| PERMIAN | Relating to the last period of the Palaeozoic era |
| ILEAC | Relating to the last division of the small intestine |
| CARNASSIAL | Of or relating to the last upper premolar and first lower molar teeth of carnivores, adapted for tearing flesh (10) |
| DEVONIAN | Fourth period of the Palaeozoic era, sometimes called the 'Age of Fishes' (8) |
| ORDOVICIAN | Second period of the Palaeozoic era of geologic time |
| CRETACEOUS | The ___ period was the last period of the Mesozoic era, between the Jurassic and Tertiary periods |
| SILURIAN | Geologic period between the Ordovician and Devonian periods of the Palaeozoic era |
| CAMBRIAN | The first geological period of the Palaeozoic era, during which marine invertebrates flourished (8) |
| NEOLITHIC | From the last period of the Stone Age (9) |
| ADAMSEDGWICK | 19th-century English geologist who proposed and named the Cambrian period, part of the Palaeozoic era (4,8) |
| NINETIES | Several obligations in possibly last period of life |
| SECOND | Unit of time defined as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom" (6) |
| LENTO | Slow or last period - zero (5) |
| WASP | Flying insect existed before the start of Palaeozoic |
| IRONAGE | Prehistory's last period |
| OTS | They may follow last periods, for short |
| TRIASSIC | The first period of the Mesozoic Era. It began 252 million years ago, at the close of the Permian Period. |
| REIGN | ... of Terror, period of the French Revolution when supreme power was in the hands of the Committee of Public Safety and the Jacobins (5) |
| ERAS | Mesozoic and Palaeozoic, for two |
| TRILOBITES | Extinct marine arthropods abundant in Palaeozoic times (10) |
