The term ‘fish’ includes hagfishes, lampreys, chondrichthyans (sharks, rays, chimaeras), actinopterygians (ray-finned fishes), actinistians (coelacanths and lungfishes) containing about 25,000 species. This is almost half the number of extant craniate species. The term ‘craniate’ (= with head) dates back to the times of non-evolutionist systematics, when creating a group because its members don't have what human beings have was an obvious and common way to classify the living things. In the intellectual context of fixism, the goal of the systematician was to find God’s plan in the puzzling diversity of his creatures. Many groupings defined organisms on the criterion of what they did not have, and thus classifications were full of groups for which there was no character exclusively shared by the members of the group. For example, fishes were craniates without limbs. Who has the limbs? The tetrapods, the group in which we find humans. Invertebrates are metazoans without vertebrae. Who has the vertebrae? . . . and so on.

After Darwin, the reason for biodiversity was thought to be genealogy, in other words phylogeny. Classifications were required to reflect descent of species from other species, not the God’s creation anymore. The purpose of groupings was not anymore to celebrate the perfection of humans but to demonstrate common ancestry. However, during the century between Darwin and Hennig, systematicians did not have efficient tools to fully reach this aim. They all recognized the need to abandon polyphyletic groups that include no common ancestor to all its members. But they remained in the old tradition in being unable to reject paraphyletic groups that contain a common ancestor to all its members, but this ancestor is also shared by organisms that are not included in the group. A true monophyletic group contains one ancestor and all his descendants. At that time however, both types of groups were recognized as valid. As before, paraphyletic groups were not defined for themselves, but to express a step in the increasing complexity of life, with human beings at the top. Such groups are called grades, always defining something else (complexity level, adaptation, ecology) than the organisms we put in it. The grade of reptiles would not exist as distinct from birds if one would not have the will to stress the extreme adaptation to flight in birds. Without the tetrapods, fishes would not exist and would simply be part of craniates (animals with a cranium). Without the eukaryotes, prokaryotes would not exist. What group has the nucleus in the cell? The group that includes human beings. Many other examples could be added.

With Hennig, it became possible to distinguish paraphyletic groups (containing an ancestor and only some of its descendants) from monophyletic ones (containing an ancestor and all its descendants). Hennig thus gave birth to modern systematics, where the paraphyletic groups are finally rejected. For example, the old group Pisces (‘fishes’) is clearly paraphyletic as there is no character that can exclusively define fishes. There is a common fish ancestor: it is the animal that had the first cranium, between 500 and 600 million years ago. But half of the living descendants of this ancestor are not put in ‘fishes’. These are the tetrapods. If we decided to make fishes a monophyletic group, we would have to include tetrapods, and humans would be fishes. Another way to point out paraphyly is to stress that some members of a group are more closely related to other organisms than to members of their group. For example, actinistians (coelacanths) and dipnoans are more closely related to tetrapods than to actinopterygians. Actinopterygians, as ‘bony fishes’ are more closely related to tetrapods than to chondrichthyans. The term ‘fish’ therefore disappears from modern systematics and more precise terms are now available, all related to monophyletic groups. These terms are given here only for extant taxa! Craniates have the cranium. They are made of two sister-groups, the hagfishes (mixinoids) and vertebrates, which are divided into petromyzontoids (lampreys) and gnathostomes, the jawed vertebrates. In jawed vertebrates, the chondrichthyans (defined by prismatic calcified cartilage and pelvic claspers) are the sister-group of the osteichthyans (defined by a typical pattern of dermal bones: premaxillar, maxillar, frontals, parietals, etc.). Osteichthyans are divided into two sister-groups, actinopterygians (defined by the acrodine cap on teeth and other characters) and sarcopterygians (monobasal paired fins found in lobe-finned fishes and tetrapods). Sarcopterygians contain actinistians (coelacanths) and rhipidistians defined by the sinuous aortic trunk and many other characters. Rhipidistians are made of two sister-groups, dipnoans and tetrapods.

The rise of cladistics in ichthyology starting from 1967 brought tremendous and sudden advances in systematic ichthyology. In about five years, half the teleostean tree passed from a bush to a cladogram. Today, the ‘bush at the top’ (a term due to Don Rosen and Gareth Nelson) persists, and much work remains within the terminal crown of the teleostean tree.

Guillaume Lecointre

1. Name of the Order (e.g., Myxiniformes);
2. Common name of the Order (e.g., Hagfishes);
3. First reported occurrence in the fossil record (multiple choice fields, with Upper/Middle/Lower for both Periods and Epochs);
4. Class to which the Order belongs (e.g., Myxini);
5. Sister Order (e.g., Petromyzontiformes);
6. Order used for Comparison (e.g., Perciformes);
7. Time since shared ancestor (here: 420) million years;
8. Number of Families in the Order;
9. A comment field for free text description of the major features of the Order;
10. A list giving access to the family(-ies) in that Order.