The echinoderms are a phylum of invertebrate animals that includes starfish, sea urchins, and sand dollars. One feature that distinguishes echinoderms from other groups of organisms is their system of locomotion——the bottoms of echinoderms are covered with a number of tiny tube feet, which aid the echinoderm in feeding and slow movement on the ocean floor. Like other invertebrates, echinoderms lack a backbone and have a fairly primitive nervous system. Unlike many invertebrates, the soft bodies of which are often poorly preserved as fossils, echinoderms have hard internal skeletons made of calcite. For this reason, echinoderms are well represented in the fossils, there are several puzzles surrounding the evolution of echinoderms.

One major mystery surrounding echinoderms is how they originated. The first clearly recognizable echinoderms appeared around 540 million years ago during the Cambrian period; however, these first echinoderms were already fairly complex. Paleontologists have been unable to conclusively identify a simpler organism that could have been the ancestor of the echinoderms. One fossil that has been tentatively suggested as an ancestral echinoderm is Tribrachidium, which dates to the Ediacaran period just before the Cambrian. Tribrachidium is a round, flat, disc-shaped fossil that was discovered in 1959. Some paleontologists have suggested that Tribrachidium could have been a primitive echinoderm, as it shares the radial body plan of modern echinoderms. A radial body plan is one in which an organism’s body is composed of parts that are arranged symmetrically around the center, like the petals of a flower. Since most known echinoderms have radial body plans, the radial symmetry of Tribrachidium has been viewed as suggestive of an evolutionary relationship. However,Tribrachidium has now been discounted as an ancestral echinoderm since it has triradial ( three-point) symmetry instead of the pentaradial ( five-point) symmetry found in adult echinoderms. In 1985 Tribrachidium was placed in its own phylum, a group with no known living descendants.

Another puzzle related to echinoderm evolution is how and why these organisms acquired their distinctive radial body plan. Among animals, all but the most primitive have bilateral symmetry—the left sides of their bodies are roughly mirror images of the right sides. Two main groups of animals have radial symmetry instead. One of these groups is jellyfish, which are extremely primitive and are assumed never to have evolved bilateral symmetry. The other radially symmetrical group is the echinoderms, which are fairly sophisticated. In fact, biologists know that the echinoderms did once have bilateral symmetry because the free-swimming larvae (young) of echinoderms are bilaterally symmetrical. Once the larvae settle down on the ocean floor, they develop the radial symmetry of adult echinoderms. It is not clear why echinoderms do not retain bilateral symmetry as adults.

Paleontologists are always attempting to determine evolutionary relationships among organisms. One widely accepted hypothesis is that echinoderms are closely related to vertebrates——the group of animals that includes fish, reptiles, and mammals. Although adult echinoderms do not resemble vertebrates, biologists have based this conclusion on DNA comparisons and embryological evidence. In most non- echinoderm invertebrates, the mouth forms from the first opening in the embryo ( an early stage of development), whereas in both echinoderm and vertebrate embryos the mouth develops from a secondary opening.

A group of animals called carpoids, which are only known through the fossil record, have been considered to be intermediate between echinoderms and chordates, a group that includes all vertebrates. Carpoids share several characteristics with vertebrates, including gill slits and a complex nervous system. In fact, some experts have placed them near or at the beginning of chordate evolution. However, carpoids possess a skeleton of calcite plates identical to that of echinoderms. A calcite skeleton is not present in any chordates. Furthermore, no ancestral genes for calcite skeleton development have been identified in any present-day chordates, which indicates that chordates and echinoderms most likely followed their own evolutionary paths and that carpoids are echinoderms. In 2012 scientists reported the discovery in southern Europe of fossils from the early middle Cambrian period; these fossils show adult echinoderms with a fully bilateral body plan. The animals represented by these fossils may be the earliest echinoderms from which all other echinoderms evolved.