Table 8. Summary of changes in morphology, performance, environment, and locomotor mode during the Eocene evolution of cetacea.

Family Group	Representative Taxon	Age	Morphological Changes	Performance Changes	Environment	Predominant Locomotor Mode
Delphinidae	Orcinus orca	Recent	Shortened compact body	Lift-based propulsion	Fully aquatic	Constrained dorsoventral
			Development of tail fluke			caudal oscillation
			Loss of external hind limbs
Basilosauridae	Dorudon atrox	36 Ma	Elongated lumbus and tail	Reduced drag	Fully aquatic	Mid-body through tail
			Reduced hind limbs	Increased efficiency		dorsoventral and lateral (?)
			Flattened tail at tip			undulation
Protocetidae	Aegicetus gehennae	38 Ma	Elongated lumbus and tail	Reduced drag	Fully aquatic	Mid-body through tail
(Georgiacetinae)			Enlarged forelimbs	Increased efficiency		dorsoventral and lateral (?)
			Loss of pelvic articulation			undulation
Protocetidae	Maiacetus inuus	46 Ma	Enlarged hind feet	Drag-based propulsion	Semiaquatic	Alternate pelvic paddling with
(Protocetinae)			Interdigital webbing			caudal stabilization
			Tail as inertial stabilizer
Pakicetidae	Pakicetus attocki	48 Ma	Larger body size	Drag-based propulsion	Semiaquatic	Quadrupedal paddling with
			Elongated phalanges			caudal stabilization
			Interdigital webbing		7
Dichobunidae	Diacodexis ilicis	56 Ma	Small body size	Ricochetal cursor	Terrestrial	Terrestrial quadruped
(Diacodexeinae)			Elongated metapodials			with dorsoventral
			Hoofed terminal phalanges			flexion and extension