Popular Science Monthly/Volume 12/March 1878/Introduction and Succession of Vertebrate Life in America I
THE origin of life and the order of succession in which its various forms have appeared upon the earth offer to science its most inviting and most difficult field of research. Although the primal origin of life is unknown, and may perhaps never be known, yet no one has a right to say how much of the mystery now surrounding it science cannot remove. It is certainly within the domain of science to determine when the earth was first fitted to receive life, and in what form the earliest life began. To trace that life in its manifold changes through past ages to the present is a more difficult task, but one from which modern science does not shrink. In this wide field, every earnest effort will meet some degree of success; every year will add new and important facts; and every generation will bring to light some law, in accordance with which ancient life has been changed into life as we see it around us to-day. That such a development has taken place, no one will doubt who has carefully traced any single group of animals through its past history, as recorded in the crust of the earth. The evidence will be especially conclusive, if the group selected belongs to the higher forms of life, which are sensitive to every change in their surroundings. But I am sure I need offer here no argument for evolution; since to doubt evolution to-day is to doubt science, and science is only another name for truth.
Taking, then, evolution as a key to the mysteries of past life on the earth, I invite your attention to the subject I have chosen: "The Introduction and Succession of Vertebrate Life in America."
In the brief hour allotted to me, I could hardly hope to give more than a very incomplete sketch of what is now known on this subject. I shall, therefore, pass rapidly over the lower groups, and speak more particularly of the higher vertebrates, which have an especial interest for us all, in so far as they approach man in structure, and thus indicate his probable origin. These higher vertebrates, moreover, are most important witnesses of the past, since their superior organization made them ready victims to slight climatic changes, which would otherwise have remained unrecorded.
In considering the ancient life of America, it is important to bear in mind that I can only offer you a brief record of a few of the countless forms that once occupied this continent. The review I can bring before you will not be like that of a great army, when regiment after regiment with full ranks moves by in orderly succession, until the entire host has passed. My review must be more like the roll-call after a battle, when only a few scarred and crippled veterans remain to answer to their names. Or, rather, it must resemble an array of relics, dug from the field of some old Trojan combat, long after the contest, when no survivor remains to tell the tale of the strife. From such an ancient battle-field, a Schliemann might unearth together the bronze shield, lance-head, and gilded helmet, of a prehistoric leader, and learn from them with certainty his race and rank. Perhaps the skull might still retain the barbaric stone weapon by which his northern foe had slain him. Near by, the explorer might bring to light the commingled coat-of-mail and trappings of a horse and rider, so strangely different from the equipment of the chief as to suggest a foreign ally. From these, and from the more common implements of war that fill the soil, the antiquary could determine, by patient study, what nations fought, and perhaps when and why.
By this same method of research the more ancient strata of the earth have been explored, and in our Western wilds veritable battlefields, strewed with the fossil skeletons of the slain, and guarded faithfully by savage superstition, have been despoiled, yielding to science treasures more rare than bronze or gold. Without such spoils, from many fields, I could not have chosen the present theme for my address to-night.
According to present knowledge, no vertebrate life is known to have existed on this continent in the Archæan, Cambrian, or Silurian period; yet during this time more than half of the thickness of American stratified rocks was deposited. It by no means follows that vertebrate animals of some kind did not exist here in those remote ages. Fishes are known from the Upper Silurian of Europe, and there is every probability that they will yet be discovered in our strata of the same age, if not at a still lower horizon.
In the shore-deposits of the early Devonian sea, known as the Schoharie Grit, characteristic remains of fishes were preserved, and in the deeper sea that followed, in which the Corniferous limestone was laid down, this class was well represented. During the remainder of the Devonian, fishes continue abundant in the shallower seas, and, so far as now known, were the only type of vertebrate life. These fishes were mainly Ganoids, a group represented in our present waters by the gar-pike (Lepidosteus) and Sturgeon (Acipenser), but, in the Devonian sea, chiefly by the Placoderms, the exact affinities of which are somewhat in doubt. With these were Elasmobranchs, or the Shark tribe, and among them a few Chimæroids, a peculiar type, of which one or two members still survive. The Placoderms were the monarchs of the ocean. All were well protected by a massive coat of armor, and some of them attained huge dimensions. The American Devonian fishes now known are not so numerous as those of Europe, but they were larger in size, and mostly inhabitants of the open sea. Some twenty genera and forty species have been described.
The more important genera of Placoderms are—Dinichthys, Aspidichthys, and Diplognathus, our largest Palæozoic fishes. Others are—Acanthaspis, Acantholepis, Coccosteus, Macropetalichthys, and Onychodus. Among the Elasmobranchs were— Cladodus, Ctenacanthus, Machœracanthus, Rhynchodus, and Ptyctodus, the last two being regarded as Chimæroids. In the Chemung epoch, the great Dipterian family was introduced with Dipterus, Heliodus, and possibly Ceratodus. Species of the European genera, Bothriolepis and Holoptychius, have likewise been found in our Devonian deposits.
With the close of the Devonian, came the almost total extinction of the great group of Placoderms, while the Elasmobranchs, which had hitherto occupied a subordinate position, increase in numbers and size, and appear to be represented by Sharks, Rays, and Chimæras. Among the members of this group from the Carboniferous, were numerous Cestracionts, species of Cochliodus of large size, with others of the genera Deltodus, Helodus, Psammodus, and Sandalodus. Of the Petalodonts there were Antliodus, Chomatodus, Ctenoptychius, Petalodus, and Petalorhynchus; and of the Hybodonts, the genera Cladodus, Carcharopsis, and Diplodus. These Elasmobranchs were the rulers of the Carboniferous open sea, and more than one hundred species have been found in the lower part of this formation alone. The Ganoids, although still abundant, were of smaller size, and denizens of the more shallow and confined waters. The latter group of fishes was represented by true Lepidostidæ, of the genera Palæaniscus, Amblypterus, Platysomus, and Eurylepis. Other genera are—Rhizodus, Megalichthys, Ctenodus, Edestus, Orodus, Ctenacanthus, Gyracanthus, and Cœlacanthus. Most of these genera occur also in Europe.
From the Permian rocks of America no vertebrate remains are known, although in the same formation of Europe Ganoids are abundant; and with them are remains of sharks, and some other fishes, the affinities of which are doubtful. The Palæozoic fishes at present known from this country are quite as numerous as those found in Europe.
In the Mesozoic age, the fishes of America begin to show a decided approach to those of our present waters. From the Triassic rocks, Ganoids only are known, and they are all more or less closely related to the modern gar-pike, or Lepidosteus. They are of small size, and the number of individuals preserved is very large. The characteristic genera are—Catopterus, Ischypterus, Ptycholepis, Rhabdolepis, and Turseodus. From the Jurassic deposit no remains of fishes are known, but in the Cretaceous, ichthyic life assumed many and various forms; and the first representatives of the Teleosts, or bony fishes, the characteristic fishes of to-day, make their appearance. In the deep open sea of this age, Elasmobranchs were the prevailing forms, Sharks and Chimæroids being most numerous. In the great inland Cretaceous sea of North America, true osseous fishes were most abundant, and among them were some of carnivorous habits and immense size. The more sheltered bays and rivers were shared by the Ganoids and Teleosts, as their remains testify. The more common genera of Cretaceous Elasmobranchs were—Otodus, Oxyrhina, Galeocerdo, Lamna, and Ptychodus. Among the osseous fishes, Beryx, Enchodus, Portheus, and Saurocephalus, were especially common, while the most important genus of Ganoids was Lepidotus.
The Tertiary fishes are nearly all of modern types, and from the beginning of this period there was comparatively little change. In the marine beds, Sharks, Rays, and Chimæroids, maintained their supremacy, although Teleosts were abundant, and many of them of large size. The Ganoids were comparatively few in number. In the earliest Eocene fresh-water deposits, it is interesting to find that the modern Gar-pike, and Amia, the dogfish of our Western lakes, which by their structure are seen to be remnants of a very early type, are well represented by species so closely allied to them that only an anatomist could separate the ancient from the modern. In the succeeding beds, these fishes are still abundant, and with them are Siluroids nearly related to the modern catfish (Pimelodus). Many small fishes, allied apparently to the modern herring (Clupea), left their remains in great numbers in the same deposits, and, with them has been recently found a landlocked ray (Heliobatis).
The almost total absence of remains of fishes from the Miocene lake-basins of the West is a remarkable fact, and perhaps may best be explained by the theory that these inland waters, like many of the smaller lakes in the same region to-day, were so impregnated with mineral matters as to render the existence of vertebrate life in them impossible. No one who has tasted such waters, or has attempted to ford one of the modern alkaline lakes which are often met with on the present surface of the same deposits, will doubt the efficiency of this cause, or the easy entombment of the higher vertebrates that ventured within their borders. In the Pliocene lake-basins of the same region, remains of fishes were not uncommon, and in some of them are very numerous. These are all of modern types, and most of them are Cyprinoids, related to the modern carp. The Post-pliocene fishes are essentially those of to-day.
In this brief synopsis of the past ichthyic life of this continent, I have mentioned only a few of the more important facts, but sufficient, I trust, to give an outline of its history. Of this history, it is evident that we have as yet only a very imperfect record. We have seen that the earliest remains of fishes known in this country are from the lower Devonian; but these old fishes show so great a diversity of form and structure as to clearly indicate for the class a much earlier origin. In this connection, we must bear in mind that the two lowest groups of existing fishes are entirely without osseous skeletons, and hence, however abundant, would leave no permanent record in the deposits in which remains of fishes are usually preserved. It is safe to infer, from the knowledge which we now possess of the simpler forms of life, that even more of the early fishes were cartilaginous, or so destitute of hard parts as to leave no enduring traces of their existence. Without positive knowledge of such forms, and considering the great diversity of those we have, it would seem a hopeless task at present to attempt to trace successfully the genealogy of this class. One line, however, appears to be direct, from our modern gar-pike, through the lower Eocene Lepidosteus to the Lepidotus of the Cretaceous, and perhaps on through the Triassic Ischypterus and Carboniferous Palæoniscus; but beyond this, in our rocks, it is lost. The living Chimæra of our Pacific coast has nearly allied forms in the Tertiary and Cretaceous, more distant relatives in the Carboniferous, and a possible ancestor in the Devonian Rhynchodus. Our sharks likewise can be traced with some certainty back to the Palæozoic; and even the Lepidosiren, of South America, although its immediate predecessors are unknown, has some peculiar characters which strongly point to a Devonian ancestry. These suggestive lines indicate a rich field for investigation in the ancient life-history of American fishes.
The Amphibians, the next higher class of vertebrates, are so closely related to the fishes in structure that some peculiar forms of the latter have been considered by anatomists as belonging to this group. The earliest evidence of Amphibian existence, on this continent, is in the Sub-Carboniferous, where footprints have been found which were probably made by Labyrinthodonts, the most ancient representatives of the class. Well-preserved remains are abundant in the Coal-Measures, and show that the Labyrinthodonts differed in important particulars from all modern Amphibians, the group which includes our frogs and salamanders. Some of these ancient animals resemble a salamander in shape, while others were serpent-like in form. None of those yet discovered were frog-like, or without a tail, although the restored Labyrinthodont of the text-books is thus represented. All were protected by large pectoral bony plates, and an armor of small scutes on the ventral surface of the body. The walls of their teeth were more or less folded, whence the name Labyrinthodont. The American Amphibians known from osseous remains are all of moderate size, but the footprints attributed to this group indicate animals larger than any of the class yet found in the Old World. The Carboniferous Amphibians were abundant in the swampy tropical forests of that period, and their remains have been found imbedded in the coal then deposited, as well as in hollow stumps of the trees left standing.
The principal genera of this group from American Carboniferous rocks are—Sauropus, known only from footprints, Baphetes, Dendrerpeton, Hylonomus, Hylerpeton, Paniceps, Pelion, Leptophractus, Molgophis, Ptyonius, Amphibamus, Cocytinus, and Ceraterpeton. The last genus occurs also in Europe. Certain of these genera have been considered by some writers to be more nearly related to the lizards, among true reptiles. Some other genera known from fragmentary remains or footprints in this formation have likewise been referred to the true reptiles, but this question can perhaps be settled only by future discoveries.
No Amphibia are known from American Permian strata, but in the Triassic a few characteristic remains have been found. The three genera, Dictyocephalus, Dispelor, and Pariostegus, have been described, but, although apparently all Labyrinthodonts, the remains preserved are not sufficient to add much to our knowledge of the group. The Triassic footprints which have been attributed to Amphibians are still more unsatisfactory, and at present no important conclusions in regard to this class can be based upon them. From the Jurassic and Cretaceous beds of this continent no remains of Amphibians are known. A few only have been found in the Tertiary, and these are all of modern types.
The Amphibia are so nearly allied to the Ganoid fishes that we can hardly doubt their descent from some member of that group. With our present limited knowledge of the extinct forms, however, it would be unprofitable to attempt to trace in detail their probable genealogy.
The authors to whom especial credit is due for our knowledge of American fossil Fishes and Amphibians, are Newberry, Leidy, Cope, Dawson, Agassiz, St. John, Gibbes, Wyman, Redfield, and Emmons, and the principal literature of the subject will be found in their publications. Reptiles and birds form the next great division of vertebrates, the Sauropsida, and of these the reptiles are the older type, and maybe first considered. While it may be stated with certainty that there is at present no evidence of the existence of this group in American rocks older than the Carboniferous, there is some doubt in regard to their appearance even in this period. Various footprints which strongly resemble those made by lizards, a few well-preserved remains similar to the corresponding bones in that group, and a few characteristic specimens, nearly identical with those from another order of this class, are known from American Coal-Measures. These facts, and some others which point in the same direction, render it probable that we may soon have conclusive evidence of the presence of true reptiles in this formation, and in our overlying Permian, which is essentially a part of the same series. In the Permian rocks of Europe true reptiles have been found.
The Mesozoic period has been called the Age of Reptiles, and during its continuance some of the strangest forms of reptilian life made their appearance, and became extinct. Near its commencement, while the Triassic shales and sandstones were being deposited, true reptiles were abundant. Among the most characteristic remains discovered are those of the genus Belodon, which is well known also in the Trias of Europe. It belongs to the Thecodont division of reptiles, which have teeth in distinct sockets, and its nearest affinities are with the Crocodilia, of which order it may be considered the oldest known representative. In the same strata in which the Belodonts occur, remains of Dinosaurs are found, and it is a most interesting fact that these highest of reptiles should make their appearance, even in a generalized form, at this stage of the earth's history. The Dinosaurs, although true reptiles in all their more important characters, show certain well-marked points of resemblance to existing birds of the order Ratitæ, a group which includes the ostriches; and it is not improbable that they were the parent-stock from which birds originated.
During Triassic time, the Dinosaurs attained in America an enormous development both in variety of forms and in size. Although comparatively few of their bones have as yet been discovered in the rocks of this country, they have left unmistakable evidence of their presence in the footprints and other impressions upon the shores of the waters which they frequented. The Triassic sandstone of the Connecticut Valley has long been famous for its fossil footprints, especially the so-called "bird-tracks," which are generally supposed to have been made by birds, the tracks of which many of them closely resemble. A careful investigation, however, of nearly all the specimens yet discovered has convinced me that there is not a particle of evidence that any of these fossil impressions were made by birds. Most of these three-toed tracks were certainly not made by birds; but by quadrupeds, which usually walked upon their hind-feet alone, and only occasionally put to the ground their smaller anterior extremities. I have myself detected the impressions of these anterior limbs in connection with the posterior footprints of nearly all of the supposed "bird-tracks" described, and have little doubt that they will eventually be found with all. These double impressions are precisely the kind which Dinosaurian reptiles would make, and, as the only characteristic bones yet found in the same rocks belong to animals of this group, it is but fair to attribute all these footprints to Dinosaurs, even where no impressions of fore-feet have been detected, until some evidence appears that they were made by birds. I have no doubt that birds existed at this time, although at present the proof is wanting.
The principal genera of Triassic reptiles known from osseous remains in this country are—Arnphisaurus (Megadactylus), from the Connecticut Valley; Bathygnathus, from Prince Edward's Island; Belodon, and Clepsysaurus. Other generic names which have been applied to footprints and to fragmentary remains need not here be enumerated. A few remains of reptiles have been found in undoubted Jurassic rocks of America, but they are not sufficiently well determined to be of service in this connection. Others have been reported from supposed Jurassic strata, which are now known to be Cretaceous. It will thus be seen that, although reptilian life was especially abundant during the Triassic and Jurassic periods, but few bones have been found. This is owing in part to the character of most of the rocks then formed, which were not well fitted for preserving such remains, although admirably adapted to retain footprints.
During the Cretaceous period, reptilian life in America attained its greatest development, and the sediments laid down in the open seas and estuaries were usually most favorable for the preservation of a faithful record of its various phases. Without such a perfect matrix as some of these deposits afford, many of the most interesting vertebrates recently brought to light from this formation would probably have remained unknown. The vast extent of these beds insures, moreover, many future discoveries of interest.
In the lowest Cretaceous strata of the Rocky Mountain region, the Dakota group, part of which at least represents the Wealden of Europe, remains of Chelonia, or Turtles, Crocodiles, and Dinosaurs occur, the last being especially abundant. The Chelonia; although known from the Jurassic of Europe, here appear for the first time in American rocks. Some of the earliest forms are allied to the modern genus Trionyx. In the higher Cretaceous beds, some Chelonians of enormous size have been found. They belong to the genus Atlantochelys, which has the ribs separate, as in the existing Sphargis, and presents other embryonic characters. A few genera appear to be related to the modern genus Chelone. The remaining Cretaceous species were mostly of the Emydoid type; and others were related to Chelydra. The more important genera of Cretaceous Chelonians known from characteristic specimens are—Atlantochelys (Protostega), Adocus, Bothremys, Compsemys, Plastomenus, Osteopygis, Propleura, Lytoloma, and Taphrosphys. Most of these genera were represented by several species, and the individuals were numerous. No land-tortoises have as yet been found in this formation. In American Tertiary deposits, Chelonians are abundant, especially in the fresh-water beds. They all show near affinities with modern types, and most of them can be referred to existing genera. In the Tertiary lake-basins of the West, land-tortoises are very numerous, and with them are many fresh-water forms of Trionyx and allied genera.
A striking feature of the American Cretaceous fauna, as contrasted with that of Europe, is the almost entire absence in our strata of species of Ichthyosaurus and Plesiosaurus, which abound in many other regions, but here seem to be replaced by the Mosasaurs. A few fragmentary remains have indeed been referred to these genera, but the determination may fairly be questioned. This is more than true of the proposed new order Streptosauria, which was founded wholly on error. The order Plesiosauria, however, is well represented, but mainly by forms more nearly related to the genus Pliosaurus than to the type of the group. These were marine reptiles, all of large size, while some of them attained vast dimensions. So far as at present identified, they may be referred to the genera Climoliosaurus, Discosaurus (Elasmosaurus), and Pliosaurus. The number of species is comparatively few, and none are known above the Cretaceous. The important suggestion of Gegenbaur, that the Halisauria, which include the Plesiosaurs, branched off from the fishes before the Amphibians, finds some support in American specimens recently discovered.
The Reptiles most characteristic of our American Cretaceous strata are the Mosasauria, a group with very few representatives in other parts of the world. In our Cretaceous seas, they ruled supreme, as their numbers, size, and carnivorous habits, enabled them to easily vanquish all rivals. Some were at least sixty feet in length, and the smallest ten or twelve. In the inland Cretaceous sea from which the Rocky Mountains were beginning to emerge, these ancient "sea-serpents" abounded; and many were entombed in its muddy bottom. On one occasion, as I rode through a valley washed out of this old ocean-bed, I saw no less than seven different skeletons of these
monsters in sight at once. The Mosasaurs were essentially swimming lizards, with four well-developed paddles, and they had little affinity with modern serpents, to which they have been compared. The species are quite numerous, but they belong to comparatively few genera, of which Mosasaurus, Tylosaurus, Lestosaurus, and Edestosaurus, have alone been identified with certainty. The genus Mosasaurus was first found in Europe. All the known species of the group are Cretaceous.
The Crocodilia are abundant in rocks of Cretaceous age in America, and two distinct types are represented. The older type, which is foreshadowed by Belodon of the Trias, has biconcave vertebras, and shows marked affinities with the genus Teleosaurus, from the Jura of Europe. The best-known genus is Hyposaurus, of which there are several species, all more or less resembling in form the modern gavial of the Ganges. A peculiar intermediate form is seen in Diplosaurus, from the Wealden of the Rocky Mountains. The second type, which now makes its appearance for the first time, has procœlian  vertebræ, and in other respects resembles existing crocodiles. The genera described are Bottosaurus, Holops, and Thoracosaurus, none of which, so far as known, pass above the Cretaceous. Of Crocodilia with opisthocœlous vertebræ, America, so far as we know, has none. Specimens similar to those so termed in Europe are not uncommon here, but they pertain to Dinosaurs.
In the Eocene fresh-water beds of the West, Crocodilians are especially abundant, and all, with the exception of Limnosaurus, belong apparently to the genus Crocodilus, although some species show certain points of resemblance to existing alligators. The Miocene lake basins of the same region contain no remains of crocodiles, so far as known, and the Pliocene deposits have afforded only a single species. The Tertiary marine beds of the Atlantic coast contain comparatively few Crocodilian remains, and all are of modern types; the genus Gavialis having one Eocene species, and the alligator being represented only in the latest deposits.
It is worthy of special mention, in this connection, that no true Lacertilia, or Lizards, and no Ophidia, or Serpents, have yet been detected in American Cretaceous beds; although their remains, if present, would hardly have escaped observation in the regions explored. The former will doubtless be found, as several species occur in the Mesozoic of Europe; and perhaps the latter, although the Ophidians are apparently a more modern type. In the Eocene lake-basins of Western America, remains of lizards are very numerous, and indicate species much larger than any existing to-day. Some of these, the Glyptosauridæ, were protected by a highly-ornamented bony coat of-mail, and others were covered with scales, like recent lizards. A few resembled, in their more important characters, the modern iguana. The genera best represented in the Eocene are—Glyptosaurus, Iguanavus, Oreosaurus, Thinosaurus, Tinosaurus, and Saniva. Some of these genera appear to have continued into the Miocene, but here, as well as in the Pliocene, few remains of this group have been found. It is not improbable that some of our extinct Reptiles may prove to belong to Ryncocephala, but at present this is uncertain. The genus Notosaurus, from Brazil, has biconcave vertebræ, and some other characters which point to that group. No Dicynodonts or Theriodonts have as yet been found in this country.
The first American serpents, so far as now known, appear in the Eocene, which contains also the oldest European species. On the Atlantic border, the genus Titanophis (Dinophis) is represented by several species of large size, one at least thirty feet in length, and all doubtless inhabitants of the sea. In the fresh-water Western Eocene, remains of snakes are abundant, but all are of moderate size. The largest of these were related to the modern boa-constrictors. The genera described are Boavus, Lithophis, and Limnophis. The Miocene and Pliocene snakes from the same region are known only from a few fragmentary remains.
The Pterosauria, or flying-lizards, are among the most interesting reptiles of Mesozoic time, and many of them left their remains in the soft sediments of our inland Cretaceous sea. These were veritable dragons, having a spread of wings of from ten to twenty-five feet. They differed essentially from the smaller Pterodactyls found in the Old World, in the entire absence of teeth, showing in this respect a resemblance to modern birds; and they possess other distinctive characters. They have therefore been placed in a new order, Pteranodontia, from the typical genus Pteranodon, of which five species are known. The only other genus is Nyctosaurus, represented by a single species. All the specimens yet found are from essentially the same horizon, in the chalk of Kansas. The reported discovery of remains of this order from older formations in this country is without foundation.
The strange reptiles known as Dinosauria, which, as we have seen, were numerous during the deposition of our Triassic shales and sandstones, have not yet been found in American Jurassic, but were well represented here throughout the Cretaceous, and at its close became extinct. These animals possess a peculiar interest to the anatomist, since, although reptilian in all their main characters, they show clear affinities with the birds, and have some features which may point to mammals. The Cretaceous Dinosaurs were all of large size, and most of them walked on the hind-feet alone, like modern struthious birds. Two well-marked types may be distinguished among the remains discovered in deposits of this age: the herbivorous forms, represented mainly by Hadrosaurus, a near ally of the Iguanodon of Europe; and their carnivorous enemies, of which Dryptosaurus (Lœlaps) may be considered typical in this country, and Megalosaurus in Europe. Near the base of our Cretaceous formation, in beds which I regard as the equivalent of the European Wealden, the most gigantic forms of this order yet discovered have recently been brought to light. One of these monsters (Titanosaurus montanus), from Colorado, is by far the largest land-animal yet discovered; its dimensions being greater than was supposed possible in an animal that lived and moved upon the land. It was some fifty or sixty feet in length, and, when erect, at least thirty feet in height! It doubtless fed upon the foliage of the mountain forests, portions of which are preserved with its remains. With Titanosaurus, the bones of smaller Dinosaurs, one (Nanosaurus), not larger than a cat, as well as those of crocodiles and turtles, are not uncommon. The recent discovery of these interesting remains, many and various, in strata that had long been pronounced by professional explorers barren of vertebrate fossils, should teach caution to those who decline to accept the imperfection of our knowledge to-day as a fair plea for the supposed absence of intermediate forms.
In the marine Cretaceous beds of the West, only a single Dinosaur (Hadrosaurus agilis) has been found, but in the higher fresh-water beds, which mark the close of this formation, their remains are numerous, and indicate several well-marked species if not genera. In the marine beds on the Atlantic coast, the bones of Dinosaurs are frequently met with, and in the Upper Cretaceous Greensand of New Jersey, the type specimens of Hadrosaurus and Dryptosaurus were found. In Cretaceous fresh-water deposits on the coast of Brazil, remains of this order occur, but the specimens hitherto discovered are not sufficiently characteristic for accurate determination. This is unfortunately true of many Dinosaurian fossils from North America, but the great number of these reptiles which lived here during the Cretaceous Period promises many future discoveries, and substantial additions to our present knowledge of the group.
The first appearance of birds in America, according to our present knowledge, was during the Cretaceous Period, although many announcements have been made of their existence in preceding epochs. The evidence of their presence in the Trias, based on footprints and other impressions, is, at present, as we have seen, without value; although we may confidently await their discovery there, if not in older formations. Archæopteryx, from the European Jura, the oldest bird known, and now fortunately represented by more than a single specimen, clearly indicates a much higher antiquity for the class. The earliest American forms at present known are the Odontornithes, or birds with teeth, which have been exhumed, within the last few years, from the chalk of Kansas. The two genera Hesperornis and Ichthyornis are types of distinct orders, and differ from each other and from Archæopteryx much more than do any existing birds among themselves; thus showing that Birds are now a closed type, and that the key to the history of the class must be sought for in the distant past.
In Hesperornis, we have a large aquatic bird, nearly six feet in length, with a strange combination of characters. The jaws are provided with teeth, set in grooves; the wings were rudimentary and useless; while the legs were very similar to those of modern diving birds. This last feature was merely an adaptation, as the more important characters are struthious, showing that Hesperornis was essentially a carnivorous swimming ostrich. Ichthyornis, a small flying bird, was stranger still, as the teeth were in sockets, and the vertebræ biconcave, as in fishes and a few reptiles. Apatornis and other allied forms occur in the same beds, and probably all were provided with teeth. It is strange that the companions of these ancient toothed birds should have been Pterodactyls without teeth. In the later Cretaceous beds of the Atlantic coast various remains of aquatic birds have been found, but all are apparently distinct from those of the West. The known genera of American Cretaceous birds are— Apatornis, Baptomis, Graculavus, Hesperornis, Ichthyornis, Laornis, Lestornis, Palæotringa, and Telmatomis. These are represented by some twenty species. In Europe, but two species of Cretaceous birds are known, and both are based upon fragmentary specimens.
During the Tertiary period, birds were numerous in this country, and all yet discovered appear to have belonged to modern types. The Eocene species described are mostly wading birds, but here, and in the later Tertiary deposits, some characteristic American forms make their appearance, strongly foreshadowing our present avian fauna. The extinct genera are the Eocene Uintornis, related to the woodpeckers, and Aletornis, which includes several species of waders. Among the existing genera found in our Tertiary beds are—Aquila, Bubo, Meleagris, Grus, Graculus, Puffinus, and Catarractes. The great auk (Alca impennis), which was once very abundant on our northeast coast, has become extinct within a few years.
In this brief summary of the past life of reptiles and birds in America, I have endeavored to exclude doubtful forms, and those very imperfectly known, preferring to present the conclusions reached by careful study, incomplete though they be, rather than weary you with a descriptive catalogue of all the fossils to which names have been applied. Even this condensed review can hardly fail to give you some conception of the wealth of our continent in the extinct forms of these groups, and thus to suggest what its actual life must have been.
Although the Trias offers at present the first unquestioned evidence of true reptiles, we certainly should not be justified in supposing for a moment that older forms did not exist. So too in considering the different groups of reptiles, which seem to make their first appearance at certain horizons, flourish for a time, and then decline, or disappear, every day brings evidence to show that they are but fragments of the unraveled strands which converge in the past to form the mystic cord uniting all life. If the attempt is made to follow back any single thread, and thus trace the lineage of a group, We are met by difficulties which the science of to-day can only partially remove. And yet the anatomist constantly sees in the fragments which he studies hints of relationship which are to him sure prophecies of future discoveries.
The genealogy of the Chelonia is at present unknown, and our American extinct forms, so far as we now have them, throw little light on their ancestry. This is essentially true, also, of our Plesiosauria, Lacertilia, and Ophidia, although suggestive facts are not wanting to indicate possible lines of descent. With the Crocodilia, however, the case seems to be different, and Huxley has clearly pointed out the path for investigation. It is probable that material already exists in our museums for tracing the group through several important steps in its development. We have already seen that the modern procœlian type of this order goes back only to the Upper Cretaceous, while the Belodonts, of our Triassic rocks, with their biconcave vertebrae, are the oldest known Crocodilians. Our Jurassic, unfortunately, throws but little light on the intermediate forms, but we know that the line was continued, as it was in the Old World through Teleosaurus. The beds of the Rocky Mountain Wealden have just furnished us with a genuine "missing link," a saurian (Diplosaurus) with essentially the skull and teeth of a modern crocodile, and the vertebræ of its predecessor from the Trias. This peculiar reptile clearly represents an important stage in the progressive series, and evidently one soon after the separation of the crocodile branch from the main stem. The modern Gavial type appears to have been developed about the same time, as the form was well established in the Upper Cretaceous genus, Thoracosaurus. The Teleosaurian group, with biconcave vertebræ, evidently the parent stock of Crocodilians, became extinct with Hyposaurus of the same horizon, leaving the crocodile and gavial, with their more perfect procœlian vertebræ, to contend for the supremacy. In the early Eocene, both of these types were abundant, but some of the crocodiles possessed characters pointing toward the alligators, which do not appear to have been completely differentiated until later.
Nothing is really known to-day of the earlier genealogy of the Pterosauria; but our American forms, without teeth, are clearly the last stage in their development before this peculiar group became extinct. The oldest European form, Dimorphodon, from the Lower Lias, had the entire jaws armed with teeth, and was provided with a long tail. The later genus, Pterodactylus, retained the teeth, but had essentially lost the tail; while Ramphorhynchus had retained the elongated tail, but had lost the teeth from the fore-part of both jaws. In the genus Pteranodon, from the American Cretaceous, the teeth are entirely absent, and the tail is a mere rudiment. In the gradual loss of the teeth and tail, these reptiles followed the same path as birds, and might thus seem to approach them, as many have supposed. This resemblance, however, is only a superficial one, as a study of the more important characters of the Pterodactyls shows that they are an aberrant type of reptiles, totally off the line through which the birds were developed. The announcement made not long since in Europe, and accepted by some American authors, that the Pterosauria, in consequence of certain points in their structure, were essentially birds, is directly disproved by American specimens, far more perfect than those on which the conclusion was based.
It is now generally admitted, by biologists who have made a study of the vertebrates, that birds have come down to us through the Dinosaurs, and the close affinity of the latter with recent struthious birds will hardly be questioned. The case amounts almost to a demonstration, if we compare, with Dinosaurs, their contemporaries, the Mesozoic birds. The classes of Birds and Reptiles, as now living, are separated by a gulf so profound that a few years since it was cited by the opponents of evolution as the most important break in the animal series, and one which that doctrine could not bridge over. Since then, as Huxley has clearly shown, this gap has been virtually filled by the discovery of bird-like reptiles and reptilian birds. Compsognathus and Archæopteryx of the Old World, and Ichthyornis and Hesperornis of the New, are the stepping-stones by which the evolutionist of to-day leads the doubting brother across the shallow remnant of the gulf once thought impassable.
[To be continued.]
- An address delivered before the American Association for the Advancement of Science, at Nashville, Tenn., August 30, 1877, by Prof. O. C. Marsh, Vice-President.
- (Ceslracion = sharp tool)—a group of sharks, so named from their denticulated dorsal spines.
- (Hybodus = hump-tooth)—a group of sharks.
- A species of Ceratodus has recently been found in the upper Jurassic of Colorado, and named by the writer C. Güntheri.—O. C. M.
- Since this address was delivered, I have determined the beds containing gigantic Dinosaurs, on the flanks of the Rocky Mountains, to be upper Jurassic, and called them Atlantosaurus Beds. (See frontispiece, section.) These strata were formerly referred to the Dakota group, or base of the Cretaceous.—O. C. M.
- Sphargis, a genus of turtles inhabiting the Atlantic and Mediterranean. They are the largest of all turtles, and have the body covered with a thick coriaceous skin instead of a hard shell.
- Emys, a genus of small land and fresh-water tortoises.
- Vertebræ which have centra concave at each end have been conveniently termed amphicœlous; those with a cavity in front and a convexity behind, procœlous; where the position of the concavity and convexity is reversed, they are opisthocœhus.—(Huxley, "Anatomy of Vertebrated Animals.")
- Dicynodonts (two canine teeth), a singular group of extinct reptiles from South Africa, India, and the Ural Mountains. The family name alludes to the two enormous canine teeth which grow from the upper jaw.
- Theriodonts (beast-tooth), a group of extinct reptiles, having, according to Owen, some characters which point toward the Mammalia.
- See note on page 520.
- This generic name proved to be preoccupied, and I have substituted for it, Atlantosaurus.—O. C. M.
- A new order of huge reptiles (Stegosauria), apparently allied to the Dinosaurs and Chelonia, and two new genera of Dinosaurs (Apatosaurus and Allosaurus), have since been described by the writer from the same Upper Jurassic horizon.—(American Journal of Science, December, 1877.)
- See note on page 520, also section.