Paleontologists have finally determined what hyoliths — a group of extinct marine creatures — actually are.
Reconstruction of the hyolith Haplophrentis on the Cambrian sea floor. Image credit: D. Dufault / Royal Ontario Museum.
Hyoliths evolved over 530 million years ago during the Cambrian period and are among the first animals known to have produced mineralized external skeletons.
Long believed to belong to the same family as snails, squid and other mollusks, a new study shows that hyoliths are instead more closely related to brachiopods — a group of animals which has a rich fossil record, although few living species remain today.
Brachiopods have a soft body enclosed between upper and lower shells (valves), unlike the left and right arrangement of valves in bivalve mollusks. Brachiopods open their valves at the front when feeding, but otherwise keep them closed to protect their feeding apparatus and other body parts.
Although the skeletal remains of hyoliths are abundant in the fossil record, key diagnostic aspects of their soft-anatomy remained critically absent until now.
“Hyoliths are abundant and globally distributed ‘shelly’ fossils that appear early in the Cambrian period and can be found throughout the 280 million year span of Paleozoic strata,” the study’s authors said.
“The ecological and evolutionary importance of this group has remained unresolved, largely because of their poorly constrained soft anatomy and idiosyncratic scleritome.”
Joseph Moysiuk, an undergraduate student at the University of Toronto and corresponding co-author on the study,” said: “our most important and surprising discovery is the hyolith feeding structure, which is a row of flexible tentacles extending away from the mouth, contained within the cavity between the lower conical shell and upper cap-like shell.”
“Only one group of living animals – the brachiopods – has a comparable feeding structure enclosed by a pair of valves.”
“This finding demonstrates that brachiopods, and not mollusks, are the closest surviving relatives of hyoliths.”
“It suggests that these hyoliths fed on organic material suspended in water as living brachiopods do today, sweeping food into their mouths with their tentacles.”
All hyoliths had an elongated, bilaterally symmetrical cone-shaped shell and a smaller cap-like shell which covered the opening of the conical shell (known as an operculum).
Some species also bore a pair of rigid, curved spines (known as helens) that protruded from between the conical shell and operculum — structures with no equivalents in any other group of animals.
Examination of the orientation of the helens in multiple hyolith specimens from the renowned Cambrian Burgess Shale suggests that these spines may have been used like stilts to lift the body of the animal above the sediment, elevating the feeding apparatus to enhance feeding.
“We examined over 1,500 specimens of the mid-Cambrian hyolith Haplophrentis from the Burgess Shale and Spence Shale Lagerstätten,” they said.
“We reconstructed Haplophrentis as a semi-sessile, epibenthic suspension feeder that could use its helens to elevate its tubular body above the sea floor.”
“Exceptionally preserved soft tissues include an extendable, gullwing-shaped, tentacle-bearing organ surrounding a central mouth, which we interpret as a lophophore, and a U-shaped digestive tract ending in a dorsolateral anus.”
“Together with opposing bilateral sclerites and a deep ventral visceral cavity, these features indicate an affinity with the lophophorates (brachiopods, phoronids and tommotiids), substantially increasing the morphological disparity of this prominent group.”
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.
“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 co-author Dr. Martin Smith of Durham University, UK.
“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 were published online this week in the journal Nature.
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Joseph Moysiuk et al. Hyoliths are Palaeozoic lophophorates. Nature, published online January 11, 2017; doi: 10.1038/nature20804
