Cetaceans are an order of animals that include all whales, dolphins and porpoises. They are secondarily aquatic mammals, their ancestors once walked on land; they need to return to the surface to breathe air; they have a pentadactyl limb in the form of flippers and their spines move vertically, rather than side to side. All these features indicate that they indeed are highly derived mammals. Most species are marine, although some dolphin species are found in the Yangtze, Amazon, Paraná, Indus and Ganges Rivers. They have streamlined bodies with highly compressed neck vertebrae, dorsal fins and a tail with two finlike flukes arranged horizontally. Modern whales have greatly elongated anterior skull bones, and the nostrils are located on the top of the head, forming the blowhole. The forelimbs are specialized to form flippers, and the hind limbs and pelvis are extremely small and do not normally extend out of the body wall of the animal (Slijper, 1979). The mystery of how these creatures returned to the sea began to unravel when discoveries were made in the late 1970s, revealing several stages in the transition of cetaceans from land to sea.
The evolution of cetaceans was once thought to originate with mesonychids, an extinct taxon of carnivorous ungulates, which resembled wolves with hooves. These first appeared in the Early Palaeocene and saw a sharp decline at the end of the Eocene. The last genus to go extinct was Mongolestes, who went extinct in the Early Oligocene. Mongolestes is found in the Ulan Gochu formation of Inner Mongolia. This is different to other mesonychids in several dental features, including the loss of the M3, a mandibular symphysis that is steeper and overall large teeth (Szalay and Gould, 1966). Below you can see a restoration along side the fossil of this animal’s jaw. Notice the large canines and the steep mandibular symphysis.
The most primitive mesonychid, Yangtanglestes, is known from the early Palaeocene of Asia (Jehle, Martin, 2006). These creatures were most likely the dominant predator of Palaeocene Asian environments. However, one genus seemed to have migrated to Europe and North America successfully; Dissacus has been found all over the northern hemisphere (Jehle, Martin, 2006). The fossil record of the European Dissacus, Dissacus europaeus, is fragmentary and includes a mandible, a complete radius and fragments of a humerus. A morphological study of these bones suggests this animal was digitigrade and more cursorial than is usually assumed for the genus. You can see here the fossil mandible.
Mesonychids possessed triangular molar teeth, similar to that of whales and dolphins, as well as many other morphological traits. This is why it was believed that mesonychids were direct ancestor of the Cetacea. However, the discovery of preserved hind limbs of ancient cetaceans (Geisler, and Uhen, 2003) now indicate that cetaceans are more closely related to hippopotamids and other ancient whales, than they are to mesonychids (Boisserie et al., 2005). Most palaeontologists now agree that whales probably did not evolve from mesonychids; it is now suggested that whales descended from, or share a common ancestor with the anthracotheres– semi-aquatic hippo ancestors (Geisler, Theodor, 2009).
Skulls of a 9 million-year-old anthracothere, Merycopotamus medioximus, from Pakistan’s Siwalik Hills (above) and a contemporaneous fossil hippopotamus (Hexaprotodon sivalensis) from the same area exhibit many similarities, including the eye socket, which protrudes above the skull to let the animals see above water while most of their head is submerged. (Anthracothere skull courtesy of Harvard University & Geological Survey of Pakistan; hippo skull from the Natural History Museum, London)
The oldest genus appeared in the Asian Eocene. After this, they thrived in Eurasia and Africa and even some entered North America in the Oligocene. They saw their demise in the Miocene, probably due to climate change and competition (Lihoreau et al., 2006). There is very little fossil evidence that shows these are the true ancestors of whales. What fossil evidence does show, however, is that they are related to hippos. Below is an image of a Microbunodon skull, an ancestor of modern hippopotamuses. You can see that there are several similar features in the lower jaw, such as the structure of their premolars. The palaeoenvironment that Microbunodon was found in, indicated that this animal was amphibious, just like modern hippos. Gene sequencing now gives us evidence that hippos are close living relatives of whales (Gatesy,1997). Therefore, anthracotheres are ancestors of whales also. Nevertheless, the earliest known anthracotheres appear in the fossil record in the middle Eocene, a long time after archaeocetes became aquatic. Cetaceans arose about 50 million years ago in Asia, whereas the family Hippopotamidae is only 15 million years old (Thewissen et al., 2007); clearly there are still a lot of gaps and unanswered questions in this area.
To add to this, there was still not enough strong evidence for a clade combining cetaceans and even-toed ungulates, until the discovery of Pakicetus in 1981. It was found in the Eocene of Pakistan and most palaeontologists consider it to be the most basal whale. The first fossil was an incomplete skull cap and a broken mandible with some teeth. The molars preserved show that this animal could tear flesh and scrapes on the teeth suggested that the food was ground and chewed (Madar, 2007). The key characteristic that links this animal to cetaceans is the large auditory bulla; it is formed from the ectotympanic bone only. The bulla is the bone of the skull that forms the floor of the cavity that houses the middle ear ossicles (Thewissen and Hussain, 1993). The thickened part of the auditory bulla was suspended from the skull, allowing it to vibrate in response to sound waves. Underwater sound would have entered the skull of Pakicetus and caused its bulla to vibrate. The bulla was in turn connected to the chain of middle ear bones, which transmitted the sound to the organ of hearing. Thus the thickened bulla of Pakicetus is interpreted as a specialization for hearing underwater sound.
“Pakicetus is the only cetacean in which the mandibular foramen is small, as is the case in all terrestrial animals. It thus lacked the fat pad, and sounds reached its eardrum following the external auditory meatus as in terrestrial mammals. Thus the hearing mechanism of Pakicetus is the only known intermediate between that of land mammals and aquatic cetaceans.” (Thewissen and Hussain, 1993).
Auditory bulla that is only loosely attached to the rest of the skull. http://www.geol.umd.edu
Following this, proto-whales had to make the transition from terrestrial and fresh water environments, to fully marine. Ambulocetus natans was discovered in Pakistan and could walk as well as swim. The fossils of ambulocetids are always found in near-shore shallow marine deposits associated with abundant marine plant fossils and littoral molluscs (Thewissen et al., 2002). Its back legs were better equipped for swimming and chemical analysis of its teeth shows that it was able to move between salt and fresh water (Thewissen et al., 1996). Ambulocetus is considered as an early whale because it had an adaptation in the nose that enabled it to swallow underwater and its periotic bones had a structure like those of whales, enabling it to hear well underwater. In addition, its teeth are similar to those of early cetaceans. The mandibular foramen in ambulocetids had increased in size, indicating that a fat pad was likely to be housed in the lower jaw. In modern whales, this fat pad in the mandibular foramen extends posteriorly to the middle ear. This allows sounds to be received in the lower jaw, and then transmitted through the fat pad to the middle ear (Thewissen, 2001). Also, they probably swam by pelvic paddling and caudal undulation. (Thewissen, 1997). Whales swim by caudal oscillation, which is more energy efficient, but this is a clear intermediate stage.
Approximately 48 to 35 million years ago, a diverse and heterogeneous group lived in Eurasia, North America and Africa. Protocetids, unlike earlier cetaceans, include open marine forms (Thewissen and Williams, 2002). These were also the first whales to leave India and disperse to all shallow subtropical oceans of the world (Thewissen and Williams, 2002). It has been suggested that these creatures were still amphibious; this is supported by the discovery of a pregnant Maiacetus fossil, in which the fossilised foetus was positioned for a head-first delivery- suggesting it gave birth on land (Gingerich et al., 2009). Whales generally give birth tail first, whilst land mammals give birth head first. This suggests that this animal would go back and forth to the ocean, like a sea lion. However, J. G. M. Thewissen has questioned this, suggesting that the smaller skeleton could be a partially digested meal. Even if the small skeleton is a foetus, Thewissen writes that it may not have been preserved in its normal in-vivo position (Thewissen and McLellan, 2009). These animals’ orbits moved to the sides of the head and increased in size- suggesting that it caught underwater prey, and are similar to the eyes of modern cetaceans (Thewissen et al., 2001). The nasal openings were also now positioned halfway up the snout and their teeth were varied, showing a broad diet (Fordyce and Barnes; 1994). The external ear canal, which is missing in modern cetaceans, is present here. Hence, the method of sound transmission present in them combines aspects of pakicetids and modern toothed whales (Nummela, 2004).
Adult female and fetal (in blue) Maiacetus
It is possible that some protocetids had flukes. However, it is clear that they are adapted even further to an aquatic life-style. But, the pelvis was still connected to one of the sacral vertebrae. The ungulate ancestry of these early whales is still underlined by characteristics like the presence of hooves at the ends of the toes in animals such as Rodhocetus. Terrestrial locomotion of Rodhocetus was very limited due to their hindlimb structure. It is thought that they moved in a way similar to how eared seals move on land.
Basilosaurids were initially mistaken for reptiles, hence the name. These lived in the late Eocene and are the oldest obligate aquatic cetaceans (Thewissen et al., 2001). Basilosaur fossils are usually found in fully marine deposits lacking any freshwater influx. Basilosaurus is significant because it is known to have retained small but well-developed hind limbs that projected from the body, although there was no joint between the pelvic bones and the vertebrae. Living whales retain only tiny splint-like bones as remnants of the pelvis and hind limbs (Gingerich et al., 1994). Modern toothed whales have a melon in their skulls- a mass of fat tissue found in the forehead that is focus and modulate the animal’s vocalisations, which basilosaurids did not have. Basilosaurs also had a smaller brain, suggesting that these animals did not have social skills, like modern Cetacea. In 2011, Fahlke et al. concluded that Basilosaurus’ skull was asymmetrical, just like modern toothed whales, and not symmetrical like in baleen whales and artiodactyls. This asymmetry is associated with high-frequency sound production and echolocation, neither of which is thought to present in Basilosaurus. This cranial torsion probably evolved in protocetids and basilosaurids together with directional underwater hearing and the sound receiving apparatus in the mandible (Falke et al., 2011). Basilosaurs could also hear directionally in water and the ear is much more derived than that of its ancestors. They did, however, have a large external auditory meatus, which is very much reduced in modern cetaceans (Nummela et al., 2004). The anatomy of this creatures tail, suggests there was probably a fluke, which would have aided vertical motion and restoration either show a small dorsal fin or ridge.
Skull and neck of Basilosaurus as reconstructed by Gidley 1913. Notice the heterodont teeth. The synapomorphic cetacean air sinus system is partially present in basilosaurids, including the pterygoid, peribullary, maxillary, and frontal sinuses.
Restoration of Basilosaurus cetoides from carnivoraforum.com . It has an elongate, eel-like body with a fluked tail.
As previously mentioned, toothed whales, or the Odontocetes, have the ability to echolocate by creating clicks at different frequencies and this is what the melon in the forehead is used for. The first fossil evidence of this is in the skull of a 33-14 million year old Squalodon (below is a partial skull of Squalodon bariensis). This animal shows both ancestral and modern features, the teeth being the most ancestral; they are widely spaced apart; their cheek teeth are triangular and serrated for grasping and cutting. In comparison to toothed whales at that time, the squalodontids were likely more mobile. It is believed that the dorsal fins were reduced but larger than that of the ancestors (Kowinsky, J.). Squalodontids also possessed many modern features. Their crania were well compressed, their rostrums were telescoped outward, and their skulls show proof of the origin of echolocation (Kowinsky, J.).
All modern whales are split into two subgroups- the Odontoceti, or the toothed whale, and the Mysticeti, or the baleen whales. Odontocetes have numerous, peg like teeth, although sometimes this is modified, such as the narwhals tusk. Examples of odontocetes are Dolphins, porpoises, belugas, narwhals, killer whales, sperm whales, and beaked whales. Baleen whales lack teeth completely as adults. They feed by straining small marine organisms out of the water using plates of baleen- a hornlike substance that forms filaments that hang down from the roof of the mouth. Blue, right, humpback, minke, gray, and fin whales are well-known examples of baleen whales. Some baleen whales, most famously the humpack whales, are known for the strange and complex songs they produce; their function is not clear, but unlike toothed whales, baleen whales do not use their songs for echolocation (Feldhamer et al., 1999). Different baleen whales use their baleen for different methods of feeding such as gulp-feeding with balaenopterids, skim-feeding with balaenids, and bottom plowing with eschrichtiids. Some members of modern baleens appeared in the middle Miocene. The demise of archaic forms and the radiation of modern ones may have been due to a change in ocean current and temperature. The early baleen whales still had teeth and a baleen and Aetiocetus cotylalveus is considered to have the transition between teeth and the baleen (Fordyce, 1998).
Aetiocetus skull, evolution.berkeley.edu
Another transitional feature is that the blowhole was located half way up its snout, rather than on the top of its head (Berkeley University, 2008). The genus, though more cranially reminiscent of archaic whales, with its pronounced snout and flat cranium, had a loose jaw like later baleen whales (Wallace, 2007). Fossil evidence of Janjucetus hunderi showed that baleen whales were predators, feeding on squid, fish and dolphin-like cetaceans, and that they evolved into gentle, toothless whales. The first true toothless baleen whales appeared in the late Oligocene and lacked the ability to echolocate.
During the Miocene, echolocation developed into its modern form and lots of dolphin-like fauna flourished. Early dolphins belong to Kentriodontidae, which were small to medium-sized toothed cetaceans with largely symmetrical skulls, and thought likely to include ancestors of some modern species. In October 2006, an unusual bottlenose dolphin was captured in Japan with small fins on each side of its genital slit, which scientists believe to be an unusually pronounced development of these vestigial hind limbs (Lovett, 2006).
The sad part is that this dolphin was found in Taji, an area notorious for dolphin fishing. This poor dolphin was sent straight to the ‘whale museum’.
The evolution of cetaceans seems to have an excellent fossil record, apart from linking them to hippo ancestors. Luckily, genetic information has bridged this gap. All whales differ from land mammals in having ear bones encased in structures attached to their jaws only by a flange of bone. This lets them perceive underwater sounds, an incredible adaptation for underwater life. Ceteacea are overall very well equipped for an aquatic lifestyle; their bodies are padded out and streamlined, they have lost substantial body hair and they have developed a fluke at the end of their tail, allowing them to swim with great energy efficiency. Some of the fossil ancestors of these creatures were thought to be sea serpents, but, after studying their clearly mammalian teeth and even a preserved pregnant ancestor, it is obvious that these creatures returned to the sea after millions of years on land. Even in modern whales and dolphins, we can still find the reduced pelvis and hind limbs- remnants of their terrestrial past.
Cladogram of Cetacea within Artiodactyla
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