Interpreting Dinosaur Tracks

GEOS 106 – LIFE IN THE AGE OF DINOSAURS

LAB – INTERPRETING DINOSAUR TRACKS

Introduction

Testing hypotheses regarding the history of the Earth can be problematic. Many processes (e.g. organic evolution) take hundreds, thousands, or millions of years to replicate. To make things worse, it may be impossible to reproduce conditions – or creatures – that existed on the prehistoric Earth. Therefore, we have two options when designing geological experiments:

1) We can test hypotheses by collecting additional evidence from the rock and fossil records, or

2) We can test hypotheses by conducting controlled experiments that approximate the biological and physical environment of the primordial Earth as closely as possible.

This exercise explores the formation and interpretation of dinosaur tracks using option 2. Although it is easy to reproduce the types of substrates (sediment that dinosaurs could have stepped in) that existed during the Mesozoic, an obvious problem is that dinosaurs have been extinct for 65 million years. Therefore, we will use chickens to make our tracks.

Chickens? Why Chickens?

Although it could be said that we are using chickens to approximate dinosaurs, a host of fossil discoveries and recent genetic experiments indicate that this is not technically correct. The fossil record tells us that birds are the direct descendents of small theropod dinosaurs. Genetic sequences reveal that one of the closest living relatives of the enormous Tyrannosaurus rex is the chicken. Therefore, chickens are not just similar to dinosaurs; in an evolutionary sense, chickens are dinosaurs! Naturally, our domestic chickens differ from the hadrosaurs and theropods that once roamed Alaska. The chickens are much smaller than the typical Mesozoic dinosaur, and they have wings. Although chickens are not capable of sustained flight, they do make “wing-assisted leaps” that wingless dinosaurs could not have imitated. However, like their carnivorous dinosaurian ancestors, chickens are 3-toed bipeds. Luckily for us, they are vegetarians, which means we may be able to convince them to walk or run with the promise of greens or grain, instead of squirming prey.

Materials

The experimental apparatus consists of:

· 6 wooden trays, or runways

· 6 wood and chicken wire tunnels

· 3 substrates: potter’s clay, damp sand, and wet mud

· 6 rulers

· 6 stopwatches or wristwatches

· string and scissors

· 6 small bags of greens and/or grain

· 4 aprons

· and 2 chickens

Make a Prediction

Before conducting the experiment, examine samples of the 3 substrates. Which one do you think will make the best tracks? For that matter, what determines the quality of a track? Depth? Potential for preservation? Anatomical detail? Consider that the definition of a good track may vary, depending on what you want to know.

What features should a “good” track possess? ______

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What substrate do you predict will make the best tracks? ______

Why do you think so? ______

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Experimental Procedure

Working in teams of 5, each team should choose a runway and tunnel apparatus. Select a Chicken Wrangler and a Timer. The timer should have a wristwatch with a digital time or a second hand. The Wrangler should put on an apron.

1. Preparing the Substrate:

Clay: Use the rolling pin and the “spacers” to roll out a slab of potter’s clay ¼ inch thick. Use the plastic knife to trim the slab so that it fits in the runway tray. You will need to roll out 2 tubes to fill the runway. Be sure to minimize the number of slabs to that your trackway will not be obsucured by numerous seams.

Sand: Damp sand is available for two of the empty runway trays. Open the plastic container and spread the sand evenly over the tray. A smooth surface is desirable, but do not pack the sand too tightly or it will be resistant to the weight of the chicken.

Mud: If you are working with the mud, you will need to mix it and pour it into the tray before wrangling any chickens. Add water to the powder in the bucket provided until your mud forms a “spreadable glop”. Mix the two using the battery-operated drill, and scoop or pour the mud carefully into the runway tray. Leave the plastic bag in the runway as a liner! If you remove it, it will be very difficult to remove the mud once it sets. Remember that the idea is to compare different substrate consistencies. This is not a competition to make the best tracks. So please create a soupy mixture. The mud will set in 90 minutes, so if you are disappointed with the resulting tracks, you can smooth it out and try again once everyone has had a chance to see the results.

2. Wrangling Your Feathered Dinosaur

Select a chicken from the crate. Chickens do not like new environments, so they will be skittish. The Chicken Wrangler should grasp the chicken by the front edge of the wings, holding them gently but firmly against its body. Introduce the chicken to the far end of the runway opposite the greens. Timer, be prepared! If your chicken is motivated by the greens, time its progress to the other end of the runway. There is no guarantee that all (or any) of the chickens will do this. If you chicken does not proceed in an orderly and continuous fashion down the runway, prompt it a bit by pushing, prodding, or waving your hands. Don’t worry if it stops, starts, or sits down half way. It is not essential that the chicken make steady progress. However, it is essential that you observe and describe its behavior.

Notes on chicken’s behavior on the runway: ______

______

Time (for chickens that cross the runway without prolonged stops or reversals): ______

Before putting your bird back in the crate, use the tape measure to determine the bird’s height at the hip. It’s hard to see a chicken’s hips, but this will be the joint that allows the bird to bend forward. It is located a little above the point where the skinny leg emerges from the feathered belly.

Height at hip: ______

3. Evaluate the Trackways

Look at the tracks your feathered dinosaur made, and compare them to the tracks left by other birds on other runways. Use the diagram below to assign each substrate a rank (A-E) based on the depth and detail of the tracks.

Potter’s Clay: _____ Damp Sand: _____ Mud: _____

Figure 1: Variations in the appearance of tracks of the same animal based on

firmness of the substrate and depth of the footprint.

Refer to your criteria for good tracks, above. Which substrate has the best tracks?

______

What aspects of the substrate make it superior in terms of track formation?

______

Was your prediction correct? If not, what surprised you about the substrates?

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4. Reconstruct a bird’s anatomy based on tracks

Foot Bones

In some cases, it may be possible to reconstruct the anatomy of the foot based on several tracks of appropriate depth and detail. Surprisingly, this is easiest to do with relatively shallow tracks, because the knuckles and joints make noticeably deeper and wider impressions in the substrate than the bones in between. Find a track or two (on your runway or another group’s runway) that preserves details of the toes. If the wide spots are joints, and the narrow spots are the bones between joints, how many bones appear to be present in the chickens’ middle toes? Keep in mind that the final “bulge” is usually the tip of a toe bone or a claw – not a joint. Examine the figure at right if you need help using the “Peabody Method”. Figure 2: Right front and rear feet of a dinosaur from New Jersey. A.Tracing of tracks. B. Reconstruction of foot anatomy.

Number of bones in the middle toe: ______

Look at a chicken’s feet. Is your reconstruction correct? ______

If not, what went wrong? Is the method flawed, or did your track lack detail?

______

Posture (Sprawling vs. Upright)

Posture can also be determined from tracks, but not from a single track. A trackway, defined as 3 or more tracks, is needed. To reconstruct posture, you will need to measure the pace angle: the angle formed by line segments that connect consecutive prints of the hind feet or front feet (not both). It is important to use only front or hind feet to determine pace angle. Luckily for you, your chicken’s forelegs are wings, so you have only hind footprints to contend with.

Find three tracks in a row. Fine detail is not necessary. Taking care not to touch or disturb the tracks, cut a piece of string to the pace length, or the distance from one print to the next print. Lay the string over the tracks, so that it runs from the middle of one print to the middle of the next print. Then cut another piece of string to the next pace length and lay it over the tracks. You should end up with two pieces of string that intersect at an angle, as in the diagram at right. The angle formed by these two pieces of string is the pace angle.

Is your pace angle obtuse or acute? ______

If the pace angle is obtuse, your chicken has upright posture, with legs under the body and knees pointing forward. If it is acute, your chicken has sprawling posture, with the knees pointing off to the sides. Did pace angle allow you to reconstruct the chicken’s posture accurately? Or is the trackway wider (left footprints far to the left of right footprints) or narrower (left and right footprints form a nearly straight line) than you would expect, based on the anatomy of your bird? Explain.

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Height

In general, big animals have big feet, and small animals have small feet. Examination of dinosaur bones allows us to figure out the average ratio of foot length to height at the hip. This ratio is not constant. Different groups of dinosaurs have different foot length/hip height ratios. To make things worse, individual dinosaurs within the same group do not have the same ratio! But once the average ratio has been established for a particular group of dinosaurs (see chart below), we can estimate the height of the animal by measuring the length of its foot – or its footprint.

Table 1: Estimates of hip height (h) from footprint length (FL)

Dinosaur group Footprint length Height at hip

Small theropods FL < 25 cm h = 4.5 FL

Large theropods FL > 25 cm h = 4.9 FL

Small ornithopods FL < 25 cm h = 4.8 FL

Large ornithopods FL > 25 cm h = 5.9 FL

(after Thulborn, 1990)

Use Table 1 and Figure 4 to determine which group of dinosaurs most closely resembles (and is probably most closely related) to your chickens. Then use the group ratio to estimate your chicken’s height at the hip by measuring the length of a well-preserved footprint.

FL: ______x Ratio: ______= Hip Height: ______

Compare this estimate to the actual measured hip height of your bird

Is your bird taller or shorter than its ancestors? ______

5. Reconstruct Behavior

Gait

When an animal runs, it covers more ground with each step. Therefore, stride length (Fig. 3) increases as an animal picks up speed. If the animal’s height at the hip is known – or can be estimated from footprint length – then it is possible to calculate relative stride: stride length divided by the animal’s height at the hip. Relative stride tells us about the animal’s gait (see Table 2 and Figure 5).

Figure 5: Relative stride versus speed (dimensionless) for various mammals and birds

Return to your three good, consecutive footprints and measure the stride length. If you like, you can measure the stride in several places on a trackway to compute an average stride. Then divide by the chicken’s height at the hip (measure above) to calculate the relative stride.

Relative Stride (Stride Length ______/ Hip Height ______) = ______

According to the relative stride and the information in Table 2 on the previous page, was your chicken walking, trotting, or running?

______

Does this calculation agree with your behavioral notes? Explain.

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Dinosaurs, Rest and Motion

On a substrate of uniform consistency (such as the ones used for this exercise), the depth of the footprints tends to be rather uniform, too. However, differences in an animal’s behavior can produce deeper or shallower tracks. Compare the prints to your notes on behavior. Did your bird do anything that left particularly deep or clear footprints? If not, can you think of anything a dinosaur might do (feathered or otherwise) that would leave deeper prints than merely strolling down the runway?

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