Mysterious sea creature part of a new family
(12 January 2017)
One branch on the tree of life is a bit more crowded today as a team of scientists have revealed what a bizarre group of cone-shaped sea creatures actually are, as reported in Nature.
Known as hyoliths, these extinct marine creatures were long believed to belong to the same family as snails, squids and other molluscs, but the researchers have shown that they are instead more closely related to brachiopods – a group which has a rich fossil record but with only a few living species known today.
The research team involved paleontologists from Durham University, the University of Toronto and the Royal Ontario Museum in Toronto.
It was the way the hyoliths fed that gave the team the strongest clues about their family ties.
Brachiopods possess a soft body enclosed between upper and lower shells, unlike the left and right arrangement of shells in clams and other bivalve molluscs. Brachiopods open their shells at the front when feeding but otherwise keep them closed to protect their feeding apparatus and other body parts inside the shells.
In typical hyolith fossils, only the shells are preserved. But exceptional new fossils allowed the researchers to identify hyoliths’ soft parts – including a feeding structure with a row of flexible tentacles extending away from the mouth, contained within the cavity between a conical shell and a cap-like structure covering the opening of the mouth. Brachiopods are the only group of living animals with a comparable feeding structure which is covered by a pair of shells.
Strange extinct creatures
Co-author Dr Martin Smith, a lecturer in paleontology in the Department of Earth Sciences at Durham University, said: “These rather strange creatures survived for more than 250 million years, only to go extinct 252 million years ago. What their bodies looked like has always been a mystery, so this analysis is key to finally getting a clearer classification for this group of animals.
“It demonstrates that brachiopods and not molluscs are the closest surviving relatives of hyoliths.”
Hyoliths first appeared 530 million years ago during the early Cambrian period and diversified globally in many marine environments. They are among the first animals to have evolved an exterior shell-like mineralized skeleton, but little has been known about key diagnostic elements of their soft-anatomy until now.
Tentacles and food
Lead author and 20-year old undergraduate student, Joseph Moysiuk of the University of Toronto, said: “Examining particularly the lateral spines, called helens, in a sub-group of hyoliths, suggests that they may have used these structures to lift their bodies above the sediment, elevating the feeding apparatus to enhance feeding.
“It suggests that hyolithids fed on organic material suspended in water as living brachiopods do today, sweeping food into their mouths with those tentacles.”
Orphaned tree branch
The research team was able to do their analysis using newly discovered fossils from the renowned Burgess Shale Formation in British Columbia, one of the earliest fossil beds in the world containing soft-part imprints.
The key specimens came from new deposits near Stanley Glacier and Marble Canyon in Kootenay National Park, about 40km southeast of the original Burgess Shale site in Yoho National Park.
Co-author Dr Jean Bernard Caron of the Royal Ontario Museum and University of Toronto, commented: “Although a molluscan affinity was proposed by some authors, such a hypothesis remained based on insufficient evidence. Hyoliths became an orphaned branch on the tree of life, an embarrassment to paleontologists.
“Our most recent field discoveries were key in finally cracking their story, around 175 years after the first description of a hyolith.”
The distinctive rigid skeleton has up to four parts: an elongate, bilaterally symmetrical cone-like shell; a cap-like covering over the opening of the conical shell, known as an operculum; and, two lateral spines in hyolithids – a sub-group of hyoliths represented at the Burgess Shale – known as helens, that protrude from between the conical shell and operculum.
The Burgess Shale is one of the most important fossil deposits for studying the origin and early evolution of animals that took place during the Cambrian period, starting about 542 million years ago. Hyoliths are just one of the profusion of animal groups that characterize the fauna of the ‘Cambrian Explosion’. They became a diverse component of marine ecosystems around the globe for more than 280 million years, only to go extinct 252 million years ago, prior to the evolution of the first dinosaurs.
Shedding light on evolutionary history
“Resolving the debate over the hyoliths adds to our understanding of the Cambrian Explosion, the period of rapid evolutionary development when most major animal groups emerge in the fossil record,” said Dr Martin Smith, who started this research at the University of Cambridge and is now based at Durham University. “Our study reiterates the importance of soft tissue preservation from Burgess Shale-type deposits in illuminating the evolutionary history of creatures about which we still know very little.”
The findings are described in the paper “Hyoliths are Palaeozoic lophophorates”, published in Nature. Funding for the research was provided primarily by the Royal Ontario Museum and a Natural Sciences and Engineering Research Council Discovery Grant.
If you are interested in postgraduate research in palaeobiology and phylogenetic methods (Masters by Research or PhD), please contact Dr Martin Smith.