fossils

Giant marine reptiles gave live birth in open ocean

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Nick
Garland

Founder and Editor
For more than 100 years dust collected on fossils in Yale’s Peabody Museum marked as unidentified bird specimens. It wasn’t until recently that paleontologists discovered these were no bird fossils at all. They were baby marine reptiles called mosasaurs whose details hinted at an unexpected behavior.

Mosasaurs were marine reptiles related to today’s monitor lizards such as the Komodo dragon, the largest lizard in the world. They were the dominant marine predators of the Late Cretaceous, some growing to a length in excess of 50 feet.

Their fossils are somewhat plentiful around the world and the group as a whole – including the genera Mosasaurus, Tylosaurus and Platecarpus – is well-studied. But an abundance of fossil discoveries were missing one crucial ingredient in understanding the life and death of these large ocean-going reptiles. Where were their babies?

Daniel Field, a doctoral candidate in the lab of Jacques Gauthier in Yale’s Department of Geology and Geophysics, “rediscovered” some fossils hiding in plain sight in the Peabody Museum of Natural History. Misidentified as marine birds, Field updated their identity as baby mosasaurs, the youngest mosasaurs ever found.

With Aaron LeBlanc, a doctoral candidate at the University of Toronto at Mississauga, Field demonstrated the jaw and teeth features present in the fossils are only found in mosasaurs. The fossils were also deposited in the deep ocean.

When you’re dealing with reptiles it’s generally a safe bet to assume the adults lay eggs in what is described as “oviparous” birth. But some modern reptiles, notably some lizards and snakes, exhibit viviparous birth, or birth completely without an egg. Still others exhibit ovoviviparous birth where there is still an egg but the young remain inside the mother’s body until they hatch.






So which form of birth did mosasaurs employ? They could have come ashore to lay eggs but no egg-laying sites had ever been discovered. Because these baby mosasaur fossils were found in the open ocean where egg-laying is not possible, the circumstances suggest that mosasaurs may have been viviparous. Along with pregnancy discoveries in other marine reptile groups like plesiosaurs, this also suggests that viviparity evolved in reptiles earlier than expected, or even independently multiple times.


Unfortunately for us, and for mosasaurs, they went extinct some 66 millions years ago in the mass extinction event that also killed the dinosaurs.


Reference:

Field, D. J., LeBlanc, A., Gau, A., Behlke, A. D. (2015), Pelagic neonatal fossils support viviparity and precocial life history of Cretaceous mosasaurs. Palaeontology, 58: 401–407. doi: 10.1111/pala.12165


Image Credit: Julius T. Csotonyi

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Nick
Garland

Founder and Editor


http://sulc.us/6m5ak
http://www.eartharchives.org/articles/a-new-beginning-for-baby-mosasaurs/