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Cetacean Evolution
Lisa T. Ballance
Marine Mammal Biology – SIO 133
Spring, 2012
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Mammalian Phylogeny
• Pinnipeds, sea otters, polar bears
• Sirenians
http://whozoo.org/mammals/mammalianphylo.htm
• Cetaceans
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Order Cetartiodactyla
Gatesy et al. 1999
Deer, Elk, Moose, Caribou
Giraffe, Okapi
Hippopotami
Bison, Buffalo, Antelope, Gazelle, Sheep, Goat
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The Archaeocetes (“ancient whales”)
Epochs in the Tertiary Period, Cenozoic Era
Millions of years before present
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Early Eocene (50-60 mya)
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Pakicetidae • Early – Middle Eocene (50 mya) • Wolf/fox-size • Predominantly terrestrial • Partially aquatic?
– Fossils in modern-day Pakistan, near coastal Tethys Sea
– Earbones adapted for underwater hearing
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Ambulocetidae
• Ambulocetus natans “walking whale that swims” (most complete specimen found to date)
• Middle Eocene (45 mya) • 3 m, 300 kg; thick limbs • Amphibious (back legs more adapted
to swimming than walking) • Shallow, swampy marine and fresh
water environments • Ecological niche similar to today’s
crocodiles
• Cetacean features: early cetacean-like teeth; cetacean-like ear bones; nose adapted for underwater swallowing; no external ears
• Transition: fresh-marine; land-water
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Remingtonocetidae
• Middle Eocene (45 mya)
• Amphibious • Coastal marine
environments • Long, narrow skull
and jaw (suggesting diet of fast-swimming prey)
• Small orbits (suggestion vision not important)
• Ears adapted for underwater hearing
Kutchicetus
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Middle Eocene (45 mya)
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Protocetidae
• Middle Eocene (45 mya) • Widespread (fossils from
Europe, Africa, Asia, N America) – first Archaeocetes to disperse to all shallow sub-tropical oceans
• Lagoon, coastal, *and* open marine forms
• Nasal openings large and half-way up the snout
• Some possibly with flukes
Rodhocetus
Protocetus
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Basilosauridae “king lizard”
• Late Eocene – Early Oligocene (41-35 mya)
• Initially mistaken for a marine reptile • Oldest known (with Dorudontids)
obligate marine forms (no freshwater forms)
• Fully recognizable cetaceans – Nostrils close to position of modern-
day blowholes – Functionally modern cetacean ear
structure (incl air-filled sinus between ear bones and skull)
• Small brains (suggesting non-social) • Tiny external hind limbs but pelvic
bones not connected to vertebral column
• Widely distributed in tropical and sub-tropical oceans of the world
• Of note: – Large body size (12-25 m!) – Extreme elongation of lumbar vertebrae – Tail with fluke but likely not the main
propulsive organ
• Fish remains found with fossils
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Dorudontidae
• Late Eocene – Early Oligocene (41-35 mya)
• Very similar to Basilosaurids *except*: – Dolphin-like (~5 m)
– Tail with fluke that formed main propulsive organ
• Modern mysticetes and odontocetes likely diverged from these forms
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Cetacean Phylogeny
35 mya
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Oligocene (35 mya)
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Early Mysticetes
• Aetiocetidae – Late Oligocene
– Toothed • Cetotheriidae – Late Oligocene – Late Pliocene
– Skull similar to Balaenopterids
– Toothless (some with baleen?)
– 3-10 m
– Diverse (>60 species described)
Cetotherium
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Early Odontocetes
• Mid Oligocene – Mid Miocene (33-14 mya)
• Rostrum of various lengths
• Primitive dentition
• Squalodontidae
– Telescoping of skull
– first hypothesized echolocation
Squalodon
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Odobenocetops “walrus whale”
• Early Pliocene
• Convergent on walrus
• Asymetrical tusk length (1.4m vs 25 cm)
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Phylogeny of modern cetaceans
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Major Features Associated with the Evolution of Cetaceans:
1. Reduction of the hind limbs (adaptive significance: increased hydrodynamic efficiency)
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Major Features Associated with the Evolution of Cetaceans:
2. Hyperphalangy (adaptive significance: use of forelimbs for locomotion in water) – Increased
number of bones in digits
Greyhound
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Major Features Associated with the Evolution of Cetaceans
3. Telescoping of the skull (adaptive significance: nares on top of head)
– Maxillae - rostrum
– Pre-maxillae – nares
– Nasals
– Occipital
– Parietals
– Frontals
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Major Features Associated with the Evolution of Odontocete Cetaceans:
4. Homodonty and Polydonty (adaptive significance: prey capture)
– Teeth of the same type
– Increase in tooth number
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Potential Term Paper Topics
• What is the evolutionary history of modern-day sperm whales? Beaked whales? River dolphins?
• What major evolutionary features (changes from terrestrial ancestors to modern-day forms) do cetaceans have in common with other secondary marine forms (marine reptiles, marine birds)?