Not evaluated Cetaceans — whalesdolphins and porpoises — are placental marine mammals. All modern members of the infraorder are fully aquatic and live in the open ocean except a few species of dolphin which inhabit rivers and estuaries. Cetaceans mate, give birth, suckle their young, and feed exclusively underwater.
Paleontology and classification Paleontology Cetaceans are distant descendants of a group of poorly defined mammals known as condylarths. There is debate as to whether the first cetaceans archaeocetes descended from an extinct group of large carnivores called mesonychids or from a group of hoofed herbivores artiodactyls.
The earliest archaeocetes were huge dolphinlike creatures 6 to 10 metres long. Basilosaurus Zeuglodon was an unusual genus that was up to 34 metres long, but it apparently gave rise to no descendant groups.
Pakicetus, an ancestral whale, the first cetacean discovered with functional legs. As the fossil record becomes more complete, the pattern will emerge as to which condylarth is ancestral to archaeocetes and which archaeocete is ancestral to living cetaceans.
The first fossil cetacean, Pakicetus, is known from the Early Eocene Epoch It has recently become clear that archaeocetes rapidly diversified during the Eocene, and at least five now-extinct families are recognized.
One subfamily of the Basilosauridaethe Dorudontinaeis thought to have given rise to both living suborders of cetaceans baleen whales and toothed whales sometime during the Late Oligocene Epoch.
Evidence of the evolution of bony structures involved in echolocation appears in toothed whales from the Late Oligocene. The earliest toothed whale to display them, Echovenator sandersi, emerged some 27 million years ago.
The first baleen whales had wide, flat skulls bordered by a reduced number of teeth in the archaeocete pattern. The roof of the mouth widened between these borders, and grooves for blood vessels supplying the emerging baleen are seen inside the tooth rows.
By the middle of the Miocene Epoch some 16 million to 11 million years agothere were several families of baleen whales, including the right whales and rorquals.
The Miocene was the epoch during which modern ocean circulation began; regional areas of upwelling and increased productivity developed, setting the stage for the evolution of large whales with seasonal migratory distribution.
At the same time, the modern toothed whales began to emerge, developing into nine families during this period; four of these have since become extinct.
Sperm whales were among the first toothed whales and were present during the Middle Miocene as large and well-defined as they are now. Classification Though there is some consensus among taxonomists that Cetacea should be treated as one order as they are in this articleothers believe they are actually two or three.
This depends on the evaluation of the degree of shared ancestry, which remains controversial. However, the absence of intermediate fossils linking baleen whales with toothed forms supports the use of separate orders. Resolution of this problem awaits the discovery of relevant fossils. Further disagreement occurs at many points below the ordinal level.
Although there is no doubt that any recent cetacean is either toothed suborder Odontoceti or baleen suborder Mysticetithe relationships of many genera are in doubt.
For example, the long-snouted dolphins are classified by some authorities as a separate family, Stenidae, rather than family Delphinidae. A similar situation exists for the porpoises family Phocoenidae. At the species level there is uncertainty about the specific or subspecific status of many populations.
It must be borne in mind that all classifications are, to an extent, artificial. Over time one species merges with another, and some classification issues actually get more complex as the fossil record improves. Current cetacean classification is the result of an improving fossil record that reveals more taxa near the origins of the three established suborders the archaeocetes, mysticetes, and odontocetes.
It has become apparent that a major diversification was associated with each major adaptive branch, and species are currently being shuttled back and forth between the latest representatives of one family and the earliest representatives of its descendant family.
Although species relationships have historically been based on morphological anatomic characteristics, biologists have begun comparing DNA sequences of different cetaceans and thereby causing morphologists to reexamine their taxonomic data.
Chromosome count karyotype can be quite variable between related mammals, but it is remarkably stable among cetaceans. All baleen whales and most toothed whales have 44 chromosomes, and sperm whales and beaked whales have Techniques such as DNA sequencing have provided different ways of evaluating the relationships of species.
Applications of these and older techniques should provide a clearer understanding of the evolutionary history of Cetacea. Annotated taxonomy The three cetacean suborders two living, one fossil recognized below share the same basic body plan but differ in their degree of specialization.
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Suborders and families are separated primarily on the basis of the following characteristics: The classification presented here is based on research by cetologists F. Kellogg, and a number of other modern authorities.Cetaceans – whales, dolphins and porpoises – are placental marine iridis-photo-restoration.com modern members of the infraorder are fully aquatic and live in the open ocean (except a few species of dolphin which inhabit rivers and estuaries).Cetaceans mate, give birth, suckle their young, and feed exclusively underwater.
Cetacea (/ s ɪ ˈ t eɪ ʃ ə /) is a widely distributed and diverse clade of aquatic mammals that today consists of whales, dolphins, and iridis-photo-restoration.comans are carnivorous and finned. Most species live in the sea, some in rivers. The name is derived from the Latin cetus "whale", itself from the Greek κῆτος kētos "huge fish"..
There are around 89 extant species, which are divided.
Cetacean - Paleontology and classification: Cetaceans are distant descendants of a group of poorly defined mammals known as condylarths. There is debate as to whether the first cetaceans (archaeocetes) descended from an extinct group of large carnivores called mesonychids or from a group of hoofed herbivores (artiodactyls).
The earliest archaeocetes were huge dolphinlike creatures 6 to As I write this unit, I realize that at first glance it might not appear to be useful to many educators because of its rather narrow focus.
Its central subject, dolphins . Cetacean A whale, dolphin or porpoise. Cetaceans A group comprising all whales, dolphins and porpoises.
Dorsal fin The unpaired fin found on the back of the body of fish, or the raised structure on the back of most cetaceans. Healthy marine species like whales, sea turtles, coral, and salmon are important for maintaining balanced and thriving ocean ecosystems. We work to prevent marine species populations from decline and extinction, protect them from human activities, and monitor activities that might impact them so they are around for future generations.