The late Carboniferous-aged (309-307 Ma) Mazon Creek lagerstätte produces some of the earliest tetrapod fossils, including those of major Paleozoic lineages such as the second oldest reptile. Despite this, the Mazon Creek lagerstätte has remained a difficult and unproductive vertebrate locality for researchers to utilize in tetrapod diversity studies due to the scarcity of fossils of this kind. Over the past decades, several new terrestrial tetrapod fossils collected from Mazon Creek have come to light. These include several new virtually-complete fossils of the earliest fossorially adapted recumbirostrans. Here I provide a revised systematic study of the Mazon Creek pan-amniote fauna, in an attempt to reassess the terrestrial ecosystem diversity present at the late Carboniferous lagerstätte. The results accumulate to systematic descriptions of four new and unique recumbirostran taxa (Diabloroter bolti, Infernovenator steenae, FMNH 1309, and MPM VP359229.2) and a re-description of the basal eureptile Cephalerpeton ventriarmatum leading to the anointment of the oldest parareptile Carbonodraco lundi (formerly Cephalerpeton aff. C. ventriarmatum from Linton, Ohio). Descriptions are aided by modern imaging techniques and updated phylogenetic analyses using Maximum parsimony and Bayesian methods where applicable. Across the newly described terrestrial fauna there is an unexpected ecomorphological diversity of bauplans present. These range from the short-bodied Diabloroter to the serpentine, long-bodied, and limb-reduced MPM VP359229.2. This diversity in combination with an array of unique cranial specializations present in the new taxa highlight the early experimentation with fossoriality in terrestrial vertebrate ecosystems. Exceptional preservation of soft tissues in FMNH 1309 even reveal integumentary adaptations to a fossorial lifestyle. Collectively, I am able to provide a revised taxonomic list for the Mazon Creek Tetrapoda that reveals the fauna is compositionally similar to that of most Permo-Carboniferous tetrapod localities. Given recent studies that suggest that recumbirostrans are crown-group reptiles, the results of this thesis imply a more rapid mid-Carboniferous diversification of amniotes into a variety of terrestrial niches and consequently an earlier establishment of terrestrial vertebrate ecosystems.