Marine centres of origin as evolutionary engines

Abstract

The world ocean supports a dynamic system in which living organisms undergo constant movements. Although some would appear to be sedentary, all are capable of invading new territory at some stage in their life cycle. Underlying these comparatively rapid changes is a much slower evolutionary system whereby new species are formed and spread out. Depending on their place of origin and genetic resources, some of the new species may give rise to continuing phyletic lines. At the same pace, some older species approach extinction by continuing to lose territory. Over time, this evolutionary system appears to be no less dynamic than the contemporary one. In recent years, considerable attention has been paid to the East Indies as a centre of origin for the marine tropics. While other centres of origin in the Antarctic and the North Pacific have been recognized, little attention has been paid to their external influence. Yet in the cooler waters of the oceans, they are as important to those areas as the East Indies is for the tropics. Evidence indicates that evolutionary flows from all three centres contribute to a dynamic system that extends throughout the world ocean. Each of the three centres and its salient features is discussed in turn, the information is summarized, and then stated in the form of a hypothesis. Antarctica has a long history of harbouring a temperate marine fauna. That assemblage first become apparent in the Devonian when the Antarctic continent was still attached to the southern part of Gondwana. At that time, Antarctica belonged to the Malvinokaffric Realm that was distinguished by its lack of many common tropical animal groups (Boucot, 1988; Crame, 1994). This latitudinal position, suggesting the presence of relatively cool conditions, continued throughout the Mesozoic and into the early Tertiary. From the early Cretaceous to the Oligocene, a warm-temperate Weddellian Province extended from the tip of South America to Antarctica and to Australia/New Zealand. That Province disappeared with the development of colder temperature and the Antarctic Circumpolar Current (Crame, 1999). A dramatic change in the temperature zonation of the earth began with the global climatic deterioration of the middle Eocene. A new cold-temperate zone, with winter temperatures from 12 °C down to 2 °C, began to form at the highest latitudes. As a consequence, the warm-temperate waters were forced away from the poles and into lower latitudes. At one time, there was a general belief that Antarctic ice sheets did not form until the early Miocene when there was a circulation of a deep current through Drake Passage between Antarctica and Australia. But there is now evidence that ice sheets probably first formed in the Eocene c. 42 Ma (Keller et al., 1992). At Seymour Island, the La Meseta Formation of the middle-late Eocene has yielded a rich fauna of marine invertebrates as well as fossils of fishes, penguins, whales and marsupials. Some of the bivalves and decapods represent first occurrences of taxa that appeared later on at lower latitudes (Crame, 1994). At first, the Antarctic ice may have waxed and waned with minor climatic cycles, but by late Oligocene time, c. 25 Ma, it appears that a major icepack grounding event took place (Bartek et al., 1992). This is consistent with terrestrial evidence indicating the presence of glaciers and tundra by the early Oligocene (Janis, 1993). The presence of a major icepack in the Antarctic in the late Oligocene suggests that the modern, cold sea surface temperature regime (+2 °C to −2 °C) had become established. By that time, much of the rich Eocene fauna had disappeared, except for animals such as the penguins, whales, fishes, and several of the bivalves and gastropods that remained to become ancestral elements of the modern marine fauna. Late Oligocene–early Miocene fossils from other parts of Antarctica show surprisingly close resemblances to living taxa (Clarke & Crame, 1989). The shelf waters of the Antarctic and Sub-Antarctic are now occupied by a highly distinctive fauna that owes its origin to four historical factors: (1) persistence of a small ancestral group of Mesozoic and early Cenozoic taxa, (2) extinction of many early Tertiary warm-temperate species, (3) geographical isolation produced by the opening of Drake Passage, and (4) invasions by cold-temperate species from the northern hemisphere. The Antarctic Biogeographical Region is divisible into three provinces. The South Polar Province includes the Antarctic continent itself plus the South Shetland, South Orkney and South Sandwich Islands. South Georgia Island and Bouvet Island have many endemic species and each is placed in a province of its own. The Sub-Antarctic is the one biogeographical region on the world that is entirely made up of small, oceanic islands. Within that Region, The Kerguelen Province includes not only Kerguelen Island itself, but also the McDonald, Heard, Prince Edward and Crozet Islands. Macquari