Sunday 15 February 2015

Trying to understand hearing in an Eocene Basilosaurid Whale.

Whales are highly specialized carnivorous Marine Mammals, with morphologies highly adapted to an aquatic lifestyle. Their closest relatives, the Artiodactyls, are terrestrial herbivores (and occasionally omnivores), with a very different lifestyle and hence anatomy. This means that almost every aspect of the Whales’ biology has had to undergo severe modification, to enable them to live in their new environment. All modern Whales appear to have highly specialized auditory (hearing) systems, though these are not the same in all Whales. In the Odontocetes (Toothed Whales), the group that includes Dolphins, the auditory system is adapted to hear very high pitched sounds in an aquatic environment, and in some cases used as part of a sonar system, detecting reflected high pitched sounds emitted by the Whale to help locate prey. In the Mysticetes (Baleen Whales) the auditory system is modified to detect extremely low sounds, and used as part of a communication system in which the Whales emit very deep sounds (Whale song) which enables them to communicate over vast distances.

This has led to two rival theories about the hearing in the early Whales; either they could have had low frequency hearing, with the Odontocetes subsequently developing high frequency hearing as a specialization, or they could have had high frequency hearing, with the Mysticetes subsequently developing low frequency hearing as a specialization to their lifestyle. Studies of the anatomy of the middle and inner ears of modern Whales suggest that low frequency hearing might have been the ancestral state, however studies of the jaws and skulls of Archaeocete Whales (Whales that lived before the split between the Odontocetes and Mysticetes) suggest that these animals were more similar to modern Toothed Whales, some even having slightly asymmetric skulls, an feature possibly indicative of echolocation.

In a paper published on the Journal of Anatomy on 14 November 2014, Eric Ekdale of the Department of Biology at San Diego State University and the Department of Paleontology at the San Diego Natural History Museum and Rachel Racicot of the Department of Geology and Geophysics at Yale University and the Department of Biology at Howard University describe the results of a study in which a right petrosal of a Late Eocene Basilosaurid Whale, Zygorhiza kochii, was CT scanned at the University of Texas CT facility in Austin in order to build up a 3D computer model of the inner ear, enabling a morphometric analysis (a method used by palaeontologists to sort bones and shells into taxonomic and ecological groups) in comparison to the inner ears of other fossil and modern Whales. The Basilosaurids are a group which appeared early in the history of the Cetaceans, and are thought to have been amongst the first fully aquatic Whales, and should therefore provide a useful model for hearing in the earliest Whales.

Digital endocast of right bony labyrinth of Zygorhiza kochii rendered from CT data in (A) anterior, (B) lateral, (C) dorsal, and (D) vestibular view. Anatomical abbreviations: aa – anterior ampulla; ac – anterior semicircular canal; ant – anterior direction; ap – apical tip of cochlea; at – apical turn of cochlear spiral; av – bony channel for endolymphatic duct (vestibular aqueduct); bl – basal end of bony laminae (represents origin of basilar membrane); bt – basal turn of cochlear spiral; cc – canaliculus cochleae for membranous perilymphatic duct; cn – canal for cranial nerve VIII within modiolus; co –cochlea; cr – common crus; Cra – radius of apical turn of cochlea; Crb – radius of basal turn of cochlea; dor – dorsal direction; er – elliptical recess of vestibule; fc – fenestra cochleae; fv – fenestra vestibuli; la – lateral ampulla; lat – lateral direction; lc – lateral semicircular canal; lg – laminar gap (distance between primary and secondary bony laminae); med – medial direction; nc – nerve canals for branches of cranial nerve VIII; pa – posterior ampulla; pc – posterior semicircular canal; pl – primary bony lamina; pos – posterior direction; sg – canal for spiral ganglion within primary bony lamina; sl – secondary bony lamina; sr – spherical recess; st – stapes; vb – vestibule; vn – canal for vestibular branch of cranial nerve VIII; w – wall separating successive turns of cochlea. Ekdale & Racicot (2014).

In order to provide comparisons for the study Ekdale and Racicot also carried out CT scans of the inner ears of a variety of modern Baleen Whales (a Bowhead Whale, Balaena mysticetus, a Minke Whale, Balaenoptera acutorostrata, one new-born and one adult Gray Whales, Eschrichtius robustus, a Right Whale, Eubalaena glacialis, and a Humpback Whale, Megaptera novaeangliae) as well as three fossil species, the Miocene ‘Megaptera’ miocaena (this is no longer considered to be a member of the genus Megaptera, but has not been re-assigned to another genus) and two undescribed fossils from the Pliocene San Diego Formation, one Balaenopterid and one Eschrichtiid. In addition they used previously published data on the inner ears of Toothed and Baleen Whales, notably the Dolphin Tursiops truncates and an undescribed Balaenopterid from the Pliocene Yorktown Formation of North Carolina, as well as data from a number of non-Cetacean Mammals.

Digital endocasts of bony labyrinths of (A) Odontocete and (B–K) Mysticete cetaceans in anterior, lateral, dorsal, and vestibular view for comparison with the Archaeocete Zygorhiza. Labyrinths for most Mysticetes are from the right side of the body, but those from the left side (D, G, H, and J) are reversed for ease of comparison with the labyrinth of Zygorhiza. Ekdale & Racicot (2014).

The exact function of the anatomy of the ears of modern Whales is not fully understood, preventing any interpretation of anatomical function in Zygorhiza kochii. Nevertheless, comparison of the inner ear of the Basilosaurid to those of modern Whales placed it firmly with the Baleen Whales, despite the fact that it was the smallest Whale in the study, with an estimated body mass of 3400 kg, and was a Whale with teeth interpreted to have had a predatory raptorial predatory lifestyle (i.e. hunting down single large prey items). This strongly suggests that an auditory system adapted for hearing low frequency noises was the ancestral state in Whales, and that the high frequency hearing of modern Toothed Whales is a recent innovation.

Only in one important regard did the ear of Zygorhiza kochii differ from that of modern Whales; the semicircular canal planes of all modern whales are held at an angle of close to 90˚ to the angle of the jaw, whereas the angle in the Basilosaurid is much more acute. Ekdale and Racicot suggest that the arrangement in modern Whales makes their ears much more sensitive to neck movements (something modern Whales tend not to do), but the arrangement in Zygorhiza kochii meant that its’ ears were able to cope much better with such movements.

See also…

Porpoises, Phocoenidae, are small Dolphins, Delphinida, found today across most of the world’s oceans, but with a fossil record restricted almost entirely to the North Pacific. Only a single fossil species from outside the Pacific Basin has been described, Septemtriocetus bosselaersi from Pliocene sediments at Verrebroek Dock in Antwerp Harbour, Belgium, with...

The first Whale (Cetacean) fossils appear in the early Eocene of Pakistan and India,. Early groups of Whales such as the Pakicetids, Ambulocetids and Remingtonocetids were apparently restricted to the Indian continent (which was an island unconnected to Eurasia in the earliest Eocene). The first group of Whales to be found on other continent were the Protocetids, which have been recorded from Africa and North and...

The Earliest Whales appeared in what is now Pakistan and India at the eastern margin of the Tethys Sea then spreading along the southern margin of the Tethys (North Africa) and eventually reaching the Atlantic...

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