Chapter 10 of 16 · 2040 words · ~10 min read

CHAPTER VII

REPTILIA

DEFINITION AND CHARACTERS–POSITION OF THE CLASS REPTILIA IN THE PHYLUM VERTEBRATA–CLASSIFICATION–SKULL AND VERTEBRAE.

The recent Reptiles comprise, broadly speaking, the Crocodiles, Tortoises, Lizards, and Snakes. They are the only Vertebrates which are cold-blooded, breathe by lungs, and have a median occipital condyle. Another equally sufficient diagnosis is the following:–Tetrapoda, with a median occipital condyle, with nucleated red blood-corpuscles, and with complete right and left functional aortic arches. A still shorter diagnosis is:–Monocondylia with a scaly skin.

If our diagnosis is to include the fossil Reptiles we have not only to discard the characters drawn from the soft parts as unavailable, but we are forced to treat the condition of the occipital condyle with caution, since there exist, or must have existed, transitional stages between Reptiles and Amphibia and Mammals; and the winged class Pterosauria does not permit us to use the wings as a differential character for the Birds. In fact, while the Reptilia are sufficiently separated from the Amphibia by their absolutely gastrocentrous vertebrae, it is difficult to distinguish them as a class from the Birds; hence the term SAUROPSIDA, which is intended to indicate the close relationship of the Reptiles to the Birds in opposition to the Mammalia, and to the ICHTHYOPSIDA or Amphibia and Fishes. However, the Reptilia take up a very central position in the evolution of the main classes of the Vertebrata. On the one hand, there is not the slightest doubt that they are evolved from some branch of {278}the Stegocephali, whilst on the other hand the Reptiles, probably through some branch of the Theromorpha, have given rise to the Mammals; some other Reptilian branch, at present unknown, has blossomed out into the Birds.

PRINCIPAL CHARACTERS OF THE REPTILIA.

1. The vertebrae are gastrocentrous.

2. The skull articulates with the atlas by one condyle, which is formed mainly by the basioccipital.

3. The mandible consists of many pieces and articulates with the cranium through the quadrate bones.

4. There is an auditory columellar apparatus fitting into the fenestra ovalis.

5. The limbs are of the tetrapodous, pentadactyle type.

6. There is an intracranial hypoglossal nerve.

7. The ribs form a true sternum.

8. The ilio-sacral connexion is post-acetabular.

9. The skin is covered (_a_) with scales, but (_b_) neither with feathers nor with hairs; and there is a great paucity of glands.

10. Reptiles are poikilothermous.

11. The red blood-corpuscles are nucleated, biconvex, and oval.

12. The heart is divided into two atria and an imperfectly divided ventricle. It has no conus, but semilunar valves exist at the base of the tripartite aortic trunk.

13. The right and left aortic arch are complete and remain functional.

14. Respiration is effected by lungs; and gills are entirely absent, even during embryonic life.

15. Lateral sense-organs are absent.

16. The kidneys have no nephrostomes. Each kidney has one separate ureter.

17. There is always a typical cloaca.

18. The eggs are meroblastic.

19. Fertilisation is internal, and is effected, with the single exception of _Sphenodon_, by means of male copulatory organs.

20. An amnion and an allantois are formed during development.

Numbers 1, 2, 6, 7, 8, 14, 16, 18, 20 separate the Reptiles from the Amphibia. Cf. also pp. 4 and 5.

Numbers 9 (_b_), 10, 12, and 13 separate them from the Birds and Mammals.

Numbers 3, 8, and 11 separate them from the Mammals.

The EVOLUTION OF THE CLASSIFICATION OF THE REPTILES has to a certain extent been already treated on pp. 7-9. For a long time only Chelonia or Tortoises, Ophidia or Snakes, and Saurii were recognised as their principal divisions. Then the Crocodiles were separated from the Lizards; later the Coeciliae were removed from the Snakes and referred to the Amphibia, {279}and ultimately _Sphenodon_ was recognised as deserving a separate position, equal in rank to the other groups. Stannius showed that the Crocodiles and Tortoises are relatively near allies in opposition to the likewise closely allied Lizards and Snakes (_Sphenodon_ was then unknown), and he expressed this by the term Monimostylica, or creatures with fixed quadrate bones, for the former, and Streptostylica, creatures with movable quadrates, for the latter combination. The fossil Reptiles were hardly allowed proper places in the system. In various zoological text-books they were, or are even now, treated as inconvenient, outlying, or supernumerary members. A long time elapsed before, thanks to the labours of H. von Meyer, Owen, Huxley, Marsh, Cope, Zittel, and Seeley, it was recognised that the extinct groups form the preponderant mass of Reptiles, and that it is the recent groups which, in spite of the bewildering number of species of Lizards and Snakes, are the comparatively few and much-reduced members of a once flourishing class. With the exception of the Lizards and Snakes, which are on the ascending branch, the modern _Sphenodon_, the Crocodiles and the Tortoises are a mere fraction, comprising a few survivals of richly-developed groups, while all the others, the overwhelming majority, have died out.

The classification adopted in this volume is as follows:–

CLASS REPTILIA.

Sub-Class I. Proreptilia.

" II. Prosauria. Orders: Microsauri, Prosauri.

" III. Theromorpha. Orders: Pareiasauri, Theriodontia, Anomodontia, Placodontia.

" IV. Chelonia. Orders: Athecae, Thecophora.

" V. Dinosauria. Orders: Sauropoda, Theropoda, Orthopoda, Ceratopsia.

" VI. Crocodilia. Orders: Pseudosuchia, Parasuchia, Eusuchia.

" VII. Plesiosauria. Orders: Nothosauri, Plesiosauri.

" VIII. Ichthyosauria.

" IX. Pterosauria.

" X. Pythonomorpha. Orders: Dolichosauri, Mosasauri.

" XI. Sauria. Orders: Lacertilia, Ophidia.

The eleven principal groups are here called "sub-classes" to emphasise the undeniable fact that these Reptilian groups are of much greater morphological value than those which are most generally called "Orders" in the Mammalia, that class which we consider as the standard or model of classificatory units.

{280}[Illustration: FIG. 54.–Diagrams of skulls, showing especially the composition of the bony arches of the orbito-temporal region.

A, C, D, E, THEROMORPHA. A, _Elginia_, p. 305; C, _Cynognathus_, p. 306; D, _Gordonia_, p. 310; E, _Dicynodon_, p. 310.

B, G, PROSAURIA. B, _Sphenodon_, p. 294; G, _Palaeohatteria_, p. 291.

F, CROCODILIA, p. 434.

H, I, K, CHELONIA, p. 316. H, _Chelydra_, p. 338; I, _Chrysemys_, p. 346; K, _Cistudo_, p. 361.

_E_, Epiotic; _F_, frontal; _IT_, infratemporal fossa; _J_, jugal, shaded vertically; _L_, lacrymal; _M_, maxillary; _N_, nasal groove; _Na_, nasal bone; _O_, orbit; _P_, parietal; _Po_, postorbital, dotted; _Pf_, post-frontal; _Pm_, premaxillary; _Pr_, prefrontal; _Ptg_, pterygoid; _Q_, quadrate; _Qj_, quadrato-jugal; _So_, supra-occipital; _Sq_, squamosal, shaded obliquely; _St_ (in B-E), supratemporal fossa; _St_ (in A), Supratemporal bone.]

{281}[Illustration: FIG. 55.–Diagrams of skulls, showing especially the composition of the bony arches of the orbito-temporal region.

L, PYTHONOMORPHA. _Clidastes_, p. 490.

M, N, O, LACERTILIA, p. 496. M, _Varanus_, p. 543; N, _Uromastix_, p. 524; O, _Lacerta_, p. 550.

P, ICHTHYOSAURIA, p. 479. _Ichthyosaurus_, p. 483.

Q, PTEROSAURIA, p. 484. _Dimorphodon_, p. 486.

R, AVES, generalised, for comparison.

S, MAMMALIA, generalised, for comparison.

T, OPHIDIA, p. 581.

_C_, Condyle of mandible; _Col_, columella cranii; _F_, frontal; _I_, interparietal or pineal foramen; _I.A_, Inner angle of mandible; _J_, jugal, shaded vertically; _L_, lacrymal; _M_, maxillary; _N_, nasal groove; _Na_, nasal bone; _O_, orbit; _O_{1}_, preorbital fossa; _P_, parietal; _Pf_, postfrontal; _Pm_, premaxillary; _Pr_, prefrontal; _Ptg_, pterygoid; _Q_, quadrate; _Qj_, quadrato-jugal; _Sq_, squamosal, shaded obliquely; _St_, supratemporal bone.]

{282}The families cannot well be changed, and terms like super-families and super-orders are sometimes resorted to by those who do not like to look stern facts in the face.

The sequence of the groups, although arranged as much as possible in ascending order, is of necessity as unnatural as that of the maps in an atlas. We cannot yet construct a satisfactory phyletic tree of the Reptiles. The Proreptilia connect them with the Amphibia. Next follow the Prosauria with _Sphenodon_ among the Prosauri as the key to most other groups. Then follow the Theromorpha, and it is probable that from various branches of these have arisen the Chelonia, Dinosauria, Crocodilia, and Plesiosauria. The descent of the Ichthyosauria is very problematic. The same applies to the Pterosauria and to the Pythonomorpha, but it is possible that they, together with the Sauria, are connected with the Prosauria.

With all reserve these hypothetical affinities may be expressed by the following diagram:–

The eleven sub-classes of the Reptilia present so many important differences that it is not advisable to give here a further general account of their structure. The diagrammatic figures A to T on pp. 280, 281, representing various types of skulls, are intended to explain their chief modifications, all referable to Proreptilian and to certain Theromorphous conditions. One of the most important features is that the mandible, which is always composed of many pieces (cf. Fig. 142, p. 550), is invariably carried by the quadrate bone. Diagrams of the generalised skulls of a Bird and a Mammal have been added for comparison.

{283}[Illustration: FIG. 56.–Composition of vertebrae of Reptiles, illustrated by the first and second cervical vertebrae. (1) Atlas (first cervical) and axis (second) vertebra of _Crocodilus_. (2) Atlas and axis of _Metriorhynchus_, a Jurassic Crocodile. (3) Analysis of the first two cervical vertebrae of a Crocodile; 2, second basiventral complex or "intercentrum" continued upwards into the meniscus or intervertebral pad. (4) Diagram of the fundamental composition of a Reptilian vertebra; compare this and (6) with Fig. 1 (8 and 9) on p. 13. (5) The first three cervical vertebrae of _Sphenodon_. (6) Trunk-vertebrae of _Eryops_, a Permian Proreptile; typically temnospondylous; _cp_, articular facet of the capitulum of a rib. (7) The complete atlas of an adult _Trionyx hurum_; the second basiventral (intercentrum) is attached to the posterior end of the first centrum, which, not being fused with the second centrum, is not yet an odontoid process. (8) The complete atlas of an adult _Trionyx gangeticus_; still typically temnospondylous. (9) The first and second cervical vertebrae of an adult _Platemys_. (10) The complete atlas of a _Chelys fimbriata_. _Az_, Anterior zygapophysis; _B.D_, basidorsal; _B.V_, basiventral; _C_{1}_, _C_{2}_, _C_{3}_, first, second, and third centra, formed by the interventralia; _Cp^1_, _Cp^2_, articular facets of the capitular portions of the first and second ribs; _I.V_, interventral; _N_{1}_, _N_{2}_, _N_{3}_, first, second, and third neural arch, formed by basidorsalia (_B.D_); _Od_, odontoid process = first centrum; _Pz_, posterior zygapophysis; _R_{1}_, _R_{2}_, ribs; _Sp_, detached spinous process of the first neural arch; _t_{1}_, _t_{2}_, tubercular attachments of the first and second ribs; 1, 2, 3, 4, "intercentra" = basiventrals; _I_, _II_, _III_, position of the exit of the first, second, and third spinal nerves.]

As mentioned on p. 278 the vertebrae of the Reptilia and those of all other Amniota are _gastrocentrous_; that is to say the centra or bodies of the vertebrae are formed by the pairs of interventralia, while the basiventralia are reduced, persisting either as so-called intercentra or wedge-bones, or as intervertebral pads, or disappearing altogether. At the earlier stages of development the gastrocentrous vertebrae behave in the {284}same way as that described on p. 12 (Fig. 1), except that the interdorsal elements are suppressed from the beginning. If the remaining three pairs of constituent elements of each vertebra (the basidorsalia, forming the neural arch; the interventralia, forming the body or centrum; and the basiventralia) remain separate, the vertebrae are called _temnospondylous_ (τέμνω, I cut, σπόνδυλος, a vertebra). If the neural arches and the centra are suturally united or are fused with each other, the vertebrae are called _stereospondylous_ (στερεός, solid). In many Amniota the atlas or first vertebra remains in a relatively primitive, embryonic condition, and is temnospondylous but for the usual modification that its centrum becomes attached to that of the second vertebra, and forms the odontoid process of the latter. The composition of gastrocentrous vertebrae (cf. p. 282) is best illustrated by the first and second cervical vertebrae of the Crocodile (Fig. 56, 3, p. 283).

Concerning GEOGRAPHICAL DISTRIBUTION, even a cursory study shows that the sub-classes have come into existence at very different geological periods, and have each followed their own lines of dispersal.

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