Tetrapods—including the modern forms of amphibians, reptiles, birds, and mammals—are loosely defined as vertebrates with four feet, or limbs. Many species we see today, like the snakes or whales, may not appear to be tetrapods, but their...

Tetrapods (Greek tetrapoda, Latin quadruped, "four-legged") are vertebrate animals having four feet, legs or leglike appendages. Amphibians, reptiles, dinosaurs, birds, and mammals are all tetrapods, and even the limbless snakes are tetrapods by...

YOUR INNER FISH: A JOURNEY INTO THE 3.5-BILLION-YEAR HISTORY OF THE HUMAN BODYBy Neil ShubinPantheon, 229 pages, $24 When the renowned paleontologist Neil Shubin announced in 2006 that he’d discovered an ancient fossil with an uncanny resemblance to a “missing link” between fish and land-dwellers,...