Abstract
Cetacean evolutionary history is a complex puzzle. Cetaceans are highly specialized fully aquatic mammals, and it appears that overtime their physiology became well adapted for the marine environment. Fossils found from the Paleogene Period, cetaceans have continued to develop until the present day. Evidence for their origins and evolution comes from fossils which possessed transitional morphologies not found in extant cetaceans. Study of these fossils presented some confusion or puzzle. For example, evidence of cetacean ankle bones and the analysis of sperm and stomach tissue led to conflicting ancestry theories for evolution from terrestrial creatures. Marine food sources may provide a reason for the development of cetaceans' fully aquatic physiologies. Discovery of the exact origins and trajectories of cetacean evolution is ongoing.
Introduction
Cetaceans are mammals. They nurse their young and they have hair yet these mammals are fully aquatic. It seems that long ago the best available food source was life in the water and all that went to drink there, so cetaceans developed a life in the water to exploit this. Over time their bodies adapted through evolution. Changes gradually appeared in cetaceans' body shape, limbs, breathing mechanisms, ears, and teeth to facilitate the transition from terrestrial to aquatic mammals. Finding documentation of the progressive changes through fossil evidence has not been easy nor is it complete. Fossils millions of years in the making of creatures that lived deep underwater had to be discovered and analyzed. Scientist puzzled over the ancestry lineage and how far back each fossil dates. Pondering over fossils, untangling ancestry lines, and figuring out possible answers to the questions posed by cetacean evolution is an on going challenge.
Modern Cetaceans
Modern cetaceans are well adapted to aquatic environments, both salt and freshwater (Milinkovitch and Lambert 2006). They are found living in both river (e.g. river dolphins) and ocean (e.g. all modern whales) habitats (Waggoner 2005). They appear underwater in every climate in the world (Milinkovitch and Lambert 2006). They consume small prey such as crustaceans, fish, and cephalopods as well as large prey as seen in Orcas who eat seals and other larger marine mammals (Milinkovich and Lambert 2006). Cetaceans have become shaped for a totally aquatic existence. Modern cetacean form shows just how evolved cetaceans have become for swimming and diving as compared to their walking terrestrial ancestors. Their current bodies are long and hydrodynamic for easy mobility through water with a narrowing skull to help them forge on through the ocean, with their body to producing less resistance and by doing so conserving energy (Milinkovitch and Lambert 2006). A shortened neck almost indistinguishable from the torso has become smaller and evolved to a shape similar to fish (Milinkovitch and Lambert 2006), On some cetaceans' backs as seen in other marine animals such as sharks and most fish, is a erect dorsal fin, other cetaceans without the dorsal fin have dorsal ridges, for balance and maneuverability when swimming (Milinkovitch and Lambert 2006). There are two additional fins called flippers on the front half of their bodies and these are used for directionality (Waggoner 2005). A layer of blubber surrounds these mammals' bodies, who can weigh anywhere from three hundred and fifty pounds to one hundred and fifty tons, to prevent heat loss (Milinkovitch and Lambert 2006). There final appendage is a horizontal tail that is characteristic of cetaceans that ungulates to move massive amounts of water in order to propel these immense creatures (Waggoner 2005). And since these animals are mammals they need air to breathe with their lungs this is accomplished by use of a blowhole, which are their nasal openings functioning as noses do in people, positioned on top of their head (Milinkovitch and Lambert 2006). Although cetaceans appear fishlike from the outside, their skeleton includes a vestigial hind pelvis and hind limbs, hinting at a slow decent from terrestrial mammals. (Thewissen et al. 1998). Modern whales, especially odoncetes who consume large prey, possess very intricate stomachs used to digest whole food since they do not masticate their food and normally swallow it whole (Newton 1875). Some cetaceans have an adapted trait called "disruptive coloration" which helps them hide in the water (Milinkovitch and Lambert 2006). An example of the is seen in the extant odonocetes Orcas which are "color shaded" with dark coloration on its back and light coloration on its stomach (Milinkovitch and Lambert 2006). This provides the Orca with camouflage from above because it will blend in with the dark colors of the deep water and when looking up from below it will blend in with the light colors of the sky seen through shallower water (Milinkovitch and Lambert 2006).
Another significant and defining feature of these mammals is their ability to dive to incredible depths without being affected by decompression syndrome (Beatty and Rothschild 2008). Decompression syndrome is when diffused gases are pulled out of the fluids in the body in the form of bubbles after depressurization as sometimes seen in human divers (Beatty and Rothschild 2008). All types of modern whales share the same characteristic of repetitive deep diving for food, while their ancestors do not (Beatty and Rothschild 2008). Whales repeat this action every day and show no symptoms of decompression syndrome (Beatty and Rothschild 2008). Cetaceans are physiologically tailored for overcoming decompression syndrome. One physiological attribute discovered by Uhen in 2004 is sinuses around the ear that help with pressure adjustment during diving (2010). The reason why these whales dive so deep is for cephalopods, also known as squid, which is the majority of the odoncetes' diet and are plentiful deep below visible levels of the ocean (Lindberg and Pyenson 2007).
The two currently living Cetacea orders are the Odontoceti, and Mysticeti. Both are part of the clade called Neoceti, or crown group Cetacea, which includes all living whales (Newman 1875). Odontecetes and mysticetes have different anatomical features. Odonocetes have homologous cylindrical teeth which are not used to chew since they normally swallow their prey whole (Newman 1875). The exception is the Orca which will use its teeth to tear its prey into pieces and swallow those pieces whole since the prey it pursues is sometimes larger than can be swallowed entirely whole (Waggoner 2005). Their blowhole is also different in that it is singular, identified by a single muscular skin flap that covers it when the blowhole is closed, and it sits atop a asymmetric skull also differing from mysticete (Newman 1875). Odoncetes have certain ribs that also diverge from mysticete anatomy because they are two headed rather than one headed (Newman 1875).
Odoncetes make and perceive noise as useable sounds due to their ability to use echolocation, and this enables them to maneuver through the water and find food in adverse conditions (Waggoner 2005). Sounds made by the odonocetes emanate from their blubbery foreheads, or melons, passing through frontal face pouches and their skull (Milinkovitch and Lambert 2006). Sounds vibrate the thin tissue that divides the cochlea, which is a spiral shaped space at the base of the head (Uhen 2010). Odonocetes have smaller gaps or spaces in their heads than mysticetes, which do not use echolocation (Uhen 2010).Echolocation is extremely useful because of its accuracy which makes it possible to view small objects at very far distances as well as very short distances, which can be a crucial tool when hunting squid hundreds of feet below the surface of the ocean with no light present (Milinkovitch and Lambert 2006).
Mysticetes are the clade of living baleen whales. In fetal mysticetes there are teeth present but are replaced during maturation with baleen (Newman 1875). Baleen is made of keratin, which is also found in human fingernails and hair, in flat plates similar to a comb with many teeth crowded together that fray out at the bottom and side similar to bristles of a toothbrush (Waggoner 2005). As opposed to odonocetes who swallow their food whole with water included, mysticetes use baleen to filter feed by rushing toward a group of prey, such as crustaceans or a school of fish, with their mouth wide open and then close their mouth with prey and water inside (Milinkovitch and Lambert 2006). The mysticetes then push their tongues to the roof of there mouth and this action filters out the water while the prey are kept inside the mouth by the comb like baleen acting as a filter (Milinkovitch and Lambert 2006). Baleen whales have two blowholes, signaled by two muscle aided skin flaps one over each hole, leading to a different respiratory system than odonocetes that goes to the trochea and then its lungs they are positioned on top of its symmetrical skull, which is another difference from odonocetes (Milinkovitch and Lambert 2006). Another anatomical difference between mysicetes and odonocetes is that mysticetes only possess one headed ribs (Newman 1875). Baleen whales produce complex songs but they do not seem to use echolocation and they have a larger basilar gap in their cochlea (Waggoner 2005).
Cetacea Order/Ancestry
Ceatceans first appeared during the Paleogene Period, when simple mammals became larger and more diverse (O'Leary and Gatesy 2008). [Figure 1] Some mammals would live on land, others such as cetaceans would become capable of living in water, and others would come to live in mostly airborne environments (O'Leary and Gatesy 2008). The Paleogene Period is made up of the Paleocene, Eocene, and Oligocene Epochs (Uhen 2010). Most cetacean fossils come from the Eocene Epoch, from fifty-six to thirty-four million years ago, and this epoch is known for the creation of the first modern mammalian orders (O'Leary and Gatesy 2008). Cetaceans continued to develop throughout the Oligocene Epoch, from thirty-four to twenty-three million years ago, ending the Paleogene Period and leading into the Neogene Period (Uhen 2010). The Neogene Period consists of the Miocene and Pliocene Epochs and leads into the Quaternary Period which continues into the present (O'Leary and Gatesy 2008). Study of early and middle Eocene skeletons since the nineteen eighties to the present with exponential findings of different scientific evidence has developed a plethora of ideas about cetacean orgins, that are constantly being changed with the addition of new found data (Uhen 2010). All of this fossil evidence makes it possible to compare molecular evidence (e.g. DNA), physiological and morphological evidence (e.g. their anatomies), and other chemical tests (e.g. isotopic analyses) all to find out things like where, when, how earlier cetaceans lived, and how closely related their are to modern cetaceans and this data indicates changes within cetaceans (Bajpai et al. 2009).
Figure 1: Evidence of emergence and evolution of early Cetacea during the Paleogene Period
Determining the extinct and extant related families and orders of cetaceans has been a long struggle (O'Leary and Gatsey 2008). Analyses done by morphologist O'Leary in 2000 seemed to show the extinct Mesonychia as closely related to Cetacea (O'Leary and Gatesy 2008), and Luo and Gingerich supported O'Leary's hypothesis (O'Leary and Gatesy 2008). Mesonychia had been believed to be placed outside of the Cetartiodactyla, Cetacea and Artiodactyla, taxa but if they were close sister taxa of cetaceans then this would include mesonychia in this group (Spalding et al. 2009). Before this all data taken from tests on cetaceans and other spcecies had shown a definite relationship between Cetacea and Artiodactyla with no other relationship of this kind between another mammal and Cetacea existing (O'Leary and Gatesy 2008). But new data showed that deleting Mesonychia entirely from phylogenetic analysis would leave an evolutionary gap. (O'Leary and Gatesy 2008).
Then an exciting discovery changed their thinking. Another piece was added to the puzzle. Hypothesizing that Mesonychia could be a sister group to whales was proven wrong by the discovery of astragali, or ankle bones, from pakicetid and ambulocetid cetacean fossils discovered in Pakistan (Uhen 2010). The morphologist Thewissen analyzed the discovered whale ankle bone and stated that by including the new ankle bone data Cetacea grouped as sister taxa of the monophyletic, with descent from a single ancestor, Artiodactyl, and that Mesonychia are positioned outside of the cetaceans (Thewissen et al. 1998). Geisler and Uhen furthered this study on the astragali and showed that the ankle bone had barely any proof of there being a strong hippopotamid and cetacean relationship exclusively and no other artiodactylans (O'Leary and Gatesy 2008). [Figure 2] Other results added to the confusion about where cetaceans fall on the phylogenetic tree, including the analysis of sperm and stomach tissue from Cetacea and other Artiodactyla in which showed no real similarities between stomach tissue and sperm of hippopotamids and whales ( O'Leary and Gatesy 2008). There is also contrasting evidence as to whether there is common ancestry of Hippopotamids and cetaceans in tested areas of the skull were morphology studies were done,;illustrating the difficulty of determing through fossils the evolutionary puzzle of these mammals (O'Leary and Gatesy 2008).
Figure 2: a) Represents presently accepted phylogenetic analysis of cetacean origins
b) Represents not currently accepted view of Mesonychia as sister taxon to cetaceans
Figuring out the evolution of cetaceans is confusing because the available data can be conflicting,as was seen when evidence of Mesonychia possibly being the sister group to Cetacea was introduced. It is difficult to match living cetacean anatomy as having a similar structure to their land mammal ancestors (O'Leary and Gatesy 2008). Extinct stem, or ancestral link, cetaceans have transitional systems not existent in modern cetacean anatomies that show evolution from prior ancestors and eventually develop into the biological systems present in today's whales (O'Leary and Gatesy 2008).
Molecular data analysis may indicate evolution from land species and a relation between cetaceans and Hippopotamidae (Thewissen et al. 2007). One question when cetaceans were first being studied was, "Why go to such lengths, overcoming such physiological hurdles, to breathe air and live in water?" (Beatty and Rothschild 2008). Access to more food seem to be the answer (Beatty and Rothschild 2008).
Cetaceans appear to come from one of many different groups of four limbed species that used the aquatic environment for food resources (Uhen 2010). Cetaceans are the only fully aquatic mammals, no other known mammals have gone through such an evolutionary change as to become completely separated from land (Uhen 2010). This may have been a slow evolutionary process, with cetaceans becoming secondarily aquatic over fifty million years ago during the Eocene Epoch (Uhen 2010). They have developed systems for aquatic breathing, movement, reproduction, eating and speech (Milinkovitch and Lambert 2006). Most stem cetacean differences are found in the head, with changes to the body and limbs appearing more slowly during cetacean evolutionary history (Uhen 2010).
Origin of Cetaceans
If whales are descendend from Artiodactyls, they are likely related to Hippopotamidae which are also artiodactyla (Thewissen et al. 2007). Artiodactyla is the even-toed order of mammals including hippopotami and cows, and they are the proposed sister taxa to Cetacea (Uhen 2010). A connection between artiodactyls and modern whales suggested by William H. Flower in 1883 saw similarities in the soft tissues of modern Cetacea and Artiodactyls and he proposed Artiodactyla as possible ancestors of cetaceans (Uhen 2010). For a long time this did not add to the debate over ceataceans ancestry and was thought to be incorrect,,until in 1950 an immune reactivity test proved Artiodactyla to be related to Cetacea (Uhen 2010). Then in 1990 studies based on DNA sequences seemed to show a strong relationship between Cetacea and Artiodactyla (Uhen 2010). Further tests showed cetaceans as significantly related to Hippopotamidae and that they could be placed within Artiodactyla (Thewissen et al. 2007). A new clade called Whippomorpha was made, and it contained Hippopotamidae and Cetacea. together (Thewissen et al. 2007).
Puzzle pieces for cetacean origins continue to be pondered. Characterizing different cetaceans and determining their ancestors had been dependent on discovering fossils and skeletons to support theories, and it has not been easy to find enough usable fossilized material from enough time periods. Working with what they do have, some scientists have played cetacean families into corresponding time periods.
Cetacean ancestor fossils from fifty-two million years ago show land animals which were able to eat and walk underneath water (Uhen 2010). In between the oldest and modern cetaceans are the Archeocetes (Uhen 2010). Archeocetes show many different characteristics as compared to modern whales such as teeth which were unlike those of odontocetes because they were not identical in shape or size but were each different, just as land mammals have (Milinkovitch and Lambert 2006). Archaeocetes lacked modern type blowholes and instead had nostrils at the end of the nose like land mammals (Waggoner 2005) They had differentiated teeth in functional groups, and two types of teeth-one deciduous and one permanent, these functions of teeth are not seen in crown Cetacea, who have one set of teeth except mysticete loose their teeth when baleen develops in the gums (Uhen 2010). Archaeocete skulls were different shapes from those of Mysticeti or Odonoceti, seen with early arachaeocete possessing snouts which modern cetaceans do not (Uhen 2010).
The oldest related fossils are from Himalayacetus the only cetacean the family Ambulocetidae to come before all other archeocetes, originating fifty-two million years ago from the early Eocene period (Milinkovitch and Lambert 2006). [Figure 3] The fossils have limb bones which indicate that these mammals were semi-aquatic, able to move around on land as well as water (Thewissen et al. 2007). The next family seems to be the Pakicetidae which is an arechaeocete from the middle of the Eocene period (Uhen 2010). These pakicetids had bodies that let them spend time walking on the bottom and swimming while drinking freshwater and eating freshwater prey (Uhen 2010).
Figure 3: Cetacean Phylogeny Schematic
Next in the progression would be the Ambulocetidae family (Uhen 2010). Their skeletons are larger than those of Pakicetids a trait showing them becoming more like modern cetaceans (Uhen 2010). Some of them drank freshwater but had a marine diet, another sign of evolution toward modern cetaceans (Uhen 2010). The next family, Remingtonocetidae, had short limbs and long skulls like modern Cetacea, and they stayed near the shore to eat a marine diet (Uhen 2010). Then the Protocetidae,came about, they walked on land but fed in and traveled great distances by water showing the growing dependence of marine environments (Uhen 2010).
Uhen (2010) gave name to a new clade, Pelagiceti, which includes Basilosauridae, the next emerging family, and Neoceti. [Figure 4.] Basilosauridae and Neoceti are both fully aquatic. and have shared sinuses around the ears enabling deep diving (Uhen 2010).
Figure 4: Age of oldest cetacean fossil, along with a measure of how much the oldest fossil might underestimate the actual origin of whales over historical time.
There is a lack of fossils from the time of transition between the Eocene and Oligocene ages when a large drop in sea level happened on Earth (Uhen 2010). So the ancestors of modern crown group cetaceans are not positively known. Odonoceti and Mysticeti, are currently believed to have one common ancestor, the basilosaurid genera Chrysocetus, as the feature shared by them is the single set of teeth called monophyodonty (Uhen 2010).
The Mysticeti family may have started with the genera Llanocetus in the Eocene period (Uhen 2010). Llanocetus was a filter feeder and is of a size between basilosaurids and the later mysticetes (Uhen 2010). Later Llanocetus fossils from the Oligocene period show more teeth than its ancestors, with progressive movement, seen in fossils over time, to the hundreds of baleen seen in Mysticeti today (Uhen 2010). The later Mysticeti are the Mammalodontidae, with teeth that were worn flat similar the baleen seen today (Uhen 2010).
The next family of Mysticeti to occur is the Aetiocetidae from the late Oligocene, who show the beginning development of features for filter feeding which is how they ate (Uhen 2010). The earliest specimen of baleen does not show up until the fossils from the Miocene Epoch (Uhen 2010). The first toothless mysticetes are from the family Eomysiticetidae from the late Oligocene of North America (Uhen 2010). These animals were thought to have baleen present used for filter feeding but the gum line is not well kept so it is not proven (Uhen 2010).
The first members of the other suborder of Neoceti, Odontoceti, are from the late Oligocene of North America but no found odonocete fossils have been catergorized by family so they cannot be specifically named (Uhen 2010). Primitive odonocetes had possessed structures used for both productions of high-frequency sounds and to perceive the echoes of those sounds, meaning they used echolocation as do present odontocetes (Uhen 2010). Odonocete ancestors ate nocturnal cephalopods,known as squid, that travel near the surface at night making the use of echolocation a selective advantage when hunting these squid in the dark, eventually echolocation become so advanced that it could be used at great depths for hunting where there is no light and numerous cephalopods live (Lindberg and Pyenson 2007).
The progression from terrestrial to aquatic life styles can be seen through the different families changes described above such as in there living habits (Milinkovitch and Lambert 2006). Cetaceans were first land mammals who waded in shallow water to feed on aquatic creatures and wait for land animals to come for a drink and then strike as crocodiles do (Uhen 2010). Then, becoming more aquatic, they began hunting and feeding in marine environments but drinking and living in freshwater (Uhen 2010). Developing into even less of terrestrial animals cetaceans then only go on land to give birth and mate (Uhen 2010). And then become fully aquatic never returning to land. Echolocation and deep diving without being affected by decompression syndrome were evolutionary changes helped make cetaceans into the mammals they are today.
Conclusion
Working with fossils as they became available, scientists puzzle back through the mysteries presented. From what cetaceans are today one would never guess that they were once terrestrial mammals. But with the information gathered from fossil ancestors it is possible to see hints of progression from land mammals to fully aquatic cetaceans that lead to the intricate modern cetaceans of today. As more fossils and skeletons are studied, more detail will emerge to help with understanding the Cetacea order and the importance of its background.
