Old fossils reveal new surprises when we know what to look for
In 2000, palaeontologists Carrie Schweitzer and Rodney Feldmann described an unusual fossil crustacean from Alaska, Pristinaspina gelasina, which lived in deep-water sediments about 100 million years ago during the Cretaceous Period. Pristinaspina clearly belonged amongst the crustaceans called Squat Lobsters, but its significance was unknown at the time because almost nothing was known about their phylogeny - the evolutionary tree stretching back into deep-time. Moreover, its modern descendents would not be discovered for another half decade and it would take a further half decade to make the connection. Uncovering the big evolutionary picture revealed that Pristinaspina not only belonged to family new to science, but that it was a pre-adapted precursor to the remarkable Yeti Crabs living on deep-sea hydrothermal vents today.
An important part of my research is reconstructing evolutionary trees (phylogenetic analysis), including that of the Squat Lobsters and their Hermit Crab relatives, collectively called Anomura (Squat Lobsters are part of Anomura just as Kangaroos are part of the Marsupials). This was no easy task, however. The enigmatic Anomura are famously difficult to understand scientifically but an international research effort using fossil, molecular and anatomical data from many living species succeeded in reconstructing the underlying evolutionary tree of the group.
Equipped with new knowledge of how anomurans are related, we then zoomed-in on the Squat Lobsters and after detailed consideration of the fossil record, discovered that Pristinaspina belonged to a family new to science, which we formally scientifically named, Pristinaspinidae. Moreover, we pin-pointed Pristinaspina as part of the stem-lineage of the modern Yeti Crabs (Kiwa, in the family Kiwaidae). These strange, hairy, deep-sea animals first hit the news in 2006 as poster species for the international Census of Marine Life.
Stem-lineage is a technical term for the extinct ancestral groups that helped shape the groups living today. Importantly, they can reveal evolutionary steps to show how and when modern lineages evolved. The stem lineage Pristinaspinidae diverged from other Yeti Crabs (Kiwaidae) during the Cretaceous, and then, in the Oligocene (about 30 million years ago), the kiwaids moved into hydrothermal vent habitats where they live today.
The modern Yeti Crabs are well adapted to the toxic waters of hydrothermal vents and cold seeps (think of undersea volcanoes). Here, toxic sulfurous hydrocarbons poison most animals and reduce dissolved oxygen. It is very difficult to breathe, let alone find food. Remarkably, Yeti crabs grow their own food by farming sulfur-"consuming" vent bacteria on dense mats of body "hair" on their bellies. Moreover, they have a very important adaptation – greatly enlarged gill chambers that improve respiration. The large size of the gill chambers of modern Yeti Crabs is revealed by the special groove patterns on the carapace.
When we look at their ancestor, Pristinaspina, their corresponding grooves reveal that the gill chambers were already becoming enlarged. Although Pristinaspina did not live near hydrothermal vents or cold seeps, it could already tolerate low oxygen conditions. Pristinaspina was thus pre-adapted, with an ability over other animals to move into vent habitats where its descendents flourish today as the Yeti Crab, Kiwa.
Pin-pointing the stem lineage of the Yeti Crabs through analysis of modern genes and ancient fossils revealed that more than 100 million years ago, their ancestors were already acquiring the ability to thrive in one of the harshest marine environments. This tells us that the adaptations that are so useful on the harsh hydrothermal vents today had their beginnings well before they were needed for that purpose – such is the nature of pre-adaptation.
Dr Shane Ahyong
Senior Research Scientist