Cyprinodontidae

Cyprinodontidae View in CoL

Divergence of the MRCA of Floridichthys – J. pulchra 42.3–21.6 Mya ( Fig. 3 View Figure 3 , node 8; Table 2 View Table 2 ) is consistent with our hypothesis that Cyprinodontidae originated in the northwestern Gulf (above). This divergence estimate spans a dramatic cooling event at the Eocene–Oligocene boundary 33.9 Mya, which initiated the Oligocene icehouse climate that lasted until ~26 Mya ( Zachos et al. 2001, Miller et al. 2020b). Sustained icehouse conditions possibly facilitated southward invasion of Nearctic Cyprinodontidae ( Fig. 7 View Figure 7 ), consistent with predictions of the intothe-tropics paradigm ( Vrba 1995, Eronen et al. 2012, Meseguer and Condamine 2020) and with our ancestral-areas analysis that included the potential for jump dispersal between realms ( Fig. 5 View Figure 5 ). The point estimate for this divergence (31.8 Mya) is aligned with Oligocene isotope zone 1b (abbreviated Oi1b), dated 31.8 Mya ( Boulila et al. 2011). Oxygen isotope zones are periods of increased global glaciation (at which times, sea level is lowered), indicated by maximum relative abundance of δ 18 O ( Boulila et al. 2011). Concurrent timing of sea-level fall with divergence of the Floridichthys –J.a pulchra lineage agrees with the hypothesis that sea-level fall isolates coastal fishes among disjunct estuaries ( Dolby et al. 2016, 2018). Hence, we hypothesize that climatic cooling and periodic sea-level rise during Oligocene oscillations ( Boulila et al. 2011) allowed southward expansion of ancestral cyprinodontids, with one or more periods of sea-level fall (potentially Oi1b) subsequently isolating populations that had settled on the Yucatán platform.

Because living Floridichthys have a disjunct distribution across the Gulf of México between Yucatán and Florida, the MRCA of Floridichthys – Jordanella could be from Florida, Yucatán, or points between. Presence of the sister taxon J. pulchra in Yucatán favours Yucatán as the ancestral area for Floridichthys and is compatible with the hypothesis (above) that Cyprinodontidae originated in the western Gulf of México (closer to Yucatán than to Florida). Furthermore, the Caribbean Loop Current could have facilitated later oceanic dispersal of Floridichthys from Yucatán to Florida (as seen in several cyprinodontiform taxa; MassipVeloso et al. 2024), whereas the current would have hampered immigration in the reverse direction. Hence, we suggest that the Oligocene ancestor of Floridichthys – J. pulchra dispersed along the western Gulf Coast from the Río Grande delta to the Bay of Campeche, where presence of coastal and shallow-marine areas (( Villagómez et al. 2022) could have provided suitable habitats ( Fig. 7 View Figure 7 ). However, Floridichthys – J. pulchra has uncertain phylogenetic placement (compare Fig. 3 View Figure 3 with Piller et al. 2022), hence further study is needed.

Our estimate for vicariance of Cualac tessellatus Miller, 1956 in the Río Pánuco 29.9–14.6 Mya ( Fig. 3 View Figure 3 , node 9) provides further support to the hypothesis that ancestral cyprinodontids inhabited the western Gulf Coast in the Oligocene. Presence of a nascent Río Pánuco ( Beltrán-Triviño et al. 2021) suggests that ancestral Cualac tessellatus ranged inland along this route ( Fig. 8 View Figure 8 ). Late Oligocene–Early Miocene uplift of the Chicontepec Basin ( Roure et al. 2009) could have stranded this population inland ( Table 2 View Table 2 ).

As already mentioned, our phylogeny indicates that Jordanella is polyphyletic ( Fig. 3 View Figure 3 ). Separation of J.floridae from Megupsilon – Cyprinodon 23.3–10.8 Mya ( Fig. 3 View Figure 3 , node 10; Table 2 View Table 2 ) suggests that an ancestral cyprinodontid dispersed eastwards from the northwestern Gulf of México during the Early Miocene, when the nascent Florida Peninsula was an island (i.e. Ocala High). Our 16.8 Mya point estimate for time of divergence closely follows the onset of the MMCO, a period of high temperatures, reduced global glaciation, and elevated sea levels spanning 17.0–13.8 Mya ( Miller et al. 2020a). This timing suggests that an ancestral cyprinodontid was distributed along the northeastern Gulf Coast, ranging between the Ocala High and the mainland while the Gulf Trough, which separated these lands, was narrowed ( Fig. 8 View Figure 8 ). Sea-level rise during the MMCO greatly broadened and deepened the Gulf Trough, shrinking the Ocala High to a small island ( Popenoe 1990). We hypothesize that J. floridae descends from a peripheral cyprinodontid population persisting around this island.

Notably, the genus Jordanella has never been resolved as monophyletic with molecular data ( Miller et al. 2005, Echelle and Echelle 2020). This, along with the biogeographical evidence provided here, could justify resurrection of the name Garmanella pulchra for the Yucatán species. In this case, morphological similarities between species presently recognized as Jordanella might reflect ancestral traits that persisted in ancient peripheral-isolate taxa sequestered in ancestral cyprinodontid niches on either side of the Gulf of México. The taxonomy of these species needs further study.

Late Miocene divergence of Megupsilon (14.9–7.4 Mya; Fig. 3 View Figure 3 , node 11) might represent Late Miocene emergence of ancestral Río San Fernando (Río Bravo in the work of Snedden and Galloway 2019) as a corridor for inland invasion ( Table 2 View Table 2 ; Fig. 8 View Figure 8 ). This species (extinct in the wild) inhabited a spring-fed habitat ( Miller et al. 2005) and might have speciated as a spring endemic. It is possible that episodes of aridity, tectonism, or volcanism isolated the spring system, but this needs further study.

The rate of Cyprinodon speciation increased upon separation from Megupsilon (node 11; Figs 3 View Figure 3 , 4 View Figure 4 ). Several factors are likely to have contributed to this trend. Initially, inland invasions into five Late Miocene rivers subdivided Cyprinodon into as many upland lineages, four of which dispersed far across the desert region ( Hoagstrom and Osborne 2021). A relative of this western inland radiation also dispersed to Yucatán, founding an endemic lineage there ( Cyprinodon artifrons species group; Figs 2 View Figure 2 , 5 View Figure 5 ). Once inland invasions were underway, barrier displacement via drainage rearrangements, climate fluctuations, tectonism, and volcanism caused widespread allopatric diversification. Meanwhile, a maritime lineage of Cyprinodon remained along the Gulf Coast ( Echelle et al. 2005, 2006). This geography is unclear in our reconstruction of ancestral habitats ( Fig. 2 View Figure 2 ), because the same widespread ancestor produced sequential upland invasions from the coast, as already described. Furthermore, during the Pleistocene, maritime Cyprinodon made new invasions into the desert region ( Lozano-Vilano and Contreras-Balderas 1999, Hoagstrom and Osborne 2021) and immigrated to Caribbean islands and South America ( Haney et al. 2007, 2009). The Yucatán lineage produced a species flock ( Strecker 2006; Fig. 3 View Figure 3 ). Beyond this, our analysis might underestimate the Cyprinodon speciation rate, because a species flock from San Salvador, Bahamas ( Martin and Wainwright 2013) and the subspecies Cyprinodon variegatus hubbsi Carr, 1936 (a potential distinct species; Brix and Grosell 2013, Jung et al. 2019) are absent from our phylogeny, which also leaves out seven Cyprinodon species for which we had no genetic data. Finally, six of the recognized species included in the analysis are likely to be polyphyletic ( Echelle and Echelle 2020) but are represented as one taxon here.