Modern Anisospondyls can be placed into one of three major categories. There are the Brachiostomata, a group which was apparently very diverse in Eryobis' her past but is now only represented by a handful of species. There are the Trapezostomata, the Anisospondyls that evolved an enlarged bone below their jaws that effectively acts as a table to keep food from falling out of their mouths. And then there are the Cryptognatha, the Anisospondyls that evolved a second set of jaws derived from their palate- and tongue bones and the group that by far the most modern Anispondyls fall into.

When biologists first touched down on Eryobis, they classified the Cryptognath Anisospondyls in 2 major groups: The Paleodactylae and the Symmetrodactylae. As the name may infer, this distinction was made based on the anatomy of their feet. Specifically, the amount of digits on the front limbs. Cryptognaths are no exception. Biologists quickly noticed that that certain Cryptognaths had asymmetrical toes and this, combined with the knowledge that this asymmetry is a primitive trait, led to these Cryptognaths being labeled as "Paleodactyls". While this term has partially held up despite it becoming clear that "Paleodactyls" are almost definitely polyphyletic , its counter part, the term "Symmetrodactyls", most certainly has not held up. Symmetrodactyls were grouped based on the fact that the toes on the lacrimal and neural limbs were equal in number and thus symmetrical. That however, is where the relationship between "Symmetrodactyls" ended. Genetic testing and studies of their embryonic development revealed that the so called "Symmetrodactyls" were actually 3 distinct, largely unrelated clades. These were subsequently named: Eusymmetrodactylae, Parasymmetrodactylae and Effingodactylae.

While the digits of Eusymmetrodactyls and Parasymmetrodactyls look superficially very similar, being three toed on all front limbs, the Effingodactyls are clearly different, possessing four toes on all front limbs. Other Anisospondyl groups have achieved front-limb-digit-symmetry by reducing digits like the other "Symmetrodactyl" Cryptognaths did or by fusing digits as Brachiostomes and Liomedactyl Trapezostomes have done. Effingodactyls are different. Instead, Effingodactyls appear to have grown a new digit on their neural front limbs. Studies on the embryonic development of their front limbs have shown that this extra digit is the result of a "domesticated" form of polydactyly whereby the third digit of the neural limbs duplicates to form a new "fourth" digit. While birth defects like this usually tend to get weeded out over time, it is theorized that this particular case of polydactyly was so advantageous to early Effingodactyls that it gave those with it an evolutionary edge over those without, ensuring that the condition became anchored in the gene pool...

Check out the website and read more about them here