The Ancient Life History of the Earth Part 16

Euomphalus. Myophoria. Pachyrisma.

(The last two are Thecidium.

princ.i.p.ally but not exclusively Tria.s.sic.)

Thus, to emphasise the more important points alone, the Trias has yielded, amongst the Gasteropods, the characteristically Palaeozoic _Loxonema, Holopella, Murchisonia, Euomphalus_, and _Porcellia_, along with typically Tria.s.sic forms like _Platystoma_ and _Scoliostoma_, and the great modern groups _Chemnitzia_ and _Cerithium_. Amongst the Bivalves we find the Palaeozoic _Megalodon_ side by side with the Tria.s.sic _Halobia_ and _Myophoria_, these being a.s.sociated with the _Carditoe, Hinnites, Plicatuloe_, and _Trigonioe_ of later deposits. The Brachiopods exhibit the Palaeozoic _Athyris, Retzia_, and _Cyrtina_, with the Tria.s.sic _Koninckia_ and the modern _Thecidium_. Finally, it is here that the ancient genera _Orthoceras, Cyrtoceras_, and _Goniat.i.tes_ make their last appearance upon the scene of life, the place of the last of these being taken by the more complex and almost exclusively Tria.s.sic _Cerat.i.tes_, whilst the still more complex genus _Ammonites_ first appears here in force, and is never again wanting till we reach the close of the Mesozoic period. The first representatives of the great Secondary family of the _Belemnites_ are also recorded from this horizon.

[Ill.u.s.tration: Fig. 146.--a, Dental plate of _Ceratodus serratus_, Keuper; b, Dental plate of _Ceratodus altus_, Keuper; (After Aga.s.siz.)]

[Ill.u.s.tration: Fig 147.--_Ceratodus Fosteri_, the Australian Mud-fish, reduced in size.]

Amongst the _Vertebrate Animals_ of the Trias, the _Fishes_ are represented by numerous forms belonging to the _Ganoids_ and the _Placoids_. The Ganoids of the period are still all provided with unsymmetrical ("heterocercal") tails, and belong princ.i.p.ally to such genera as _Paloeoniscus_ and _Catopterus_. The remains of Placoids are in the form of teeth and spines, the two princ.i.p.al genera being the two important Secondary groups _Acrodus_ and _Hybodus_. Very nearly at the summit of the Trias in England, in the Rhaetic series, is a singular stratum, which is well known as the "bone-bed," from the number of fish-remains which it contains. More interesting, however, than the above, are the curious palate-teeth of the Trias, upon which Aga.s.siz founded the genus _Ceratodus_.

The teeth of Ceratodus (fig. 146) are singular flattened plates, composed of spongy bone beneath, covered superficially with a layer of enamel. Each plate is approximately triangular, one margin (which we now know to be the outer one) being prolonged into p.r.o.ngs or conical prominences, whilst the surface is more or less regularly undulated. Until recently, though the master-mind of Aga.s.siz recognised that these singular bodies were undoubtedly the teeth of fishes, we were entirely ignorant as to their precise relation to the animal, or as to the exact affinities of the fish thus armed. Lately, however, there has been discovered in the rivers of Queensland (Australia) a living species of _Ceratodus_ (_C. Fosteri_, fig. 147), with teeth precisely similar to those of its Tria.s.sic predecessor; and we thus have become acquainted with the use of these structures and the manner in which they were implanted in the mouth. The palate carries two of these plates, with their longer straight sides turned towards each other, their sharply-sinuated sides turned outwards, and their short straight sides or bases directed backwards. Two similar plates in the lower jaw correspond to the upper, their undulated surfaces fitting exactly to those of the opposite teeth. There are also two sharp-edged front teeth, which are placed in the front of the mouth in the upper jaw; but these have not been recognised in the fossil specimens. The living _Ceratodus_ feeds on vegetable matters, which are taken up or tom off from plants by the sharp front teeth, and then partially crushed between the undulated surfaces of the back teeth (Gunther); and there need be little doubt but that the Tria.s.sic _Ceratodi_ followed a similar mode of existence. From the study of the living _Ceratodus_, it is certain that the genus belongs to the same group as the existing Mud-fishes (_Dipnoi_); and we therefore learn that this, the highest, group of the entire cla.s.s of Fishes existed in Tria.s.sic times under forms little or not at all different from species now alive; whilst it has become probable that the order can be traced back into the Devonian period.

[Ill.u.s.tration: Fig. 148.--Footprints of a Labyrinthodont (_Cheirotherium_), from the Tria.s.sic Sandstones of Hessberg, near Hildburghausen, Germany, reduced one-eighth. The lower figure shows a slab, with several prints, and traversed by reticulated sun-cracks: the upper figure shows the impression of one of the hind-feet, one-half of the natural size. (After Sickler.)]

[Ill.u.s.tration: Fig. 149.--Section of the tooth of _Labryinthodon (Mastodonsaurus) Joegeri_, showing the microscopic structure.

Greatly enlarged. Trias.]

[Ill.u.s.tration: Fig. 150.--a, Skull of _Labyrinthodon Joegeri_, much reduced in size; b, Tooth of the same. Trias Wurttemberg.]

The _Amphibians_ of the Trias all belong to the old order of the _Labyrinthodonts_, and some of them are remarkable for their gigantic dimensions. They were first known by their footprints, which were found to occur plentifully in the Tria.s.sic sandstones of Britain and the continent of Europe, and which consisted of a double series of alternately-placed pairs of hand-shaped impressions, the hinder print of each pair being much larger than the one in front (fig. 148). So like were these impressions to the shape of the human hand, that the at that time unknown animal which produced them was at once christened _Cheirotherium_, or "Hand-beast." Further discoveries, however, soon showed that the footprints of _Cheirotherium_ were really produced by species of Amphibians which, like the existing Frogs, possessed hind-feet of a much larger size than the fore-feet, and to which the name of _Labyrinthodonts_ was applied in consequence of the complex microscopic structure of the teeth (fig. 149). In the essential details of their structure, the Tria.s.sic Labyrinthodonts did not differ materially from their predecessors in the Coal-measures and Permian rocks. They possessed the same frog-like skulls (fig.

150), with a lizard-like body, a long tail, and comparatively feeble limbs. The hind-limbs were stronger and longer than the fore-limbs, and the lower surface of the body was protected by an armour of bony plates. Some of the Tria.s.sic Labyrinthodonts must have attained dimensions utterly unapproached amongst existing Amphibians, the skull of _Labyrinthodon Joegeri_ (fig. 150) being upwards of three feet in length and two feet in breadth. Restorations of some of these extraordinary creatures have been attempted in the guise of colossal Frogs; but they must in reality have more closely resembled huge Newts.

Remains of _Reptiles_ are very abundant in Tria.s.sic deposits, and belong to very varied types. The most marked feature, in fact, connected with the Vertebrate fauna of the Trias, and of the Secondary rocks in general, is the great abundance of Reptilian life. Hence the Secondary period is often spoken of as the "Age of Reptiles." Many of the Tria.s.sic reptiles depart widely in their structure from any with which we are acquainted as existing on the earth at the present day, and it is only possible here to briefly note some of the more important of these ancient forms.

Amongst the group of the Lizards (_Lacertilia_), represented by _Protorosaurus_ in the older Permian strata, three types more or less certainly referable to this order may be mentioned. One of these is a small reptile which was found many years ago in sandstones near Elgin, in Scotland, and which excited special interest at the time in consequence of the fact that the strata in question were believed to belong to the Old Red Sandstone formation. It is, however, now certain that the Elgin sandstones which contain _Telerpeton Elginense_, as this reptile is termed, are really to be regarded as of Tria.s.sic age. By Professor Huxley, _Telerpeton_ is regarded as a Lizard, which cannot be considered as "in any sense a less perfectly-organised creature than the Gecko, whose swift and noiseless run over walls and ceilings surprises the traveller in climates warmer than our own." The "Elgin Sandstones" have also yielded another Lizard, which was originally described by Professor Huxley under the name of _Hyperodapedon_, the remains of the same genus having been subsequently discovered in Tria.s.sic strata in India and South Africa. The Lizards of this group must therefore have at one time enjoyed a very wide distribution over the globe; and the living _Sphenodon_ of New Zealand is believed by Professor Huxley to be the nearest living ally of this family. The _Hyperodapedon_ of the Elgin Sandstones was about six feet in length, with limbs adapted for terrestrial progression, but with the bodies of the vertebrae slightly biconcave, and having two rows of palatal teeth, which become worn down to the bone in old age. Lastly, the curious _Rhynchosaurus_ of the Trias is also referred, by the eminent comparative anatomist above mentioned, to the order of the Lizards.

In this singular reptile (fig. 151) the skull is somewhat bird-like, and the jaws appear to have been dest.i.tute of teeth, and to have been encased in a h.o.r.n.y sheath like the beak of a Turtle or a Bird. It is possible, however, that the palate was furnished with teeth.

[Ill.u.s.tration: Fig. 151.--Skull of _Rhynchosaurus articeps_. Trias.

(After Owen.)]

The group of the Crocodiles and Alligators (_Crocadilia_), distinguished by the fact that the teeth are implanted in distinct sockets and the skin more or less extensively provided with bony plates, is represented in the Tria.s.sic rocks by the _Stagonolepis_ of the Elgin Sandstones. The so-called "Thecodont" reptiles (such as _Belodon, Thecodontosaurus_, and _Paloeosaurus_, fig. 152, c, d, e) are also nearly related to the Crocodiles, though it is doubtful if they should be absolutely referred to this group. In these reptiles, the teeth are implanted in distinct sockets in the jaws, their crowns being more or less compressed and pointed, "with trenchant and finely serrate margins" (Owen).

The bodies of the vertebrae are hollowed out at both ends, but the limbs appear to be adapted for progression on the land. The genus _Belodon_ (fig. 152, c) is known to occur in the Keuper of Germany and in America; and _Paloeosaurus_ (fig. 153. e) has also been found in the Trias of the same region. Teeth of the latter, however, are found, along with remains of _Thecodontosaurus_ (fig. 153, d), in a singular magnesian conglomerate near Bristol, which was originally believed to be of Permian age, but which appears to be undoubtedly Tria.s.sic.

[Ill.u.s.tration: Fig. 152.--Tria.s.sic Reptiles. a, Skull of _Nothosaurus mirabilis_, reduced in size--Muschelkalk, Germany; b, Tooth of _Simosaurus Gaillardoti_, of the natural size--Muschelkalk, Germany; c, Tooth of _Beladon Carolinensis_--Trias, America; d, Tooth of _Thecodontosaurus antiquus_, slightly enlarged--Britain; e, Tooth of _Paloeosaurus platyodon_, of the natural size--Britain.]

The Trias has also yielded the remains of the great marine reptiles which are often spoken of collectively as the "Enaliosaurians"

or "Sea-lizards," and which will be more particularly spoken of in treating of the Jura.s.sic period, of which they are more especially characteristic. In all these reptiles the limbs are flattened out, the digits being enclosed in a continuous skin, thus forming powerful swimming-paddles, resembling the "flippers"

of the Whales and Dolphins both in their general structure and in function. The tail is also long, and adapted to act as a swimming-organ; and there can be no doubt but that these extraordinary and often colossal reptiles frequented the sea, and only occasionally came to the land. The Tria.s.sic Enaliosaurs belong to a group of which the later genus _Plesiosaurus_ is the type (the _Sauropterygia_). One of the best known of the Tria.s.sic genera is _Nothosaurus_ (fig. 152, a), in which the neck was long and bird-like, the jaws being immensely elongated, and carrying numerous powerful conical teeth implanted in distinct sockets. The teeth in _Simosaurus_ (152, b) are of a similar nature; but the orbits are of enormous size, indicating eyes of corresponding dimensions, and perhaps pointing to the nocturnal habits of the animal. In the singular _Placodus_, again, the teeth are in distinct sockets, but resemble those of many fishes in being rounded and obtuse (fig. 153), forming broad crushing plates adapted for the comminution of sh.e.l.l-fish. There is a row of these teeth all round the upper jaw proper, and a double series on the palate, but the lower jaw has only a single row of teeth. _Placodus_ is found in the Muschelkalk, and the characters of its dental apparatus indicate that it was much more peaceful in its habits than its a.s.sociates the Nothosaur and Simosaur.

[Ill.u.s.tration: Fig. 153.--Under surface of the upper jaw and palate of _Placodus gigas_. Muschelkalk, Germany.]

The Tria.s.sic rocks of South Africa and India have yielded the remains of some extraordinary Reptiles, which have been placed by Professor Owen in a separate order under the name of _Anomodontia_.

The two princ.i.p.al genera of this group are _Dicynodon_ and _Oudenodon_, both of which appear to have been large Reptiles, with well-developed limbs, organised for progression upon the dry land. In _Oudenodon_ (fig. 154, B) the jaws seem to have been wholly dest.i.tute of teeth, and must have been encased in a h.o.r.n.y sheath, similar to that with which we are familiar in the beak of a Turtle. In _Dicynodon_ (fig. 154, A), on the other hand, the front of the upper jaw and the whole of the lower jaw were dest.i.tute of teeth, and the front of the mouth must have const.i.tuted a kind of beak; but the upper jaw possessed on each side a single huge conical tusk, which is directed downwards, and must have continued to grow during the life of the animal.

[Ill.u.s.tration: Fig. 154.--Tria.s.sic Anomodont Reptiles. A, Skull of _Dicynodon lacerticeps_, showing one of the great maxillary tusks; B, Skull of _Oudenodon Bainii_, showing the toothless, beak-like jaws. From the Trias of South Africa. (After Owen.)]

It may be mentioned that the above-mentioned Tria.s.sic sandstones of South Africa have recently yielded to the researches of Professor Owen a new and unexpected type of Reptile, which exhibits some of the structural peculiarities which we have been accustomed to regard as characteristic of the Carnivorous quadrupeds. The Reptile in question has been named _Cyanodraco_, and it is looked upon by its distinguished discoverer as the type of a new order, to which he has given the name of _Theriodontia_. The teeth of this singular form agree with those of the Carnivorous quadrupeds in consisting of three distinct groups--namely, front teeth or _incisors_, eye teeth or _canines_, and back teeth or _molars_.

The canines also are long and pointed, very much compressed, and having their lateral margins finely serrated, thus presenting a singular resemblance to the teeth of the extinct "Sabre-toothed Tiger" (_Machairodus_). The bone of the upper arm (humerus) further shows some remarkable resemblances to the same bone in the Carnivorous Mammals. As has been previously noticed, Professor Owen is of opinion that some of the Reptilian remains of the Permian deposits will also be found to belong to this group of the "Theriodonts."

[Ill.u.s.tration: Fig. 155.--Supposed footprint of a Bird, from the Tria.s.sic Sandstones of the Connecticut River. The slab shows also numerous "rain-prints."]

Lastly, we find in the Tria.s.sic rocks the remains of Reptiles belonging to the great Mesozoic order of the _Deinosauria_. This order attains its maximum at a later period, and will be spoken of when the Jura.s.sic and Cretaceous deposits come to be considered.

The chief interest of the Tria.s.sic Reptiles of this group arises from the fact that they are known by their footprints as well as by their bones; and a question has arisen whether the supposed footprints of _birds_ which occur in the Trias have not really been produced by Deinosaurs. This leads us, therefore, to speak at the same time as to the evidence which we have of the existence of the cla.s.s of Birds during the Tria.s.sic period. No actual bones of any bird have as yet been detected in any Tria.s.sic deposit; but we have tolerably clear evidence of their existence at this time in the form of _footprints_. The impressions in question are found in considerable numbers in certain red sandstones of the age of the Trias in the valley of the Connecticut River, in the United States. They vary much in size, and have evidently been produced by many different animals walking over long stretches of estuarine mud and sand exposed at low water. The footprints now under consideration form a double series of _single_ prints, and therefore, beyond all question, are the tracks of a _biped_--that is, of an animal which walked upon two legs. No living animals, save Man and the Birds, walk habitually on two legs; and there is, therefore, a _prima facie_ presumption that the authors of these prints were Birds. Moreover, each impression consists of the marks of three toes turned forwards (fig. 155), and therefore are precisely such as might be produced by Wading or Cursorial Birds. Further, the impressions of the toes show exactly the same numerical progression in the number of the joints as is observable in living Birds--that is to say, the innermost of the three toes consists of three joints, the middle one of four, and the outer one of five joints. Taking this evidence collectively, it would have seemed, until lately, quite certain that these tracks could only have been formed by Birds. It has, however, been shown that the Deinosaurian Reptiles possess, in some cases at any rate, some singularly bird-like characters, amongst which is the fact that the animal possessed the power of walking, temporarily at least, on its hind-legs, which were much longer and stronger than the fore-limbs, and which were sometimes furnished with no more than three toes. As the bones and teeth of Deinosaurs have been found in the Tria.s.sic deposits of North America, it may be regarded as certain that _some_ of the bipedal tracks originally ascribed to Birds must have really been produced by these Reptiles. It seems at the same time almost a certainty that others of the three-toed impressions of the Connecticut sandstones were in truth produced by Birds, since it is doubtful if the bipedal mode of progression was more than an occasional thing amongst the Deinosaurs, and the greater number of the many known tracks exhibit no impressions of fore-feet. Upon the whole, therefore, we may, with much probability, conclude that the great cla.s.s of Birds (_Aves_) was in existence in the Tria.s.sic period.

If this be so, not only must there have been quite a number of different forms, but some of them must have been of very large size. Thus the largest footprints. .h.i.therto discovered in the Connecticut sandstones are 22 inches long and 12 inches wide, with a proportionate length of stride. These measurements indicate a foot four times as large as that of the African Ostrich; and the animal which produced them--whether a Bird or a Deinosaur--must have been of colossal dimensions.

[Ill.u.s.tration: Fig. 156.--Lower jaw of _Dromatherium sylvestre_.

Trias, North Carolina. (After Emmons.)]

[Ill.u.s.tration: Fig. 157.--a, Molar tooth of _Micro estes antiquus_, magnified; b, Crown of the same, magnified still further. Trias, Germany.]

[Ill.u.s.tration: Fig. 158.--The Banded Ant-eater (_Myrmecobius fasciatus_) of Australia.]

Finally, the Trias completes the tale of the great cla.s.ses of the Vertebrate sub-kingdom by presenting us with remains of the first known of the true Quadrupeds or _Mammalia_. These are at present only known by their teeth, or, in one instance, by one of the halves of the lower jaw; and these indicate minute Quadrupeds, which present greater affinities with the little Banded Anteater (_Myrmecobius fasciatus_, fig. 158) of Australia than with any other living form.

If this conjecture be correct, these ancient Mammals belonged to the order of the Marsupials or Pouched Quadrupeds (_Marsupialia_), which are now exclusively confined to the Australian province, South America, and the southern portion of North America. In the Old World, the only known Tria.s.sic Mammals belong to the genus _Microlestes_, and to the probably identical _Hypsiprymnopsis_ of Professor Boyd Dawkins. The teeth of _Microlestes_ (fig. 157) were originally discovered by Plieninger in 1847 in the "bone-bed"

which is characteristic of the summit of the Rhaetic series both in Britain and on the continent of Europe; and the known remains indicate two species. In Britain, teeth of _Microlestes_ have been discovered by Mr Charles Moore in deposits of Upper Tria.s.sic age, filling a fissure in the Carboniferous limestone near Frome, in Somersetshire; and a molar tooth of _Hypsiprymnopsis_ was found by Professor Boyd Dawkins in Rhaetic marls below the "bone-bed" at Watchet, also in Somersetshire. In North America, lastly, there has been found in strata of Tria.s.sic age one of the branches of the lower jaw of a small Mammal, which has been described under the name of _Dromatherium sylvestre_ (fig. 156). The fossil exhibits ten small molars placed side by side, one canine, and three incisors, separated by small intervals, and it indicates a small insectivorous animal, probably most nearly related to the existing _Myrmecobius_.

LITERATURE.

The following list comprises a few of the more important sources of information as to the Tria.s.sic strata and their fossil contents:--

(1) 'Geology of Oxford and the Valley of the Thames.' Phillips.

(2) 'Memoirs of the Geological Survey of Great Britain and Ireland.'

(3) 'Report on the Geology of Londonderry,' &c. Portlock.

(4) "On the Zone of Avicula contorta," &c.--'Quart. Journ. Geol.

Soc.,' vol. xvi., 1860. Dr Thomas Wright.

(5) "On the Zones of the Lower Lias and the Avicula contorta Zone"--'Quart. Journ. Geol. Soc.,' vol. xvii., 1861. Charles Moore.

(6) "On Abnormal Conditions of Secondary Deposits," &c.--'Quart.

Journ. Geol. Soc.,' vol. xxiii., 1876-77. Charles Moore.

(7) 'Geognostische Beschreibung des Bayerischen Alpengebirges.'

Gumbel.

(8) 'Lethaea Rossica.' Pander.

(9) 'Lethaea Geognostica.' Bronn.

(10) 'Petrefacta Germaniae.' Goldfuss.

(11) 'Petrefaktenkunde.' Quenstedt.

(12) 'Monograph of the Fossil Estheriae' (Palaeontographical Society).

Rupert Jones.

(13) "Fossil Remains of Three Distinct Saurian Animals, recently discovered in the Magnesian Conglomerate near Bristol"--'Trans.

Geol. Soc.,' ser. 2, vol. v., 1840. Riley and Stutchbury.

(14) 'Die Saurier des Muschekalkes.' Von Meyer.

(15) 'Beitrage zur Palaeontologie Wurttembergs.' Von Meyer and Plieninger.

(16) 'Manual of Palaeontology.' Owen.

(17) 'Odontography:' Owen.

(18) 'Report on Fossil Reptiles' (British a.s.sociation, 1841). Owen.

(19) "On Dicynodon"--'Trans. Geol. Soc.,' vol. iii., 1845. Owen.

(20) 'Descriptive Catalogue of Fossil Reptilia and Fishes in the Museum of the Royal College of Surgeons, England.' Owen.

(21) "On Species of Labyrinthodon from Warwickshire"--'Trans. Geol.

Soc.,' ser. 2, vol. vi. Owen.

(22) "On a Carnivorous Reptile" (Cynodraco major), &c.--'Quart.

Journ. Geol. Soc.,' vol. x.x.xii., 1876. Owen.