The ammonoids are a sub-class that forms, with the sub-class of the nautiloids and that of the colloids, the cephalopod class. Squids are cephalopods. The ammonoids were sea dwelling animals that formed successive shells made of calcium carbonate. These shells increased in size during growth along a regular spiral situated in one plane. The ammonites originated about 425 million years ago at the end of the Ordovician period (fig. 5.1). They thrived during 80 million years; then a near extinction of the clymenide family occurred 345 million years ago, at the end of the Devonian period. The ammonites recovered thereafter as goniatitinae, which flourished for about 110 million years, to disappear at the end of the Permian period, 230 million years ago. The stock was reconstituted again and developed during the 50 million years that form the Triassic period. Again mass destruction occurred at the end of that period but the stock managed to survive. The total disappearance of this animal sub-class that twice reconstituted itself, after a loss of 99% of its existing families, occurred 63 million years ago at the end of the Cretaceous period. The new era, devoid of ammonites and of dinosaurs, was called the Tertiary era. Paleontologically speaking, the disappearance was sudden and dramatic: of the 14 families that survived at the beginning of the Superior Cretaceous period, 9 died out before the end of that period, whereas the five remaining families survived until the end of the Cretaceous period. The "dramatic" disappearance lasted 15 million years.
It is not because the end of the Permian, Trias or Cretaceous periods occurred that the ammonites were destroyed but because the ammonites disappeared from the living scene that the new periods were recognized. The possibility to discern different periods in the earth’s history demonstrates that this earth went through sometimes brutal changes, "brutal" being of course evaluated in millions of years.
At the moment of their appearance, about 450 million years ago, corals were very profuse. They suffered a severe reduction in the number of their families 100 million years later, at the end of the Devonian period. Thereafter, they slowly recuperated and built up the number of their families again during the next 120 million years. At the end of the Permian period however, 99% of these families were wiped out. They recovered very fast thereafter, very fast in this case meaning about 10 million years, and they have survived without problems ever since.
The echinoids (sea urchins and sea stars) are certainly more evolved than corals but less than ammonites. They appeared about 425 million years ago at the beginning of the Silurian period. After a slow build-up, they lived profusely during 100 million years. They suffered a strong decline during the Pennsylvanian period, i.e. the 30 million years that initiated about 310 million years ago. At the beginning of the Permian period (280 million years ago), the trend was reversed, the building up in number of families resumed and has continued with some small variations up till the present day.
If geological periods are considered that are closer to us, the same story repeats itself with the vertebrates. The Amphibians were the first vertebrates to leave the water and populate the earth. They invaded almost empty continents in force at the beginning of the Mississippian period, 345 million years ago. Ever since, the number of their families has fluctuated from 100% to near 0% in the most dramatic manner. Reptiles did not have a less tortured history. Mammals appearing in great numbers only during the middle of the Jurassic period, 200 million years ago, were almost extinct during the middle of the Cretaceous period, 100 million years ago. The build-up of new species was due to the appearance of placental mammals. These mammals, whose females nourish the embryos through a placenta, appeared at the beginning of the Paleocene period, 63 million years ago.
4.5.5 Mammals: the quaternary extinction
Mammals are, with some exceptions, in decline today. About one fifth of all presently existing animal species will probably disappear within the next 30 years. It is the great Quaternary Extinction. The blue whale will not survive much longer than our own days. Wolves have already disappeared from most of Western Europe while European buffaloes and wild horses are surviving only in small reserves located in Poland. The camel originated in North America and has disappeared from there as well as from most of South America, most of Africa, the whole of China, India and Europe. The Asiatic lion is mentioned in tales such as the Thousand and One Nights and roamed until 1800 AD over large parts of Saudi Arabia, Turkey, Iraq, Iran, Pakistan, up the Indus valley and down most of the Ganges’ valley. Today this animal species is found only in a small game reserve located in Western India. The North African elephant utilized by Hannibal against Rome disappeared 2,000 years ago. In the US, the destructive effects insecticides have on the solidity of various predatory birds’ eggshells will rapidly cause the disappearance of these birds.
Manatees were observed along the Hispaniola shores, 500 years ago, during the Spanish conquest of America, and these are now gone. The conquistadors could not measure themselves against mastodons but American Indians living 6,000 years ago could. Eight thousand years ago, they faced mammoths, dire wolves, camels, horses, giant armadillos and Bison occidentalis. Nine thousand years ago, Amerindians hunted in addition another type of bison (Bison antiquus) and the giant ground sloth. Ten thousand years ago, the animal collection comprised the ground sloth and a kind of tapir; fourteen thousand years ago the saber-toothed cat, a kind of peccary, a dwarf elephant and two other kinds of mammoths were to be added to the list. Man is most likely the culprit for their disappearance.
The intensity of the Quaternary extinction is non-existent or mild in the oceans, in Africa and South East Asia but very severe in the Americas, where 73% of all genera weighing more than 44 kg were lost, in Madagascar and the Oceanic islands, and in Australia that lost every vertebrate species larger than a human, as well as all the flightless birds weighing more than about a kilo. The Australian birds disappeared 50,000 years ago, at the moment of the arrival of Humans on the continent. There is a possibility that either they were hunted or else their eggs were eaten. In New Zealand, the birds (11 species of Moa) survived until just 800 years ago.
The argument that the initial human colonization of North America and Australia led rapidly to massive extinction is subject to debate, primarily for sentimental reasons: how could those people, reputed to be lovers of nature and ecologists, who later on were themselves subject to massive overkill, be accused of megafaunal mass extinctions? Primitive and modern Man do not bear alone responsibility for the catastrophes but it is now recognized that, as far as the Great Quaternary extinction is concerned, he mightily helped, even when he was at the most primitive cultural level. It was recognized that the game hunted by Neanderthal Man in the Near East and Italy was small and sluggish: e.g. tortoises, shellfish and ostrich eggs. The type of the game did not change when Homo sapiens arrived but a switch to lagomorphs (hares) and birds has been observed when the density of the human sapiens population abruptly increased, with a rapid depletion of tortoises and shellfish populations, due to over hunting.
In America, the median age of the extinction event falls 1229 years after the initial human invasion, which is set at 13,400 years before the present. There are kill sites for mammoths, mastodons and giant tortoises, indicating that humans were capable of hunting all terrestrial species. All mammals less than 10 kg were spared and the extinction occurred during a deglaciation that doubled the habitable range of the continent. The extinctions did not occur during any of a half-dozen comparable, earlier deglaciation events. This extinction pulse ranged from Alaska to Patagonia, without a contemporary effect in Old World continents and on islands, although extinctions did invariably follow invasion of these places by humans, at earlier and later times.
In the Caribbean islands, species extinctions resulting from human pressures proceed today on a massive scale. Of the 197 endemic mammals and birds across the islands, at least 43 have become extinct over the last 500 years. This equates to nearly 500 extinctions per year per million species, 1000 times higher than expected given species’ lifetimes in the fossil record. A high probability of extinction in the near future expects 84 more extinctions of Caribbean endemic mammals and birds. The sixth mass extinction in our planet’s history appears imminent, precipitated, unlike the previous episodes, by human activity.