Fossil soft-bodied worms such as nematodes are rare, and, because they often have few features preserved, difficult to interpret. A number of worm-like specimens from the late Carboniferous Mazon Creek fossil site were originally identified as a species of free-living nematode, called Nemavermes mackeei, which was among the oldest and largest fossil free-living nematodes. Here we reinvestigate these specimens, and determine that they encompass multiple species, including worms (but not free-living nematode worms) and cyclostome fish. In particular, some of these specimens belong to a new species of cyclostome fish, which we name Squirmarius testai.

Nemavermes mackeei Schram, 1973, found in the Mazon Creek fossil site and the Bear Gulch Limestone, was described initially as a free-living marine nematode. Here we investigate 13 specimens of N. mackeei from the Mazon Creek to reassess its morphology and identity, and also two specimens originally identified as Gilpichthys greenei Bardack and Richardson, 1977. Based on the extensive morphological variation among these specimens, N. mackeei encompasses multiple species that are only distantly related. The holotype of N. mackeei is a proboscis of Tullimonstrum gregarium Richardson, 1966, making N. mackeei a junior synonym of T. gregarium. However, the other specimens that we investigated could not be attributed to T. gregarium. We name a new species from these specimens: Squirmarius testai new genus new species, a cyclostome. One specimen is likely a juvenile G. greenei. Other specimens were not identified during this study but represent a variety of vermiform bilaterians.