Although it might seem logical to discuss the dominant land reptiles ofthe Me-sozoic Era immediately after considering the diversification on land of Palaeozoic reptiles, it is more convenient first to dismiss those reptilian taxa that swam in the seas and flew in the air. Colbert (1966) followed the same procedure, as did Palmer (1999) amongst others. Land-dwelling vertebrates did not turn to life in the sea until the Triassic period, but this trend has been repeated several times since then. Many amphibians were aquatic in the Palaeozoic Era and in the Triassic period, but few of them were marine. The reason for this probably lies mainly in their osmotic relations. Reptiles were better equipped to evolve the adaptations necessary for a marine existence. Not only did they have efficient lungs which had replaced their long-lost ancestral gills, but their limbs were easily transformed into paddles. These, like their tails, in some cases, acquired functions very similar to those of the fins and tails of fishes. Furthermore, reptiles had cleidoic eggs (Sect. 2.3): some came ashore to bury them in the sand,but others evolved ovoviviparity - the birth of living young which had developed from eggs in the body of the mother where they were relatively safe.

The factors that most influence the ways in which animals have evolved and specialised are, first, the environment in which they live and move and, second, what they feed on. Tetrapod reptiles evolved and first diversified on land. Some of them then returned to water and became adapted for swimming; others developed wings and took to the air like birds. (The theropod origin of the latter forms the subject matter of Sect. 11.5.) The majority of reptiles, however, remained on land. These will be discussed in Chapters 7-11. The above is, of course, a very great over-simplification: between such broad groupings are many intermediate forms. Some of the land-living reptiles were shore-dwelling but semi-aquatic or amphibious forms that spent some parts of the day on land and some in water as modern crocodilians do. They moved reasonably well on land (Sect. 4.6),yet were equally at home in water where they swam, often with webbed hands and feet or by means of flattened tails.

Not only were many Mesozoic reptiles amphibious, but several became truly aquatic. These included some of the turtles (Testudines or Chelonia), the pla-codonts, nothosaurs, and plesiosaurs ('near lizards'; Sauropterygia),sea croco diles (geosaurs and thalattosuchians: Archosauria) and the Ichthyosauria, which were the most dramatically adapted of them all. A total of seven subclasses or orders - either entirely or in a large portion - became aquatic and returned to the waters in which their distant rhipidistian ancestors originally evolved. One of these was the family Mesosauridae, an example of which, Mesosaurus (Fig. 7) from the Lower Permian of southern Africa and Brazil, has already been mentioned in this book (Sect. 2.4). Another mesosaur, Stereo-sternum, has only been found in southern Brazil. Were any needed, this distribution provides unequivocal evidence for continental drift. Although the mesosaurs have sometimes been regarded as suspension feeders, this reconstruction depends upon interpretation of the mandibular teeth as being small, marginal upper teeth. According to Spinar (1995), as well as Collin and Janis (1997), it is therefore more probable that the mesosaurs were piscivorous, living in fresh water and capturing fish and other small individual prey items selectively, rather than by filtering large volumes of water through their teeth -but neither hypothesis is yet fully substantiated. Benton (2004), among others, has favoured the latter, Collin and Janis (1997) the former.

Was this article helpful?

0 0
Pregnancy And Childbirth

Pregnancy And Childbirth

If Pregnancy Is Something That Frightens You, It's Time To Convert Your Fear Into Joy. Ready To Give Birth To A Child? Is The New Status Hitting Your State Of Mind? Are You Still Scared To Undergo All The Pain That Your Best Friend Underwent Just A Few Days Back? Not Convinced With The Answers Given By The Experts?

Get My Free Ebook

Post a comment