odontocetes

Echolocation allows odontocetes to pursue agile prey, even in dark or murky waters. Porpoises and dolphins produce ultrasonic clicks by forcing air between the phonic lips in their nasal passages. The lips open and close, which causes the nearby tissue to vibrate and create sound waves. These bounce off the bony cribriform plate at the front of the skull and are then focused into a beam by the melon, which the cetacean aims at the prey. Dolphins use the melon, an oily lump of tissue behind the forehead, to focus their sound waves into a beam. Echoes returning from an animal or object are transferred through oil filled sinuses in the lower mandible to the left ear. Sperm whales (Physeter macrocephalus) have a different anatomy for producing echolocation clicks. The left nasal passage is used for breathing, while the right is for sound production. Sound waves pass through an oil-filled spermaceti organ, rebound off an air sac at the rear of the head, and are focused into a beam of sound through many fatty lenses. Experiments with bottlenose dolphins (Tursiops truncatus) show that they can identify submerged objects by size, shape, composition, and many other factors. This allows then to learn the echo signatures of their preferred prey species. Dolphins also make an array of low frequency sounds to communicate with each other, like whistles and squeaks. Each dolphin has a signature whistle. The picture shows a long beaked common dolphin (Delphinus capensis) hunting fish during the sardine run. Photo source here.

Whalevolution Month #22 – Janjucetus

Living around southeast Australia 25 million years ago, the 3.5m long (11′6″) Janjucetus had large eyes and jaws full of sharp teeth, and may have been an active hunter in a similar niche to modern leopard seals.

But despite sounding like it should be some sort of proto-orca, it wasn’t even an odontocete. This was actually a baleen whale, albeit a member of an odd group that split off before the development of baleen and kept their teeth. Both Janjucetus and its close relative Mammalodon had wide blunt snouts and very large mouths for their size, suggesting they were specialized for suction feeding – using water pressure to draw prey straight into their waiting jaws.

Post #1: Whales

Whales are fully aquatic marine mammals. This means they live in the sea and give labor to living creatures, so they don’t lay eggs. Another example of a mammal that lives under water is a dolphin. Whales belong to the clade Cerartiodactyla (even-toed ungulates) and a very close relative is the hippo! There are 40 species of whales, for example: the gray whale, the beluga, the beaked whales, … Whales can be separated in two suborders: Odontocetes and Mysticetes. The Odontocetes are the toothed whales and the Mysticetes are the baleen whales. This means they have a whole other way of eating and living!



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Whalevolution Month #15 – Squalodon

An early member of the odontocetes, Squalodon was a basal member of the Platanistoidea branch of the toothed whales – represented today by the river dolphins, but originally widespread throughout marine habitats before the rise of the modern oceanic dolphins.

At least seven different species of Squalodon are known from around the world, including Europe, eastern North America, New Zealand, and Argentina, dated to between 34 and 15 million years ago. The largest would have been around 3m long (9′10″), and the tips of their jaws featured protruding tusk-like teeth. Their skulls also show some of the earliest evidence of the development of echolocation.

Whalevolution Month #17 – Simocetus

Simocetus lived around the Pacific Northwest of the United States about 32 million years ago. Roughly the side of a modern bottlenose dolphin – estimated about 2.5m long (8′2″) – this odontocete is known from a fossil skull and a few other fragments discovered in the 1970s but not officially described until 2002. It had a mixture of primitive and specialized features, indicating it was closely related to the most basal of the toothed whales, but its skull structure suggests it already had well-developed echolocation ability.

Its jaws were also partly toothless and oddly downturned, which has been interpreted as an adaptation for suction feeding – sucking in a mixture of sediment and soft-bodied invertebrates from the seafloor and straining the food out between its sieve-like teeth.

Yes. A filter-feeding dolphin.

Whalevolution Month #19 – Cotylocara

Only discovered last year (2014), Cotylocara is known from South Carolina of the United States, about 28 million years ago. Roughly 3m long (9′10″), it was a member of a somewhat obscure clade of early odontocetes called the Xenorophidae – one of the very first groups to diverge, and probably about as distantly related to modern forms as it’s possible to be while still being a member of the toothed whales.

Its skull indicates it possessed at least a rudimentary form of echolocation. Since it was so distantly related to other toothed whales, this gives strong evidence that echolocation was a truly ancestral trait for all odontocetes, originally developing in their common ancestor at least 34 million years ago.