Hippos are (almost) definitely whales, not pigs

Hippos are strange mammals. They lack hairs and sweat glands, and have an unusually thick skin. The only other mammals that share these features with hippos are whales, but they look nothing alike, except they’re also huge and live in water. Coincidence?

Traditionally hippos were included in the Suidae (pigs) branch of the mammalian evolutionary tree, but molecular data unambiguously shows that they're closely related to cetaceans (whales, dolphins and porpoises). This not only sounds unlikely (hippos look much more like pigs than whales), but it's also quite difficult to test—there is simply not enough fossil evidence. So the origin of hippos has remained something of a mystery. Now, a new fossil discovery by a team of French and Kenyan palaeontologists may have tipped the balance of the hippo evolutionary history.

Common hippo showing off its mandibles.

Fossils of hippo are rare. Every now and then a tooth pops up, but bones are nearly impossible to find. “To make a comparison between whales and hippos we need to find their ancestors. We had the whale ancestor but until now the hippo ancestor was unknown,” says Fabrice Lihoreau, a palaeontologist at the University of Montpellier, in France.

In 2005, Lihoreau and colleagues discovered a mandible with teeth of unusual morphology in the paleontological collection of the National Museum of Kenya, in Nairobi. Lihoreau is an expert on anthracotheres, a diverse group of semi-aquatic herbivorous mammals that lived in Africa from around two to 40 million years ago. For some time palaeontologists had suspected that anthracotheres could be the ancestor of hippos. “We published many studies suggesting hippo is related to anthracotheres, and not to pigs. This new discovery not only supports that, but it tells us precisely to which lineage of anthracotheres hippos originated from,” Lihoreau explains.

The newly found teeth have morphological features of both anthracotheres and hippos. They belonged to a large herbivorous mammal that thrived in Lokona, Kenya, around 28 million years ago. The discovery of this new hippo-like anthracothere, named Epirigenys lokonensis for ‘hippo’ (Epiri) and ‘origin’ (genys), shows that hippos are definitely not pigs—they originated from an old lineage of antrachotheres, the bothriodontines. And not only that, Lihoreau says, “we added a bit more to the history of mammals in saying that hippos are African, they were born in Africa.”

Evolutionary transition of the upper molar from an anthracothere (left),
Epirigenys (middle) and a primitive hippo (right). 

Many African mammals (rhinos, elephants, giraffes…) originated in Eurasia and then migrated to Africa in two large waves of migration, around 35 and 20 million years ago. Because the oldest fossils of a ‘true’ hippo are about 16 millions years old, palaeontologists have assumed they crossed into Africa on a land bridge during the second wave of migration. But Epirigenys lived 28 million years ago, so hippos must have originated from their anthracotheres ancestor in Africa. This also explains why fossils of hippo ancestors hadn’t been found before: palaeontologists were looking in the wrong place.

But are hippos whales? The discovery of Epirigenys doesn’t prove that hippos and whales came from the same ancestor, but it makes any different scenario rather unlikely. “This study is very important because now we have a hippo ancestor. And we know that the ancestors of hippos are from South-East Asia, and the ancestors of whales are also from South-East Asia, from the same period”, Lihoreau says.

Phylogenetic relationships between hippos, anthracotheres and cetaceans.

Lihoreau and colleagues are now going to focus on searching for the ancestor of anthracotheres in South-East Asia, to then compare it with the ancestor of whales, which is well known. If the team gets lucky, they might find their  ‘holy grail’—the common ancestor of hippos and whales.

Jonathan Geisler, a palaeontologist at the New York Institute of Technology who studies the evolution of dolphins and whales says “About 15 years ago there was a big gap between the age of the earliest hippos and the oldest whales. These authors, and their collaborators, have been steadily filling in this gap through the discovery of new fossils, as well as detailed studies that have moved known fossil species into this gap.”

Many questions remain unresolved. Lihoreau suspects that hippo ancestors hopped into Africa around 30 million years ago alone and… swimming. “This is somewhat speculative but certainly seems possible,” says Geisler. “There is evidence to suggest some anthracotheres were semi-aquatic, and were able to make this crossing.” This hypothesis implies that the hippo-whale ancestor already lacked hairs and sweat glands, which would have “constrained the evolution of the hippo group to get into water”, Lihoreau says. His team is going to collaborate with geologists and geochemists to try and figure out in what sort of environment hippo ancestors were living. This should help us understand what shaped the evolution of hippos towards their semi-aquatic lifestyle, which is very rarely seen for herbivorous mammals (capivaras and beavers are the only other exceptions).

Reference:

Lihoreau F., Fredrick Kyalo Manthi & Stéphane Ducrocq (2015). Hippos stem from the longest sequence of terrestrial cetartiodactyl evolution in Africa, Nature Communications, 6 6264. DOI: http://dx.doi.org/10.1038/ncomms7264

An edited version of this article was published in Lab Times on the 17-03-2015. You can read it here.

Rare embryo discovery gives hints on how dinosaurs reproduced

Sometime in the Late Jurassic era, a dinosaur nest was hit by a fatal tragedy and its eggs never hatched. Whatever killed the baby dinos - perhaps a hungry predator or a flood - was a stroke of luck for the team of paleontologists who, about 150 million years later, stumbled on the crushed eggs and embryo remains in the Lourinhã geological formation, in Portugal.  

“Most of the time what happens is that you find eggs without embryos, to find them together is really a matter of chance,” says Ricardo Araújo, lead author in the recent Scientific Reports study describing the Lourinhã fossils.

 

Lourinhã geological formation (Credit: wikipedia commons)

Findings of fossilised eggs with embryos are extremely rare - no more than a handful have ever been found. But without embryos it is virtually impossible to link an egg to a specific dinosaur species. So although many dinosaur eggs have been discovered all around the world, sadly we still know little about how dinosaurs reproduced and looked after their babies.

Araújo and colleagues at the Museum of Lourinhã didn’t immediately realise the importance of their fossil discovery; it wasn’t until they prepared the specimens in the lab that they saw tiny teeth and bones amongst the broken eggshells. “That’s when the good news really happened,” Araújo says.

The dinosaur baby teeth had nothing cute about them - they were long, pointy and sharp. Together with other bone features, the shape of the fossilised teeth gave away their identity: they belonged to Torvosaurus, the top meat-eating predator of the Late Jurassic era.

Sketch of the anterior part of the embryonic maxilla, showing the sharp teeth.
(Credit: Museu da Lourinhã)

Torvosaurus is an older, or 'primitive', dinosaur of the theropod family, which includes large carnivorous predators like the famous Tyrannosaurus and also modern birds. Even though Torvosaurus lived (and was extinct) well before Tyrannosaurus was around, it looked a lot like its cousin- it had huge jaws and walked on two legs, but it had longer and stronger arms. Torvosaurus was also nearly as big as Tyrannosaurus, measuring up to 11 meters and weighing about two tons.

Partial fossils of adult Torvosaurus have been found in North America and in the Lourinhã formation, but no embryos had ever been discovered. Up until now that is. The new specimens are the oldest theropod embryos found to date.

"Before we had examples of eggs and embryos of very advanced theropod dinosaurs, but we didn’t know anything at all of what was happening at the base of the family tree," says Araújo “[…] this finding is one of the oldest in the world, and it’s certainly the oldest for theropod dinosaurs,” he adds.

So what do these new fossils tell us about how primitive theropods lived?

By using a bunch of high tech methods, like high-power electron microscopy, the Lourinhã researchers looked in extreme detail at the microstructure of the eggshell pieces. They found that Torvosaurus’ eggs had a single structural layer, in contrast to advanced theropods that have more complex eggshells with two or even three layers (including modern birds, which are technically living dinosaurs).

It was known that primitive dinosaurs from other families had single-layered eggs, but theropods were the missing piece in the puzzle. “Now we have the evidence that eggshells of primitive dinosaurs only have one structural layer,” Araújo says. It appears that eggshell complexity increased throughout dinosaur egg evolution. But this isn’t all.

Torvosaurus is a basal or 'primitive' member of the theropod dinosaur family (Credit: Vladimir Bondar
& GEAL - CIID - Museu da Lourinhã)

The Torvosaurus’ eggshells have another interesting (and strange) feature- they have huge pores, or holes. Bird, reptile and dinosaur eggshells have pores so gases can be exchanged between the inside and the outside of the egg, so the embryos can breath. As a rule of thumb, eggs with larger pores are laid in a moist substrate, while eggs with smaller pores are incubated in nests exposed to air.

The Torvosaurus eggshells have large pores that “interconnect in a network towards the top of the eggshells,” explains Araújo “this is really different from what was found to date”. The eggshell pore size suggests that Torvosaurus buried their eggs, just like most modern reptiles. 

Another indication that Torvosaurus’ eggs were buried is the fact that the fossilised eggshells and embryos are exceptionally well preserved. "The eggshells are nearly exactly the same as they were 150 million years ago," says Araújo. Being underground would have protected the fossils from bacteria and atmospheric erosion.

“Dinosaur embryos are very rare and also challenging to identify,” says David Varricchio, a paleontologist from Montana State University “This study provides an important addition to our understanding on dinosaur reproduction.”

The Lourinhã formation is very rich in Late Jurassic fossils; it has many dinosaur nests and footprints, and countless invertebrate fossils. Araújo notes “There have been more discoveries and […] they will give further insights into dinosaurs and other types of vertebrates from 150 million years ago in Portugal. Finding other eggshells and embryo associations from other groups of dinosaurs would be really helpful to complete the picture.”

Reference:

Araújo R., Castanhinha R., Martins R.M.S., Mateus O., Hendrickx C., Beckmann F., Schell N. & Alves L.C. (2013). Filling the gaps of dinosaur eggshell phylogeny: Late Jurassic Theropod clutch with embryos from Portugal, Scientific Reports, 3 DOI: 10.1038/srep01924

This article was publish in Lab Times on the 13-08-2013. You can read it here.