In this study, we assess thermoregulation in mosasaurs by comparing δ 18 O data from tooth enamel, with some additional results from well‐preserved cortical bone, of three genera Clidastes , Platecarpus and Tylosaurus , chosen on the basis of their adult size to provide context for small, medium and large mosasaurs respectively (Fig. 1 ). These data were further compared to stable oxygen data from Enchodus , as the representative of small poikilothermic fish in the Late Cretaceous, Toxochelys , a medium‐sized marine turtle with a shell typically less than 1 m in length that provided an analogue for an atmospheric breathing marine ectotherm, and Hesperornis and Ichthyornis , which are tooth‐bearing seabirds and chosen as coeval endothermic analogues.

The closest living relatives of the mosasaurs are varanid (monitor) lizards (Conrad 2008 ), which are a group of terrestrial (including arboreal) to semi‐aquatic squamate reptiles found naturally in certain tropical to temperate regions of Africa, Asia, Australia and western Pacific islands (Koch et al . 2010 ). Present‐day monitor lizards are ectothermic and regulate their body temperature primarily by using alternating periods of basking in sunlight and cooling in shade. Studies of Australian monitor lizards showed an optimal midday temperature range of 36.4–28.2°C for some wild, tropical to subtropical species, with ambient atmospheric temperatures (maximum/minimum) ranging from 30.4/19.3°C in the dry season to 33.1/25.3°C during the wet season (Christian & Weavers 1996 ). The variation in recorded body temperatures was dependent on the species, geographical region and season in which the temperatures were observed. The majority of present‐day monitors are ‘sit‐and‐wait’, ambush‐style predators, however, some are ‘widely foraging’, expending considerable effort in search of food (Clemente et al . 2009 ).

Bernard et al . ( 2010 ) assessed the potential of thermoregulation in a variety of Mesozoic marine reptiles using the δ 18 O of biophosphates (teeth) of coeval poikilothermic fish, from the same collection locality as the marine reptiles, as a proxy for ambient seawater temperatures. Poikilothermic animals have internal temperatures that vary significantly, typically fluctuating with the temperature of their surrounding environment. This approach indicated that ichthyosaurs and plesiosaurs were likely to be endothermic homeotherms, animals with a constant body temperature (homeotherms) that regulate that temperature metabolically (endotherms). Mosasaurs were more difficult to constrain and were thought to be potentially more influenced by ambient sea temperatures, or even that they could be gigantothermic, maintaining a high internal body temperature by virtue of their smaller surface area to volume ratio (Bernard et al . 2010 ; Motani 2010 ). Part of the problem that prevented constraining the potential for thermoregulation in mosasaurs was the absence of an endothermic taxon (e.g. toothed seabird) for comparison. Additionally, the whole range of body sizes in mosasaur genera was not evaluated in the Bernard et al . ( 2010 ) study for its potential influence in thermoregulation, and instead focused primarily on larger‐bodied taxa (of the specimens that were identified to the generic level).

In spite of these derived characteristics, the ichthyosaurs went extinct during the mid‐Cretaceous and were quickly replaced by the early mosasaurs, which probably evolved from small, near‐shore marine squamates (Dutchak 2005 ; Jacobs et al . 2005 ; Diedrich et al . 2011 ; Polcyn et al . 2014 ). From a morphological standpoint, the evolutionary success of the mosasaurs is often attributed to specialized intracranial mobility and other characters found in the skull, including streptostyly and a flexible mid‐mandibular joint, which allowed a better grasp on prey items (Russell 1964 , 1967 ; Callison 1967 ). As mosasaurs further evolved, they began to develop some of the same morphological features of their ichthyosaur predecessors, such as a hypocercal tail (Lindgren et al . 2007 , 2010 , 2011 ). By the time of their extinction, highly‐derived mosasaurs such as Plotosaurus bennisonni (Camp 1942 ) had acquired a number of additional features present in basal ichthyosaurs, including hyperphalangic paddles, a long slender muzzle with small sharp teeth, and a probable dorsal fin (Russell 1967 ; Lindgren et al . 2007 , 2008 ). This trend toward convergent evolution has led some researchers to hypothesize that mosasaurs may have similarly developed elevated thermoregulation (Bernard et al . 2010 ), which would have helped facilitate a possible transition from ambush predation to active, pursuit predation. Endothermy (the ability to regulate body temperature via metabolic control versus ambient or passive methods such as sunning) would also have enabled the mosasaurs to exploit cooler polar waters to expand their potential habitat range (Martin 2002 ; Chin et al . 2008 ) and provided them with an evolutionary advantage in the progressively cooling oceans during the Late Cretaceous (Friedrich et al . 2012 ; Polcyn et al . 2014 ).

Mosasaurs were a diverse group of highly successful marine reptiles that evolved in the mid‐Cretaceous and died out during the Cretaceous–Palaeogene (K/Pg) extinction event. This evolutionary success has previously been attributed to the opportunistic exploitation of ecological niches left vacant by the extinction of the ichthyosaurs during the mid‐Cretaceous (Sharma 2005 ; Everhart 2007 ). Prior to the mosasaurs, ichthyosaurs (and to a lesser extent plesiosaurs) were the dominant marine reptiles for a large portion of the Mesozoic Era (Motani 2005 ), their highly‐derived fusiform shape enhancing their capabilities as pursuit predators (Massare 1988 ). This advanced dolphin‐like or tuna‐like shape and a probable active lifestyle led some researchers to suggest the possibility that ichthyosaurs may have also had endothermic thermoregulation in order to maintain a high activity level and increase their chances of feeding success (de Buffrénil & Mazin 1990 ; Massare 1994 ; Motani 2002a , b , 2010 ; Bernard et al . 2010 ).

Powder samples were transported to the Department of Geosciences and the Stable Isotope Lab at the University of Arkansas (USA) for chemical processing and analysis. Methods were adapted from O'Neil et al . ( 1994 ) and Vennemann et al . ( 2002 ). Approximately 300–500 μg of material from each sample was weighed, placed in individual microcentrifuge tubes with 1 mL of 4% NaOCl solution, and allowed to sit overnight to remove organic material. Samples were rinsed five times with nanopure deionized water, using a vortex and centrifuge with each rinse, and allowed to dry. The samples were then dissolved in 100 μL of 0.5 M HNO 3 . Exactly 75 μL of 0.5 M KOH was then added to increase pH and decrease solubility of CaF 2 . Next, 200 μL of 0.17 M KF was added to precipitate out Ca 2+ as CaF 2 for 1–2 h. Samples were centrifuged and supernatant was transferred to clean, low‐binding microcentrifuge tubes using disposable glass pipettes for each sample. Each sample then received 250 μL of silver amine solution and was placed placed in an oven at 46.5°C to heat for 20 h. Yellow Ag 3 PO 4 crystals precipitated during the heating process, and samples that did not produce Ag 3 PO 4 crystals were discarded. The samples were then centrifuged and the supernatant decanted and disposed. Samples were rinsed five times with deionized water, and allowed to dry overnight. Ag 3 PO 4 crystals were weighed and then transferred to 3.5 × 5 mm silver capsules. The capsules were loaded into a zero‐headspace autosampler attached to a Finnigan ® MA TC/EA (thermochemical/elemental analyser) operated at 1400°C. The resulting CO was measured on line by a Thermofinnigan ® DeltaPlus XL gas chromatograph‐IRMS. Results (Table 1 ) were normalized to internal standards (returned values) consisting of USGS 24 (−28.1 ± 0.2), USGS 35 (57.1 ± 0.7), ANU Sucrose (37.1 ± 0.08), and Alpha Aesar Ag 3 PO 4 (8.4 ± 0.8) and reported in standard delta notation (δ 18 O) relative to V‐SMOW. NIST 120c was analysed as a quality control standard rather than a calibration standard and returned an average value of 22.4 ± 0.4‰. The lab reported external precision is ~0.3‰ (1σ). Statistical software was used to test the significance of the results using paired t ‐tests.

Fossil specimens were mechanically processed in the Department of Geological Sciences at the University of Alabama (USA). Specimens were first washed in an ultrasonic bath to remove any remaining traces of matrix, rinsed with deionized water and dried overnight. Each specimen was successively placed in a cleaned and dried agate mortar under a binocular dissecting microscope to collect the sample powders. A Dremel ® rotary tool equipped with diamond drill bits was used to obtain approximately 1–2 mg of sample powder, which collected in the mortar. Sampling was restricted to the outer enamel of tooth specimens because of its resistance to diagenetic alteration, and restricted to the outer cortical region of bone specimens to minimize contamination from infilling matrix and secondary minerals. Each sample powder was then transferred to weigh paper and measured on an electronic balance. Sample powders were placed in individually labelled microcentrifuge tubes for transport. Between specimens, the drill bits and mortar were cleaned with dilute trace metal grade nitric acid (HNO 3 ), rinsed with deionized water and dried with Kimwipes ® to prevent cross contamination.

The Sharon Springs Formation is a component of the Pierre Shale Group in South Dakota and consists of 6–14 m of dark, thinly bedded, organic‐rich shale with numerous bentonite interbeds (Martin et al . 2007 ). In the Black Hills region of South Dakota, the Sharon Springs Formation ranges from the lower to middle Campanian and represents a comparatively deeper water depositional environment than other formations of the Pierre Shale Group (Patrick et al . 2007 ). Although the macroinvertebrate fauna is mostly restricted to cephalopods, the vertebrate fauna of the Sharon Springs Formation is relatively diverse, with skeletons often preserved in articulation. Bone preservation can be poor, especially for weathered specimens at or near the surface, which typically have a veneer of selenite crystals.

The lower, unnamed member of the Mooreville Chalk in Alabama consists of beds of yellowish‐grey to bluish‐grey chalk and chalky marl that typically weather to a lighter white colour (Raymond et al . 1988 ). This relatively complete upper Santonian to lower Campanian geological section ranges in thickness from 107 to 122 m in western and central Alabama and is representative of an inner or middle shelf depositional environment (Mancini et al . 1996 ; Liu 2009 ). Water depth at the time of deposition is estimated to have ranged from 35 to 90 m based on the number of ‘sighted ostracodes’ (ostracodes with light‐sensing organs) preserved in the chalk and marl beds (Puckett 1991 ). The marine vertebrate and invertebrate fauna of the Mooreville Chalk is very diverse (Stephenson & Monroe 1940 ; Zangerl 1953 ; Thurmond & Jones 1981 ; Ikejiri et al . 2013 ) and well‐preserved, with little diagenetic alteration due to the shallow burial depth and impermeability of the chalk (Liu 2009 ).

Fossil specimens sampled in this study were collected from the lower unnamed member of the Mooreville Chalk in Greene and Dallas counties in Alabama, and the Sharon Springs Formation along the southeast flank of the Black Hills in Fall River County, South Dakota. Although many of the Mooreville Chalk specimens were previously collected by the Alabama Museum of Natural History, some were collected by TLH at the Harrell Station locality in Dallas County, a site that is well‐known for its rich vertebrate fauna (Zangerl 1948 , 1953 ; Langston 1960 ; Applegate 1970 ; Russell 1970 ; Thurmond & Jones 1981 ; Ikejiri et al . 2013 ). All of the Sharon Springs Formation specimens analysed in this study were collected by TLH from a landfill facility near Buffalo Gap, South Dakota. The numerous outcrops in this area are known for producing a variety of Cretaceous marine vertebrate fossils, as well as pterosaurs (Hargrave 2007 ). These two geological formations were selected for study because of their latitudinal variation and similar temporal occurrence (early Campanian).

Late Cretaceous birds were used as an analogue for an endothermic organism for comparison with contemporaneous mosasaurs, because all modern birds are endotherms and generate a relatively high body temperature (Prinzinger et al . 1991 ). Well‐preserved cortical bone of Hesperornis and Ichthyornis was used for isotopic analyses as their teeth were too small and fragile for sampling. Hesperornis was a fully‐marine, flightless seabird that inhabited the Western Interior Seaway, ranging from present‐day Arkansas to the Canadian arctic (Wilson & Chin 2014 ). Ichthyornis was a seabird that superficially resembled a modern seagull and was fully capable of flight, however its fossils are presently known only from offshore marine deposits in Kansas, Alabama, Texas and New Mexico (Clarke 2004 ; Shimada & Fernandes 2006 ). Hesperornis and probably Ichthyornis obtained water from the sea and therefore acquired oxygen isotopes from the same source as marine reptiles.

Fossils sampled in this study were either obtained with permission from the Alabama Museum of Natural History (ALMNH) collections from documented localities, or were field collected by TLH (Table 1 ). Taxonomic identifications were either based on museum records or were made by TLH using reference literature. All specimens sampled were in the adult to sub‐adult ontogenetic range to minimize effects of possible habitat partitioning by juveniles and adults. Mosasaur material for isotopic analyses (δ 18 O from the phosphate) was extracted mainly from well‐preserved tooth enamel, because the apatite is considered to be resistant to diagenesis (Koch 2007 ). For further assessment of the preservation, visual and SEM observations of tooth enamel were conducted and rare earth element (REE) data were evaluated from two previous studies using similar fossils from the same localities (Harrell & Pérez‐Huerta 2015a , b ). REE content suggests minimum diagenetic alteration in Mooreville Chalk fossils to the point of using the material to evaluate habitat preference based on light/medium/heavy REE ratios (Harrell & Pérez‐Huerta 2015a ). In addition, δ 18 O data from well‐preserved cortical bone were compared to tooth enamel results to determine the possible influence of diagenesis and to evaluate whether δ 18 O‐temperature derived data would reflect body temperatures (Table 1 ). The same procedure used to evaluate mosasaur samples was also used for Enchodus and Toxochelys . Finally, δ 18 O results from mosasaur and fish from the Mooreville Chalk in Alabama, which represent the bulk of data for this study, were compared to samples from the Sharon Springs Formation in South Dakota, which are known to be partially altered by diagenesis (Cicimurri & Everhart 2001 ).

The data presented here for mosasaurs correspond well with the regression line reported by Bernard et al . ( 2010 ). The small mosasaur, Clidastes , produced mean δ 18 O PO4 values of 20.3 ± 0.2‰ with a corresponding body temperature (T b ) of 33.1°C. The single medium‐sized mosasaur specimen from the Mooreville Chalk, Platecarpus , produced the lowest mean δ 18 O PO4 value and highest body temperature of the mosasaurs in the study (δ 18 O PO4 = 19.5 ± 0.2‰, T b = 36.3°C). The large mosasaur, Tylosaurus , produced temperature estimates (δ 18 O PO4 = 20.0 ± 0.2‰, T b = 34.3°C) warmer than Clidastes , but cooler than the medium‐sized Platecarpus . Specimens plotting closer to the zero region of the graph (Fig. 3 ) were considered to be poikilothermic while those with more negative values were considered to be more endothermic. The Ichthyornis specimens produced the most negative values and were designated as the endothermic end‐member of the graph. Four of the mosasaurs ( Tylosaurus specimens AL‐10, AL‐11; Platecarpus AL‐8; Clidastes AL‐6) plotted close to the Ichthyornis specimens, indicating high body temperatures. The remaining three mosasaurs ( Clidastes specimens AL‐5, AL‐7; Tylosaurus AL‐9) were less negative but still plotted away from the poikilothermic zero point on the graph. The Toxochelys turtle specimen (AL‐4), an air‐breathing poikilotheric reptile, plotted well within the range of poikilothermic fish (Figs 2 – 3 ).

The δ 18 O of body water was assumed to be approximate to the δ 18 O of seawater for marine vertebrates, and probably so for pelagic birds (see 4 below). The δ 18 O water values were determined based on the turtle bone analysis of Coulson et al . ( 2011 ) using clumped isotopes, and the turtle bone results of the present study (see 4 ). The δ 18 O PO4 of each Mooreville Chalk specimen was averaged and subtracted from the δ 18 O PO4 of fish and plotted on a graph modified from Bernard et al . ( 2010 ) to show the relationship of sampled taxa between poikilothermic fish and endothermic birds (Fig. 3 ).

The δresults for all taxa are summarized in Fig. 2 and Table 2 . Most of the data is obtained from the analysis of specimens collected from the Mooreville Chalk, since many of the samples from the Sharon Springs Formation did not produce sufficient AgPOcrystals. δvalues range from a maximum of 22.2‰ for(specimen AL‐4b), to a minimum of 18.7‰ for(specimen AL‐12a). Overall, the δvalues of bone samples correlate well with those of the enamel samples, showing no clearly differing trend. Therefore, the δof bone and enamel samples for each specimen were averaged together and body temperatures were calculated using the revised biogenic apatite formula by Pucéat. () (1), with δvalues of −1.4‰, −0.4‰, 0‰, and 0.2‰ (Table 2 ).

Discussion

The specimens analysed in this study were restricted to two geological formations in relatively small geographical areas in order to minimize possible effects of differing depositional environments on the δ18O water . The Mooreville Chalk was deposited in a relatively shallow, open marine environment (Liu 2009) unlikely to be affected by nearshore sources of meteoric water; the Sharon Springs Formation of South Dakota was also probably deposited in similar offshore, open marine conditions (Patrick et al. 2007). Previous studies on stable carbon isotopes in fossils from southwestern Africa, using different mosasaur taxa than those presented here, suggested habitat partitioning related to distance from shore and variation in adult size (Strganac et al. 2015a). Habitat partitioning could potentially influence the oxygen isotope signature in biophosphates because of differences in the oxygen isotope content of seawater in relation to proximity to shore, which has been observed in present‐day marine mammals (Clementz & Koch 2001). However, rare earth element data for the three mosasaur genera analysed in this study (Clidastes, Platecarpus, Tylosaurus) suggest that they inhabited the same environment despite their difference in adult size (Harrell & Pérez‐Huerta 2015a), limiting the potential variation in δ18O of water between taxa.

The mosasaur taxa from the Mooreville Chalk analysed in the present study produced a range of δ18O PO4 values (Figs 2–3). However, the mean of each genus (Clidastes = 20.3 ± 0.2‰, P = 1.3E−4; Platecarpus = 19.5 ±0.2‰, P = 2.8E−6; Tylosaurus = 20.0 ± 0.2‰, P = 3.5E−6; p‐values derived from paired t‐tests relative to mean Enchodus δ18O PO4 value) is significantly less than that of poikilothermic fish (Enchodus = 21.4 ± 0.1‰). The corresponding temperature calculations for each mosasaur genus (Clidastes = 33.1°C; Platecarpus = 36.3°C; Tylosaurus = 34.3°C) are substantially higher than that of poikilothermic fish and the ambient sea temperature (28.3°C). As the Mooreville Chalk sea was less than 90 m deep (Puckett 1991) and therefore unlikely to have a significant thermal gradient, the observed differences in body temperatures are not attributable to possible habitat partitioning in the water column. Sea water temperatures in present‐day warm, tropical environments are nearly uniform to depths of 100 m (Wallace & Hobbs 2006).

The present study uses a δ18O water value of 0‰ for the Mooreville Chalk as a good approximation of the difference between the δ18O water calculated from the turtle bone average (0.2‰) and the δ18O water calculated from the presumably more accurate turtle sample (−0.4‰, AL‐4a). Clementz & Sewall (2011) reported δ18O seawater for the Eocene ranging from 0.00‰ to −0.4‰ at latitudes of 30o–35o N, which are comparable to the geographical position of central and southern Alabama during the Late Cretaceous (Vrielynck & Bouysse 2003).

For palaeotemperature calculations, it is vital to determine the δ18O of body water (δ18O bodywater ) rather than the δ18O of ambient seawater (δ18O seawater ). Whereas the δ18O bodywater of terrestrial organisms will vary based on the δ18O of meteoric water, marine organisms should have δ18O bodywater that is relatively homogenous with seawater (Schaffner & Swart 1991; Koch 2007). The δ18O bodywater in fish is determined by the δ18O of ingested water rather than the δ18O of ingested phosphates from food items (Kolodny et al. 1983), and so in marine fish body water is in equilibrium with the δ18O seawater . Little research has been done on the δ18O bodywater of other aquatic vertebrates. Mosasaurs were large, marine reptiles incapable of traveling onto land (Field et al. 2015), with no modern ectothermic tetrapod analogue. Studies performed on crocodilians, which are relatively large ectothermic aquatic reptiles, showed as much as 2‰ enrichment of δ18O bodywater relative to environmental water (Amiot et al. 2007). However, crocodilians are only semiaquatic, with the ability to travel onto land and cool themselves through evaporative panting, which would likely enrich their body water in 18O, and range in habitats from freshwater rivers to coastal marine environments that have a wide range of environmental δ18O. Cetaceans are a possible modern endothermic tetrapod analogue for mosasaurs as they are fully aquatic tetrapods incapable of traveling onto land. Cetaceans ingest water from food items, which have similar δ18O bodywater to that of δ18O seawater , and so cetacean δ18O bodywater is also in isotopic steady state with δ18O seawater (Luz et al. 1984; Yoshida & Miyazaki 1991). As derived mosasaurs were fully marine reptiles incapable of travelling onto land, and ingested food from marine sources (Martin & Bjork 1987; Strganac et al. 2015a,b), they are here assumed to have δ18O bodywater approximately equal to δ18O seawater .

The δ18O bodywater of Cretaceous sea birds is more difficult to constrain. Whereas Hesperornis was a fully marine bird incapable of flight and could only walk on land with great difficulty (Reynaud 2006), Ichthyornis both flew and had legs that could support it in terrestrial environments (Clarke 2004). Ichthyornis is currently known only from offshore marine deposits (Clarke 2004; Shimada & Fernandes 2006) and is superficially similar to a gull or albatross in appearance (Clarke 2004), so it is therefore assumed to have had a similar pelagic lifestyle. Although no gastric residues have been identified with any described Ichthyornis specimen, based on its small size and tooth morphology, it most likely fed on small fish and/or soft‐bodied marine invertebrates. As these marine food items probably had δ18O bodywater approximating to that of seawater, it is here assumed that Ichthyornis had an initial δ18O bodywater approximating to seawater as well. Although the δ18O bodywater of modern birds has not been reported extensively, Kerstel et al. (2006) reported that stable oxygen isotopes in the body water of shorebirds occur at ‘natural abundances’ while Hobson & Koehler (2015) suggested a close association between δ18O bodywater and meteoric water ingested by migrating terrestrial birds. Schaffner & Swart (1991) reported that ingested water is the only source of δ18O bodywater in terrestrial vertebrates including birds, with only minor short term fluctuations due to respiration and metabolism. Because Ichthyornis could potentially cool itself through evaporative panting, unlike the other fully marine tetrapods and fish in this study, the possibility exists that its δ18O bodywater could have been enriched with 18O by evaporative fractionation. Modern seagulls sometimes cool by panting when they overheat during nesting but at other times they also cool by immersing their unfeathered legs in water (Ehrlich et al. 1988). Although there is no exchange of oxygen with ambient water vapour during respiration in birds (Hobson & Koehler 2015), Amiot et al. (2007) noted an enrichment of δ18O bodywater in semiaquatic crocodiles, which are distantly related to birds (archosaurs) and also use evaporative cooling, of up to 2‰. These δ18O bodywater enrichments were greatest in crocodiles with higher mass whereas small crocodiles had δ18O bodywater approximate to their ambient δ18O water . Amiot et al. (2007) concluded that the enrichment was due to metabolic processes related to mass. However, they also noted that uncertainties of up to ±2‰ in determining the δ18O of source waters and in determining the temperature of bone formation, suggesting that crocodile δ18O bodywater could still be used to determine the δ18O of environmental water. Because of these factors and its small body mass, the δ18O bodywater of Ichthyornis in this study is assumed to be the same as the δ18O seawater used in this study (0‰), although evaporative cooling may have caused slight enrichment of δ18O bodywater .

When 0‰ δ18O water is used in the fish palaeothermometry equation of Pucéat et al. (2010), revised from an earlier equation of Kolodny et al. (1983), the average temperature of poikilothermic fish (mean δ18O PO4 = 21.4 ± 0.1‰) from the Mooreville Chalk is 28.3°C. This temperature is comparable with the temperature range of ~26–29°C (Fig. 2) calculated for the middle Mooreville Chalk by δ18O CO3 carbonate analysis (Liu 2009). The average fish temperature reported here is also close to the temperature range of ~27.1–27.8°C calculated for the Mississippi Embayment region using clumped isotope analysis (Coulson et al. 2011). The depositional environment of the Mooreville Chalk sea is estimated to have been between 90 and 35 m deep (Puckett 1991) so there would have been little thermal gradient between surface and bottom waters affecting the fish. However, seasonal migrations of fish to other geographical regions with different water temperatures or δ18O water values that might have affected their δ18O PO4 values cannot be excluded. In contrast to fish data, and using 0‰ δ18O water , Ichthyornis specimens, with a mean 19.0 ± 0.1‰ δ18O PO4, produce an average body temperature of 38.6°C. This temperature falls within the measured resting and active (r/a) temperature range of modern birds (38.5°C/41°C), and more specifically of modern Charadriiformes (38.5°C/40.9°C) that include many of the pelagic seabirds (Prinzinger et al. 1991). Enriching the δ18O water of Ichthyornis to 1‰ (unlikely to be greater due to the small mass of Ichthyornis) to account for fractionation due to evaporative cooling during overheating produces a higher body temperature (T b = 42.8°C) that is still within the high activity temperature range (41.0–43.6°C) of modern Charadriiformes (Prinzinger et al. 1991). As with the fish, the migration of birds to other geographical regions with potentially differing δ18O water values cannot be discounted. However, as the fish and bird palaeotemperatures reported here are consistent with temperatures reported elsewhere, it would suggest that the δ18O PO4 values acquired from this analysis are relatively accurate and suitable for comparison between different taxa for determining thermoregulation by proxy.

The analysis of other organisms that included both tooth enamel and bone from the same individual showed good correlation between the two biophosphate samples, with no trend of the enamel δ18O PO4 being greater or lesser than that of the bone (Fig. 2). This is also an indication of the resistance to diagenetic alteration of fossils from the Mooreville Chalk because of the impermeable nature of the clay‐rich chalk and the relatively shallow burial depth of the formation (Liu 2009). Comparisons by Coulson et al. (2011) of δ18O PO4 with δ18O CO3 values in turtle bone samples from the Mississippi Embayment region (that includes the present study region in Alabama) suggested that there is little diagenetic alteration and that the original δ18O PO4 values have been retained. The bone phosphates of the Ichthyornis specimens from the Mooreville Chalk are therefore believed to be suitable for comparison with other taxa, and serve as the endothermic endmember.

Two samples of turtle bone were analysed in this study in order to provide an analogue for a likely ectothermic reptile for comparison with the mosasaur genera. Turtle bone can also be used to determine the δ18O water independent of the temperature at which it formed (Coulson et al. 2008, 2011). Whereas the turtle specimens from the Sharon Springs Formation did not produce any Ag 3 PO 4 crystals for testing, the samples from the Mooreville Chalk produced δ18O PO4 values comparable to those of the fish (Fig. 2). The mean δ18O PO4 turtle values were inserted into the formula of Coulson et al. (2008) and produced a δ18O water value of 0.23‰. This value is slightly greater than the hypothesized δ18O water value of Late Cretaceous ocean water of between 0 and −1.00‰ (Lécuyer et al. 2003). Of the two turtle samples from the Mooreville Chalk analysed in the present study, one (AL‐4b) produced a δ18O PO4 value that is significantly greater than the other turtle sample (AL‐4a) and the coeval fish. If this higher sample is omitted, and the lower turtle sample is inserted into the Coulson et al. (2008) equation, the δ18O water is −0.4‰. Coulson et al. (2011) determined a δ18O water value of −1.37‰ for the Mississippi Embayment region using toxochelyid and protostegid turtle fossils. However, their study involved a much wider geographical area and also included fossils from the underlying Tombigbee Sand Member of the Eutaw Formation, a geological unit that contains transgressive lag deposits with some reworked fossils (Mancini & Soens 1994). When the Coulson et al. (2011) δ18O water value of −1.37‰ for the Mississippi Embayment region is used, the calculated average fish temperature is 22.6°C (Table 2), which is lower than the temperature reported by Liu (2009) for the middle of the Mooreville Chalk. Using the Coulson et al. (2011) δ18O water value also results in a T b for Ichthyornis that is lower than any presently known bird. The very warm temperatures and shallow depth of the Mooreville Chalk sea in the Mississippi Embayment probably made the region one of high evaporation potential in the Late Cretaceous, which would have enriched the water in 18O and potentially produced δ18O water values close to 0‰. Therefore, the temperature calculations in this analysis were influenced by the δ18O PO4 present in turtle specimen AL‐4 rather than the much more negative value determined by Coulson et al. (2011).

To corroborate further whether calculated body temperatures are accurate, data from Late Cretaceous specimens collected from Alabama and South Dakota were compared. Most samples from the Sharon Springs Formation in South Dakota failed to produce Ag 3 PO 4 crystals or produced an insufficient mass of Ag 3 PO 4 crystals for reliable analysis. The likely reason for the lack of crystal production is that many of the specimens from the Sharon Springs Formation have poor preservation on their outer surfaces and the original biophosphate has been lost during diagenesis. The diagenetic effect is obvious in the lower average body temperature of Hesperornis (34.7°C) in comparison to Icthyornis data (Table 2). Nevertheless, the relationships between the taxa from the Sharon Springs Formation that did produce sufficient Ag 3 PO 4 crystals and δ18O PO4 values are the same as those from the Mooreville Chalk, and overall mosasaur and bird body temperatures are higher than those of fish (Table 2).

Endothermy in mosasaurs would have provided the taxonomic group with several distinct evolutionary advantages over their ectothermic prey and competitors. Among these advantages would be the possible transition from ambush predation, in which the predator uses surprise and quick bursts of energy to subdue prey over short distances, to pursuit predators, in which prey is chased over longer distances or with rapid changes in direction at the expense of greater energy consumption. Morphological adaptations in derived mosasaurs like Plotosaurus are similar to those present in early, possibly endothermic, pursuit‐style predatory ichthyosaurs (Massare 1988; Lindgren et al. 2007, 2008; Bernard et al. 2010), suggesting that some mosasaurs may have been adapting the same foraging behaviour. Endothermy would have also enhanced the ability of mosasaurs to forage over larger areas in search of prey, especially in cooler water environments, potentially increasing their chances of feeding success (Scharf et al. 2006). A second advantage for warm‐blooded mosasaurs would have been the ability to exploit high‐latitude polar waters, increasing their potential habitat range and foraging areas. A few occurrences of high‐latitude mosasaurs have previously been documented. Several species of mosasaur were reported from the Antarctic Peninsula by Martin (2002, 2006) along with a partial skeleton of a plesiosaur, another taxon of marine reptile that potentially possessed endothermy (Bernard et al. 2010). Mosasaur teeth and plesiosaur fossils were reported by Chin et al. (2008) from Devon Island (Canada), which is located at approximately 75° N near the northern end of Baffin Bay. Ectothermic reptile fossils in these polar regions are rare, as no marine turtles were reported from the Devon Island locale and only a single specimen has since been reported from the Antarctic Peninsula (de la Fuente et al. 2010). Finally, endothermy in mosasaurs would have provided a long‐term evolutionary advantage in the changing marine environment of the Late Cretaceous. Isotopic studies have shown that a relatively steady decrease in mean ocean temperatures occurred during the Late Cretaceous, from the Cretaceous thermal maximum (CTM) during the Cenomanian, falling to a low during the Maastrichtian (Friedrich et al. 2012; Strganac et al. b). During this same time interval mosasaur generic diversity increased, achieving its greatest richness during the Maastrichtian (Polcyn et al. 2014). If mosasaurs were ectothermic, it would be expected that diversity would decrease over time as global climatic conditions became more unfavourable, as is observed in Late Cretaceous marine turtles after the Cretaceous thermal maximum (Hirayama 1997; Nicholson et al. 2015).