Frederickson and Cifelli—Cretaceous lungfishes
well-known Late Cretaceous vertebrate faunas) by the Western Interior Seaway. Given the general scarcity of dipnoan fossils and inadequate knowledge of early Late Cretaceous assemblages, little more can be said about the decline and extinction of lungfishes in North America. It is worthwhile noting, however, that the major drop in abundance and diversity occurs within the context of a broader turnover event among aquatic taxa at the Cenomanian- Turonian boundary (Eaton et al., 1997; Brinkman et al., 2013). North American Ceratodus species are known from a
variety of aquatic environments. Most commonly found in freshwater lake or stream deposits (Kirkland, 1987), it is likely that many of these species had lifestyles similar to the modern Australian lungfish (Neoceratodus forsteri), inhabiting diverse freshwater waterways, but preferring straight, flowing streams and rivers bordered by heavy vegetation and containing woody debris (Kemp, 1986; Arthington, 2009). Although often found in a range of low-quality habitats, members of this species are largely salt intolerant, incapable of living in or migrating through saline waters (Arthington, 2008). Similar to the modern Neoceratodus, Jurassic NorthAmerican ceratodontids appear to be limited to terrestrial environments. The Early Jurassic species (P. guentheri group) are only known from freshwater deposits in the Moenave and Kayenta formations. The holotype of C. stewarti, for example, was discovered as part of a relatively diverse lacustrine fauna (Milner and Kirkland, 2006). Similarly, Late Jurassic species are found in terrigenous rocks deposited under generally arid conditions (Engelmann et al., 2004), with most fossils deriving from fluvial (Kirkland, 1987) or lacustrine (Pardo et al., 2010) facies, far from marine habitats (Fig. 5.1). By contrast, Early Cretaceous occurrences (Fig. 5.2) of
North American Ceratodontidae occur in a wider range of depositional settings, varying from fluvial (Ostrom, 1970) to nearshore marine (Schultze, 1981). Species from the Late Cretaceous (Fig. 5.3, 5.4) all occur in close proximity to the paleo-shoreline, and several sites lie in horizons that were probably deposited under brackish or marine settings. Within the C. frazieri group, for example, at least two specimens derived from nearshore marine deposits (Schultze, 1981; Parris et al., 2004), and one is from a possibly paludal setting (Frederickson et al., 2016). Other large ceratodontids may have also had some degree of salt tolerance. For example, fossils of C. molossus (C. robustus group) and C. gustasoni (C. frazieri group) were found in the near-coastal sediments of the Naturita Formation (Kirkland, 1987; this study). Similarly, a Late Cretaceous member of the P. guentheri group (C. carteri)is known from a deltaic plain or coastal wetland environment (Main et al., 2014), suggesting that salt tolerance may have developed among gracile species groups as well. Notably, the P. guentheri and C. frazieri groups are also the only dipnoans known from Appalachia during the Cretaceous, although it is likely that they inhabited the area prior to completion of the Western Interior Seaway. At least one Australian species of Ceratodus, wide-ranging C. diutinus, occurs in both freshwater and marine units (Kemp, 1993). Ceratodus also occurs in the marine Westbury Formation (Late Triassic) of Britain, but the vertebrate fossils from this unit apparently represent a mixed assemblage (Storrs, 1993), so the origin of the lungfish specimens is unclear.
159 Alternatively, it could be argued that all of these later
brackish to saltwater occurrences represent reworked material from freshwater settings. Indeed, the Campanian C. frazieri fossil from New Jersey was said to be “taphonomically mature” (Parris et al., 2004, p. 65), indicating that this specimen was transported prior to final burial. In most cases, however, reworking hypotheses are based on behavioral observations of salt intolerant modern lungfish and not paleontological evidence (Kirkland, 1987; Kemp, 1993). In isolated cases, the reworking hypothesis may seem more parsimonious, but taken as a whole, the Ceratodus record of the Late Cretaceous of North America appears to include more coastal, or at least coastal adjacent, settings, than that of the preceding Jurassic. Besides a functional lung, Dipnoi possess a variety of traits
that allow them to survive in an array of environments, including ephemeral bodies of water. For example, aestivation allows living lepidosirenids (Lepidosiren and Protopterus) to lie dormant in an underground cocoon of mucus during drought. Trace fossils interpreted as aestivation chambers belonging to the basal dipnoan Gnathorhiza can be locally quite common in Paleozoic rocks of North America (e.g., Carlson, 1968; Berman, 1976), implying that this behavior has a deep history within the evolution of the clade. The most basal living species of lungfish (Neoceratodus forsteri), however, does not aestivate. By analogy, and noting the absence of identified aestivation burrows in geologically younger rocks, Kirkland (1987; see also Bakker and Bir, 2004) suggested that post-Triassic North American lungfish did not aestivate either. Conversely, in their phylogeny of Dipnoi, Cavin et al. (2007) hypothesized that all taxa more derived than Neoceratodus (including Ceratodus) would have been exclusively freshwater fish that were capable of aestivating during dry spells. Burrows of the correct size and shape to accommodate a full-grown Ceratodus would hypothe- tically be conspicuous, especially for some of the larger species, but to date none has been found. Thus we tentatively reject the hypothesis of Cavin et al. (2007) because: (1) North American ceratodontids may not have been limited to freshwater habitats, and (2) there is no evidence for dipnoan aestivation chambers in the Jurassic and Cretaceous of NorthAmerica. The second point also argues against an alternative hypothesis: that marine- dwelling Ceratodus may have burrowed during low tide (see Schultze and Chorn, 1997).
Conclusions
In the three decades since the last systematic treatment of North American ceratodontids (Kirkland, 1987), the number of recognized species has more than doubled, from five to 12, with most of the new taxa coming from the middle part of the Cretaceous (Albian–Cenomanian). Still, few of these species are known from more than a few specimens, and only one is represented by material other than tooth plates and the bones that bear them. This limitation still makes direct phylogenetic study difficult, and it remains debatable as to whether included species belong to single or multiple genera (see Pardo et al., 2010). Nonetheless, a phenetic approach helps organize species into useful species groups, which, in some cases at least, likely represent clades. Using this technique, it is apparent that all of the Late Jurassic species have at least one morphological
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