Fossils are windows into the past. They reveal what ancient creatures looked like, where they lived and something of how they lived. Palaeontology is the branch of science that looks into the past by studying fossils. The stomatopod crustaceans, better known as mantis shrimps, are well known for their superb vision, bright colours and super-fast rapacious claws. Today, they are abundant and occur throughout the warmer waters of the world. Fossil mantis shrimp, however, are rather less common and any fossil finds are of great interest. For example, we know of about 3000 species of fossil decapods (crabs, lobsters and prawns), but only about 30 species of fossil stomatopods.
One such species was recently discovered in quarries in the Taulignan region, southeastern France. The well preserved fossil, which is about 16–20 million years old (of Miocene age), is about 10 cm long, and its diagnostic characters (special features of particular scientific importance) mark it as new to science. We have now named the species Squilla taulignanus. Southeastern France today conjours images of vineyards and pastures, but back in the Miocene, the region was part of the eastern margin of the expanding body of water that would become the modern Atlantic Ocean.
The genus Squilla includes one of the best known mantis shrimps, Squilla mantis, which is fished in the Mediterranean Sea, and served in pasta or paella. Today, Squilla occurs only off the coasts of the Americas, Europe and West Africa, that is, the Atlantic Ocean and eastern Pacific (the region known as the Atlanto-East Pacific). Strangely, no species of Squilla occurs in the region of highest marine biodiversity, the western Pacific and Indian Ocean (the Indo-West Pacific). Why is this so? Did they previously live in the Indo-West Pacific but go extinct there?
Phylogenetic analysis (reconstructing the evolutionary tree of life) tells us that Squilla and a cluster of its close Atlantic relatives stands in a separate lineage within the family tree. The existence of the Miocene fossil Squilla taulignanus is direct evidence that the Squilla lineage had already diverged from other mantis shrimp lineages by then (and some other potential Squilla fossils may push it back to the Eocene). By this time, under the action of plate tectonics, the Atlantic Ocean had already separated from the Indian and Pacific oceans, taking with it a largely separate mantis shrimp fauna that remains separate to this day.
In fact, all of the fossils accurately ascribed to Squilla are from palaeo-Atlantic locations. This tells us that Squilla did not go extinct in the Indo-West Pacific, but has always only lived in the Atlanto-East Pacific. From what we know of palaeontology and phylogenetics, it will remain so into the future.
Our research explains why Squilla lives where it does today, and not off Australia or India, for instance. Geological and evolutionary events of the past leave a clear mark on the fauna of today, which we make sense of through the combined evidence of palaeontology and phylogenetics. Paradoxically, looking into the past helps us understand the present and maybe also the future.
Dr Shane Ahyong
Senior Research Scientist