Titanichthys, an Ancient Fish that Fed Like a Basking Shark

Scientists have found that Titanichthys, a giant armoured fish (or placoderm) that lived 380 million years ago, used a feeding strategy similar to modern day basking sharks (Cetorhinus maximus), as published in Royal Society Open Science earlier this week. The research team was formed from a collaboration between paleontologists at the University of Bristol and University of Zurich as part of a post-graduate thesis.

Titanichthys, covered in its tough, armoured plates, was one of the largest animals of the Devonian period. It could reach an overall length of over 5 m (16.5 ft) and the length of its lower jaw exceeded 1 m (3 ft).

Artist's interpretation of Titanichthys. Image credit: Mark Witton

However, before now, there was no evidence to suggest how this massive fish fed. It's lower jaw was narrow and lacked dentition or sharp edges that would make it suitable for cutting. As a result, it has long been assumed that Titanichthys was a suspension feeder, filtering large amounts of plankton from the water column by swimming slowly with its mouth wide open. This is a technique known as continuous ram feeding.

But, to further complicate things, there is no fossil evidence to confirm this feeding strategy. Modern suspension feeders, such as basking sharks, have long projections covering their gills, known as gill rakers, to assist in filtering plankton, but no fossilised suspension feeding structures have ever been found for Titanichthys.

Instead, the research team focused on fossilised jaws collected from the Moroccan part of the Sahara Desert for their study. They used biomechanical analysis to compare the lower jaw of Titanichthys to those of other species, testing jaw resilience using a technique known as Finite Element Analysis (FEA). This allowed them to apply forces virtually to each jaw and assess how likely they were to break or bend.

Of their findings, lead author, Sam Coatham, said in a press release, "We have found that Titanichthys was very likely to have been a suspension feeder, showing that its lower jaw was considerably less mechanically robust than those of other placoderm species that fed on large or hard-shelled prey. Consequently, those feeding strategies (common amongst its relatives) would probably have not been available for Titanichthys."

The FEA revealed that the lower jaw of Titanichthys was far less resistant to stress and, therefore, more likely to break than the jaws of other placoderms, such as the better-known Dunkleosteus. As such, the jaws would not have been able to withstand feeding on larger prey as this would exert too much mechanical stress for them to handle.

This is similar to what is seen in both sharks and whales. The jaws of modern suspension feeders are less resistant to stress than their actively hunting relatives. Building on this finding, further analysis that compared the distribution of stress in the jaws showed similar patterns in both Titanichthys and the basking shark.

Basking shark suspension feeding at the water's surface

The team believes that several other extinct species would have also been suspension feeders, including other placoderms and even a species of plesiosaur. They have already identified promising areas for future research to better understand the development of suspension feeding.

"Our methods could be extended to identify other such species in the fossil record and investigate whether there were common factors driving the evolution and extinction of these species. We suggest a link between oceanic productivity and the evolution of Titanichthys, but this should be investigated in detail in the future. An established link could have implications for our understanding of the conservation of modern suspension feeders," Mr Coatham explained.

With so many of today's large suspension feeders being either vulnerable or endangered, the findings of this study, and of the future research it inspires, could prove useful in better protecting these species and minimising the factors that are currently impacting them.