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
Cairo spiny mouse Red List of Threatened Species, lists it as being of "Least Concern". A recent study, still under peer review at the time of writing, indicates that this species could become a candidate animal model for the human menstrual cycle. It exhibits a 9-day cycle, and is the first rodent found to have such a cycle. Gene sequencing is currently underway to investigate this, and other unique physiological traits displayed by this species. Cairo spiny mouse The Cairo spiny mouse ("Acomys cahirinus"), also known as the common spiny mouse, Egyptian spiny mouse, or Arabian spiny mouse, is a nocturnal species of