Journal of Paleontology, 91(1), 2017, p. 146–161 Copyright © 2016, The Paleontological Society 0022-3360/16/0088-0906 doi: 10.1017/jpa.2016.131


New Cretaceous lungfishes (Dipnoi, Ceratodontidae) from western North America


Joseph A. Frederickson,1,2 and Richard L. Cifelli1,2


1SamNoble OklahomaMuseumof Natural History, 2401 ChautauquaAvenue, Norman,Oklahoma 73072,USA ⟨joseph.a.frederickson-1@ou.edu⟩ ⟨RLC@ou.edu⟩ 2Department of Biology, University of Oklahoma, 730 Van Vleet Oval, Norman, Oklahoma 73019, USA ⟨joseph.a.frederickson-1@ou.edu⟩ ⟨RLC@ou.edu⟩


Abstract.—Ceratodontid lungfishes are generally rare, poorly represented elements of North America’sMesozoiceco- systems, with previously known maximum diversity in the Late Jurassic. Herein we describe four new species of the form genus Ceratodus, from the Cretaceous of the Western Interior, considerably expanding fossil representation of post-Triassic dipnoans in North America. To model taxonomic and morphologic diversity, we adopt a four-fold system of phenetically based species groups, named for exemplars from the Morrison Formation. Ceratodus kirklandi n. sp. (Potamoceratodus guentheri group) and C. kempae n. sp. (C. frazieri group) represent a hitherto unsampled time interval, the Valanginian. Ceratodus nirumbee n. sp. and C. molossus n. sp. extend the temporal ranges of the C. fossanovum and C. robustus groups upward to the Albian and Cenomanian, respectively. These new occurrences show that ceratodontids maintained their highest diversity from the Late Jurassic through the mid-Cretaceous (Albian–Cenomanian), an interval of ~60 Myr. The existing record suggests that some of the later (mid-Cretaceous) ceratodontids may have been tolerant of salt water; to date, there is no evidence that they aestivated. Only a few occurrences are known from horizons younger than Cenomanian. Demise of ceratodontids appears to be part of a broader pattern of turnover that occurred at the Cenomanian-Turonian boundary in North America.


Introduction


Though now restricted to southern landmasses, lungfishes were present in North America for much of the Paleozoic and Mesozoic, dating back to the Devonian Period. Early repre- sentatives display broad diversity in skull and dental morpho- logy, suggesting differences in trophic adaptations and representation of multiple clades (e.g., Long, 2010; Pardo et al., 2010). Post-Triassic dipnoans of North America, however, are represented almost entirely by small samples (or unique occur- rences) of isolated tooth plates—a fossil record that is poorly suited to detailed phylogenetic or paleobiologic understanding, and that is therefore permissive of contrasting perspectives. For example, Martin (1982) proposed family-level distinction of the two major morphs (flat and sharp-crested, referred to Ceratodontidae and Ptychoceratodontidae, respectively) seen among North American Jurassic–Cretaceous varieties. Alternate hypotheses of relationships have been suggested, however (e.g., Kirkland, 1987, 1998), and in view of the fact that the tooth plates are broadly similar in gross anatomical and histological features, they are generally referred to Ceratodus (Agassiz, 1838; see diagnosis in Kemp, 1993), which has long been regarded as a form genus (Schultze, 1981; Kirkland, 1987). That this fossil record imperfectly reflects taxonomic diversity is illustrated by cranial remains recently described for one species (named Ceratodus guentheri by Marsh, 1878; and compared favorably to Ptychoceratodus by Martin, 1982 and some


subsequent workers) from the Morrison Formation, which is distinctive enough to warrant placement in a separate genus, Potamoceratodus (Pardo et al., 2010). On the other hand, tooth plates of the closest living species, Neoceratodus forsteri, are sufficiently variable (particularly according to growth stage) that fossils dating as far back as the Early Cretaceous are attributable to it (Kemp and Molnar, 1981; see also Kemp, 1997). The opposing issues of general morphologic conservatism and intraspecific variability pose challenges to interpreting a fossil record that consists mainly of rare, widely scattered occurrences of isolated tooth plates. Yet the record of North American Jurassic–Cretaceous


lungfish is informative in spite of its imperfection. Recognizable species-level distinctions can often be made between isolated specimens or small samples of Ceratodus tooth plates (see Kemp, 1993 and references therein; Kirkland, 1987, 1998). Moreover, the contrasting morphology between the major types of tooth plates (flat versus high-crested) suggests diversity in dietary preference. Species of Ceratodus (and Potamoceratodus) are inferred to have been faunivorous to omnivorous; low-crowned, flat tooth plates are interpreted to reflect adaptation for durophagy, suggesting more reliance on mollusks and, perhaps, vertebrates (Kirkland, 1987; Bakker, 2009; Shimada and Kirkland, 2011). It is also noteworthy that some of the flat-crowned (and presumably durophagous) taxa were the largest fishes of their respective communities; one unnamed species attained an estimated length


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