Module 13 Investigation-2

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Anthropology

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Dec 6, 2023

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1 Estimating Dinosaur Speed from Trackways Testing the Method of R. McNeill Alexander (1989) Fossil footprints are of great interest to paleontologists because they record information about what an extinct animal was doing while it was alive and moving around. Careful analysis of fossil tracks has the potential to reveal several aspects of the behavior of the trackmaker, including posture, herding behavior, and walking or running speed. A basic fact of vertebrate motion is that the faster an animal is moving the farther apart its footprints fall along a trackway (Figure 1). By measuring the spacing of fossil footprints it is possible to estimate the speed of the trackmaker, but only after making several assumptions based on footprint size and the behavior of a wide range of living animals. A widely applied method for estimating speed from trackways was developed through the research of R. McNeill Alexander, an expert in biomechanics. This exercise is designed to lead you step-by-step through the methods and principles involved in estimating speed of movement from trackway data using Alexander’s method. We will apply it to measurements taken from a variety of dinosaur trackways. Establishing the Nature of Trackway Data Footprints / tracks: individual foot impressions. In a continuous line these form a trackway. Footprint size is also important because it can be used to estimate leg length, which is an important parameter for estimating speed. Estimating leg length from footprint size. How do we arrive at an estimate of how long an extinct animal’s leg was from the size of its footprint? We note a basic physiological fact - larger animals have larger feet. This makes sense because the larger you are the more you weigh and the larger your feet need to be to support that weight. The best way to test this idea is to measure foot size and leg length for a wide variety of individuals (and species) and examine the data to see if there is a simple relationship. For dinosaurs, Alexander (1989) notes that most dinosaurs have leg lengths that are about 4 x the length of the foot (based on his own copious measurements of a variety of dinosaur skeletons).

2 Establishing the relation between speed and stride length. Stride: The distance between footprints from the same leg / foot. Dynamic Similarity and Dimensionless Speed Alexander’s method is base d on the concept of dynamic similarity , which is the idea that structurally similar animals (for example, tetrapods) function equivalently according to the laws of physics. Put simply, the dynamic differences between a Chihuahua and an elephant when they are both walking come from their different sizes. If we can adjust for size, they will show the same dynamic behavior. How do we adjust for size? We need to calculate a quantity called dimensionless speed . This is speed that has been adjusted for the size of an organism based on its leg length and the force of gravity. Two animals moving at the same dimensionless speed will be dynamically similar. Alexander (1989) measured speed and stride length for a variety of living animals, including large and small mammals, ostriches, and humans. When he calculated relative stride (stride / leg length) and dimensionless speed, he discovered a consistent linear relationship between these two quantities (Figure 4). In other words, regardless of what kind of tetrapod you are, your stride is related to your speed in the same way once you make adjustments for your size. If we make the reasonable assumption that extinct tetrapods like dinosaurs functioned the same way as the great variety of living tetrapods, then we can apply this same relationship between stride and speed to dinosaurs. Measuring stride from a fossil trackway and estimating leg length from the size of the footprint, we can arrive at an estimate of speed for different types of dinosaurs.

3 Estimating Speed from Trackways – Steps to Calculate Measurements

4 Part I - Estimating Dinosaur Speed from Trackways - Applying the Method Having demonstrated that Alexander’s method works, we can now apply it to actual fossil trackways to learn something about the behavior of dinosaurs. Figures 5 and 6 show five different dinosaur trackways made by different species of dinosaurs during different periods of the Mesozoic. For each trackway, use the scale bar provided to measure footprint length and stride length. Try to be as accurate as possible, measuring as many strides as possible for each trackway to obtain an average. Note that some trackways only permit measurement of a single stride. Trackway #5 (Figure 6) is shown in a photograph with a meter stick lying between tracks of the left and right foot. Estimate this distance (the pace) and then double it to estimate the stride. Recall that there are 100 centimeters in a meter and that a meter stick is divided into 10 cm segments (these are enhanced with black lines in the photograph). Because of the angle at which the photo was taken the apparent distances are somewhat distorted – for example, the footprint in the foreground is about 30 cm in length but looks larger relative to the measuring stick. Make your best estimate! 1. Complete the table below: Trackway Measured foot size Estimated leg length Measured stride length Relative stride length Estimated dimensionless speed Estimated speed MPH 1. Cret. Ornithopod x4 2. Jur. Sauropod x4 3. Trias Theropod x4 4. Trias Prosauropod x4 5. L. Cret. Theropod x4 Meters/sec x 2.24 = miles/hour (MPH) STEP 1 STEP 2 STEP 3 STEP 4 STEP 5

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