TOEFL Experts Reading Practice 16
Scientists have long been fascinated with the evolution of pterosaurs—warm blooded, flying reptiles that flourished during the Jurassic and Cretaceous periods, the time of the dinosaurs. Pterosaurs were the first animal after insects to develop powered flight. This ability was enabled by the pterosaur’s hollow bone structure, a structure more similar to modern birds than to ancient dinosaurs. Because of their ability to fly, pterosaurs were able to expand their range and fill many ecological niches1, eventually evolving into dozens of different species that ranged in size from a small bird to a small airplane. However, hollow bones decay more easily than solid bones, making complete pterosaur fossils hard to find. Known fossils are difficult to categorize into evolutionary families. How did the pterosaur evolve over time into the many different species that roamed the Earth? Hidden from scientists’ view were the linking fossils that would indicate how closely related many of the different species were.
Fossil discoveries allowed scientists to theorize that pterosaurs first split into two subgroups: short-tailed and long-tailed. Short-tailed pterosaurs are categorized into four distinct but evolutionarily related families. Theories about which species of pterosaur belonged in which family have changed over time as new evidence emerges. One of the four families, Gallodactylidae, was created specifically because the swan-beaked pterosaur was thought to be distinct from the members of the other three families. Gallodactylidae are characterized by having fewer than 50 teeth, all of which are present only in the tip of the jaw. As new fossils were found, it was determined that swan-beaked pterosaurs were, in fact, more similar to species from other families than was originally thought. As a result, the family Gallodactylidae was largely disregarded.
An exciting discovery in China soon reversed that trend. In 2012, a sword-headed pterosaur was discovered that also had teeth only in the front tip of the jaw. Swan-beaked and sword-headed pterosaurs were thought, for this reason, to be similar enough that they likely both directly evolved from a single common ancestor. The new discovery reenergized the belief that Gallodactylidae was its own unique family. Scientists quickly flocked back to the practice of classifying the swan-beaked pterosaur in the family Gallodactylidae, this time along with the sword-headed pterosaur. Like many pterosaur fossils, the 2012 sword-headed pterosaur fossil was incomplete; only the skull and lower jaw were recovered. By categorizing the sword-headed pterosaur as a close relative of the swan-beaked pterosaur, scientists vastly expanded the geographical range of this family. No specimen of Gallodactylidae had ever been discovered in China before this point. Researchers also concluded that the family was far more varied than was originally thought.
However, a major discovery in 2016 again forced scientists to reevaluate their classification of pterosaurs. Another specimen of the sword-headed pterosaur was discovered, but this time it was nearly complete. The entire skull was present, along with the jawbone and much of the postcranial skeleton2. The fossil was almost perfect, missing only one of the rear legs and two vertebrae3. Analysis of this new fossil had some surprising results. The sword-headed pterosaur was actually different from the swan-beaked pterosaur in important ways. In fact, researchers determined that the sword-headed pterosaur was far more similar to species located in other families than to the swan-beaked pterosaur, with which it had been previously paired.
Again, the families were reorganized. The sword-headed pterosaur was placed in an entirely different family called Ctenochasmatoidea. This family has been well-studied and is already known to have members in China. While a tentative organization of pterosaurs has helped to clarify which species evolved from a common ancestor and which are less closely related, it is apparent that the classification is far from final. Additional discoveries are likely to further alter scientists’ perspectives on this reptile.
niches1: positions or fun
postcranial skeleton2: all or part of the bones behind the skull
vertebrae3: bones found in the spine
Scientists have long been fascinated with the evolution of pterosaurs—warm blooded, flying reptiles that flourished during the Jurassic and Cretaceous periods, the time of the dinosaurs. Pterosaurs were the first animal after insects to develop powered flight. This ability was enabled by the pterosaur’s hollow bone structure, a structure more similar to modern birds than to ancient dinosaurs. Because of their ability to fly, pterosaurs were able to expand their range and fill many ecological niches1, eventually evolving into dozens of different species that ranged in size from a small bird to a small airplane. However, hollow bones decay more easily than solid bones, making complete pterosaur fossils hard to find. Known fossils are difficult to categorize into evolutionary families. How did the pterosaur evolve over time into the many different species that roamed the Earth? Hidden from scientists’ view were the linking fossils that would indicate how closely related many of the different species were.
Fossil discoveries allowed scientists to theorize that pterosaurs first split into two subgroups: short-tailed and long-tailed. Short-tailed pterosaurs are categorized into four distinct but evolutionarily related families. Theories about which species of pterosaur belonged in which family have changed over time as new evidence emerges. One of the four families, Gallodactylidae, was created specifically because the swan-beaked pterosaur was thought to be distinct from the members of the other three families. Gallodactylidae are characterized by having fewer than 50 teeth, all of which are present only in the tip of the jaw. As new fossils were found, it was determined that swan-beaked pterosaurs were, in fact, more similar to species from other families than was originally thought. As a result, the family Gallodactylidae was largely disregarded.
An exciting discovery in China soon reversed that trend. In 2012, a sword-headed pterosaur was discovered that also had teeth only in the front tip of the jaw. Swan-beaked and sword-headed pterosaurs were thought, for this reason, to be similar enough that they likely both directly evolved from a single common ancestor. The new discovery reenergized the belief that Gallodactylidae was its own unique family. Scientists quickly flocked back to the practice of classifying the swan-beaked pterosaur in the family Gallodactylidae, this time along with the sword-headed pterosaur. Like many pterosaur fossils, the 2012 sword-headed pterosaur fossil was incomplete; only the skull and lower jaw were recovered. By categorizing the sword-headed pterosaur as a close relative of the swan-beaked pterosaur, scientists vastly expanded the geographical range of this family. No specimen of Gallodactylidae had ever been discovered in China before this point. Researchers also concluded that the family was far more varied than was originally thought.
However, a major discovery in 2016 again forced scientists to reevaluate their classification of pterosaurs. Another specimen of the sword-headed pterosaur was discovered, but this time it was nearly complete. The entire skull was present, along with the jawbone and much of the postcranial skeleton2. The fossil was almost perfect, missing only one of the rear legs and two vertebrae3. Analysis of this new fossil had some surprising results. The sword-headed pterosaur was actually different from the swan-beaked pterosaur in important ways. In fact, researchers determined that the sword-headed pterosaur was far more similar to species located in other families than to the swan-beaked pterosaur, with which it had been previously paired.
Again, the families were reorganized. The sword-headed pterosaur was placed in an entirely different family called Ctenochasmatoidea. This family has been well-studied and is already known to have members in China. While a tentative organization of pterosaurs has helped to clarify which species evolved from a common ancestor and which are less closely related, it is apparent that the classification is far from final. Additional discoveries are likely to further alter scientists’ perspectives on this reptile.
niches1: positions or fun
postcranial skeleton2: all or part of the bones behind the skull
vertebrae3: bones found in the spine
- The classification of pterosaurs is an area of interest to scientists.