The Tree of Life
As more and more fossil groups were found and recognized in the late 19th and early 20th century, palaeontologists worked to understand the history of animals through the ages by linking together known groups The Tree of life was slowly being mapped out, with fossil groups taking up their position in the tree as understanding increased.
These groups still retained their formal Linnaean taxonomic ranks, giving rise to a number of units that were paraphyletic, i.e. where the descendants were considered a part of the daughter group rather than that of the ancestral group. Particularly on the level of orders and classes, most of the traditional vertebrate systematic units are paraphyletic, representing natural evolutionary grades rather than clades.
Vertebrate palaeontology had mapped out the evolutionary sequance of vertebrates as currently understood fairly well by the closing of the 19th century, followed by a reasonable understanding of the evolutionary sequence of the Plant kingdom by the early 20th century. The tying together of the various trees into a grand Tree of Life only really became possible with advancements in microbiology and biochemistry in the Interwar period. With the modern evolutionary synthesis of the early 1940s, an essentially modern understanding of evolution of the major groups was in place.
Read more about this topic: Evolutionary Taxonomy
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