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seismic sauropods & other fractalizations



The chronology of the original literature:
C.E. O'Connell, B.T. Arnason, L.A. Hart, 1997. Seismic transmission of elephant vocalizations and movement. Jour. Acoustical Society of America 102(5):3124 [abstract]
Tom Reuter, Simo Hemila, Sirpa Nummela, 1998. Elephant hearing. Jour. Acoustical Society of America 104(2):1122-1123
C.E. O'Connell, L.A. Hart, B.T. Arnason, 1999. Comments on "Elephant hearing". Jour. Acoustical Society of America 105(3):2051-2052
C.E. O'Connell-Rodwell, B.T. Arnason, L.A. Hart, 2000. Seismic properties of Asian elephant (Elephas maximus) vocalizations and locomotion. Jour. Acoustical Society of America 108(6):3066-3072
The literature on bioacoustics among extant dinosaurs is sparse but worthy of scholarly exegesis. Sound production/hearing are intricately connected to predator-avoidance behavioural systems and predator/prey sexual displays. The larynx is a protective valve, its mucosa immediately shutting the larynx when any object touches  it. Among mammalians, the larynx often penetrates the nasopharyngeal openings. However: in living dinosaurs the larynx is protective, but the syrinx has only one purpose: sound production. Moreover, the larynx among theropods is quite variable, ranging from zero to nine muscles and is in different positions. Syringeal variability seems to play an important role in speciation processes via reproductive isolation, unlike mammals where the larynx is rarely variable. The theropod syrinx is, as K.F. Liem has observed, a "key innovation", enabling theropods to avoid the multi-use constraints seen in mammals (although I think it probable pre-K/T thero! po! ! ds displayed the same variability as in flying dinosaurs today).
Sound is also heard, and sensed, so to speak. Among elephants, the sensed sound, in correlation with infrasonics, is pivotal in an array of communication systems, the body size of these megaherbivores a major parameter. Their infrasonics is possible because of  the requirements of impedance matching, the production of low-frequency oscillations assisted by the elephant's ability to convert oscillation energy into acoustic energy, i.e., oscillator size > wavelengths of generated sound. Among dinosaurs, voices emanate from the tracheal base, the vocal tract being oral cavity + nasal cavity + trachea. Regardless of the size of the theropod, the vocal tract is relatively longer than that of elephants and reptiles, and it is interesting how a small dinosaur can bellow, as it were, because of tracheal elongation.
Among lambeosaurs, as David Weishampel et al. have illustrated, the elongated nasal passages lengthened their vocal tracts, decreasing frequences. Similarly, among mammals with proboscises (elephants, elephant shrews, proboscis primates, oreodonts, some pinnipeds), the nasal vocal tract is analogously lengthened, lowering nasal formant frequencies. I believe all of this -- the long necks of sauropods, elephant trunks, lambeosaur crests, e.g. -- exaggerate the size of the body producing the sounds. Among elephant seas and proboscis primates, these are sexually dimorphic traits, making one wonder whether or not neck/crest sizes among pre-K/T dinosaurs were similarly derived.
Several taxa use, as well, "seismic" cues. Caitilin O'Connel-Rodwell and team note that elephants, unlike other therian mammals, have ears with a cochlear anatomy homologous to reptilian vibrational sensitivity (e.g., snakes), their 1999 paper providing the interested student to primary references. The 2000 paper is, as others have noted, "pregnant" with inferences re: sauropods (and hadrosaurs and ceratopsians), it being probable sauropods used infrasonics and "seismic" communications over long distances because their long necks enabled heightened visual acuity, and ceratopsians/hadrosaurs may have used group size and "seismic" sound to compensate for not being able to see farther, as it were.
I would also refer to the research of Marc Hauser and W. Tecumseh on the interrelationships between acoustic signalling and how sound is produced.