALS HAVE SIMILAR STRCTURAL REQUIREMENTS
1. SAUROPOD AND ELEPHANT
III. INTERNAL CONSTANCY VS VARIABILITY OF TEMP
A. WHAT ABOUT HEAT PRODUCTION
1. RESTING METABOLIC RATE GREATER IN MAMMALS
AND BIRDS THAN IN REPTILES
SLIDE OF RESTING METABOLIC RATE FROM MC A
2. SIZE CONFERS SOME THERMAL INERTIA
1. SURFACE AREA IS WHERE HEAT IS LOST
2. SURFACE AREA INCREASES AS THE SQUARE OF linear DIM
3. BUT VOLUME INCREASES AS THE CUBE
4. THEREFORE FOR THINGS OF SIMILAR SHAPE,
LARGE THINGS LOSE HEAT SLOWER
5. BIG ANIMALS HAVE GREAT THERMAL INERTIA
SO BIG DINOS WERE COULD HAVE BEEN INERTIAL
SLIDE DATA ON LITTLE VS BIG TEMPS
3. ALSO THIS MEANS THAT LARGE THINGS WITH
HIGH METABOLIC RATES NEED TO DUMP HEAT
4. BUT SMALL THINGS LIKE SHREWS NEED TO
CONSERVE IT ALWAYS
5. SOMETIMES BIRDS WITH GOOD INSULATION NEED
TO COOL - AIR SAC SYSTEM
6. AIR SAC SYSTEM PRESENT IN GIANT SAUROPODS
- PROBABLY FOR COOLING
B. OF COURSE, IF YOU HAVE HIGH METABOLIC RATES
YOU NEED TO EAT MORE FOOD
SLIDE OF BAKKERS DOG VS LIZARD PG 376
1. THUS WE WOULD EXPECT THE SAME NUMBER OF PREY
ANIMALS TO BE ABLE TO FEED A LARGE NUMBER OF
LOW METABOLIC RATE ANIMALS THAN HIGH ONES
2. IN FACT IF WE LOOK AT THE DISTRIBUTION OF
PREDATOR-PREY RATIOS WE CAN SEE THAT DINOSAURS
HAVE A LOWER PREDATORY/PREY RATIO THAN EOCENE MAMMALS
SLIDE OF PREDATOR/PREY RATIOS DRAWING FROM BAK PG 351
3. THUS IT EVIDENTLY TOOK MANY, MANY HERBIVOROUS
DINOSAURS TO FEED ONE CARNIVOROUS ONES
IV. ACTIVITY LEVEL VS. BONE STRENGTH
1. IT HAS LONG BEEN OBSERVED THAT THE STRUCTURE
OF THE BONES IN MAMMALS AND BIRDS IS DIFFERENT
THAN THAT SEEN IN OTHER LIVING REPTILES
ADAPTED SLIDE FROM BAKKER PG 352
HAVERSIAN SYSTEM PRESENT IN BIRDS AND MAMMALS
SLIDE OF HAVERSIAN SYSTEM AND NOT
EXPLAIN CROMPTON'S ARGUMENT
SHARED DERIVED CHARACTERS OF DINOSAURS AND BIRDS
FOUR CHAMBERED VS THREE CHAMBERED HEART
A. DINOSAURS COULD RUN VERY FAST
B. DINOSAURS WERE BUILT FOR LONG PERIODS OF ACTIVITY
C. LARGE DINOSAURS COULD HAVE EASILY BEEN INERTIAL
HOMEOTHERMS BUT SMALL ONES HAD TO PRODUCE EXTRA
HEAT OR WOULD HAVE TO ADAPT THEIR BEHAVIOR TO
RECEIVE IT FROM ENVIRONMENT
D. LARGE DINOSAURS NEEDED TO LOSE HEAT
E. PREDATOR/PREY RATIOS SUGGEST THAT CARNIVOROUS
DINOSAURS NEEDED AS MUCH FOOD AS CARNIVOROUS MAMMALS
F. DINOSAUR BONES REMODELED AS OFTEN AS MAMMAL AND
BIRD BONE SUGGESTING HIGH ACTIVITY LEVELS
G. BIRD ARE DINOSAURS AND MAY HAVE INHERITED THEIR
AT LEAST SOME DINOSAURS WERE AS ACTIVE AND
"WARM BLOODED" AS ANY MAMMAL OR BIRD
AND SO ALTHOUGH WE CANT GO BACK IN TIME AND
MEASURE THEM WE CAN INFER, MAKE PREDICTION AND
TEST THEM IN THE FOSSIL RECORD
NOTE THAT COMBINED WITH THEIR LOCOMOTOR ADAPTATIONS
IT BECOMES EASY TO SEE WHY DINOSAURS REMAINED
DOMINANT FOR SO LONG OVER MAMMALS
Dinosaur Sociality from Tracks
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