A New Whale with Crazy Teeth

This week, a [paper] came out that described a new fossil whale from South Carolina (USA). The fossil is from the Oligocene (~ 30 million years ago) and it has a basically complete skull, some vertebrae, and a few ribs.

A reconstruction of Coronodon by A. Gennari.

The authors named the fossil Coronodon havensteini, meaning ‘crown tooth.’ It was found by Mark Havenstein, so the specific epithet (the second part of the name) is in his honor. The teeth of this fossil are particularly interesting.

Figure 2 from the paper showing the teeth of Coronodon.

Instead of simple, conical teeth (like in dolphins), or baleen (like in the blue whale), Coronodon has teeth with many bumps, giving each tooth the appearance of a little crown. Food and other particles left little scrape marks on the teeth, which indicate the direction of water flow through the mouth, and how the teeth were used during feeding. When the mouth was closed, the upper teeth sat on the outside of the lower teeth, providing just enough space for water to escape through the teeth, leaving delicious food bits inside the mouth.

Figure 2f from the paper showing how water would have flowed between the teeth.

Coronodon was using its crazy teeth to filter feed! Why is this important? Because understanding how filter feeding began in whales is an ongoing question. Coronodon is one of the earliest relatives of the mysticetes (baleen whales), but it has no baleen itself. It used its teeth the same way mysticetes use their baleen. Later on in mysticete evolution, baleen began to develop and finally took over as the dominant feeding structure. Coronodon represents the first step in that process. Another idea is that whales went through a toothless, suction-feeding phase before filter-feeding with baleen came about, but Coronodon shows that suction-feeding wasn’t part of the evolution of filter-feeding.

We can learn a lot about how extinct animals ate their food through looking at the shape and tiny scrape marks on the teeth (called microwear), and Coronodon is new amazing example of that!

The Jurassic King of Scotland

Last week, a [paper] was published that described a new fossil of a previously known mammaliaform from Scotland. This animal is called Wareolestes rex, meaning Ware’s brigand King and it’s from the middle Jurassic Period (specifically the Bathonian, 168 to 166 million years ago). Mammaliaforms were starting to diversify in the middle Jurassic, and were relatively small, so finding any fossils of them is important to our understanding of how they lived.

A reconstruction of Wareolestes by E. Panciroli.

The new specimen of Wareolestes is a lower jaw with teeth. This fossil was CT scanned and the details of the teeth were recreated with computer software. The authors found that the jaw had two molars preserved in place, and several teeth that were hiding in the bone, waiting to erupt.

Figure 4 from the paper showing the CT scan of the jaw. The large molars are already erupted, but several premolars and another molar are still in the jaw. The purple is the mandibular nerve. The scale is 1mm.

The original fossil of Wareolestes is an isolated molar, but similarities with the molars preserved in this new specimen show that the two specimens come from the same species. The shape of the teeth indicate that Wareolestes is a morgonucodontan (an early mammal relative).

The un-erupted teeth show that Wareolestes replaced their teeth once in life. Most mammals have a set of baby teeth (also called ‘Milk’ teeth) and a set of adult teeth. This happens for two main reasons: 1) our mouths grow over time, but teeth cannot grow once they are formed, so we grow adult teeth to fit adult-sized mouths. And 2) our teeth fit precisely together so that we can chew our food really well (this is called precise occlusion), by only replacing our teeth once, we ensure that our teeth will fit together appropriately.

A photo showing how the different cusps of the premolars (labeled P4, for premolar 4) and the molars (labeled M1 and M2 for molar 1 and 2) fit together. From P.D. Polly.

By comparison, other animals (like crocodiles, for example), do not have a final adult size – they grow continuously as they age. They also replace their teeth as many times as they need to ensure they always have teeth for grabbing prey, but they don’t chew like mammals do, so their teeth don’t have to fit together.

This new specimen shows that morganucodontans had a similar tooth replacement pattern as modern mammals, and are the most basal mammaliaforms that have it.

Dinosaur Teeth and Eggs

At the start of 2017, an [article] was published that reported on a new discovery about dinosaur teeth. The authors looked at the teeth of two species of dinosaur – Protoceratops andrewsi and Hypacrosaurus stebingeri.

proto and hypa

Protoceratops on the left by Z. Chuang and Hypacrosaurus on the right by V. Kontantinov.

But not just any teeth. The authors examined the teeth of embryos (an embryo is a baby still inside its egg). Teeth grow day by day, and as they do, they leave lines inside the tooth – much like tree rings inside a tree. By counting these lines (called Von Ebner’s growth lines), we can see how old an animal is.

Fig 1

Figure 1 from the paper showing the Von Ebner lines in Hypacrosaurus in A and in Protoceratops in B, and a CT image of the jaw with a functional tooth in C.

But the authors did more than that. We know that embryos do not begin to form teeth immediately, and that sometimes ‘practice’ teeth can form and fall out before the final teeth (called functional teeth) are made. In fact, in crocodiles, functional teeth start to grow about 42% through their incubation period, after the jaws have formed. Using this information, and by counting the growth lines in the teeth, the authors estimated that Protoceratops babies incubate for a minimum of 83 days. The same math gives a calculation of 171 days for Hypacrosaurus.

ILLUSTRATION-PROTOCERATOPS WITH EGGS

Are you ready yet?? (Protoceratops with eggs by M. Kelly).

Today’s birds incubate their eggs for a maximum of 39-83 days (based on the size of the egg and other factors). This means that these dinosaurs incubated their eggs for over TWICE the time that birds today do. In the case of Hypacrosaurus, it’s almost HALF OF THE YEAR.

The authors point out that because of these long incubation times, adults and babies were more at risk of environmental changes and predators and could have contributed to their extinction.

Beaks, Seeds, and Extinction

Now that the summer is here, many paleontologists are out in the field and the time between paleontological articles gets bigger. This week, we’re going to talk about an [article] that was published back in May 2016. The authors wanted to see if the diet of theropod dinosaurs changed over the last 18 million years of the Cretaceous.

To do this, they examined the shape of the teeth of over 3100 theropod teeth from four groups – dromaeosaurs (the raptors), troodontids (smaller carnivores related to dromaeosaurs), Richardoestesia (a medium sized theropod from North America), and the toothed members of Aves (birds and their closest relatives). They analyzed how the shapes of the teeth changed over the last part of the Cretaceous to figure out why the birds made it through the extinction, but the small carnivores did not.

The shape of the tooth and the number and shape of the denticles (little ridges) on the tooth can tell you what the animal was eating. Now, modern birds have beaks of all different shapes and no teeth so at some point in their history, a beak became a more useful food-getting tool than teeth.

troodon tooth Science Mus of minnesota

A troodontid tooth (from the Science Museum of Minnesota).

The authors found that tooth shape remained stable (did not change) in these groups during those last 18 million years. The stability they found means that the environment was not changing or worsening during that time. The authors found that instead of dying off little by little, these groups of theropods went extinct all at once. But why?

When the meteorite hit at the end of the Cretaceous, it had many world-wide consequences. One of those consequences may have been wildfires that destroyed much of the habitat and food for many dinosaur species. The animals that used teeth to catch food had less food to catch. The animals that had beaks were able to use a different resource, completely unused by toothed-dinosaurs: seeds.

seeds

A variety of seeds (by K.L.Moses).

Seeds are amazing because, like an egg, they contain all of the nutrients a baby plant needs to grow until it can eat (or gather sunshine) on its own. Seeds are resistant to fire, can be scattered on the ground for years and years, and they are packed with nutrients. The birds, with their beaks, could easily crack the seeds open and eat them, whereas the toothed dinosaurs could not, and so the birds didn’t go extinct at the end of the Cretaceous.

Bird-Behavior-Blue-Jay-Marie-Read

A blue jay holding a seed. (From here).

We can even see this today when fires destroy forests. The birds that eat seeds are the first to return! They eat the exposed seeds off the ground, and help to disperse them, creating new forest wherever they go.

The Snail-Crunching Australian Marsupial

In May (2016), a [study] was published describing a new marsupial from Australia with an interesting set of teeth. Let’s dive right in!

Firstly, what is a marsupial? A marsupial is a type of mammal that has a pouch. When a marsupial baby is born, it crawls into the mom’s pouch to continue growing before it’s ready to live in the world.

kangaroo pouch

Kangaroos have pouches.

When most mammals are born, they are instead born straight into the world, and their parents take care of them until they are ready to live on their own. This type of mammal is called placental, and humans belong to this group. A few mammals still lay eggs, but that’s a story for another day.

Marsupials live in the Americas and Australia and come in a variety of shapes and sizes. Interestingly, marsupial and placental mammals seem to be copying each other in living environments, diets, and lifestyles. There are burrowers, fast-movers, tree-dwellers, large herbivores, and carnivores in both marsupials and placentals. There were even marsupial lions and wolves (see video at the bottom)! This is an example of convergent evolution, where animals get the same characteristics independently of each other.

marsupial vs placental

Similar ‘niches’ or lifestyles in placental and marsupial mammals.

Even more interestingly, there are no aquatic or flying marsupials (so nothing like a marsupial whale or bat) because having a pouch makes those environments difficult to live in.

Now that we know what a marsupial is (and how cool they are!) we can talk about the new one. The fossil, Malleodectes mirabilis (meaning “hammer-tooth”), was found in the Northwest of Queensland, Australia, in rocks of Miocene age (around 14.6 million years ago). It is a left maxilla (the bone that holds teeth in the upper jaw) and it preserves several teeth, including one that had not yet come up.

Fig 4

Figure 4 from the paper. A and B showing the new specimen and it’s unique tooth. C, D, E, and F showing the other Malleodectes specimens and teeth.

Teeth are really important in mammal paleontology because each species has its own unique set of bumps so you can identify species from only finding individual teeth. This fossil has a tooth still in the jaw that is cone-shaped, with a wide base and a small point. The same type of tooth found in other animals that eat snails. The shapes of the other teeth tell us that this animal was able to eat other types of food as well. This combination of tooth types tells us that Malleodectes was the only mammal able to take advantage of this variety of diet in Australia’s Miocene rainforests and makes it completely unique.

Fig 6

Malleodectes by P. Schouten.

Video of a Thylacine, the Tasmanian Wolf, also known as the Tasmanian Tiger.