The orchestra has arrived at this complement through centuries of evolution; the present size is needed to perform music from the Baroque, Classical, and Romanticperiods, as well as the varied repertoires that followed. The various sections, with the exception of percussion, divide themselves in somewhat the same manner as a choir. The woodwinds, for example, divide into flutes sopranosoboes altosclarinets tenorsand bassoons bassesalthough this distinction must be greatly qualified. Instrumental range is larger than vocal range, and the clarinets of an orchestra may play higher than the flutes in a woodwind passage.
Whereas living turtles are toothless, many ancestral forms possessed teeth. Many of the oldest and most primitive forms not only lacked a shell but also lacked a plastron and a carapace.
However, early turtles did possess precursors to these structures. As these structures evolved and the turtle's carapace emerged in later forms, the flexibility of the spine was reduced and crawling speed declined.
Eunotosaurus was toothed, and its midsection contained nine elongated trunk vertebraenine pairs of broad T-shaped dorsal ribs, and five pairs of gastralia ventrally located abdominal ribs. Collectively, these modified bones may have served as a type of intermediate shell structure from which the carapace and plastron evolved.
Similar structures also appear in Pappochelys, a younger, toothed species dating from about million ago during the Middle Triassic Epoch. These broad-ribbed structures likely evolved to provide these early forms with more stability and leverage for burrowing.
Odontochelys semitestacea, a species dating from about million years ago, during the Late Triassic, is the oldest species to possess a complete plastron, broad dorsal ribsand a series of neural plates, though it lacked a fully developed carapace.
Authorities contend that this species is evidence that the carapace evolved after the plastron. This evidence also suggests that the carapace of later turtles arose from neural plates that hardened over time to become flat sections of bone osteoderms supported by wide dorsal ribs.
Teeth were still present in turtles at this time, appearing in both the upper and lower jaws of Odontochelys. A slightly younger fossil species, Proganochelys quenstedi, also had teeth, but the teeth were located on the roof of the mouth, not on the upper or lower jaw.
In contrast to Odontochelys, the shell of Proganochelys possessed most of the features of modern turtles, and it completely encased the shoulder and pelvic girdles.
Although Odontochelys, Proganochelys, and Eunotosaurus offer insight into early anatomy, the origin of turtles remains a strongly debated issue. There are three main hypotheses concerning their origins, and existing evidence is such that there is a lack of overwhelming support for any one of them.
One hypothesis relies heavily on DNA analysis, whereas the others are based on morphological studies of fossils.
The DNA hypothesis suggests that turtles were a sister group to the archosaurs the group that contains the dinosaurs and their relatives, including crocodiles and their ancestors and modern birds and their ancestors.
A second hypothesis posits that turtles were more closely related to lizards and tuataras. A third hypothesis suggests that turtles arose as anapsids—a lineage whose skull contained no openings temporal fenestrae in the side of the head.
The Anapsida is thought to have been ancestral to the Diapsida —a lineage of reptiles whose skull was characterized by two temporal fenestrae and would subsequently include all archosaurs as well as lizards and tuataras.
Proterocheris is another ancient fossil turtle that lived at the same time as Proganochelys. Proterocheris has many features that suggest that it is a side-necked turtle.
If this is true, the two major taxonomic groups of living turtles, suborders Pleurodira side-necked turtles and Cryptodira hidden neckshad their origins some million years ago during the Triassic Period Proterocheris and two later-appearing Triassic genera are likely not true side-necks but turtles that share some pleurodire characteristics.
Unquestionable pleurodires do not appear until the Early Cretaceous about million to In tracing back the history of the other turtle suborder, Cryptodira, Kayentachelys aprix of the Late Jurassic some million years ago is almost assuredly a cryptodire; it is also the oldest known North American turtle.
Other cryptodires are known from the Late Jurassic, although they are not representative of existing families.
The largest known turtle, Archelon, a genus of sea turtles that lived during the Late Cretaceous, reached a length of about 3. Softshell turtle s family Trionychidae are the first modern turtles found in the fossil recordappearing in the Cretaceous Period.Get the latest slate of VH1 Shows!
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