Factors That Drove Cetacean Ancestors to the Sea

 

Introduction:

A series of climatic and biogeographic feedbacks could potentially explain some of the factors that drove this family of once-terrestrial predators to the seas. This essay will discuss the climatic variables constituent during the late Paleocene to early Eocene that precipitated cetacean ancestor’s exodus from land to water. A series of teleconnections—or long distance interconnected anomalies resultant from climatic perturbations—will be examined and discussed. Additionally, geologic processes—namely, the position of the continents—of the time period need to be evaluated. Together, the climatic variables as well as the geologic occurrences at the time of this terrestrial-aquatic transition might explain why cetacean ancestors moved to the water.

The Paleocene-Eocene Thermal Maximum that occurred 55 million years ago was a unique event in Earth’s climatic history. During this time, sea surface temperatures of the Tethys Sea—which was located near present-day India, Pakistan, and Saudi Arabia—would have been between 32-34 degrees Celsius (Figure 1)—ten degrees higher than modern SST’s (Winguth et al 2012). Such high SST’s would undoubtedly facilitate the thermal expansion of the oceans. According to Sluijs et al. (2008), sea levels during this time would have been 70-80m higher than they are currently (p. 4216) and modeling by Miller et al. (2008) even suggests that the sea levels at the beginning of the Eocene, at which point global annual temperatures were at about 23 degree Celsius (Figure 2), reached a maximum of 150m above current levels (p. 211). It is at this point that the expanded seas likely made contact with inland freshwater sources and began mixing.

With the oceans expanding and encroaching on inland water sources, interaction between the saline and freshwater ecosystems undoubtedly occurred. As oxygen isotopes have shown, cetacean ancestors spent considerable time feeding in the shallows of river and semi-marine systems  (Gingerich et al. 2001, p. 2240). The brackish environments that were likely created as a result of the heightened sea levels facilitated vegetative growth and diversification to the point that—over a period of time—the ancestors may have undertaken prey switching. By this time, it is also likely that the ancestors had undergone morphological transitions that are more in keeping with an aquatically adapted organism. The ancient cetacean ancestor that is heralded as the vanguard for cetacean ancestry is Pakicetus. According to Thewissen and Williams, pakicetid fossils are consistently located in now dry river channel deposits and they account for 60% of the animal fossils recovered in those deposits (p. 77).  The later adaptation known as Ambulocetus, is commonly found in near-shore marine deposits (Thewissen & Williams 2002, p. 79). This lends support to the idea that the heightened sea levels created pathways through which the slowly evolving cetacean ancestors could find their way to open water.

Given the extent of estuarine productivity, it is likely that pakicetids occupied estuarine environments. Estuarine productivity is undoubtedly an attractive ecosystem from which predators can feed. Once freshwater and saline water resultant from the expanded bodies of ocean water began mixing and began yielding productivity, pakicetids began their slow transformation. Isotopic records of fossil remains recovered from Balochistan Province,Pakistan show that the morphological divergence in the artiodactyl family began during the Eocene temperature maximums (Figure 3 and Figure 4) (Gingerich et al. 2001, p. 2239).

The climatic conditions in the Eocene resulted in the highly productive estuaries and warm, broad, shallow seas which produced nutrient rich environments that shaped and altered the feeding behavior of early cetaceans. This new dietary behavior change drove aquatic evolutionary adaptations and progressions that eventually facilitated the transition into marine environments, completely independent from land. Fossil records show that mammals have re-entered marine environments on seven separate occasions. Uhen 2007 proposes that though each of these groups adapted different morphological solutions for marine conditions, all groups exhibit aquatic adaptations directly related to feeding. Ancestors of the cetacea entered the marine environment during the Eocene which was a time of high productivity in aquatic environments, with warm, broad, and shallow seas (Uhen 2007). Later mammalian clades such as the Desmostylia and Pinnipedia entered aquatic environments during the Oligocene, a period of rapid glaciation which dramatically changed global oceanic circulation, temperature gradient from the equator to the poles, and lowered the sea level affecting shallow oceans (Uhen 2007) removing many archipelago-like aquatic systems. These changes during the Oligocene shifted the productivity of the marine environments most likely forcing the Desmostylia and Pinnipedia mammalian clades to form feeding habits and behaviors still reliant on terrestrial environments, being less conducive for a complete transition from land to the sea. Had the Oligocene been more productive and conducive for a complete transition to marine environments, the Desmostylia and Pinnipedia clades, like the cetaceans, might also have completely become independent from land. The dietary behavior that drove aquatic adaptations in mammals was an outcome of the productivity and resource availability in marine environments which was a direct result of the climate. The evolutionary progression, driven by dietary behavior, which led to modern cetaceans (Thewissen et al 2011) was due to the high productivity in the marine environments, which was a direct result from the climate of the Eocene.

By the end of the Eocene the cetaceans had become completely independent from land. (Uhen 2007). Through a series of key evolutionary steps, driven by climate change and feeding behaviors, these land dwelling quadrupedal ungulates were able to solidify their expansion into marine environments, further exploit the resources of aquatic ecosystems, and eventually never return to land. One of the first evolutionary steps that led the cetaceans toward a life independent from land was the shrinking of the semicircular canal system, one of the main sensory organs located in the inner ear that records angular motion of the head and is important in neural control and locomotion (Spoor et al 2002). By body mass the modern cetacean is comparatively much more agile than terrestrial mammals, has much less neck mobility, and requires much less sensory from this organ to match fast body rotations that characterize cetacean behavior (Spoor et al 2002). This adaptation of a small semicircular canal system is the first cetacean organ to reach modern morphology, and evolutionary speaking, adjusted instantaneously opposed to the gradual physical modifications which took much longer. This adaptation appeared in the early to mid-cetacean families Ambulocetidae and Remingtonocetidae (figure 5), just five million years into the origin of the cetacea order and represents the “point of no return” in early cetacean aquatic evolution (Spoor et al 2002).

The next step of intermediate adaptations to aquatic life occurred in the mid Eocene 55.8-48.6 mya involving the families Remingtonocetidea and Protocetidae (figure 5). These early families of cetacea began the early development of many important aquatic traits such as the shift of the nasal cavity from the tip of the snout to the top of the head (Bajpai et al 2011) and displayed the first genuine underwater ear (Nummela 2007). Dramatic structural and physiological evolutionary shifts and adaptations also began with these groups; however, the morphology was much slower and occurred over a much longer period of time. These adaptations include a physical and structural shift in the neck, spine, vertebrae, pelvis, and legs that allowed for faster and more efficient movement in the water. The radical shift from paraxial locomotion to axial locomotion was completely necessary for increased agility, efficiency, and movement in water (Buchholtz 2000). Structural evolutionary adaptations such as extension of the spine, increase in spacing between the vertebrae, increase in vertebrae count, shrinking of the chest region, conversion of thoracic vertebrae into lumbar, reducing vertebrae centrum length are all aquatic adaptations that enhance the hydrodynamic shape, efficiency, flexibility, and mobility in water (Buchholtz 2000). Multiple types of mammals took advantage of the very nutrient rich aquatic environments as a result of the climate in the Eocene, but these important intermediate evolutionary steps allowed the cetacean ancestors to developed unique adaptations that allowed them to further exploit the resources and minimize reliance and eventually abandon land.

With an increased evolutionary emphasis on swimming, the development and subsequent elimination of limbs is one of the most unique factors in the development of modern whales. The pakicetid and many of its subsequent diverging families still featured limbs, but this dramatically changed with the basilosauridae. The step from pakicetid to protocetid to basilosaurid represents the most pivotal steps in the evolution of cetaceans from land dwelling mammals to aquatic cetaceans.

The first phylogenic family of cetaceans to display a strong swimming ability were the protocetids, which existed about 47 to 41 million years ago during the mid eocene. Fossil evidence of protocetids have been discovered in Africa, North America, and South America. Their skeletons showcase the presence of strong hind limbs aided by a powerful tail. This, combined with their ability to colonize the world’s oceans implies that they were strong swimmers. The diet of protocetids is believed to have been a combination of plants and animals. The structure of their jaws and the robustness of their teeth hints at an omnivorous diet. However, the protocetids were not primarily ocean dwelling. Fossils found in carbonate platforms indicated by sediments show that protocetids dwelled mainly in shallow, clear, and relatively warm water (Bajpai et al. 676, 2009).

Following protocetids, basilosaurids came to dominate the oceans in the late Eocene. While the protocetids were still able to walk on land, the basilosaurids were obligately aquatic. They had tiny hind limbs and flipper shaped forelimbs. They swan in a similar manner to protocetids, but with more emphasis on powering trough the water with their tail and forelimbs. (Thewissen 2002) While having bodies very similar to modern cetaceans (figure 6), they still lacked the specializations of echolocation and filter-feeding, as well as a fluke on their tail. (Thewissen, 2009)

The adaptions of filter-feeding and echolocation are not entirely understood, but it is observed that they have played a major role in the development of body-type of cetaceans. In some species of Basilosauridae, it has been found that they had “lost” one tooth in each upper jaw, bringing their total number of teeth to 42. This is interesting in that their molars differed from other early cetaceans in the lack of a central depression surrounded by three cusps in the upper molars. As a result, those molars were not suitable for crushing food (Thewissen 2009). This may be point to an early stage in the development of filter-feeding.

While species of basilosauridae preferred tropical and sub-tropical waters (Thewissen, 2009), modern cetaceans have shown to have the largest range of any mammal, both in latitude and longitude, with the exception of humans (Croll, 2008).

The development of filter-feeding favored a larger body-type, leading to cetaceans specializing in this method of foraging to become much larger in size than their smaller, hunter cousins. This allows them to go extended periods without food when the densities of schooling prey in the summer at the mid-latitudes all but disappear in the winter months. Their size allows them the blubber needed to survive in the frigid waters in the higher latitudes (Croll, 2008).

Climate during and following the Paleocene-Eocene Thermal Maximum undoubtedly acted as a driving force for early cetacean evolution. Higher global temperatures, the thermally expanded oceans, and the position of the continents at the time of the Thermal Maximum drove cetaceans from the land to the water. 

 

References:

 

• Gingerich, P.D., (2003). Land-to-Sea Transition in Early Whales: Evolution of Eocene Archaeoceti (Cetacean) in Relation to Skeletal Proportions and Locomotion of Living Semiaquatic Mammals, Paleobiology, 29(3), 429-454.

 

• Gingerich, P.D., Haq, M., Zalmout, I.S., Khan, I.H., Malkani, M.S., (2001). Origin of Whales from Early Artiodactyls: Hands and Feet of Eocene Protocetidae from Pakistan, Science, 293(5538), 2239-2242.

 

• Winguth, A., Winguth, C., Franklin, M., Cope, J., Wright, A., Asher, V. (2012). Paleocene Eocene Thermal Maximum. Retrieved from University of Texas Arlington, Climate Research Group, http://www.uta.edu/faculty/awinguth/petm_research/petm_home.html

 

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• Thewissen, J.G.M., & Williams, E.M. (2002). The Early Radiations of Cetacea (Mammalia): Evolutionary Pattern and Developmental Correlations, Annual Review of Ecology and Systematics, 33, 73-90.

 

• Uhen, M. (2010). The Origin(s) of Whales, The Annual Review of Earth and Planetary Sciences, 38, 189-219.

 

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• Uhen, M. D. (2007). Evolution of Marine Mammals: Back to the Sea After 300 Million Years. The Anatomical, 290(6), 514-22.

 

• Thewissen, J. G. M., Sensor, J. D., Clementz, M. T., Bajpai S. (2011). Evolution of dental wear and diet during the origin of whales. Paelobiology, 37(4), 655-669.

 

• Spoor, F., Bajpai, S., Hussain, S.T., Kumar, K., Thewissen, J. G. M. (2002). Vestibular evidence for theevolution of aquatic behavior in early cetaceans. Nature. 417(6885), 163-166.

 

 

• Buchholtz, E. A. (2000) Vertebral osteology and swimming style in living and fossil whales (Order: Cetacea). Journal of Zoology, 2001, Vol.253(2), pp.175-190

 

• Bajpai, S., Thewissen, J. G. M., Conley, W. (2011). Cranial anatomy of middle Eocene Remingtonocetus (Cetacea, Mammalia) from Kutch, India. Journal of Paleontology, 85(4), 703-718.  

 

• Nummela, S., Thewissen, J. G. M., Bajpai, S., Hussain, T., Kumar, Kishor. (2007). Sound transmission in archaic and modern whales: anatomical adaptations for underwater hearing. Anatomical Record, 716-733.

 

• Croll, Donald A., Tershy, Bernie R., and Newton, Kelly M. (2008) Filter Feeding. Encyclopedia of Marine Mammals. 429 – 432

 

• Thewissen, J.G.M., and Williams, E.M., (2002) The Early Radiations of Cetacea (Mammalia): Evolutionary Pattern and Developmental Correlations. Anni. Re. Ecol. Syst. 33:73-90 

 

• Thewissen, J.G.M., Cooper, Lisa N., George, John C., Bajpai, Sunil, (2009) From Land to Water: the Origin of Whales, Dolphins, and Porpoises. Evo Edu Outreach 2:272-288