T. rex holotype specimen. Photo by Scott Robert Anseimo, CC BY-SA 3.0.
But how close to reality are these sounds? Do we have any ways of using science to figure out what dinosaurs and other stem-birds may have sounded like? Do we have evidence that they made sounds at all?
Take a look at the Vulture article linked above. It lists some of the amazing places sound effects artists went to create the sounds for the dinosaur movie Jurassic Park in 1993. The iconic T. rex roar was created by playing with the speed and frequency of elephant and dog sounds. The raptor sounds were created using tortoises, horses, and geese. Only one of these sources is an animal anywhere close to being related to dinosaurs--the goose. Many of the other sounds are re-mixed from various mammals, the kinds of sounds we expect large predators (which today are almost all mammalian) to have, thus increasing the scare factor for audiences.
In reality, mammalian or even tortoise sounds are probably not the best analogue for real-life dinosaur sounds. But then what are? The only real evidence for stem-bird vocalizations in fossils comes from two sources: good old phylogenetic bracketing, and correlations between bone structure and soft tissue vocal chords. Phil Senter reviewed this evidence in his 2009 paper on evidence for vocalization in prehistoric animals.
Many groups of animals today can make noise using vocal chords of some kind, and most of these do so using some kind of larynx or larynx-like vocal organ. Most groups of mammals vocalize, so the prince of a larynx is probably ancestral for our own major group, though how far back down the stem-mammal lineage the larynx might go is hard to say. One line of evidence cited by Senter is the presence of a tympanic ears, which would be necessary to hear any complex vocal communication being made. The earliest stem-mammals lacked ears, which don't appear until almost the mammalian crown group, though the grade leading up to the crown may have been able to hear some low-frequency airborne sounds.
The almost random distribution of tympanic ears and the larynx in modern tetrapods indicates that these features evolved multiple times, for example in frogs, some salamanders, mammals, turtles, gekkotan lizards, and crocodiles. This is supported by the fact that birds lack the larynx entirely, and vocalize with an entirely different organ called the syrinx. While it is possible that the ancestors of birds had a larynx and it was later replaced with the syrinx, there is currently no evidence to support or test this possibility, and it should be assumed that stem-birds never had a larynx.
Can we tell how far down the bird family tree the syrinx evolved? It turns out that the presence of a sound-making syrinx is strongly tied with the presence of a clavicular air sac, without which the syrinx could not function (Olsen & Joseph, 2011). Like other air sacs, the clavicular sac creates hollow spaces in the bones of the forelimb and shoulder girdle. There are three known stem-bird clades that have such air sacs: Pterosauria, Aerosteon, and Ornithothoraces. The latter, of course, is the one that includes birds, so the air sacs in non-avian ornithothoracines are probably homologous with those of modern birds. Aerosteon and pterosaurs, though, are problematic. Aerosteon are bracketed on both sides by theropods known to definitely lack clavicular air sacs, so the presence in the former taxon is probably an independent evolution possibly unrelated to vocalization. The same goes for pterosaurs, which may have independently evolved air sacs to help cope with flight, and independently developed the air sacs in their forelimbs, possibly without any relation to a syrinx (Senter, 2009).
Does this mean T. rex and her dinosaurian buddies were all silent? The answer is no, because animals can make plenty of sounds by means other than a larynx or syrinx, just not the mammal- or bird-like sounds we are used to associating with them in pop culture. Hisses, drumming sounds, clicking or rattling sounds can all be produced just by moving air and not vocalizing. Non-ornithothoracine stem-birds could also have made physical sounds, like whip-cracking tails in diplodocids, beak-clacking, jaw-grinding, water-slapping, etc. Some birds can also make low booming and drumming calls using only passages through the trachea and cervical air sac, like emu and cassowary (Olsen & Joseph, 2011).
What about the famous hooting horns and crests of lambeosaurines? Senter noted that even in modern animals, the presence of resonating chambers don't always correlate with the presence of vocal chords. Some snakes, for example, have massive resonating chambers in their skulls which amplify and modify the sound of their hiss. It's entirely plausible, Senter argued, and more in line with the other evidence, to think that Parasaurolophus used their crests to project loud hissing or drumming sounds, rather than trumpeting (Senter, 2009). However, it's also possible that the complex internal, trumpet-like tubes of such crests evolved because these animals lacked vocal chords. After all, a trumpet player doesn't vocalize into his instrument, he simply blows air through, and the internal "anatomy" of the trumpet create the musical sound. I have to disagree with Senter here and suggest that lambeosaurines did hoot and trumpet, but were probably one-note instruments, unless some kind of weird soft tissue allowed them to hit different "keys" and modify the sound. Either way, no syrinx or larynx should be required for this to work.
It needs to be noted that the larynx evolved many times in tetrapods, and it's entirely possible that some stem-bird lineages also independently evolved such an organ. But absent any kind of testable, direct evidence, or unusual sound-related structures like the ones found in lambeosaurines, any good scientist needs to presume that most of them could not vocalize the way depicted in movies.
So how did these animals communicate if their were largely silent and limited in vocal repertoire? Stem-birds, more than possibly any other group of tetrapods, are well-known for outlandishly flamboyant visual display structures. Huge, flashy crests, plates, spikes, sails, and frills abound in nearly every stem-bird group. The idea that these animals were very limited in audible communication may help explain why they developed such a dazzling array of visual communication devices. It is only in the ornithothoracines that we see direct evidence for acoustic communication and, maybe not coincidentally, this is the first group of stem-birds to become fully arboreal. For very small species living mostly hidden from each other in the dense leaves of trees, visual communication would become more difficult, and there would be selective pressure for alternate means of communication, such as sound, to evolve.
Bird calls may have developed initially to keep in touch between animals that had become hidden from each there in the trees and could no longer regularly maintain visual signals. Like songbirds today, which use visual but mainly vocal communication to find and attract mates, basal ornithothoracine protobirds may have become the first stem-birds to call to each other through the foliage.
So, what did T. rex say? It may have hissed and boomed like crocodiles, snakes, and emus. It may have been the strong, mostly-silent type. But it probably couldn't roar. Except, of course, in the next Jurassic Park film. Movie producers will never let science get in the way of a good scare!
- Olsen, P., & Joseph, L. (2011). Stray Feathers: Reflections on the Structure, Behaviour and Evolution of Birds. Csiro Publishing.
- Senter, P. (2008). Voices of the past: a review of Paleozoic and Mesozoic animal sounds: review. Historical Biology, 20(4): 255-287.