Almost-Living Fossils Month #27 – Those Giant Sharks

For the final entry this month, let’s look at a particularly famous lineage: the megatooth sharks.

More formally known as the otodontids, the megatooths were a group of sharks that first appeared in the Early Cretaceous, about 115 million years ago. They were a branch of the mackerel shark lineage – making them evolutionary cousins to a variety of modern species like the great white shark, basking shark, and goblin shark – and had a near-worldwide distribution, with fossils known from every continent except Antarctica.

Early otodontids in the Cretaceous were usually small-to-medium sized, around 2-3m long (6′6″-9′10″), but after surviving through the end-Cretaceous extinction they took over the marine apex predator niches left vacant by the vanished mosasaurs and plesiosaurs and began to get very big. Species of Otodus in the Paleocene and early Eocene may have reached sizes of at least 9m long (29′6″), twice the size of an average great white.

Their teeth gradually became proportionally larger in their jaws, losing their side cusplets and taking on a chunky triangular shape with finely serrated edges. This gave them an incredibly powerful bite force, and they would have probably fed on pretty much any other large marine vertebrates they could catch, including bony fish, smaller sharks, turtles, and early penguins – and then when marine mammals like early whales and sirenians appeared in the mid-Eocene, they adapted to this new food source too.

By the Late Eocene (~35 mya) the Otudus lineage was still developing even chunkier and more serrated teeth, and by the Early Oligocene (~28 mya) Otodus chubutensis reached even larger sizes rivaling the modern whale shark at around 12m long (39′4″).

But the most well-known member of the group evolved just a few million years later in the Early Miocene (~23 mya) – the absolutely enormous “megalodon”.

There’s some debate about what genus name megalodon should be assigned to – at the moment its formal name is usually considered to be Otodus megalodon, but some paleontologists place it in Carcharocles or Megaselachus or Procarcharodon instead. Whatever you want to call it, it was a ridiculously big shark – size estimates range up to about 18m (59′), which would make it potentially the largest fish to have ever lived.

Since these huge sharks are all known mostly from just their fossilized teeth (and occasionally a few exceptionally preserved cartilaginous vertebrae), it’s hard to tell what they actually would have looked like in life. Megalodon is frequently depicted as simply a scaled-up great white, but it’s unclear how accurate that really is – it may have convergently resembled a giant great white due to their similar predatory habits, or it could have had a build more like the larger basking shark or whale shark.

A preserved megalodon skull has actually been found, but no studies of it have been published yet. It might give us some important clues about the head shape of this giant shark, but until there’s some official information all we can do is continue to speculate.

Megalodon was a highly successful species, living all around the world in warm and temperate ocean waters for around 20 million years. Its teeth have been found in association with the bones of many different smaller whale species, suggesting it frequently ate marine mammals, and the patterns of the bite marks indicate it probably used different hunting strategies than modern great whites. Some whales seem to have been heavily rammed and then had their ribcages bitten into, targeting their hearts and lungs, while others had their flippers ripped off to immobilize them.

During the Pliocene (~5-2.6 mya), however, megalodon began to struggle. Cooling oceans and changes in the abundance of the marine mammals it ate began to restrict its available prey. Baleen whales started to grow too large for it to effectively hunt, since it preferred to target smaller species, and they also shifted their ranges towards the cold polar waters that megalodon didn’t seem to be able to survive in. In addition, dropping sea levels may have destroyed most of its shallow warm-water nursery sites, making it harder for newborn young to survive into adulthood.

By the end of the Pliocene, somewhere between 3.6 and 2.6 million years ago, megalodon went completely extinct. Despite some very pseudoscientific claims, there’s definitely no living “Meg” out there anymore – if there was, we’d be constantly finding freshly-shed teeth and whales with giant bite marks on their bodies!

Almost-Living Fossils Month #21 – More Sharks

First appearing in the Early Permian, about 290 million years ago, the synechodontiformes were an early branch of the neoselachian lineage of cartilaginous fish, slightly closer related to living sharks and rays than to the hybodontiformes featured earlier this month.

They originated in the Paleo-Tethys Ocean and survived through the devastating end-Permian “Great Dying” mass extinction (~252 mya), then went on to quickly spread around most of the world and also survive through the Triassic-Jurassic extinction (~201 mya). During the Jurassic and Cretaceous they became quite common and diverse, taking over some of the niches previously occupied by the hybodontiformes and adapting to a range of marine environments from shallow coastal waters to open ocean.

Most known synechodontiform fossil remains are just their teeth, since cartilage skeletons don’t preserve very often, but there are a few rare body fossils that show they were varied in appearance with differing arrangements of dorsal fins and spines.

Paraorthacodus jurensis here was one of the species known from the Late Jurassic of Germany (~155-150 mya). Reaching lengths of at least 1.3m (4′2″), it had only one dorsal fin far back on its body, along with large pectoral fins and a low asymmetrical tail that gave it a superficial resemblance to the modern sixgill sharks.

Its teeth were close in shape to those of sand tiger sharks, and it may have had a similar lifestyle opportunistically hunting prey just above the sea floor in the waters around the continental shelf and slope. Remains of a chimaera in the mouth and gut contents of a couple of Paraorthacodus jurensis fossils suggest that smaller cartilaginous fish were fairly common elements of its diet.

A few synechodontiformes managed to survive the end-Cretaceous extinction 66 million years ago – but while the ancestors of moderns sharks thrived in the Cenozoic, the synechodontiformes never recovered anything close to their Mesozoic levels of success and instead began to decline.

The last known synechodontiforme was a currently-unnamed member of the Paraorthacodus genus, hanging on in the waters around Antarctica in the Late Eocene (~37 mya). If they managed to survive past that time it probably wasn’t for very much longer, and it’s likely they finally disappeared during another extinction event at the Eocene-Oligocene boundary.

Almost-Living Fossils Month #09 – Horned Sharks

All modern species of sharks and rays are part of a single lineage of cartilaginous fish known as neoselachians, and the closest evolutionary “cousins” to all of them were the hybodontiformes.

First appearing way back during the Devonian, about 400 million years ago, these early sharks were widespread around the world and incredibly successful as a group, living in both marine and freshwater environments.

Although due to their cartilaginous skeletons hybodontiformes are mostly known from fossilized teeth, there are still some complete specimens known that show us their overall body shape. They had two dorsal fins, each with a long spine in front, and an asymmetrically-shaped tail. Some of them also had small horn-like spines on their heads – this seems to be a sexually dimorphic trait, since the ones with “horns” also have claspers which show they were males – and they generally had powerful jaws with teeth specialized for crushing.

They were probably fairly slow swimmers most of the time, but would have still been capable of occasional bursts of higher speed, and various species were adapted to a wide range of food sources. Some had wider flatter teeth for cracking open hard-shelled seafloor invertebrates, and others were more opportunistic hunters that would have crunched on pretty much anything they could fit in their mouths.

Hybodontiformes were the dominant type of shark around the world before the end-Permian “Great Dying” mass extinction (~252 mya), and then went on to recover and flourish once again up until the mid-Jurassic.

Hybodus hauffianus was one of the Early Jurassic species, living around 183 million years ago in Europe. About 2m long (6′6″), it had two different types of teeth in its mouth – sharper ones in the front and flatter ones in the back – suggesting it was a generalist predator eating whatever it could catch. We do know its diet at least included the fast-swimming squid-like belemnites, since some fossils preserve clusters of their internal hard skeletons in Hybodus’ stomach region.

Towards the end of the Jurassic neoselachians began to diversify and take over most of the marine shark ecological niches, and the hybodontiformes became increasingly restricted to freshwater. During the Cretaceous they continued to do fairly well in those environments, but most of them still disappeared around the time of the end-Cretaceous extinction (~66 mya). Since most other sharks weren’t actually particularly affected by the extinction event, it’s not clear whether the hybodontiformes were more vulnerable for some reason or whether it was the ongoing competition from neoselachians that drove the majority of them extinct at that time.

Still, a few of them did seem to make it through to the Cenozoic, although they were absent from the fossil record until the Miocene. Freshwater deposits in Sri Lanka have evidence of a late-surviving member of the group living perhaps as recently as 5 million years ago – so they would have only gone completely extinct sometime after that, and we probably missed seeing them alive by only a few million years at most.