5. The Evolution of Metazoa

5.4 The Reptiles

Amphibians lay eggs in water. The development and hatching of these eggs depend on constant humidity and an optimal temperature. The hatched animal is still an embryo that can only live in water. Reptiles, on the other hand, protect their eggs with a solid capsule. These eggs are laid on solid ground and, within the egg, the embryo breathes thanks to a special membrane, the amnion, and feeds on abundant reserves. The emergence of reptiles at the end of the Carboniferous period coincides with a glaciation that was followed by a period that was dry and cool. These were conditions the reptiles were ready to face. The reptiles were favored by a dry climate that restrained the efflorescence of the amphibians.

5.4.1 The evolutive lines

The first two reptile groups to appear at the end of the Carboniferous period, about 230 million years ago, were the anapsids and the synapsids. These primitive groups reached their apogee during the Trias period and, after 30 million years, started to decline (fig. 5.18). It was during a long time thought that the primitive anapsids are still represented today by turtles, but it now appears that the turtles are diapsids, appearing late (about 200 million years ago, in the Triassic, not shown in this figure) and related to the crocodilians, despite the obvious lack of support from morphology6.

Figure 5.18. Genealogy tree of the mammals.

Anapsids and synapsids were the first reptilian groups to appear. All the synapsids are gone. This group represented a trend towards a higher organizational order that could not express itself because of the presence of the Dinosaurians. (I did not depict the genealogy tree of the Dinosaurians, to not clutter the picture). The synapsids evolved in monotremes, allotherians and architherians. Among the architherians, panthotherians occupied a niche neglected by the dinosaurians: the panthotherians fed on insects and fruits. From them evolved the Marsupials and the true placentary mammals, i.e. the eutherians. There is no representative left of the synapsids. Yet, these are at the origin of the mammals. The mammalian characters of Cynognathus (fig. 5.19) are countless.

Figure 5.19. Cynognathus is a Therapsid reptile. It belongs to the synapsid group and lived 200 million years ago. Teeth and jaws developed for mastication purposes, while reptiles swallow their prey. The eardrum of Cynognathus is loca­ted at the end of an external auditive conduct, which is not the case with reptiles. Most important is that the axial skull has opened (see fig. 5.20).

A reconstruction of the skull of a crossopterygian shows, on the outside, the various dermic plates that form the cephalic shield inherited from the agnates. Many teeth are present at the outer edge. Inside this heavy armor, the brain i.e. the encephalon, is protected once more by an axial skull that envelops not only the encephalon but also the organs of smell and hearing. A comparison of the skull patterns of a crossopterygian, a synapsid and a mammal reveals that the pleurosphenoid bone is disappearing in the synapsid model, and has completely disappeared in the mammal. This stage is already reached by Cynognathus. By opening, the axial skull leaves a place for additional nervous material that may invade the totality of the head.

Figure 5.20. Transversal slides through the skull (at the height of the basipterygoid apophyse) demonstrate that the skull of a crossopterygian fish encloses an axial skull that protects the nervous material. This axial skull opens up during evolution, leading from fish to mammal. This stage is reached by Cynognathus, a synapsid reptile. In this way, more room is left for nervous tissue. In mammals (dog), the pleurosphenoïd bone no longer exists.

Within the synapsid group, at least four evolutive lines tended towards such a superior organizational order. Since these lines originated early in the course of reptilian evolution, one would expect the passage to the mammalian order to occur fast, provided such an order were really giving a definite survival edge over the reptiles. This was not the case. Mammals exist for 190 millions years. Yet, during the first 120 million years, they were fully dominated by the dinosaurians. One can only guess that these reptiles were superior to the mammals during all that time.

The synapsids disappeared almost totally at the end of the Trias period. The reason for this disappearance might be correlated with an event that is not that extraordinary. At least 150 impacts of asteroids with the earth are known, of which the vast majority occurred more than 200 million years ago. Spray et al. report 7 that several chromium-laden meteorites hit the earth within hours at extreme velocities, 215 million years ago. Most of these meteorites were lost in the oceans but at least five, and possibly seven, fell on solid ground (fig. 5.21).

 

Figure 5.21. Meteorites. Late Triassic impact sites (about 215 million years ago). In those days, North America and Europe-Asia were united and surrounded by the Tethys Sea.

The meteorite that fell at Obolon (Ukraine) had a diameter of 15 km. The Rochechouart structure that hit France was about 25 km in diameter, the two which fell on Canada had a diameter of 40 km (St. Martin) and 100 km (Manicouagan), and the impact structure of Red Wing, in the US, was 9 km in diameter. Coincident with these devastating impacts 8, that heralded the advent of the Jurassic period, the synapsides disappeared. They briefly surfaced again 50 million years later under the form of architherians, but otherwise were replaced by other reptiles, which became the undisputed masters of the earth during 100 million years. At the Triassic-Jurassic boundary, plants disappeared and were replaced by ferns, plants known to rush in when the landscape is devastated. The Triassic reptiles that had been around for 20 million years disappeared within 20,000 years and the first distinctive tracks of dinosaurs appeared within 10,000 years after the event. Dinosaurs jumped from 20% to more than 50% of taxa. At the same time, meat-eating dinosaurs ballooned to twice their previous mass.

These new rulers, the parapsids and the diapsids, were absolutely magnificent in their adaptive power. Together, they re-invaded the high seas at least three times, differentiated into huge carnivores, such as the tyrannosaurus, which had acquired night-vision, and herbivores such as the iguanodon. They solved the problem associated with great distances between the heart and the brain, but not quite. The diplodocus did not feed on the cymes of the trees because it could not raise its neck high enough. Its heart was not powerful enough to send blood to the erected head. To raise its head, the heart of the animal should have weighed more than 2 tons. They produced the presently living lizards, snakes and crocodiles, of which the serpents are now in full efflorescence and attempting to re-invade the seas again, and they twice invaded the air, the second time under the form of birds. They showed an extraordinary diversity and an aptitude to invade the totality of a given environment, with the exception of the cold regions. But at that time there were no cold places on the earth.

5.4.2 Homeothermy

Living reptiles are poikilotherms. They rely on external heat sources for the regulation of their temperature. This poikilothermy is, however, not absolute, even in fishes. The skipjack tuna fish can be as much as 9°C warmer than the ambient temperature and the large bluefin tuna can be even 21°C warmer. Reptiles such as pythons are able to maintain themselves 7°C above the external temperature and the largest known turtle, Dermochelis coriacea, reaches 18°C above the ambient temperature. This heat retention is obtained by muscular activity and through a rearrangement of the blood circulatory system, so that the heat gained would not be so easily lost. The regulation is thus favored by a large body size.

The huge dinosaurian reptiles of the Mesozoic era must have needed enormous amounts of energy to move and, under this condition, one may envisage that they managed in some way to acquire homeothermy, that is, they were liberated from the necessity of solar radiations to keep their body at an elevated temperature. This would have permitted them a sustained activity denied to poikilotherms and would have helped in their success. Note that these enormous beasts were in general moving slowly. Tyrannosaurus rex weighed about 6000 kilos. To move such a mass at a speed of 70 kilometers per hour would have required that 86% of the muscle mass be packed into the legs. The fastest a tyrannosaur and other massive dinosaurs could have traveled was probably 40 km/hour.

Evidence for homeothermy in dinosaurs comes from several directions. Lizards and other poikilothermic reptiles are unable to reconstitute their reserves of energy as fast as do mammals. Except for short bursts of about 20 seconds, the maximum speed of a lizard is 1 to 2 kilometers per hour at 35°C. It is thus 10 to 20 times slower than a mammal of the same size. Mongooses prey on deadly cobras because the reptile tires and is exhausted after a few strokes. The body length of a terrestrial snake presents a severe handicap. When the animal, e.g. a rattlesnake, stands erect, its tail part thickens by about 4% while its arterial pressure diminishes in the head by as much as 69%.

Several features that favor the utilization of larger amounts of oxygen in mammals are found in dinosaurs also. Dinosaurs have a well-developed secondary palate that allows continuous breathing through the nose while mastication goes on. This is not possible for lizards. The vertebrae of brontosaurs have been found to contain air sacs. This permits an extraction of oxygen as efficacious as in birds and much more efficient than in mammals. The shape of the skeleton of the dinosaurs also points to the possibility that they were homeotherms. The hugest of them must have been able to reach speeds of 40 km per hour and the small ostrich-like dinosaurs (fig. 5.22) had such long limbs that their top speed could have been as much as 80 km.

Figure 5.22. Ornitholestes is a dinosaur whose limbs and general features are such that the animal should be able to reach top speeds of 80 km/hour.

If they had only lizard-like energetics, their speed would have been restricted to no more than 3 km per hour and their anatomy would not be justified. Finally, certain dinosaurs have a bone-histology that is mammal-like and very different from lizards.

Homeothermy requires not only larger amounts of oxygen, but also larger amounts of fuel. Wild dogs for example consume ten times more in prey than do the Komodo dragons. It is obvious that large mammalian predators feeding on herbivores cannot be too numerous, lest the reserves of food would very quickly be hopelessly depleted. Poikilothermic predators may be more numerous. The predator/prey ratio for various animal communities has been analyzed and recording the frequency of skeletons of predators versus prey for various geological periods can prudently extend this. Today, the carnivores of a mammal tropical community represent 1% to 4% of the prey biomass. In mammal communities of the Oligocene period, it was 3% to 5%. This ratio increased however to 23% for the reptilian fauna of the Permian period. Yet, during the Dinosaurian dominance, it was again only 1% to 5% of the prey. Tyrannosaurs and allosaurs are indeed much rarer than are iguanodons, brontosaurs, stegosaurs, etc.

All these presumptions do not amount to the certitude that dinosaurs were perfectly homeothermic. Yet, on many accounts, they do not resemble living reptiles but mammals and birds. In fact, they are hybrids. The forefathers of the alligator, caiman and crocodile were the great bipedal reptiles. Their descendants have kept a trace of this, as their forelimbs are shorter than their hind legs. Ruben et al 9 showed that the lungs of dinosaurs are similar to those of crocodiles. They were thus basically poikilotherms. However, these simple lungs were able to power periods of high metabolism and intense activity. Contrary to other reptiles, that use only their ribs to ventilate their lungs, the dinosaurs used an additional ventilating piston, their liver. The liver of crocodiles extends from the top to the bottom of the abdominal cavity. An airtight diaphragm separates the lung from the liver. A muscle, anchored on the pubic bone, moves the liver back and forth like a piston, causing the lung to expand and contract. In dinosaurs, this well-ventilated lung was capable of rates of gas exchange as high as those of mammals. The crocodile possesses the same system of ventilation but it lost the use of it because of a secondary adaptation to aquatic life.

5.4.3 Bipedalism

The first reptiles to leave the marshes of the Permian period had, like the batracians, their limbs placed on the side of their body, where they served for propulsion in water. On solid ground, these animals were crushed by their weight and could move only with great difficulty. The dinosaurs, which appeared distinctly later, were initially of a small size. The world’s oldest dinosaurian fossils found- originating from thecodont reptiles- are about 2.5 meters in length, weigh about 90 kg and appeared 225 million years ago in Arizona. These animals had their legs placed directly underneath their bodies. This authorized a bipedal posture and an evolution towards gigantic forms. Among mammals, bipedal locomotion has been only rarely exploited (kangaroo, bear, ground sloth, apes, man).

The dinosaurs should occupy a separate class, which subdivides into three: the saurischians (brontosaur, diplodocus, brachiosaur and tyrannosaur), the ornitischians (iguanodon, triceratops) and the birds, which are the last living representatives of the dinosaurian class. These dinosaurs, all of a respectable size, had during their reign an advantage over other reptiles and animals in mobility as well as in the capacity to unload endogenous heat production during sustained vigorous daylight activity in a warm climate. The biggest of them developed nasal cavities to gigantic proportions. The inner enormous cavity of their noses was lined with extensive mucous membranes suffused with a huge amount of blood that helped them unload the excess heat that would potentially damage their brains.

Debate rages about the origin of birds. People studying dinosaurs are delighted to learn that they are still around but ornithologists do not like the theory of dinosaurian origin of birds. There are two main problems: first, the bird-like dinosaurs appear about 75 million years after Archaeopteryx, which is the first bird that looks like a feathered dinosaur. Second, on the basis of evidence from embryos, the wing digits of birds are the fingers II, III and IV, whereas the forelimbs of Archaeopteryx are made of digits I, II and III. However, according to Chiappe10, the question is settled: birds are short-tailed, feathered dinosaurs. Birds have usually short tails tipped by a few vertebrae fused into a rod. Dinosaur’s tails consist of at least 22 individual bones. This kind of tail adorns the end of the 145-million-year-old Archaeopterix from Germany, as well as that of Rahonavis from the late Cretaceous in Madagascar, and also that of Jeholornis, a bird living in the early Cretaceous, some 120 million years ago. Birds are really feathered dinosaurs, which evolved their front limbs first for powering flight and only later modernized their tails into part of the flight gear. The most recent evidence traces the origin of modern birds in the Gondwana, at a time when the southern continents were amassed around the South Pole.

References

6. In this scheme (Hedges and Poling: a molecular phylogeny of reptiles, Science 283: 998-1001, 1999), birds appeared 228 million years ago.

7. Nature 392, p. 171, 1998

8.It is quite possible that the impacts tilted the axis of the earth. As a result, seasonal contrasts could occur.

9. Pulmonary function and metabolic physiology of theropod dinosaurs. Science 1999; 283: 514-516

10. Nature, 378, 23 November 1995: The first 85 million years of avian evolution

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