How do the wrist and ankle bones make perissodactyl limbs more rigid?

In perissodactyls, the wrist (carpus) and ankle (tarsus) bones are structured to increase limb rigidity, contrasting with the flexibility found in most other mammalian orders.1

This rigidity is achieved through an interlocking arrangement of bones. In the forefoot, the third distal carpal (the magnum) is enlarged and interlocks with the proximal carpals, while the elongated third metacarpal thrusts upward against these interlocked bones. A similar mechanism exists in the hindfoot of advanced forms, where the third metatarsus thrusts against an enlarged, flattened third tarsal (ectocuneiform). This bone then contacts a large, flat navicular, which in turn abuts the flattened astragalus.1

This configuration creates a stable, weight-bearing column that supports the mesaxonic condition, where the limb's axis of symmetry passes through the third digit. By reducing the number of bones and utilizing these interlocking surfaces, the perissodactyl limb is specialized for efficient, fore-and-aft movement.1