Living fossils

Given its large size, broad, flat face and slow, deliberate motion, divers could be forgiven for assuming the bluntnosed sixgill shark is a throwback to an earlier point in evolution. In fact, these deep-sea sharks aren’t very primitive at all. The shark’s sixth gill, a unique feature of its family the Hexanchiformes, and other parts of its well-adapted anatomy, are recent, specialized innovations. “They don’t have a second dorsal fin,” says Mark Wilson, a professor at the University of Alberta’s Laboratory for Vertebrate Paleontology. “That’s undoubtedly not a primitive feature of sharks in general.” And the lone fin, positioned far back on the shark’s long body, doesn’t have a fin spine (a bony extension at the leading edge of the fin) that would have been present in primitive sharks, Wilson says. The sixgill’s rows of comb-shaped teeth are distinctive and not particularly primitive either.

Count the gills: the bluntnosed sixgill belongs to the genus Hexanchidae, along with one other six-gilled and two seven-gilled species

So why is swimming next to these large sharks like swimming back in time? They’re simply different from the majority of today’s modern sharks – more than 370 species of them. In fact, according to the most accepted version of the shark family tree, they belong to a whole other branch: the squalomorph sharks. The squalomorph’s living representatives are limited to dogfish, the Greenland shark, and the six and seven-gilled sharks. The other, bigger branch is the Galeomorph sharks, the group which includes most familiar, shallow water sharks, such as the mako and the great white. In the best estimation of evolutionary biologists, these two branches split from a common ancestry around 200 million years ago, the age in which the oldest Hexanchiform fossils are found.

“There is nothing remarkable in the separation of the lineage,” says sixgill researcher Dr. Robert Dunbrack, an associate professor at Memorial University. “Lineages split continually. Since most lineages go extinct, the only surprising thing is that there are still Hexanchiform species around today, 200 million years later.” And that’s what makes the sixgill a fascinating specimen. Fossils of what are believed to be the shark’s close relatives date back about 100 million years, to the mid-Cretaceous period, and they were found right here in Canada, in a deposit near Vancouver. “The mid-Cretaceous teeth from B.C. are very similar to the teeth of living Hexanchus and have been assigned to the fossil species Hexanchus microdon,” says Dunbrack.

Scientists guess at the size of another ancient shark, the megalodon, based on the size of its teeth. 

The process of categorizing fossils isn’t an exact science, but sometimes paleobiologists get a break. “You can’t tell exactly what the body would have looked like from having the teeth alone, you have to associate it with a body,” says Wilson, from the University of Alberta. “You’d have to find a body with similar teeth, and in fact, in the case of the sixgilled sharks … there are fossils from Europe that are extremely well preserved, which show what the body was like, even in some of the oldest members of that group.” The European Hexanchiform specimen is an entire body preserved in a lagoon from the Jurassic period. It’s not as closely related to the bluntnosed sixgill as the fossils from the mid-Cretaceous, but belongs to the same larger family.

From past to present

Amanda Heath and Rob Zielinski of Hornby Island Diving take tourists and other divers to swim with the sixgill

Scientists may know more about the sixgill shark‘s past than they do about its current biology. Like other northern water sharks, and especially the deep water ones, basic information about the sixgill’s lifecycle isn’t known. Scientists aren’t sure how long it lives, how fast it grows, how often it reproduces, or even why it migrates from its usual depths of around 2500 metres to the shallower British Columbia waters each year. Thanks to the yearly visit however, researchers and eco-tourists are able to get up close to the shark – making observations to expand current data. One key project includes underwater still cameras which record the various individual sharks so scientists can track when specific sharks come and go at Hornby Island. Further work is being done on the shark’s heritage as well. Dunbrack, the sixgill researcher at Memorial University, is hoping to take another look at the family tree, using the molecular systematics approach (comparing DNA samples) to get a clearer picture of where the sixgill fits in the evolutionary history. To see the swimming sixgill for yourself, check out its profile on the Sharks in Canada page.

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