Anomalocaridids had long, spiny head limbs presumably used to snag prey, and a series of blade-like filaments in segments across the animal’s back, which scientists think might have functioned as gills.
Credit: Esben Horn
The anomalocaridid fossil shows this ancient sea predator existed for much longer and grew to much larger sizes than previously thought.
Credit: Peter Van Roy
LONDON: The discovery of a collection of bizarre, fossilised sea creatures in southeast Morocco suggests that a large, iconic sea predator survived for 30 million years longer than previously thought.
The fossils are of anomalocaridids - a family of giant free-swimming invertebrate predators with mouths shaped like pineapple slices - and at almost one metre in length, this as-yet-unnamed species is the largest type of anomalocaridid ever found.
The team who found the fossils reported in the current issue of Nature that they are so exceptionally preserved they provide new clues as to when these animals became extinct - suggesting it was much later than previously thought.
"The anomalocaridids are one of the most iconic groups of Cambrian animals. [They] have come to symbolise the unfamiliar morphologies displayed by organisms that branched off early from lineages leading to modern marine animals, and then went extinct," said lead author and palaeontologist Derek Briggs from Yale University. "Now we know that they died out much more recently than we thought."
A fierce predator
In the 1800s anomalocaridids were first described as relatives of the shrimp because of their curved segmented bodies. However, in 1985 Briggs and a colleague distinguished them as unique due to their size and predatory features.
Prowling the oceans while hunting and feeding with the help of two long, spiny limbs that protruded from their head, they were well-known to be successful predators and at the top of the food chain.
Previous estimates had placed their extinction in the Cambrian period, an era which extended from about 540 to 501 million years ago and known for the sudden explosion of an array of new animal and plant species.
The right conditions for preservation
In 2008, Briggs and his colleague Peter Van Roy, a palaeontologist from Ghent University in Belgium, discovered the new anomalocaridid fossils in southeast Morocco.
When concretions are formed around a dead animal and a cavity in sedimentary rock, they often get flattened in the mudstones and therefore do not preserve well. But the sheer size of the Moroccan specimens and the preserving qualities of the silica in the rock appears to have prevented this.
"The picture from normal shelly fossils is very incomplete. More exploration in the right kind of settings to preserve soft-bodied fossils might well reveal more examples," said Briggs, whose analysis of the specimens suggests for the first time that the species survived until the early Ordovician Period - about 488 to 472 million years ago.
"Only about 40% of marine animals have hard parts, such as shells - the rest are soft-bodied and normally decay and are therefore not fossilised."
