October 8, 2016:

by Research: Mosasaurs: Cold or Warm Blooded?

Thermoregulation, the ability of organisms to regulate their own body temperature, has been a subject of great interest in paleontology for more than a century. It is unknown precisely how and when endothermy (“warm-blooded” or the ability to self-regulate temperature) evolved in different lineages of vertebrate organisms, including mammals. Whether it evolved before the mammal-lineage broke off from their reptilian ancestors, or whether endothermy evolved within the reptilian lineage is debated. For example, fish and reptiles are considered as cold-blooded organisms, but several studies have shown that leatherback turtles and some fish, such as tuna and shark, can have some active control over body temperature (Holland et al., 1992; Thums et al., 2012; Casey et al., 2014). Dinosaurs are evolutionary related to archosaurian reptiles, with cold-blooded modern representatives, but birds that are warm-blooded evolved from a dinosaur lineage and it is believed that many dinosaurs were able to regulate their temperature (Amiot et al., 2006; Eagle et al., 2011). The goal of our project, initiated by Dr. Lynn Harrell of the University of Alabama as part of his dissertation research, was to investigate the thermoregulating capabilities of the swimming reptiles, mosasaurs, by analyzing the oxygen isotopic composition of their teeth and comparing them to the oxygen isotopic composition of coeval warm-blooded sea birds, and cold-blooded fish and turtles.

Oxygen is found in the mineral apatite, the mineral that makes up vertebrate bones and teeth. There are three types of oxygen based on the number of protons found in their nucleus. These different numbers of protons result in different masses of oxygen called oxygen isotopes. An analogue to this would be milk. Milk can be full fat milk, 2%, or even skim. It is still milk but whole milk is “heavier” than skim milk. This mass difference allows oxygen to partition itself in different ways based on temperature. Thus, temperature is a major control on which types of oxygen are incorporated into the mineral. At higher temperatures, the lighter isotope is incorporated into apatite, in cooler temperatures the heavier is incorporated into apatite. These values are preserved in teeth and bone and allow paleontologist and geochemists to investigate thermoregulation in extinct animals.

A study published by Bernard et al. (2010), attempted to explore thermoregulation in the most iconic marine reptiles – ichthyosaurs, pleisiosaurs, and mosasaurs – that lived in oceans and seas during the Mesozoic. The main conclusion was that ichthyosaurs and pleisiosaurs could have controlled their body temperature, but mosasaurs were cold-blooded. This study only had access to a limited number of fossils, which did not cover the whole spectrum of body sizes in mosasaurs, and only cold-blooded organisms (e.g., fish) for comparison. Thus, we initiated a more complete study of mosasaurs using the rich collections of the Alabama Museum of Natural History. After a careful scrutiny of fossil preservation, we analyzed the oxygen isotopic composition of teeth and bone at the University of Arkansas under the supervision of Dr. Suarez. Results were published in the journal Palaeontology early this year (Harrell et al., 2016) and remarkably show that mosasaurs regulated their temperature. Based on oxygen isotope composition of Cretaceous birds, we calculated body temperatures (~38 oC) very similar to those of modern pelagic seabirds. Mosasaurs oxygen isotope values were independent of size, and were more similar in composition to birds rather than fish. This suggests that they precipitated their apatite at higher body temperatures. Based on estimated oxygen isotopic composition of sea water calculated from the turtle oxygen isotopic composition, mosasaur body temperatures were > 30 oC, much higher than seawater temperatures of ~28 oC suggesting some ability for thermoregulation. Independence between body size and oxygen isotopic composition may also imply that body temperature control is not behavioral but rather a characteristic of the group itself. Our findings provide a better understand of the fossil record of mosasaurs. Their thermoregulating ability may have allowed their dispersal to cooler waters which is supported by the fossil record of at high latitudes. Thus, their ability to thermoregulate influenced their evolutionary success during the Cretaceous, allowing them to disperse and occupy a wide variety of ecological niches.

References:

Amiot, R., Lécuyer, C., Buffetaut, E., Escarguel, G., Fluteau, F., Martineau, F., 2006. Oxygen isotopes from biogenic apatites suggest widespread endothermy in Cretaceous dinosaurs. Earth and Planetary Science Letters 246, 41–54.

Eagle, R.A., Tütken, T., Martin, T.S., Tripati, A.K., Fricke, H.C., Connely, M., Cifelli, R.L., Eiler, J.M., 2011. Dinosaur body temperatures determined from isotopic (13C-18O) ordering in fossil biominerals. Science 333, 443–445.

Bernard, A. et al. 2010. Regulation of body temperature by some Mesozoic marine reptiles. Science 328, 1379-1382.

Casey, J.P., James, M.C. & Williard, A.S. 2014. Behavioral and metabolic contributions to thermoregulation in freely swimming leatherback turtles at high latitudes. Journal of Experimental Biology 217, 2331-2337.

Harrell, T. L., Perez-Huerta, A. & Suarez, C.A. 2016. Endothermic mosasaurs? Possible thermoregulation of late Cretaceous mosasaurs (reptilia, squamata) indicated by stable oxygen isotopes in fossil bioapatite in comparison with coeval marine fish and pelagic seabirds. Palaeontology 59, 351-363.

Holland, K.N., Brill, R.W., Chang, R.K.C., Sibert, J.R. & Fournier, D.A. 1992. Physiological and behavioural thermoregulation in bigeye tuna (Thunnus obesus). Nature 358, 410-412.

Thums, M., Meekan, M., Stevens, J., Wilson, S. & Polovina, J. 2012. Evidence for behavioural thermoregulation by the world’s largest fish. Journal of the Royal Society Interface 10, 20120477.