Evolution of marine mammals: Back to the sea after 300 million years

Abstract

The fossil record demonstrates that mammals re-entered the marine realm on at least seven separate occasions. Five of these clades are still extant, whereas two are extinct. This review presents a brief introduction to the phylogeny of each group of marine mammals, based on the latest studies using both morphological and molecular data. Evolutionary highlights are presented, focusing on changes affecting the sensory systems, locomotion, breathing, feeding, and reproduction in Cetacea, Sirenia, Desmostylia, and Pinnipedia. Aquatic adaptations are specifically cited, supported by data from morphological and geochemical studies. For example, analysis of oxygen isotopes incorporated into fossil tooth enamel indicates whether these mammals foraged in (and, therefore, ingested) fresh water or sea water. Comparisons between groups are made to see if there are any common patterns, particularly relating to adaptations to aquatic life. Results show that aquatic characteristics evolved in mosaic patterns and that different morphological solutions to aquatic conditions were achieved separately in each of these groups. Changes in the axial and appendicular skeleton assist with locomotion for aquatic foraging. Nostril and eye placement modifications accommodate wading versus underwater foraging needs. All groups exhibit aquatic adaptations directly related to feeding, particularly changes in the dentition and rostrum. The earliest representatives of these clades all show morphological features that indicate they were feeding while in the water, suggesting that feeding ecology is a key factor in the evolution of marine mammals.Anat Rec 290:514-522, 2007.2007 Wiley-Liss, Inc.Key words: cetacea; sirenia; pinnipedia; carnirora; desmostylia; feeding ecology; aquatic adaptation A recurrent theme in tetrapod evolution is a return to an aquatic habitat. Whereas amphibians never quite parted with the water, all other tetrapod groups (reptiles, birds, and mammals) have several representatives that have returned to the water for at least an amphibious lifestyle of some degree, if not a fully aquatic existence. Mammals in particular have returned to the water in at least seven separate lineages (Cetacea, Sirenia, Desmostylia, Pinnipedia, Ursus maritimus [polar bear], Enhydra lutris [sea otter], and Thalassocnus spp. [aquatic sloths]). Some of these lineages have retained most of their terrestrial form while spending a great deal of time in the water, whereas others have changed their morphology dramatically and spend almost all, if not all, of their time in the water.Before exploring how the anatomical changes for life in water came about, one must know how marine mammals are related to one another, and particularly to their terrestrial relatives. A brief introduction to the phylogeny of each group of marine mammals is presented here, based on the latest studies using both morphological and molecular data. CETACEAN PHYLOGENYThe relationships of cetaceans to terrestrial mammals and other marine mammals was a great mystery for many

Examining predator–prey body size, trophic level and body mass across marine and terrestrial mammals

“…These characteristics make mammals ideal to investigate patterns in trophic level. Mammals have re-entered the marine environment on seven separate occasions, and there are five extant clades: Cetacea, Sirenia, Pinnipedia, Ursus maritimus and Enhydra lutris [3]. This provides a unique opportunity to explore the possible changes that have occurred as mammals moved into an environment where not only physiological and morphological modifications have taken place, but also additional behavioural changes associated with foraging ecology.…”

Genital morphology of the male South American fur seal (Arctocephalus australis) and biological implications

“…Reviews of anatomy and its relationship with biology and ecology have been recently published for some marine mammals (Reidenberg 2007, Uhen 2007. The relationship between morphology and seasonal functionality of the reproductive organs in the Otariidea family has been scientifically demonstrated (Colegrove et al 2009a, Colegrove et al 2009b.…”

Comparative feeding kinematics and performance of odontocetes: belugas, Pacific white-sided dolphins and long-finned pilot whales

“…This transition resulted in a radiation of adaptations for feeding (O'Leary and Uhen, 1999;Uhen, 2007), and feeding modes that presumably converge with feeding modes of basal aquatic vertebrates. Among cetaceans, certain lineages have specialized for filter feeding (baleen whales, suborder Mysticeti), biting (killer whales, Orcinus orca), ram feeding (some delphinids) and suction feeding (kogiids, porpoises, belugas and some delphinids) (Kastelein et al, 1997;Werth, 2000a;Bloodworth and Marshall, 2005).…”