No bones about it: sharks evolved cartilage for a reason

Sharks are one of the oldest and least changed of all the living back-boned jawed creatures. But because their skeletons are made of cartilage much of their early fossil record is poor.

Cartilage is a rubbery tissue that forms the framework for bones to ossify (harden) upon. It’s why babies have rubbery legs when they begin to walk, as the bones haven’t fully ossified around the cartilage cores. Our ears and noses have cartilage frameworks too, which lack bone, but still support the soft structures we hear and smell with.

Cartilage doesn’t preserve as well as bones, so the early shark fossil records are based mostly on isolated scales and teeth.

Although the oldest of these shark-like scales is 480 million years old, the oldest complete shark fossil, Cladoselache, is only about 360 million years old.

Older but quite incomplete fossil sharks are known, such as Doliodus from Canada, around 400 million years old. But the simple truth is that most sharks of this age are known only from isolated teeth or scales.

Cladoselache, one of the oldest complete fossil sharks, is dated at around 360 million years old, but its remains are compressed, unlike the new fossil shark from Gogo in Western Australia.John Long, Author provided

This poor fossil record is partly responsible for scientists thinking that sharks must represent a primitive condition in vertebrate evolution compared to all other fishes and land animals (tetrapods) which have a well-ossified bony skeleton.

But this idea has just been challenged due to the discovery, announced today in the journal PLOS One, of a 380-million-year-old fossil shark from Western Australia named Gogoselachus lynbeazleyae that shows remnant bone cells present in its cartilaginous skeleton.

Finding the fossil

Finding a very rare fossil in the field gives one a kind of euphoric rush and I recall it well the day I found the Gogo shark, at 11am on July 7, 2005. I was searching for fossils on Gogo Station in the Kimberley, near Fitzroy Crossing, about a four-hour drive inland from Broome.

I had just split a limestone nodule with my hammer and saw a vague outline of a pair of jaws staring at me. Examining the specimen with my hand lens revealed the teeth had multiple cusps fixed onto a broad bony base – a feature unique to sharks at this time. I was overjoyed at finding the first fossil shark in more than 60 years of collecting from the site.

This photo of me holding the Gogo shark was snapped minutes after the discovery on July 7, 2005.Lindsay Hatcher, Author provided

So why the big deal about finding a shark at Gogo? The Gogo Formation is undoubtedly one of the world’s best sites for studying the early evolution of fishes as it yields superb three-dimensional specimens that lived 380 million years ago, a very important time in fish evolution.

Gogo has a diverse fauna of many kinds of ancient armoured placoderm fishes as well as early bony fishes (osteichthyans), but no sharks.

Finding a shark at Gogo has been a bit of a holy grail for fish palaeontologists as we all expected a shark from this site would have extraordinarily good preservation. It should reveal something new about early shark evolution, as nearly all other sharks of this age were flattened and poorly preserved.

Back in the lab, I prepared the specimen in dilute acetic acid, which slowly dissolved away the limestone rock surrounding the fossil. I was surprised to find the cartilaginous elements of the shark easily came out of the rock. This suggested that the skeleton was made of a special kind of highly mineralised cartilage.

Although mostly incomplete, the specimen comprised the complete lower jaws, shoulder girdles which support the pectoral fins, some isolated gill-arch elements and many small teeth and scales.

Top, the Gogo shark specimen in rock as it was found. Below, after three weeks of dilute acetic acid preparation the large lower jaw cartilages are seen emerging in perfect 3D form.John Long, Author provided

The teeth were highly unusual, with many small cusps surrounding the larger fangs. From the distinctive teeth we knew we had a new species of shark, as every living shark on the planet has its characteristic teeth that can identify the species from teeth alone.

Gogoselachus was clearly a fast-swimming predator that hunted other fishes using its jagged teeth to snare prey. Gogoselachus lived on an ancient reef that teemed with many kinds of large predatory placoderm fishes, so had to hold its own in this piscine rat race.

Teeth of Gogoselachus are distinctive with many small cusps. The image far right is a CT-scanned tooth showing internal structure.John Long, Flinders University, and Tim Senden, ANU, Author provided

Fossil shark cartilage with bone cells

Professor Per Ahlberg is a palaeontologist at Uppsala University in Sweden who was not involved in the study but is an expert on early fish evolution. He acknowledges that this discovery about early sharks is interesting.

It fills an ecological gap in our understanding of the Gogo reef. We know from other fossil localities that sharks had evolved and were already quite diverse by this time, so it has always been a puzzle that they were absent from the Gogo fauna. Now we can see that they were there after all, even though they seem to have been quite rare.

What Gogoselachus might have looked like, as restored from the scant fossil remains.John Long

Yet the most significant thing about the find was in the detail of its cartilage microstructure. We analysed the specimen using thin-sections, micro-CT scanning and scannning electron microscopy.

While these tools allowed us to confirm the cartilage was like modern shark cartilage, made up of little bundles called tesserae, the matrix holding these cartilage units together retained a cellular structure with remnant bone cells visible.

This implied that sharks most likely evolved from ancestors that had much more bone in the skeleton. The evolution of modern sharks was driven by their loss of bone, which suggested they are not as primitive as previously thought.

Gogoselachus cartilage showing the separate units called tesserae making the up the lower jaw (left), and a thin section showing bone cells (red line) inside the matrix which binds the tesserae together (right). Image on the left is 0.5mm across, image on right is about 0.1mm across.John Long, Flinders University, and Carole Burrow, Queensland Museum, Author provided

Per told me the other exciting thing about this shark is the light it throws on the evolution of the skeleton.

Modern sharks have skeletons of a peculiar tissue called prismatic calcified cartilage: cartilage that is mineralised, not as solid sheets, but as a mosaic of tiny mineral prisms.

The new Gogo shark shows what seems to be an early version of prismatic calcified cartilage: unlike the modern kind, the gaps between the prisms contain cells that resemble bone cells. This may help to explain the relationship between prismatic calcified cartilage and bone.

Modern sharks most likely evolved their lighter cartilaginous skeletons to become faster swimmers, to evade predators and swiftly catch their prey. The loss of bone in their skeleton is also supported by the fact the oldest and most basal of all jawed vertebrates, the placoderms, had heavy bony skeletons. In the most recent phylogenetic analysis of lower vertebrates, the placoderms appear as being basal – or at a common evolutionary level – to sharks.

This study further supports the idea that sharks must have evolved from bony primitive ancestors and lost their bone early on in the race as they acquired their predatory body shape.

Today when we see the sleek form of a shark in water we see a triumph of evolution, a masterpiece of fine tuning at the cellular level, resulting in their current ecological success.

John Long receives funding from The Australian Research Council