Read Online Dinosaurs Without Bones by Anthony J. Martin - Free Book Online
Authors: Anthony J. Martin
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(hadrosaurs), paleontologists started wondering how such large animals kept themselves upright on land without also placing incredible stress on their muscles, bones, and joints. So allpaleontologists needed was a little bit of suggestive evidence to nudge these big animals into the water, where their weights would have been supported through buoyancy.
For hadrosaurs, this evidence was scanty but persuasive for those who wanted these dinosaurs to be aquatic. For example, one hadrosaur trace fossil specimen had skin impressions around its hand that stretched between its fingers. This led paleontologists to conclude that this skin was webbing that aided it in paddling around in bodies of water. Only later did paleontologists realize this “webbing” was actually a result of skin drying around its bones after the dinosaur had died. Another hadrosaur, Paralophosaurus , also had a tall hollow crest on its skull, which was explained as a “snorkel” that allowed the dinosaur to breathe while most of its body was hidden underwater from predators. A major flaw in this seemingly marvelous adaptation was that the hollow tube in the center of the crest, once studied in more detail later, actually makes a U-turn which would have constituted a perfectly inept snorkel. (If you don’t believe me, try making one like this and let me know how that worked out for you.) Yet another anatomical trait was an elongated snout that led to the nickname of “duck-billed dinosaurs” for hadrosaurs, which imagines them as favoring soft aquatic plants as food. Again, a reexamination of their teeth and jaws as well as their trace fossils (coprolites and microwear on their teeth, explained in a later chapter) revealed that hadrosaurs could eat all sorts of land plants. In short, just calling a hadrosaur “duck-billed” doesn’t make it a duck.
This explanation of body fossil evidence favoring aquatic lifestyles for dinosaurs was even extended to dinosaur tracks. In 1938, paleontologist Roland Bird of the American Museum of Natural History learned that the area around Glen Rose, Texas, had lots of dinosaur tracks. Once he investigated, he confirmed the presence of exquisitely preserved three-toed theropod tracks, but also made an astonishing discovery: the first known sauropod dinosaur tracks from the geologic record. These huge tracks faithfully matched the size and anatomy of sauropod feet: five toes in the rear, and arounded pad in the front. However, among these sauropod track-ways were ones in which only the front feet registered. Why would the weightiest part of a sauropod—its rear end, with long tail—not connect with the sediment surface? Bird surmised that this was a result of a sauropod floating along, only touching the bottom with its front feet.
Later, a closer look at these tracks showed that the missing tracks in the sequence of steps could be attributed to differences in track preservation. If these sauropods had applied more pressure in the front while walking on land, these would have been more likely to be preserved as undertracks than the rear feet. Hence, Bird had not been looking at tracks from the original surface where sauropods placed their feet (or not), but more at the ghostly prints below. Once this alternative explanation caught hold, people realized that Bird was likely wrong about “swimming sauropods” at the Texas site.
Ironically, Bird’s recognition of sauropod tracks in the first place led from an initial view of sauropods as aquatic dinosaurs that, with more such discoveries, shifted them onto the land. Once paleontologists had the right search images for sauropod trackways, they started finding them outside of Texas. In the U.S., sauropod tracks are also in Colorado, New Mexico, and Utah, as well as in Argentina, Australia, China, France, Korea, Mexico, Morocco, Switzerland, the United Kingdom, and Zimbabwe, among other places. These tracks are also in rocks from near the start of sauropods in the
For hadrosaurs, this evidence was scanty but persuasive for those who wanted these dinosaurs to be aquatic. For example, one hadrosaur trace fossil specimen had skin impressions around its hand that stretched between its fingers. This led paleontologists to conclude that this skin was webbing that aided it in paddling around in bodies of water. Only later did paleontologists realize this “webbing” was actually a result of skin drying around its bones after the dinosaur had died. Another hadrosaur, Paralophosaurus , also had a tall hollow crest on its skull, which was explained as a “snorkel” that allowed the dinosaur to breathe while most of its body was hidden underwater from predators. A major flaw in this seemingly marvelous adaptation was that the hollow tube in the center of the crest, once studied in more detail later, actually makes a U-turn which would have constituted a perfectly inept snorkel. (If you don’t believe me, try making one like this and let me know how that worked out for you.) Yet another anatomical trait was an elongated snout that led to the nickname of “duck-billed dinosaurs” for hadrosaurs, which imagines them as favoring soft aquatic plants as food. Again, a reexamination of their teeth and jaws as well as their trace fossils (coprolites and microwear on their teeth, explained in a later chapter) revealed that hadrosaurs could eat all sorts of land plants. In short, just calling a hadrosaur “duck-billed” doesn’t make it a duck.
This explanation of body fossil evidence favoring aquatic lifestyles for dinosaurs was even extended to dinosaur tracks. In 1938, paleontologist Roland Bird of the American Museum of Natural History learned that the area around Glen Rose, Texas, had lots of dinosaur tracks. Once he investigated, he confirmed the presence of exquisitely preserved three-toed theropod tracks, but also made an astonishing discovery: the first known sauropod dinosaur tracks from the geologic record. These huge tracks faithfully matched the size and anatomy of sauropod feet: five toes in the rear, and arounded pad in the front. However, among these sauropod track-ways were ones in which only the front feet registered. Why would the weightiest part of a sauropod—its rear end, with long tail—not connect with the sediment surface? Bird surmised that this was a result of a sauropod floating along, only touching the bottom with its front feet.
Later, a closer look at these tracks showed that the missing tracks in the sequence of steps could be attributed to differences in track preservation. If these sauropods had applied more pressure in the front while walking on land, these would have been more likely to be preserved as undertracks than the rear feet. Hence, Bird had not been looking at tracks from the original surface where sauropods placed their feet (or not), but more at the ghostly prints below. Once this alternative explanation caught hold, people realized that Bird was likely wrong about “swimming sauropods” at the Texas site.
Ironically, Bird’s recognition of sauropod tracks in the first place led from an initial view of sauropods as aquatic dinosaurs that, with more such discoveries, shifted them onto the land. Once paleontologists had the right search images for sauropod trackways, they started finding them outside of Texas. In the U.S., sauropod tracks are also in Colorado, New Mexico, and Utah, as well as in Argentina, Australia, China, France, Korea, Mexico, Morocco, Switzerland, the United Kingdom, and Zimbabwe, among other places. These tracks are also in rocks from near the start of sauropods in the
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