Cetacea

Cetaceans, (Whales dolphins, and porpoises) are in the order of mammals that originated 50 Million years ago in the Eocene

Cetaceans are mammals,

all similar in shape,

horizontal tail fluke

forelimbs are flippers

No external hind limbs, neck short, body stream lined.

cetaceans lack fur but have a few hairs on the face

From the Eocene of Pakistan

Long snout, long tail, long slender limbs, 4-5 toes end in a hoof

closest relative to cetaceans

bone is thick, small marrow cavity, bone is heavy, pacystotic, common in aquatic mammals.

stable isotope study of teeth suggest indohyus lived in water

Pakicetids are the oldest cetaceans known

known from middle eocene of northern pakistsan and west india

there are 3 genera of pakicetids

Pakicetids are related to artiodactlys, have an astragalus with two trachleas, and an involucrum, thick bone over the ear space

orbits located close together near the top of the skull

bones are osteosclerotic, hand and dense

aquatic animals,

stable isotopes also suggest aquatic 

Ambulocetus fossils are approx 49ma (Middle Eocene)

Ambulocetus is larger than pakicetids

short powerful legs, 5 fingers four toes

tail verts robust, muscular tail

osteosclerotic limb bones

ambush preditor

eyes located high pn skull but more towards the sides

long snouted cetaceans

found at same locations as Ambulocetus, 48-41ma india and pakistan

has a mandular foramen perforation on the lower jaw behind the teeth

in modern cetaceans this foramen allows for transmission of sound underwater

49-40ma 

world wide distribution

15 genera described

eyes large and face laterally, set far from the middle of the skull

Nasal opening located futher posterier on the snout, foreshadowing the foremation of the blowhole

protocetids lacked a fluke

swimming was a combination of paddling with hind limbs and dorsoventral undulations of the tail

found in near shore marine deposits often associated with carbonate platforms such as reefs

7 genera

41ma ago

nasal opening forms a blowhole

flippers for forelimbs, fluke at the end of the tail

tiny hind limbs, did not leave the ocean

first obligate cetaceans

hetrodont detentition, modern cetaceans have homodont/ isodont dentition

3 phalanges per finger, modern cetaceans have added more bones,

pelvis not attached to the vertibral column

Can be split between Odontoceti and Mysticeti

34ma and monophyletic

Mysticeti are filter feeders, have baleen plates