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Fossil Friday: The Explosive Origin of Mosasaurs in the Cretaceous

Photo: Plesioplatecarpus planifroms, MCDinosaurhunter via Wikimedia, CC BY-SA 3.0.

This fossil Friday features the mosasaur Plesioplatecarpus planifroms from the Late Cretaceous of North America. Mosasaurs were large carnivorous marine reptiles with a very elongate body, which resembled the mythical sea serpents of ancient sailor legends. Mosasaurs appeared in the Late Cretaceous about 90 million years ago and quickly developed a large diversity of some 42 known genera, filling different ecological niches. The largest species reached enormous lengths of more than 15 meters.

Even though there is an extinct group of shore-dwelling semiaquatic lizards called Aigialosauridae, considered the walking relatives of mosasaurs, they are of about equal Late Cretacaous age. There are no putative ancestors of mosasaurs known prior to the Late Cretaceous. This leaves only a very short window of time for the marine adaptations of mosasaurs to evolve, which creates yet another of the notorious waiting-time problems for the required genetic changes to originate and spread in an ancestral population.

Substantial Genetic Changes Imply New Coding

These genetic changes must have been substantial as it was not enough to just grow in length and reduce the limbs to flippers to make a mosasaur. They also developed biological novelties such as shark-like flukes, which implies a different mode of swimming from that of lizards that just undulate their tail. But there is more: well-preserved fossils show that mosasaurs shared a unique aquatic adaptation with modern whales, that is absent in terrestrial vertebrates (e.g., the possibly related monitor lizards) and thus also absent in the assumed mosasaur ancestors: their paired bronchi leading to the lungs run parallel to each other instead of splitting from a common tracheal tube (Lindgren et al. 2010).

Such substantial changes of the internal organs and physiology cannot be explained with simple mutations but required significant new genetic coding. The math of population genetics precludes a Darwinian origin of these new genes in such a short time. But evolutionary biologists generally don‘t bother about the mathematical feasibility of their fancy just-so stories.