The Mosasaurs were a group of Lizards related to Varanids (Monitor Lizards) that adopted to a marine lifestyle in the Middle Cretaceous, after the extinction of the Icthyosaurs. They were a highly successful group producing a range of species ranging from under a meter to over 10 m in length, and are thought to have been the top predators in many Late Cretaceous marine environments. Mosasaurs were fully aquatic, clearly incapable of leaving the water, and known to have given birth to live young; later forms show convergence with Icthyosaurs and Dolphins, with streamlined bodies and hypocercal tails (tails with enlarged lower lobes), indicating that they were most likely highly active animals. This, combined with their presence in deposits in cold regions such as Antarctica and the Canadian Arctic has led some researchers to sugest that they may have been endothermic ('warm blooded', capable of maintaining a warm body in a cold environment by metabolic means), as has been suggested for othe Mesozoic Marine Reptile groups.
In a paper published in the journal Palaeontology on 1 April 2016, Lynn Harrell of the Department of Geological Sciences at the University of Alabama, Alberto Pérez-Huerta of the Department of Geological Sciences at the University of Alabama and the Alabama Museum of Natural History and Celina Suarez of the Department of Geosciences at the University of Arkansas, describe the results of a study of oxygen isotopes from Mosasuar, Fish, Turtle and Seabird specimens from the Late Cretaceous Mooreville Chalk of Alabama and Sharon Springs Formation of South Dakota, with the aim of determining the approximate body temperatures of these animals, and assessing whether those of the Mosasaurs were closer to those of the cold blooded Fish and Turtles or warm blooded Birds.
Oxygen has several stable isotopes, all of which can be included in any oxygen compound. Importantly heavier isotopes, particularly oxygen-18, are included in biominerals (mineralized tissues produced by living things, such as shell, bone and tooth) at higher levels at lower temperatures and at lower levels at higher temperatures, providing an approximate thermometer for the time when these tissues were laid down.
Three different Mosasaurs were used in the study, the small Clidastes, the medium-sized Platecarpus and the large Tylosaurus, as well as a small, cold blooded Fish, Enchodus, a medium sized, air-breathing, cold blooded Turtle, Toxochelys, and two Seabirds, the flightless Hesperornis and the Seagull-like Ichthyornis.
Body outlines and skeletons of Mosasaur genera analysed in this study for scale (left) and representative dentaries for each genus (right). Scale bars represent 3 m (left) and 10 cm (right). Harrell et al. (2016).
Both the Mooreville Chalk and the Sharon Springs Formation are thought to have been laid down in open marine environments, as opposed to closed bays where lack of free circulation can influence water temperature. The Moorville Chalk is inturpretted as a shallower environment than the deeper Sharon Springs Formation.
Harrell et al. found that the (warm blooded) Birds Hesperornis and Ichthyornis contained consistently lower levels of oxygen-18 levels than the (cold blooded) Fish, Enchodus, or Turtle, Toxochelys, suggesting that these animals (as predicted) had higher body temperatures, and that the methodology was able to detect this. The Mosasaur specimens fell between these two groups, with the lowest oxygen-18 levels found comparible with those of Birds and even the highest levels lower than those found in the cold-blooded Fish and Turtles, suggesting that the Mosasaurs had higher body temperatures than members of these groups.
Within the Mosasaurs the large Tylosaurus had on average the lowest levels of oxygen-18, suggesting the highest body temperature, while the medium-sized Platecarpus had the on average highest oxygen-18 levels, suggesting the lowest body temperatures, though there was considerable overlap between specimens of the different species.
δ18OPO4 data of Mooreville Chalk specimens relative to Fish (left) and Birds (right). The 0 position on the line graph indicates the mean δ18OPO4of all Mooreville Chalk Fish. Harrell et al. (2016).
Harrell et al. conclude that this does provide support for (if not actual proof of) the theory that Mosasaurs were endothermic. The lack of a direct connection between size and oxygen-18 levels would appear to rule out the alternative hypotheis, that the Mosasaurs were gigantothermic, not able to generate heat metabolically, but able to retain more heat due to a large volume compared to their surface area (larger objects, including animals, have larger volumes compared to their surface areas).
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