Today, four new ones paper published in Nature continues this tradition by revealing the world’s oldest well-preserved jawfish, dating between 436 million and 439 million years ago to the beginning of the Silurian period.
The fossil finds are all from new fossils in Guizhou and Chongqing provinces in China. The Chongqing site was found in 2019, when three young Chinese paleontologists were fighting, and one was kicked into the rock with kung fu. Rocks tumbled down revealing a spectacular fossil inside.
The research teams behind the papers are led by: Zhu Mino from the Institute of Vertebrate Paleontology and Paleoanthropology in Beijing. Min told me: “The discovery of the Chongqing bearing status (a “lagerstatte” is a fossil site that is exceptionally well preserved) is indeed an incredible marvel of fossil hunting. Suddenly we realized that we had found an overwhelming lagerstatte. We are now close to the crux of untangling the fish tree of early jawed vertebrates.”
What kind of fish were they?
Most fish today fall into two main groups:
- chondrichthyans (including sharks, rays, and chimaerids) have cartilaginous skeletons
- osteichthyans (bony fish such as trout) have bone that forms the skeleton.
The origin of these living fish groups is now much clearer with new finds of the oldest complete fish from China.
These were shark-like fish. Some were placodermenan extinct class of armored fish with bony plates that formed a solid shield around the head and torso.
Others were ancestral species of sharks called acanthodians. These are extinct forms of “stock sharks” that evolved as a separate branch — or tribe — of the evolutionary lineage that led to modern sharks.
Placoderms are the earliest known jawed vertebrates. It is important to examine them, as they help reveal the origin of many parts of the human body (including our heart and face).
A small flattened placoderm called Xiushanosteusabout three inches long, is the most common fish at the new Chongqing site.
Its skull shows paired bones that mirror those on our own heads. Frontal and parietal bones originate in these fish. Zhu You-an, who led the research on these fishes, told me: “All things are still like dreams. Today we are staring at complete early Silurian fish, 11 million years earlier than the previous oldest finds! Both of these are the most exciting.” , as well as the most challenging fossils I’ve had the privilege of working on!”
The oldest sharks and teeth in the world
The new papers also describe the oldest complete shark-like fish, called shenacanthus. It has a body shape similar to other prehistoric ones acanthodians (or stem sharks) – but differs in having thick plates that form armor around them, as seen in placoderms.
The fact that Shenacanthus shares the characteristics of both acanthodians and placoderms suggests that these two groups evolved from similar ancestral tribes. That said, Shenacanthus retains typical shark-like fin spines, so it’s not considered a placoderm, but a chondrichthyan (the group including today’s cartilaginous sharks).
The research also reveals the oldest known teeth of any vertebrate — at least 14 million years older than any previous findings. Originating from a fossil chondrichthyan called Qianodus, the teeth are arranged as coiled rows called “wreaths.” Such ring gears are common at the junction of the jaws in many ancient sharks and some early bony fish such as Onychodus.
The researchers also found another early tribal shark called Fangjinshania at the new site in Giuzhou. More than 300 kilograms of rock were collected and dissolved in weak acetic acid to free thousands of microscopic pieces of bone and teeth.
Fangjinshania resembles a tribal shark called Climatius that is known to have lived in Europe and North America about 30 million years later. Fangjinshania lived as far back as 436 million years ago, which tells us that the fossil record of such sharks is much older than we previously thought.
Both Fangjinshania and Qianodus were about 10 to 15 cm long, making them many times larger than the placoderms and the Shenacanthus. They would have been the best predators in their ancient ecosystem and the world’s first predators armed with sharp teeth.
Plamen Andreev, the lead author of two of the new papers, told me, “These new finds support the idea that older fossil shark-like scales found in the Ordovician period can now really be called sharks.”
From fins to limbs
Another interesting discovery from these fossils concerns how paired limbs in vertebrates first evolved. A new jawless fish called Tujiiaspis now shows the primitive state of paired fins before they separated into pectoral and pelvic fins — the precursor to arms and legs.
Pectoral fins were thought to have evolved into jawless fish called osteostracans, then pelvic fins later in placoderms. But the new Tujiiaspis fossil suggests that both sets of fins may have evolved on the at the same time of fin folds which run along the body and end at the caudal fin.
When was the first irradiation of the jawfish?
Finally, all of these discoveries reveal that the first major “radiation” from the vertebrate jaws (referring to an explosion in diversity) happened much earlier than anyone could have imagined. Ivan Sansom of the University of Birmingham co-authored one of the articles. As Sansom points out, “We’ve had hints of older material before, but the appearance of clearly defined remains of vertebrate jaws so close to the base of the Silurian suggests that jaws and jawless fish coexisted longer than previously thought. evidence for an earlier radiation of sharks and other jawfish in the Ordovician period.”
The four papers have shaken up the evolutionary tree, and new diagrams show revised hypotheses about the relationships between living fish. Zhu Min told me it will take many years to complete the studies on the new fossils, and several new species have not yet been described in the papers.
We will have to wait patiently for the next exciting discoveries to be announced at these extraordinary fossil sites.
Dawn of fishes: Early Silurian jaws of vertebrates revealed from head to tail
Quote: A kung fu kick led researchers to the world’s oldest complete fish fossils. Here’s What They Found (2022, October 1), retrieved October 1, 2022 from https://phys.org/news/2022-09-kung-fu-world-oldest-fish-fossils.html
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