Cetacean Evolution • The Cetacea probably originated in the Palaeocene, and had an Eocene differentiation. • We have 2 questions: • 1) From which mammalian group did the Cetacea evolve? • 2) Do the 2 modern suborders share a common ancestor?
Cetaceans: Evolution • The earliest cetacean fossils date to the Eocene of Pakistan and belong to the suborder: Archaeoceti. • Other early fossils are from the middle Eocene of Egypt and southern Nigeria. • These fossils are members of the suborder Archaeoceti (sometimes referred to as Zeuglodont).
This suborder includes Pakicetus and Ambulocetus, species associated with shallow seas. • The ancestral group, as we noted in our discussion of ungulates and subungulates, is probably the Condylarthran family Mesonychidae.
Cetaceans: Evolution • Recall that the Condylarthrans also gave rise to the subungulates and ungulates, particularly the Perissodactyla. Condylarthran Mesonychids were carnivorous - scavenging ungulates.
Cetacean Evolution • By the mid to late Eocene, most Archaeocetes were so specialized that they were probably not ancestral to the Odontocetes and Mystecetes. • Archaeocete skulls are characteristic of early Eocene Creodonts (ancestral group for the Carnivora - wait, what is going on?).
Cetacean Evolution • Archaeocete skull characteristics: • Slightly modified tribosphenic teeth. • Presence of turbinal bones • Incisors, canines, premolars, and molars are primitive: 3/3, 1/1, 4/4, 3/3. • Posterior extension of palate via pterygoid and palatines. • Sagital crest on parietals.
Cetacean Evolution • External nares lie halfway to orbit, inline with first premolars. • Rostrum is narrowed posteriorly. • Nasals are much narrower than Creodonts. • Now, what is the connection with the Creodonts?
Cetacean Evolution • In the Cretaceous and Paleocene, there was considerable differentiation in important mammalian groups, probably derived from the insectivores. • These groups were probably closely related to the Ungulata. • Suborder Arctocyonia was probably ancestral to the Ungulates.
Cetacean Evolution • A related order, the Mesonychia, was probably ancestral to the Cetacea (NOTE: taxonomy has changed - now the order containing the Arctocyonia and Mesonychia is the Condylarthra, containing the family Mesonychidae. • As early as 1969, VanValen (Evol. 23: 118-130) did serological studies demonstrating a close affinity between Artiodactyla and Cetacea.
Cetacean Evolution • Zeuglodonts (and perhaps all other Cetacea) probably diverged from the Mesonychidae at the end of the Cretaceous, taking to the sea in the early Paleocene. • Skulls of Zeuglodonts and Mesonychidae are very similar in cranial and dental characters. • Mesonychids were differentiated and widespread in the late Cretaceous.
Cetaceans: Evolution • Basilosaurus had functional hind limb elements. Other species were clearly transitional between terrestrial and aquatic. By the mid-Miocene, the Archaeoceti were fully aquatic.
2 Zeuglodonts: Basilosaurus and Zeuglodon osiris. Note the remnants of the pelvic girdle and hind-limb elements in Zeuglodon, elongation in Basilosaurus, dentition, and elongation of both skulls.
Cetacean Evolution • Conclusion: • Archeoceti (Zeuglodonts) probably diverged from Mesonychids at the end of the cretaceous. Mesonychids were closely related to the Arctocyonia, which probably gave rise to the Ungulates. Mesonychids actually gave rise to the Perissodactyla.
Colonization of the Sea • Early Zeuglodont fossils are associated with relatively restricted western arm of the Tethyan Sea (approximately Mediterranean - Persian Gulf) in the Paleocene, and dispersed through the warm shallow coastal waters of the greatly re-enlarged Tethys during the Eocene.
Colonization of the Sea • Tethys sea was shallow warm water basin throughout the Mesozoic. • During the Paleocene, western arm of Tethys became constricted and semi-enclosed. • Condylarthrans probably utilized riverbanks and shores of the Tethys, feeding on aquatic invertebrates and fish.
Colonization of the Sea • Natural selection may have favored those individuals which avoided intense inter- and intra-specific competition by foraging in deeper mud and waters. • Those individuals which had forms of variation which enabled them to exploit food resources in deeper waters probably had greater reproductive success.
Colonization of the Sea • Perissodactyls graze, and are limited by availability of food - or so we imagine. • Diversity of Perissodactyls was much greater in the Eocene than it is now. • Warm shallow seas are extremely productive for both plant and animals. • Foraging in shallow water makes sense if other resources are limiting.
Colonization of the Sea • If you forage in the water, what kinds of morphological attributes might be favorable? • Longer and narrower rostrum for use in catching fish. • Webbed appendages. • Migration of nares to top of the skull.
Major morphological developments in the transition from terrestrial to fully aquatic marine mammal.
Colonization of the Sea • Could a small rodent or insectivore have done this?
Colonization of the Sea • Why are there no transitional forms to bare out this hypothesis? • Evolutionary event took place over a very restricted area. • Event was probably very rapid (in geological time scale). • Fragmentation of skeletons after death. • Perhaps limited sediment deposition.
Cetaceans: Evolution • The transition to aquatic feeders is not difficult to imagine. It has been done before: • Ichthyosaurs • Plesiosaurs • Other reptile groups. • Aquatic reptilian groups went extinct by the end of the Cretaceous.
Colonization of the Sea • Last Archaeocetes are from the middle Miocene of France. • Early Odontocetes and Mysticetes were present in the middle Oligocene, and completely replaced the Archaeocetes by the middle Miocene.
Cetacean Evolution • Characteristics of the suborders with living representatives: • Resistance to lactic acid accumulation. • Tolerance of oxygen debt in muscle tissue. • High titre of muscle myoglobin for rapid transfer of oxygen to the cells. • Hypodermal blubber layer for energy storage, thermoregulation (?)
Cetacean Evolution • Oil storage in bones for energy. • Development of flukes for locomotion. • Development of dorsal fin for stability and thermoregulation in smaller forms. • External nares located on top of skull with means of sealing out water. • Modification of tracheal system and lungs to withstand high pressure.
Cetacean Evolution • Modification of the eyes to tolerate salt water and extreme pressure. • Modification of sound conducting routes and sound production routes. • Modification of dentition to reflect a filterfeeding or piscivorous diet.
Genital grooves in a) male and b) female. Forelimbs of c) pilot whale, d) right whale, and e) human.
Cetaceans: Evolution • Unresolved is the question of how the 2 extant cetacean suborders are related to one another, or how either suborder is related to the Archaeoceti. • Are they polyphyletic? Probably not. • Odontocetes and Mysticetes are clearly differentiated by the Oligocene.
Evolutionary Patterns Within the Odontoceti • How do odontocetes differ from the Zeuglodonts? • Odontocete lachrymal bones abut onto the ventral area of the maxillaries, not on to their lateral surfaces. • The maxillaries have migrated posteriad to lie over the supraorbital region of the frontal bones.
Evolutionary Patterns Within the Odontoceti • Significant telescoping of skull with accomodation for melon, nasal diverticula, and spermaceti organ associated with sound production and sound reception. • Odontocetes have homodont dentition.