A Procetid, the ancestor of modern whales, and the model for the Ceti on Thalassa. They swam the seas and walked on land. on earth 40 million years ago. How did they end up on Thalassa?

As a marine biologist, I am always spellbound watching whales on the open sea. Witnessing their majestic movements, they are the most marine of all creatures, but I never fully appreciated their origins until I researched the topic before writing SOT. I focused on the ancestors of cetaceans because they are super-cool and are a great hook to get people to read my book. They also use sonar and sing songs and thus are perfect to play a major part of the “songs” of Thalassa (but they are not the only ones).

Modern whales also play an important role in Polynesian (and Hawaiian) culture and have a tragic history of exploitation. For all of these reasons, I used the Nesoi, Ceti, and Baleena as central figures in SOT. Please remember that these are not the same as their counterparts on earth for two reasons: first, they are of uncertain origin; and second, they have evolved on Thalassa for millions of years. For more on these amazing creatures, you’ll have to wait for my next book, Songs of Hina.

Illustration of Dorudon, a prehistoric whale ancestor, and model for the Nesoi. Source: Mikkel Juul Jensen.

So to begin, imagine an ocean with no whales, no dolphins, and no porpoises. If you could travel through time and swim in Earth’s warm Tethys Sea of 50 million years ago (mya) you’d be startled by the early ancestors of whales — the walking whales; the model for the Nesoi.

Whales and their relatives, the cetaceans, are related to terrestrial (even-toed) ungulates like the hippopotamus. They are all descendants of semi-aquatic animals that invaded the empty sea in Eocene times to prey on rich marine resources (and the absence of marine reptiles). To truly understand the story of cetaceans we need to place their ancestors, deduced from fossil evidence, in the context of continental drift and the formation and fate of the Tethys, and (natural) long-term climate change. For the full story, go here.

Drivers of Whale Evolution: Continental Drift and Climate

Fossils of the walking whales show they evolved in the Tethys’ swamp-like seas then spread through what would become the Mediterranean and Caribbean seas and eventually to the Pacific coasts and worldwide oceans. Much of their success was driven by climate. In the beginning, when the ancestors of whales invaded the sea, it was super warm –the warmest seen in the last 65 million years. But 15 million years later, Earth shifted to a cool, ocean-rich ice age. It was the perfect climatic driver for the success and spread of early cetaceans.

To help you understand the evolution of cetaceans, and the appearance of the major species in SOT, I’ve included a brief illustrated description of their journey to global prominence on earth.

Pakicetus — the first cetacean on Earth (49 mya)

Illustration of Pakicentus, the mammalian ancestor of all whales that invaded the shallow Tethys sea 49 million years ago. Source: Lucas Lima, https://252mya.com.

Pakicetus was a dog-sized, mostly terrestrial mammal that occasionally hunted fish in the shallow Tethys sea. It had several unique characteristics adapted to a partial aquatic existence: upward-looking eyes, thick bones (which assist in floating), and a thickened skull bone to improve underwater hearing. This species was hugely successful in exploiting the rich marine resources of the shallow seas. As it became adapted to a more aquatic existence, over a million years it gave rise to another more ocean-oriented species.

Ambulocetus — the walking whales (48 mya), model for the Nesoi

Fossils of Ambulocetus were first found in Pakistan in 1994 and made headlines as the “walking whale” due to its combined aquatic and terrestrial features. Why Pakistan you might ask? If you go back to the Eocene, the continent that would become India was surrounded by ocean and was slowly rafting towards Asia. As got close it created the warm, shallow, island-rich Tethys Sea which persisted for millions of years. It was the perfect setting for a mammal to invade the sea. Today, the fossils of early cetaceans are found in the continental shelves of the former Tethys, which are exposed in modern-day Pakistan, Afghanistan, and even the Himalayas; all former seafloor that was created by India’s collision with Asia.

Illustration of the continents during the early Eocene.

Ambulocetus was up to 10 feet long and its crocodile-like shape included an elongated snout with upward-facing eyes. Studies suggest it was mostly aquatic, using its front and hind legs and tail to swim, but occasionally walked on land to drink fresh water and give birth. It was a big change from Pakicetus and one that supported the eventual spread of its descendants out of the Tethys corridor.

The Nesoi in SOT is based mostly on these walking whales with a few modifications. Unlike what we know about these ancestral species, the Nesoi were capable of a unique set of traits due to their millions of years of evolution on Thalassa: 1) sonar for navigation (their “clicks:” and “creaks”); 2) the ability to communicate by whistling in the air, and 3) the ability to sing beautiful songs like baleen whales to communicate to each other and to other creatures on the planet (they could even be heard in space). Given these abilities, their heads would have a different morphology than Ambulocetus to be able to create and receive sonar, whistles, and songs. Their whistles are based on Silbo Gomero, a whistling language used by humans of La Gomera in the Canary Islands to communicate by whistles across the deep ravines and narrow valleys that radiate throughout the island. Their sounds can be heard up to a distance of 3 miles (Wikipedia)

Procetids spread across the Tethys (47-39 mya), the model for the Ceti

Procetids were a big step towards a more aquatic existence than the walking whales but with their hind legs, they could still walk (and give birth) on land. But over 8 million years on earth multiple adaptations arose to a more-marine existence with some species walking on land and others being fully aquatic. Major changes include their eyes shifting to the side for better aquatic vision, their nasal openings moving closer to their eyes, and their ears becoming more adapted to underwater hearing. These major adaptations were key to the ultimate success of these cetaceans.

I used the Procetids as a model for the Ceti on Thalassa, the first one Sage encounters in the cave of light. Part of including the Ceti was to show that the ancestors of the Nesoi had diversified on Thalassa, as they would have across an ocean mostly devoid of large animals. Based on what we know on Earth, over millions of years on Thalassa I believed the Ceti would exploit large mantis squid in the deep seas and submarine canyons of Thalassa. As such, they would move away from a land-based existence and become more fully aquatic and give rise to critters more similar to our modern whales, but with a twist (no pun intended). Expect more about the Ceti in the next book.

Modern whales emerge as Climate Shifts– (33-28 mya), Model for the Effects of Hina

On earth, the ancestors of toothed and baleen whales diversified and eventually diverged as the world’s climate cooled and opened up new opportunities for their ancestors, the basilosaurids, the descendants of Procetids. Shifting continents 34 mya on earth created large-scale changes in ocean currents and temperatures that coincided with this diversification. Principle among them was the isolation of Antarctica and the openings of the Tasmanian Seaway and the Drake Passage resulted in the largest current on the planet, global cooling, and the Antarctic circumpolar current that created the richest marine resources on earth.

Illustration of the development of circumpolar Antarctic current in late Eocene. From Blakey (2020), Geology b102, Historical Ecology.

I included the same process in the history of Thalassa where the arrival of Hina warmed the planet from increased volcanism, which raised sea level, and opened up two circumpolar currents, just as did on earth. As discussed in the book, the new currents created the perfect whale feeding grounds: Cetacean heaven! These changes helped create the next phase of diversification for the ceti and new species on Thalassa, as it occurred on Earth.

Baleen whales tap the world’s plankton (36 mya-present), the model for the Baleena (and others to come)

On earth, the emergence of crown-shaped teeth 30 mya show an early transition from teeth to baleen, the filter-feeding system inside the mouths of all modern baleen whales. Filter feeding is beneficial and allowed baleen whales to tap huge planktonic energy resources, such as Antarctic krill, which eventually resulted in the massive body size of modern species.

The jaws and serrated teeth of Coronodon, a toothed mysticetes from the early Oligocene (30 mya). The serrated teeth may have been an early form of suction feeding, which led to the development of baleen.

Early species were suction feeders and may have used their serrated teeth to feed on plankton. As the planet cooled even further, baleen, sheets of fingernail-like teeth hanging from the roofs of their mouths, evolved and baleen whales diversified into many species, including the modern-day skimmers (e.g., right whales), bottom feeders (e.g., Gray whales), and the rorqual whales, which are lunge feeders (e.g., humpback and blue whales). Their hearing organs became adapted to send and receive long-range sounds, which became the basis for the melodic songs of modern species used for communication.

The mouth of a Gray whale with 300 baleen plates attached to the roof of their mouth to strain food from water and sediment. Photo by Christopher Swann/Minden Pictures.
Blue whales lunge feeding. Try to picture Sage’s sight of the Baleen feeding in a similar way but in a spiral fashion.

Toothed whales exploit the deep sea (34 mya-present), the Model for Other Thalassa Cetaceans

As baleen whales evolved ways to tap into the ocean’s abundant plankton, the ancestors of toothed whales developed sonar (echolocation) and became the largest predators on the planet. Echolocation involves emitting a series of clicks at varying frequencies using an expansion of the head to send sound waves, bounce them off potential prey or surroundings, and receive the signals with their elongated lower jaw. This key adaptation made them more efficient predators and allowed them to dive deeper in search of food which opened up the rich resources of the deep sea on earth (e.g, squid).

Illustration of echolocation in dolphins, from Lubis (2016).

On earth, the success of these early species eventually gave rise to dolphins and porpoises, sperm whales, killer whales, and beaked whales. On Thalassa, recall that the team from the Duke only had a short period of time (and one submersible dive) to explore its oceans. The open ocean, the deep sea, and the mysterious nearshore splashes Sage observed are all potential sources of new species to be found in the next book, Songs of Hina.

Interestingly, early sperm whales on earth, such as Livyatan, hunted other whales with their monster teeth. Could be a cool critter to include in the next book. Let me know what you think.

Livyatan, the sperm whales that hunted whales. Left: Livyatan jaws, center: illustration of mouth with teeth; right: preying on a whale.

Cetaceans’ Status and Conservation on Earth

Clearly, cetaceans have a spectacular evolutionary history of successfully invading the sea. Within 15 million years they went from a terrestrial lifestyle to a fully marine existence and are now the most aquatic and widely distributed of all marine mammals. Similar invasions of the sea by the marine, but coastal, manatees and dugongs (40 mya), the semi-aquatic seals and sea lions (24 mya), and the coastal sea otters (2 mya) and polar bears (130k) occurred but with less success.

From Cressey, 2015.

Tragically, these magnificent animals, have a dark history of human exploitation and most all are on the UN’s endangered species list. Pre-human global cetacean populations were…well, we don’t know and never truly will. Based on genetics, the current populations of the remaining great whales are estimated at >10% of their pre-contact population sizes in most species. Prior to whaling, Antarctic blue whales were thought to number about 250,000 individuals but were reduced to fewer than 400 animals by 1972 — about 1% of their former population size (Roman et al., 2014). As quoted by Halina in SOT, researchers estimate that in the 20th century alone, three million whales were killed by the whaling industry (Cressey, 2015).

Without a doubt, these magnificent, intelligent animals with their beautiful songs, amazing sonar capabilities, and role as ecosystem engineers which enhance the productivity of the world’s oceans, deserve our utmost respect and the highest level of protection. In an effort to promote their conservation, this is the principal reason they were included in SOT. Is Milo and Moshe’s treatment of the Nesoi typical of what we would expect after we discover a new species on a virgin planet? I leave you with this question and ask you to ponder the wisdom of Sage’s talk at the Oceanarium.

References and further reading:

  • Cressey, D. 2015. World’s whaling slaughter tallied. Nature 519: 140-141.
  • Gingerich, P. 2012. Evolution of Whales from Land to Sea. Proceedings of the American Philosophical Society. 2012 vol: 156 (3)
  • Lambert, O. et al. 2019. An Amphibious Whale from the Middle Eocene of Peru Reveals Early South Pacific Dispersal of Quadrupedal Cetaceans. Current Biology 29, 1–8, https://doi.org/10.1016/j.cub.2019.02.050.
  • Lubis, M. Z. 2016. Behavior and echolocation of male Indo-Pacific Bottlenose dolphins. In: Male Info-Pacific Bottlenose Dolphins Captive
  • in Indonesia. Chapter: 3, Publisher: Lap Lambert Academic Publishing, Editor: C. Evans.
  • Marx F, Fordyce R. 2015. Baleen boom and bust: A synthesis of mysticete phylogeny, diversity, and disparity. Royal Society Open Science, 2015 vol: 2 (4)
  • Marx F., Hocking D, Park T, Ziegler T, Evans A, Fitzgerald E. 2016. Suction feeding preceded filtering in baleen whale evolution. Memoirs of Museum Victoria vol: 75 pp: 1447-2554.
  • Marx, F., O. Lambert, and M.D. Uhen, editors. 2016..Cetacean Paleobiology (TOPA Topics in Paleobiology). Wiley Blackwell. 319 pp.
  • Roman et al. 2014 Whales as ecosystem engineers. Front Ecol. Environ. 12(7): 377–385, doi:10.1890/130220.
  • Steeman M, Hebsgaard M, Fordyce R, Ho S, Rabosky D, Nielsen R, Rahbek C, Glenner H, Sørensen M, Willerslev E. 2009. Radiation of extant cetaceans driven by the restructuring of the oceans. Systematic Biology. vol: 58 (6) pp: 573-585
  • Thewissen, J. G. M. 2014. The Walking Whales: from land to sea in eight million years. University of California Press. 245 pp.
  • Uhen, M. 2010. The Origin(s) of Whales. Annual Review of Earth and Planetary Sciences, vol: 38 (1) pp: 189-219.

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