Paleontology - Classifying Ancient Organisms

Classifying Ancient Organisms

Tetrapods	Amphibians Amniotes Synapsids Extinct Synapsids Mammals Reptiles Extinct reptiles Lizards and snakes Archosaurs ? Extinct Archosaurs Crocodilians Dinosaurs ? Extinct Dinosaurs ? Birds

Simple example cladogram
Warm-bloodedness evolved somewhere in the
synapsid–mammal transition.
? Warm-bloodedness must also have evolved at one of
these points – an example of convergent evolution.

Naming groups of organisms in a way that is clear and widely agreed is important, as some disputes in palaeontology have been based just on misunderstandings over names. Linnean taxonomy is commonly used for classifying living organisms, but runs into difficulties when dealing with newly-discovered organisms that are significantly different from known ones. For example: it is hard to decide at what level to place a new higher-level grouping, e.g. genus or family or order; this is important since the Linnean rules for naming groups are tied to their levels, and hence if a group is moved to a different level it must be renamed.

Palaeontologists generally use approaches based on cladistics, a technique for working out the evolutionary "family tree" of a set of organisms. It works by the logic that, if groups B and C have more similarities to each other than either has to group A, then B and C are more closely related to each other than either is to A. Characters that are compared may be anatomical, such as the presence of a notochord, or molecular, by comparing sequences of DNA or proteins. The result of a successful analysis is a hierarchy of clades – groups that share a common ancestor. Ideally the "family tree" has only two branches leading from each node ("junction"), but sometimes there is too little information to achieve this and palaeontologists have to make do with junctions that have several branches. The cladistic technique is sometimes fallible, as some features, such as wings or camera eyes, evolved more than once, convergently – this must be taken into account in analyses.

Evolutionary developmental biology, commonly abbreviated to "Evo Devo", also helps paleontologists to produce "family trees". For example the embryological development of some modern brachiopods suggests that brachiopods may be descendants of the halkieriids, which became extinct in the Cambrian period.