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
Spiny mouse organism to study menstruation and reproductive disorders, as Mus musculus, the common laboratory mouse, does not menstruate. At least two species of African spiny mice, "Acomys kempi" and "Acomys percivali", are capable of autotomic release of skin upon being captured by a predator. They are the only mammals known to do so. They can completely regenerate the autotomically released or otherwise damaged skin tissue - regrowing hair follicles, skin, sweat glands, fur and cartilage with little or no scarring. It is believed that the corresponding regeneration genes could also function in humans. In a research article published on May 16,