The complete nucleotide sequence of the mitochondrial DNA of the fin whale,Balaenoptera physalus

Summary The composition of the mitochondrial DNA (mtDNA) of the fin whale,Balaenoptera physalus, was determined. The length of the molecule is 16,398 bp, and its organization conforms with that of other mammals. The general similarity between the mtDNA of the fin whale and the cow is greater than the similarity between the fin whale and other species (human, mouse, rat) in which the composition of the entire molecule has been described. The D-loop region of the mtDNA of the fin whale is 81% identical to the D-loop of dolphin DNA, and the central portion of the D-loop is similar to the bovine D-loop. The accumulation of transversions and gaps in the 12S and 16S rRNA genes was assessed by comparing the fin whale, cow, and human. The sequence difference between human and the whale and human and the cow was at the same level, indicating that the rate of evolution of the mtDNA rRNA genes is about the same in artiodactyls and cetaceans. In the 12S rRNA gene an accumulation rate of 0.05% per million years places the separation of cetaceans and artiodactyls at about 55 million years ago. The corresponding figure for human and either the whale or the cow is about 80 million years. In the 16S rRNA gene a 0.08% accumulation rate of transversions and gaps per million years yields concurring figures. A comparison between the cytochromeb gene of the fin whale and cytochromeb sequences in the literature, including dolphin (Stenella) sequences, identified the cetaceans as monophyletic and the artiodactyls as their closest relatives. The comparison between the cytochromeb sequences of the fin whale andStenella showed that differences in codon positions one or two were frequently associated with a change in another codon position.     

Episodic Molecular Evolution of Pituitary Growth Hormone in Cetartiodactyla

The sequence of growth hormone (GH) is generally strongly conserved in mammals, but episodes of rapid change occurred during the evolution of primates and artiodactyls, when the rate of GH evolution apparently increased substantially. As a result the sequences of higher primate and ruminant GHs differ markedly from sequences of other mammalian GHs. In order to increase knowledge of GH evolution in Cetartiodactyla (Artiodactyla plus Cetacea) we have cloned and characterized GH genes from camel (Camelus dromedarius), hippopotamus (Hippopotamus amphibius), and giraffe (Giraffa camelopardalis), using genomic DNA and a polymerase chain reaction technique. As in other mammals, these GH genes comprise five exons and four introns. Two very similar GH gene sequences (encoding identical proteins) were found in each of hippopotamus and giraffe. The deduced sequence for the mature hippopotamus GH is identical to that of dolphin, in accord with current ideas of a close relationship between Cetacea and Hippopotamidae. The sequence of camel GH is identical to that reported previously for alpaca GH. The sequence of giraffe GH is very similar to that of other ruminants but differs from that of nonruminant cetartiodactyls at about 18 residues. The results demonstrate that the apparent burst of rapid evolution of GH occurred largely after the separation of the line leading to ruminants from other cetartiodactyls.     

Evolution of the cytochromeb gene of mammals

Summary With the polymerase chain reaction (PCR) and versatile primers that amplify the whole cytochromeb gene (∼ 1140 bp), we obtained 17 complete gene sequences representing three orders of hoofed mammals (ungulates) and dolphins (cetaceans). The fossil record of some ungulate lineages allowed estimation of the evolutionary rates for various components of the cytochromeb DNA and amino acid sequences. The relative rates of substitution at first, second, and third positions within codons are in the ratio 10 to 1 to at least 33. For deep divergences (>5 million years) it appears that both replacements and silent transversions in this mitochondrial gene can be used for phylogenetic inference. Phylogenetic findings include the association of (1) cetaceans, artiodactyls, and perissodactyls to the exclusion of elephants and humans, (2) pronghorn and fallow deer to the exclusion of bovids (i. e., cow, sheep, and goat), (3) sheep and goat to the exclusion of other pecorans (i. e., cow, giraffe, deer, and pronghorn), and (4) advanced ruminants to the exclusion of the chevrotain and other artiodactyls. Comparisons of these cytochromeb sequences support current structure-function models for this membrane-spanning protein. That part of the outer surface which includes the Q_o redox center is more constrained than the remainder of the molecule, namely, the transmembrane segments and the surface that protrudes into the mitochondrial matrix. Many of the amino acid replacements within the transmembrane segments are exchanges between hydrophobic residues (especially leucine, isoleucine, and valine). Replacement changes at first and second positions of codons approximate a negative binomial distribution, similar to other protein-coding sequences. At four-fold degenerate positions of codons, the nucleotide substitutions approximate a Poisson distribution, implying that the underlying mutational spectrum is random with respect to position.     

Structure and evolution of opossum,guinea pig,and porcupine cytochrome b genes

Summary We have sequenced the mitochondrial cytochrome b gene from the guinea pig, the African porcupine, and a South American opossum. A phylogenetic analysis, which includes 22 eutherian and four other vertebrate cytochrome b sequences, indicates that the guinea pig and the porcupine constitute a natural clade (Hystricomorpha) that is not a sister group to the clade of mice and rats (Myomorpha). Therefore, the hypothesis that the Rodentia is paraphyletic receives additional support. The artiodactyls, the perissodactyls, and the cetaceans form a group that is separated from the primates and the rodents. The 26 sequences are used to study the structure/function relationships in cytochrome b, whose function is electron transport. Most of the amino acid residues involved in the two reaction centers are well conserved in evolution. The four histidines that are believed to ligate the two hemes are invariant among the 26 sequences, but their nearby residues are not well conserved in evolution. The eight transmembrane domains represent some of the most divergent regions in the cytochrome b sequence. The rate of nonsynonymous substitution is considerably faster in the human and elephant lineages than in other eutherian lineages; the faster rate might be due to coevolution between cytochrome b and cytochrome c. Offprint requests to: W.-H. Li     

Evolutionary genetics of the suiformes as reconstructed using mtDNA sequencing

We have amplified and sequnced the entire mitochondrial DNA cytochromeb gene from four species of Suidae: babirusa, warthog, bearded pig, and some specimens belonging to different subspecies and populations of wild and domestic pigs (Sus scrofa). These sequences were aligned with additional mammalian sequences retrieved from the literature and were used to obtain phylogenetic trees of the Suiformes (Artiodactyla). Several species of Carnivora, Perissodactyla. Cetacea, and other Artiodactyla were used as outgroups. Molecular phylogenetic relationships among the Suiformes reflect their current taxonomy: Hippopotamidae, Tayassuidae, and Suidae are separated by deep genetic gaps, and the division of the Suidae into the subfamilies Babyrousinae., Phacochoerinae, and Suinae has strong genetic correlates. Cytochromeb sequences show differences among Asian and Western populations ofSus scrofa, agreeing with other genetic information (karyotypes blood groups, and protein variability). The two Italian subspecies of wild boar have unique mtDNA cytochromeb haplotypes. The evolutionary rates of cytochromeb sequences are different at transitions versus transversions as well as at first, second, and third positions of codons. Therefore, these classes of substitutions reached different levels of mutational saturation. Only transversions and the conservative first and second position substitutions are linearly related to genetic distances among the Suiformes. Therefore, divergence times were computed using unsaturated conserved nucleotide substitutions and calibrated using paleontological divergence times between some Artiodactyla. Transversions apparently evolve at remarkably regular rates in ungulate taxa which have accumulated less than 20% estimated sequence divergence, corresponding to about 40–45 million years of independent evolution. Molecular, information suggests that Hippopotamidae and Tayassuidae are not closely related (as stated by Pickford, 1986, 1989, 1993) and that the origin of babirusa and warthog (about 10–19 and 5–15 million years ago, respectively) is more recent than supported by current evolutionary reconstructions. The inferred origin of bearded pig is about 2.1 million years old, and genetic divergence among differentSus scrofa populations is probably a Pleistocene event. The addition of new sequences of Suiformes does not help in resolving the phylogenetic position ofHippopotamus amphibius, which shows weak but recurrent linkages with the cetacean evolutionary lineage.To whom correspondence should be addressed.     

The position of Cetacea within mammalia: phylogenetic analysis of morphological data from extinct and extant taxa

Knowledge of the phylogenetic position of the order Cetacea (whales, dolphins, and porpoises) within Mammalia is of central importance to evolutionary biologists studying the transformations of biological form and function that accompanied the shift from fully terrestrial to fully aquatic life in this clade. Phylogenies based on molecular data and those based on morphological data both place cetaceans among ungulates but are incongruent in other respects. Morphologists argue that cetaceans are most closely related to mesonychians, an extinct group of terrestrial ungulates. They have disagreed, however, as to whether Perissodactyla (odd-toed ungulates) or Artiodactyla (even-toed ungulates) is the extant clade most closely related to Cetacea, and have long maintained that each of these orders is monophyletic. The great majority of molecule-based phylogenies show, by contrast, not only that artiodactyls are the closest extant relatives of Cetacea, but also that Artiodactyla is paraphyletic unless cetaceans are nested within it, often as the sister group of hippopotamids. We tested morphological evidence for several hypotheses concerning the sister taxon relationships of Cetacea in a maximum parsimony analysis of 123 morphological characters from 10 extant and 30 extinct taxa. We advocate treating certain multistate characters as ordered because such a procedure incorporates information about hierarchical morphological transformation. In all most-parsimonious trees, whether multistate characters are ordered or unordered, Artiodactyla is the extant sister taxon of Cetacea. With certain multistate characters ordered, the extinct clade Mesonychia (Mesonychidae + Hapalodectidae) is the sister taxon of Cetacea, and Artiodactyla is monophyletic. When all fossils are removed from the analysis, Artiodactyla is paraphyletic with Cetacea nested inside, indicating that inclusion of mesonychians and other extinct stem taxa in a phylogenetic analysis of the ungulate clade is integral to the recovery of artiodactyl monophyly. Phylogenies derived from molecular data alone may risk recovering inconsistent branches because of an inability to sample extinct clades, which by a conservative estimate, amount to 89% of the ingroup. Addition of data from recently described astragali attributed to cetaceans does not overturn artiodactyl monophyly.     

Marine mammal chemoreception

The presence or absence of a chemoreceptive capacity in marine mammals has drawn relatively little attention from the research community outside the Soviet Union. Toothed whales are typically labelled anosmic (lacking a sense of smell) since they do not have the peripheral olfactory structures typically associated with terrestrial mammals. Baleen whales are known to possess reduced olfactory tracts; their olfactory bulbs also may be reduced or absent. Although the neural structures that mediate taste in terrestrial mammals have been reported to be present in both groups of whales, cetaceans have been considered to have a poor sense of taste because typical mammalian taste receptors have been thought to be absent. Soviet researchers, however, recently have reported that gustatory receptors are present on some cetacean tongues and that the tongue of Tursiops truncatus appear to be well innervated. These workers also have been conducting investigations which seem to be aimed at describing a specialized gustatory capability in cetaceans. No experimental work has been reported by Western scientists. Little work has been done by either Western or Soviet researchers with regard to chemoreception among the other orders of marine mammals (Pinnipedia, Carnivora and Sirenia). Pinnipedia are typically labelled microsmatic (having a poor sense of smell); research has been restricted to histological examination of the nasal pathways, and neural anatomy. Sea otters are credited with a keen sense of smell, but no quantitative work has been reported. The chemosensory abilities of Sirenia remain unknown. The tongues of non-cetacean marine mammals have been histologically examined and found to resemble those of terrestrial mammals. No other investigations of gustation in non-cetacean marine mammals have been reported.     

Analyses of mitochondrial genomes strongly support a hippopotamus-whale clade.

Although the sister-group relationship between Cetacea and Artiodactyla is widely accepted, the actual artiodactyl group which is closest to Cetacea has not been conclusively identified. In the present study, we have sequenced the complete mitochondrial genome of the hippopotamus, Hippopotamus amphibius, and included it in phylogenetic analyses together with 15 other placental mammals. These analyses separated the hippopotamus from the other suiform included, the pig, and identified the hippopotamus as the artiodactyl sister group of the cetaceans, thereby making both. Artiodactyla and the suborder. Suiformes paraphyletic. The divergence between the hippopotamid and cetacean lineages was calculated using this molecular data and was estimated at ca. 54 Ma BP.     

Subordinal artiodactyl relationships in the light of phylogenetic analysis of 12 mitochondrial protein-coding genes

Ursing, B. M., Slack, K. E. & Arnason, U. (2000) Subordinal artiodactyl relationships in the light of phylogenetic analysis of 12 mitochondrial protein-coding genes. — Zoologica Scripta , 29 , 83–88.
Extant artiodactyls (even-toed hoofed mammals) are traditionally divided into three main lineages: Suiformes (pigs, peccaries and hippopotamuses), Tylopoda (camels and llamas) and Ruminantia (bovids, deer, tragulids and giraffes). Recent molecular studies have not supported a close relationship between pigs and hippopotamuses, however, instead grouping hippopotamuses with Cetacea (whales, dolphins and porpoises). In this study we have sequenced the complete mitochondrial genome of a tylopod — the alpaca (Lama pacos), the only artiodactyl suborder not previously represented by a complete mitochondrial sequence. This sequence was included in phylogenetic analyses together with the complete mitochondrial protein-coding sequences of other artiodactyls plus two cetaceans. Despite the length of the data set, the relationship between Suina (Suiformes sine Hippopotamidae), Tylopoda and Ruminantia/Hippopotamidae/Cetacea could not be fully resolved, however, a basal position of the alpaca (Tylopoda) relative to the other artiodactyls/cetaceans was unsupported.     

Evolution of the MHC-DQB exon 2 in marine and terrestrial mammals

On the basis of a general low polymorphism, several studies suggest that balancing selection in the class II major histocompatibility complex (MHC) is weaker in marine mammals as compared with terrestrial mammals. We investigated such differential selection among Cetacea, Artiodactyla, and Primates at exon 2 of MHC-DQB gene by contrasting indicators of molecular evolution such as occurrence of transpecific polymorphisms, patterns of phylogenetic branch lengths by codon position, rates of nonsynonymous and synonymous substitutions as well as accumulation of variable sites on the sampling of alleles. These indicators were compared between the DQB and the mitochondrial cytochrome b gene (cytb) as a reference of neutral expectations and differences between molecular clocks resulting from life history and historical demography. All indicators showed that the influence of balancing selection on the DQB is more variable and overall weaker for cetaceans. In our sampling, ziphiids, the sperm whale, monodontids and the finless porpoise formed a group with lower DQB polymorphism, while mysticetes exhibited a higher DQB variation similar to that of terrestrial mammals as well as higher occurrence of transpecific polymorphisms. Different dolphins appeared in the two groups. Larger variation of selection on the cetacean DQB could be related to greater stochasticity in their historical demography and thus, to a greater complexity of the general ecology and disease processes of these animals.     

Sequence variation in the Tbx4 gene in marine mammals

The amino-acid sequences of the T-domain region of the Tbx4 gene, which is required for hindlimb development, are 100% identical in humans and mice. Cetaceans have lost most of their hindlimb structure, although hindlimb buds are present in very early cetacean embryos. To examine whether the Tbx4 gene has the same function in cetaceans as in other mammals, we analyzed Tbx4 sequences from cetaceans, dugong, artiodactyls and marine carnivores. A total of 39 primers were designed using human and dog Tbx4 nucleotide sequences. Exons 3, 4, 5, 6, 7, and 8 of the Tbx4 genes from cetaceans, artiodactyls, and marine carnivores were sequenced. Non-synonymous substitution sites were detected in the T-domain regions from some cetacean species, but were not detected in those from artiodactyls, the dugong, or the carnivores. The C-terminal regions contained a number of non-synonymous substitutions. Although some indels were present, they were in groups of three nucleotides and therefore did not cause frame shifts. The dN/dS values for the T-domain and C-terminal regions of the cetacean and artiodactylous Tbx4 genes were much lower than 1, indicating that the Tbx4 gene maintains it function in cetaceans, although full expression leading to hindlimb development is suppressed.     

Postcranial morphology and locomotion of the Eocene raoellid Indohyus (Artiodactyla: Mammalia)

Raoellids are small, raccoon-sized Eocene artiodactyls, closely related to archaic cetaceans (archaeocetes) that have poor representation of postcranial elements in the fossil record. Little is known of the aquatic and terrestrial locomotor affinities of raoellids due to the paucity of their fossil record, leaving a critical gap in our understanding of the earliest portion of the artiodactyl marine invasion. To address this gap, a comparative morphological analysis of the postcranial elements was undertaken based on newly recovered elements of the raoellid Indohyus, archaeocetes and extant artiodactyls. Greater than 200 postcranial elements of Indohyus were described, and some limb element cross-sections were visualised via paleohistological thin sections and CT scans. Results show that during terrestrial locomotion, Indohyus probably had a digitigrade posture and mediolaterally stabilised limbs that functioned mostly in flexion and extension within the parasagittal plane. Quantification of midshaft cross-sectional area for some elements of Indohyus showed an osteosclerotic cortex, a skeletal characteristic associated with aquatic behaviours. Indohyus may represent a critical intermediate in the evolution of the cetacean terrestrial-to-aquatic body plan, as it bears gracile postcranial element proportions similar to a terrestrial artiodactyl but also an incipient form of osteosclerosis compared to pakicetid archaeocetes.     

More DNA support for a Cetacea/Hippopotamidae clade: the blood-clotting protein gene gamma-fibrinogen

Recent phylogenetic analyses of DNA sequences suggest that cetaceans (whales) and hippopotamid artiodactyls (hippos) are extant sister taxa. Consequently, the shared aquatic specializations of these taxa may be synapomorphies. This molecular view is contradicted by paleontological data that overwhelmingly support a monophyletic Artiodactyla (even-toed ungulates) and a close relationship between Cetacea and extinct mesonychian ungulates. According to the fossil evidence, molecular, behavioral, and anatomical resemblances between hippos and whales are interpreted as convergences or primitive retentions. In this report, competing interpretations of whale origins are tested through phylogenetic analyses of the blood-clotting protein gene gamma- fibrinogen from cetaceans, artiodactyls, perissodactyls (odd-toed ungulates), and carnivores (cats, dogs, and kin). In combination with published DNA sequences, the gamma-fibrinogen data unambiguously support a hippo/whale clade and are inconsistent with the paleontological perspective. If the phylogeny favored by fossil evidence is accepted, the convergence at the DNA level between Cetacea and Hippopotamidae is remarkable in its distribution across three genetic loci: gamma-fibrinogen, the linked milk casein genes, and mitochondrial cytochrome b.      

Comparative molecular cytogenetics in cetartiodactyla

Cetartiodactyla comprises Artiodactyla (even-toed ungulates) and Cetacea (whales, dolphins and porpoises). Artiodactyla is a large taxon represented by about 200 living species ranked in 10 families. Cetacea are classified into 13 families with almost 80 species. Many publications concerning karyotypic relationships in Cetartiodactyla have been published in previous decades. Formerly, the karyotypes of closely related species were compared by chromosome banding. Introduction of molecular cytogenetic methods facilitated comparative mapping between species with highly rearranged karyotypes and distantly related species. Such information is a prerequisite for the understanding of karyotypic phylogeny and the reconstruction of the karyotypes of common ancestors. This study summarizes the data on chromosome evolution in Cetartiodactyla, mainly derived from molecular cytogenetic studies. Traditionally, phylogenetic relationships of most groups have been estimated using morphological data. However, the results of some molecular studies of mammalian phylogeny are discordant with traditional conceptions of phylogeny. Cetartiodactyls provide several examples of incongruence between traditional morphological and molecular data. Such cases of conflict include the relationships of the major clades of artiodactyls, the relationships among the extant families of the suborder Ruminantia or the phylogeny of the family Bovidae. The most unexpected aspect of the molecular phylogeny was the recognition that Cetacea is a deeply nested member of Artiodactyla. The largest living order of terrestrial hoofed mammals is the even-toed hoofed mammals, or Artiodactyla. The artiodactyls are composed of over 190 living species including pigs, peccaries, hippos, camels, llamas, deer, pronghorns, giraffes, sheep, goats, cattle and antelopes. Cetacea is an order of wholly aquatic mammals, which include whales, dolphins and porpoises. Cetartiodactyla has become the generally accepted name for the clade containing both of these orders.     

From Land to Water: the Origin of Whales, Dolphins, and Porpoises

Cetaceans (whales, dolphins, and porpoises) are an order of mammals that originated about 50 million years ago in the Eocene epoch. Even though all modern cetaceans are obligate aquatic mammals, early cetaceans were amphibious, and their ancestors were terrestrial artiodactyls, similar to small deer. The transition from land to water is documented by a series of intermediate fossils, many of which are known from India and Pakistan. We review raoellid artiodactyls, as well as the earliest families of cetaceans: pakicetids, ambulocetids, remingtonocetids, protocetids, and basilosaurids. We focus on the evolution of cetacean organ systems, as these document the transition from land to water in detail.     

Astragali of Pakicetidae and other early-to-middle Eocene archaeocetes (Mammalia,Cetacea) of Pakistan: locomotion and habitat in the initial stages of whale evolution

Richard Dehm and colleagues of the Bayerische Staatssammlung für Paläontologie und Geologie in Munich made an important collection of early-to-middle Eocene mammals at Ganda Kas in Pakistan during the winter of 1955/56. The genera and species Ichthyolestes pinfoldi and Gandakasia potens were named from this collection. Both are now recognized as early and primitive archaeocete cetaceans. In addition, Dehm’s group collected 16 complete or partial astragali of archaeocetes that were misidentified as artiodactyls. These bring the total number of archaeocete astragali known from Ganda Kas to 28. They separate clearly into four species distinguished by size: from smallest to largest Ichthyolestes pinfoldi Dehm and Oettingen-Spielberg, Pakicetus attocki (West), Gandakasia potens Dehm and Oettingen-Spielberg, and Ambulocetus natans Thewissen et al. Ganda Kas artiodactyls are smaller and rare in comparison. Ichthyolestes and Pakicetus are pakicetid archaeocetes, Gandakasia is presently indeterminate to family, and Ambulocetus is an ambulocetid. Tooth size and astragalus size are highly correlated, corroborating reference of astragali to the first three archaeocete taxa based on teeth. Multivariate morphometric comparison (Auto3Dgm) shows that pakicetid astragali overlap almost completely in shape with those of early artiodactyls. Middle Eocene protocetid astragali are divergent from both. Retention of an astragalus indistinguishable from that of artiodactyls shows that pakicetids are closely related to artiodactyls phylogenetically, but does not make Ichthyolestes and Pakicetus terrestrial or cursorial. Other skeletal elements and bone microstructure indicate that pakicetids were semiaquatic like later protocetids. Tropical riverine and marginal marine facies of the Kuldana Formation are likely habitats for initial stages of the transition from terrestrial artiodactyls to semiaquatic and fully aquatic archaeocetes.     

Evolution of the Gibbon Subgenera Inferred from CytochromebDNA Sequence Data

DNA sequences for the mitochondrial cytochromebgene from the four extant gibbon subgenera are described. The data confirm that the gibbon subgenera evolved from a common hylobatid ancestor and suggest that they diverged from each other after the divergence of the extant African great ape species. The cytochromebgene does not resolve the evolutionary relationships between the gibbon subgenera themselves.     

Phylogenetic Relationships among the Major Lineages of the Birds-of-Paradise (Paradisaeidae) Using Mitochondrial DNA Gene Sequences

Complete mitochondrial cytochromebgene sequences were determined from 12 species of the Australo-Papuan birds-of-paradise (Paradisaeidae) representing 9 genera. Phylogenetic analysis of these and 5 previously published sequences reveals a radiation of the main paradisaeinine lineages that took place over a relatively short evolutionary time scale. The core paradisaeinines are resolved as the monophyletic sister-group to the crow-like manucodines. The genusParotiais basal to other paradisaeinines and is not closely related to the morphologically similar generaPtilorisandLophorina.Three major clades within the paradisaeinine ingroup include: (1)CicinnurusandDiphyllodes,(2)PtilorisandLophorina,and (3) the genusParadisaea.The monotypic genusSeleucidisis apparently closely related to clades (1) and (2). Cytochromebsequences did not provide evidence for the monophyly of the sicklebill generaEpimachusandDrepanornis.The paradisaeid tree is characterized by short internodal distances. Thus, some clades cannot be strongly resolved by cytochromebsequences alone.     

The Mitochondrial Genome of the Sperm Whale and a New Molecular Reference for Estimating Eutherian Divergence Dates

Extant cetaceans are systematically divided into two suborders: Mysticeti (baleen whales) and Odontoceti (toothed whales). In this study, we have sequenced the complete mitochondrial (mt) genome of an odontocete, the sperm whale (Physeter macrocephalus), and included it in phylogenetic analyses together with the previously sequenced complete mtDNAs of two mysticetes (the fin and blue whales) and a number of other mammals, including five artiodactyls (the hippopotamus, cow, sheep, alpaca, and pig). The most strongly supported cetartiodactyl relationship was: outgroup,((pig, alpaca),((cow, sheep),(hippopotamus,(sperm whale,(baleen whales))))). As in previous analyses of complete mtDNAs, the sister-group relationship between the hippopotamus and the whales received strong support, making both Artiodactyla and Suiformes (pigs, peccaries, and hippopotamuses) paraphyletic. In addition, the analyses identified a sister-group relationship between Suina (the pig) and Tylopoda (the alpaca), although this relationship was not strongly supported. The paleontological records of both mysticetes and odontocetes extend into the Oligocene, suggesting that the mysticete and odontocete lineages diverged 32–34 million years before present (MYBP). Use of this divergence date and the complete mtDNAs of the sperm whale and the two baleen whales allowed the establishment of a new molecular reference, O/M-33, for dating other eutherian divergences. There was a general consistency between O/M-33 and the two previously established eutherian references, A/C-60 and E/R-50. Cetacean (whale) origin, i.e., the divergence between the hippopotamus and the cetaceans, was dated to ≈55 MYBP, while basal artiodactyl divergences were dated to ≥65 MYBP. Molecular estimates of Tertiary eutherian divergences were consistent with the fossil record. Received: 12 July 1999 / Accepted: 28 February 2000