Tag Archives: Late Cretaceous

Fossil tells a new tail

Mosasaurs lived in the world’s oceans during the Late Cretaceous, the last Period from the Age of Dinosaurs (see the geologic time scale). They are close relatives of modern snakes and lizards, and during the Cretaceous they become fully aquatic sea monsters, growing to tremendous sizes, and were the top predators of their environments.

Their fossil remains occur in great numbers in the marine chalk deposits of the Central Plains, and numerous specimens have been preserved in museums all over the world (see posts on the chalk formation and on rock formations in general). Yet despite the great numbers of specimens collected, we still have much to learn about these great beasts.

For example, examination of their bones shows that they are elongate animals, with enlarged tails for propelling their bodies through the water. Their limbs are modified into flippers, useful in controlling the direction and orientation of their bodies in the fluid medium. So, it is clear that they are primarily tail-swimmers.

Early restorations based upon this evidence imagined a tail sort of like a modern crocodile, a thick tail that was slightly compressed laterally, making it taller than thick, but remaining relatively snake-like. Early restorations of the skeleton articulated the tail as a long chain of vertebrate, continuous from base to tip without any remarkable difference along the way.

Here is an illustration of the skeleton of the mosasaur Platecarpus from a classic work on mosasaurs (Williston 1898). Note the rod-like straightness of the back.

Mosasaur Platecarpus from Williston

Mosasaur Platecarpus from Williston

And here is an artist’s illustration of Tylosaurus, the largest of the mosasaurs, from Mike Everhart’s Book, Oceans of Kansas, showing the tail with a slight thickening near the end, but mostly being straight (Everhart 2005, recommended in the Boneblogger store).

Mosasaur Tylosaurus

Mosasaur Tylosaurus from Oceans in Kansas by Mike Everhart

However, frequently the skeletons of mosasaurs were found preserved in the rock with the last third of the tail bent downward, away from the main axis of the base of the tail. And this was not just found in a few skeletons, but it was found frequently enough that scientists speculated, at least in conversations with each other, that perhaps the down turned tip was not an artifact of preservation, but maybe meant something.

Well, a newly described mosasaurs fossil, which has exceptional preservation, provides the answer. This specimen collected in Kansas and now at the L.A. County Museum, preserves not only the bones, but also impressions of skin, impressions of internal organs, and even some of the body outline. The bones of the tail are clearly down-turned, giving the authors of this new study enough confidence to state what has been quietly talked about before—mosasaurs had a bi-lobed tail fluke (Lindgren et al. 2010).

Mosasaur Platecarpus

Mosasaur Platecarpus showing revised body outline

It only takes a single fossil to help overturn past notions about prehistoric life. The next big discoveries are out there, in the rocks and sitting in the museum drawers, waiting to be examined in detail. What will we find next?

Everhart, M. J. 2005. Oceans of Kansas: A Natural History of the Western Interior Sea. Indiana University Press, Bloomington.

Lindgren, J., M. W. Caldwell, T. Konishi, and L. M. Chiappe. 2010. Convergent evolution in aquatic tetrapods: insights from an exceptional fossil mosasaur. PLoS ONE 5(8):e11998.

Williston, S. W. 1898. Mosasaurs. University of Kansas Geological Survey 4(1):81-347.

Shark bites in the Cretaceous Sea

One of the most exciting things in paleontology is being able to definitively establish the interaction of two species from the fossil record. It is thrilling to picture a moment in time, millions of years ago, when two animals were at the same place, at the same time, and be able from fossil evidence to glean something about their interaction and behavior.

One dramatic example of this is finding a fossil with clear evidence that it was bitten by a shark. During the Late Cretaceous, North America was cut in half by an interior sea that extended the Gulf of Mexico across the mid-continent to connect with the Arctic Ocean in the north, effectively creating two land masses where today there is one.

In this last period from the Age of Dinosaurs, fantastic and strange creatures swam the seas. Today, the sediments from that ocean are exposed in badlands across much of western Kansas, Nebraska, and South Dakota. These geologic formations, like the Niobrara Formation, preserve a rich record of the ocean life, and clearly show what a scary ocean it was.

Tylosaurus model from the Carnegie Collection

Tylosaurus model from the Carnegie Collection

Giant marine lizards thrived in the sea. These beasts, close relatives of modern snakes and lizards, were called mosasaurs. There were several kinds that likely had different modes of life, some making use of resources close to the surface, and other species specializing in deep-water feeding, with the largest of them reaching 50 feet in length. They were joined by another group of marine reptiles called plesiosaurs. Plesiosaurs occur in two basic body plans, with the unimaginative names of long-necked and short-necked for obvious reasons.

Long-necked plesiosaur Styxosaurus

Long-necked plesiosaur Styxosaurus

The long-necked plesiosaurs have been described as looking like a turtle with a snake threaded through its shell. They had a stocky, turtle-like body, enormously long necks capped by a remarkably small head, and stumpy tails. They had four large flippers that helped to propel them through the water as well.

Short-necked plesiosaurs had large heads attached to short, thick necks. The long-necked forms most likely specialized in eating smaller fish with their small heads, maybe using their long necks to “snake” their way amongst their prey before being noticed. The short-necked forms obviously ate large prey, as evidenced by their massive heads and powerful jaws. (You can find models of both long and short-necked forms, as well as mosasaurs as part of the collection of dinosaur toys).

Living alongside these giants of the sea were animals that we would easily recognize, at least for their general body plan—these were the sharks. There was a significant amount of shark diversity in the Interior Sea as well, from relatively small forms that likely ate near the sea floor, to mid-sized forms that ate smaller fish and scavenged on dead carcasses, to several very large species that rivaled the modern great white shark in size and ferocity.

On occasion, when finding remains of fish or the marine reptiles, we find evidence of those remains having been bitten by sharks. The most compelling evidence is when teeth are found embedded in the fossil remains, but also punctures and tooth scratches can be a telltale sign.

Several plesiosaurs have been found as partial skeletons, with bites in several areas of their body, suggesting that after they died and settled to the ocean floor their carcass was scavenged by mid-sized sharks.

Cretoxyrhina bites the back of a mosasaur in the Late Cretaceous

Cretoxyrhina bites the back of a mosasaur in the Late Cretaceous. Painting by Dan Varner.

And in one dramatic example, the great white of the Kansas seas bite the back of a mosasaurs, cutting a section of vertebrae completely out of the giant lizard. The section of back, with its included vertebrae, was later spit out by the shark after having been mostly digested. The gristly remains settled to the ocean floor to lie there for millions of years before being found and placed in a museum.

Today we are fascinated by tales of shark attack, with the movie Jaws being a prime example. You can learn about these dangerous animals in another post, but perhaps it gives you some comfort to know that the denizens of the ancient seas also were subject to shark bites!

Additional information about this specimen can be found at Oceans of Kansas.

Bonnerichthys

In modern oceans, the very largest organisms specialize in filter feeding, or living on the very small plankton in the water. (Read more about the filter feeding niche). Up until now, it has appeared to researcher that during the Age of Dinosaurs, when the oceans were dominated by large, toothy reptiles, there were no marine animals specializing in the niche of large-bodied filter feeding, despite ample evidence that the oceans were rich in planktonic resources.

However, this niche was in fact filled during the Mesozoic as demonstrated in a recent paper in the journal Science (Friedman et al., 2010). Turns out that several species of fish did specialize in filter feeding, and they too grew quite large. Most of the specimens were already sitting in drawers in museums, having been misunderstood for many years, until Friedman and his colleagues re-evaluated them.

For example, one species has been known for over 100 years—having been named by E. D. Cope in 1873 as ‘Portheus’ gladius from a specimen collected from the Niobrara Chalk formation in western Kansas. The Niobrara Chalk was deposited during the Late Cretaceous period (see a geologic time scale). The species has a long and complex taxonomic history, mostly of interest to professionals, but it does clearly show that many scientists reviewed the fossil material and scratched their heads in wonder about this strange set of fossils.

Friedman and his colleagues have finally put the pieces together, and it fills in much about the history of life in the oceans. They have created a new genus in which to place the species, so now it is known as Bonnerichthys gladius. The genus was named for the Kansas fossil-collecting family that collected the most complete specimen found to date.

Bonnerichthys would have been about 20 to 25 feet in length with a huge, gaping mouth. You can see an artist’s reconstruction of Bonnerichthys at Oceans of Kansas. And you can listen to an interview with Matt Friedman at NPR.

This discovery opens up a whole new understanding of the paleoecology of the Mesozoic oceans, and shows that filter feeding was utilized for at least 100 million years longer as a major life strategy than previously recognized.

FRIEDMAN, M., K. SHIMADA, L. MARTIN, M. J. EVERHART, J. LISTON, A. MALTESE, AND M. TRIEBOLD. 2010. 100-million-year dynasty of giant planktivorous bony fishes in the Mesozoic seas. Science, 327:990-993.