OK, everyone uses that weak pun, so why shouldn't I get into it? I use worse puns than that every day. And now that we are headed into Shark Week (cue Jaws music), I put together a few interesting tidbits.
You often hear sharks called "living fossils." Well, that depends how you define the term. There are no shark species today that saw the dinosaurs come and go, but there are many of long lineage (up to tens of millions of years). The bizarre goblin shark (Mitsukurina owstoni), for example, can trace back its family about 125MY. The frilled shark (Chlamydoselachus anguineus) is almost as weird and may be as old. There's no questioning that sharks, as a group, are survivors. They are older than dinosaurs, older than flowers, older than mammals, older than trees.
One of the challenges in tracing the lineage of sharks is that they don't fossilize well. Shark teeth fossilize wonderfully, but the shark's skeleton is all cartilage. When enough calcium has built up in the skeleton, most often in an older shark, you can find the vertebrae partially preserved. For a full, articulated shark fossil, you need fine-grained sediments and a great deal of luck.
There are few remains of everyone's favorite fossil whale-chomper, Carcharocles megalodon (formerly put in the genus Carcharodon with the great white) except teeth. They are some teeth, up to 17cm or more in length, finely serrated, and often found well-preserved enough to cut your hand on. The lack of other remains has contributed to the uncertainty about Megalodon's size. Scientists have found two partial spinal columns, with vertebra up to 23cm across.
Megalodon jaws (considerably oversized)
A famous reproduction of the jaws, using a modern great white as a model, led to speculation of sharks 30m (100 feet) long or even more. However, this assumed the teeth were the same size all through the jaws and thus got the size of the jaws and the overall shark much too large. Modern estimates tend to cluster around 15m - 18 on the outside - which is still, well, one giant shark. They may be larger than the current champion, the harmless whale shark. The great white is still "great" by anyone's standards, but it's been whittled down from estimates of 10 or even 12m to around 6.5m for the biggest females (males are smaller). Seven meters has been claimed for one Australian catch (estimated in the water - it was too big to get in the boat) and for a never-caught South African shark nicknamed "The Submarine." Seven meters is not impossible, but likely represents the extreme upper bound.
Meg lived from about 28MYA to perhaps as late as 1.5MYA, though it's pretty firmly in the "Extinct" category, which is too bad. It was presumably outcompeted by the nimbler (perhaps smarter) great whites and the pack-hunting orcas, which popped up in the last few million years of its reign (orca beginnings are rather fuzzy.) Meg lived at the same time as the sperm whale ancestor Livyatan melvillei, which was about the same size: their battles must have been awesome spectacles.
Steve Alten used the hundred-foot size in his popular Meg novels: Charles' Wilson's Extinct, which in my opinion is the inferior of the two, makes them much larger. (I was pretty hard on Alten for his science, but I agree he's gotten better, and Wilson's was so bad I gave it away after one reading.) I just downloaded a new novel with the truth-in-advertising name Big Ass Shark, but haven't read it yet.
Male Great White Shark, Farallone Islands, 2010 (NOAA)
There are sharks that beat the odds and fossilize very well. The Canadian Fossil Discovery Centre in Manitoba knew they had an 80-90
million year old fossil of a Squalicorax or crow shark but didn’t look closely at it
until 2014, when they realized they had the largest specimen ever found (3m) as
well as one of the most complete set of remains of this modern-looking shark. Peter
Cantelon, Executive Director, said, “With shark fossils… what you'll typically
find are just the teeth. In this instance, we have an entire shark from tip to
tail. The spine is visible all the way through; we can see a large portion of
its skull, jaw, possible fin material, gill material. It's incredible how well
this was preserved."
So as you watch Shark Week (a mixed bag of very good shows, average shows, and terrible fake Megalodon "documentaries"), remember that these really are incredible creatures, worth of respect, fascination, and protection. (They have more to fear from us than vice versa. Sharks may kill ten people a year: people may kill 10,000 sharks an hour.) There are some 400 species and counting - a single researcher who traveled with a commercial fishing fleet turned up as many as eight new species (not yet confirmed and described) in the "bycatch."
Now, time to watch Sharknado 2! I'm waiting for the obvious sequel - Sharknado 3: Megalodon.