An unusual fossil from the Yunnan Province in China

Exquisitely-preserved soft body parts found in a 525 million year old fossil may help shed new light on the evolution of vertebrates, that is, animals with backbones. It’s been identified as a pterobranchia, a class of small animals resembling a worm with tentacles. Pterobranchs live inside hard tubes built around their soft bodies. Scientists have named the newly discovered species Galeaplumosus abilus, which means “feathered helmet from beyond the clouds,” a reference to its well-preserved feathery tentacles and to the location of its discovery in the Yunnan province of China. Yunnan translates as “south of the clouds.”

The entire 525 million year old Galeaplumosus abilus, showing the lower tube and feathery tentacles attached to a long ‘arm.’. Photo credit: Professor Derek Siveter, Oxford University.

Details of the discovery were published in the journal Current Biology by scientists from Yunnan University and the universities of Leicester and Oxford. Professor David Siveter from the University of Leicester’s Department of Geology commented on the fossil in a press release,

Amazingly, it has exceptionally preserved soft tissues – including arms and tentacles used for feeding – giving unrivaled insight into the ancient biology of the group.

Only about thirty species of pterobranchs survive today, many living colonially on the ocean floor. But 490 to 400 million years ago, they and other closely-related creatures called graptolites lived in great abundance.

Today’s pterobranchs are structured in much the same way as their ancestors: the soft body of each animal is protected by a hard tube made from a secreted collagen-like material. Depending on the species, there are one to nine “arms” that stick out of each tube. Each appendage has a row of tiny tentacles along it, used for catching plankton from the surrounding water.

Pterobranchs are of particular interest to evolutionary biologists because they belong to a phylum (a high-level taxonomic classification of life) called Hemichordata. These are creatures that lie between two major categories of animals: chordates and invertebrates. Chordates include vertebrates – animals with backbones, such as mammals, birds, fish, and humans.

Some invertebrates are also classified as chordates. They have a flexible rod called a notochord that supports the body, a hollow nerve cord that runs down the back, and “pharyngeal slits” which are like primitive gills. Similar features are seen in the embryonic stages of vertebrates.

Invertebrates, on the other hand, are animals that don’t have a backbone or notochord. Insects and mollusks are invertebrates.

“Hemichordates,” as the name suggests, are not quite chordate. They do not have a notochord but do have pharyngeal slits. Hemichordates appear to be creatures caught between two major evolutionary categories of animals, chordates and invertebrates.

A 525 million year old fossil pterobranch, a hemichordate that lived before the emergence of vertebrates in the fossil record, is quite a find for scientists studying the evolution of complex animals.

Pristiograptus, a type of graptolite showing the tube structure. Photo Credit: Dr. Jan A. Zalasiewicz, University of Leicester.

Until this discovery, fossil hemichordates were only known from their outer tubes, the hard part of the animal that remained after the soft body had decayed. These were called graptolites. Now extinct, they were once very abundant in the fossil record, between about 490 to 400 million years ago.

Galeaplumosus abilus, from 525 million years ago, is the oldest known hemichordate fossil, and the only known specimen containing preserved soft body parts. Compared to modern-day pterobranchs that are typically 1 millimeter (less than a twelfth of an inch) in length, this fossil specimen is a behemoth, measuring almost 4 cm in length (just over an inch and a half). The excellent state of preservation shows many details of the animal’s soft body – there are two arms, but only one arm, 22.5 mm in length, is fully intact. It has 36 clearly visible tentacles, used for catching food in the water. The hard protective tube is about 14 mm in length, and there are a few other features that remain unidentified. It’s not known if this specimen was part of a larger colony of animals. Except for its enormous size, this 525 million year old pterobranch looks quite similar to its modern-day counterparts.

Galeaplumosus abilus was found at a fossil site in China slightly older than, and similar in fauna to, a famous Cambrian fossil site called the Burgess Shale. At both locations, soft-bodied animals were preserved, which is extremely rare for fossils. It’s usually the hard remains like shells and skeletons that are preserved. Dr. Mark Williams of the University of Leicester, a co-author of the paper in Current Biology that describes Galeaplumosus abilus, told EarthSky,

The fossil locality is in southern Yunnan. This is a famous fossil deposit called the “Chengjiang biota.” It is half a billion years old, and records the first flowering of complex life on Earth. Many elements of the fauna are preserved with soft tissues. For this to happen you need animals to be captured and killed, and buried in sediment. And you need to switch off mechanisms of bacterial decay. You also need the right kind of sediment to template the soft tissues (otherwise they decay, and only the hard skeletal tissues – the tube in this case – remain).

Locked in the rock for 525 million years, Galeaplumosus abilus, the “feathered helmet from beyond the clouds,” has emerged in extraordinarily pristine condition, with even its soft body parts preserved in the fossil. It’s the oldest-known hemichordate, a type of animal between vertebrates and invertebrates. Compared to modern-day pterobranchs (a sub-category of hemichordates) which typically measure about 1 mm in length, the fossil was a giant, measuring almost 4 cm (40 mm) long. This unique discovery in Yunnan Province, China, is a valuable new clue to understanding the evolution of complex life on Earth.

A magnified view of the feather-like arm of Galeaplumosus abilus, showing details of the tentacles attached to it. Photo Credit: Professor Derek Siveter, Oxford University.

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