Marattialean ferns and medullosan seed ferns are two dominant groups in late Carboniferous lowland tropical floras. Differences in their foliage suggest that some seed ferns may have had a higher preservation potential when compared to marattialean ferns. Should this be accurate, it would suggest that when the two groups are found together, marattialean ferns would be better preserved and potentially better represented than seed ferns. Such taphonomic biases would have direct implications for evaluating plant diversity in deep time. To evaluate this hypothesis, we analyzed more than 880 fern and seed fern fossils encased in siderite (FeCO₃) concretions from the ~ 308 Ma Mazon Creek Lagerstätte to test if seed fern foliage is better preserved than fern foliage with respect to taxonomic information. For comparative purposes, we conducted a series of decay experiments on two modern ferns and a cycad (the last providing a seed fern analogue) for 160 days. Despite anatomical and compositional differences, fossil analyses demonstrate that ferns and seed ferns are equally well preserved in the Mazon Creek Lagerstätte with respect to venation, which represents one of the most important taxonomic characters of the flora. The decay experiments support this conclusion: despite having a more robust leaf, cycad leaves decayed at a similar rate to the thinner, more hypothetically fragile ferns. Together, our results indicate that our ability to discriminate within and between ferns and seed ferns is not biased by differential preservation in the Mazon Creek flora.