ab3n+@andrew.cmu.edu (Alexander Bernard Brands) (11/21/89)
>If true, would this hypothesis support the contention that >birds (and therefore dinosaurs) are more closely related to >the mammals than to the reptiles? I'm not sure that I follow your reasoning. Are you saying that Dinosaurs stopped growing when they reached maturity? I've never heard that before, but if there is evidence to support that, I would like to hear about it. -Alex
davidp@dbrmelb.dbrhi.oz (David Paterson) (11/27/89)
> Are you saying that Dinosaurs stopped growing when they > reached maturity? I've never heard that before, but > if there is evidence to support that, I would like to hear about it. Yes, so would I. Let me put it this way. The closest living relatives of the dinosaurs are the birds and many species of birds stop growing when they reach maturity. So, in the absence of any other evidence, there is a better that 50% chance that dinosaurs stopped growing when they reached maturity. It is widely believed that the second closest living relatives of the dinosaurs are the snakes and lizards (see, for instance, the back cover of every second issue of New Scientist). But, a few years ago, a study of molecular evolution revealed a closer link between the birds and mammals than between these and reptiles. There are other peices of evidence supporting this result. Mammals, birds and dinosaurs are warm blooded but reptiles are not. Mammals, birds and dinosaurs have an upright stance but reptiles do not. Pterosaurs are thought to be closely related to birds and dinosaurs but pterosaurs and mammals have hair. Mammals and birds stop growing when they reach maturity but reptiles do not. Mammals and birds have toenails but reptiles do not. So, if mammals are the second closest living relatives of the dinosaurs then there is an excellent chance that the dinosaurs stopped growing when they reached maturity. If it is true that ALL mammals and birds stop growing when they reach maturity then the chance that dinosaurs stop growing when they reach maturity approaches certainty. However, this takes me a long way beyond my present knowledge. Some readers will be temped to write in and say that the second closest living relatives of the dinosaurs MUST be the snakes and lizards because snakes and lizards, birds and dinosaurs share the same skull type which is different from that of mammals. But it is possible to arrange an evolutionary sequence: fish skull to tortoise skull to Ichthyosaur skull to lizard and bird skull to mammal skull. And mammal skull may have evolved from bird skull AFTER the evolutionary split between birds and lizards. If I'm wrong, please write in and tell me. There is a further possibility. Some Pterosaurs have hair; feathers are structurally more similar to hairs than scales; birds and mammals both have toenails. These are pieces of evidence that suggest that some dinosaurs were covered in hair. These would probably be small dinosaurs; some large dinosaurs are known to have been naked like some large mammals and some large lizards. Keep a lookout for hairy dinosaurs. David Paterson CSIRO, Highett, Australia.
davidp@dbrmelb.dbrhi.oz (David Paterson) (11/28/89)
OK, hands up who spotted my deliberate mistakes. What I said about snakes and lizards also applies to crocodiles. I haven't the foggiest idea if they apply to tuataras. My reference to the back covers of New Scientist applies to issues of one to two months ago. I don't really know if some reptiles have finger nails. David Paterson CSIRO, Highett, Australia.