on other sensory stimuli. For example, the internal configuration of nasal cavities within the dorsally located cranial crests in some hadrosaurids may also indicate whether they were used for crooning, an auditory means of wooing. Hadrosauridine cranial crests can be categorized as solid or hollow, based on the presence or absence of bone in the crest. Solid-crested forms, such as Saurolophus, have large nasal chambers under the solid bone; non-crested hadrosaurids (such as Edmontosaurus and Kritosaurus) have a similar anatomical situation. In contrast, the hollow-crested forms, represented by the lambeosaurines, have tube-like structures that emanate from the nasal cavities and extend dorsally and posteriorly.
Initial analyses of these structures resulted in hypotheses that fit the former view of ornithopods and most large dinosaurs, such as sauropods (Chapter 10), as aquatic animals. In these hypotheses, the tubes were thought to be snorkels. However, more complete lambeosaurine skulls, such as those of the Late Cretaceous Parasaurolophus and Tsintaosaurus, show the tubes to be U-shaped, having no discernible openings at the top. This configuration meant that the structures did not aid in breathing underwater, effectively falsifying the snorkel explanation. Thus far, the simplest hypothesis devised on the basis of the evidence is that these tubes, which connected with the nasal cavities, were resonating chambers used for changing the sounds produced, by moving air through the lambeosaurine skull.
If a tree falls in the forest and no one is there to hear it, does it make a sound? The answer is no, because the tree does not make a sound, as sound is a mental translation made from perception of a compressional wave that traveled to the animal. Sound not only depends on a host that can perceive it but, because of its wave properties is also affected by:
1 the density of the compressed medium;
2 temperature, and
3 any obstacles that might interfere with the transmission of the sound.
For example, a compressional wave will travel faster through a denser medium: at 20°C, sound waves in water move at about 1470 m/s, but in air they move at 343 m/s. In gases, with higher temperatures, the speed of sound increases noticeably, at about 30 cm/s for each 1°C. So during times of higher global temperatures, which were common during much of the Cretaceous Period, sound traveled slightly faster. For example, on a typical equatorial day in the lowland tropics of the Mesozoic the temperature may have been 40°C, which would have caused the speed of sound to have been:
V = (343 m/s) + (0.3 m/s x 20) = 343 m/s + 6.0 m/s = 349 m/s (11.1)
where V is the speed of sound.
Because warmer and denser air is normally close to the surface of the Earth, sounds travel faster for ground-dwelling animals living near sea level, and slower for those at higher and colder elevations. For a Mesozoic example that relates to survival, the cracking sound made by a stalking theropod stepping on a branch would have reached the ears of an ornithopod slightly faster on a warm day near a beach than during a cold day in the mountains. This simplistic scenario is complicated by:
1 obstacles that reflect the sound, exemplified by echoes off canyon walls;
2 refraction of the sound as it passes from local air masses of different temperatures; and
3 wind direction and velocity.
An understanding of how resonating chambers worked in these skulls requires a discussion of the fundamental physics of sound.
With regard to the latter, animals downwind of sounds will hear them more easily than animals upwind or standing in still air. This is because the sounds have been carried to the downwind animals as compressional waves in the air moving toward them. Sound also will travel farther if it has a low frequency, with frequency defined as:
where n is frequency, V is velocity of sound (m/s), and l is wavelength (meters). Frequency is measured as the number of vibrations that pass a point per second, as indicated by the equation that has the length units (meters) canceled out. For example, notice how the frequencies differ for sound waves that have wavelengths of 60 cm and 9 cm at the same speed of sound:
Step 2. n = 572 vibrations per second (Hertz)
Step 2. n = 381 vibrations per second
The longer wavelength for the second sound also results in a lower frequency. Using the human voice as an example, a baritone will sing using lower frequency sounds (longer wavelengths) than will a tenor, and a soprano will have a higher frequency sound (shorter wavelengths) than a tenor. Modern wind instruments also illustrate these principles: a bass tuba emits lower frequencies than a flute.
In the case of Parasaurolophus and its unusual U-shaped cranial tube, sound would have been caused by air moving through the tube as it was inhaled and exhaled through the nares. Sound would have been compressed in the tube and thus the length would have controlled the wavelength and the frequency of any sound emitted. In other words, the longer the tube, the longer the frequency. This regulation of frequency to change the quality of a sound is resonance. Parasaurolophus had a long tube (about 2 meters) bent around like a trombone, which apparently served as a resonance chamber. Models of the tubes scaled to the same size make low-frequency sounds of about 85 vibrations per second when air is blown through them. Slightly different lengths of tubes, which might have been inherent with sexually dimorphic Parasaurolophus (although this dimorphism is not firmly established), correspondingly would have produced slightly different sounds. These differences could have been important from a mating perspective because they would have helped the animals to distinguish genders of the same species from a distance without requiring any visual contact. Furthermore, juvenile Parasaurolophus had shorter tubes, so their sounds would have had a higher pitch (frequency) that would have been distinct from the adult sounds, which certainly would have aided active parenting. Other hadrosaurids, whether they were non-crested, solid-crested, or hollow-crested, may have supplemented their enlarged nasal cavities with soft tissues that also would have affected the resonance of any sounds that they made.
Once mates were attracted and successful mating occurred, the development and laying of eggs, as well as nest building, were the next steps in the perpetuation of ornithopod species. Egg shapes confirmed for some hadrosaurids are spheroidal to oblate spheroids; egg sizes are 12 to 20 cm wide, with calculated volumes of 1250
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