Read Online Dinosaurs Without Bones by Anthony J. Martin - Free Book Online Page B
Authors: Anthony J. Martin
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the “lone dinosaur” is not so likely in many species. Because of this combination of trackway and skeletal evidence, we now nonchalantly discuss the ecological effects of vast herds of sauropods, hadrosaurs, and ceratopsians, or the pack-hunting behavior of theropods, on Mesozoic ecosystems. “Strength in numbers” is a strategy used by many animals today, from schooling in fish to herding in caribou to pack hunting in wolves.
So let’s say that paleontologists find a dinosaur tracksite with hundreds of tracks preserved in it. They can then test whether these dinosaur trackmakers moved together as a large group or not. This is based on whether the following questions receive an answer of “yes” or “no”: (1) Do the tracks look alike? (2) If the tracks look alike, do they also show variations in sizes? (3) Are the tracks all heading in (more or less) the same direction? (4) Do the tracks show any other harmony of movement, such as staying parallel to or following one another? (5) Do the tracks seem to have been made at about the same time?
Here’s what those questions are testing: The first—do the tracks look alike—is examining whether they belong to the same dinosaur species or not. If they do have the same basic form but also show a range of sizes (question 2) from small to large, then they also could represent growth stages of the same species, from babies to full-grown adults, and perhaps gender differences as well. The third question is key, then, as this sorts out whether the dinosaurs were truly moving together or not, and not just randomly milling about. The fourth question further clarifies the third, as it asks about more nuanced behavior such as whether dinosaurs in the group maintained a consistent “personal space” from one another or whether they were following leaders. Finally, the fifth question addresses whether these tracks were all made by a sizeable group of dinosaurs moving through the area, as opposed to, say, a dinosaur family consisting of just two adults and two juveniles that neurotically walked through the same spot every day for several weeks.
Given this idealized dinosaur tracksite in mind, do any fulfill all of the criteria? I think you suspect the answer to that, but let’s look at a few examples anyway. The first recognized example of herding behavior shown by dinosaur tracks, and still one of the best, stems from a series of Early Cretaceous sauropod tracks at Davenport Ranch in Texas, found by Roland Bird in 1941. This site has trackways of more than twenty sauropods walking in the same direction and apparently made at about the same time. A Late Jurassic sauropod tracksite near La Junta, Colorado, also shows the tracks of five sauropods moving in the same direction, spaced at regular intervals, and turning in harmony along the length of their trackways. One time, when visiting this site with students, I asked them to walk alongside the tracks. By observing them this way, the lesson became much more visceral for these students as they could experience the subtle changes in movement made by the sauropods, rather than just gazing at them from afar or listening to me babble on about them.
Sauropod tracks, in fact, provide the best evidence that these dinosaurs herded. So far, sauropod trackways indicating group behaviors are documented from Jurassic–Cretaceous rocks of theU.S. (Arkansas, Colorado, Texas, Utah), Brazil, Bolivia, China, Portugal, and the U.K. The tracksite in the U.K., preserved in Middle Jurassic (about 165 mya ) rocks, was likely made by dozens of sauropods moving together, a truly awesome spectacle to imagine. Even better, some of the same sauropod tracksites show smaller tracks along with larger ones, suggesting that sauropods of different ages were traveling with one another, perhaps with multiple families.
For ornithopods, similar sorts of trackways also point toward their herding. These include several sites from the Cretaceous of Korea, one of
So let’s say that paleontologists find a dinosaur tracksite with hundreds of tracks preserved in it. They can then test whether these dinosaur trackmakers moved together as a large group or not. This is based on whether the following questions receive an answer of “yes” or “no”: (1) Do the tracks look alike? (2) If the tracks look alike, do they also show variations in sizes? (3) Are the tracks all heading in (more or less) the same direction? (4) Do the tracks show any other harmony of movement, such as staying parallel to or following one another? (5) Do the tracks seem to have been made at about the same time?
Here’s what those questions are testing: The first—do the tracks look alike—is examining whether they belong to the same dinosaur species or not. If they do have the same basic form but also show a range of sizes (question 2) from small to large, then they also could represent growth stages of the same species, from babies to full-grown adults, and perhaps gender differences as well. The third question is key, then, as this sorts out whether the dinosaurs were truly moving together or not, and not just randomly milling about. The fourth question further clarifies the third, as it asks about more nuanced behavior such as whether dinosaurs in the group maintained a consistent “personal space” from one another or whether they were following leaders. Finally, the fifth question addresses whether these tracks were all made by a sizeable group of dinosaurs moving through the area, as opposed to, say, a dinosaur family consisting of just two adults and two juveniles that neurotically walked through the same spot every day for several weeks.
Given this idealized dinosaur tracksite in mind, do any fulfill all of the criteria? I think you suspect the answer to that, but let’s look at a few examples anyway. The first recognized example of herding behavior shown by dinosaur tracks, and still one of the best, stems from a series of Early Cretaceous sauropod tracks at Davenport Ranch in Texas, found by Roland Bird in 1941. This site has trackways of more than twenty sauropods walking in the same direction and apparently made at about the same time. A Late Jurassic sauropod tracksite near La Junta, Colorado, also shows the tracks of five sauropods moving in the same direction, spaced at regular intervals, and turning in harmony along the length of their trackways. One time, when visiting this site with students, I asked them to walk alongside the tracks. By observing them this way, the lesson became much more visceral for these students as they could experience the subtle changes in movement made by the sauropods, rather than just gazing at them from afar or listening to me babble on about them.
Sauropod tracks, in fact, provide the best evidence that these dinosaurs herded. So far, sauropod trackways indicating group behaviors are documented from Jurassic–Cretaceous rocks of theU.S. (Arkansas, Colorado, Texas, Utah), Brazil, Bolivia, China, Portugal, and the U.K. The tracksite in the U.K., preserved in Middle Jurassic (about 165 mya ) rocks, was likely made by dozens of sauropods moving together, a truly awesome spectacle to imagine. Even better, some of the same sauropod tracksites show smaller tracks along with larger ones, suggesting that sauropods of different ages were traveling with one another, perhaps with multiple families.
For ornithopods, similar sorts of trackways also point toward their herding. These include several sites from the Cretaceous of Korea, one of
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