1.7" Starfish (Petraster?) & Edrioasteroid (Spinadiscus)- Ordovician

Starfish (Petraster?) & Edrioasteroid (Spinadiscus)- Ordovician #100503

This is a 1.7" wide, Ordovician aged starfish of the genus Petraster that's associated with more than three Edrioasteroid fossils of the species Spinadiscus lefebvrei. There also looks to be two Carpoid fossils of an unidentified species, that preserved on the rock. It comes from the Tioririne Formation near Blekus, Morocco.

There are multiple repaired cracks through this rock, one of which goes through the starfish. There is some minor surface restoration at this point of breakage.

Edrioasteroids are an extinct class of echinoderms that lived from the Ediacaran until the Permian, about 300 million years ago. The living animal would have resembled a pentamerously symmetrical disc or cushion.

Carpoids are enigmatic organisms. They are early echinoderms related to starfish and sea urchins. Unlike modern echinoderms, carpoids lack the five fold radial symmetry and water vascular system. Most were asymmetrical or bilaterally symmetrical. But like other echinoderms, they did possess a skeleton of calcite plates. Depending on the species, carpoids had one or two ambulacra (filtering appendages), a theca (“head”) and a three-section stele (“tail”). They are likely early ancestors of starfish and other echinoderms, but they may also be an early ancestor of Chordates, the phylum that humans belong to. This is a contentious theory, based on the presence or absence of early gills in carpoids. Because these organisms are only known from fossils, gills are hard to identify accurately.

Carpoids evolved during the Middle Cambrian Period and disappeared from the fossil record during the early Devonian Period.

Carpoids were unconventional-looking organisms. Their vital organs were contained in a theca, and internal organs were surrounded by calcite plates. Depending on the species of carpoid, one or two ambulacrum extended from one end of the theca. These were appendages for filtering water for food. A mouth was located at the base of the ambulacrum if there was just one, or between them if there were two. From the other end of the theca, the three section stele (“tail”) extended. This was used either for locomotion or to hold the organism in place if fixed in one location. Carpoids grew up to about 4 inches (10cm).