Marine predators from the Cretaceous

Article | Updated 7 years 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.

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.

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. 

 

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. 

 “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.

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.

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.