Pterichthyodes is a genus of placoderm fishes from the Devonian period, now all extinct. They were one of the first species recognized for what they were, as their fossils are common in the Old Red Sandstone formation studied by geologists in the early 1800s. Due to their extreme divergence from modern-day fishes, they were a puzzle unsolved until Charles Darwin brought forward his theories on evolution. See genus (mathematics) for the use of the term in mathematics. ... Orders Antiarchi † Arthrodira † Petalichthyda † Phyllolepida † Ptyctodontida † Rhenanida † The Placodermi are fish known from fossils dating to the Devonian period. ... The Devonian is a major division of the geologic timescale that extends from the end of the Silurian period (360 million years ago (mya)) to the beginning of the Mississippian subperiod of the Carboniferous (408. ... In biology and ecology, extinction is the ceasing of existence of a species or group of species. ... In biology, a species is a kind of organism. ... FOSSIL is a standard for allowing serial communication for telecommunications programs under DOS. FOSSIL stands for Fido Opus Seadog Standard Interface Layer and was made by a group of Fidonet sysops to make their software work on different machines. ... The Old Red Sandstone is a rock formation of considerable importance to early paleontology. ... A geologist is a contributor to the science of geology. ... Charles Darwin, about the same time as the publication of The Origin of Species. ... Charles Darwin, the father of evolutionary theory Although generally, evolution is taken to mean any process of change over time, in the context of life science, evolution is a change in the traits of living organisms over generations, including the emergence of new species. ...

Pterichthyodes had heavily armored heads and front bodies, while their tail ends were uncovered. As placoderms, they were members of one of the first group of animals to possess jaws, though they had grinding plates rather than teeth. The Pterichthyodes are distinguished easily from other placoderms by their odd wing-like appendage where fins would be found on a modern fish ("pterichthys" is Ancient Greek for "wing-fish") -- strictly speaking, these are not fins as we normally think of them, which evolved in another group of fishes, the Actinopterygii. For other uses of the word jaws, see jaws (disambiguation). ... Orders See text The Actinopterygii are the ray-finned fish. ...

The appearance of Pterichthyodes is sufficiently reminiscent of a modern animal that they are one of the few extinct species to have a popular name: "sea scorpions". They are, however, completely unrelated to modern-day sea scorpions, which are fully modern fish, or the other fossil animal class with the same popular name and similar appearance, the Eurypterids (which are believed to be ancestors of actual scorpions). Orders many, all extinct The eurypterids were the largest known arthropods that ever lived. ... A scorpion is an invertebrate animal with eight legs belonging to the order Scorpiones in the class Arachnida. ...

External Links

• A page devoted to Asterolepidoidei, the parent group of the pterichthyodes according to cladistics (http://www.palaeos.com/Vertebrates/Units/Unit060/060.200.html#Asterolepidoidei)