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Journal of Paleontology 91(1):146–161
large, robust-plated ceratodontid, most similar in appearance to KUVP 16262 (see Schultze, 1981) from the Kiowa Shale. USNM 508543 is one of only two described Cretaceous dipnoan fossils from eastern North America, and the only one from the Early Cretaceous. Thurmond (1974) reported a sharp-crested species of
Ceratodus in the Butler Farm local fauna, Paluxy Formation, Texas. He compared then-available specimens favorably to C. americanus (herein regarded as a nomen dubium) from the Morrison Formation. Kirkland (1987) regarded the Trinity species as an unnamed relative of what he later (Kirkland, 1998) described as C. fossanovum, also from the Morrison Formation. The Trinity species was formally described as Ceratodus texanus Parris, Grandstaff, and Banks, 2014, based on eight specimens (four pterygopalatine and four prearticular tooth plates) from localities in the Twin Mountains and Paluxy formations (Aptian–Albian, see Winkler et al., 1990), north- central Texas. Parris et al. (2014) compared C. texanus favorably with both Potamoceratodus guentheri and C. fossanovum, distinguishing the species on the basis of size and differences in angles formed by major crests.
Late Cretaceous.—Non-marine vertebrates of North
America’s Late Cretaceous are incomparably better sampled than those of the Early Cretaceous (e.g., Kielan-Jaworowska et al., 2004; Weishampel et al., 2004; Benson et al., 2013). In this context, the fact that only four ceratodontids are known for the Late Cretaceous suggests a real decline in diversity of the group. (Two additional taxa from the Late Cretaceous, Cer- atodus cruciferus Cope, 1876 and C. hieroglyphus Cope, 1876, were transferred to Acipenser by Estes [1964]). Three of the four are of Cenomanian (early Late Cretaceous) age. Ceratodus molossus n. sp., a large species with flat-crowned tooth plates (described herein), is from the Cedar Mountain and Naturita formations of Utah and is a probable relative of Late Jurassic C. robustus. Ceratodus gustasoni Kirkland, 1987, somewhat smaller but also flat-crowned, also comes from the Cenomanian of Utah. The holotype,UCM54074, and paratype,UCM54073, are both left prearticular plates. Although other specimens have since been discovered (Fig. 2.14, 2.15, and SupplementalData 9), pterygopalatine plates for this species are still unknown. Fossils of juveniles have been referred to the species (Brinkman et al., 2013, fig. 10.30), but these are uninformative. Kirkland (1987) hypothesized that C. gustasoni may be closely related to C. frazieri, differing most noticeably from this species by its posteriorly expanded lingual margin. A specimen of a juvenile, identified as Ceratodus sp. indet. and recently described from a Santonian level in the Iron Springs Formation, may belong to C. gustasoni (Eaton et al., 2014). Regardless, the specimen is of interest because it and an undescribed species from Los Peyotes, Coahuila Mexico (Coniacian–Santonian) (González-Rodríguez et al., 2016) represent the geologically youngest published ceratodontids from the Western Interior. However, putative lungfish remains have also been discovered from the lower Campanian part of the Aguja Formation of Western Texas (Wick et al., 2015; personal communication, A.A. Brink, 2016). If confirmed through future study and publication, this material would significantly extend the known range of lungfish from Laramidia (the western part of North America, separated from
Appalachia by the Western Interior during much of the Late Cretaceous; Archibald, 1996). Ceratodus carteri Main, Parris, Grandstaff, and Carter,
2014 is represented by a relatively large sample, with seven known tooth pates (one pterygopalatine and the remainder prearticulars) from the Woodbine Formation (Cenomanian) of Tarrant County, Texas. The sample also includes a wide size range, presumably reflecting different ontogenetic ages. In general, this species is most similar to the tall-crested P. guentheri. Ceratodus carteri also resembles tall-crested C. texanus (as noted by Parris et al., 2014), C. nirumbee (both Early Cretaceous), and C. fossanovum (Late Jurassic), but differs in its smaller size, more obtuse C1Cp angle, and lack of a crushing platform. A right prearticular and overlying plate (NJSM 18774),
found at the Big Brook locality in Monmouth County, New Jersey, represents the geologically youngest occurrence of Ceratodontidae in North America, and is one of only two specimens from the eastern seaboard of the continent. Speci- mens from the Big Brook locality probably come from the Mount Laurel Formation and are of Campanian age (Gallagher et al., 1986; Lauginiger, 1986). NJSM 18774 is structurally indistinguishable from prearticular plates of Ceratodus frazieri from the Cloverly Formation. Parris et al. (2004) identified the Big Brook specimen as Ceratodus sp. aff. C. frazieri, commenting that it probably represents a different species owing to its occurrence.Wedo not disagree, but because there is no anatomical basis for this distinction, we simply refer the specimen to C. frazieri without issue. Just as Ceratodus is regarded as a form genus among Ceratodontidae (e.g., Schultze, 1981), so might C. frazieri well be taken as a form species within that genus.
Diversity and paleoecology.—As noted, the nature of the fossil record of post-Triassic lungfishes in North America—which consists almost entirely of rather simple tooth plates—places serious constraints on phylogenetic and paleobiologic inference. In this case, the path of reasoned commentary lies perilously
close to a chasm of fantasy. These caveats notwithstanding, we find ourselves largely in agreement with phylogenetic and ecomorphologic interpretations proposed by previous authors (Kirkland, 1987, 1998; Parris et al., 2014), although we are unconvinced that existing data support recognition of ancestor-descendant relationships. The welcome new additions to the fossil record help span temporal and geographic gaps, and add considerably to diversity of North American Mesozoic ceratodontids.Wepropose a simple model, consisting of four species groups, to serve as a framework for discussing the post-Triassic evolutionary history of dipnoans in North America. As noted by Kirkland (1987), tooth plates of North American ceratodontids span a morphological continuum, from
small, high-crested Potamoceratodus guentheri to large, flat-crowned Ceratodus robustus. Our four species groups are named for the four species from the Upper Jurassic Morrison Formation: P. guentheri, C. fossanovum, C. frazieri, and C. robustus. The categories are phenetic: we do not intend these groups as definable phylogenetic units, although in most cases a close relationship of within-group species represents a
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