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Journal of Paleontology 91(1):146–161
Figure 5. Maps showing approximate North American lungfish distribution during the: (1) Kimmeridgian to Tithonian, Late Jurassic (Kirkland, 1987, 1998; Pardo et al., 2010); (2) Valanginian to Albian, Early Cretaceous (Ostrom, 1970; Schultze, 1981; Kirkland, 1987; Oreska et al., 2013; Parris et al., 2014; Frederickson et al., 2016); (3) Cenomanian, Late Cretaceous (Kirkland, 1987; Main et al., 2014); (4) Santonian to Campanian, Late Cretaceous (Parris et al., 2004). Numbers and color grouping correspond to Figure 4; abbreviations for species groups (1): Fr, Ceratodus frazieri; Fo, C. fossanovum; Gu, Potamoceratodus guentheri; and Ro, C. robustus. Maps modified from © Ron Blakey, Colorado Plateau Geosystems, and were chosen based on their best fitto the depositional environments observed at all of the fossil sites, combined with the closest temporal representation available. Specimens from the Willow Tank Formation, Nevada; Los Peyotes, Mexico; and the Aguja Formation, Texas are mentioned in the discussion but not shown in this figure. Scale bar = 200 km.
P. guentheri group (C. stewarti and Potamoceratodus cf. P. guentheri; Milner and Kirkland, 2006; this study) from the Early Jurassic of the southwest (Moenave and Kayenta formations). Intriguingly, tooth plates of C. stewarti bear a twinned Cp ridge, a feature also seen in Early Cretaceous C. kirklandi (Valanginian, Utah), suggesting the possibility of a sub-clade within the P. guentheri group—which itself may or may not be monophyletic. By the Late Jurassic, diversity vastly increases, with all four groups co-occurring in the Morrison Formation. Given the current gap in knowledge for terrestrial ecosystems of the Middle Jurassic of North America (not to mention poor anatomical representation of fossil lungfishes generally), it is currently unknown whether any of these novel groups are derived from the P. guentheri group (Parris et al., 2014) or immigrants from other continents. Significantly, all of this additional diversity involves taxa characterized by larger size and increased crushing capacity. Most distinctive in this regard is C. robustus, which has been compared favorably with C. kaupi Agassiz, 1838 (Triassic, Europe) and C. diutinus Kemp, 1993, which ranges from the Aptian–Miocene of Australia (Kemp, 1993). Assuming non-convergent within- group relationships, all four species groups present in the
Morrison Formation survived into the Albian, after which the C. fossanovum group disappears, followed by the P. guentheri group and the C. robustus group after the Cenomanian (Fig. 4). Unquestionably the most noteworthy recent improvement in the record of North American ceratodontids is in the medial Cretaceous (Albian–Cenomanian), with the addition of four species, collectively representing three of the four groups (Main et al., 2014; Parris et al., 2014; this study). Among these, the most surprising is the Cenomanian C. molossus n. sp., which appears to be a close relative of C. robustus, and hence represents the only record of the C. robustus group outside of the Morrison Formation of Wyoming. The existing record, including these new occurrences, suggests
that lungfish diversity in North America remained relatively stable from the Late Jurassic through the Cenomanian—an interval of ~60 Ma (time scale after Cohen et al., 2013) (Fig. 4). To date, geologically younger horizons have yielded only two described specimens: one from the probable Santonian of Utah (Eaton et al., 2014) and the other from the Campanian of New Jersey (Parris et al., 2004).Both belong to theC. frazieri group; the last-surviving member presumably occurred in a refugium on the east coast of Appalachia, which was then separated from Laramidia (and its
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