They look like jellyfish but they are not. They seem unpretentious but are skilled hunters – sometimes, they even eat fish. They are gelatinous and very fragile – and they rarely fossilize!
Ctenophores, also known as comb jellies, are colorful, translucent animals that float through ocean water. Unlike jellyfish, ctenophores do not have stinging cells, and typically capture prey using long, sticky tentacles.
Our research describing a fossil ctenophore from eastern Canada recently published scientific report, shows that our creature was alive very late from the dawn of the beasts. It also means that a very controversial idea about early animal evolution cannot be dismissed by the fossil record.
Common today but rare as a fossil
There are about 200 species of living ctenophores, and many are locally abundant. Some well-known modern comb jellies include the sea gooseberry (sea gooseberry).pleurobrachia pilus) is found in open waters in the North Atlantic Ocean, the North Sea, the Baltic Sea and the Black Sea, and the ribbon-like Venus girdle (sestum veneris) can be seen in tropical and subtropical oceans worldwide.
However, their delicate bodies generally lack hard parts, meaning that very few fossil ctenophores have been preserved and discovered: only about a dozen species have been found globally. The fossilization of these soft-bodied animals requires exceptional conditions such as very rapid burial in an oxygen-poor aquatic environment with very fine sediments, which suppress the decomposition and scavenging activities of the organisms. Other environmental parameters also play an important role in conservation.
Until the early 1980s, comb jellies were unknown from the fossil record. The first comb jelly fossil to be discovered came from the Early Devonian Hunsrück slate in Germany, deposited about 405 million years ago.
Since then, records of superbly preserved early relatives of comb jellies were described from the 518-million-year-old Chengjiang Biota in southern China, the 505-million-year-old Burgess Shale of British Columbia in western Canada, and other similar deposits.
In August, two new species of Cambrian comb jellies were also reported from Utah. Our new fossil, named dihuoides jacobwinteri, adds substantially to this low record.
strange body symmetry
Most living ctenophores have a translucent spherical or cylindrical body, often showing brightly colored bioluminescence, reminiscent of colored disco mirror balls. Most use a pair of long tentacles equipped with non-venomous sticky cells (coloblasts) to trap smaller prey and deliver them to their mouths at the top of their bodies.
Ctenophores propel themselves using comb rows: pulsating hairs (cilia) arranged in longitudinal bands. The appearance, number and organization of these comb rows are taxonomically important. single specimen of our fossil daihuoids Reveals a circular disc-shaped body (calyx), about six centimeters in diameter, with 18 radiating comb rows, each distinguished by a clear zigzag pattern.
The presence of comb rows allowed us to identify this fossil as a ctenophore, but their high numbers were puzzling. This number is unusual in a living ctenophore, but rather common in very ancient Cambrian ctenophores. Cambrian comb jelly from the Chinese Chengjiang fauna, belonging to the genera dahua, xianguangia And denomyscus, share a hexaradiate-based symmetry, meaning six-fold or multiples thereof, such as 18-fold.
Our new fossil comes from the well-documented Devonian fossil site off Migusha along the southern coast of the Gaspe Peninsula in eastern Canada.
It is a UNESCO World Heritage Site because it preserves an extraordinary diversity of early fishes, including a transitional form between fish and land vertebrates (tetrapods). This group of fossils, known as the Escuminac Assemblage, is 375 million years old – and was once an estuary near the equator!
Since 1842, more than 21,000 fossil fish belonging to 20 different species have been found. Many of these fossils represent nearly complete skeletons with most of the bones still present.
Unlike the plethora of fish, invertebrates are rarer and less diverse. In fact, only 10 species have been found. Most of them are known from only a handful of specimens, and mainly arthropods (hard-bodied invertebrates with articulated legs, today represented by things like crabs and insects).
base of the tree of life
The Cambrian Explosion shows the near-simultaneous appearance of major groups of animals in the fossil record from 540 to 520 million years ago.
Read more: Exquisite fossil sheds new light on ‘Cambrian explosion’, when oceans first filled with complex animal life
Before this, animals were very simple and largely microscopic, but in the geological blink of an eye, most modern animals (metazoans) appeared, including arthropods, mollusks and vertebrates. Ctenophores have long been thought to be near the base of the animal tree of life, resembling other primitive forms such as cnidarians (corals and jellyfish). Sponges look primitive because they lack a nervous system and organized tissues, and they have only a few types of cells.
Ctenophores and cnidarians, despite their relative simplicity, are much more complex than sponges, so it was traditionally believed that sponges were the absolute basis of the animal family tree—the “sponge-first hypothesis.”
However, some recent genomic studies have proposed that comb jellies are actually even lower on the family tree than sponges, a “Ctenophores-first” hypothesis. This radical idea remains highly controversial as sponges have been considered more primitive than ctenophores for over 150 years.
Read more: Is our furthest animal relative to sponges or comb jellies? Our study provides an answer
If true, this could mean that many traits shared with normal animals (such as nervous system, gut and complex muscles) ctenophores may have evolved twice: once in comb jellies and separately in all other animals. From.
Comb jellies would be true evolutionary aliens compared to all other animals.
In the light of our finding, we tested whether the anatomy of fossil ctenophores better supports the sponge-first or ctenophores-first hypothesis. Strikingly, and in contrast to the previous study, the fossils were equally compatible with both views.
According to the Bible, Jesus resurrected Bethany’s Lazarus four days after her death. In paleontology, a “Lazarus taxon” is an organism that disappears from the fossil record for a long period of time, only to reappear much later.
our new fossil ctenophore, daihuoids, is a perfect example of such a Lazarus taxon and postdates its Cambrian relatives by more than a hundred million years. Our organism resembles a primitive type of cetenophore with 18 sets of radially arranged organs. These forms were known from the Cambrian (more than 500 million years ago) and then were believed to soon become extinct.
daihuoids suggests that these primitive comb jellies lived about 375 million years ago in the Devonian and for 140 million years. The discovery demonstrates a huge gap in the known fossil record, and implies that many amazing fossils remain to be discovered.
Parc National de Miguasha researcher Johan Kerr is a co-author of this article.