A new species of the lungfish Ceratodus (Dipnoi) from the Early Cretaceous of the eastern U.S.A.

http://zoobank.org/urn:lsid:zoobank.org:pub:CFECA48B-7833-472B-8FA8-D746B3A6D9CE SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP Citation for this article: Frederickson, J. A., T. R. Lipka, and R. L. Cifelli. 2016. A new species of the lungfish Ceratodus (Dipnoi) from the Early Cretaceous of the eastern U.SA. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1136316.

single specimen attributed to Ceratodus aff. C. frazieri, from the Campanian of New Jersey. Until now, this specimen has been the only record of a lungfish from the Cretaceous of eastern North America (Parris et al., 2004).
Herein we add to the known diversity of Cretaceous lungfishes by describing a new species from the Arundel Clay facies of the Potomac Formation (Lower Cretaceous) of Maryland, USA (Fig. 1). Although known by a single, incomplete (yet diagnostic) tooth plate, Ceratodus kranzi sp. nov. represents the only known lungfish taxon from the eastern coast of North America during the Early Cretaceous. As such, it provides new data regarding lungfish diversity and biogeography during this time.
For lungfish tooth plates and dentin, we have adopted terminology from various sources (Schultze, 1981;Kirkland, 1987;Kemp, 1992Kemp, , 1998Kemp, , 2001b. Previous studies have relied on metrics (e.g., angles defined by borders and major ridges of tooth plates; Kirkland, 1987;Main et al., 2014;Parris et al., 2014). For comparative purposes and to maintain uniformity in treatment, we provide these data as far as possible; abbreviations for tooth plate ridges and other features are shown in Fig. 2. However, given the above-mentioned issues related to variability, small samples, and fossil incompleteness, our systematic treatment and comparisons rely heavily on qualitative characters.
Institutional Abbreviations-KUVP, Kansas University Vertebrate Paleontology Lab, Kansas Museum of Natural History, Lawrence, Kansas; NJSM, New Jersey State Museum, Trenton, New Jersey; USNM, National Museum of Natural History (formerly United States National Museum), Washington, D.C.; YPM, Yale Peabody Museum, New Haven, Connecticut.

AGE AND GEOLOGIC SETTING
The Arundel Clay lies within the terrigenous Cretaceous System deposited on the coastal plain of the mid-Atlantic region. The unit, originally defined as a formation within the Potomac Group (Clark and Bibbins, 1897), is sometimes regarded as a unique facies within the middle of the Potomac (e.g., Glaser, 1969;Lipka et al., 2006;Jud and Hickey, 2013), intermittently exposed in Washington, D.C., and Maryland. The Potomac is well known for its palynological record (Brenner, 1963;Doyle and Robbins, 1977;Hochuli et al., 2006;Doyle, 2012), which indicates a late Aptian to early Albian age for the Arundel Clay facies. The specimen described herein was recovered from USNM locality 41614, in what is now Dinosaur Park in Prince George's County, Maryland (Fig. 1A).
The Potomac is interpreted as representing a large, meandering, eastward-flowing fluvial system. Lithologically, the Arundel Clay facies consists mainly of lignite-rich bluish-gray claystones, reaching a maximum thickness of just under 40 m (Clark and Bibbins, 1897). The Arundel Clay facies, developed as a series of lenticular to elongate, localized deposits at or near the upper surface of the Patuxent member, is believed to represent floodplain or paludal depositional environments (Hansen, 1969;Kranz, 1998;Lipka et al., 2006). SYSTEMATIC PALEONTOLOGY SARCOPTERYGII Romer, 1955DIPNOI M€ uller, 1846CERATODONTIDAE Gill, 1872 CERATODUS Agassiz, 1838 CERATODUS KRANZI, sp. nov. ( Fig. 2A Etymology-The species is named for Peter Kranz, in recognition of his tireless efforts to study, publicize, and preserve vertebrate fossils from the Arundel Clay of Maryland (Kranz, 1989(Kranz, , 1998. Diagnosis-A large species of Ceratodus with an originally five-crested pterygopalatine tooth plate characterized by a highly obtuse inner angle (<ABC), with the last two ridge crests well defined and subequal in length; a large mass present on the mesiolingual side of the occlusal surface; and a posterior margin that is sublinear and suborthogonal to the lingual margin. The pterygopalatine tooth plate of C. kranzi can be distinguished from those of all other comparable species of Ceratodus based on a combination of its large size, square posterior margin, and the sizable mass present on the occlusal surface of the inner angle.
Description-The holotype and only known specimen of C. kranzi (USNM 508543) is an incomplete pterygopalatine tooth plate that is missing the rostral-most third. Ridge crest C 1 is not preserved as such, but its base can be distinguished as separate from a swelling that represents C 2 on the broken rostral edge of the tooth plate. The occlusal surface is heavily worn, with multiple islands of circumdenteonal dentine within an interdenteonal mass. These islands give the occlusal surface a cratered appearance (occlusal pits of Kemp, 2001a); however, it does not differ texturally from other large ceratodontids with robust tooth plates ("ceratodontid morphology" of Parris et al., 2014:280). The occlusal surface is generally flat, with a prominent convexity present on the mesiolingual side (see Supplementary Data). The pterygopalatine is present but is broken 10 mm caudal to the margin of the tooth plate. On the dorsal surface, there is a massive ascending pterygopalatine process medial to the space between the C 3 and C 4 ridge crests. The preserved portion of the pterygopalatine is morphologically indistinguishable from that seen in other Ceratodus species from North America. nov. (USNM locality 41614). Regional map (lower right) shows approximate positions of sites yielding C. kranzi (A) and Ceratodus aff. C. frazieri (B). Maps modified from Kranz (1989) and Weishampel and Young (1996).
Ceratodus kranzi is most similar to C. frazieri Ostrom, 1970, specifically KUVP 16262, a pterygopalatine plate described by Schultze (1981) from the Lower Cretaceous Kiowa Shale of Kansas (Fig. 2). Both specimens are relatively large and broad, with wide, low-angle ridge crests. Ceratodus kranzi differs from C. frazieri in having a conspicuous, caudolingually placed eminence on the occlusal surface and a straighter posterior margin. Additionally, the notches between the ridge crests are wider in C. frazieri, presumably allowing for the occlusion of wider ridges on the prearticular plate (unknown for C. kranzi). Ceratodus frazieri, originally described from the Albian Cloverly Formation of Wyoming (Ostrom, 1970), is also known from the Late Jurassic of South Dakota and, possibly, the Late Cretaceous of New Jersey (Kirkland, 1987;Parris et al., 2004). Although the range of this species might hypothetically overlap with that of C. kranzi, we consider them morphologically distinct, based on differences in the occlusal surface and variations in ridge crest angles.
Ceratodus kranzi can also be compared favorably with C. robustus Knight, 1898, known by broad, robust tooth plates from the Upper Jurassic Morrison Formation (Kirkland, 1987(Kirkland, , 1998. Pterygopalatine tooth plates of both C. robustus and C. kranzi possess a convexity on the mesiolingual side of the occlusal surface; however, this mass is much larger and broader in C. kranzi. Similarly, both species have sublinear, suborthogonal posterior and lingual margins, and are relatively large for ceratodontids. Ceratodus kranzi can be differentiated from C. robustus based on its longer C 3 ridge crest, a less developed notch between the last two ridge crests, and lingual and posterior borders that meet at an angle closer to 90 , in contrast to the more rounded intersection in C. robustus (which creates a more obtuse angle). Finally, and most definitively, C. robustus is unique among North American ceratodontids in that it only has four ridge crests on the pterygopalatine plate. Although incomplete, the type specimen of C. kranzi has three complete ridge crests and the base of a fourth ridge crest preserved. As noted, the base of this crest (C 2 ) is distinct from a more mesial area, where we interpret C 1 to have originated, giving a total count of five crests.
Other ceratodontids from the Cretaceous of North America include Ceratodus texanus Parris et al., 2014 from the Trinity Group (Aptian-Albian) of Texas, C. carteri Main et al., 2014 from the Cenomanian Woodbine Formation of Texas, and C. gustasoni Kirkland, 1987 from the Cenomanian Naturita Formation of Utah (see Carpenter, 2014). Both species from Texas, C. texanus and C. carteri, have small, high-crested tooth plates similar to those of Potamoceratodus guentheri (Marsh, 1878) from the Morrison Formation (see Kirkland, 1998) and are therefore fundamentally dissimilar to C. kranzi. Cenomanian C. gustasoni is more problematic because it is known only from prearticular plates. These plates are low crowned, as in C. kranzi, but the available sample suggests an adult size noticeably smaller than C. frazieri, C. robustus (see Kirkland, 1987), and C. kranzi. Furthermore, the thin ridge crests on the prearticular plates of C. gustasoni contrast with the broader, relatively shorter ridge crests of C. kranzi. Shimada and Kirkland (2011) described a giant lungfish pterygopalatine tooth plate found on the surface of Ogallala Group (Mio-Pliocene) rocks of west-central Nebraska and presumed to derive originally from Jurassic-or Cretaceous-aged units of the Western Interior. This specimen is anatomically distinct from C. kranzi based on its substantially larger size and the presence of a C p ridge crest that extends more labially than any of the other ridge crests. Further, the mesiolingual edge is rounded in shape and does not possess the squaredoff caudal margin (i.e., sublinear and suborthogonal to the lingual margin) seen in C. kranzi.
Measurements of ridge crest angles have been widely used to describe Ceratodus species (e.g., Kirkland, 1987;Parris et al., 2014). Although this technique has been justifiably criticized as inappropriate for small sample sizes (Kemp, 1997), these measurements remain one of the most widely used ways to diagnose species within the genus. To maintain consistency with previous studies, we present comparable data for C. kranzi here. Based on the relative incompleteness of this specimen, only two of the most relevant measurements can be determined. The inner angle (<ABC) of USNM 508543 is 169 -the highest of any North American Ceratodus species. The next greatest inner angle from a comparable specimen is that of KUVP 16262, a specimen of C. frazieri from the Kiowa Shale, with an angle of 152 (Kirkland, 1987; see Fig. 2). The C 3 -C p angle in C. kranzi is 31 , which is comparable to values found in multiple other species of Ceratodus (e.g., C. frazieri, C. guentheri, and C. fossanovum; see Kirkland, 1987).

Size
The holotype of Ceratodus kranzi is relatively large by comparison to other Mesozoic lungfishes of North America. Though incomplete, an estimate based on the length between FIGURE 2. Ceratodont lungfish tooth plate terminology/basis for measurements and Ceratodus kranzi, sp. nov. Top (A and B), system of crest terminology and angle measurements, after Kirkland (1987), showing upper (pterygopalatine) tooth plates of Ceratodus kranzi (USNM 508543, holotype, left) and C. frazieri (KUVP 16262, right tooth plate reversed to appear as left). Crests are numbered C 1 -C p (posteriormost crest); lines B-A and B-C approximate the mesial and lingual borders of the tooth plate respectively. Bottom (C and D), photographs of Ceratodus kranzi, sp. nov., USNM 508543 (left pterygopalatine plate, holotype) from USNM locality 41614, Arundel Clay facies, Potomac Formation (Aptian-Albian), Prince George's County, Maryland. Left, occlusal view; right, dorsal view. All scale bars equal 10 mm. the C 3 -C p ridge crests indicates that this species would have been subequal in length to C. frazieri (KUVP 16262;Schultze, 1981 estimated total length at approximately 60 mm) and between 77% and 120% the length of C. robustus (UCM 54248, see Kirkland, 1987). By using a direct comparison of tooth lengths, it is possible to approximate total body length of C. kranzi. Kirkland (1987) used published data for the Australian lungfish to hypothesize that large individuals of C. frazieri and C. robustus would have achieved sizes up to 1.5 m in length. Using Kirkland's (1987) estimates, C. kranzi would have ranged somewhere between 1.5 and 2.0 m in total body length. Using Kemp's (2005) regression equation for pterygopalatine tooth plates of N. forsteri (and again estimating the length of USNM 508543 at 60 mm) yields an estimated length of 2.2 m for C. kranzi. By any measure, it would appear that Ceratodus kranzi was one of the largest aquatic faunivores in the Arundel ecosystem.
Biogeography Parris et al. (2004) reported the first unambiguous occurrence of a Cretaceous lungfish from the east coast of North America (Fig. 1B). The specimen is a prearticular tooth plate (NJSM 18774), which the authors suggest may represent a new species and which they provisionally attributed to Ceratodus aff. C. frazieri. As noted, C. frazieri was originally described from the Cloverly Formation (Albian) and has been reported from the Kimmeridgian (with all previous reports based on fossils from the Western Interior). Specimen NJSM 18774 hails from the Campanian of New Jersey, implying great temporal and geographic range extensions, if referable to C. frazieri. Although not directly comparable, the holotype of C. kranzi and NJSM 18774 share some noteworthy similarities. Both are from large lungfish with robust, flat-crested tooth plates. The ridge crests on C. kranzi correspond somewhat more favorably with the narrow, finger-like valleys in NJSM 18774 than with the wider valleys seen in the holotype of C. frazieri, YPM 5276. Parris et al. (2014) noted that small Ceratodus species with high-crested tooth plates, which date back to the Early Jurassic (Milner and Kirkland, 2006) and are most characteristic of the Late Jurassic (Kirkland, 1998) of North America, make their last appearance in the Cenomanian of Texas (Main et al., 2014), on the eastern side of the then newly formed Western Interior Seaway. Given the limitations of the fossil record, relationships among North American species of Ceratodus remain almost wholly conjectural. It is conceivable, as Parris et al. (2014) noted, that C. aff. frazieri from New Jersey evolved from a clade of high-crested lungfishes that were restricted to eastern North America after completion of the seaway. However, the occurrence of C. kranzi in the Arundel Clay of Maryland shows that large-bodied forms with robust, flat-crested tooth plates were in eastern North America by the Aptian-Albian.