Pentremites sp. is a member of the class Blastoidea of the phylum Echinodermata ("echino", meaning spine, "derm", meaning skin). This name comes from the fact that all members of the phylum possess a dermal endoskeleton composed of calcium carbonate. The extent to which this skeleton is developed varies from group to group and influences to a large extent the overall flexibility of a given species. This phylum include many species to which you are probably familiar, including the starfish, the brittle stars, the sea urchins, the sand dollars, and the sea cucumbers, as well as some groups that you may not be, including the basket stars, the feather stars, the sea-lilies, and the sea-daisies (all of which are extant, meaning that there are modern-day survivors). There are many, many other groups of extinct echinoderms, and they provide us with a rich fossil history. The blastoids (to which Pentremites sp. belongs) are a group of extinct, benthic (bottom dwelling), attached echinoderms that flourished in the Paleozoic seas. The blastoids (the only common name that I am aware of for the group) first appeared in the middle Ordovician period (about 460 million years ago), reached their peak in the Mississippian (about 330 million years ago), followed by a decline in the Pennsylvanian (about 280 million years ago), but making a strong comeback in the Permian (about 270 million years ago) before going extinct by the end of the Paleozoic Era (about 245 million years ago). I'm glad to hear that you are interested in Pentremites sp., as the blastoids are one of my favorite groups of fossil echinoderms. There are many other unique characteristics of the phylum Echinodermata. See how many you can discover on your own using available texts and the Internet. One last thing to note is that embryologically, we share many things in common with the echinoderms and lots of useful information regarding fertilization biology and early human development has been learned from studying such species as the local Purple Urchin (Strongylocentrotus purpuratus) and the Bat Star (Asterina miniata). Who would have guessed?

:)

Pentremites obesus is an echinoderm. Here's some background on echinoderms, plus some of their more recent common names:

"Echinoderms are known from the Cambrian until today. The extant echinoderms comprise sea lilies, sea cucumbers, sea stars (starfish), brittle stars and sea urchins. They are all marine animals. The skeleton of an echinoderm consists of calcite plates covered by a thin skin. The calcite plates are therefore an inner skeleton. The animals breathe by assimilating oxygene from the sea water being pumped through a vascular canal system. Food is brought to the mouth through the movement of tube-feet along special food-furrows. The tube-feet are external extensions of the water vascular system. In addition, the starfish, brittle stars and sea urchins use the tube-feet for locomotion. Most echinoderm adults have a five-rayed symmetry. They do not posses a head, and in most free-living species, the anus is placed on the upper surface of the animal. Attached forms have both the mouth and the
anus on the upper surface, close to each other. The echinoderms and the chordates probably share a common ancestor, and that the carpoids, which are now extinct, were the link. " from http://www.toyen.uio.no/palmus/galleri/montre/english/m_pigghud1_e.htm

and these guys were alive during the age of the Carboniferous, here's a brief description of that age:

"Carboniferous
The first land vertebrates were meat-eaters, while leaves and plants were food for the insects. Many insects developed specialised jaws to open cones and seed pods, others had a kind of sucking straw to drink plant juices. Gigantic dragon flies, with a wingspan of over 60 centimetres, flew among the tree tops. The flying insects show an explosive development towards the end of the Carboniferous. Many kinds of spiders lived on the ground and from the late Carboniferous a millepede with a length of more than 1.8 metres is known. The reptiles and the first plant-eating vertebrates appeared towards the end of the Carboniferous, when changes in climate lead to a drying-out of the swamps, and changes in the vegetation. " from http://www.toyen.uio.no/palmus/galleri/montre/english/m_karbon_e.htm ( a very good website for this type of thing!)

Hopefully these notes and links to these website will be helpful.