Holotypes are reference specimens kept in museums as a guarantee of the definition of a species. Horseshoe Crab A horseshoe crabs are not actually crabs they are primitive arthropods. Their ancestry can be traced back to the Cambrian period, but in wasn't until the Jurassic period that the true horse-shoe shape developed. By the Late Cretaceous period, they were less common although five species of horseshoe crab still exist today. Horsetail A group of plants that originated in the Devonian period and were particularly successful in the Carboniferous period when some species grew up to 30 metres tall.
Horsetails are regular symmetrical plants with jointed stems and circles of leaves.
Some species still exist today but none grows taller than a couple of metres. It inhabited both deep and freshwater sea, searching for fish, baby Ophthalmosaurus and small Cryptoclidus . It could grow up to 2.5 metres long. Although sharks evolved in the Silurian period the Hybodus was one of the first of the modern sharks. Hylaeosaurus was one of the first dinosaurs to be discovered and was one of the three animals on which Richard Owen based his idea of a group of extinct giant reptiles that he called Dinosauria .