Early Cambrian fossil: Bizarre half-billion-year-old worm with tentacles solves evolutionary mystery

Date:
August 27, 2020
Source:
Royal Ontario Museum
Summary:
A half-a-billion years old fossil species of marine animal sheds
light on how the anatomies of the two main types of an animal group
called the hemichordates are related, and provides new evidence
in the historical debate among zoologists. The fossils are over
half-a-billion years old and were discovered at a Burgess Shale
site in the Canadian Rockies.



FULL STORY ==========================================================================
New research undertaken by scientists at the Smithsonian National Museum
of Natural History, Royal Ontario Museum (ROM) and University of Montreal,
has uncovered fossils of a new species of marine animal, Gyaltsenglossus
senis, (pronounced Gen-zay-gloss-us senis) that provides new evidence in
the historical debate among zoologists: how the anatomies of the two main
types of an animal group called the hemichordates are related. The fossils
are over half-a-billion years old and were discovered at a Burgess Shale
site in the Canadian Rockies. This discovery was published August 27,
2020, in the science journal Current Biology.


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With the early evolution of hemichordates being contentious among
researchers the discovery of Gyaltsenglossus senis is significant. It
provides direct fossil evidence connecting the two major groups of hemichordates: the enteropneusta and pterobranchia.

Although enteropneusts and pterobranchs appear to be quite different
types of animals they are closely related. This close relationship is
supported by DNA analysis of present-day species. More broadly, the
role of Gyaltsenglossus in understanding hemichordate evolution helps us understand the origins of a larger group of animals called deuterostomes
(which includes humans) by clarifying what characteristics they may have
shared with hemichordates early in their history.

The enteropneusta are a group of animals known commonly as acorn worms,
which are long, mostly mud-burrowing animals, that can be found today
in oceans around the world from the tropics to Antarctic. The other
main group of animals within hemichordates are pterobranchs, which are microscopic animals that live in colonies, each protected by tubes they construct and which feed on plankton using a crown of tentacled arms.

"Acorn worms and pterobranchs look so different from each other that understanding the origins of their evolutionary relationship has
been a major historical question in zoology," said Dr. Karma Nanglu,
Peter Buck Deep Time post-doctoral fellow at the Smithsonian National
Museum of Natural History and lead author on this paper. "Answering this question has been made much harder by the extreme lack of fossils of these soft-bodied hemichordates. Throughout the half-billion-year-long history
of hemichordates you can count on one hand the number of exceptional
preserved fossil species." Despite being just two centimeters in length,
the remarkably preserved soft tissues of the Gyaltsenglossus fossils
reveal incredibly detailed anatomical structures. These details include
the oval-shaped proboscis of acorn worms and a basket of feeding tentacles similar to those of pterobranchs. The age of these fossils, combined
with the unique morphological combination of the two major hemichordate
groups, makes this discovery a critical find for understanding early hemichordate evolution.



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"An ancient animal with an intermediary anatomy between acorn worms and pterobranchs had been hypothesized before but this new animal is the
clearest view of what the ancestral hemichordate may have looked like,"
says Dr.

Christopher Cameron, Associate Professor at the University of Montreal and
a co-author on this study. "It's exciting to have so many new anatomical details to help drive new hypotheses about hemichordate evolution."
In the case of Gyaltsenglossus, the exceptional preservation of these
fine details can be attributed to the unique environmental conditions of
the Burgess Shale, which rapidly entombed ancient animals in underwater mudslides. Through a combination of factors, including slowing the rate of bacteria decaying the entombed animals' bodies, the fossils of the Burgess Shale are preserved with far greater fidelity than typical fossil sites.

"The Burgess Shale has been pivotal in understanding early animal
evolution since its discovery over 100 years ago," says co-author
Dr. Jean-Bernard Caron, Richard M. Ivey Curator of Invertebrate
Palaeontology at the ROM and Associate Professor at the University of
Toronto. Dr. Caron led the field expedition in 2010 which collected the
33 fossils of Gyaltsenglossus.

"In most localities, you would be lucky to have the hardest parts of
animals, like bones and teeth, preserved, but at the Burgess Shale
even the softest body parts can be fossilized in exquisite detail,"
says Dr. Caron. "This new species underscores the importance of making
new fossil discoveries to shine light on the most stubborn evolutionary mysteries." In this particular case, Gyaltsenglossus suggests that the ancestral hemichordate may have been able to use the feeding strategies of
both of the modern groups. Like acorn worms, the long proboscis may have
been used to feed on nutrient-filled marine mud, while at the same time,
and like the pterobranchs, the array of six feeding arms was probably
used to grab suspended food particles directly from the water above
where it was crawling.

Hemichordates belong to a major division of animal life called
Deuterostomia, which includes chordates like fish and mammals, and not
the division of animal life called Protostomia, that includes arthropods
such as insects and annelids such as earthworms. Dr. Nanglu explains,
when looking at Gyaltsenglossus, we're actually looking at a very,
very distant relative of our own branch of vertebrate and human evolution.

"The close relationship between hemichordates and our own evolutionary
group, the chordates, is one of the first things that made me excited to research them," Nanglu explains. "Understanding the ancient connections
that join animals like fish and even humans to their distant cousins
like sea urchins and acorn worms is such an interesting area on the evolutionary tree and Gyaltsenglossus helps bring that link into focus
a little bit more clearly." The original 1909 discovery and research
about the Burgess Shale was made by Charles Walcott, who was Secretary
of the Smithsonian Institution at the time.

The Burgess Shale fossil sites are located within Yoho and Kootenay
National Parks and are managed by Parks Canada.


========================================================================== Story Source: Materials provided by Royal_Ontario_Museum. Note: Content
may be edited for style and length.


========================================================================== Journal Reference:
1. Karma Nanglu, Jean-Bernard Caron, Christopher B. Cameron. Cambrian
Tentaculate Worms and the Origin of the Hemichordate Body
Plan. Current Biology, 2020; DOI: 10.1016/j.cub.2020.07.078 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/08/200827141305.htm

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