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Journal of Paleontology 91(1):146–161
Table 2. Post-Triassic North American lungfish recognized in this study; C. = Ceratodus; P. = Potamoceratodus. Epoch
Species
Early Jurassic Late Jurassic
C. stewarti P. guentheri
Early Cretaceous C. kempae C. kirklandi C. kranzi
C. frazieri Late Cretaceous C. carteri
C. frazieri C. robustus P. guentheri
Distribution
C. fossanovum Morrison Fm. (Kimmeridgian–Tithonian), WY Morrison Fm. (Kimmeridgian–Tithonian), SD, OK Morrison Fm. (Kimmeridgian–Tithonian), WY
Moenave Fm. (Hettangian), UT; Kayenta Fm. (Sinemurian), UT Kayenta Fm. (Sinemurian–Pliensbachian), AZ
Morrison Fm. (Kimmeridgian–Tithonian), CO, OK, UT, WY Cedar Mountain Fm. (Valanginian), UT Cedar Mountain Fm. (Valanginian), UT Potomac Fm. (Aptian), MD
C. nirumbee C. texanus
C. gustasoni C. molossus C. frazieri
Trinity Group (Aptian–Albian), TX
Cloverly Fm. (Albian), MT; ?Willow Tank Formation (Aptian–Albian), NV
Cloverly Fm. (Albian), MT, WY, Kiowa Fm. (Albian), KS Woodbine Fm. (Cenomanian), TX
Principal reference(s)
Kirkland (1987, 1998); Pardo et al. (2010) This work This work
Kirkland (1987, 1998) Kirkland (1987)
Frederickson et al. (2016) Bonde (2008); This work
Parris et al. (2014)
Ostrom (1970); Schultze (1981); Kirkland (1987); This work
Naturita Fm. (Cenomanian), UT, Iron Springs Fm. (Santonian), UT
Cedar Mountain Fm. (Cenomanian), UT; Naturita Fm. (Cenomanian), UT This work Mount Laurel Fm. (Campanian), NJ
Kirkland (1987); Eaton et al. (2014) Parris et al. (2004)
Main et al. (2014)
Milner and Kirkland (2006) Milner and Kirkland (2006) Kirkland (1998)
appearance—an appearance enhanced by the fact that the tips lie in a linear configuration (rather than rounded, as is typically the case). Also, the Cp tip is distinctive in being elongate and slen- der. The occlusal surface is worn and much resembles that of other flat-crowned plates, such as those of C. frazieri or C. gustasoni, in having a cratered appearance owing to the presence of circumdenteonal dentin islands. Wear is also distinct on the near-vertical surfaces in the notches between ridges. The dorsal surface of the fossil reveals the presence of a now-broken but originally large pterygopalatine process above C2-C3, similar to the condition in OMNH 04033, referred to C. frazieri (Kirkland, 1998), and differing from C. kranzi, in which it lies above C3-C4 (Frederickson et al., 2016).
Etymology.—For Anne Kemp, in recognition of her many contributions to understanding the evolution and biology of lungfishes, past and present.
Remarks.—Ceratodus kempae is clearly a flat-crested lungfish, with tooth plates that presumably emphasized crushing function (Parris et al., 2014): the occlusal aspect of the tooth plate has little topography, ridges lack denticles, and the surface has a pock-marked texture owing to dentin structure. On the other hand, C. kempae resembles sharp-crested species such as P. guentheri and C. kirklandi in its relatively small size, low inner (ABC) angle, presence of ridge crests (though faint), and slender, angulated labial ridge tips that are separated by angular notches. In these respects, C. kempae is structurally intermediate between flat-crowned and sharp- crested (“ceratodont” and “ptychoceratodont,” Parris et al., 2014, p. 279) morphologies. This combination of characters is novel in the North American record, prompting us to give it formal taxonomic recognition. Three other fragments of lungfish tooth plates have thus far
been recovered from the surface at BYU locality 1783. These are small, suggesting that they may belong to juvenile individuals, and are too fragmentary for confident identification, but the presence of sharp ridges on each suggests that they may belong to C. kirklandi, also known from the Yellow Cat Member of the Cedar Mountain Formation.
Discussion
North American ceratodontids in space and time.—Given the general scarcity of dipnoan fossils in the Jurassic–Cretaceous of North America, coupled with intraspecific variability and low information content of most specimens (tooth plates), some identifications and taxonomic issues are problematic. Formal systematic revision cannot be attempted here (nor do we believe that revision will be worthwhile until substantial new dis- coveries are made). In its stead, we provide a brief review of fossil occurrences, as a basis for discussion of diversity patterns through time. The review is arranged by geologic age; lacking a repeatable systematic framework, species are listed alphabeti- cally within each epoch (Table 2). One occurrence is noted in this introductory paragraph because age and provenance are uncertain. This is UNSM 50993, a gigantic (maximum dimension=117mm) pterygopalatine plate described by Shimada and Kirkland (2011). The specimen, which undoubt- edly represents a distinct, unnamed species of flat-crowned ceratodontid, was found in the outcrop belt of Miocene- to Pliocene-age deposits of the Ogallala Formation in western Nebraska, and was probably transported from a Jurassic or Cretaceous unit to the west.
Early Jurassic.—North America’s earliest ceratodontids
are of Early Jurassic age; two taxa are currently known, one not positively identified. Ceratodus stewarti Milner and Kirkland, 2006 was originally recognized on the basis of a single prearticular plate (UMNH 16027) from the Whitmore Point Member of the Moenave Formation, Utah. The only known pterygopalatine plate, described herein, hails from the strati- graphically higher Kayenta Formation, also in Utah. The species generally resembles Potamoceratodus guentheri (below), although it appears to be more closely related to Early Cretac- eous C. kirklandi n. sp. The known range of C. stewarti is Hettangian–Sinemurian. The other Early Jurassic occurrence is an unidentified
taxon, Potamoceratodus cf. P. guentheri, based onMCZ 13865, a prearticular plate distinguished from the otherwise similar Ceratodus stewarti in having higher crests and more acute angulation of C1Cp (Milner and Kirkland, 2006), as well as
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