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Evolution of cetaceans

File:Evolutionary pathway.jpg
A phylogeny showing the relationships among cetacean families.[1]

The Cetaceans (whales, dolphins and porpoises) are mammalian marine descendants of land mammals. Their terrestrial origins are indicated by:

  • Their need to breathe air from the surface;
  • The bones of their fins, which resemble the limbs of land mammals
  • The vertical movement of their spines, characteristic more of a running mammal than of the horizontal movement of fish.

The question of how land animals evolved into ocean-going leviathans was a mystery until recent discoveries in Pakistan revealed several stages in the transition of cetaceans from land to sea.

DNA sequence analysis

After many years when palaeontologists thought whales had evolved from mesonychids, DNA sequence analysis showed the closest relationship was with artiodactyls, the even-toed ungulates. A new clade was created to include Cetacea and their nearest relatives, the hippopotamus family. This clade is called the Cetartiodactyla.

Fossil record

Further information: Archaeoceti
File:Raoellids&cetaceans.jpg
Possible relationships between cetaceans and other ungulate groups.[1] [2]
Reconstruction of Indohyus

Hippo fossils are not found until the Miocene, but whale ancestors have been found from the Eocene. This leaves a gap of nearly 30 million years where no hippo ancestors are found. The most recent hypothesis is that hippos and whales shared a common semi-aquatic ancestor which branched off from other Artiodactyls around 60 million years ago (mya).[3][4] This ancestral group probably split into two branches around 54 mya.[5] One branch evolved into cetaceans, possibly beginning with the proto-whale Pakicetus from 52 mya. [6] These early cetaceans gradually became adapted to life in the sea. They became the completely aquatic cetaceans.[7]

Indohyus

File:Mystice pelvis (whale).png
Some modern whales have traces of their land-dwelling ancestors. The skeleton of a Bowhead whale shows its hind limb and pelvic bone structure (circled in red). This bone structure stays inside its body its entire life: it is a vestigial structure.

Indohyus is a small deer-like creature, which lived about 48 million years ago in Kashmir. It belongs to the artiodactyls family Raoellidae, and is believed to be the closest sister group of Cetacea.[1]

About the size of a raccoon or domestic cat, this herbivorous creature shared some of the traits of whales.[1] It also showed signs of adaptations to aquatic life, including a thick and heavy outer bone coating. This is similar to the bones of modern creatures such as the hippopotamus,[8][9] and reduces buoyancy so that they can stay underwater. This suggests a similar survival strategy to the African mousedeer or water chevrotain which, when threatened by a bird of prey, dives into water and hides beneath the surface for up to four minutes.[10][11][12]

Size

Scientists from the University of Plymouth studied many animals that dive and hold their breath, from insects to whales, and they found that larger animals can hold their breath longer than smaller animals because they can store more oxygen for their size, and this difference was much bigger for warm-blooded animals than for cold-blooded animals. They said this may be why modern whales and extinct diving animals like plesiosaurs became so large.[13][14]

The blue whale is not the largest ever whale, and perhaps by quite a long way. 39 million years ago Perucetus colossus was between 85 and 340 tons, so up to 3x heavier than the blue whale.[15][16]

Evolution Of Cetaceans Media

  • Species of the infraorder Cetacea

  • Cladogram showing the position of Cetacea within Artiodactylamorpha

  • Archaeocetes (like this Basilosaurus) had a heterodont dentition

  • VMNH megalodon.jpg

    Artistic impression of two Eobalaenoptera pursued by the giant shark Otodus megalodon

  • Acrophyseter deinodon skull.jpg

    Acrophyseter skull

  • Ichthyosaur vs dolphin.svg

    Dolphins (aquatic mammals) and ichthyosaurs (extinct marine reptiles) share a number of unique adaptations for fully aquatic lifestyle and are frequently used as extreme examples of convergent evolution

  • Pakicetus attocki skeleton

  • The-Ecological-Conditions-That-Favor-Tool-Use-and-Innovation-in-Wild-Bottlenose-Dolphins-(Tursiops-pone.0022243.s002.ogv

    Researchers pushed a pole with a conical sponge attached along the substrate to simulate sponging behavior by dolphins. They videotaped this experiment to learn what prey was available on the seafloor and why a sponge would be beneficial to foraging rather than echolocation.

References

  1. ↑ 1.0 1.1 1.2 1.3 Lua error in Module:Citation/CS1/Identifiers at line 630: attempt to index field 'known_free_doi_registrants_t' (a nil value).
  2. ↑ Lua error in Module:Citation/CS1/Identifiers at line 630: attempt to index field 'known_free_doi_registrants_t' (a nil value).
  3. ↑ "Scientists find missing link between the dolphin, whale and its closest relative, the hippo". Science News Daily. 2005-01-25. Archived from the original on 2007-03-04. Retrieved 2007-06-18.
  4. ↑ Lua error in Module:Citation/CS1/Identifiers at line 630: attempt to index field 'known_free_doi_registrants_t' (a nil value).
  5. ↑ Lua error in Module:Citation/CS1/Identifiers at line 630: attempt to index field 'known_free_doi_registrants_t' (a nil value).
  6. ↑ Pakicetus and other early whale ancestors are collectively known as the Archaeoceti.
  7. ↑ Boisserie J.R; Lihoreau F. & Brunet M. (2005). "The position of Hippopotamidae within Cetartiodactyla". Proceedings of the National Academy of Sciences. 102 (5): 1537–1541. doi:10.1073/pnas.0409518102. PMC 547867. PMID 15677331.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  8. ↑ University Of California, Berkeley (2005). "UC Berkeley: French scientists find missing link between the whale and its closest relative, the hippo". ScienceDaily. Retrieved 2007-12-21.
  9. ↑ University Of Michigan (2001). "New fossils suggest whales and hippos are close kin". ScienceDaily. Retrieved 2007-12-21.
  10. ↑ Carl Zimmer (2007). "The loom: whales: from so humble a beginning". ScienceBlogs. Archived from the original on 2007-12-21. Retrieved 2007-12-21.
  11. ↑ Ian Sample (2007). "Whales may be descended from a small deer-like animal". Guardian Unlimited. Retrieved 2007-12-21.
  12. ↑ Myers P.Z. (2007). "Pharyngula: Indohyus". Pharyngula. ScienceBlogs. Archived from the original on 2007-12-20. Retrieved 2007-12-21.
  13. ↑ Eurekalert.org (May 26, 2020). "Scientists reveal new fundamental principles governing diving in animals". Press release. https://www.eurekalert.org/pub_releases/2020-05/uop-srn052620.php. Retrieved June 1, 2020. 
  14. ↑ Lua error in Module:Citation/CS1/Identifiers at line 630: attempt to index field 'known_free_doi_registrants_t' (a nil value).
  15. ↑ Source: Eli Amson, State Nuseum of Natural History, Stuttgart.
  16. ↑ Bianucci G.; Lambert O.; Urbina M.; Merella M.; Collareta A.; Bennion R.; Salas-Gismondi R.; Benites-Palomino A.; Post K.; de Muizon C.; Bosio G.; Di Celma C.; Malinverno E.; Pierantoni P.P.; Villa I.M.; Amson E. 2023. A heavyweight early whale pushes the boundaries of vertebrate morphology. Nature doi:10.1038/s41586-023-06381-1. PMID 37532931

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