How did early land animals differ from those common today?

Early land animals, which started to appear during the Paleozoic era, about 500 million years ago, differed from those common today in several significant ways:

1. Anatomy: Early land animals had simpler body structures compared to modern animals. For example, the first land vertebrates, amphibians, had different respiratory systems and limb structures designed for both aquatic and terrestrial life. Their limbs were less specialized than those of modern terrestrial animals, with more fish-like features such as fin-like extensions.

2. Size: Many of the earliest land-dwelling animals were smaller compared to some of the terrestrial animals today. Over time, land animals have evolved various sizes, from tiny insects to massive land mammals like elephants.

3. Diversity: The diversity of early land animals was relatively low in the beginning, with a limited number of species. Throughout millions of years of evolution, the number of species and the variety of forms have increased dramatically, leading to the vast array of land animals we see today.

4. Adaptations: Early land animals had a limited range of adaptations to the terrestrial environment because they were some of the first to make the transition from water to land. As time went on, animals developed more specialized adaptations, such as better limb structures for locomotion, more efficient lungs for breathing air, and different reproductive strategies that didn't rely on water.

5. Reproduction: Many early land animals still depended on water for reproduction (like modern amphibians), laying eggs in water or in very moist environments. In contrast, most modern terrestrial animals use various strategies to reproduce on land, such as amniotic eggs in reptiles and birds or internal fertilization and live birth in mammals.