Fins to Feet

Last of the Sea Dragons

A prodigious mass of bone, scales and flesh lies parked on a submarine atoll in a Cretaceous sea. Swirling shoals of scavenging sharks gorge upon it without ceremony. From nose to tail, the dead creature runs a length of 15 meters – and none of these sharks would have dared approach it in life. He was the king of his world – an apex predator plying a great inland sea way that once ran through the heart of North America. Scientists call him Tylosaurus poriger – member of a group of marine lizards called Mosasaurs that represent the final chapter in the incredible history of giant reptiles at sea.

What are mosasaurs, exactly?

One of the first things you ought to know about Mosasaurs is what they are not. That is, they are not Dinosaurs – nor are they particularly close relatives. They are squamates – and more closely related to modern day lizards (particularly monitor lizards) and snakes than to crocodilians, dinosaurs or birds (the so-called “archosaurs”).

Mosasaurs were a highly successful and diverse group of sea-going predators that lived during the final age of the dinosaurs, the Cretaceous period. Mosasaur fossils have been found on every continent – and they dominated the world’s oceans for a space of 27 million years.

They appeared on the scene after the demise of one group of large marine reptiles, the Ichthyosaurs, and a drastic reduction in the diversity of another, the Plesiosaurs. Sea levels were higher during the Cretaceous period than at any other time in the Phanerozoic eon (“the age of multicellular life”) and vast competition-free spaces lay open for the Mosasaurs to radiate into. They ranged from 3 meters to 15 meters in length. While they did not quite attain the awe-inspiring dimensions of the largest modern baleen whales, the biggest Mosasaur is somewhat comparable in size to the Sperm Whale, the largest extant toothed whale. Modern toothed whales can, in many ways, be seen as the ecological analogues of these reptillian sea-beasts.

Reptiles play a relatively minor role in modern marine ecosystems, but in the Mesozoic they filled an impressive suite of predatory roles – from bivalve-munching placodonts to the large game hunting mosasaurs. The idea of oceans ruled by gigantic sea monsters excited the Victorian scientists and fossil hunters who first unearthed their remains. “In the mosasaurids”, wrote celebrated Paleontologist Edward Cope in 1869, “we almost realize the fictions of snake-like dragons and sea serpents, which men have have been ever prone to indulge”.

Whence cometh the mosasaurs?

It is clear that, like the Whales of today, Mosasaurs trace their evolutionary origins to a terrestrial ancestor. It is generally supposed that they are descended from a family of semi-aquatic lizards called Aigialosaurs – but gaps in the fossil record make drawing up a precise account of the transition from land-based lizard to aquatic reptile a little problematic. The occurence of Algiasaurs overlaps with the appearance of the earliest mosasaurs. Aigialosaurs display an anatomy that is “intermediate” between that of modern monitor lizards (the closest living relatives of mosasaurs) and primitive mosasaurs.  What changes did the shift to life at sea involve?

We observe a dramatic increase in the length of vertebral column and a reduction in the relative size of the limbs in the varanid-aigialosaur-mosasaur transition. These changes may represent an increasing commitment to an aquatic mode of life.

We also see a pronounced change in the structure of the tail – from the varanid (monitor lizards) condition where the tail shows little segmentation and all the tail vertebrae are morphologically uniform to the mosasaurine condition, where the tail can be segregated into separate functional units (each unit has a complement of vertebrae that are anatomically distinct from vertebrae in other units): a tail-base that provides the force for a propulsive stroke, an intermediate portion that sways and is displaced during the stroke, a “hinge” section that joins the main body of the tail to the fin and a set of downturned terminal vertebrae that supports a tail fin. This segmentation is diagrammed below.

The ancestors of mosasaurs would have propelled themselves through the water by laterally undulating their entire bodies – as eels and sea-snakes do. In Mosasaurs, however, the front third of the body is stiffened while the rest of the body is flexible – it is the latter portion that undulates when the animal swims.

As far as the limbs go, Aigialosaurs are virtually indistinguishable from monitor lizards. As mosasaur evolution progressed, the five-fingered terrestrial lizard limb-plan had to be traded in for the paddle-like arms seen in later forms. This was likely achieved by changes in embryonic development. The development of the skeletal elements of the limb involves the formation of cartilage from dense connective tissue and the subsequent replacement of cartilage by bone (except at the joints). Genetic changes in the timing of the steps in this process or in the patterning of bone formation can result in major changes to the number and the morphology of bones in the limb – later forms show incomplete ossification (bone formation) and an increase in the number of finger elements in the limb. The evolution of webbed or paddle-like feet is related to the incomplete separation of the digits to form fingers during development (something that can be achieved in a laboratory via mutations to certain genes).

Primitive mosasaurs have five-fingered limbs that are not dissimilar to those of Aigialosaurs or monitor lizards. The close resemblance between the fore-limbs of certain later mosasaur species and the fore-limbs of whales is remarkable and is a splendid example of two very distant animal groups answering an environmental challenge with nearly identical anatomical solutions.

How did Mosasaurs move? Could they come ashore?

The transfer of locomotory function from the limbs to the tail seems integral to understanding how early Mosasaurs took to the open oceans. As noted earlier, it is the undulation of the tail that drives Mosasaur motion – this form of motion is also seen in Trout and is known as carangiform swimming. Mosasaur tails are deep and bear large tail fins, providing a large surface area to displace water and generate a propulsive thrust. The base of the tail is stiffened and well-muscled to help maximize the force generated.

The elongated bodies of mosasaurs are not optimized for reducing friction drag and they were probably not pursuit predators. It is more likely that they were ambush hunters – capable of quick targeted bursts of speed.

It is clear that Mosasaurs – so well adapted for life in the sea – could not haul themselves ashore like seals or walruses. The anatomy of the limbs and trunk simply does not permit it. This poses a problem: where, exactly, does a mama mosasaur deposit her eggs? Reptile eggs are not designed to survive or hatch underwater. Modern female sea-turtles solve the problem by clambering ashore to nest. Leatherback Turtle hatchlings are born in the beach sands and make their way towards the sea en masse (a rather dramatic natural event). But this could not have been the case with mosasaur hatchlings. The issue of mosasaur birth befuddled Paleontologists for several decades, until fossilized prenatal embryos were discovered amid the remains of the mosasaur Plioplatecarpus. 4 embryos were also discovered in the posterior trunk of an adult aigialosaur. It is now apparent that Mosasaurs gave birth to live young – a trait observed in a number of extant lizards and in other large marine reptiles (like Plesiosaurs and Ichthyosaurs). The orientation of the embryos in the aigialosaur specimen suggests that the tail came out first and the nostrils last – this minimizes the possibility of drowning.

What did Mosasaurs eat?

Probably just about anything that moved in the water. Mosasaur jaws bear a row of conical, pointed teeth (the complexity of which varies from species to species) designed to tear into large fleshy quarry. Another set of teeth called Pterygoidal teeth – also observed in modern snakes – rises out of the animal’s hard palate (the roof of the mouth) and serves to hold struggling prey in place. Mosasaurs swallowed their food whole without masticating it, so the identity of ingested prey can sometimes be determined from the the stomach contents of fossilized mosasaurs. This gives us a tantalizing window into their feeding habits. We know, for example, that Tylosaurus poriger fed on bony fishes, sharks, birds and even smaller Mosasaurs!

Some mosasaurs had bony “rams” on their snouts that projected out beyond the teeth that they could use to batter and stun prey.

Certain Mosasaur species had more rounded teeth – well-suited for crushing – and it is thought that they fed on hard-shelled animals like ammonites, which were ubiquitous in the Cretaceous seas.

Could Mosasaurs dive deep?

Deep diving mammals typically have bones of lower density than those that inhabit shallower waters. Animals with dense bones achieve neutral bouyancy (that is, they neither sink nor rise) at shallow depths, while animals with more porous and lighter bones (as well as compressible lungs) have access to wider range of depths. Thus, bone density can be used to determine the depths at which different Mosasaur species might have swam and hunted. Tylosaurus, for example, had a low bone density and was likely a deep diver.

Human divers ascending too quickly from high-pressure depths in the ocean are susceptible to a disease commonly known as the “Bends”. The rapid depressurization leads to inert gases dissolved in the blood, like nitrogen, coming out of solution as bubbles. These bubbles could potentially block blood vessels that supply the bones, leading to cell death or “necrosis”. This sort of permanent bone injury has been observed in many Mosasaurs – and is a telltale sign of the bends or, if we are to be medically proper, decompression syndrome. This may imply that these Mosasaurs were members of a deep-water species and these decompression events occured when they ascended to shallow waters too quickly.

Scientists believe that, like other marine reptiles, Mosasaurs were at least partially endothermic (endothermy is the ability to generate body heat) – deep-divers like Tylosaurus would probably have to have been to deal with the cold depths of the continental seas they inhabited.

What became of the Mosasaurs?

The end of the reign of the Mosasaurs coincided with the demise of the Dinosaurs on land in the aftermath of KT event, 65 million years ago. The asteroid impact that put an end to the age of reptiles and sparked a worldwide nuclear winter, would have severely affected the productivity of phytoplankton in the world’s oceans. Such a productivity decline would result in the extermination of a very large number of animal groups, particularly those in the upper ranks of the food chain. It is unclear, however, why the extinction event was so selective: why did the Mosasaurs and non-avian dinosaurs perish, but crocodiles, sea-turtles and birds persist? Perhaps we will explore the issue in a future post.

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