The Invertebrate Fossils of the Alamo Site
Invertebrates compose the groups of animals without a backbone. In the fossil record invertebrates are represented by both extant organisms (clams, snails, crabs, lobsters) which are familiar to most people, and more interestingly by a vast array of extinct and bizarre creatures many of which went extinct millions of years ago. Some of these invertebrates lived as far back as the Cambrian Period over half a billion years ago!
Many of them developed hard shells made from the minerals calcite or aragonite. Luckily, the seafloors spanning the Phanerozoic Eon have been continuously populated by huge numbers of these shelly creatures leading to an amazingly diverse fossil record. Trilobites are some of the first invertebrates to appear in the fossil record and were a marine arthropod that scurried along Paleozoic Era seafloors.
Other early invertebrates include archaeocyathid and stromatoporoid sponges, which formed extensive reefs similar to the corals of today! Corals of the past took many forms no longer present in modern oceans. The rugose corals, for example, grew to be fairly large, horn-shaped organisms and contained within them a single polyp.
Extant echinoderms, such as sea urchins, have existed since the Paleozoic as well. This prickly group gets its name from the Greek word echinos, meaning "spine", and derm, meaning "skin". This group includes the star fish and the crinoids, also known as sea lilies because of their resemblance to flowers.
Although they may not be as massive and striking as some of the ancient and massive megafauna, the invertebrate fossil record is a primary resource for the study of evolution and our current understanding of modern life.