The missing link in cetacean evolution? John Girard




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PAKICETUS –

THE MISSING LINK IN CETACEAN EVOLUTION?





John Girard

Fall Semester 2006

Evolution


Illustration by Carl Buell, and taken from http://www.neoucom.edu/Depts/Anat/Pakicetid.html.

Modern whales from the order Cetacea are marine mammals that are believed to have evolved from Artiodactyls approximately 45 to 49 million years ago (Thewissen et al. 2001; Graur and Higgins 1994) with paleontological, anatomical, and molecular evidence supporting this conclusion (Gingerich et al. 1990). Complex anatomical and physiological changes must have been coordinated to accommodate the transition from land to water. The morphology of intermediate cetaceans from the genus Pakicetus will be examined to determine how terrestrial artiodactyls transitioned into aquatic mammals. A comparison of the anatomical structures of the ear, teeth, mandible, cranium, ankle, and pelvis reveal early cetaceans were likely amphibious mammals living in fresh water coastal regions (Gingerich et al. 1983). Osmoregulation can be predicted due to oxygen isotopes found in the enamel of Pakicetus teeth and habitat can be assumed due to these isotopes as well as the organisms found within the same strata. The swimming mechanics of modern cetaceans will be compared to modern otters and Eocene cetaceans to demonstrate the locomotor transition from land to water. Lastly, the phylogenetic derivation of cetaceans will be discussed as several clades within Artiodactyla warrant support.

Classification of Pakicetus comes from the fossil record and comparative anatomy. Pakicetidae is the oldest of six families of ancient cetaceans collectively known as archaeocetes. Ambulocetidae, Remingtonocetidae, Protocetidae, Dorudontidae, and Basilosauridae are the remaining families of archaeocetes. Thewissen et al. (2001) report Pakicetidae lack aquatic characteristics common to the other five families such as: “presence of short neck vertebrae; thoracic and lumbar vertebrae that are similar in length; unfused sacral vertebrae; lack of a sacro-iliac joint; presence of a short tail with a ball vertebra; broad fan-shaped scapula with anterior acromion and small supraspinous fossa; an ulna with a large and transversely flat olecranon; a wrist and distal forearm flattened in the plane of the hand; and tiny hind limbs.” There are three genera in the Pakicetidae, Pakicetus, Nalacetus, and Ichthyolestes; with all fossils being found in the lower Kuldana Formation of Pakistan and the upper Subathu Formation in India (Thewissen and Bajpai 2001, Bajpai and Gingerich 1998). Pakicetus and Nalacetus species were approximately the size of a wolf while Ichthyolestes were 29% smaller, almost as big a fox (Thewissen et al. 2001).

Ear morphology in Eocene cetaceans resembled an intermediate stage between terrestrial quadrapedal mammals and modern cetaceans. The semicircular canal arcs were found to be reduced in size compared to land mammals, yet larger than most modern whales (Spoor et al. 2002). This reduction in semicircular canal arc size was hypothesized to correlate with its diminished function in aquatic mammals as the semicircular canals are responsible for “sensing” angular motion, which would limit agility while swimming as the head must be stabilized (Spoor et al. 2002). This reduction of semicircular canal arc size was considered a vestigial condition, as mechanoreceptors were lost due to limited neck and eye movement of cetaceans while swimming. Spoor et al. (2002) found Pakicetus semicircular canals were closest in canal arc size to modern artiodactyls. Cochlear size in Eocene cetaceans is nearly the same as some modern mammals which suggested to Spoor et al. that only the semicircular canals became reduced and vestigial, not the entire inner ear. Gingerich et al. (1983) reported finding a prominent fossa that would house the tensor tympani muscle in Pakicetus. This is significant because the tensor tympani muscle keeps the tympanic membrane taut in all land mammals. This allows sound waves traveling through the air to vibrate the tympanic membrane which in turn causes a vibration in the ossicles of the inner ear, which strike the oval window of the cochlea transferring the otic wave through the cochlea, thereby creating action potentials interpreted by the brain as sound. The presence of the tensor tympani fossa suggests that Pakicetus had a functional eardrum, which implies that underwater directional hearing was not plausible as they had anatomical structures homologous to other land mammals (Gingerich et al. 1983). Thewissen and Hussain claim the hearing mechanism of Pakicetus is truly an intermediate between land mammals and cetaceans as the inner ear retains land mammal characteristics described by Spoor et al. (2002) and Gingerich et al. (1993), as well as an inflated incus, which is well represented in modern cetaceans. The inflated incus has a greater mass and more inertia, which allows for easier movement through the fluid filled inner ear. The ventral position of the joint between the malleus and incus is another intermediate characteristic as most terrestrial mammals have the same joint facing ventral-medially and modern cetaceans have the malleus-incus joint facing ventral-laterally (Thewissen and Hussain 1993). Thewissen and Hussain (1993) also report the incus proportions of Pakicetus are most consistent with those from modern day Artiodactyls, not modern day cetaceans.

Molar tooth structure in ancient terrestrial mammals consisted of three elevated areas or cusps, arranged in a triangle with deep depressions in between the cusps, while modern cetacean molars lack any depression between cusps (Thewissen and Bajpai 2001). Pakicetus had molars with shallow depressions between the three cusps demonstrating an intermediate character. Thewissen et al. (2001) reported deep nearly vertical gouges in Pakicetus molars and suggested these gouges are a result of a diet consisting primarily of fish. Molars from other mammal species were shown to have similar gouges due to a diet of fish.

The mandible of Pakicetus was more like land mammals due to a small mandibular foramen. Modern cetaceans have an enlarged mandibular foramen which houses a fat pad used in sound wave reception and conduction (Thewissen and Bajpai 2001). This fat pad extends to the inner ear in modern whales and is more sensitive to sound waves than the external ear. Terrestrial mammals and the Pakicetidae have a small mandibular foramen with no fat pad, giving the mandible no function in sound wave reception or conduction (Thewissen and Bajpai 2001). Thewissen and Bajpai (2001) report an increase in size of the mandibular foramen in Ambulocetus and further increase in remingtonocetids and protocetids. The mandibular foramen of basilosaurids and dorudontids extends through the entire mandible as it does in modern whales.

The nasal opening of Pakicetus was found on the distal end of the snout which is similar in terrestrial mammals (Thewissen et al. 2001). Other archaeocetes demonstrate a progressive dorso-caudal movement of the nasal opening toward the modern day cetacean blowhole position (Thewissen et al. 2001). Fossil odontocetes and mysticetes have blowholes that are more caudal than the archaeocetes, but not as caudal as modern cetaceans (Thewissen and Bajpai 2001).

The eye socket position in Pakicetus was found to be on the dorsal midline allowing for binocular vision. Thewissen et al. (2001) suggest this placement is similar to crocodiles which would allow Pakicetus to see prey out of the water while they themselves are submerged. No other cetacean has this structural arrangement of ocular placement, as the orbits of all other archaeocetes and modern cetaceans exhibit a lateral facing ocular placement. Because this arrangement is common in amphibious organisms, Pakicetus is believed to have been amphibious in nature. Lacrimal foramen are present in the Pakicetidae and modern terrestrial mammals, but are lacking in all modern aquatic mammals such as whales and pinnipeds (Thewissen et al. 2001).

The tarsal morphology of Pakicetus was described by Thewissen and Madar (1999) as well as Gingerich et al. (2001) and compared to artiodactyls. They found that both exhibited dorso-plantar mobility and had mediolateral restricted movement due to similar astragalus morphology. Thewissen and Madar (1999) described, “The sustentacular facet of pakicetid cetaceans (H-GSP 97227) is long and narrow, limited to the medial third of the astragalus, but is proximodistally convex, as in artiodactyls.” The expanded sustentacular facet on the astragalus is the only structural character that directly links archaeocetes to artiodactyls and excludes Mesonychids phylogenetically (Thewissen and Madar 1999). Mesonychids are an extinct order of even toed carnivorous ungulates which looked like wolves. These animals possesed unusual triangular teeth that are similar to those of cetaceans and have been phylogenically linked due to the tooth morphology.

The pectoral girdles of modern cetaceans have mobile shoulders and immobile elbows wrists and phalanges. Modern whales are different from all other mammals as the last bone in each finger lacks a dorsal keratin cover which is homologous to nails, hooves, or claws (Thewissen and Bajpai 2001). Ambulocetus has been found by Thewissen and Bajpai (2001) to have movable elbows, wrists, and phalanges. The phalanges of Ambulocetus were not embedded in a flipper and had dorsal keratin covers in the shape of hooves. Believing Pakicetus to be older than Ambulocetus based on their positions in the strata, it can be assumed that Pakicetus had similar characteristics. The forelimbs of remingtonocetids and protocetids have not been discovered, while dorudontids and basilosaurids have forelimbs which resemble modern cetaceans with movable phalanges similar to the ambulocetids (Thewissen and Bajpai 2001). The Pakicetus humerus was found by Thewissen et al. (2001) to have been long and thin but lacking a deltopectoral crest which is common in running land mammals.

The pelvic morphology of Pakicetus is remarkably different than modern whales. Pakicetus had a sacrum consisting of four fused vertebrae with a strong sacro-iliac joint (Thewissen et al. 2001). Terrestrial mammals also have a fused sacrum which attaches to the ilial, ischial, and pubic bones at the sacro-iliac joint to form a rigid pelvic girdle for support during locomotion (Thewissen and Bajpai 2001). The sacrum is attached to the pelvis bearing two acetabulum, or sockets, for attachment of the femurs. Modern cetaceans lack a recognizable sacrum and the pelvis has been reduced to a small bar of bone lacking an acetabulum and discernable ilium, ischium, and pubis (Thewissen and Bajpai 2001). The femurs of Pakicetus were reported by Thewissen and Bajpai (2001) to be longer than modern cetaceans, yet shorter than most land mammals, suggesting that Pakicetus is an early intermediate in cetacean evolution. Femur length decreases and pelvis reduction with discernable acetabulum is evident in younger archaeocetes, a trend leading to the modern whale pelvis morphology. Thewissen and Bajpai (2001) report, “The pelvis forms an excellent transitional series, in which pakicetids, ambulocetids, and remingtonocetids retain all elements of land mammals; protocetids lose the fused sacrum and the iliosacral joints and have short femurs. Basilosaurids and dorudontids have greatly reduced hind limbs and reduced ilia, while still retaining the acetabulum and modern whales have vestiges of those structures.” Gingerich et al. (1990) suggest the reduction of the hind limbs may have changed their function from one of locomotion, to guides for copulation.

Locomotion of Pakicetus has been experimentally determined by Thewissen and Fish (1997) comparing modern aquatic mammal swimming patterns and anatomical structures to format a locomotion model. Pakicetus was found to have anatomical structures consistent with both terrestrial and aquatic locomotion. Basilosaurus species were the first cetaceans to swim with a dorsoventral oscillation using its caudal fluke. Protocetids were reported by Gingerich et al. (1994) to swim in the same manner. Modern cetaceans swim using the same mechanics as basilosaurids and protocetids, but what of the older archaeocetes? Thewissen et al. (1994) examined the limb skeleton of Ambulocetus natans and concluded it to be morphologically different than other younger cetaceans. The skeletal framework of Ambulocetus natans suggested the power stroke during swimming came from the feet, not the caudal tail although the propulsive strokes probably occurred in the dorsoventral plane. Fish (1996) proposed a model for the evolution of the three most derived modes of swimming in mammals. His model described the lateral pelvic oscillation in phocid seals, pectoral oscillations in otariid seals, and dorsoventral oscillation in cetaceans. Fish (1996) described how cetacean swimming evolved, beginning with quadrapedal paddling, followed by alternate pelvic paddling, followed by simultaneous pelvic paddling, which was fine tuned into dorsoventral undulation, which evolved into the very efficient dorsoventral caudal oscillation. The quadrapedal swimming mode is common of many terrestrial mammals and mimics the dog paddle. Thewissen and Fish (1997) compare quadrapedal paddling to a modified terrestrial gait. Pelvic paddling resulted in an effort to reduce frictional drag due to the forelimbs and to increase aerobic efficiency. Quadrapedal and pelvic paddling occurs when a mammal swims on the water surface, whereas dorsoventral undulation and oscillation occur when a mammal swims under the waters surface. Thewissen and Fish (1997) proposed that Pakicetus were quadrapedal swimmers as they have chiefly terrestrial mammal skeletal morphology. Thewissen and Fish (1997) compared the metatarsal length of Pakicetus to Ambulocetus and interpreted the elongation of the metatarsals in Ambulocetus as an adaptation for foot propulsion. They also found the caudal vertebrae morphology to be larger as well as longer in Ambulocetus which would support a pelvic paddling and caudal undulation similar to river otters.



Another independent line of evidence, which reinforces the paleontological data which claims that Pakicetus is a transitory cetacean, indicates habitat changed during early whale evolution. Excess salt is detrimental to mammals, thus the mammalian kidney dissolves excess salt in water and excretes the solution as urine. The ability to maintain the homeostasis of salt in mammals is called osmoregulation. Modern cetaceans have adapted to high salt ion concentrations in seawater with minimal freshwater intake (Thewissen and Bajpai 2001). This derived characteristic is believed to have evolved in the archaeocetes. Water drinking behaviors were evaluated using Oxygen-16 and oxygen-18 isotopes from the calcium phosphate found in fossilized bones and teeth. Thewissen and Bajpai (2001) state that the oxygen used in calcium phosphate was derived from drinking water. These isotopes were compared to determine the original habitats of the archaeocetes. Oxygen-18 is heavier than oxygen-16 and doesn’t evaporate as readily, thus freshwater has a high concentration of 16O and saltwater has a high concentration of 18O.

The relative percentage of 18O is significantly higher in the teeth and bones of marine cetaceans than in those of freshwater cetaceans. The oxygen isotope concentration in mammal bone was due to the incorporation of the particular ratio of oxygen isotopes in fresh vs. salt water ingested. Thewissen et al. (1996) reported that biogenic phosphate found in tooth enamel retains the original oxygen isotope ratio present during enamel formation. Thewissen et al. (1996) tested oxygen isotope concentrations in extant marine and freshwater cetacean species to support their hypothesis that ingested water contains oxygen isotopes which are utilized in enamel and bone formation. They found that marine cetaceans had high concentrations of 18O in the enamel and bone matrix while freshwater whales had a high concentration of 16O, which indicates that oxygen isotope present in bone matrix and tooth enamel reflects water ingested. The earliest archaeocetes (e.g. Pakicetus and other Pakicetids) have lower 18O ratios, associated with a freshwater habitat, while remingtonocetids, protocetids, and dorudontids all have higher ratios, indicating a fully marine habitat (Bajpai and Gingerich 1998). Ambulocetus have a wide range of 18O levels, suggesting they lived in a wide range of salinities, as one might expect for a clearly transitional form (Thewissen et al. 1996). The oxygen isotope ratio evidence suggests that cetacean osmoregulation evolved rapidly from the obligatory fresh water Pakicetus to an estuary tolerant Ambulocetus and finally the fully marine remingtonocetids, protocetids, and dorudontids. Other evidence supporting the habitat placement of the archaeocetes is the strata in which the fossils were unearthed. The Pakicetus fossils have only been found in fluvial red sediments amongst opossum, rodent, fish, snail, turtle, and crocodilian fossils which suggested to Gingerich et al. (1983) that Pakicetus spent a considerable amount of time on land as well as in fresh water. Gingerich et al. (1983) hypothesized that Pakicetus was picivorous and fed from oceanic upwellings in epicontinental waters. Thewissen et al. (1996) describe finding Pakicetus and Nalacetus fossils in only deposits associated with shallow freshwater environments. They also discuss finding Ambulocetus fossils only in intertidal zone beds and Indocetus fossils only in neritic beds. The high concentration of 16O in Ambulocetus fossils perplexed Thewissen et al. (1996) as all Ambulocetus fossils have been found in “Unambiguous marine beds high in the Kuldana Formation.” Thewissen et al. (1996) proposed two possible hypotheses for this enigma; Ambulocetus had not fully developed osmoregulation to handle the salt concentrations in marine water and had to seek out fresh water to ingest; or Ambulocetus lived in freshwater during the early stages of its life when mineralization of the tooth enamel would take place. Indocetus was found to have a high concentration of 18O in the bone matrix and tooth enamel. The evidence suggests that Indocetus had a fully marine existence given the neritic bed locations of Indocetus fossils and the concentration of 18O in the bone and enamel (Thewissen et al. 1996).

Phylogenetic placement of Cetacea was debated between morphological structures linking whales to the now extinct Mesonychidae and molecular evidence placing Cetacea within Artiodactyla. The Artiodactyla placement is commonly accepted in the literature, but debate over which clade Cetaceans belong yet again pits morphological evidence versus molecular evidence, with neither side of experts backing from their stance. The Order Artiodactyla is historically divided into three suborders: Suiformes which includes pigs, peccaries, and hippopotamuses; Tylopoda which includes camels and llamas; and Ruminantia which includes deer, cows, giraffes, goats, and sheep (Graur and Higgins 1994). Graur and Higgins (1994) constructed phylogenetic trees based on protein and DNA sequence comparison of pig, cow, camel, and several cetacean species and found the sequences of cow DNA were most similar to those of extant cetaceans. Using DNA evolution rate calculations Graur and Higgins (1994) estimated the divergence between cetaceans and ruminant artiodactyls occurred ~45-49 million years ago based on divergence time of pig and cow as a reference and assuming that DNA sequences evolve at the same rate in all lineages.

Ursing and Arnason (1998) sequenced the complete mitochondrial DNA of the cyctochrome b gene Hippopotamus amphibius and used it to compare the genomes of cytochrome b in fifteen other placental mammals, including cow, pig, sheep, mouse, harbor seal, grey seal, cat, horse, donkey, white rhinoceros, Indian rhinoceros, armadillo, human, fin whale and blue whale. They found that Suiformes and the hippopotamus were paraphyletic; and that the hippopotamus and cetaceans were monophyletic based on the sequence of the cytochrome b mammalian gene. Ursing and Arnason (1998) claimed that the hippopotamus-whale clade demonstrated an accelerated evolution rate compared to ruminants based on amino acid sequence changes. The calculated evolution rate and the cytochrome b sequence supported the sister group monophyletic placement of the Hippopotimidae with Cetacea.

SINE’s, or Short Interspersed Nuclear Elements and LINE’s, or Long Intersperced Nuclear Elements were described by Nikaido et al. (1999) as “Mobile genetic elements that have been amplified and integrated into a host genome by retroposition.” which is the integration of a reverse transcribed copy of RNA. The significance of SINE’s and LINE’s is that the integration of either at a given locus is considered unique and an irreversible mutation as it is easier to mutate a sequence than it is to fix that specific sequence. Nikaido et al. (1999) stated, “Because the probability that a SINE/LINE will be lost once it has been inserted into the genome is extremely small, and the probability that the same SINE/LINE will be inserted independently into an identical region of the genomes of two different taxa is also very small, the probability that homoplasy will obscure phylogenic relationships is, for all practical purposes, zero.” This makes SINE’s and LINE’s an excellent tool to construct phylogenetic trees. Nikaido et al. (1999) found the CHR-1 SINE of Odontoceti, Mysticeti, and hippopotamuses to be monophyletic to the exclusion of ruminants as ruminants lacked the chromosome 1 SINE. Short interspersed nuclear elements and long interspersed nuclear elements clarified the Artiodactyl phylogeny to Nikaido et al. (1999) by placing Cetacea in a sister-group with Hippopotamidae deep within the artiodactyls. Other evidence of this monophyletic placement given by Nikaido et al. (1999) included morphological similarities of the aquatic mammals including a lack of hair and oil glands, as well as the behavioral similarity of underwater vocalizations used to communicate. These findings suggested to the authors that cetaceans evolved from immediate artiodactyls, not mesonychians.

The usage of SINE’s in phylogeny, which bolsters the molecular view that cetacean evolved from artiodactyls, is discussed by Shedlock et al. (2000). They point out that SINE’s are found dispersed throughout the entire genome and that there is no known process which removes SINE’s. The evidence suggests to Shedlock et al. (2000) that copies of SINE’s insert into the DNA one time and stay put with random mutation possibly changing them over millions of years. The authors find that all SINE’s to date indicate that they arise in, and are limited to particular phylogenetic groups. Shedlock et al. (2000) state, “In the absence of incomplete lineage sorting, copies of the same fixed SINE at a given locus in different host taxa define a clade, and the absence of SINE insertion at that exact locus is an ancestral condition that defines an outgroup.” They discuss that there is no known mechanism which would insert SINE’s at the exact same loci in different taxa, nor remove SINE’s from those same loci. Shedlock et al. (2000) also point out the absence of a SINE at a given locus does not mean that the absence of that SINE was detected or measured at that locus, merely that a SINE was not present. Shedlock et al. (2000) describe the probability of four independent SINE’s inserting into the exact same loci in the hippopotamus-whale clade as 1 in 1036.

Support for the usage of SINE’s in cladistics was given by Lum et al. (2000) as they describe the independent insertion of SINE’s and their irreversible nature which allow them to be used as shared derived characters in cladogram construction. They also describe how the SINE flanking sequences could potentially be used to date historical SINE insertion. Evolution would not play a role in changing SINE flanking sequences as they occur in introns and are therefore not expressed. Lum et al. (2000) state, “The amount of divergence between nonfunctional SINE flanking sequences at orthologous loci is proportional to elapsed time.” The SINE flanking sequence lengths could potentially be interpreted as time as longer similar sequences would be found in families which are closely related. The DNA sequences flanking seven SINE insertions were analyzed and Lum et al. (2000) found that the M11 locus and the total DNA sequence suggests a (Tylopoda, Suidae, (Rumantia, (Hippopotamidae, Cetacea))) phylogeny with a 98% bootstrap statistical support for a hippopotamus-whale clade to the exclusion of the other artiodactyls. The radiation of Cetartiodactyla is estimated to be ~65 million years ago which is supported by the fossil record, while the divergence of hippopotamuses and whales from ruminants is estimated to by ~60 million years ago. This means that Tylopoda and Suidae taxa diverged rapidly and early from the other artiodactyls setting up a paraphylogenetic tree. The ~52 million year old Pakicetus fossils suggest to Lum et al. (2000) that the ancestor of hippopotamuses and whale lived between 52-60 million years ago.

Further molecular comparison studies were done by Gatesy et al. (1999) as they added the DNA sequences for IRBP, or Interphotoreceptor Retinoid Binding Protein, which is found on exon 1, and vWF, or von Willebrand Factor which is a blood clotting protein sequenced on exon 28. The authors did this study to see the effect that adding new DNA sequences had on the cladistic stability of Artiodactyla. Gatesy et al. (1999) described cladistic stability as “The resistance to change with the addition of new data.” They found that the addition of IRBP and vWF DNA sequences to the data sets from previous molecular studies did not disrupt any of the previously examined relationships within cladistic models as of 1999. The DNA sequences of more than 1,400 characters collected from 17 data sets suggest three possible stable clades; (Cetacea + Hippopotamidae), (Cetacea + Hippopotamidae + Ruminantia), and (Cetacea + Hippopotamidae + Ruminantia + Suina). Nodal data set influence, or NDI, was calculated by Gatsey et al. (1999) and the statistical tests favored a (((((Cetacea + Hippopotamidae) Rumantia) Suina) Camelidae) outgroup) phylogeny and strongly contradicted that Artiodactyla and Suina are monophyletic. The authors also discuss Graur and Higgins 1994 phylogenetic findings and attribute the whale-cow clade to an incomplete data set, as Hippopotamidae were not included in their study. The addition of the hippopotamus DNA sequence data by Ursing and Arnason in 1998 and Nikaido et al. in 1999 clarified the phylogenetic relationships of Artiodactyls on a molecular level, and with the addition of IRBP and vWF Gatesy et al. (1999) find the cladistic relationships to be stable.

The known morphology and molecular evidence were synthesized and examined by O’Leary (2001) to corroborate the phylogeny of Artiodactyla. Criticisms of the previous phylogenies were that none had included ancient cetacean DNA for systematic construction, soft tissue characters have not been examined, and behavioral data were inconclusive or missing. The author’s aim was to correlate the morphological and molecular evidence to form a phylogeny and explained the inferences made based on the gathered evidence. O’Leary (2001) explained that although Pakicetus fossils lack a preserved sacro-iliac articulation it can be inferred that Pakicetus indeed had a sacro-iliac joint based on other younger archaeocetes with preserved sacro-iliac joints. Swimming style of Pakicetus was inferred from morphological design and its hypothesized aquatic environment due to the location of excavated fossils in intertidal rock deposits. O’Leary (2001) arrived at the same phylogenetic conclusion as previous authors, but warned that until all evidence is synthesized, it may be premature to describe the phylogeny as truly supported. O’Leary and Geisler (1999) also estimate that 89% of the artiodactyl ingroup consists of extinct clades, none of which has been DNA sequence tested. They did conclude, based on 123 anatomical characters from 10 extant and 30 extinct taxa, that cetaceans are more closely related to artiodactyls than perissodactyls. They also excluded mesonychians as they only share one derived character, tooth morphology with cetaceans. O’Leary and Geisler (1999) propose that mesonychians are a paraphyletic extinct group from artiodactyls.

In conclusion the anatomical structures associated with Pakicetid fossils demonstrate a derived state which would allow Pakicetus to live on terrestrial and within aquatic environments. Sensory structures such as the eye placement and the ear morphology as well as the migration of the blowhole placement all strongly suggest that Pakicetus is the most common semi-terrestrial ancestor to all cetacean species. Further molecular evidence supports the origination of cetaceans from the Order Artiodactyla, not Mesonychids which was proposed by scientists based on the unique anatomical structure of their teeth. Short interspersed nuclear element and long interspersed nuclear element insertions provide geneticists with a powerful cladistic tool to create phylogenetic trees. The molecular evidence provides several possible artiodactyl clades each including cetacea, but the family to which cetacea belongs is still under debate. A paraphyletic relationship can be supported with whales, hippopotamuses, and ruminants forming a sister group to the exclusion of camels and pigs. A monophyletic relationship also can be supported with the Cetacean-Hippopotamidae sister group deeply placed in Artiodactyla based on several derived anatomical characteristics and molecular data.

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