A Fossil Champsosaur Population from the High Arctic: Implications for Late Cretaceous Paleotemperatures ⁎ Deborah Vandermark A, John A

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 useful for assignment of possible whereas the remaining limb bones are broken mid-shaft, relationships between the Axel Heiberg champsosaur leaving only the proximal or distal end intact. The lateral limb bones. Working with fragmented limb bones can be relationships of the limbs established a base from which to problematic: similarly sized examples are lacking in the exclude commonalities between elements (i.e. an indi- literature. Using the measured dimensions of proximal vidual can possess only one right humerus or one left and distal ends, complete length estimates of fragmented femur). Measurements of the limb bones (complete length bones were scaled using documented measurements of and the maximum proximal and distal end diameters) complete specimens. were compared with those from the literature to determine Measurement comparisons of the fossils suggest that uniqueness and to infer an adult or juvenile status. only two of the available elements could have come from the same individual. Thus, the minimum number of champsosaur individuals in the assemblage repre- Table 1 sented by the limb bones is twenty-one. The com- Measured parameters of champsosaur fossil limb bones including lateral designation and estimated maturity levela plete length measurements of humerus UR 97.013 (113.15 mm) and tibia UR 00.018 (101.90 mm) Element CL PMXD DMXD Lateral Est. maturity designation status correspond closely with the bone lengths from an articulated adult Champsosaurus albertensis (humerus, Humerus UR 97.013 113.15 34.40 39.80 Left Adult 114 mm; tibia, 102 mm)(Parks, 1927). Femora UR UR 00.036 97.55 26.65 n/a Left Sub–adult 00.052 and UR 00.055 are left and right elements, NUFV 212 94.90 27.10 31.60 Right Sub–adult respectively. Only the proximal end of UR 00.052 is NUFV 242 144.50 40.80 46.20 Left Adult preserved, whereas UR 00.055 is only the distal end. – NUFV 244 n/a 29.10 n/a Left Sub adult The difference between the distal and proximal end Radius diameters of these two femoral sections is similar to UR 97.030 40.90 8.30 7.95 Left Sub–adult differences between those of SMM P71.2.1 and SMM UR 00.020 n/a 12.80 n/a Right Sub–adult P60.2004 shown in Erickson (1972). The remaining limb bones show no close relationship to each other and Femur clearly represent separate individuals. UR 97.015 n/a n/a 33.20 Right Adult UR 97.018A n/a 21.50 n/a Left Sub–adult Maturity estimations are based on comparisons UR 97.019A n/a 25.50 n/a Left Sub–adult with materials described in Brown (1905), Parks UR 97.027 n/a n/a 21.35 Right Sub–adult (1927), Russell (1956), Erickson (1972), and Erickson UR 97.038 n/a n/a 19.45 Left Sub–adult (1985). Taking into consideration that two limb bones – UR 97.047 67.50 n/a n/a Right Sub adult possibly belong to the same individual, 67% of the UR 00.052 n/a 35.20 n/a Left Adult UR 00.055 n/a n/a 37.40 Right Adult limb bones from the Axel Heiberg assemblage came from subadults. This is consistent with other fossil Fibula communities. Assemblages containing large numbers UR 97.023 n/a n/a 9.80 Right Sub–adult of Champsosaurus, such as those from the Late Creta- UR 97.035 n/a 17.16 n/a Left Adult ceous Dinosaur Park Formation, have well represented UR 97.054 n/a n/a 7.90 Right Sub–adult numbers of immature individuals, suggestive of cohab- Tibia itation between adults and juveniles and the young UR 97.014 114.35 20.90 16.20 Right Adult being hatched within the normal habitat of adults UR 97.016 79.55 18.60 11.40 Right Sub–adult (Erickson, 1972). UR 97.029 n/a 24.60 n/a Right Adult UR 00.018 101.90 22.50 n/a Left Adult NUFV 216 82.30 19.20 11.50 Right Sub–adult 3. Size estimates aMeasurements unobtainable due to partial preservation of elements are represented as n/a. Body size estimates based on isolated elements Abbreviations: CL, complete length; PMXD, proximal maximum has been achieved through models developed around diameter; DMXD, distal maximum diameter. linear relationships between morphological features (i.e. 52 D. Vandermark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 248 (2007) 49–59 D. Vandermark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 248 (2007) 49–59 53 between limb length and total length). For extant fauna, using the A. mississippiensis regression model and these relationships can easily be ascertained from measured total lengths for several species as a result of readily available specimens. Extinct faunas typically varying femoral consistencies with A. mississippiensis. lack a large database of specimens from which to For example, total lengths for Pristichampsus were over- develop a reliable regression model. In some cases, estimated using the A. mississippiensis based regression. related extant lineages, which share similar morpholog- This was attributed to a relatively longer femur in Pristi- ical arrangements, are reasonable analogs for estimating champsus when compared to A. mississippiensis of body size of extinct faunas. While it is impossible to similar total lengths, a functional adaptation of a more eliminate all uncertainty due to morphological variabil- terrestrial lifestyle in Pristichampsus. On the other hand, ity, or the degree of relatedness of species used in the total lengths were underestimated for Gavialis, a highly models (Damuth and MacFadden, 1990), there are some aquatic modern crocodilian (Farlow et al., 2005). Gavials practical tests that can be made using comparisons with have been used as a functional analog for Champsosaurus limited complete fossil specimens. Here we will first due to their superficial similarity in appearance. Modern propose and test a body size estimate based on gavials rarely come out of the water and large individuals crocodiles, and later utilize the crocodile model to actually push themselves forward using their hind legs. estimate the size-frequency distribution of champso- Like gavials, species of Champsosaurus have much saurs from the Axel Heiberg locality. We follow Farlow longer hind legs compared to their forelimbs (Erickson, et al. (2005) in using total length, rather than snout-vent 1972), which are better designed for horizontal movement length, in our measure of length. While the use of total than vertical support (Russell, 1956). The predominance length can reduce the sample size available, we feel that of an inferred aquatic habit of Champsosaurus may this is compensated for by being able to directly equate to somewhat reduced femoral dimensions in com- compare with the larger data set presented by Farlow parison to A. mississippiensis; thus estimates based et al. (2005). on A. mississippiensis would represent minimal total lengths. 3.1. Testing application of Alligator mississippiensis To further examine the uncertainties involved with regression model utilizing the regression model of Farlow et al. (2005) to estimate champsosaur total length, we applied the model Most estimates of crocodilian body size have to complete Champsosaurus specimens with measur- concentrated on cranial measurements (Webb and able total lengths and femur lengths. Ideally, we would Messel, 1978; Woodward et al., 1995). A strong prefer a large number of specimens with which to test correlation exists across species between body size the model, however this is limited due to the paucity of and cranial dimensions, a relationship helpful in both complete skeletons. fossil and in vivo studies. In the case of champsosaurs, Brown (1905) described a relatively complete the skull is an extremely fragile part of the skeleton and skeleton of C. laramiensis (No. 981) with a total length complete preservation is rare. Therefore, models based of 1.5 m. The total length of the hind limb was reported on alternate skeletal elements are needed for total length to be 217 mm from which we estimate the femur to be estimation. Recent investigations into femoral dimen- approximately 75 mm (femur length was not reported). sions of Alligator mississippiensis show a tight Using the estimated femur length for No. 981, the correlation with total body length (total length from predicted total length based on the regression model of snout to the end of the tail)(Farlow et al., 2005). This Farlow et al. (2005) is 1153 mm, about 25% shorter than provides a promising means of assessing size range for the actual length. fossil collections of limited material. A complete juvenile specimen of C. laramiensis Champsosaurus and A. mississippiensis share similar from the Science Museum of Minnesota (SMM P65.3.1) body forms and limb proportions, thus A. mississippiensis has a reported femur length of 93.5 mm and total length is a reasonable functional analog. Farlow et al. (2005) of 1610 mm (Erickson, 1972). Using the reported femur noted some discrepancies between estimated total length length, the regression model predicts an average total

Fig. 1. Champsosaur limb bones collected from Late Cretaceous deposits on Axel Heiberg Island. Humerus: (a) UR 97.013 (b) NUFV 212 (c) NUFV 242 (d) UR 00.036 (e) NUFV 244 Radius: (f) UR 97.030 (g) UR 00.020 Femur: (h) UR 97.047 (i) UR 97.018A (j) UR 97.019A (k) UR 97.038 (l) UR 97.015 (m) UR 00.052 (n) UR 00.055 (o) UR 97.027 Fibula: (p) UR 97.014 (q) UR 97.016 (r) UR 00.018 Tibia: (s) NUFV 216 (t) UR 97.029 (u) UR 97.035 (v) UR 97.023 (w) UR 97.054 Bar scales equal 20 mm. UR signifies fossil repository at the University of Rochester. NUFV signifies fossil repository at the Canadian Museum of Nature. 54 D. Vandermark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 248 (2007) 49–59 length of 1380 mm, approximately 15% smaller than the mum of 25%. The smaller values obtained using the actual total length. A. mississippiensis regression may indeed be a reflec- An estimate for the champsosaurs from the Axel tion of a reduced femur length due to a predominately Heiberg assemblage was originally scaled from a aquaticmodeoflifeforChampsosaurus. The variation complete tibia found during one of the earliest in the amount of underestimation of total lengths expeditions (UR 97.014). A comparison of the total among the Champsosaurus used in the comparison length of tibia UR 97.014 with published data and a may stem from interspecific differences among the similarly sized specimen from the Royal Tyrrell champsosaurs, or sexual dimorphic and ontogenetic Museum of Paleontology (RTMP 86.12.11) suggested differences. Regardless of these uncertainties, we feel that champsosaurs from this Arctic locality reached at trends in the size distribution will be preserved. least 2.4 m (Tarduno et al., 1998). Using the relationship of Farlow et al. (2005), the total body length 3.2. Estimating femur lengths corresponding to tibia UR 97.014 is 2155 to 2392 mm. We plotted the total lengths for the champsosaur The Arctic assemblage contains a single complete fossils for which we have direct femur measure- femur. In order to obtain sizes for the other individuals ments, and estimates using other methods (i.e. com- represented in the assemblage, it was necessary to parison of tibia length with a complete specimen) develop a means of estimating femur length based on versus the predicted total lengths calculated using the other limb bone types. This was achieved using limb regression model of Farlow et al. (2005).Thisis bone ratios (Table 2). The A. mississippiensis regression also shown compared to a trendline representing the requires the total length of the femur as well as one of A. mississippiensis relationship (Fig. 2). All points fall several femoral dimensions. These include the distal above the A. mississippiensis relationship. Using the height, distal width, and proximal maximum and mini- actual total lengths of the champsosaurs, we can pre- mum diameters. As these cannot be measured directly, dict a femur length and total length relationship. The we assume a reasonable range for these dimensions to be regression of Farlow et al. (2005) underrepresented between 10 and 20 mm in subadults and 20 and 40 mm the actual total lengths in Champsosaurus by a maxi- in adults. Incorporating these values into the regression equation provided in Farlow et al. (2005) produced a range of total lengths for each individual, the longest and shortest length deviating from their mean by only 5.2% (Table 2; Fig. 3). The length of the one complete femur from the fossil bed was directly applied in the size regression. To roughly scale the lengths of the seven fragmented femora from their measurements, the femoral dimensions from a complete specimen of C. albertensis described in Parks (1927) were used. As before, these estimates were incorporated into the regression based on femoral measurements to acquire total length ranges (Table 3; Fig. 3). There is a larger degree of uncertainty with femur length estimates based on femoral dimensions, as there is no clear relationship identified in Champsosaurus. The possibility of varying growth rates and a nonlinear relationship between femur lengths and end diameters are important factors. Total bone lengths associated with the two femora (UR 00.052 and UR 00.055) suspected of belonging to the same individual were calculated independently using the regression of Farlow et al. (2005). The femur Fig. 2. Plot of champsosaur total lengths, predicted and actual, in segments were scaled to full length, UR 00.052 based on relation to trendline representing A. mississippiensis relationship of Farlow et al. (2005). Dashed line indicates the predicted champsosaur proximal end dimensions and UR 00.055 based on distal relationship based on a 75% reduction in total length in comparison to end dimensions. The femur lengths estimated are similar an alligator of similar femur length. (154.1 mm and 155.4 mm). The average total body D. Vandermark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 248 (2007) 49–59 55

Table 2 Table 3 Estimated femur length and total length based on complete limbsb Estimated femora lengths and total lengths determined based on partial femorac Complete Estimated femur Calculated total limb length (mm) length (mm) Femur PMXD DMXD Estimated femur Calculated total (mm) (mm) length (mm) length (mm) UR 97.013⁎ Humerus 136.33 1969–2184 UR 00.036 Humerus 117.53 1573–1746 UR 97.015 n/a 33.20 137.9 1987–2205 NUFV 212 Humerus 114.34 1539–1707 UR 97.018A 21.50 n/a 94.1 1314–1458 NUFV 242 Humerus 174.10 2400–2663 UR 97.019A 25.50 n/a 111.6 1509–1674 UR 97.014 Tibia 152.47 2155–2392 UR 97.027 n/a 21.35 88.7 1253–1390 UR 97.016 Tibia 106.07 1448–1607 UR 97.038 n/a 19.45 80.8 1161–1289 UR 00.018⁎ Tibia 135.87 1963–2178 UR 00.052⁎ 35.20 n/a 154.1 2174–2412 NUFV 216 Tibia 109.73 1488–1651 UR 00.055⁎ n/a 37.40 155.4 2189–2429 UR 97.030 Radius 80.68 1160–1287 cMeasured dimensions from femoral sections. Femur lengths scaled UR 97.047 Femur 67.50 (actual) 1004–1114 based on measurements of C. albertensis specimen (2.3–2.5 m total bEstimated femur lengths based on complete limb specimens length) described in Parks (1927). Elements marked as (⁎) indicate calculated using limb bone ratios in champsosaurs. The humero- possibly same individual. femoral ratio (0.83) and tibia-femoral ratio (0.75) for C. laramiensis as cited in Parks (1927) and the radius-femoral ratio (0.50) calculated from a corresponding specimen (No. 982; Brown, 1905) were used. 3.3. Estimated Arctic champsosaur lengths Elements marked as (⁎) indicate possibly same individual. Most Champsosaurus species from the fossil record length corresponding to femur UR 00.052 is 2.293 m range from 1.0 to 2.5 m (Erickson, 1972), although and to femur UR 00.055 is 2.309 m. The similarities of some isolated elements found are suggestive of these estimates further suggest that these femora came individuals upwards of 4 to 5 m long (Parks, 1927). from the same individual, but also affirm our estimation The smallest champsosaur represented by limb bones methods. from the Axel Heiberg assemblage corresponds to femur Humerus UR 97.013 and tibia UR 00.018 have UR 97.047. The estimated total length for this lengths similar to that of a complete articulated speci- champsosaur ranges from 1.004 to 1.114 m. The largest men described in Parks (1927), which suggest they individual represented by humerus NUFV 242, ranged could have originated from the same individual. We in size from 2.400 to 2.663 m. Following our estimated the corresponding femur lengths using comparison of actual total lengths with total lengths humero-femoral and tibia-femoral ratios of Parks predicted by the regression given in Farlow et al. (2005), (1927) to be 136.33 mm and 135.87 mm, respectively. the total lengths described above represent minimal Average total body lengths calculated from these femur estimates. Even so, these estimates are consistent with lengths are 2.077 and 2.071 m. This coherence further complete specimens described in the literature. A femur supports our estimation methods. from a 1.61 m long C. laramiensis juvenile measured

Fig. 3. Total length vs. femur length. Solid points correspond with the average calculated total lengths for the champsosaur based on estimated femoral lengths from other complete limb bones as described in the text. Open points are average total lengths based on estimated femoral lengths scaled from fragmented femur fossils. Error bars show the range of total lengths calculated for each femur. Trendline based on Farlow et al. (2005) regression equation. 56 D. Vandermark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 248 (2007) 49–59

Fig. 4. Histogram showing the number of individuals for given total lengths ranges represented by the Axel Heiberg champsosaur limb bones.

93.5 mm (SMM P65.3.1; Erickson, 1972), longer than sample. The similarity in the size distributions within the femur of our smallest total length estimate. Parks the Royal Tyrrell Museum collection and the Axel (1927) described a specimen of C. albertensis with a Heiberg champsosaurs suggest that population demo- total length 2.3 to 2.5 m, having a humerus 138 mm graphics are comparable at both high and lower long, smaller than our largest humerus. latitudes. This similarity implies that the Axel Heiberg The Arctic champsosaur total lengths estimated using assemblage was also a breeding population despite the the femur relationship based on A. mississippiensis absence of hatchling fossils. The absence of hatchling (Farlow et al., 2005) appears to have a bimodal fossils is likely a sample size/preservation effect. distribution, with adults offset from subadults (Fig. 4). Total length estimates for the adult Champsosaurus are 4. Discussion all greater than 2 m. Subadults are smaller than 1.8 m. The number of specimens is limited but the subadults Champsosaurus thrived from the Cretaceous into the form a distribution around an average total length of early Tertiary (Erickson, 1972). In North America, 1.4 m. The total lengths calculated using the same established populations extended from New Mexico regression for a collection of Champsosaurus femora northward into Canada. Well-represented fossil records from the Royal Tyrrell Museum exhibit a similar of Cretaceous age have been described from the Lance, distribution (Fig. 5). A higher frequency of individuals Hell Creek, Judith River (Erickson, 1972), and Laramie less than 1.6 m long and greater than 2.0 m long is offset Formations (Carpenter, 1979) of western United States by a conspicuous decline in the number of champso- and the Frenchman Formation, Edmonton Group, and saurs within the size range of 1.6 and 2.0 m. The femora Belly River Group of Canada (Erickson, 1972; Gao and from the Royal Tyrrell Museum come from various Fox, 1998). localities within the late Campanian Dinosaur Park The genus Champsosaurus has no extant members, Formation and can be viewed as a random population thus characterization of the genus has relied exclusively

Fig. 5. Histogram showing the number of individuals for given total lengths ranges represented by champsosaur limb bones from the Royal Tyrrell Museum. D. Vandermark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 248 (2007) 49–59 57 on the fossil record in spite of preservational biases or during cooler intervals (Markwick, 1998). Today, incompleteness of specimens. Certain morphological crocodilians are limited to within ∼30° of the equator features of Champsosaurus parallel modern gavials, in regions defined by a mean annual temperature (MAT) suggesting they occupied similar niches. The delicate of 14.2 °C and a coldest month mean temperature nature of the champsosaur skull and zones for muscle (CMMT) no less than 5.5 °C (Markwick, 1998). attachment outlined from fossil specimens suggest the Modern crocodilians live within a finite temperature jaw was too weak and vulnerable for crushing and range and preferentially select for temperatures between chewing large prey. Rather, a long, slender jaw lined 25 and 35 °C with no appreciable ontogenetic difference with numerous stout, sharp teeth was better adapted for (Lang, 1987). Tropical species are limited to the higher delivering an effective quick snap for catching small fish limits of this range and are fatally vulnerable at and pelagic invertebrates (Russell, 1956), supporting temperatures below 15–20 °C, whereas temperate speculation that they were piscivorous. Although species are able to thrive in climates with mild seasonal thought of as semi-aquatic, champsosaurs probably changes (Lang, 1987). Such seasonality, however, must spent most of their time in the water. Incomplete be characterized by a relatively low temperature ossification of the articular surfaces of the limbs bones gradient (Markwick, 1998). High crocodilian abundance and the absence of fusion between the neural arches and corresponds with lower seasonal fluctuations and as centra (Russell, 1956), coupled with a highly stream- seasonal temperature gradients increase, the survival lined body form, granted Champsosaurus more agility rate of hatchlings and juveniles drops off sharply in water that on land (Erickson, 1972). In effect, (Markwick, 1998). This is a clear limiting factor to the Champsosaurus were formidable piscavores, which northward expansion of crocodile populations. dominated widespread ranges in relatively large The extinction of the large bodied champsosaurs numbers. in the Early Tertiary took with them knowledge of their Morphological structures and function can be teased physiology; it is impossible to classify them as tropi- out and defined from fossil material, however determi- cal or temperate reptiles. Locality correspondence of nation of physiological condition is difficult to recon- fossil materials from species typified as temperate struct on the basis of bone alone. In such cases, compels this designation and it provides a conservative comparison with a related extant lineage is sometimes estimation. Temperate species are able to cope and the only guide to understanding the physiology of an adjust to seasonal changes more efficiently than their extinct species, as well as the environmental require- tropical counterparts, but their northward migration ments necessary for success and geographical expansion today is still limited by temperature of a lesser degree. A of such populations. The use of extant analogs carries reduction in activity occurs in A. mississipiensis when the assumption that these characteristics are not its body temperature falls below 25 °C. This has been significantly distanced by evolution. observed to result in a reduction in feeding which Body size, limb proportions and stature are similar to ultimately limits growth and increases the animals modern crocodilians and depositional environments of susceptibility to disease (Lang, 1987). Temperatures as fossil assemblages suggest Champsosaurus inhabited low as 4–5 °C can be tolerated but only for short the same types of freshwater to brackish water intervals of time and the probability of survival from systems (Erickson, 1972) as their modern crocodilian such temperatures is much lower for juveniles and counterparts. hatchlings than it is for adults (Brisbin et al., 1982). Although modern temperate crocodilians, principally 5. Temperature constraints adults, can survive low temperatures in controlled experiments for short intervals, there is no record of The ectothermic condition of reptiles places their populations sustained in high latitudes. The length of survival directly dependent on their environment. winter seasons and the inability of hatchlings and Reptiles rely on ambient air and water temperatures to juveniles to survive even short cold intervals are the regulate their own body temperatures. Global climatic crucial limitations. zones directly control the geographical range of reptiles. The presence of numerous subadult champsosaurs in This is evident in the latitudinal fluctuations observed in the vertebrate assemblage is consistent with other the distributional patterns of fossil crocodilians, syn- reconstructed champsosaur populations. It suggests chronous with global climatic trends (Markwick, 1998). that breeding grounds were within or adjacent to adult Distributions during “greenhouse” intervals extend into habitats (Erickson, 1972) a characteristic similar to higher latitudes and similarly retreat toward the equator modern crocodilian populations (Mazzotti, 2002). 58 D. Vandermark et al. / Palaeogeography, Palaeoclimatology, Palaeoecology 248 (2007) 49–59

Modern crocodilians exhibit no migratory behaviors so 2004). Warm ocean temperatures can explain how species live and breed in the same environments; thus bordering landmasses experienced a moderated climate, year-round climate must be suitable for the survival of which supported diverse and temperature-sensitive flora hatchlings and juveniles into adulthood for a population and faunal communities. Paleobotanical records suggest to persist. floras dominated by broad-leaved plants stretched into While Champsosaurus characterizes the Axel Hei- regions north of 80° N latitude during the Cretaceous, berg assemblage, true crocodilians have yet to be flora which today are excluded to regions south of 55° N identified. The significance of this apparent absence is latitude (Herman, 1994). Leaf analyses of fossil plants unclear. As noted previously, the Axel Heiberg site rock from the Alaskan North Slope (Coniacian) imply a MAT exposure is limited. In light of this limitation, arguably of 12.5 °C and a CMMT of 5.7 °C (Herman and Spicer, the most conservative explanation of the apparent 1997). These temperatures are consistent with what the absence is that it reflects a patchiness in the crocodile presence of the Axel Heiberg champsosaur community distribution controlled by environmental or ecological suggests. Based on modern crocodilians, the MAT was conditions. 14.2 °C and CMMT was no less than 5.5 °C, compelling Survival of hatchlings and juveniles is inhibited by the exclusion of seasonal ice formation. The established colder northern seasons and defines a latitudinal and reproductively successful community of champso- restriction. The small body size of hatchlings and saurs, suggested by the diverse size range and dominance juveniles relates a lower thermal inertia putting them at a of subadults, combined in a diverse ecosystem of turtles disadvantage to adults. Juveniles and hatchlings will and fishes lends unprecedented support to the growing lose precious body heat and approach a critical argument that high latitudes during the Late Cretaceous minimum body temperature at a faster rate than adults not only experienced warmer climates but also lacked (Lee et al., 1997). During cold intervals and even short ice-forming seasonal fluctuations. periods of ice formation, adults sustain a body temperature above a critical minimum through a behavior 7. Conclusions known as the “icing response”. The animal adopts an inclined position submerged in water with only the tip of Approximately 67% of the champsosaurs repre- the snout exposed at the water surface (Brisbin et al., sented by fossil limbs from the Late Cretaceous Axel 1982). This behavior has been observed in an adult Heiberg assemblage are from subadult individuals. This A. mississippiensis, and attributed for its survival and high proportion of subadult individuals is similar to full recovery from temperatures between 4–5°C what has been observed at lower latitude sites and has (Brisbin et al., 1982). The same behavior utilized by special importance because it indicates that a reproduc- juveniles and hatchlings is not as successful under tive population of champsosaurs was present at very similar conditions. Adults are also large enough to sub- high latitudes. Hatchling and juvenile mortality rates of merge their bodies into deeper waters that buffer the modern crocodilian analogs suggest that the Axel cold air temperatures. Individuals less than 50 cm long Heiberg site lacked even seasonal ice formation. These are unable to take advantage of the water temperature observations further support a period of extreme warmth gradient as well as maintain holes in the ice for and very low seasonality during the Late Cretaceous. breathing (Brandt and Mazzotti, 1990). Small individuals such as juveniles and hatchlings are ill equipped References to survive even short periods of ice formation, Brandt, L.A., Mazzotti, F.J., 1990. The behavior of juvenile Alligator principal to the limitation of northward expansion of mississippiensis and Caiman crocodilus exposed to low temper- populations. ature. Copeia 3, 867–871. Breithaupt, B.H., 1982. Paleontology and paleoecology of the Lance 6. Paleoclimate implications Formation (Maastrichtian), East Flank of Rock Springs Uplift, Sweetwater County, Wyoming. Contributions to Geology 21, 123–151. Paleoclimate indices from marine and terrestrial Brinkman, D.B., Tarduno, J.A., 2005. 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