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DINOSAUR PALEOBIOLOGY

DINOSAUR PALEOBIOLOGY. How do we weigh dinosaurs? There is a regular relationship between the weight of an animal and the cross-sectional area of its legs.  This relationship has been developed for modern animals, using animals that range in size from shrews to elephants.

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DINOSAUR PALEOBIOLOGY

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  1. DINOSAUR PALEOBIOLOGY

  2. How do we weigh dinosaurs? • There is a regular relationship between the weight of an animal and the cross-sectional area of its legs.  • This relationship has been developed for modern animals, using animals that range in size from shrews to elephants. • In general: Mass is proportional to Leg area2.75 • Typical weights derived using this method in metric tons (=1000 kilograms): • Big Theropods (e.g., Allosaurus, Tyrannosaurus): 2 - 7 tons • Sauropods (e.g., Diplodocus, Apatosaurus, etc.): 6 - 50 tons, biggest ~180 tons • Ornithopods (e.g., Iguanodon): ~5 tons • Stegosaurs:  2 - 3 tons • Big Ceratopsians (e.g., Triceratops): 4 - 6 tons

  3. What problems does large size create? As animals get big, their weight increases dramatically. D (10X bigger) C (4X bigger) A B (2X bigger) Animal weight is determined by the volume of the body (ignoring leg weight).  Animal volume increases as length raised to the power of 3. Although animal D is only 10x bigger than animal A in linear dimensions, it weighs 1000x more than. Volume   A B  C D                      13=1 23=8 43=64      103=1000

  4. Stress, thepressure on the legs, increases with volume. Stress = (Mass x gravity)/unit area = (Volume x density x gravity)/unit area Strength is the stress required to break an object (such as a leg). Strength depends on cross-sectional area of the objects resisting stress.  Strength = Combined Cross sectional area of 4 legs   A               B                   C              D 4(0.12)=0.04 4(0.22)=0.16   4(0.42)=0.644(1.02)=4 Stress increases much faster than Strength if shape is constant. If size increases without a change in shape, stress on legs will quickly outpace strength.

  5. How to get large without breaking? • Live in water. Originally proposed for sauropods. WRONG. • Trackways indicate they walked on relatively dry land. • Don't maintain shape. Increase cross-sectional area of legs disproportionately. • Keep you legs straight. Most breaks occur during bending, not vertical loading. • Big living animals like elephants don't bend their legs much when walking.

  6. How fast did dinosaurs move? • Speed is estimated from spacing of footprints.  • We can generate a regular relationship between foot print spacing (stride length) and speed (km/hr) in living animals.  • Each animal forms its own relationship on a plot of stride length versus speed.  • However, when both measurements are scaled for difference in body size (dividing by leg length does the trick for stride length, it's a bit more complex for speed), all animals terrestrial animals fall on the same relationship.  • We can use this general relationship between relative stride length and relative speed to calculate absolute speeds of dinosaurs.  • Of course, we need to know the leg length of the track maker, but it turns out that this can be nicely estimated from the size of the foot print.

  7. Speed Estimates: • Sauropods and ankylosaurs slowest, ~3 - 5 km/h • Ornithopods faster, ~5 - 7 km/hr • Theropods fastest, ~10 km/hr, burst to 43 km/hr, faster than human sprinter.

  8. How old were dinosaurs? • We could estimate how long it would take for an animal to grow from its hatchling to its adult size.   • If we assume a crocodile growth rate as typical, it would take 200 years.  • Of course, if dinosaurs were endotherms, particularly when they were young, they may grow faster-at bird-like rates.  • When these rates are used, we obtain more reasonable estimates of how long it would take a dinosaur to reach adult size (5 to 20 years). • Note, we're not calculating life span here, just the period to reach adult size.  • Many animals live a long time after reaching adult size, and if dinosaurs slowed down their metabolism when old, they might follow a similar pattern. 

  9. How old were dinosaurs? cont. • It turns out that, for modern animals, there is a regular relationship between life span and body weight (albeit with considerable scatter). • The life spans of living animals increase proportional to body weight0.25 • For example: • Suppose we know that a 1 kg animal lives 1 year.  • How long would a 16 kg animal live?  • To determine the scaling factor, just raise the weight to the 0.25 power: • for the small animal:  10.25 = 1 • for the larger animal: 160.25 = 2 • So the 16 kg animal lives 2x longer than the 1 kg animal.  It lives 2 years.  • If the 1 kg animal lived for 4 years, then 16 kg animal would live for 8 years (2x4 = 8).

  10. How old were dinosaurs? cont. • To calculate dinosaur life spans, we compare them to the largest living land animal, the elephant.  • Elephants weigh 6 metric tons and live 60 years.

  11. How "brainy" were dinosaurs? • As with the other aspects of animal biology that we have examined, there is a linear association between brain weight and body weight.  • Interestingly endotherm follow one relationship and ectotherms follow another. • For mammals and birds:  Brain weight = 0.07 x body weight0.67 • For living reptiles and fish:  Brain weight = 0.007 x body weight0.67

  12. How "brainy" were dinosaurs? cont. • In general, at a given body size, the average living reptile has a brain 10x smaller than the average living mammal. • For example: • Consider two animals that 100 kg • Typical Mammal:  Brain size = 0.07(1000.67)  = 1.50 kg • Typical Reptile:  Brain size = 0.007(1000.67)  = 0.15 kg • It should be clear that, to estimate dinosaur "braininess", we're not interested in absolute brain size.  • Rather, we're interested in whether or not any particular dinosaur has more or less brains than you'd expect for an animal of that body size.  • To this end, scientists working on braininess have developed a measure called the Encephalization Quotient or EQ. • EQ - is the ratio of the estimated brain weight to the brain weight predicted for a typical animal of that body size.

  13. How "brainy" were dinosaurs? cont. To estimate EQ (relative brain size) for a dinosaur: a) Estimate the body weight of the dinosaur. b) Predict its brain weight using the ectotherm equation above.  This is the brain size we would expect from a typical reptile as large as the dinosaur. c) Estimate the brain weight of the dinosaur from an endocast (or internal mold) of the inner braincase. d) Divide the Estimated Brain Weight by the Predicted Brain Weight. This gives you EQ or the relative braininess of the dinosaur.

  14. How "brainy" were dinosaurs? cont. Encephalization Quotient Estimates • Big Sauropods and Stegosaurs: EQ of ~ 0.1 to 0.2 • Ceratopsians and Ornithopods are a big brainier:  EQ of ~ 0.2 to 0.9 • Big Theropods are close to typical reptiles: EQ of ~0.9 to 1.5 • Some smallertheropods have much bigger brains than typical reptiles: EQ ~ 5

  15. How "brainy" were dinosaurs? cont. • In general, we're still comparing dinosaurs to living reptiles, that have brains an order of magnitude smaller than mammals and birds.  • So even our brainiest theropod still only has about half the head meat of a similar sized mammal. • The extremely small brain sizes of some dinosaurs has led some authors to speculate: • that dinosaurs were very stupid (unlikely) or • that dinosaurs decentralized some of their neural processing to large nerve centers down at some distance from the brain.  • There is some evidence for the latter conjecture from analysis of changes in the size of the spinal cord.

  16. How do we study aspects of animal behavior that are unlikely to fossilize? • Many of the attributes of animals that contribute to their ecological roles are things that won't leave a good fossil record.  • For example, we'd like to know about social interactions among dinosaurs, as social systems play an important role in animal ecology.  • Cladograms provide us with a tool for generating ideas about the behaviors (and soft tissue anatomies) of extinct critters.

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