Mystery of 508-million-year-old sea predator with jackknife headStaff Writer | January 6, 2018
Paleontologists at the University of Toronto (U of T) and the Royal Ontario Museum (ROM) in Toronto have entirely revisited a tiny yet exceptionally fierce ancient sea creature called Habelia optata.
History It lived during the middle Cambrian period
The research by lead author Cédric Aria, recent graduate of the PhD program in the department of ecology & evolutionary biology in the Faculty of Arts & Science at U of T, and co-author Jean-Bernard Caron, senior curator of invertebrate palaeontology at the ROM and an associate professor in the departments of ecology & evolutionary biology and Earth sciences at U of T, is published today in BMC Evolutionary Biology.
Approximately 2 cm in length with a tail as long as the rest of its body, the long-extinct Habelia optata belongs to the group of invertebrate animals called arthropods, which also includes such familiar creatures as spiders, insects, lobsters and crabs.
It lived during the middle Cambrian period approximately 508 million years ago and comes from the renowned Burgess Shale fossil deposit in British Columbia. Habelia optata was part of the "Cambrian explosion," a period of rapid evolutionary change when most major animal groups first emerged in the fossil record.
Like all arthropods, Habelia optata features a segmented body with external skeleton and jointed limbs.
What remained unclear for decades, however, was the main sub-group of arthropods to which Habelia belonged. Early studies had mentioned mandibulates - a hyperdiverse lineage whose members possess antennae and a pair of specialized appendages known as mandibles, usually used to grasp, squeeze and crush their food.
But Habelia was later left as one of the typically unresolved arthropods of the Burgess Shale.
The new analysis by the U of T-ROM researchers suggests that Habelia optata was instead a close relative of the ancestor of all chelicerates, the other sub-group of arthropods living today, named for the presence of appendages called chelicerae in front of the mouth and used to cut food.
This is mostly due to the overall anatomy of the head in Habelia, and the presence of two small chelicerae-like appendages revealed in these fossils.
"Habelia now shows in great detail the body architecture from which chelicerates emerged, which allows us to solve some long-standing questions," said Aria, who is now a post-doctoral researcher at the Nanjing Institute of Geology and Palaeontology, in China.
"We can now explain why, for instance, horseshoe crabs have a reduced pair of limbs - the chilaria - at the back of their heads. Those are relics of fully-formed appendages, as chelicerates seem to originally have had heads with no less than seven pairs of limbs."
Aria and Caron analyzed 41 specimens in total, the majority of which are new specimens acquired by ROM-led fieldwork parties to the Burgess Shale.
The research illustrates that the well-armoured body of Habelia optata, covered in a multitude of different spines, was divided into head, thorax and post-thorax, all bearing different types of appendages.
The thorax displays five pairs of walking legs, while the post-thorax houses rounded appendages likely used in respiration.
"Scorpions and the now-extinct sea scorpions are also chelicerates with bodies divided into three distinct regions," Aria explained.
"We think that these regions broadly correspond to those of Habelia. But a major difference is that scorpions and sea scorpions, like all chelicerates, literally 'walk on their heads,' while Habelia still had walking appendages in its thorax."
The researchers argue that this difference in anatomy allowed Habelia to evolve an especially complex head that makes this fossil species even more peculiar compared to known chelicerates.
The head of Habelia contained a series of five appendages made of a large plate with teeth for mastication, a leg-like branch with stiff bristle-like spines for grasping, and an elongate, slender branch modified as a sensory or tactile appendage.
"This complex apparatus of appendages and jaws made Habelia an exceptionally fierce predator for its size," said Aria.
"It was likely both very mobile and efficient in tearing apart its preys." ■