Book - Vertebrate Zoology (1928) 39

From Embryology

Vertebrate Zoology G. R. De Beer (1928)

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Chapter XXXIX The Evolution Of The Reptiles

That the Reptiles were evolved from the amphibia there is no doubt whatever, and indeed, in some cases it is difficult to decide whether a fossil is an amphibian or a reptile. The most important distinctive features are the fact that the centra of the vertebral column are formed from the pleurocentral elements while the hypocentral elements are very much reduced, and the absence of grooves for lateral-line canals on the skull. This latter point shows that the reptiles had become definitely terrestrial. They emancipated themselves from the water by overcoming the three obstacles which checked the amphibia ; viz. the necessity of water for breathing, for copulating, and for the embryo to develop in.

The first of these was countered by a better development of the lungs and the adoption of the method of expanding the thoracic cavity by means of the ribs, for replenishing their content of air. The skin was thus enabled to become dry and horny, and to be of greater efficiency in protection.

The next difficulty was surmounted by the development of copulatory organs with which the sperm could be inserted straight into the oviducts of the female, and fertilisation was internal. The sperm swam to the egg in the mucous fluid of the oviduct instead of in the pond water.

The last obstacle was overcome by a group of adaptations. The egg was laid on land, and the albumen surrounding it was itself surrounded by a shell composed of lime salts secreted by the oviducts. In this way the egg was protected from drying up, and from becoming flattened and collapsed like a " poached egg," which it would otherwise be. The embryo became surrounded by upgrowths of the blastoderm forming the amnion and enclosing the amniotic cavity. The embryo developed in the fluid contents of this cavity, which may thus be regarded as an artificial enclosed pond. The food requirements of the developing embryo were met as in lower forms by a store of yolk in the yolk-sac. This entailed no new modifications by itself, but as embryonic development took a longer time, the quantity of yolk in the egg was relatively greater and this necessitated the modification of the process of gastrulation, and the formation of a primitive streak. There remained the difficulty of breathing, for although the gill-slits were developed they opened into the amniotic cavity, the oxygen- content of which could not be renewed. The problem was solved by the development of the allantois, representing the bladder of the amphibia. The allantois became applied to the inner surface of the porous shell, and as it was highly vascularised, the respiratory exchange took place in it. At the same time, the allantois served as a receptacle for the non- volatile excretory products of the embryo during development. Because of these structural adaptations, the reptile does not pass through a metamorphosis, but hatches from the egg as a more or less perfect miniature replica of the. adult.

In the reptiles, the head is capable of extensive independent movement, and a definite neck is formed. In this connexion, the two first vertebrae become modified into the atlas and the axis.

The most primitive known reptile is Seymouria (Permian), and it is remarkable for the fact that its characteristics are not intermediate between those of amphibia and of reptiles, but some of its characters are frankly .amphibian and others reptilian. Seymouria is therefore a mosaic transitional form. It is probable that the transition from amphibia to reptiles took place in the Carboniferous.

Seymouria belongs to the group of Reptiles known as the Cotylosaurs, and they preserve the complete covering of dermal bones over the skull which they inherited from their Stegocephalian amphibian ancestors. The nature of the skull is of importance in tracing out the lines of evolution of the reptiles, and those forms in which the roof is complete and imperforate are often grouped together as the Anapsida.

The Chelonia are often classified among the Anapsida because their skull-roof is not fenestrated, although it may be reduced by emargination ; i.e. bones may be lost round the edge but there is no separation of bones by a perforation forming a fossa. The Permian fossil Eunotosaurus which had osteoscutes and expanded ribs appears to be intermediate between Cotylosauria and Chelonia. The Chelonia preserve the osteoscutes (which covered the body of the Stegocephalia) and they contribute to the formation of the carapace which is so distinctive a feature of the Chelonia. The Triassic Triassochelys still had teeth and the cleithrum ; these structures are absent from existing forms. The clavicular pectoral girdle of existing Chelonia, consisting of clavicles and interclavicle, is associated with the ventral covering of osteoscutes that form the plastron. The scapular pectoral girdle consists of a coracoid and a scapula bearing a large process as big as itself, directed forwards and inwards. This girdle and the pelvic girdle are remarkable in that they are situated within the ribs instead of outside them as in normal forms. It is interesting to note that some Chelonia have become secondarily adapted to life in water, and their limbs have been modified into paddles or flippers, as in the turtles.

The osteoscutes of the carapace are covered by corneoscutes (" tortoise-shell ") except in Sphargis, the " leathery turtle," which form is further interesting in that the carapace attached to the expanded ribs as in other Chelonia is not present. Instead there is a bony shell formed of a great many little polygonal osteoscutes bearing no relation to the ribs.

The 5th metatarsal is hook-shaped in the Chelonia, but normal in the other Anapsida (Cotylosauria).

The next group of reptiles to consider is the Synapsida. They are characterised by the fact that the skull-roof is perforated by one inferior temporal vacuity or fossa, on each side, and the 5th metatarsal is normal. Here belong the Plesiosaurs and Theromorphs.

The Theromorphs are a very important group. They appear in the Permian, and preserve many primitive characters. They may have a precoracoid as well as a coracoid in the shoulder girdle, and some even retain the cleithrum. The most highly developed forms are the Theriodonts, which are the ancestors of the mammals, and they foreshadow the characters of the latter in many respects. The skull had two occipital condyles, a false palate was present, and the teeth were modified into incisors, canines, premolars and molars. The dentary was large and beginning to take on the articulation with the squamosal, while the quadrate became small and loose. In the pelvic girdle the ilium showed the mammalian character of pointing forwards, and the limbs were long and supported the body clean off the ground. A typical Theriodont is Cynognathus, but it is probable that several of its characters were evolved parallel with the mammals, having been derived from a more primitive ancestor common to it and to the mammals.

Among Synaptosauria, the Sauropterygia or Plesiosaurs have become secondarily adapted to an aquatic mode of life. They preserve primitive features such as the gastralia, which are remnants of the ventral dermal bones or osteoscutes of the Stegocephalian amphibia, but their limbs become modified into paddles. This modification has not proceeded as far in the Triassic Nothosaurus as in the Jurassic Plesiosaurus. The Plesiosaurs reached lengths of 50 feet.

In the next group or Parapsida, the roof of the skull was perforated by a single superior temporal vacuity, above the postorbital and squamosal bones. The hind border of the vacuity is formed by a bone concerning the homology of which doubt remains (see p. 104), but which may be the supratemporal. Here belong the Ichthyosaurs and the Squamata (Lacertilia and Ophidia).

The Ichthyosaurs are primitive in retaining the gastralia, and a foramen for the pineal eye between the parietals, but otherwise they are specialised in adaptation to an aquatic mode of life. Median dorsal and tail-fins are developed, and the limbs become modified into paddles, so much so that it is impossible to determine the nature of the 5th metatarsal. A series of progressive modification can be traced from the Triassic Mixosaurus, through the Jurassic Ichthyosaurus to the Cretaceous Ophthalmosaurus. They reached lengths of 30 feet.

All the Squamata which possess limbs have a hook-shaped 5th metatarsal. The first group of these are the Lacertilia, first appearing in the Jurassic, and represented now by the lizards, geckos, and chamaeleons. In the Cretaceous, a group of Lacertilia became adapted to an aquatic life, — the Mosasauria. They reached a length of as much as 40 feet, and their limbs became modified into paddles. The second group of the Squamata are the Ophidia or snakes. It is characteristic of the Squamata that the quadrate is loose, and in the Ophidia the two halves of the lower jaw are separate, which enables relatively enormous mouthfuls to be swallowed.

The remaining reptiles form the group Diapsida, for their skull roof is perforated by two temporal fossae or vacuities on each side. So far as is known, all of them have a hook-shaped 5th metatarsal. Here belong the Rhynchocephalia, the Crocodilia, the Dinosaurs and the Pterosaurs, and the Diapsida also contained the ancestors of the birds.

The Rhynchocephalia appear in the Triassic with Rhynchosaurus, and are represented to-day by Sphenodon. They are primitive in retaining the gastralia. The Triassic Thalattosaurs, which had paddle-like limbs, were probably related to the Rhynchocephalia.

The Crocodilia form a large group of generalised reptiles, possibly dating back to the Permian. The Triassic Pseudosuchia, of which Euparkeria is an example, are regarded as related to the ancestors of the Rhynchocephalia, the Dinosaurs and Pterosaurs, the existing crocodiles, and the birds. The living crocodiles and alligators retain the gastralia, and other dermal ossifications. Mention may be made of the Jurassic Thalattosuchia, yet another group of reptiles which became secondarily adapted to aquatic life with paddle-like limbs.

The Dinosaurs were the dominant animals in the Jurassic and Cretaceous. The skull had two temporal vacuities on each side, and in addition a prelachrymal vacuity. Some were quadrupedal and herbivorous, such as Diplodocus

442 EVOLUTIONARY MORPHOLOGY (Jurassic), reaching the immense length of 90 feet. Others were carnivorous with formidable teeth, and the hind limbs larger than the fore limbs, so that they were probably bipedal. An example of such a form is Tyrannosaurus, Cretaceous, reaching almost 50 feet in length. The foregoing types of Dinosaurs had a pelvis of normal shape, and are grouped together under the term " Saurischia." The remainder, Predentata (or Ornithischia), have an additional postpubis which stretches back beneath the ischium, and a predentary bone in the lower jaw. The Predentata include the herbivorous bipedal Iguanodon from the Cretaceous, over 30 feet long ; the absurd-looking Stegosaurus with its armour of large bony plates (Jurassic, 20 feet long) ; and the horned Triceratops (Cretaceous, 25 feet long).

The Pterosaurs were closely allied to the Dinosaurs, and like them had a pair of temporal vacuities and a prelachrymal vacuity on each side. They also preserved gastralia, and while some had teeth others were toothless. The fore limbs were modified for flying, by means of a web of skin stretched from the greatly elongated fourth finger. The Cretaceous Pteranodon had an expanse of wings measuring 25 feet. Of all this enormous wealth of reptilian life which dominated the land, water, and air, in the Jurassic and Cretaceous periods, only the Squamata, the Chelonia, the Crocodilia, and the Rhynchocephalia have survived, and in very reduced numbers. The rest went extinct before the Eocene. It may be that a reduction of temperature put an end to them, or that the food supply became deficient. Certain it is that their brains were ridiculously small, and they can have been no match for the small and agile mammals in intelligence.

Two main points for consideration arise out of a study of the reptiles. The first concerns the structure of the 5th metatarsal bone. In the Cotylosaurs it was of a normal shape, as also in the Synapsida. Now, the Synapsida are to be regarded as having been derived from the Cotylosaurs, and one group of them, the Theromorphs, gave rise to the mammals. The line of mammalian descent is therefore characterised by the possession of a normal-shaped straight 5th metatarsal.

On the other hand, the other reptiles such as the Chelonia, together with the Squamata, and all the Diapsida have a hook-shaped 5th metatarsal. The birds were derived from a Diapsid stock, and so it may be said, therefore, that the line of avine descent is characterised by the possession of a hook-shaped 5th metatarsal. Actually what this modification means or what function it serves is unknown, but it is to be noticed that among the animals possessing it are forms which live on land, in the water, and in the air, so that it would seem not to have an adaptive significance, nor to be capable of modification by different modes of life. It looks, therefore, as if it could be used as a diagnostic feature inherited from a common ancestor by all the forms possessing it. This common ancestor was probably a late Anapsidan, and the importance of this matter is that from this point onwards the reptiles were divided into two main and divergent branches. One branch which may be called the Sauropsidan includes the Chelonia, the Parapsida, the Diapsida, and the birds. The other or Theropsidan branch includes the Synapsida perhaps (the Synaptosauria), and the mammals. As a result of these considerations, it appears that the term " Reptilia " is applied not so much to a unified group of related animals as to two divergent stocks. It therefore refers to a grade of structure and degree of evolution ; and when the knowledge of fossil forms is more complete, it will be possible to abolish the class " Reptilia," or to restrict it to the primitive Anapsida, and to substitute the classes " Sauropsida " and " Theropsida," containing the birds and mammals respectively.

That these conclusions are sound is shown by a consideration of the aortic arches. It is to be noticed that all the living reptiles belong to the Sauropsidan branch, and in all of them the systemic aorta is split into two right down to the ventricle of the heart. The result is that there are right and left systemic arches springing respectively from the left and right sides of the ventricle. The condition of the bird fits into this scheme, for it differs from the arrangement in the crocodile only by the loss of the left arch. Now, in the mammal, the systemic aorta is single and undivided. The point is that it is impossible to derive the Sauropsidan type of aorta from the mammalian, or vice versa, and it is necessary to go back to a primitive type like that of the amphibia where the aorta is not only undivided but the pulmonary arch has not yet become separated off. The primitive Cotylosaurs may have been of this type. It is certain that the Synapsida must have resembled the mammal (for the latter was derived from the former), and therefore differed from the Sauropsida as regards the structure of the aortic arches. This is the same divergence which appeared from a consideration of the hook-shaped 5th metatarsal.


Goodrich, E. S. On the Classification of the Reptilia. Proceedings of the Royal Society, Ser. B, vol. 89, 191 6.

Nopcsa, F. Die Familien der Reptilien. Fortschritte der Geologie und Palaeontologie, 2, 1923.

Watson, D. M. S. On Seymouria, the Most Primitive known Reptile. Proceedings of the Zoological Society, London, 1918.

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
Vertebrate Zoology 1928: PART I 1. The Vertebrate Type as contrasted with the Invertebrate | 2. Amphioxus, a primitive Chordate | 3. Petromyzon, a Chordate with a skull, heart, and kidney | 4. Scyllium, a Chordate with jaws, stomach, and fins | 5. Gadus, a Chordate with bone | 6. Ceratodus, a Chordate with a lung | 7. Triton, a Chordate with 5-toed limbs | 8. Lacerta, a Chordate living entirely on land | 9. Columba, a Chordate with wings | 10. Lepus, a warm-blooded, viviparous Chordate PART II 11. The development of Amphioxus | 12. The development of Rana (the Frog) | 13. The development of Gallus (the Chick) | 14. The development of Lepus (the Rabbit) PART III 15. The Blastopore | 16. The Embryonic Membranes | 17. The Skin and its derivatives | 18. The Teeth | 19. The Coelom and Mesoderm | 20. The Skull | 21. The Vertebral Column, Ribs, and Sternum | 22. Fins and Limbs | 23. The Tail | 24. The Vascular System | 25. The Respiratory system | 26. The Alimentary system | 27. The Excretory and Reproductive systems | 28. The Head and Neck | 29. The functional divisions of the Nervous system | 30. The Brain and comparative Behaviour | 31. The Autonomic Nervous system | 32. The Sense-organs | 33. The Ductless glands | 34. Regulatory mechanisms | 35. Blood-relationships among the Chordates PART IV 36. The bearing of Physical and Climatic factors on Chordates | 37. The origin of Chordates, and their radiation as aquatic animals | 38. The evolution of the Amphibia : the first land-Chordates | 39. The evolution of the Reptiles | 40. The evolution of the Birds | 41. The evolution of the Mammalia | 42. The evolution of the Primates and Man | 43. Conclusions | Figures | Historic Embryology

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