Palaeontologists can examine the behaviours of animals alive today to gain an insight into the potential behaviours of closely related extinct animals. However, for many types of prehistoric animal, there really isn’t a living analogy and it is challenging to determine how some types of extinct creatures moved around and kept themselves safe from predators.

For example, earlier this month a scientific paper was published in PeerJ that looked at how straight-shelled ammonites might have escaped predation. The ammonite that was studied is known as Baculites and it was geographically widespread during the Late Cretaceous. The fossil record shows that many different types of straight-shelled cephalopods evolved repeatedly through time, but how these molluscs moved and how they avoided being eaten has puzzled scientists and the lack of a modern-day analogue made research more challenging.

Baculites (foreground) with a Xiphactinus (background).
The heteromorphic ammonite Baculites compressus swims in a Late Cretaceous ocean whilst in the background a large Xiphactinus swims slowly by in the background.

Three-dimensional Models and a Water Tank

Three-dimensional models of the heteromorphic ammonite Baculites compressus were created and placed in a water tank in order to test how these molluscs moved. The regularly coiled ammonite is well-known to fossil collectors and the general public, however these types of ammonite underwent a steady decline in diversity through the Cretaceous. The heteromorphs, those ammonites that did not have regularly coiled shells, continually produced new and often bizarre species indicating a certain level of success at occupying new ecological niches.

A beautiful ammonite fossil on display.
A stunning fossil of a Jurassic ammonite on display at the London Natural History Museum. These ammonites with their regularly coiled shells went into decline in the Cretaceous and many more bizarre forms evolved with heteromorphic shells.

Vertical Propulsion

Ammonites played an important role in Mesozoic marine food chains, filling a variety of ecological niches. Throughout the long evolutionary history of the cephalopods, orthoconic conchs (straight shells) have evolved in a wide variety of families not just the Ammonoidea. For example, prior to the evolution of the ammonites, during the Ordovician nautiloid cephalopods were extremely numerous and some genera were giants, such as Cameroceras.

Orthocone/Orthoceras scale drawing.
An early scale drawing design for the Orthoceras/orthocone fact sheet produced by Everything Dinosaur. During the Ordovician numerous types of straight-shelled nautiloid cephalopod evolved.

The models tested in the water tanks revealed that orthocones required very little energy to be exerted for rapid movements and increase in velocity. These molluscs could dart vertically in the water column at a very low metabolic cost.

This suggests that avoid predation these types of cephalopod were able to make sudden vertical movements under rapid acceleration that would have permitted them to avoid the lunge of a marine predator such as a mosasaur.

Baculites avoiding predation.
The cruising mosasaur first notices the prey (i), then begins to accelerate (ii). After closing in (iii), the predator makes its final lunge for the prey (iv). Cones surrounding the predator indicate hypothetical turning radiuses. For a successful dodge, the orthocone cephalopod must wait until the last possible moment or else the incoming predator could adjust its vertical trajectory.

In the tests, the models were able to move more than two body lengths upwards in less than a second. Whilst these cephalopods likely assumed low energy lifestyles day-to-day, they may have had a fighting chance to escape from larger, faster predators by performing quick, upward dodges.

The researchers concluded that these experiments suggest that orthocones sacrificed horizontal mobility and manoeuvrability in exchange for highly streamlined, vertically-stable, upwardly-mobile shells.

The scientific paper: “Vertical escape tactics and movement potential of orthoconic cephalopods” by David J. Peterman and Kathleen A. Ritterbush and published in PeerJ.

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