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Platecarpus by megabass22
Platecarpus, a typical "first-wave" mosasaur genus that went extinct in a poorly understood middle Campanian mosasaur extinction event.

Mosasaurs were pretty cool marine reptiles. Essentially being giant marine monitor lizards, they dominated seas worldwide during the last 25 million years of the Cretaceous and reached a considerably amount of diversity with the short timespan in which they lived in mind. This journal entry is not to serve as a complete chronicle of their evolutionary history but rather to highlight some interesting points within it and give a brief overview. Here is an image showing the stages of the Late Cretaceous in case you are not familiar with them, since they are referred to quite a lot in this summary.

So how come mosasaur ancestors could go from small amphibious lizards to giant marine apex predators in just 25 million years? Well, similar to the evolution of whales, there were plenty of empty niches due to a series of anoxic events causing a quite severe marine extinction event. The ichthyosaurs were completely gone and the plesiosaurs heavily reduced which certainly provided an opportunity for new creatures to arise to fill their place. Since several groups of squamates (among them the dolichosaurids and aigialosaurids) had already diversified into semiaquatic forms, they had a bigger opportunity than say, mammals, to fill those niches. It turns out that the increased sea levels of the Cretaceous, combined with the creation of new marine habitats in the newly opened southern Atlantic (due to the separation of Africa and South America) created regions of high productivity which also helped fill the seas with life once more. In fact, the situation was quite similar to how it was for the early whales, and there are further parallels that can be drawn between the evolutionary history of both groups.

The earliest true mosasaurs (members of the mosasauridae) were plesiopedal. "Plesiopedal" refers to their "conservative adaptations" in that they were generally small in size and not very well adapted to marine life. They also retained elongated propodial elements (humerus, ulna, radius) and many also seem to have retained fully-fledged limbs and having been capable of walking on land. Some examples of such plesiopedal species are Dallasaurus, Yaguarasaurus and Tethysaurus. Now, interestingly enough, the hydropedal mosasaurs (mosasaurs well adapted to marine life with proper flippers and more streamlined bodies) do not share a common ancestor which is not plesiopedal. Hydropedality evolved at least twice (mosasaurinae, rusellosaurina) or perhaps even thrice (halisaurinae, mosasaurinae, rusellosaurina) depending on where you put the halisaurines. This is also interesting because it goes to show how much convergent evolution happened within the mosasauridae. The largely similar Mosasaurus and Tylosaurus were as far apart as you could get on the mosasaur family tree and had several more different close relatives.

Mosasaur evolution can be simplified to "waves" where adaptations appeared and spread. The first such "wave" occured in the Coniacian and Santonian when the earliest hydropedal mosasaurs appeared and the major subfamilies (mosasaurinae, plioplatecarpinae, tylosaurinae and halisaurinae) were established. The new hydropedal mosasaurs also seem to have developed endothermy (the ability to control body temperature using bodily functions and not depending on outside temperature) to a degree and had increased in size. Typical first-wave genera, like Tylosaurus, Clidastes and Platecarpus were larger than the previous plesiopedal mosasaurs but also smaller than later species (Coniacian Tylosaurus, while the biggest mosasaur at the time, were just 7-8 meters in length compared to later 12+ meter species of the same genus, similar size increases are seen in other genera as well).

Rapid size increases and diversification in the Campanian marks the beginning of the "second wave" of mosasaurs. New genera such as Hainosaurus and Plioplatecarpus reached great success, while the mosasaurinae experienced a massive diversification (with the appearance of highly specialised genera like Globidens). The "second wave" seemingly ended in an extinction event during the middle Campanian in which previously widespread genera went extinct (such as Eonatator and Platecarpus). Halisaurines and Plioplatecarpines appear to have been hit harder than other groups.

The mosasaurs recovered through their third and last "wave". The mosasaurines diversified into several forms unlike anything seen before, with ichthyosaur-like forms (Plotosaurus), pliosaur-like forms (Plesiotylosaurus) and crocodile-like forms (Goronyosaurus, probably a mosasaurine). Several new species of Globidens also appeared before the end of the durophagous (shellfish-eating) forms in the middle Maastrichtian due to the extinction of their primary food source, the inoceramids. In fact, it appears that the mosasaurines were beginning to outcompete the other groups of mosasaurs, with the tylosaurines only being found in limited number in polar environments and the mosasaurines constituting a majority of all mosasaur taxa during the Maastrichtian. The "third wave" of mosasaur evolution and diversification had just begun when they were wiped out in the K-Pg extinction along with several other prominent Mesozoic groups.

Main sources
  • Everhart, M. J., 2002. Rapid ontogenetic change in Late Cretaceous mosasaurs (Reptilia; Mosasauridae) as a model of vertebrate morphogenesis. International Conference on Morphogenesis and Pattern Formation in Biological Systems, Chubu University,  Nagoya, Japan, p. 86.…
  • Everhart, M. J. 2005. Rapid evolution, diversification and distribution of mosasaurs (Reptilia; Squamata) prior to the K-T Boundary. Tate 2005 11th Annual Symposium in Paleontology and Geology, Casper, WY, p. 16-27…
  • Polcyn, J. M., Jacobs, L.L., Araújo, R., Schulp, A.S., Mateus, O., 2014, Physical drivers in Mosasaur Evolution, Palaeogeography, Palaeoclimatology, Palaeoecology 400: 17-27…
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Stuff-by-Tyto Featured By Owner Mar 25, 2017  Hobbyist General Artist
Fascinating! I must admit that Mosasaurs have never particularly interested me, but you may just have changed my mind on that. The comparison to whale evolution is very interesting, and I can see the parallels. The morphology of the two groups is actually comparatively similar in regards to the skull and dentition, and the overall build in the case of Archaeocetes. You've got to wonder what directions they might have gone in if the K-Pg extinction hadn't happened...
megabass22 Featured By Owner Edited Mar 25, 2017  Hobbyist Traditional Artist
Yup! I find the speed in which they gained control of the oceans and diversified to be interesting, as well as the variety of forms and lifestyles they actually had. Yeah, whales seem to have evolved very convergently to mosasaurs. Without the K-Pg extinction I would expect to see mosasaurs in a lot of niches you wouldn't really expect them in and a variety of very different forms. The fact that they reached the diversity they eventually had in just 25 million years would suggest a bright future and golden age, had they survived. 
paleosir Featured By Owner Feb 18, 2017  Hobbyist Traditional Artist
Extremely interesting!
I learned quite some things here
megabass22 Featured By Owner Edited Feb 19, 2017  Hobbyist Traditional Artist
Great! Anything in particular you found interesting?
paleosir Featured By Owner Feb 19, 2017  Hobbyist Traditional Artist
The fact that they became hydropedal multiple times indepentenly.
I never expected that.
megabass22 Featured By Owner Feb 20, 2017  Hobbyist Traditional Artist
Yeah, that was pretty unexpected. There was a lot of convergent evolution happening within the mosasauridae with hydropedality happening more than once, similar species arising in different lineages, mosasaurs beginning to resembe other marine reptiles and other stuff.
Of course, whales then went on to evolve on a path very similar to mosasaurs.
paleosir Featured By Owner Feb 20, 2017  Hobbyist Traditional Artist
acepredator Featured By Owner Feb 13, 2017
The Cenomanian-Turonian event also finished off a bunch of other critters.
megabass22 Featured By Owner Feb 13, 2017  Hobbyist Traditional Artist
Yup. I did a status post about it:…
It is a surprisingly overlooked event.
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Submitted on
February 13


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