Fundulidae

Fundulidae View in CoL

As described above (Early branching of major lineages), Middle Eocene fundulids theoretically inhabited the Brazos River delta and flooded Mississippi Embayment ( Fig. 6 View Figure 6 ). Late Eocene– Oligocene divergence of Leptolucania Myers, 1924 39.4–22.5 Mya ( Fig. 3 View Figure 3 , node 12) suggests eastward immigration of an ancestral fundulid across the Suwannee Channel to the Ocala High ( Table 2 View Table 2 ; Fig. 7 View Figure 7 ), which developed peritidal landforms by this time (Avon Park and Ocala formations; Randazzo and Jones 1997, Maliva et al. 2011). During the Oligocene, any of several sea-level fluctuations could have facilitated lineage separation if a sea-level fall (detected as an oxygen isotope event) facilitated dispersal across the Suwannee Channel to the Ocala High, with subsequent sea-level rise causing vicariance of the Ocala population owing to widening and deepening of the ocean gap (as proposed above for J. floridae ). The Oi2 event ~30.0 Mya ( Boulila et al. 2011) appears best aligned in time with our divergence estimate for Leptolucania . Although modern Leptolucania is a freshwater genus, the ancestor is likely to have had high salinity tolerance ( Ghedotti and Davis 2013), consistent with this hypothesis.

Our phylogeny places Lucania within Fundulus , on an earlydiverging branch that includes Fundulus subgenus Wileyichthys ( Fig. 3 View Figure 3 ). This is contrary to the work of Ghedotti and Davis (2017), who recovered Lucania outside Fundulus , but in partial agreement with the fish tree of life ( Rabosky et al. 2018), which places Lucania inside Fundulus , albeit on a separate branch from Wileyichthys (this tree did not include Leptolucania ). Given that Wileyichthys includes only the Pacific Coast taxa Fundulus lima and Fundulus parvipinnis and given that the Coahuilan endemic Lucania interioris is the earliest diverging lineage of Lucania , we propose (following Fig. 3 View Figure 3 ) that the MRCA of Lucania – Wileyichthys originated from range expansion of fundulids westwards from the Brazos delta to the Río Grande-Río Bravo delta 31.3–19.1 Mya ( Fig. 3 View Figure 3 , node 13; Fig. 7 View Figure 7 ). During this time, there were sea-level falls of 30–40 m below modern sea level approximately every 1.2 Mya ( Miller et al. 2020b). Following Dolby et al. (2016, 2018), we postulate that sea-level fluctuations promoted cycles of dispersal and vicariance. Any one or a combination of recessions might account for vicariance of fundulids between the Brazos and Río Grande-Río Bravo deltas, but the Oi2c event dated ~25.1 Mya ( Boulila et al. 2011) best aligns in time with divergence of Lucania – Wileyichthys ( Table 2 View Table 2 ).

In the Oligocene, the common ancestor of Lucania – Wileyichthys presumably immigrated into the ancestral Río Grande drainage. This massive river system extended far across southern North America to what would become the Basin and Range geomorphic province ( Snedden and Galloway 2019) and appears to have been the only Cenozoic route to the southern Basin and Range from the western Gulf of México ( Fig. 7 View Figure 7 ). This geographical scenario comes with relatively strict time constraints. Divergence of Wileyichthys from Lucania must have occurred when Basin and Range drainages became disconnected from the Río Grande, dated ~23.0 Mya ( Snedden and Galloway 2019). This timing is compatible with our 30.7–7.5 Mya estimate for divergence of Wileyichthys ( Fig. 3 View Figure 3 , node 14; Table 2 View Table 2 ). Fossil Fundulus in the Great Basin ( Smith et al. 2002) also support this hypothesis. Finally, distribution of Wileyichthys on the Pacific Coast of southern California and south along the Baja Peninsula agrees with the Río Grande as the dispersal corridor, because the headwaters of the Oligocene Río Grande extended to the Continental Divide in what would become southern California ( Karlstrom et al. 2020). Tectonism in this region potentially resulted in stream transfers that introduced Wileyichthys to the Pacific slope. Likewise, Empetrichthyinae must have been living in Basin and Range drainages along the continental divide, after break-up of the Madrean River (above). Fossil evidence from Empetrichthyinae ( Uyeno and Miller 1962) indicates that this group was also transferred to the Pacific slope, although no representatives survived to the present time.

The major split within the Fundulus crown group separated subgenus Fundulus as a clade 24.2–17.1 Mya ( Fig. 3 View Figure 3 , node 15; Table 2 View Table 2 ). An eastward shift of the Oligocene Brazos River at the Oligocene–Miocene transition (23 Mya) from its existing delta to form a new Red River delta relatively close to the Mississippi River delta (present Sabine River delta; Galloway et al. 2011) might have provided a catalyst for this event by providing an uninhabited delta open to immigrants. During this time, there were sea-level falls of 30–40 m below modern sea level approximately every 1.2 Mya ( Miller et al. 2020b). Any one or a combination of these fluctuations might account for a dispersal–vicariance sequence in fundulids between the Mississippi River and Red River deltas ( Dolby et al. 2016, 2018). However, the Mi1a isotope zone (sea-level recession) dated ~20.8 Mya ( Boulila et al. 2011) best aligns in time with our divergence estimate for subgenus Fundulus .

We hypothesize that the Zygonectes – F. sciadicus – Plancterus clade descends from a lineage that originated in the Mississippi River delta, because Plancterus (node 18) has a Mississippian distribution and is the westernmost lineage on this branch ( Fig. 8 View Figure 8 ). By default, this implies that subgenus Fundulus descends from fundulids that immigrated into the Red River delta, then diverged into a separate lineage during sea-level recession. Later abandonment of the Red River delta ( Fig. 8 View Figure 8 ), which merged with the Mississippi River delta 15 Mya ( Snedden and Galloway 2019), mixed Red River and Mississippi River fishes ( Hoagstrom and Echelle 2022). This would have brought subgenus Fundulus into sympatry with Zygonectes . Thereafter, sea-level rise 19–17 Mya leading up to the MMCO 17.0–13.8 Mya, during which high sea levels were sustained ( Miller et al. 2020a), theoretically aided range expansion of all coastal fundulids ( Dolby et al. 2016, 2018).