Marine predators from the Cretaceous

Article | Updated 8 months ago

How palaeontologists uncovered their ancient mysteries

Imagine yourself during The Age of the Dinosaurs 100 million years ago. While the land is populated by the ferocious Spinosaurus, the oceans host massive sharks and gigantic marine reptiles. This is the geological period that Dr Siversson, Curator of Palaeontology at the Western Australia Museum, chose to specialise in. How do palaeontologists acquire their knowledge about such a distant past? What are the steps in fossils identification and how do scientists deal with uncertainty? We asked our expert.

Image of Dr Siversson comparing shark tooth specimens

Dr Siversson comparing a fossil tooth of the giant, extinct shark Carcharocles megalodon, with the jaws of a long fin mako shark (gray nurse shark jaw on the table).
Image copyright WA Museum 

Image of Dr Siversson examining a fossil fragment

Dr Siversson examining a fragment of fossil bone from a 120 million-year-old plesiosaur, found near Kalbarri. Remains of marine reptiles (plesiosaurs and, more commonly, ichthyosaurs) of Early Cretaceous age are regularly found in the lower Murchison river area.
Image copyright WA Museum 

The best part of being a palaeontologist

The main step before starting palaeontological research on fossils is to find material to study. Dr Siversson regularly leads field trips in Australia and overseas to better understand the creatures that lived on the Earth during the Cretaceous period, between 145 and 66 million years ago.

Being in the field and digging up fossils is perhaps the most enjoyable part of being a palaeontologist!

confesses the curator with a smile. For the last few years the Western Australian Museum has led almost one palaeontological expedition per year. The team, composed of Dr Siversson, additional staff and 3-5 volunteers, usually leaves in August or September to avoid harsh weather conditions that often occur in Western Australia. Dr Siversson splits up the team into two groups:

one group comes with me and digs into the rock for sampling while the other volunteers just walk along the outcrop, looking for extinct marine reptile bones.

Image of an excavation site north of Kalbarri, WA.

In 2012 Dr Siversson’s team did a bed-by-bed excavation north of Kalbarri, looking for fossil fish. As the team is digging down, they venture into progressively older rocks. Scientists are able to reconstruct the evolution of the local fish fauna by looking at changes in the fossil record. Here, several hundreds of thousands of years have elapsed from the bottom to the top of the rock. The bed-by-bed excavation was done in the Albian (Early Cretaceous) Alinga Formation in the Lower Murchison Riv
Image copyright WA Museum 

How are fossils found? 

As Dr Siversson’s research focuses on marine fossils from the Cretaceous Period, he is targeting rocks that have previously been identified as having been laid down in the ocean during this time. Global sea levels were generally much higher during the Cretaceous, which explains why rocks laid down on the shelf can now be found on dry land.

“Lamniform sharks, the main marine predators during the Cretaceous” 

Dr Siversson is specialised in lamniform sharks, an order of sharks that includes some of the most familiar modern species such as the great white shark, mako sharks, the megamouth shark and the grey nurse shark. Dr Siversson explains that,

lamniform sharks were the main predators in most shallow marine environments during the Cretaceous.

As sharks have a cartilaginous skeleton that does not fossilise as well as bone, palaeontologists study these extinct creatures primarily by looking for fossilised teeth and vertebrae.

The paleontological daily, a puzzle story

How do Dr Siversson and his palaeontologist colleagues manage to determine a shark’s length just from a single tooth?

Occasionally we may find a whole dentition from one single shark,

Dr Siversson said, but these finds are extremely rare. Sharks produce a lot of teeth. During their life time these marine predators may produce between 5,000 and 10,000 teeth that are constantly replaced. As a result palaeontologists mostly find shed teeth and may attempt to reconstruct a whole shark dentition using teeth that belong to different individuals. They essentially base the dentition reconstruction on comparisons with modern species and fossil specimens where the teeth have been preserved in their natural position. By analysing the fossilised teeth and reconstructing dentitions scientists are then able to make suggestions on the species’ average length. 

 

Image of a fossilised belonging to the extinct shark species Otodus

This fossilised tooth belongs to the extinct shark species Otodus (Megaselachus) megalodon. This species was much larger than any modern shark.
Photo by Jessica Scholle, image copyright WA Museum 

Image of an 18 million-year-old shark tooth

This 18 million-year-old shark tooth was found in Cape Range National Park, WA. Based on the tooth’s length (13 cm), palaeontologists estimate the total length of the animal. This is the biggest shark tooth ever found in WA. This shark would have been 12-13 metres!
Photo by Jessica Scholle, image copyright WA Museum 

Image of a reconstructed dentition of a Cretaceous Shark

Reconstructed dentition of the left side of the upper and lower jaws based on the associated dentition of a single Cretaceous shark (Cardabiodon ricki).
Photo by Jessica Scholle, image copyright WA Museum

Occasionally palaeontologists find associated vertebrae, which may give them a more accurate estimate of the length of the shark as

there is a relatively strong correlation between the maximum vertebral diameter and the total length of the shark,

Dr Siversson explains. Scientists start their investigation by cutting a section of the vertebrae and counting the growth bands that, as for wood, may give a good indication of the specimen’s age. 

Image of a Elasmosaur vertebra (left) and ichthyosaur vertebrae (right).

Elasmosaur vertebra (left) and ichthyosaur vertebrae (right). Can you see the differences? Elasmosaur vertebrae have a flat articulating surface and are quite tall whereas ichthyosaur vertebrae have a concave articulating surface and are very short. By looking at different features, palaeontologists are able to identify which group of animals fossilised vertebrae belong to.
Photo by Jessica Scholle, image copyright WA Museum  

Image of a fossil fragment of an Ichthyosaur specimen.

Ichthyosaurs were large, dolphin-like marine reptiles that appeared about 240 million years ago and became extinct in the Late Cretaceous, approximately 94 million years ago.
Photo by Jessica Scholle, image copyright WA Museum  

Image of two fossilised vertebrae of an ichthyosaur specimen.

These two fossilised vertebrae belong to an ichthyosaur specimen that lived in the coastal waters of Western Australia 120 million-years-ago
Photo by Jessica Scholle, image copyright WA Museum  

Image of a plesiosaur vertebra.

Can you see the two elongated parallel holes on the vertebra? They are found on all plesiosaur vertebrae. When palaeontologists find a vertebra with these characteristic holes called “foramina”, they know that is a plesiosaur vertebra.
Photo by Jessica Scholle, image copyright WA Museum 

Image of a 95 million-year-old fossilised vertebra

This 95 million-year-old fossilised vertebra belongs to an elasmosaurid plesiosaur, a huge fish-eating marine reptile that lived during the Cretaceous Period. Elasmosaurs became extinct at the same time as the dinosaurs (excluding the birds), about 66 million years ago.
Photo by Jessica Scholle, image copyright WA Museum 

 “It is up to other palaeontologists to agree or disagree”

Species, length, diet, age… Palaeontology teaches us a lot about creatures that populated the Earth millions of years ago. How can scientists be sure that they are correct? Dr Siversson gives us his opinion with a practical comparison:

It is the same as if someone is interested in cars, for example: he or she might recognise a Ferrari even if it is 500 metres away; the person would know that it is a Ferrari because of experience and because the person has looked at it so much. It is the same with everything else. In palaeontology there are certain characteristics, called ‘diagnostic features’, that occur - for example in a particular type of shark tooth but not in any other shark teeth.

Dr Siversson adds,

when we publish a discovery, it is then up to other palaeontologists to agree or disagree, that is how science works. The published paper is the starting point for other scientists to review. If you do not try to reconstruct dentitions there is not much to discuss. Sometimes you have to take risks, sticking your neck out a bit, in order to stimulate scientific progress.

To deal with uncertainty

When Dr Siversson first described the Australian species Cardabiodon ricki he estimated the shark’s total length from the teeth he had found near Exmouth, WA. But the discovery of Cardabiodon shark’s vertebrae revealed the species was much larger than expected. Dr Siversson explains:

Prior to the discovery of the Cardabiodon shark, mid-Cretaceous lamniforms were thought to be moderately large, but the discovery of their vertebrae indicated the shark was much larger than what was suggested by a simple comparison of tooth size with modern species. Using the white shark as a model, the Cardabiodon shark was estimated to be no more than 3-4 metres in length. However, when we compared the vertebrae with modern species, we estimated the shark to be 5-6 metres in length as its vertebrae are of the same size of a 5-6 metre white shark. In other words, the white shark has larger teeth for its body length compared with Cardabiodon.

Lower jaw of Cardabiodon ricki compared to the jaws of a mako shark.

Lower jaw teeth of the giant, 95 million-year-old shark Cardabiodon ricki compared to the jaws of a modern long-fin mako shark.
Image copyright WA Museum 

Science is a discipline in perpetual motion and what was true a few decades ago can be proved wrong today. Thus when palaeontologists understood that modern lamniform sharks have fewer and consequently larger teeth than extinct forms, the traditional method of determining a Cretaceous shark’s length based on tooth size was questioned. Dr Siversson explains:

This is an example of a case where an important find can change the way we look at these sharks, not only in regards to their absolute size but also their place within the food chain.

“Research is just one thing you do as a curator”

As a curator at the Western Australian Museum, Dr Siversson also works on answering public enquiries, deals with incoming and outgoing materials borrowed by other institutions, and ensures the preservation of the State’s fossil collection. He also has a large role in science communication, giving many talks and helping out in the creation of Museum exhibitions, such as the Dinosaur Discovery exhibition hosted by the Western Australian Museum – Perth in 2014 that has occupied Dr Siversson for almost seven months.

The specimens in the fossil collection belong to the people of Western Australia. It is my responsibility to make sure that they will survive long term.

Image of shark teeth

Sharks’ teeth are constantly replaced throughout life but the rate of tooth replacement slows down as the shark reaches maturity. During their life time, mako sharks can produce several thousand teeth.
Photo by Jessica Scholle, image copyright WA Museum

Image of shark teeth

Shark jaws are filled with hundreds of teeth distributed over multiple rows of which only the last one or two rows are functional in species feeding predominantly on fish and marine mammals. The soft tissue supporting the rows of replacement teeth steadily moves forward like a conveyor belt. Within a tooth file replacement with a new tooth may take a few weeks to several months depending on the species and the age of the individual.
Photo by Jessica Scholle, image copyright WA Museum

Image of a short-fin mako shark jaw specimen.

This massive jaw belongs to a short-fin mako shark specimen, a species closely related to the great white shark. The jaw is 29 cm wide and the largest teeth are 38 mm in length.
Photo by Jessica Scholle, image copyright WA Museum 

Image of shark teeth

The fourth and fifth teeth from the middle in the upper jaw are much smaller than the other teeth. Palaeontologists have suggested that these dwarfed teeth serve to break up the dentition into sections, allowing for deeper penetration by the enlarged front teeth.
Photo by Jessica Scholle, image copyright WA Museum 

Image of a grey nurse shark specimen.

Tooth shape depends on the sharks’ diet. Have you seen the flattened tiny teeth at the far end of the jaw? Sharks and rays feeding heavily on molluscs and crustaceans have small, blunt and densely spaced teeth forming a pavement-like dentition for crushing. However, grey nurse sharks mostly have needle-like teeth used for gripping slippery fish.
Photo by Jessica Scholle, image copyright WA Museum

Image of a grey nurse shark specimen.

This toothy jaw helps grey nurse sharks feeding on bony fish, crustaceans, squid, skates and even other sharks.
Photo by Jessica Scholle, image copyright WA Museum

Image of a grey nurse shark specimen.

Modern shark jaws help palaeontologists in comparison and identification of extinct species. This jaw belongs to a grey nurse shark specimen, the shark species you can observe at the Aquarium of Western Australia. The jaw is 38 cm wide and the largest teeth are 33 mm in length.
Photo by Jessica Scholle, image copyright WA Museum  

Further Information

Would you like to know more about this topic? Listen to Dr Siversson talking about super predatory sharks of the Cretaceous period.

You also can visit our website dedicated website to Marine Reptiles and Sharks from the Age of the Dinosaurs.