Palaeogeography, Palaeoclimatology, Palaeoecology 248 (2007) 49–59 www.elsevier.com/locate/palaeo A fossil champsosaur population from the high Arctic: Implications for Late Cretaceous paleotemperatures ⁎ Deborah Vandermark a, John A. Tarduno a, , Donald B. Brinkman b a Department of Earth and Environmental Science University of Rochester, Rochester, New York 14627, USA b Royal Tyrrell Museum of Paleontology Box 7500, Drumheller, Alberta T0J0Y0, Canada Received 27 February 2006; received in revised form 21 November 2006; accepted 24 November 2006 Abstract During the Late Cretaceous, Axel Heiberg Island of the high Canadian Arctic supported a sizable population of champsosaurs, a basal archosauromorph, amongst a community including turtles and a variety of freshwater fishes. Here we report that a large portion of the available champsosaur fossil assemblage is comprised of elements from subadults. This dominance of subadults is similar to that seen from low latitude sites and suggests that the champsosaur population was a well-established facet of the ecological community. Because of the sensitivity of juveniles to ice formation, the make-up of the Arctic champsosaur population further indicates that the Late Cretaceous (Coniacian–Turonian) saw an interval of extreme warmth and low seasonality. The Coniacian–Turonian date makes these choristoderes amongst the earliest in North America, apart from the Jurassic Cteniogenys and a single limb bone from the mid-Cretaceous. © 2006 Elsevier B.V. All rights reserved. 1. Introduction between 92 and 86 Ma (Turonian–Coniacian) by radiometric age data from the underlying basalts and A vertebrate fossil assemblage collected from Late marine fossils in the overlying shales (Tarduno et al., Cretaceous rocks on Axel Heiberg Island (79°, 23.5′ N, 1998). Paleomagnetic data indicate the paleolatitude for 92°, 10.9′ W) in the High Canadian Arctic contains a the site to be approximately 71° N (Tarduno et al., 1997, diverse community of freshwater fishes and reptiles. 2002). In addition to champsosaurs, the assemblage also These fossils have been extracted from a relatively thin includes an array of freshwater fishes and turtles, 3 m section of shale and siltstone overlying the subaerial analogous with other well-known fossil communities flood basalts of the Strand Fiord Formation (See described from Cretaceous deposits interpreted as Tarduno et al., 1998 for detailed map and stratigraphic subtropical floodplains and fluvial depositional systems column). The beds, exposed on opposing sides of a associated with the Western Interior Seaway (Estes and rivercut, extend for approximately 50 m. The fossil- Berberian, 1970; Carpenter, 1979; Breithaupt, 1982; bearing strata underlie marine shales of the Kanguk Fastovsky, 1987). Formation; the age of the fossils is constrained to be The fossil assemblage describes an aquatic community far removed from that found in modern high latitudes. Fish elements isolated from matrix have been described in ⁎ Corresponding author. Fax: +1 585 244 5689. great detail. Jaw bones, isolated teeth, and ganoin E-mail address: [email protected] (J.A. Tarduno). microstructure indicate the presence of both vidalamiinine 0031-0182/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2006.11.008 50 D. Vandermark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 248 (2007) 49–59 and amiinine amiids, lepisosteids, and teleosts (Friedman significance of this occurrence with respect to its latitude et al., 2003). Modern lepisosteids, warm-water temperate has been discussed previously (Tarduno et al., 1998). In to tropical fishes, are represented by seven species. this paper, we further consider the question of the paleo- Modern members are primarily found within the climatic significance of the Axel Heiberg assemblage by freshwater systems of North America through Central reconstructing the champsosaur community demograph- America, only occasionally inhabiting brackish waters ics and the approximate size range of champsosaurs in a (da Silva et al., 1998). Lepisosteid's exclusion from mod- climatically sensitive region of the Earth. The size range ern latitudes north of 50° N suggests their occurrence is and size-frequency distribution present demonstrate that controlled by climatic factors. The most northern taxon, individuals ranged from near hatchling size to full-grown Lepisosteus osseus, has abundance patterns which adults. Thus it is concluded that this was an in situ positively correlate with the number of frost-free days population that nested in the general area, rather than (Friedman et al., 2003). The family Amiidae is repre- being comprised of old individuals that migrated into this sented by a single extant species, Amia calva,isolatedto area from other localities. Since, in general, young shallow freshwater systems within temperate eastern individuals are most sensitive to climatic extremes (Lee North America (da Silva et al., 1998). The ecological et al., 1997), this further supports the previous interpre- range of A. calva is limited to areas south of the 18.3 °C tation that this occurrence of champsosaur and other (65 °F) July isotherm (50° N latitude)(Friedman et al., ectothermic reptiles provides evidence for a period of high 2003). The latitudinal restriction based on temperature temperature at this time. regimes as seen in these extant forms is transferable to the Axel Heiberg fossil locality and suggests that the Late 2. Champsosaur demographics Cretaceous saw extreme polar warmth. In addition, turtle material collected from the Axel Our comparative analysis of the fossil material focused Heiberg site has been described. From the turtle fossils, on identifying the number of individual champsosaurs Borealochelys axelheibergensis, a generically interdeter- represented and assigning each as an adult or subadult minate eucryptodire, and a trionychid have been identified based on the inferred size of the individual. The use of (Brinkman and Tarduno, 2005). A high diversity of turtles size as a proxy for age is based on the interpretation above the polar circle is unprecedented and lends further that a single species of Champsosaurus is present in support for extreme warmth at high latitudes during the the assemblage. In addition, the morphological varia- Late Cretaceous. Turtles, as all ectothermic reptiles, have tion in the degree of ossification of the limb bones is distributions limited by temperature. As a conservative consistent with the smaller individuals being less analog, the cold-water adapted species Chelydra serpen- mature. Ideally, individuals from a fossil assemblage tina and Chrysemys picta need at least 100 frost-free days caneasilybeidentifiedonthebasis of direct association for viable reproduction. Likewise, these turtles are or articulation of elements, however it follows that most excluded from areas with a mean annual temperature bone beds are complicated by some degree of dis- less than 2.5 °C and a warm-month mean temperature less articulation and loss of material. This can be caused by than 17.5 °C (Tarduno et al., 1998). various factors ranging from the activity of foraging The most prominent components of the vertebrate animals, depositional mode, level of preservation, and assemblage are a large bodied, long-snouted choristodere reworking of the beds. In the case of the Axel Heiberg tentatively referred to here as the genus Champsosaurus, assemblage, the majority of fossils extracted were dis- extinct diapsid reptiles morphologically similar to articulated (although the preservation of a few articu- gavials. The referral of this material to Champsosaurus lated specimens, including a series of aligned thoracic is based on the presence of tooth-bearing elements that vertebra, reveals that the site of deposition could not indicate it had a long slender snout. have been far removed from the site of habitation). As is observed in modern crocodilian species, lati- Elements such as vertebra, teeth and ribs were elimi- tudinal range of habitation was likely correlated to cli- nated from the estimation, as these provide no clear mate. The presence of large champsosaurs in the distinctiontoanoriginfromasoleindividualor assemblage further suggests a period of extreme polar multiple individuals. Limb bones offer promise for warmth during the Late Cretaceous. Several champsosaur our study due to the ease of distinguishing individuals species have been described in the literature from various through measurement comparisons (simple lateral rela- localities (Brown, 1905; Parks, 1927; Russell, 1956; tionships and limb bone ratios). Limb bones offer a Erickson, 1972, 1985). The initial identification of reliable representation of a sedentary population in champsosaurs from Axel Heiberg Island and the that the robust morphology and the dense matrix of D. Vandermark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 248 (2007) 49–59 51 limb bone composition enhance the probability of The lengths of complete bones were compared preservation. utilizing various limb bone ratios. The humero-femoral The assemblage available to date is represented by 23 ratio is consistent between species of Champsosaurus limb bones, consisting of five humeri, two radii, eight (Parks, 1927). The ulna-femoral and tibia-femoral ratios femora, three fibulae, and five tibiae (Table 1; Fig. 1). vary slightly (Parks, 1927), but the variation is relatively Approximately half of the limb bones are complete small and the ratios are