Starfish first appeared in the fossil record in the Cambrian. The first known asterozoans were the Somasteroidea, which exhibit characteristics of both groups. Starfish are infrequently found as fossils, possibly because their hard skeletal components separate as the animal decays. Despite this, there are a few places where accumulations of complete skeletal structures occur, fossilized in place in Lagerstätten — so-called "starfish beds". By the late Paleozoic, the crinoids and blastoids were the predominant echinoderms, and some limestones from this period are made almost entirely from fragments from these groups. In the two major extinction events that occurred during the late
Ferreira's spiny tree-rat in 1817, Anselme Gaëtan Desmarest described the species based on this specimen housed in the National Museum of Natural History (France) of Paris. Using ancient DNA technology, a portion of the mitochondrial cytochrome b gene has been sequenced from a small skin fragment of this holotype. DNA sequence comparisons then suggested that the specimen was originally obtained in eastern Amazonia north of the Amazon River, most likely in the Brazilian state of Amapá. Ferreira's spiny tree-rat Ferreira's spiny tree-rat, "Mesomys hispidus", is a spiny rat species from South America. It is found in Bolivia, Brazil, Colombia, Ecuador, French Guiana, Guyana,