Dinosaur Biology

Dinosaur Biology: A Summary of Lecture

by Dr. James Farlow

Don Chamberlain

Questions that we should consider about dinosaur biology include the following: What do we know? What can we know? How can we find out?

Inferences about dinosaur biology range from those with fairly robust amounts of evidence to those that include vast amounts of speculation.

Questions: Why were carnivorous dinosaurs (a significant number) so large? Why are there no elephant-sized carnivorous mammals today?

The numbers and sizes of species of the "modern" meat-eating mammals were compared with the numbers and sizes of species of meat-eating dinosaurs. Most mammalian carnivores are about the size of domestic cats (the largest carnivore being the bear). In general, carnivorous dinosaurs were two magnitudes of order larger than todayıs mammalian predators (about the size of elephants). Such factors as pre-predator population density, the amount of prey mass required to support a predator, and predator-prey distribution ranges should be considered as part of any hypotheses that might help answer these questions.

Generalization: a large continental area makes it easier for organisms to reach a large size: thus, large continents (or land masses) tend to have larger predators than smaller land masses.

Enough prey must be present to support the predator. Large predator organisms require a very low population density of predators to assure that enough prey can be caught. If they were "elephant sized" today, mammalian carnivores would require even larger ranges than they presently have. It would be very difficult for large warm-blooded carnivores to maintain low population density and still have enough organisms to likely find a mate needed to maintain the species.

In the late Cretaceous period, the carnivorous dinosaurs were attaining sizes ten times larger than the largest carnivores existing today AND they were doing so on smaller land masses that were divided into even smaller habitats. How is this possible?

Dr. Farlow hypothesized these reason for the larger size of carnivorous dinosaurs:

  1. These dinosaurs were cold-blooded. Therefore, they would not have to eat as often or to spend large amounts of energy maintaining a high metabolism.

  2. Larger females would be able to produce more eggs than smaller females.

  3. The Mesozoic world was a "greenhouse" world (with carbon dioxide levels four times greater than today and mean temperatures higher than today). Plant biomass could have been produced in greater quantities.

  4. Plant communities at that time were changing to larger, taller plants that would have favored larger, taller herbivores.

Questions and Answers (following the lecture):

Q. Dinosaurs have four-chambered hearts like birds and mammals (which are warm-blooded). Doesnıt this support the idea that dinosaurs, too, were warm-blooded?

A. Crocodilians and veranial lizards (monitor lizards, Komodo dragons, etc.) also have four-chambered hearts, but function as cold-blooded reptiles.

Q. Some reptiles have specialized bond patterns (including what appear like "growth rings" or Haversian systems) on certain bones. No such bone patterns appear in dinosaur bones. Bakker suggests than since dinosaurs lack these markings, this emphasizes their uniqueness among the reptiles.

A. New dinosaur bones discovered are unlike either modern reptiles or mammals. Young crocodiles have bones similar to some dinosaur bones.

Q. During the time of dinosaurs, the types of plants present included primarily conifers and cycads and not the angiosperms used by modern grazing animals. What kind of digestive system would be needed to handle such a diet? Were they not similar to "moving compost heaps"?

A.Increased Size of Animals-> Decreased Rate of Food Passing -> Increased Time for Fermentation of Food

Q. Can you comment on a reason for the demise of the giant mammals that existed in the past?

A. 1) Climate Change: hotter, drier summer = decreased vegetation = decreased number of larger mammals, and 2) Human Over Hunting probably played a major role.

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