By Sarah Boessenecker (@tetrameryx)
Happy Fossil Friday!
Recently, CCNHM received a large donation of fossils from the famous PCS mine in Aurora, NC – known to paleontologists as Lee Creek. This is a mecca for fossils from the mid-Miocene through the early Pliocene (~15 – 3 million years ago), and include fossils from pinnipeds, cetaceans, birds, sharks, and sea turtles.
Some of the most common fossils found are cetacean tympanic bullae and periotics – ear bones!
Whales hear differently from land animals, as sound travels close to 4 times faster through the water. Why does this matter? When we hear something, it is caused by sound waves that travel through the air. Sound travels fast, (332 meters per second!) but it is still slow enough that it hits each of our ears at slightly different times. While these differences are minute, our brain is able to process it and calculate which ear the sound waves hit first – enabling us to pinpoint the direction the sound is coming from.
When we hear a sound, it is caused by sound waves vibrating through the air, entering our ear canal, and striking our tympanic membrane – the ear drum. The ear drum vibrates, causing tiny bones in the inner ear to move, and these vibrations are translated into nerve impulses the brain interprets as sound.
However, water is much more dense than air, and this allows sound waves to travel much more quickly though it. If you’ve ever been swimming and submerged your head, any sounds you hear seem to be surrounding you; it’s impossible to tell what direction the sound is coming from. The sound is simply moving too quickly through the water for your brain to be able to differentiate which ear the sound wave hits first. Whales have adaptations to deal with this though.
Water, bones, and muscle all have similar densities – much higher than air. This means that when sound travels through the water and strikes an object, such as a whale, it travels at the same speed through all the bone, muscle, and connective tissue, and as such reaches both ears at the same time. To combat this, whales have adaptations that allow them to still have directional hearing.
Whales have evolved multiple sinus cavities around their ear bones – these air cavities cause acoustic impedance (sound waves bouncing) and as such some sounds are reflected back into the water and scattered. However, whales have also evolved a mandibular fat pad – a large, fatty mass in the lower jaw that acts like our own ear pinna (our outer cartilaginous ear). Sound travels up through the skull and jaw, is channeled into the mandibular fat pad, and is then focused and funneled into the tympanic bulla, which is attached to the malleus (the stirrup in human ears). The malleus vibrates into the cochlea, moving liquid in the canal (just as in humans) over little hairs (cilia) and the brain interprets these movements as sounds.
All of these adaptations also mean that whale ear bones tend to be larger and more dense than those of terrestrial mammals, and as such tend to preserve very well in the fossil record. Since they are not attached to the skull and are simply embedded in soft tissue, they often drop out of decomposing carcasses and tend to be found in concentrated horizons – bone beds. The Lee Creek Mine includes several bone beds, with a rich assortment of earbones from many different species of cetaceans. Be sure to visit CCNHM soon to see the upcoming display featuring many of the fossils from the Lee Creek Mine!
Ketten, D.R. 1994, “Functional analyses of whale ears: Adaptations for underwater hearing.” I.E.E.E. Proceedings in Underwater Acoustics 1: 264-270.
Ketten, D.R. 2000, “Cetacean Ears.” In: Hearing by Whales and Dolphins (W.W.L. Au, A.N. Popper, and R.R. Fay, eds.) Springer-Verlag, Inc., New York, NY. p. 43-108.