Part 7
=Lower Silurian (Ordovician) Period.= The life characteristics of this period are very similar to those of the Cambrian. Brachiopods have developed extensively, though they have not yet reached their height of development. The Trilobites attained their greatest development in this period; but after the Lower Silurian these creatures commenced to decline in variety and numbers, and finally underwent extinction near the close of the Palæozoic era. In America no plants have been discovered above the grade of seaweeds, but a few of the higher Cryptogams are doubtfully reported in Europe. The character of the flora in a later (Devonian) period makes it highly probable that land plants were well advanced in the Lower Silurian period. Professor Scott states that the remains of land plants may be discovered at any time, though this must remain a matter of chance, inasmuch as all known Ordovician rocks are marine, and therefore not a favorable circumstance for the preservation of land plants. Foraminifera and Radiolaria were abundant in the seas of the Lower Silurian period. Sponges and Brachiopods are numerous and varied. Among Cœlenterates the Graptolites are very numerous and varied; the few and doubtful Cambrian Corals are succeeded by a considerable number of Ordovician genera and species. They were characteristically different from the reef-builders of modern seas. The Echinodermata have greatly increased in importance, being more numerous and varied. The _Cystids_ reach their maximum development in this period. The Crinoids greatly increase in numbers and variety. The Star-fishes greatly expand; and a new and higher order of Echinoderms, the _Sea-Urchins_, make their first appearance in the later Ordovician period, but under very primitive forms.
Among Arthropods the Trilobites increase greatly in numbers and variety, and attain their maximum development in this period. The occurrence of a Centipede in Ordovician rocks indicates the interesting fact that terrestrial animal life had already begun.
One of the most striking differences between the Lower Silurian and Cambrian periods is the great advance made by the Molluscs, in variety and numbers, during the former period. Molluscs exhibit the most significant change in the great expansion of the _Cephalopods_, a few of which had appeared in the uppermost Cambrian rocks. In the Lower Silurian period the Cephalopods became one of the predominant types in the marine life. In modern times nearly all Cephalopods are naked (_Cuttlefish_ and _Squid_), only a few having a shell (_Nautilus_ and _Argonaut_). The naked forms are higher creatures than those with shells. In the Lower as well as in the Upper Silurian periods no naked forms existed, but only the lower forms with shells (Nautiloids). In modern times and throughout later geological periods only Nautiloids with coiled shells were in existence. In both of the Silurian periods the shells were all straight, and the animals were called Orthoceratites. Such animals were extremely abundant in those times, and often reached an enormous size. Specimens have been found which were ten inches in diameter and over fifteen feet long. They were the most formidable animals of those early ages, and were the rulers and scavengers of the seas; so that the Lower and Upper Silurian periods are known as the =Age of Molluscs=.
=Upper Silurian Period.= In this period great progress is made in the history of life on the globe. Upper Silurian life is the continuation and advance of the organic system that flourished in the Ordovician, certain groups expanding, others diminishing; and some new groups now appear for the first time. Sponges are still common. Among Cœlenterates the Graptolites have greatly diminished; Hydroid Corals have become important features of the seas and in the formation of the reefs. True Corals increase largely and play a more important part than in the preceding period. Honeycomb and Chain Corals are quite characteristic of this period.
Among Echinoderms there is a diminution of the Cystids, and a marked increase of the Crinoids. A new class of Echinoderms, the Blastids, now make their first appearance. The Sea-urchins and Star-fishes have increased in variety and abundance. Brachiopods still exist in multitudes, but under changed forms. Among Molluscs the Orthoceratites are still the rulers of the seas. Among Arthropods the Trilobites are still numerous, though decidedly less so than in the preceding period. The land animals are insects of low types, mainly allied to cockroaches. Scorpions are also present as fossils in the Upper Silurian rocks. These animals prove the existence of a contemporaneous land vegetation.
During all the many millions of years that constitute the Silurian periods, hosts of species lived and died; hosts of variations were induced in the living creatures by the ever-changing environment; so that at the close of this time the Silurian rocks show that some of the life-forms, lowly as they were, had yet climbed higher in the scale of organization.
The later Silurian rocks reveal the remains of _an entirely new branch of living forms, a higher branch of animals_ (=Vertebrates=) than had ever before existed. These remains are those of =Ostracoderms= and =primitive Sharks=. But at that early time they held a very subordinate position among the hosts of living creatures; the Molluscan Orthoceratites were still the rulers of the seas; it was still the Age of Molluscs.
=Devonian Period.= The next set of ages succeeding the Silurian time is known as the Devonian period. Many of the kinds of creatures living in the Silurian ages are also found in these ages, but under changed forms. The Devonian seas had a great abundance and variety of Corals and Crinoids. The Chain Corals have perished, but the Cup and Honeycomb Corals still live on with modified appearance. The Graptolites are almost extinct. The Orthoceratites still live on, much reduced in numbers and size. The Molluscan Cephalopods have been revolutionized. New forms have grown out of the variations of the past, so that now in Devonian times we have the introduction of the great Molluscan _Ammonite family_, under the forms known as _Goniatites_, which are characteristic of this and the succeeding Carboniferous ages. The Trilobites still continue under new forms, but much reduced in size and number. Brachiopods are abundant and diversified.
In this age, for the first time, land plants become conspicuous. The Devonian forests consisted of the highest flowerless plants, such as ferns, horsetails, and club mosses; and also the lowest flowering plants, such as cycads, pines, cypresses, etc.—plants which have imperfect and inconspicuous flowers. In all ages, as now, land vegetation has been closely related with _insects_. Insects, though rare as fossils, are found in connection with the forests in the Devonian period. They are among the lower orders of the class, and are somewhat allied to cockroaches and dragon flies.
The most characteristic feature of the Devonian age is the expansion of that new and higher class of animals, =the backboned animals=; for during the earliest Devonian or latest Silurian times the =Fishes= made their appearance. At first many of them were comparatively small in size, they were few in numbers, and of strange, unfishlike forms. Such were the _Ostracoderms_, which, though generally called fishes, belong to a type much below the true fishes, and more nearly allied to the Lampreys, for they were devoid of jaws and paired fins. There was a great variety and wealth of true fishes in the Devonian period. The _Elasmobranchii_ (Sharks) were well represented, _though very generalized ones_. _Crossopterygii_, _Dipnoi_ and _Ganoidei_ were important elements of the fish fauna of this period. At this period there was an entire absence of those highly specialized fishes (_Teleosts_) which in modern times make up the vast majority of fishes, both marine and fresh-water.
The true fishes soon developed to so great an extent in size and numbers that they swarmed in the Devonian seas and quickly became the rulers of the age; hence Devonian time is known as the =Age of Fishes=.
These early Devonian Fishes were of a more lowly organization than modern ones, and were what is called generalized forms; that is, they combined in themselves the characters of two distinct classes. They had distinct _amphibian characters_. From these generalized Fishes were afterwards formed, through many transitions, the _Amphibians_ and the _Fishes_ as branches from a common trunk. This illustrates what is a very general law, viz.: that the first-introduced examples of a class are not typical forms of that class, but intermediate forms or connecting links with other classes. Certain footprints recently discovered in the upper Devonian rocks of Pennsylvania indicate that the Amphibia, the lowest of air-breathing vertebrates, commenced their career in the latter part of the Devonian period; but they were inconspicuous among the monarch fishes.
=Carboniferous Age.= As in the Devonian Age so in the Carboniferous times, we find flowerless plants, such as ferns, horsetails, and club mosses; but in this coal-forming age they culminate, and have become gigantic in size, especially the two latter. At this time we again meet with the next higher order of plants, as cycads, pines, cypresses, etc.
These plants of the coal period were remarkable, generalized types, connecting classes now widely separated. During this period the insects increase in variety and numbers along with the advancing vegetation. It is interesting to find that as the highest flowering plants are not yet in existence, so those highest orders of insects, the flower-loving and honey-loving ones, such as bees, butterflies and moths, are not yet in existence. Corals and Echinoderms, Molluscs and Crustaceans, etc., continue through the ages with ever shifting forms. Brachiopods have greatly diminished. Foraminifera for the first time assume considerable importance in the earth’s economy.
The Molluscan Goniatites continue through this age with changing forms and advancing organization, and are very numerous. The Trilobites, which are characteristic of Palæozoic times, continue under new forms through this age, and then perish. In this age are introduced for the first time typical Crustaceans of the long-tailed kind, such as shrimps.
Devonian fishes still prevail. The Elasmobranchii are numerous and varied, and some of them are highly specialized. _Pleuracanthus_ is a remarkable shark which has many features in common with the lung-fishes (Dipnoi), such as the character of the pectoral fins, the shape of the tail, the bones which form a roof for the skull, while the skin is naked.
The Dipnoi continue, though in diminished numbers.
The Crossopterygians are much less abundant. The Ganoidei increase in numbers and varieties.
The fact of greatest interest concerning the Carboniferous period is the _expansion of the true land-breathing, backboned animals, such as the_ =Amphibians=. _Their expansion marks a distinct step forward in the scale of life._
The Carboniferous Amphibia all belong to the extinct order of _Labyrinthodonts_ (_Stegocephala_), in which the skull is well covered with a roof of sculptured bones. The Amphibia of the lower Carboniferous rocks are of small or moderate size, not exceeding eight feet in length and mostly much smaller. A great number of these Amphibians are known, most of them like the Salamanders in shape, but some are snake-like in form, being long and slender. An example of the Stegocephala is _Archegosaurus_.
The upper Carboniferous rocks represent the later Carboniferous period (=Permian Period=). The life of this period is transitional between that of the Palæozoic and of the Mesozoic eras. Here we meet with the last of many types which had persisted from Cambrian times, associated with forms which are prophetic of the characteristic types of the Mesozoic era. We also meet with types in the Permian that are peculiar to the period. In this section of the Carboniferous period many of the genera of the fishes are the same, while new ones are introduced. Among the lung-fishes the genus _Ceratodus_ is introduced, a creature very closely allied to the modern lung-fish of Australia. The Amphibia are still represented by the Stegocephala, several of the older genera persisting, while many new forms appear for the first time; several of the latter surpass the earlier Carboniferous genera greatly in size. The transitions from many of the Devonian fishes to the Carboniferous Labyrinthodonts are so gradual that it is sometimes difficult to say whether we are dealing with Fish or Amphibian. In other words, the Devonian fishes are generalized fishes, that is, connecting links between Fish and Amphibian. When the Amphibians finally separated from the fishes they were not the highly specialized forms of more recent times; but they were generalized Amphibians, having some reptilian characteristics. As time passed on and the creatures continued to modify, some varied more and more in the direction of true amphibians, and others more and more in the direction of reptiles, until in the Permian stage early, generalized Reptiles, of lizard-like form, appeared in large numbers. The most interesting of these primitive Reptiles are the _Theromorpha_ (beast forms), which present many remarkable approximations to the structure of the mammals. As yet no snakes or turtles, no alligators or crocodiles had come into existence. In spite of the fact that great advance in animal life was made during Carboniferous times, there was little of the life in those ancient woods that we associate with the forests of the present; they were gloomy wastes of shade, without the presence of bright flowers, no humming of the bees, no song of birds, and few sounds save the gurgling of running streams, the sighing of the wind through the leaves, the splash of waves upon the shore, and the bursting of the thunder clouds. The interest and importance of the Carboniferous Amphibians as the first land backboned animals, is so great as to cause the Carboniferous period to be spoken of as the =Age of Amphibians=.
=Triassic Period.= In this period the ferns and horsetails continue under new forms; but the next higher orders of plants, the Cycads and Conifers, now predominate. The Goniatites are replaced by the Molluscan _Ceratites_, the latter being characteristic of the Triassic period.
One of the most characteristic changes from the Palæozoic to the Mesozoic era consists in the great reduction of the Brachiopods.
The =Vertebrata= of this period are of extraordinary interest and show great progress. The fishes exhibit the least progress. The Dipnoan _Ceratodus_ is very characteristic, continuing from the Permian. The Crossopterygians have greatly declined. The Ganoidei continue to be the dominant fish-type, and are most like the existing gar-pikes. The Amphibia (Stegocephala) culminate in this period, multiplying and diversifying greatly, and far surpass in size the Carboniferous and Permian genera, and then become extinct.
It is in the =Reptilian= class that we find the most remarkable changes. The abundance and diversity of the reptiles in this and the two succeeding periods are incomparably greater than those in the Permian age. The Triassic rocks have representatives of almost all the orders of Mesozoic reptiles, though often these are comparatively small and rare forms.
The reptiles with mammalian characters (Theromorpha), which first appeared in the Permian period, culminate in the Triassic, especially in southern Africa, and then become extinct. No birds, or reptiles which can be regarded as the ancestors of birds, have been found in this period. In this period one of the greatest advances in the progress of life is indicated by the first appearance of _low mammals_,—_mammals having very decided reptilian characters_, and belonging to the orders of generalized =Monotremes= and =Marsupials= (_Dromatherium_; _Microlestes_).
=Jurassic Period.= The _Fishes_ have advanced greatly beyond those of the Triassic rocks. The Sharks have advanced practically to their modern condition, and a new order of Elasmobranchs, the broad and flat _Rays_, are introduced.
The _Chimæroids_ occur in this period, and were more numerous than in modern times. Dipnoans have become very scarce. The Crossopterygians are greatly reduced. The Ganoidei are still the dominant type; some of these latter approximate the Teleosts so closely that it seems arbitrary to call them Ganoidei. In Europe the _Reptiles_ culminate in this period and show extraordinary development in variety, huge size, number and degree of organization. They were rulers in every department of nature. They were the rulers in the air in place of birds; rulers on the land in place of mammals; and rulers in the sea instead of sharks and whales.
Immense land reptiles (_Dinosauria_) as large as our largest mammals, and in some cases larger than the elephant, moved sluggishly over the land. Some walked on all fours; others were occasionally or usually bipedal, and walked upright like birds, and had many structural features in common with the latter; some were herbivorous, feeding on plants and even reaching into the branches of trees for their food, others were carnivorous, feeding on their fellow-creatures. Huge reptiles (_Ichthyosauria_) swam about in the sea in great numbers. Immense bat-like forms (_Pterosauria_) sailed through the air like birds, being literally flying dragons.
These reptiles, which had branched off from the generalized Amphibians, were themselves very generalized creatures. It is interesting to remember that the Monotreme and Marsupial mammals found in this age were very low reptilian mammals; they were not typical, specialized Monotremes and Marsupials, like the modern creatures, but very generalized forms, being probably connecting links with low generalized Insectivora, which were the first of the true mammals.
These early Monotremes and Marsupials were quite small animals, varying in size from a mole to a rabbit. They were insignificant creatures among the mighty giant reptiles, but they carried in their warm blood the promise of future mammalian supremacy.
Birds appear for the first time in this period, but very different from modern birds. Not only did many of the reptiles of this time have bird-like characters, but all the birds of this period were distinctly _reptilian_ birds (_Archæopteryx_). Birds came off from primitive reptiles as branches from a stem (see Diagram of Development). It was only with the passing epochs that these early birds changed more and more from their reptilian characters and assumed more and more the features of modern birds. Many fossil connecting links between reptiles and birds have been discovered, and afford most useful illustrations of those changes in creatures that we call _Evolution_.
=Cretaceous Period.= In the Cretaceous period the aspect of plant life has changed greatly. Now, for the first time, we meet with ordinary hard-wood trees, such as beech, oaks, hickory, maples, poplar, etc., but of very different genera and species from those existing to-day.
In this period, for the first time, we find the highest order of Cephalopods, viz., the naked ones, allied to cuttlefishes and squids. They are known as the _Belemnites_. Sometimes the fossil ink-bags of these creatures are found so that they can be drawn in their own fossil ink.
The Ammonites proper, which were introduced in the Triassic period and culminated in Jurassic time, become extinct at the close of the Cretaceous.
The =Vertebrates= form the most characteristic features of the Cretaceous fauna. A revolution has occurred among the Fishes. Sharks of modern type are numerous. Crossopterygians and Ganoidei are rare. There has been an immense expansion of _Teleosts_ or Bony Fishes, which now become the dominant fishes. Most of the Cretaceous Teleosts belong to modern families and even genera.
The _Reptiles_, in this period, continue to be the dominant types of the land, air, and sea, and it is difficult to decide whether the Cretaceous or the Jurassic is to be regarded as the culminating period of Reptilian history. The flying Reptiles (Pterosauria) of this period are remarkable for their great size, far exceeding, in this respect, those of the Jurassic time. The land Reptiles (Dinosauria) are in greater numbers than in the preceding period. The sea Reptiles are less numerous than in the Jurassic, but are of greatly increased size.
Cretaceous _Birds_ are much more numerous and advanced than the Jurassic ones.
The _Mammals_ of the Cretaceous are much more abundant and varied than those of Jurassic times, but they are nearly all of very small size, and continue to play a very modest rôle. The lower Cretaceous mammals differ but little from those of the Jurassic, except for the larger number of genera. The mammals of the latest Cretaceous time are much more numerous and diversified than those of the early Cretaceous. They also show affinities with the mammals of the next succeeding or Tertiary period. The birds and mammals, though they have varied with the passing time and changing surroundings like all other living creatures, are very different from those around us now. They have thrown off some of their reptilian characters, yet they are still distinctly and pronouncedly reptilian.
The Triassic, Jurassic and Cretaceous periods together constitute the Mesozoic Era, or =Age of Reptiles=; for never before and never in succeeding ages were reptiles so huge, and varied, and masters in all the realms of nature.
=Tertiary Period.= In reaching Tertiary time we enter upon the threshold of our modern world. The reptiles have dwindled to a few low forms, such as alligators and crocodiles, to the small lizards, to turtles and tortoises, and to those low-grade reptiles, the snakes, which now for the first time are the most numerous of the class. In these ages, as now, the Teleost fishes vastly predominate, and the Ganoid and Crossopterygian fishes are nearly extinct. In Tertiary times, as in the present, all the reptilian birds had disappeared, and only typical birds remain. In other words, the bird-class had now separated from the reptilian class, and the connecting links became extinct.
In this age for the first time the highest flowering plants are abundant, and now for the first time also the highest orders of insects are abundant, such as bees, ants, butterflies, etc. On account of the greater warmth and moisture in the Tertiary period, insect and plant life were fuller then than now. That the climate was warmer in those times is shown by the following facts: In America, during the early Tertiary period, figs, evergreens and palms grew in Dakota, showing a temperature there at that time equal to the temperature in Florida at present. In the middle Tertiary period trees like the Redwood of California, and Magnolias, were abundant in Greenland.
Although _Reptilian Mammals_ (_Multituberculata_) lived in the preceding era, true or typical Mammals first came to view in the earliest Tertiary times. They soon succeeded the vanished giant reptiles as the rulers of the world; so that this and the succeeding (Quaternary) period are known as the =Age of Mammals=. At this period, for the first time, appear generalized _primitive flesh-eating mammals_ (_Creodonta_), and _primitive grass-eating ones_ (_Condylarthra_ and _Amblypoda_); also _primitive Primates_ (_primitive Lemuroids_, and, later, _primitive types of Monkeys_, e. g., _Anaptomorphus_).
The evolution of mammals, compared with that of other animal groups, has been so rapid that each stage of even the Eocene has its own mammalian fauna, differing from those of the succeeding and preceding stages. All through the Tertiary period, as through all the preceding ages, the species that are advancing in life are undergoing greater and greater specialization. Animals at first closely related finally become more and more separated from one another. This is well illustrated by a study of the hoofed animals. In the earliest Tertiary period all these animals seem to unite into one branch but now they consist of many widely separated sub-branches. As we trace the branch down to the earliest Tertiary period, we find that even in early Eocene times it divides into the even-toed and odd-toed ungulates. In the later Miocene times, each of these again separates,—the former, even-toed ones, into the hog and hippopotamus families with four toes, and the ruminant family with two toes; the latter, odd-toed ones, into the elephant family with five toes, the rhinoceros and tapir families with three toes, and the horse family with one toe.
[Illustration: FIG. 14.—Illustrating genesis of horse’s feet. Fore-feet (a) and hind-feet (b) of Orohippus; fore-feet (c) and hind-feet (d) of Mesohippus; fore-feet (e) and hind-feet (f) of Miohippus; fore-feet (g) and hind-feet (h) of Protohippus; fore-feet (k) and hind-feet (l) of Pliohippus; fore-feet (m) and hind-feet (n) of Equus.]