Cetartiodactyla
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Cetartiodactyla is a superorder to which whales (including dolphins) and even-toed ungulates are proposed to belong. The term was coined by merging the name for the two orders, Cetacea and Artiodactyla, into a single word. The term Cetartiodactyla is most commonly used to describe the classification that whales evolved from within the artiodactyls. Under this definition their closest living land relative is thought to be the hippopotamus. The clade formed by uniting whales and hippos is called Cetancodonta. Alternatively, the term Cetartiodactyla is used to denote a clade where Cetacea and Artiodactyla are sister groups, but where Cetacea did not actually evolve from within the Artiodactyla. Under this definition, all artiodactyls, including hippos, are more closely related to one another than any are to the whales.
Whales evolved from land mammals and appear to form a monophyletic group. The notion that all whales evolved from a single ancestor is not in dispute. The most widely accepted hypothesis prior to the 1990s was that the closest relatives to whales was the fossil group Mesonychia. These were hooved, predominantly carnivorous, mammals that are known only from fossils. Few modern authorities still consider mesonychids to be more closely related to whales than artiodactyls. Instead, they are usually considered to be the closest relative of the Cetartiodactyla as a whole.
Kin to hippos
The idea that whales evolved from within the Artiodactyla was first proposed based on the analysis of DNA sequences. In the initial molecular analyses, whales were shown to be more closely related to ruminants (such as cattle and deer) than ruminants are to pigs. In order for the order name to reflect a real evolutionary unit, the term Cetartiodactyla was coined.
Later molecular analyses included a wider sampling of artiodactyls and produced a more complete tale. Hippos were determined to be the closest relative of whales, ruminants were related to a whale/hippo clade, and pigs were more distant. In addition to producing the controversial whale/hippo clade, these analyses suggested that hippos and pigs were not closely related. This had been a popular taxonomic hypothesis (Suiformes) based on similarities in morphological (physical) characteristics.
In addition to DNA and protein sequences, researchers tracked the movement of transposons called SINEs in the genome (see the method at Retrotransposon_Marker. A transposon is a DNA sequence that will occasionally make a copy of itself and insert that copy into another part of the genome. It is considered highly unlikely that SINEs will insert themselves into the exact same part of a genome by chance. The data indicate that several transposons inserted themselves at the same point in the genomes of whales, ruminants and hippos (sometimes referred to as "pseudoruminants" because although they have four-chambered stomachs like true ruminants, they do not chew the cud). This insertion point is not shared with camels and pigs.
This hypothesis has been tested with DNA sequences from a host of genes: the complete mitochondrial genome (as well as several of its genes independently), beta-casein, kappa-casein, von Willebrand factor, breast cancer 1, recombination activating genes 1 and 2, cannabinoid receptor 1, and several others. These sequence data and the transposons converge on the same conclusion that hippos and whales are more closely related to one another than either is to other artiodactyls.
Sequences analyzed in combined analyses with morphological characters have also produced the same results as sequences alone. Some have argued that the sheer number of characters (one for each nucleotide) in sequences swamps out the effects of morphology. There have been a few morphology-based studies that have suggested (weakly) the same results as the molecular results, but overall most morphological studies have conflicted with the whale/hippo hypothesis of Cetartiodactyla.
An important exception is a recent conducted by Boisserie et al. (2005). They examined 80 hard morphological characters of fossil and extant cetartiodactylan taxa. Their results suggest that hippopotamids evolved from within a clade of anthracotheres. That anthracothere/hippopotamid clade appears to be sister to the Cetacea and supports the molecular results.
A clade apart
The morphology and paleontology-based opinion of the relationship of whales to other mammals has also undergone considerable changes since the early 1990s. Fossil such as Rodhocetus, have been discovered that refute the notion that whales are derived from or are closely related to the mesonychids. Many morphologists and paleontologists support the notion of a clade called Cetartiodactyla that unites Cetacea + Artiodactyla. Many are not, however, in support of the hypothesis that Cetacea evolved from within the Artiodactyla. Under this definition, Artiodactyla remains a valid, monophyletic clade since artiodactyls evolved from a common ancestor separate from whales. Cetartiodactyla would represent a grandorder or superorder uniting the two orders.
The vast majority of phylogenetic analyses based on morphological characters have not uncovered a whale/hippo clade, but show Cetacea and Artiodactyla as distinct from one another. Features of the bones of the astragalus in the ankle region are cited as particular evidence for a monophyletic Artiodactyla.
Hippo fossils are not observed until the Miocene, but whale ancestors have been dated to the Eocene. The whale/hippo hypothesis imposes a gap of nearly 30 million years where no hippo ancestors are present. Certain anthracotheres have been proposed as hippo ancestors, but this concept has not garnered wide support among paleontologists.
See also
References
- Boissere, J.-R., F. Lihoreau, and M. Brunet. 2005. The position of Hippopotamidae within Cetartiodactyla. Proceedings of the National Academy of Sciences of the United States of America, 102:1537-1541.
- Gatesy, J. 1997. More support for a Cetacea / Hippopotamidae clade: The blood clotting protein gene g-fibrinogen. Molecular Biology and Evolution, 14:537-543.
- Gatesy, J., C. Hayashi, M. Cronin, and P. Arctander. 1996. Evidence from milk casein genes that cetaceans are close relatives of hippopotamid artiodactyls. Molecular Biology and Evolution, 13:954-963.
- Gatesy, J., C. Mathee. R. DeSalle, and C. Hayashi. 2002. Resolution of a supertree/supermatrix paradox. Systematic Biology, 51:652-664.
- Gatesy, J., M. Milinkovitch, V. Waddell, and M. Stanhope. 1999. Stability of cladistic relationships between Cetacea and higher-level artiodactyl taxa. Systematic Biology, 48:6-20.
- Grauer, D. and D. Higgins. 1994. Molecular evidence for the inclusion of cetaceans within the order Artiodactyla. Molecular Biology and Evolution, 11:357-364.
- Milinkovitch, M. C. and J. G. M. Thewissen. 1997. Even-toed fingerprints on whale ancestry. Nature, 388:622-624.
- Montgelard, C., F. Catzeflis, and E. Douzery. 1997. Phylogenetic relationships among cetartiodactyls and cetaceans as deduced from the comparison of cytochrome b and 12S RNA mitochondrial sequences. Molecular Biology and Evolution, 14:550-559.
- Naylor, G. J. P. and D. C. Adams. Are the fossil data really at odds with the molecular data? Morphological evidence for Cetartiodactyla phylogeny reexamined. Systematic Biology, 50:444-453.
- O'Leary, M. A. and J. H. Geisler. 1999. The position of Cetacea within Mammalia: phylogenetic analysis of morphological data from extinct and extant taxa. Systematic Biology, 48:455-490.
- Nikaido M, Rooney AP, Okada N (1999) Phylogenetic relationships among cetartiodactyls based on insertions of short and long interpersed elements: Hippopotamuses are the closest extant relatives of whales. Proc Natl Acad Sci U S A 96: 10261–10266.
- Shedlock, A. M., M. C. Milinkovitch, and N. Okada. 2000. SINE evolution, missing data, and the origin of whales. Systematic Biology, 49:808-816.
- Shimamura, M., H. Yasue, K. Ohshima, H. Abe, H. Kato, T. Kishiro, M. Goto, I. Munechika, and N. Okada. 1997. Molecular evidence from retroposons that whales form a clade within even-toed ungulates. Nature, 388:666-670.
- Thewissen, J. G., E. M. Williams, L. J. Roe, and S. T. Hussain. Skeletons of terrestrial cetaceans and the realtionship of whales to artiodactyls. Nature, 413:277-281.
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