Limbs

Animal Adaptations and Locomotion

General Premise

  • Adaptations of animals typically involve trade-offs.

  • In terms of locomotion, adaptations can be classified along a spectrum based on load-bearing capacity versus speed.

Extremes of Locomotion in Land Mammals

  • Two primary extremes:

    • Graviportal Limbs

    • Adapted for supporting large loads.

    • Characteristics: Thick, column-like structure.

    • Example: Elephants have graviportal limbs.

    • Cursorial Limbs

    • Designed for rapid movement.

    • Characteristics: Longer, lower leg bones; typically walk on toes or toenails.

    • Examples: Cats, deer.

Specific Examples

  • Horses: Walk on their toenails (hooves = modified fingernails).

  • Dinosaurs:

    • Some had graviportal limbs (e.g., Argentinosaurus).

    • Others had cursorial limbs (e.g., Ornithomimus).

    • Early sauropods (e.g., Massospondylis) fell in the middle.

    • Correct cursorial dinosaur: Ornithomimus (B is the correct answer).

Trade-offs Between Limbs

  • Graviportal limbs:

    • Efficient for carrying heavy loads.

    • Sacrifices speed.

  • Cursorial limbs:

    • Allow for faster running.

    • Less efficient in load-bearing capacity.

Human Comparison

  • Locomotion Analogy:

    • Standing/walking position aligns with graviportal style (legs straight, foot soles on ground).

    • Running position aligns with cursorial style (using toes, faster but more energy-intensive).

    • This exemplifies the trade-off between speed and efficiency in limb positioning.

Investigating Dinosaur Locomotion

  • Method: Muscle reconstruction studies (e.g., Scott Persons, PhD student, University of Alberta).

Study of Tail Muscles

  • Focus on leg and tail muscles of hadrosaurs and tyrannosaurs.

  • Dissection of modern reptiles (e.g., iguanas) allows for investigation of tail muscle anatomy.

Important Muscle: Caudofemoralis

  • A tail muscle with a tendon attachment to the femur (upper leg bone).

  • Role: Retracts and pulls legs backward.

    • Major muscle for locomotion in lizards, crocodilians, and dinosaurs.

Identifying Attachment Sites

  • Observing the femur of hadrosaurs shows the attachment site for caudofemoralis: tall trochanter (bony crest).

  • Size estimations of caudofemoralis can be calculated using muscle-to-bone attachment locations.

Dinosaur Muscle Reconstruction Findings

  • Digital models show that large carnivorous dinosaurs (Tyrannosaurus, Carnotaurus) had significantly larger caudofemoralis muscles relative to body size compared to modern reptiles.

  • Implications: Likely faster locomotion than previously thought.

Load versus Speed Implications

  • Attachment Location:

    • Carnivorous dinosaurs (e.g., Tyrannosaurus) - caudofemoralis attaches high near hips.

    • Advantage: Able to swing leg quickly, covering longer strides.

    • Disadvantage: Higher energy expenditure leads to quicker fatigue.

    • Herbivorous dinosaurs (e.g., hadrosaurs) - caudofemoralis attaches centrally.

    • Advantage: More sustainable movement over longer durations.

    • Disadvantage: Slower locomotion.

Bipedal vs Quadrupedal Locomotion

  • Definitions:

    • Bipeds: Animals walking on two legs.

    • Quadrupeds: Animals walking on four legs.

Human Example

  • Humans are categorized as both due to developmental stages; infants are quadrupedal and adults are primarily bipedal.

Obligate vs Facultative Movement

  • Obligate: Must move in a certain way.

    • Obligate Biped: Primarily walks on two legs (e.g., humans).

    • Obligate Quadruped: Primarily walks on four legs (e.g., elephants).

  • Facultative: Have the option to move in more than one way.

    • Example: Bears (usually quadrupedal but can walk on two legs).

    • Example: Kangaroos (typically bounding on two legs but can walk on four legs when moving slowly).

Advantages of Bipedalism

  • Examples of advantages in facultative bipeds:

    • A. Fast runner

    • C. Ability to fly

    • D. Grasping prey

  • Correct selections: A, C, D (large body not necessarily an advantage).

Diversity in Dinosaur Locomotion

  • Sauropods: Obligate quadrupeds due to large size.

  • Theropods (e.g., Tyrannosaurus): Obligate bipeds with impractical front limbs for walking.

  • Ancestral characteristic of dinosaurs was bipedalism, subsequently leading to quadrupedal forms.

  • Notable groups: Hadrosaurs represent a transitional group.

Historical Misconceptions

  • Earlier views: Hadrosaurs were thought to stand upright on hind limbs with tails for support.

  • Current understanding indicates a different locomotion strategy.

Tail Positioning in Dinosaurs

  • Example comparisons of Tyrannosaurus rex in different stances.

    • Accurate representation: Tail off the ground (indicating active locomotion).

    • Evidence against tail dragging: Absence of tail impressions in preserved dinosaur footprints.