Evolution

Unusual teeth reveal a new species and a twist on evolution

4-million-year-old seal skulls reveal a new species.

So often when we think of our animal friends, we take it for granted that their species is a fixed, unchanging thing.

Your beloved pet dog is Canis familiaris. But sometimes, new species do come to light right under scientists' noses — the tiger you thought was like all the other tigers is actually not, for example. And things can get particularly complicated when all you have to go on are ancient traces of a creature — as is the case for dinosaurs, or the megafauna that once roamed the Earth millions of years ago.

Sometimes, it's worth taking another look at the fossil record.

In a pair of studies published on Wednesday, researchers describe how they did just that — and made a discovery that puts a serious twist on the evolution of one of the ocean's most beloved creatures.

A Sarcodectes skull from the study.

A new species — In the first study, published in the journal Royal Society Open Science, researchers focused on a set of 4-million-year-old skull fossils previously identified under the taxon Callophoca obscura — an extinct, earless seal thought to have once lived in the waters off America and Europe. But previous studies had questioned this classification as inaccurate.

Turns out these suspicions were right: These skulls actually belong to a Pliocene-era seal species that is entirely new to science.

"In the past, fossil species (including Callophoca) were often described from fragmentary and isolated bones. Today, we benefit from more complete fossil material, thanks to generations of fossil collecting," James P. Rule, lead author on the study and a postgraduate student at Monash University in Australia, tells Inverse.

By analyzing skull specimens, Rule and his colleagues realized they had a new species — an extinct monachine seal from the Pliocene era, which occurred between 5.3 and 2.6 million years ago. They named their new species Sarcodectes magnus.

"Often, these new fossils were simply ascribed to these incomplete fossil species [C. obscura], when in reality, they are different enough to represent a new species," Rule says. "This is what we found with Sarcodectes, as when we compared it to other seals, it's morphology was quite unique."

A drawing of the new seal species' skull, including its teeth.Rule et al. Royal Soc. Open Science (2020)

Evolution overload— Sarcodectes magnus was no ordinary seal. Residing in the western North Atlantic, Sarcodectes possessed a fairly big body — nearly three meters long — and sharp teeth perfect for snatching their next meal.

The seal's canines can be seen here.Rule et al, Royal Soc. Open Sci. (2020)

"We learned that Sarcodectes had teeth that were very sharp in addition to large jaw muscles, which we theorized were for helping it bite and chew large prey," Rule says.

Furthermore, while Sarcodectes may have been a big fella in ancient times, it's not super impressive compared with modern-day monachine seals -- especially the elephant seal (also known as the largest seal on planet Earth).

The seal discovery puts the evolution of other seals, like the giant elephant seal, into perspective.Tarpan / 500px/500Px Plus/Getty Images

"Sarcodectes is a large seal when compared to other extinct fossil monachines. But it is not large when compared to today's living monachines, such as the five-meter long elephant seal," Rule says.

For reference: Modern-day monachines are "earless seals that swim with their back feet, that mostly live in the Southern Hemisphere," Rule says. They include Antarctic seals, elephant seals, and monk seals.

By comparing Sarcodectes to modern-day monachine seals, the researchers gleaned some surprising information about the evolution of living monachine seals.

"However, Sarcodectes was not as big as today's living monachine seals, especially the ones living around Antarctica. Our analyses unveiled that monachine seals did not get large until late in their evolution," says Rule.

A Sarcodectes skull rendering from the study.

There's more — In the second study, published Wednesday in the Proceedings of the Royal Society B, the same team of researchers also announce another groundbreaking discovery.

In that study, Rule and his colleagues reveal they have discovered another extinct monk seal species Eomonachus belegaerensis — and even more excitingly, this is the first such discovery south of the equator.

Prior to the study, paleobiologists thought that only two monachine species — elephant seals and lobodontins — had crossed into the Southern Hemisphere from the North Atlantic. The idea was that these seals must have evolved from a common monachine seal ancestor in the north, and then crossed the equator.

But the research finds that the newly-discovered monk species also lived in the Southern Hemisphere, adding a third monachine species to the mix and charting a change in seal evolution.

Instead of evolving in the North Atlantic, as previously thought, these true seals (phocids) may actually have originated in the Southern Hemisphere and then headed north, traversing the equator multiple times throughout history.

These twin discoveries reshape our understanding of seals' lineage. They also show that while fossils may be set in stone, our understanding of ancient species' evolution is ever-changing.

Abstract: Today, monachine seals display the largest body sizes in pinnipeds. However, the evolution of larger body sizes has been difficult to assess due to the murky taxonomic status of fossil seals, including fossils referred to Callophoca obscura, a species thought to be present on both sides of the North Atlantic during the Neogene. Several studies have recently called into question the taxonomic validity of these fossils, especially those from the USA, as the fragmentary lectotype specimen from Belgium is of dubious diagnostic value. We find that the lectotype isolated humerus of C. obscura is too uninformative; thus, we designate C. obscura as a nomen dubium. More complete cranial and postcranial specimens from the Pliocene Yorktown Formation are described as a new taxon, Sarcodectes magnus. The cranial specimens display adaptations towards an enhanced ability to cut or chew prey that are unique within Phocidae, and estimates indicate S. magnus to be around 2.83 m in length. A parsimony phylogenetic analysis found S. magnus is a crown monachine. An ancestral state estimation of body length indicates that monachines did not have a remarkable size increase until the evolution of the lobodontins and miroungins.
Abstract: Living true seals (phocids) are the most widely dispersed semi-aquatic marine mammals, and comprise geographically separate northern (phocine) and southern (monachine) groups. Both are thought to have evolved in the North Atlantic, with only two monachine lineages—elephant seals and lobodontins—subsequently crossing the equator. The third and most basal monachine tribe, the monk seals, have hitherto been interpreted as exclusively northern and (sub)tropical throughout their entire history. Here, we describe a new species of extinct monk seal from the Pliocene of New Zealand, the first of its kind from the Southern Hemisphere, based on one of the best-preserved and richest samples of seal fossils worldwide. This unanticipated discovery reveals that all three monachine tribes once coexisted south of the equator, and forces a profound revision of their evolutionary history: rather than primarily diversifying in the North Atlantic, monachines largely evolved in the Southern Hemisphere, and from this southern cradle later reinvaded the north. Our results suggest that true seals crossed the equator over eight times in their history. Overall, they more than double the age of the north–south dichotomy characterizing living true seals and confirms a surprisingly recent major change in southern phocid diversity.
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