Hans Thewissen

Last updated

Hans Thewissen
Hans Thewissen 8-6-14 with Microscope.jpg
Born
Johannes Gerardus Marie Thewissen

(1959-11-28) 28 November 1959 (age 63)
Herkenbosch, Netherlands [1]
NationalityAmerican / Dutch
Alma mater University of Utrecht (MSc),
University of Michigan (PhD)
Known for Ambulocetus
Pakicetus
Indohyus
Kutchicetus
Arctic whales
Scientific career
Fields Paleontology, evolutionary biology, anatomy, embryology, Sensory ecology
Institutions Northeast Ohio Medical University,
Kent State University
Cleveland Museum of Natural History
Thesis Evolution of Paleocene and Eocene Phenacodontidae (Mammalia, Condylarthra)  (1989)
Doctoral advisor Philip D. Gingerich

Johannes Gerardus Marie (Hans) Thewissen is a Dutch-American paleontologist known for his significant contributions to the field of whale evolution. Thewissen's fieldwork has led to the discovery of key fossils that have shed light on the transition of whales from land to water, including the discovery of Ambulocetus, Pakicetus, Indohyus, and Kutchicetus. In addition to his work on fossil discoveries, Thewissen also studies modern bowhead and beluga whales in Alaska, focusing on their biology and the implications of this knowledge for management and conservation efforts. His research has been instrumental in deepening our understanding of cetacean evolution and the adaptations that allowed these mammals to transition from terrestrial to fully aquatic lifestyles.

Contents

Early life

Thewissen has always been interested in paleontology and natural history. His mother said that when Thewissen was a small boy, she had to sort through his pockets before laundry time to take out the rocks and worms he collected. His father used to take him to the town of Maastricht, and they collected fossils from the Maastrichtian period. His 12th birthday present was a rock hammer, which has accompanied him on all collecting trips since. He grew up just 2 miles from Liessel, a fossil locale that yielded the first whales he ever collected. [2]

Educational background

After finishing Gymnasium secondary education in Deurne, he completed undergraduate degrees in biology with a minor in geology in 1981 at the University of Utrecht.

Thewissen's M.S. projects involved work in three departments of the University of Utrecht. He studied a small artiodactyl from the Eocene of Pakistan in the Geology Department, the systematic position of aardvarks in the biology department, and the functional morphology of digging in the veterinary sciences.

He then earned an MSc cum laude degree in biology from the University of Utrecht in 1984. He studied for a semester at the Museum National d'Histoire Naturelle in Paris, before moving to the U.S. to earn a PhD in Geological Sciences at the University of Michigan, where he studied phenacodontids, a group of Paleogene ungulate mammals (condylarths) that is ubiquitous in North America (more than 6,000 fossils), but rare or absent in all other continents. The work with artiodactyls and phenacodontids brought familiarity with the terrestrial ancestors of cetaceans. At that time, paleontologists thought cetaceans were derived from another group of condylarths, mesonychians, even though molecular biologists later found evidence that cetaceans were closely related to artiodactyls. [3]

After graduating from the University of Michigan, he moved to a postdoctoral position at Duke University Medical Center which is where he became interested in studying whales. Thewissen's work on pakicetids in 2001, and that of his former PhD advisor Philip Gingerich in the same year provided evidence to support a re-evaluation of all fossil evidence. [4]

Career

Thewissen was an assistant professor at Northeast Ohio Medical University (then called Northeastern Ohio Universities College of Medicine) from 1993 to 1999, then associate professor from 1999 to 2008.

In 2001, he was a visiting professor at the Tokyo Institute of Technology. Since 1994 he has been a research associate at the Cleveland Museum of Natural History. In 2008 he became the Ingalls-Brown Endowed Chair, Full Professor of Anatomy at Northeast Ohio Medical University. [5] In February 2019, he was a scientist in residence at Sitka Sound Science Center, Alaska.

Research

Thewissen discovered or worked on four missing links in the evolution of cetaceans. In addition, he worked on the following.

Cetacean evolutionary missing links researched by Thewissen
Indohyus BW.jpg
Indohyus (Raoellidae cca 49–48 Ma)
Pakicetus BW.jpg
Pakicetus (Pakicetidae cca 49–48 Ma)
Ambulocetus BW.jpg
Ambulocetus (Ambulocetidae cca 49–48 Ma)
Kutchicetus BW.jpg
Kutchicetus (Remingtonocetidae cca 48 Ma)

Discovery of Ambulocetus

Thewissen and Hussain discovered a partial skeleton of a new cetacean Ambulocetus in 1992, working with and a team from the Geological Survey of Pakistan in the Kala Chitta Hills of Punjab, Pakistan. [6]

Thewissen and Ambulocetus natans Hans Thewissen and Ambulocetus.jpg
Thewissen and Ambulocetus natans

When this new ancestral whale appeared in the magazine Science in 1994, Stephen Jay Gould dubbed it 'as the smoking gun of whale evolution. [7] [8]

Ambulocetus was recovered from Pakistan [9] ( 33°36′N72°12′E / 33.6°N 72.2°E / 33.6; 72.2 , paleocoordinates 14°18′N68°18′E / 14.3°N 68.3°E / 14.3; 68.3 ) [10] [11] in 1993 by Thewissen and Muhammed Arif, and was described by Thewissen, Hussain, and Mohammad Arif in 1994. [6]

While has been known since the time of Charles Darwin that cetaceans had ancestors that lived on land, this was the first skeleton that included limb bones strong enough to walk on land. [12] [13]

Ambulocetus natans Ambulocetus new NT small.jpg
Ambulocetus natans

"I sat on the porch of a Pakistani guesthouse, puzzling over the sea lion-size skeleton that we had just dug up in the Kala Chitta Hills. I opened some of the packages containing fossil remains that I had wrapped earlier that day, and as I scraped with a dental tool, I realized that this was a whale—one that could walk around on the large hind legs that we had unearthed. It was the first such whale to be seen by a human, ever." [14] [15]

Discovery of Kutchicetus

Sunil Bajpai and Thewissen collected fossils in District Kutch, State of Gujarat, India, in the desert area close to the Pakistani border. Here, they found the skeleton of a small whale called Kutchicetus minimus. The holotype of Kutchicetus consists of some skull fragments, many vertebrae, and ribs and the limb bones, although parts of fore- and hind feet were not found. A jaw fragment allowed several whale skulls and lower jaws to be from the same species. [3]

Discovery of Pakicetus

Ambulocetus (on top) Pakicetus Skeletons comparison Pakicetus Ambulocetus Skeletons comparison.jpg
Ambulocetus (on top) Pakicetus Skeletons comparison

Teeth of Pakicetus were first found and recognized as cetaceans by the American paleontologist Robert West in 1980. [16] However, the Kala Chita Hills in Pakistan is a bone bed where cetaceans and other animals were buried together, so anatomical association between different parts is lost, and West was unable to determine which skeleton bones were associated with the teeth. [17]

Thewissen excavated the site further, discovering hundreds of bones of different mammal species. Whale teeth were the most common teeth recovered, and there were no mammals of the same sizes. This allowed the researchers to identify bones of the Pakicetus skeleton in a preliminary fashion. They later tested this identification by studying the stable isotopes of the bones, which matched the stable isotopes of the teeth and not those of the teeth of other mammals. [18]

Discovery of Indohyus

The Indian geologist A. Ranga Rao collected fossils in the 1960s and 1970s in Indian Kashmir, which he named Indohyus . Upon his death, his widow, Dr. Friedlinde Obergfell, gave the rocks to Thewissen to study. [19]

During the extraction of the fossils, the fossil preparator accidentally broke one of the skulls. In the cracked specimen, Thewissen recognized the ear structure of the auditory bulla which had a shape which is highly distinctive, found only in the skulls of living and extinct cetaceans, including Pakicetus. This suggested that Indohyus was related to cetaceans, and this was later confirmed by formal systematic analysis. [20]

Thewissen was able to extract many skeletal bones of Indohyus, which showed that the species was similar in body shape to a modern mousedeer (also called chevrotains). [21]

Thewissen postulates that the first steps whale ancestors took toward aquatic habitats may also have involved escaping predators. [22]

Thewissen's discovery of Indohyus helped refine the connection between whales and hippos and suggested that Indohyus was closely related to hippos too. [23]

Fred Spoor, an anthropologist at University College London, said the significance of the latest find was comparable to Archaeopteryx , the first fossils to show a clear transition between dinosaurs and birds. "For years, cetaceans were used by creationists to support their views because for a long time, the most primitive whales known had bodies that looked like modern whales, so there seemed to be this enormous gap in Evolution. But since the early 1990s, there's been a rapid succession of fossils from India and Pakistan that beautifully fill that gap," he said. [20]

Embryology

Thewissen acquired access to a collection of embryos of the pantropical spotted dolphin (Stenella attenuata). This dolphin has hind limbs as an embryo, but the limbs are lost as the embryo develops. This work formed the basis for a study of gene control in development. In addition, Thewissen's lab studied the unusual aspects of the dentition of these dolphins, namely the absence of replacement teeth, the similarity of the shape of teeth across the toothrow, and the significant number of their teeth.

Fetus of dolphin showing similarities between land mammals and modern cetaceans. The pelvic remnant is seen as a small blue bar below the tail, and the many bones of the forelimb match those of land mammals. The large brains of cetaceans are growing and not protected by bone at this stage. The fetal tissues were made transparent, cartilage was stained blue, and bone purple. Scale bar is 1 cm. Dolphin Fetus.jpg
Fetus of dolphin showing similarities between land mammals and modern cetaceans. The pelvic remnant is seen as a small blue bar below the tail, and the many bones of the forelimb match those of land mammals. The large brains of cetaceans are growing and not protected by bone at this stage. The fetal tissues were made transparent, cartilage was stained blue, and bone purple. Scale bar is 1 cm.

Sensory organ evolution

To gain access to modern whale soft tissues, Thewissen began traveling to Alaska's north slope. Working in collaboration with the Department of Wildlife Management of the North Slope Borough, Thewissen gained access to the small number of bowhead and beluga whales which are captured yearly by native Alaskans as an indigenous exemption to the Marine Mammal Protection Act. Here, Thewissen discovered that parts of the bowhead whale's brain are dedicated to smell (the olfactory bulbs), something which had never before been confirmed in any cetaceans, and which confirmed the long-held views of Inupiat Alaskans that bowheads have a sense of smell. [24]

The organ of balance, yellow, and that of hearing, blue, in this 43-million-year-old Andrewsiphius Whale ear scan.png
The organ of balance, yellow, and that of hearing, blue, in this 43-million-year-old Andrewsiphius
The braincase of 43-million-year-old Remingtonocetus, with brain (green) and ear (yellow and red) showing through, as based on high-resolution CT scan. The large green canal on the right carries the olfactory nerve and indicates that these whales had a developed sense of smell. CT scan of 43 million year old Remingtonocetus.jpg
The braincase of 43-million-year-old Remingtonocetus , with brain (green) and ear (yellow and red) showing through, as based on high-resolution CT scan. The large green canal on the right carries the olfactory nerve and indicates that these whales had a developed sense of smell.

[25]

Age estimation of modern whales

Thewissen's lab was instrumental in estimating the age of Arctic whales. This data is crucial to gathering data on overall population rate of increase or decrease. [26]

Some cetaceans, such as belugas, lay down layers in their teeth, analogous to tree rings. Thewissen's lab determined that there are several sets of finer repeated layers within the large-scale layers. One of these finer sets is linked to daily processes, and indeed, the thickness of 365 of these layers matches one large-scale layer, suggesting that the large-scale layers reflect annual intervals. [27]

Bowhead whales do not have teeth, but their baleen plates grow with age and can be used to estimate age in younger whales. It was already established that one of the bones of the ear, the tympanic part of the temporal bone, grows annually by laying down a layer of bone. Thewissen's lab studied this for bowhead and determined that this bone may also be used for determining age in this species. Both dental aging and the temporal bone aging are effective methods for determining age in fossil whales. [28]

Thewissen also established that in some cases the presence of earwax in bowhead whales may be used to establish age. In some baleen whales, earwax grows in annual layers that are not expelled through the ear canal and this can be used in age estimation. [29] [30]

Brain evolution research

Thewissen's current work involves counting neurons in bowhead and beluga whale brain samples, to assess brain function, in Utqiaġvik—formerly Barrow—Alaska. Though the size of the brains of sperm whales and killer whales are larger than those of any other organism, including humans, [31] a better measure of brain function is to determine how many neurons there are in the brain. Suzana Herculano-Houzel has developed a method of counting of neurons of human and other animals' brains and the relation between the cerebral cortex area and thickness and number of cortical folds. Humans and other primates pack about twice the number of neurons in a cubed inch of brain as most other mammals. [32]

Appearances in science films and TV shows

Discovery Channel, "Paleoworld", 1994."Back to the Seas" Paleoworld (Season 1)

BBC, "Walking with Beasts", 2001 (work covered with extensive interviews). [33] [34] [35] [36]

NHK (Japanese National Public Television), "The Oceans", 1996

Discovery Channel, 2001 "The Oceans".

Discovery Channel (BBC produced), 2006. Life in the Womb (Prenatal development in dolphins). [37]

Animals in the Womb, 2006 [38]

Evolutions (National Geographic Channel), 2009. [39]

Morphed, 2009 [40]

Bibliography

Books

Critical studies and reviews of Thewissen's work

The walking whales

Related Research Articles

<span class="mw-page-title-main">Cetacea</span> Infraorder of mammals

Cetaceans are an infraorder of aquatic mammals belonging to the order Artiodactyla that includes whales, dolphins, and porpoises. Key characteristics are their fully aquatic lifestyle, streamlined body shape, often large size and exclusively carnivorous diet. They propel themselves through the water with powerful up-and-down movement of their tail which ends in a paddle-like fluke, using their flipper-shaped forelimbs to maneuver.

<i>Rodhocetus</i> Genus of mammals

Rodhocetus is an extinct genus of protocetid early whale known from the Lutetian of Pakistan. The best-known protocetid, Rodhocetus is known from two partial skeletons that taken together give a complete image of an Eocene whale that had short limbs with long hands and feet that were probably webbed and a sacrum that was immobile with four partially fused sacral vertebrae. It is one of several extinct whale genera that possess land mammal characteristics, thus demonstrating the evolutionary transition from land to sea.

<span class="mw-page-title-main">Pakicetidae</span> Family of mammals

Pakicetidae is an extinct family of Archaeoceti that lived during the Early Eocene in Pakistan.

<span class="mw-page-title-main">Evolution of cetaceans</span>

The evolution of cetaceans is thought to have begun in the Indian subcontinent from even-toed ungulates (Artiodactyla) 50 million years ago (mya) and to have proceeded over a period of at least 15 million years. Cetaceans are fully aquatic marine mammals belonging to the order Artiodactyla and branched off from other artiodactyls around 50 mya. Cetaceans are thought to have evolved during the Eocene, the second epoch of the present-extending Cenozoic Era. Molecular and morphological analyses suggest Cetacea share a relatively recent closest common ancestor with hippopotami and that they are sister groups. Being mammals, they surface to breathe air; they have 5 finger bones (even-toed) in their fins; they nurse their young; and, despite their fully aquatic life style, they retain many skeletal features from their terrestrial ancestors. Research conducted in the late 1970s in Pakistan revealed several stages in the transition of cetaceans from land to sea.

<i>Ambulocetus</i> Genus of extinct mammals of the order Cetacea

Ambulocetus is a genus of early amphibious cetacean from the Kuldana Formation in Pakistan, roughly 48 or 47 million years ago during the Early Eocene (Lutetian). It contains one species, Ambulocetus natans, known solely from a near-complete skeleton. Ambulocetus is among the best-studied of Eocene cetaceans, and serves as an instrumental find in the study of cetacean evolution and their transition from land to sea, as it was the first cetacean discovered to preserve a suite of adaptations consistent with an amphibious lifestyle. Ambulocetus is classified in the group Archaeoceti—the ancient forerunners of modern cetaceans whose members span the transition from land to sea—and in the family Ambulocetidae, which includes Himalayacetus and Gandakasia.

<span class="mw-page-title-main">Ambulocetidae</span> Family of mammals

Ambulocetidae is a family of early cetaceans from Pakistan. The genus Ambulocetus, after which the family is named, is by far the most complete and well-known ambulocetid genus due to the excavation of an 80% complete specimen of Ambulocetus natans. The other two genera in the family, Gandakasia and Himalayacetus, are known only from teeth and mandibular fragments. Retaining large hindlimbs, it was once thought that they could walk on land—indeed, their name means "walking whales"—, but recent research suggests they may have been fully aquatic like modern cetaceans. Though the research has some limits that cast doubt on this conclusion.

<i>Basilosaurus</i> Prehistoric cetacean genus from the Late Eocene epoch

Basilosaurus is a genus of large, predatory, prehistoric archaeocete whale from the late Eocene, approximately 41.3 to 33.9 million years ago (mya). First described in 1834, it was the first archaeocete and prehistoric whale known to science. Fossils attributed to the type species B. cetoides were discovered in the United States. They were originally thought to be of a giant reptile, hence the suffix "-saurus", Ancient Greek for "lizard". The animal was later found to be an early marine mammal, which prompted attempts at renaming the creature, which failed as the rules of zoological nomenclature dictate using the original name given. Fossils were later found of the second species, B. isis, in 1904 in Egypt, Western Sahara, Morocco, Jordan, Tunisia, and Pakistan. Fossils have also been unearthed in the southeastern United States and Peru.

<i>Dorudon</i> Genus of ancient whales

Dorudon ("spear-tooth") is a genus of extinct basilosaurid ancient whales that lived alongside Basilosaurus 40.4 to 33.9 million years ago in the Eocene. It was a small whale, with D. atrox measuring 5 metres (16 ft) long and weighing 1–2.2 metric tons. Dorudon lived in warm seas around the world and fed on small fish and mollusks. Fossils have been found along the former shorelines of the Tethys Sea in present-day Egypt and Pakistan, as well as in the United States, New Zealand and Western Sahara.

<i>Pakicetus</i> Genus of ancient whales

Pakicetus is an extinct genus of amphibious cetacean of the family Pakicetidae, which was endemic to Pakistan during the Ypresian period, about 50 million years ago. It was a wolf-like animal, about 1 metre to 2 metres long, and lived in and around water where it ate fish and other small animals. The vast majority of paleontologists regard it as the most basal whale, representing a transitional stage between land mammals and whales. It belongs to the even-toed ungulates with the closest living non-cetacean relative being the hippopotamus.

<span class="mw-page-title-main">Archaeoceti</span> Paraphyletic group of primitive cetaceans from Early Eocene to Late Oligocene

Archaeoceti, or Zeuglodontes in older literature, is a paraphyletic group of primitive cetaceans that lived from the Early Eocene to the late Oligocene. Representing the earliest cetacean radiation, they include the initial amphibious stages in cetacean evolution, thus are the ancestors of both modern cetacean suborders, Mysticeti and Odontoceti. This initial diversification occurred in the shallow waters that separated India and Asia 53 to 45 mya, resulting in some 30 species adapted to a fully oceanic life. Echolocation and filter-feeding evolved during a second radiation 36 to 35 mya.

Nalacetus is an extinct pakicetid early whale, fossils of which have been found in Lutetian red beds in Punjab, Pakistan. Nalacetus lived in a fresh water environment, was amphibious, and carnivorous. It was considered monophyletic by Cooper, Thewissen & Hussain 2009. It was said to be wolf-sized and one of the earliest forms of the order Cetacea.

<i>Ichthyolestes</i> Genus of mammals

Ichthyolestes is an extinct genus of archaic cetacean that was endemic to Indo-Pakistan during the Lutetian stage. To date, this monotypic genus is only represented by Ichthyolestes pinfoldi.

<span class="mw-page-title-main">Whippomorpha</span> Suborder of mammals

Whippomorpha or Cetancodonta is a group of artiodactyls that contains all living cetaceans and hippopotamuses. All Whippomorphs are descendants of the last common ancestor of Hippopotamus amphibius and Tursiops truncatus. This makes it a crown group. Whippomorpha is a suborder within the order Artiodactyla. The placement of Whippomorpha within Artiodactyla is a matter of some contention, as hippopotamuses were previously considered to be more closely related to Suidae (pigs) and Tayassuidae (peccaries). Most contemporary scientific phylogenetic and morphological research studies link hippopotamuses with cetaceans, and genetic evidence has overwhelmingly supported an evolutionary relationship between Hippopotamidae and Cetacea. Modern Whippomorphs all share a number of behavioural and physiological traits; such as a dense layer of subcutaneous fat and largely hairless bodies. They exhibit amphibious and aquatic behaviors and possess similar auditory structures.

<i>Kutchicetus</i> Genus of mammals (fossil)

Kutchicetus is an extinct genus of early whale of the family Remingtonocetidae that lived during Early-Middle Eocene in what is now the coastal border of Pakistan and India. It is closely related to Andrewsiphius with which it was synonymized by Gingerich et al. 2001. Thewissen & Bajpai 2009 proposed a new clade, Andrewsiphiinae, for the two species. Later authors, however, still accept both as separate genera.

<i>Indohyus</i> Genus of extinct artiodactyl mammals from Eocene Epoch

Indohyus is an extinct genus of digitigrade even-toed ungulates known from Eocene fossils in Asia. This small chevrotain-like animal found in the Himalayas is one of the earliest known non-cetacean ancestors of whales.

<span class="mw-page-title-main">Raoellidae</span> Family of mammals

The Raoellidae, previously grouped within Helohyidae, are an extinct family of semiaquatic digitigrade artiodactyls in the clade Whippomorpha. Fossils of raoellids are found in Eocene strata of South and Southeast Asia.

<i>Remingtonocetus</i> Genus of mammals

Remingtonocetus is an extinct genus of early cetacean freshwater aquatic mammals of the family Remingtonocetidae endemic to the coastline of the ancient Tethys Ocean during the Eocene. It was named after naturalist Remington Kellogg.

<i>Maiacetus</i> Genus of mammals

Maiacetus is a genus of early middle Eocene cetacean from the Habib Rahi Formation of Pakistan.

<span class="mw-page-title-main">Remingtonocetidae</span> Family of mammals

Remingtonocetidae is a diverse family of early aquatic mammals of the order Cetacea. The family is named after paleocetologist Remington Kellogg.

<i>Dalanistes</i> Species of mammal (fossil)

Dalanistes is an extinct genus of remingtonocetid early whale known from the late early Eocene of Kutch, India and Punjab and Balochistan, Pakistan. Dalanistes is closely related to Remingtonocetus, but also shares several features with Ambulocetus, and, with its combination of terrestrial and amphibious adaptations, Dalanistes apparently is an intermediate form between these two groups. Isotopic evidence suggest that Dalanistes had a marine diet.

References

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  3. 1 2 Thewissen, Hans (November 2014). The Walking Whales From Land to Water in Eight Million Years (First ed.). University of California Press; First edition. ISBN   978-0520277069. Archived from the original on 7 April 2022. Retrieved 28 November 2018.
  4. ↑ Marx, Felix; Lambert, Oliver; Uhen, Mark (2016). Cetacean Paleobiology (TOPA Topics in Paleobiology) (1st ed.). Wiley-Blackwell. ISBN   978-1118561270.
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  18. ↑ Cooper, Lisa Noelle; Thewissen, J.G.M; Hussain, S. T (2009). "New middle Eocene archaeocetes (Cetacea: Mammalia) from the Kuldana Formation of northern Pakistan". Journal of Vertebrate Paleontology. 29 (4): 1289–1299. doi:10.1671/039.029.0423. S2CID   84127292.
  19. ↑ Ranga, Rao, A (1971). "New mammals from Murree (Kalakot Zone) of the Himalayan foothills near Kalakot, Jammu and Kashmir state, India". Journal of the Geological Society of India. 12 (2): 124–134.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  20. 1 2 Sample, Ian (19 December 2007). "How Bambi evolved into Moby-Dick". Guardian. Archived from the original on 17 August 2021. Retrieved 1 December 2018.
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  25. ↑ Thewissen, J. G. M. (April 2001). "Olfaction and brain size in the bowhead whale (Balaena mysticetus)". Marine Mammal Science. 27 (2): 282–294. doi:10.1111/j.1748-7692.2010.00406.x.
  26. ↑ Thewissen, J. G. M.; Waugh, David (January 2018). "Validation of Growth Layer Group (GLG) depositional rate using daily incremental growth lines in the dentin of beluga (Delphinapterus leucas (Pallas, 1776)) teeth". PLOS ONE. 13 (1): e0190498. Bibcode:2018PLoSO..1390498W. doi: 10.1371/journal.pone.0190498 . PMC   5770016 . PMID   29338011.
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  28. ↑ Rehorek, Susan; Thewissen, Hans (2018). "Structure of the external auditory meatus of the Bowhead whale (Balaena mysticetus) and its relation to their seasonal migration". Journal of Anatomy.
  29. ↑ Thewissen, J. G. M.; Sensor, Jennifer (April 2018). "Age estimation in bowhead whales using tympanic bulla histology and baleen isotopes". Marine Mammal Journal. 34 (2): 347–364. doi:10.1111/mms.12476.
  30. ↑ Berger, Matthew (18 June 2016). "The Story of the Arctic Is Written in Whale Earwax". Newsweek. Newsweek. Archived from the original on 16 December 2018. Retrieved 9 December 2018.
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  32. ↑ Herculano-Houzel, Suzana (March 2016). The Human Advantage A New Understanding of How Our Brain Became Remarkable. MIT Press. ISBN   9780262034258.
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