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Description

A selection of basal forms of the odontocetes, to scale.

Sonar is a technique used to navigate and find objects through emitting pulses of sound and listening for the echo. It is thought that the first use by humans can be attributed to Leonardo da Vinci in 1490 when he inserted a tube into water and could hear the sound of vessels. Technology has moved on since then! However humans were far from the first animals to develop sonar, with various unrelated groups of vertebrates all showing forms of echolocation to navigate and forage in the dark. Simple versions are used by shrews, tenrecs, oilbirds and swiftlets, but highly developed forms are best known in among bats. Two separate methods are utilised to generate sound, either laryngeal clicks (most 'microbat' species) or tongue-clicking (rousette bats). The other major group which uses echolocation is the toothed whales. By passing air from the blowhole through a unique structure called the phonic lips (which act like human vocal cords) the air is vibrated creating a sound beam. Focusing the sound beam through the melon at the front of the head gives it shape and direction. Returning sound beams are picked up by the throat and passed to the inner ear. Lower frequencies are used for long distance, while higher frequencies give better definition to an object. So versatile is echolocation, that whales are able to determine distance, size, shape, speed and vector of movement, composition between two similar objects, and avoiding interference from other pod members. Echolocation is not known in the primitive 'archaeocetes' (see Bone-crunching proto-whales - Basilosaurids ) or the mysticetes (see Shark fodder - Cetotheriid whales ). As to why it developed in odontocetes is hard to pinpoint, but may be linked with cooler waters in the Oligocene (formation of the Southern Ocean) which created opportunities such as hunting in turbid rivers and below the photic zone in oceans.

Early odontocetes show a wide diversification and specialisation following the Late Eocene extinction of the basilosaurids. While they generally resemble modern dolphins and beaked whales, the basal forms show more primitive features, such as a bulky caudal peduncle (the base to the tail flukes; indicative of slower swimmers), smaller more forward-placed melons, and often have long snouts with interlocking double rooted teeth. Some like Ankylorhiza had an array of large teeth, with those at the front pointing forwards. It has been suggested that this formidable predator behaved much like a modern orca, ramming prey with the teeth causing fatal injuries. Inticetus had differentiated dentition with pointed teeth at the front and multi-lobed teeth towards the rear. A lack of wear on the teeth means that this cetacean didn't chew food, possibly sieving smaller prey. In contrast Inermorostrum and Simocetus are notable for a shortened rostrums. Inermorostrum goes a step further lacking any teeth, and is thought to have preyed upon invertebrates through suction-feeding.

Description

A selection of basal forms of the odontocetes, to scale.

Sonar is a technique used to navigate and find objects through emitting pulses of sound and listening for the echo. It is thought that the first use by humans can be attributed to Leonardo da Vinci in 1490 when he inserted a tube into water and could hear the sound of vessels. Technology has moved on since then! However humans were far from the first animals to develop sonar, with various unrelated groups of vertebrates all showing forms of echolocation to navigate and forage in the dark. Simple versions are used by shrews, tenrecs, oilbirds and swiftlets, but highly developed forms are best known in among bats. Two separate methods are utilised to generate sound, either laryngeal clicks (most 'microbat' species) or tongue-clicking (rousette bats). The other major group which uses echolocation is the toothed whales. By passing air from the blowhole through a unique structure called the phonic lips (which act like human vocal cords) the air is vibrated creating a sound beam. Focusing the sound beam through the melon at the front of the head gives it shape and direction. Returning sound beams are picked up by the throat and passed to the inner ear. Lower frequencies are used for long distance, while higher frequencies give better definition to an object. So versatile is echolocation, that whales are able to determine distance, size, shape, speed and vector of movement, composition between two similar objects, and avoiding interference from other pod members. Echolocation is not known in the primitive 'archaeocetes' (see Bone-crunching proto-whales - Basilosaurids ) or the mysticetes (see Shark fodder - Cetotheriid whales ). As to why it developed in odontocetes is hard to pinpoint, but may be linked with cooler waters in the Oligocene (formation of the Southern Ocean) which created opportunities such as hunting in turbid rivers and below the photic zone in oceans.

Early odontocetes show a wide diversification and specialisation following the Late Eocene extinction of the basilosaurids. While they generally resemble modern dolphins and beaked whales, the basal forms show more primitive features, such as a bulky caudal peduncle (the base to the tail flukes; indicative of slower swimmers), smaller more forward-placed melons, and often have long snouts with interlocking double rooted teeth. Some like Ankylorhiza had an array of large teeth, with those at the front pointing forwards. It has been suggested that this formidable predator behaved much like a modern orca, ramming prey with the teeth causing fatal injuries. Inticetus had differentiated dentition with pointed teeth at the front and multi-lobed teeth towards the rear. A lack of wear on the teeth means that this cetacean didn't chew food, possibly sieving smaller prey. In contrast Inermorostrum and Simocetus are notable for a shortened rostrums. Inermorostrum goes a step further lacking any teeth, and is thought to have preyed upon invertebrates through suction-feeding.