Possible evidence of endothermy in Mosasaurs.

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. 

δ¹⁸O_PO4 data of Mooreville Chalk specimens relative to Fish (left) and Birds (right). The 0 position on the line graph indicates the mean δ¹⁸O_PO4of 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).

See also...

Phosphorosaurus ponpetelegans: A new species of Halisaurine Mosasaur from the Late Cretaceous of Hokkaido.               Mososaurs were a distinctive group of large...

A Freshwater Mosasauroid from the late Cretaceous of Hungary.                       Mosasauroids were large marine Lizards that became dominant marine predators in the Late Cretaceous after the disappearance...

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A Freshwater Mosasauroid from the late Cretaceous of Hungary.

Mosasauroids were large marine Lizards that became dominant marine predators in the Late Cretaceous after the disappearance of the Ichthyosaurs. They were related to Monitor Lizards, but fully adapted to an aquatic environment, with flipperlike limbs and elongate, streamlined bodies, probably coming ashore only to lay eggs like modern marine Turtles. The smallest members of the group reached about three meters in length, the largest were over seventeen meters long.

In a paper published in the journal PLoS One on 19 December 2012, László Makádi of the Department of Paleontology and Geology at the Hungarian Natural History Museum, Michael Caldwell of the Department of Biological Sciences at the University of Alberta and Attila Ősi of the MTA-ELTE Lendület Dinosaur Research Group at the Department of Physical and Applied Geology at Eötvös University describe a new species of Mosasauroid from the Late Cretaceous Csehbánya Formation of western Hungary.

The new species is named Pannoniasaurus inexpectatus, meaning the unexpected Lizard from Pannonia; Pannonia being the Roman name for the region where the fossils were found. The species is described from a large number of disarticulated bones from a flood breccia (conglomerate deposit formed by a flash flood) exposed at the Iharkút open-pit bauxite mine in the Bakony Hills of Western Hungary since 2000. Such disarticulated remains are difficult to work with, but in this case are so numerous as to allow a reasonable reconstruction of the living animal.

Skeletal anatomy of Pannoniasaurus inexpectatus. (A) Skull drawing shows known elements in grey. (B) Skeletal reconstruction showing preserved bones in white. Scale bar 1 m. (C) Premaxilla in dorsal view. Scale bar 1 cm. (D) Right maxilla in lingual view. Scale bar 1 cm. (E) Left postorbitofrontal in dorsal view. Scale bar 1 cm. (F) Right quadrate in lateral view. Scale bar 1 cm. (G) Left dentary in lingual view. Scale bar 1 cm. (H) Isolated teeth without and with base preserved. Scale bar 1 cm. (I) Left splenial in lingual view. Scale bar 1 cm. (J) Right coronoid in lingual view. Scale bar 1 cm. (K) Left angular in labial view. Scale bar 1 cm. (L) Right surangular in labial view. Scale bar 1 cm. (M) Left articular in dorsal view. Scale bar 1 cm. (N) Mid-cervical vertebra in lateral view. Scale bar 1 cm. (O) Dorsal vertebra in dorsal view. Scale bar 1 cm. (P) First sacral vertebra in dorsal view. Scale bar 1 cm. (Q) Second sacral vertebra in dorsal view. Scale bar 1 cm. (R) Anterior caudal vertebra in lateral view. Scale bar 1 cm. (S) Rib fragment in dorsolateral view. Scale bar 1 cm. (T) Proximal end of left humerus in flexor view. Scale bar 1 cm. (U) Distal end of right humerus in flexor view. Scale bar 1 cm. (V) Left ilium in lateral view. Scale bar 1 cm. Makádi et al. (2012)

Pannoniasaurus inexpectatus appears to have been a Crocodile-like animal reaching up to seven meters in length, though most of the remains appear to come from animals of between three and four meters. It has a flattened skull, suggesting it lived as an ambush predator operating in shallow water, similar to modern Crocodiles. 

This is the first known Mosasauroid that appears to have lived entirely in freshwater (some previous specimens have been found from estuarine deposits). As wells as the sedimentological evidence that this is a fluvial flood deposit Makádi et al. carried out a number of geochemical tests, all of which point to a freshwater, rather than a marine, origin for the deposits where the specimens where found.

Apart from being disarticulated the fossils show little sign of wear and tear; this implies that the specimens were not moved far in the flood, but that they were from animals that had pre-deceased the flood event (allowing them to become disarticulated), but not attacked by scavengers (which leave distinctive marks on bones. This suggests that the animals were either quickly buried or submerged in anoxic waters, then exhumed and redeposited by the flood event.

See also A Venomous Lizard from the Late Cretaceous of Mongolia, A giant Monitor Lizard from the Miocene of Samos, Greece and Helodermatid Lizard from the Late Miocene-Early Pliocene of Tennessee.

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