The order Cetacea is one of many aquatic vertebrate groups (e.g., ictiosaurs, monosaurs, sirenians, pinnipeds, cetaceans) that have terrestrial ancestors. In fact, the aquatic Cetacea is sister to the terrestrial Artiodactyla (e.g., pig, peccaries, camels). Both groups are each other closest relatives, forming a monophyletic group called CETARTIODACTYLA. For cetaceans the transition from land to water occurred approximately 50 millions years ago (early Eocene). Among the oldest fossils are Ambulocetus natans which morphology resembles that of more recent fossils such as Himalayacetus, Nalacetus, and Pakicetus. Three suborders are recognized:
Archeoceti: The archeocetes were very similar in appearance to modern cetaceans. The group was relatively diverse with animals ranging between 2 to 21 meters. Their body was relatively hydrodynamic and had paddle-like extremities. Approximately 30 million years ago they disappeared.
Mysticeti and Odontoceti
Adaptive Radiation and Speciation
Paleocene: The most antique cetaceans lived during this period. Their body was heavier and specialized for an aquatic life style than modern hippos. Their habitats consisted of estuaries were they fed on mollusks (riverine and estuarine species) and fish that were buried in the mud.
Paleocene-Eocene: During this time mammals radiated. Possible competition for territories and prey on land stimulated the return to the sea. Many niches along the coast of Tethys exploited previously by reptiles (e.g., ictiosaurs, plesiosaurs, and monosaurs), became open when they disappeared at the end of the Cretaceous. Thus, free of competition these habitats were later colonized by cetaceans.
Eocene: The appearance of Archeocetes occurred during this period. These animals occupied warm waters were they performed short dives. Apparently due to several morphological limitations, archeocetes were less competitive against more derived forms. Although they dominated this period, in general the Eocene was a period of low diversity for cetaceans. Due to an increase in the competition for resources, natural selection favored those species with dentition and fast movements that could prey on benthonic and nektonic fish. These specialized species outperformed generalist species. In addition, evasive behaviors were favor. However, selection over behaviors was not as intense as selection in diet and habitat.
Oligocene: Occurs the transition of archeocetes to modern cetaceans of the suborders Mysticeti and Odontoceti. The radiation of these two groups coincided with:
Global climate change
Oceanic productivity
New patters in water circulation (due to the break of Gondwanaland, opening of the South Ocean, opening and close of the isthmus).
Miocene-Present: Warmer climate seems to have promoted an increase in the population size and distribution of many cetacean species. During this time of toothed whales (Odontoceti) and baleen whales (Mysticeti) diversified.
Phylogeny
The evolutionary history of Cetacea is perhaps one of the most controversial topics of the past 20 years. The debate roots on the type of characters used to reconstruct the evolutionary history of the group (Morphology, DNA nuclear and mitochondrial) and the inappropriate use of “outgroups”. The controversies can be summarized as follows:
-The phylogenetic position of Cetacea as sister group of Artiodactyla or embedded within Artiodactyla, a clade called Cetartiodactyla.
-The phylogenetic relationships between baleen and toothed whales
-The phylogenetic relationships among delphinoids (Phocoenidae, Monodontidae, Delphinidae).
-The phylogenetic position of river dolphins.
Many of these debates are considered solved thanks to the use of matrices combining morphology and molecules, the more extensive use of outgroups, and new methods for phylogenetic analyses. Some of the less debated topics include:
The phylogenetic position of Cetacea. The sister group of Cetacea is Hippopotamidae (hippos) which makes Artiodactyla paraphyletic (meaning that the group is artificial because it does not include the ancestor and all its descendants) if Cetacea is not embedded within it. Today many scientists acknowledge the clade Cetartiodactyla that includes both Cetacea and Artiodactyla.
The monophyly (a clade that contains the ancestor and all its descendants) of each suborder Mysticeti and Odontoceti.
The monophyly of Delphinoidea and each of its families Phocoenidae, Monodontidae, and Delphinidae.
However, the evolutionary history of river dolphins, ziphiids, and some species within delphinids remains unclear. Furthermore, within Delphinidae several genera are paraphyletic, such as Stenella, Tursiops, and Lagenorhynchus.
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