Zygonectes (Zygonetes) Agassiz 1854

Subgenus Zygonectes – F. sciadicus – Plancterus Our tree uniquely groups F. sciadicus as sister to Plancterus (Fundulus kansae Girard, 1859 – Fundulus zebrinus Jordan & Gilbert, 1883). Divergence of this group from Zygonectes 20.4– 13.0 Mya (Fig. 3, node 17; Table 2) and a transition from coastal to upland habitat (Fig. 2) suggest that the MRCA of F. sciadicus – Plancterus immigrated up the Mississippi River (Fig. 8), which extended to the nascent Great Plains (Snedden and Galloway 2019). Fundulus sciadicus and Plancterus species are grassland associates (Cross and Moss 1987, Fausch and Bestgen 1997). We hypothesize that the MRCA diverged from eastern relatives via adaptation to grassland habitats on the northern Great Plains. Accordingly, the 16.6 Mya point estimate for divergence of this group corresponds to a time when the Great Plains region was already ~68% open habitat (Edwards et al. 2010, Andermann et al. 2022). Also, the Ogallala Formation, which forms the substrate of the Great Plains, was forming at this time (Chapin 2008, Galloway et al. 2011). The 5.0–2.6 Mya fossil † Fundulus detillae Hibbard & Dunkle, 1942, recovered from the Ogallala formation in Kansas (Ghedotti and Davis 2017, Cashner and Echelle 2020), is consistent with the hypothesis that this lineage originated on the northern plains. Altogether, this evidence suggests that the F. sciadicus group might be a case of ecological isolation.