Futurismo Biologists Team
Figure 1: A: Pakicetus, Ref: 1; B: Basilosaurus, Ref: 2; C: Odontecete; D: Misticete
What is Biogeography?
Biogeography is the area of science that tries to understand the distribution of species and ecosystems. The current distribution of the species is due to two factors:
Their ancestors, species that no longer exist today, which have evolved / adapted in order to become the species that live in our days.
The geological features of our planet that led species to their current distribution.
Archaeoceti, cetaceans’ ancestors
Archaeoceti (ancestral whales) are the ancestors of the cetaceans that we know nowadays, but they are extinct since many millions of years ago.
The family Pakicetidae is considered the most recent ancestral of all cetacean species. In 1970, the first fossil from these species was found. It´s estimated that they have lived about 55Ma ago, being the hippopotamus the actual land mammal more related with them. Since that time Earth has changed, through many geological transformations that influenced the actual cetacean distribution.
Planet Earth Transformations?
Since 55Ma ago, there have been described 5 different epochs (Eocene, Oligocene, Miocene, Pliocene and Pleistocene) till our days, being the epoch that we live now described as Holocene (Figure 2).
Figure 2: Planet Earth at 55Ma, arrows and circumferences show the areas where major geological transformations occur, numbers shown the order of this occurrence (ref 3)
1) During the Eocene (55-35Ma), the first cetaceans appeared in estuarine and coastal areas, in what was known as Tethys Sea. The planet was a much warmer place compared to the one that we live in our days.
It was during this time that possibly the greatest morphological transformations occurred in the Archaeocetes. Their adaptation to the marine environment included 5 main transformations in their body (Figure 3):
- Nose migration, from the anterior position on the face, to the top of the head. It is called now blowhole.
- Hind limb regression to the interior of the body. Now it is a vestigial organ.
- Fore limb transformation for a swimming support organ, known as flippers.
- The tail became the main propulsion mechanism, frequently called fluke.
- All these transformations led to a more streamlined body development, more adapted to the marine environment.
Figure 3: Archaeoceti evolution, since the family Pakicetidae until the family Basilosauridae, ref 4.
2) & 3) During the Oligocene (35-23Ma) and Miocene (23-5Ma), there were three major geological transformations that greatly transformed the oceans. During the first, what is today India, joined Asia, leading to the continuous closing of the Tethys Sea. This transformation was followed by the opening of the Drake Passage between Antarctica and Patagonia. Afterwards, Australia (Tasmania) with Antarctica, led to the creation of an Antarctic current, leading to a progressive cooling of the oceans in the Southern Hemisphere, and possibly increasing the primary production, which might be the responsible of new feeding techniques by cetaceans. Finally, all Oceans have become practically as we know them today, mainly with the continuous opening of the Atlantic Ocean.
These epochs were fundamental in cetacean history, as it was during these epochs when a large part of the Archaeocetes got extinct, probably leading to the appearance of the Neocetes. It was during this time when the two main feeding techniques from the actual cetaceans started to evolve:
Development of filter feeding (Figure 4). During these epochs, the ancestors of the misticetes evolved from a biting, intraoral suction and simple sieving, to a baleen filtering, due to appearance and development of baleen plates, which are made of keratin the same amino acid of our hair and nails.
Figure 4: Filter feeding evolution, from the common ancestor of mysticetes and odontocetes, Basilosauridae family, showing several mysticetes ancestral families Aetiocetidae and Eomysticetidae until a living mysticete species, adapted from Marx, Hocking et al. (2016).
The ancestors of the odontocetes, developed structures that allow the production of high frequency signals, which possibly would enable the production of echolocation (Figure 5). A fundamental characteristic for the capture of food, which has been shown to be quite effective during the evolution of odontocetes, as it led this sub-order to become the most abundant with 73 species of cetaceans described today.
Figure 5: Earing evolution since the Archaeocetes until the living odontocetes, with cochlea images (anterior (a,c,e,g) and vestibular (b,d,f,h)), adapted from Park, Fitzgerald et al. 2016.
4) & 5) During Pliocene (5-2Ma) and Pleistocene (2Ma-10000anos), mostly on the last one, occurred the famous ice ages. Our Planet, mainly on the Northern Hemisphere had successive warmer and cooler phases that greatly influenced cetacean species distribution as we know today. In the Azores we can observe some of these examples.
- Anti-tropical species: sister species that exist in both hemispheres but does not exist in tropical areas. The northern bottlenose whale is one of these species and occurs in Azorean waters.
- Baleen whale’s migration with different timing in each hemisphere might be explained by this consecutive ice ages.
Article by Rui Peres dos Santos
Bibliography:
Marx, F. G., D. P. Hocking, T. Park, T. Ziegler, A. R. Evans and E. M. Fitzgerald (2016). “Suction feeding preceded filtering in baleen whale evolution.” Memoirs of Museum Victoria 75.
Park, T., E. M. Fitzgerald and A. R. Evans (2016). “Ultrasonic hearing and echolocation in the earliest toothed whales.” Biol Lett 12(4).
References:
- https://pt.wikipedia.org/wiki/Pakicetus#/media/Ficheiro:Pakicetus_BW.jpg
- https://pt.wikipedia.org/wiki/Basilosaurus#/media/Ficheiro:Basilosaurus_BW.jpg
- https://deeptimemaps.com/global-paleogeography-and-tectonics-in-deep-time-series/
- https://evolutionnews.org/2018/07/from-bears-to-whales-a-difficult-transition/
Futurismo Biologists Team
Sandrina Rodrigues
