Among marine Mesozoic reptiles, each of the groups broadly known as ichthyosaurs, plesiosaurs, mosasaurs and crocodiles were polyphyletic in a marked degree.
The polyphyletic law was early demonstrated among invertebrates by Neumayr (1889) when he showed that the ammonite genus Phylloceras follows not one but five distinct lines of evolution of unequal duration.
When two organs can be traced along the same line of descent to one primitive form, that is when they are found to be mono phyletic, their homology is complete; when, however, they are traceable to two primitive forms, though these forms belong to the same morphological series, they are polyphyletic and therefore only incompletely homologous.
The real difficulty of classification of the fungi lies in the polyphyletic nature of the group. There is very little doubt that the primitive fungi have been derived by degradation from the lower algae.
Among land animals striking illustrations of this local polyphyletic law are found in the existence of seven or eight contemporary series of rhinoceroses, five or six contemporary series of horses, and an equally numerous contemporary series of American Miocene and Pliocene camels; in short, the polyphyletic condition is the rule rather than the exception.
There are other remarkable and distinctive features of structure which hold the Arthropoda together, and render it impossible to conceive of them as having a polyphyletic origin, that is to say, as having originated separately by two or three distinct lines of descent from lower animals; and, on the contrary, establish the view that they have been developed from a single line of primitive Gnathopods which arose by modification of parapodiate annulate worms not very unlike some of the existing Chaetopods.
He was, however, the first to show clearly that the Ratitae are the retrograde descendants of flying ancestors, that the various groups of surviving Ratitae are, as such, a polyphyletic group, and he has gone fully into the interesting question of the development and subsequent loss of the power of flight, a loss which has taken place not only in different orders of birds but also at various geological periods, and is still taking place.
Knight, made under the direction of Professor Osborn.) Laws of Local Adaptive Radiation and Polyphyletic Evolution, illustrated by two Upper Miocene Horses of the Plains Region of North America.
A collective polyphyletic or heterogeneous group, originally cosmopolitan; with certainty existing since the Miocene.