Finding rare fossilized comb jelly reveals new gaps in fossil record
They look like jellyfish but they are not. They appear harmless but are efficient predators – sometimes they even eat fish. They are gelatinous and very delicate – and they fossilize extremely rarely!
Ctenophores, also known as comb jellies, are colorful, translucent animals that drift in ocean waters. Unlike jellyfish, ctenophores do not have stinging cells and usually capture their prey using long, sticky tentacles.
Our research describing a fossilized ctenophore from eastern Canada, recently published in Scientific reports, suggests that our creature was a very late survivor of the very dawn of the animals. 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 live ctenophores, and many are locally abundant. Some well-known modern comb jellies include gooseberry (Pleurobrachia pileus) found in the open waters of the North Atlantic Ocean, North Sea, Baltic Sea and Black Sea, and the ribbon-shaped belt of Venus (Cestum veneris) that can be observed in tropical and subtropical oceans around the world.
However, their delicate bodies generally lack hard parts, which means that very few fossil ctenophores have been preserved and discovered: only a dozen species have been found in the world. The fossilization of these soft-bodied animals requires exceptional conditions such as very rapid burial with very fine sediments in an oxygen-poor aquatic environment, which suppresses the activities of decaying organisms and scavengers. Other environmental parameters also play an important role in preservation.
Until the early 1980s, comb jellies were unknown in the fossil record. The first comb jelly fossil to be discovered comes from German slate from the Lower Devonian Hunsrück, deposited around 405 million years ago.
Since then, records of spectacularly preserved early parents of comb jellies have been described in the 518 million year old Chengjiang biota in southern China, the 505 million year old Burgess Shale in British Columbia. in western Canada and other similar deposits.
In August, two new species of Cambrian comb jellies were also reported in Utah. Our new fossil, named Daihuoides jakobvintheri, adds considerably to this meager record.
Strange anatomical symmetry
Most living ctenophores have a translucent spherical or cylindrical body, frequently showing brightly colored bioluminescence, vaguely reminiscent of colored disco mirror balls. Most use a pair of long tentacles, armed with sticky, non-venomous cells (colloblasts), to trap small prey and transport them to their mouths on top of their bodies.
Ctenophores are propelled using rows of combs: beating hairs (cilia) organized in longitudinal bands. The presence, number and organization of these rows of combs are taxonomically important. The only specimen of our fossil Daihuoides reveals a circular disc-shaped body (chalice), about six centimeters in diameter, with 18 rows of radiating combs, each distinguished by a clear zigzag pattern.
The presence of rows of combs allowed us to identify this fossil as a ctenophore, but their high number was puzzling. This number is unusual in a living ctenophore, but quite common in very ancient Cambrian ctenophores. Cambrian comb jellies of the Chinese fauna of Chengjiang, belonging to the genera Daihua, Xianguangia and Dinomischus, share a symmetry based on hexaradiate, i.e. being six times or a multiple thereof, such as 18 times.
Our new fossil comes from the well-documented Devonian fossil site of Miguasha along the south coast of the Gaspé Peninsula in eastern Canada.
It is a UNESCO World Heritage Site because it preserves an exceptional diversity of primitive fish, including a form of transition between fish and terrestrial vertebrates (tetrapods). This fossil treasure, 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 almost complete skeletons with most of the bones still in place.
Unlike the plethora of fish, invertebrates are rare and less diverse. In fact, only 10 species have been found. Most of them are only known to a handful of specimens and are primarily arthropods (hard-bodied invertebrates with jointed legs, represented today by things like crabs and insects).
The base of the tree of life
The Cambrian explosion refers to the almost simultaneous appearance of the main groups of animals in the fossil record, between 540 and 520 million years ago.
Read more: Exquisite fossil finds shed new light on ‘Cambrian explosion’, when oceans first filled with complex animal life
Before that, animals were very simple and largely microscopic, but in a geological blink of an eye, most modern animal phyla (metazoa) have appeared, including arthropods, mollusks, and vertebrates. Ctenophores have long been thought to be found near the base of the animal tree of life, resembling other primitive forms such as cnidarians (corals and jellyfish). Sponges appear primitive because they lack a nervous system and organized tissue, and they only have a few types of cells.
Ctenophores and cnidarians, despite their relative simplicity, are much more complex than sponges, so it was traditionally assumed that sponges were the absolute basis of the animal family tree – the “first sponge hypothesis” .
However, some recent genomic studies have suggested that comb jellies are actually even lower in the family tree than sponges, a “cenophores first” hypothesis. This radical idea remains highly controversial as sponges have been thought to be more primitive than ctenophores for over 150 years.
Read more: Is our furthest animal parent a comb sponge or jelly? Our study provides an answer
If true, it could mean that many traits that ctenophores share with typical animals (such as a nervous system, intestines, and complex muscles) could have evolved twice: once in comb jellies and separately. in all other animals.
Comb jellies are said to be true evolutionary aliens compared to all other animals.
In the light of our discovery, we tested whether the anatomy of the fossil ctenophores better supported the hypothesis of sponges first or ctenophores first. Surprisingly, and unlike a previous study, the fossils were also consistent with both ideas.
Fossil of Lazarus
According to the Bible, Jesus brought Lazarus from Bethany back to life four days after his death. In paleontology, a “Lazarus taxon” is an organism which disappears from the fossil record for a long time, only to reappear much later.
Our new fossil ctenophore, Daihuoides, is a perfect example of such a taxon of Lazarus and postdates its Cambrian parents by over one hundred million years. Our creature looks like a primitive type of ctenophore with 18 sets of organs arranged radially. These forms were known since the Cambrian (over 500 million years ago) and believed to have died out soon after.
Daihuoides shows that these primitive comb jellies survived for another 140 million years, until the Devonian, about 375 million years ago. This discovery demonstrates the huge gaps in the known fossil record and implies that many wonderful fossils remain to be discovered.
Johanne Kerr, researcher at Miguasha National Park, is co-author of this article.