What Was a Thecodont? A Group That Turned Out Not to Exist

The term meant socket-toothed and was applied to early archosaurs whose teeth sat in sockets rather than being fused to the jaw, which is a real anatomical feature shared across the group. In practice it collected a varied assemblage of Triassic reptiles that were clearly related to dinosaurs, crocodiles and pterosaurs without belonging to any of them, ranging from small lightly built runners to heavy armoured predators. The category was useful descriptively, since it named a recognisable set of animals occupying a particular time and grade of organisation, and it was treated as ancestral, with the more familiar groups understood to have arisen from it. Textbooks through the middle of the century presented the arrangement as settled, and the word appears in a great deal of older literature.

The group failed a test that classification began applying systematically from the 1970s onward:

• •A valid group must include an ancestor and all of its descendants, which is what makes it a real branch of the tree rather than a selection from it
• •This group was defined by what its members lacked, namely the specialisations of dinosaurs, crocodiles and pterosaurs
• •It therefore excluded the descendants of its own members, which makes it a grade rather than a branch
• •Such groups appear to be real because their members resemble each other, and the resemblance reflects shared primitive features rather than shared ancestry
• •The animals concerned are still real and still studied, distributed among properly defined branches instead
• •The same reasoning retired several other familiar categories at around the same time

The same revision removed several categories that remain in popular use. Reptiles as traditionally understood excluded birds while including the ancestors of birds, which makes it the same kind of group and is why birds are now placed within the reptiles. Fish as a category excludes everything that came out of the water while including its ancestors. Apes excluded humans on the same basis. Invertebrates is defined entirely by the absence of a feature and covers organisms less related to each other than some are to vertebrates. In each case the traditional category names a recognisable grade of organisation and does not name a branch, and the response has been to retain such words informally while using properly defined groups in technical work, which is a reasonable compromise and is why both usages appear.

The shift in method has a traceable history and was contested at the time. The approach was set out by a German entomologist whose work was published in the 1950s and reached a wide audience only after English translation in 1966, and it proposed that classification should reflect branching order and nothing else, which meant grouping by shared derived features rather than by overall similarity. The argument was that similarity is not evidence of relationship where it reflects features inherited from a distant ancestor and retained, so a group defined by such features collects animals that have merely stayed the same. Adoption was slow and produced substantial disagreement through the 1970s and 1980s, with defenders of the older approach arguing that it discarded useful information about degree of difference. The method won, and the resulting reorganisation of familiar groups is what most people notice.

The modern treatment names branches defined by common ancestry, and the Triassic animals formerly collected under the abandoned label are now distributed among them. Several are placed on the crocodile branch, several on the bird branch, and a number sit near the base of the whole radiation in positions that are still being revised as new specimens appear. Those placements are hypotheses derived from analyses of anatomical characters and are revised when the analyses change, which happens regularly. The practical consequence for a reader is that older literature uses a vocabulary that no longer maps onto current classification, so a paper from the 1960s describing a thecodont is describing a real animal in a framework that has been replaced, and translating between the two requires knowing which animal is meant rather than looking up the group.

The name covered Triassic archosaurs that were not yet dinosaurs, crocodiles or pterosaurs, defined by what they lacked rather than by shared ancestry. That makes it a grade rather than a branch, and modern classification names only branches including all descendants. The same reasoning placed birds inside reptiles and retired fish, apes and invertebrates as technical categories.