FIG. 2c Bathymetrical sketch-chart of the Atlantic Ocean
CHAPTER 2 EVOLUTION AND THE NORTH ATLANTIC SEA-BIRDS
GEOLOGISTS DIFFER in their opinions of the origin of the Atlantic Ocean. The followers of the geomorphologist Alfred Wegener believe that it is a real crack in the earth’s crust whose lips have drifted away from each other, and this opinion is lent verisimilitude by the neat way in which the east coast of the Americas can be applied to, and will fit with extraordinary exactitude, the west coast of Europe and Africa. It must be stated that, while the present opinion of most geographers is that the resemblance of the Atlantic to a drifted crack is purely coincidental, this is not shared by all students of animal distribution and evolution, some of whom, find the Wegener theory the most economical hypothesis to account for the present situation.
Whatever the truth is, there is no doubt that the boundaries of the Atlantic, and their interconnections, have varied considerably; thus halfway through the Cretaceous Period, about ninety million years ago (during this long period nearly all the principal orders of birds evolved), there were bridges between Europe, Greenland and Eastern North America cutting the Arctic Ocean from the North Atlantic completely; and from then until the late Pliocene—perhaps only two million years ago—there was no continuous Central American land bridge, but a series of islands.
Our present knowledge of the tree of bird evolution owes much to Alexander Wetmore and his school, who have so notably added to our knowledge of fossil birds during the last twenty years, especially in North America. Birds do not appear very frequently in the sedimentary rocks—their fossil population does not generally reflect their true population in the same way as that of mammals is reflected. However, if land-birds are rare in the beds, water-birds are relatively common, and the periods and epochs in which all our sea-bird orders, and many of our sea-bird families and genera, originated are quite well known. A recent paper by Hildegarde Howard (1950), of the school of Wetmore, enables us to show a diagrammatic family tree of birds (Fig. 3), with special reference to sea-birds, and to collate its branching with the approximate time scale of the epochs, so cleverly established by geomorphologists in recent years from studies of sedimentation-rate and the radioactivity of rocks. It will be seen that the primary radiation of birds and the great advances into very different habitats consequent upon the first success of the new animal invention—feathered flight—took place in the Cretaceous period, the first birdlike feathered animals having been found as fossils in Jurassic deposits of the previous period, over a hundred and twenty million years old. In the Cretaceous period—the period of reptiles—ostriches were already foreshadowed, as were grebes and divers, and the pelican-like birds, and the ducks.
In the Cenozoic period—the period of mammals—the radiation of birds into all nature’s possible niches continued rapidly, especially in the first two of its epochs—Eocene and Oligocene—from sixty to thirty million years ago. In these epochs grebes can be distinguished from divers, and a bird of the same apparent genus (Podiceps, or, as the North Americans have it, Colymbus) as modern grebes has been found. Gannet-boobies of the modern genus Sula have been found in the Oligocene, as have cormorants of the modern genus Phalacrocorax. The only penguin fossils known are later—of Miocene age—but it seems probable that they share a common stem with the tubenoses, which would mean that their ancestors branched off in the Eocene. The tubenoses diversified in the Oligocene—from this epoch we have a shearwater of the modern genus Puffinus; and from the Miocene Fulmarus and albatrosses. The ducks started their main evolution in the Cretaceous, and by the Oligocene we find modern genera such as Anas (mallard-like) and Aythya (pochard-like); in the Pliocene we have Bucephala (Charitonetta)—one of the tribe of sea-ducks.
For the Lari-Limicolae, the order which includes waders, gulls and auks, the fossil record is rather indefinite, mainly owing to the difficulty of distinguishing the present families by bones alone. However, we know that the auk family was early—an Eocene offshoot; that the waders and gulls diverged in the Oligocene; and that the gulls, terns and skuas probably diverged in the Miocene—which means that an important part of the adaptive radiation of this order was comparatively late. One of the early auks, the Pliocene Mancalla of California, out-penguined the great auk, Alca (Pinguinus) impennis, for it had progressed far beyond it in the development of a swimming wing.
FIG. 3
Diagrammatic family tree of sea-birds, mainly after Hildegarde Howard (1950)
According to Howard (1950) a few living species of birds have been recorded from the Upper Pliocene, but large numbers of modern forms occurred in the Pleistocene. Of course in the Pleistocene the oceans approximated very closely to what they are today, with the Central American land-bridge closed, the Norwegian Sea wide open between Arctic and Atlantic Oceans, the Mediterranean a blind diverticulum of the North Atlantic. We need this picture as a background to a consideration of the North Atlantic’s present sea-bird fauna, for we shall find that it has few sea-bird species of its own, and only two genera; for the primary sea-bird species which now breed in the Atlantic (and Mediterranean) and in the neighbouring parts of the Arctic, and nowhere else in the world, are no more than twelve: the Manx shearwater Puffinus puffinus*; the very rare diablotin and cahow of the West Indies and Bermuda (Pterodroma hasitata and P. cahow); the storm-petrel Hydrobates pelagicus; the North Atlantic gannet Sula bassana; the shag Phalacrocorax aristotelis; the lesser black-back Larus fuscus; the great blackback L. marinus; the Mediterranean gulls L. melanocephalus and L. audouinii; the Sandwich tern Thalasseus sandvicensis; the razorbill Alca torda, the puffin Fratercula arctica; besides the extinct Alca impennis, the great auk. The two present genera peculiar to the North-Atlantic-Arctic are Hydrobates and Alca.
The sea-birds which qualify by birth and residence to be members of the North Atlantic fauna (excluding purely Arctic and Mediterranean species) include thirteen tubenoses, seventeen cormorant-pelicans, fourteen gulls, nineteen terns, two skimmers, four skuas and five auks (besides various secondary sea-birds, notably about eighteen ducks, three divers and two phalaropes). If we are to understand how these have got into the North Atlantic we should analyse the present distribution of the sea-bird orders and groups as between the different oceans.
The most primitive group of sea-birds, yet the most specialized, is that of the penguins. The Sphenisci have fifteen species in all, of which eight breed in the South Pacific, seven in the Antarctic Ocean, five in the South Atlantic and two in the Indian Ocean. One (and one only) reaches the Equator, and thus the North Pacific, at the Galapagos Islands. No live wild penguin has ever been seen in the North Atlantic.* It seems certain that the evolution of this order of birds has taken place in Antarctica and in the neighbouring sectors of the South Pacific.
The great order of Tubinares the albatrosses,