An Amphibious Whale or a Terrestrial Swimmer?

A recent article published by Olivier Lambert et al.[1] reports on the finding of a new fossil vertebrate in the Peruvian desert that has been named Peregocetus pacificus. The fossil was discovered in fine-grained sediments of the Yumaque Member of the Eocene Caballas Formation in the East Pisco Basin. From the very beginning of the article, the authors describe the fossil as a “cetacean”, a “protocetid cetacean”, a “four-limbed whale” and “the first indisputable quadrupedal whale record from the Pacific Ocean and the Southern Hemisphere.”

Protocetids are a diverse group of extinct tetrapod vertebrates known from Europe, Asia, South and North America, and Africa, with long and strong fore and hind limbs that supported their body on land. It is thought that they also lived amphibiously as they have some traits that suggest they were adapted to amphibious life.

The newly reported fossil consists of a partial adult skeleton of a protocetid “preserving the mandibles and most of the postcranial skeleton.” An examination of the skeletal parts of Peregocetus clearly suggests that this was a terrestrial animal with some capability to swim in water or walk in areas of shallow waters. The vertebrae and limbs are clearly designed to support the weight of the body on land. Tail vertebrae have bifurcated flat transverse processes, which are similar to those observed in terrestrial animals capable of swimming, like otters, and beavers.

Since the finding of this fossil was first published in the scientific literature, numerous reports have come out in popular online magazines and science news with claims nearing the assertion that this is the perfect fossil link between a terrestrial artiodactyl the size of a large dog and fully swimming whales.[2] Other fossil protocetids have been found in Pakistan, so this specimen in Peru is remarkable because it is interpreted to show that land animals made their way from Asia to the Pacific coast of South America while in the process of gradually evolving into whales.

Our experience in the study of fossils tells us to be cautious with these kinds of claims. First of all, the fossil skeleton found in Peru is, as the authors say, a terrestrial animal with amphibious capacity. Having traits indicative of amphibious lifestyle does not make a fossil an intermediate form. Several fossils of sea lions and seals (Fig. 1) have been found in rocks above the layers containing the Peregocetus pacificus.[3] Those animals are amphibious as well, and display a combination of adaptations for both terrestrial and aquatic lifestyle, as Peregocetus does. However,  we have modern species of sea lions and seals, showing that they represent a functional and stable body plan adapted to a specific lifestyle rather than intermediates in an evolutionary sequence. Because we don’t have a modern animal correlative of Peregocetus, some find it convenient to place it in an evolutionary sequence from land to water mammals instead of considering the alternative of functional design.

Fig. 1: An example of exquisitely preserved fossil seal recovered from the sediments of the PIsco Basin, Peru.
Fig. 1: An example of exquisitely preserved fossil seal recovered from the sediments of the PIsco Basin, Peru.

But what about the presence of traits that seem to indicate a mixture of characteristics? Extinct animals often display mosaic traits, which are a combination of features typically found in different groups of animals. The bird Archaeopteryx and the amphibian Tiktaalik are two examples of mosaic fossils. The extant mudskipper (an amphibious fish) is also an example of mosaic animal. Rather than seeing these mosaic forms as transitional, they should be evaluated as representing specific combinations of behavioral/physiological adaptations. In the case of “transitions” from sea to land, the word “amphibious” applied to the mudskippers and Peregocetus must not be understood as a behavior or anatomy in evolutionary transition from one type of environment to another, but meaning a different and specific lifestyle that is independent of evolution.

Having the capacity to swim does not make an animal a transitional form from land to water. Peregocetus was a terrestrial animal with the capacity to venture into the water. However, this capacity must have been somehow limited and very different from prolonged submergence or complex subaqueous motion. For the latter, mammals need to have a vertebral column with unfused vertebrae, as modern whales have. Peregocetus lacked such feature, showing fused sacral vertebrae. The article indicates that the first three caudal vertebrae “show unleveled anterior and posterior epiphyses, indicating that the tail was ventrally bent just behind the sacrum, leading to a tail not as horizontal as in fully aquatic cetaceans.”

The concept of transition from land to a fully aquatic environment entails extreme changes in the behavior, anatomy and physiology of an animal. Many body systems would require a substantial overhaul with profound changes. The respiratory, circulatory, renal, and reproductive systems of fully aquatic animals would have to be very different from their putative land ancestors. How would the alleged transitional forms work? How would a transitional heart, kidney, or eyes and ears work? In a design perspective these adaptations are the product of intelligence. The genetic changes involved in going from a fully terrestrial or even amphibious animal to a fully aquatic animal like whales would have to be coordinated to a level that defies statistical imagination.

A creationist perspective interprets Peregocetus as an extinct animal, like many others occurring in the sediments of the Pisco Basin, that has a unique combination of traits not commonly found in fossils. Its putative amphibious behavior needs not to be considered transitional in any sense. In the layers of the Pisco Basin, there are multiple fossil specimens of amphibious animals (sloths, sea lions, seals, crocodiles) that represent well documented stable body plans and ecological strategies. I suppose that if one assumes a priori the theory of evolution and the idea that cetaceans evolved from land animals, then finding a fossil like Peregocetus with a combination of land and aquatic traits fits the needs of the theory. But what if we don’t assume evolution? Then Peregocetus becomes just what it was: an animal that lived on land, derived from one of the many kinds that God created, that occasionally would venture into water to fish or bathe, and, in the case of our specimen, was carried over (alive or dead) by a water current until it sunk to the seafloor, to be buried in sediment.

REFERENCES

[1] Lambert, O., Bianucci, G., Salas-Gismondi, R., Di Celma, C., Steurbaut, E., Urbina, M., and de Muizon, C., 2019, An Amphibious Whale from the Middle Eocene of Peru Reveals Early South Pacific Dispersal of Quadrupedal Cetaceans: Current Biology (2019), https://doi.org/10.1016/j.cub.2019.02.050.

[2] See for example, Kimberly Hickok, Ancient Four-Legged Whale Swam Across Oceans, Walked Across Continents, LiveScience, livescience.com, accessed 4 April 2019; Michael Le Page, Amazing four-legged fossil shows how walking whales learned to swim, New Scientist, newscientist.com, accessed 4 April 2019.

[3] Muizon, C.d., 1978, Arctocephalus (Hydrarctos) lomasiensis, subgen, nov. et nov. sp., un nouvel Otariidae du Mio-Pliocene de Sacaco, (Perou): Bulletin de L'Institut Francais D'Etudes Andines, Tome XV, Muizon, C.d., 1986, Un nouveau Phocoenidae (Odontoceti, Mammalia) du Miocene superior de la Formation Pisco (Perou): Comptes Rendus Academy Science Paria, v. 303, p. 1509-1512.