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Rare Fossil Teeth from China Overturn Long-held Views about Evolution of Vertebrates

An international team of researchers has discovered 439-million-year-old remains of a toothed fish that suggest the ancestors of modern osteichthyans (ray- and lobe-finned fish) and chondrichthyans (sharks and rays) originated much earlier than previously thought.

Related findings were published in Nature on Sept. 28.

Rare Fossil Teeth from China Overturn Long-held Views about Evolution of Vertebrates. Fig. 1 Life reconstruction of Qianodus duplicis. (Image by ZHANG Heming)

A remote site in Guizhou Province of south China, containing sequences of sedimentary layers from the distant Silurian period (around 445 to 420 million years ago), has produced spectacular fossil finds, including isolated teeth identified as belonging to a new species (Qianodus duplicis) of primitive jawed vertebrate. Named after the ancient name for modern-day Guizhou, Qianodus possessed peculiar spiral-like dental elements carrying multiple generations of teeth that were added throughout the life of the animal.

The tooth spirals (or whorls) of Qianodus turned out to be one of the least common fossils recovered from the site. They are small elements that rarely reach 2.5 mm and as such had to be studied under magnification with visible light and X-ray radiation.

A conspicuous feature of the whorls is that they contained a pair of teeth rows set into a raised medial area of the whorl base. These so-called primary teeth show an incremental increase in size towards the inner (lingual) portion of the whorl. What makes the whorls of Qianodus unusual in comparison with those of other vertebrates is the clear offset between the two primary teeth rows. A similar arrangement of neighboring teeth rows is also seen in the dentitions of some modern sharks but has not been previously identified in the tooth whorls of fossil species.

The discovery indicates that the well-known jawed vertebrate groups from the so-called “Age of Fishes” (420 to 460 million years ago) were already established some 20 million years earlier.

“Qianodus provides us with the first tangible evidence for teeth, and by extension jaws, from this critical early period of vertebrate evolution,” said LI Qiang from Qujing Normal University.

Unlike the continuously shedding teeth of modern sharks, the researchers believe that the tooth whorls of Qianodus were kept in the mouth and increased in size as the animal grew. This interpretation explains the gradual enlargement of replacement teeth and the widening of the whorl base as a response to the continuous increase in jaw size during development.

For the researchers, the key to reconstructing the growth of the whorls was two specimens at an early stage of formation, easily identified by their noticeably smaller sizes and fewer teeth. A comparison with the more numerous mature whorls provided the palaeontologists with a rare insight into the developmental mechanics of early vertebrate dentitions. These observations suggest that primary teeth were the first to form whereas the addition of the lateral (accessory) whorl teeth occurred later in development.

Fig. 2 Volumetric reconstruction of a tooth whorl viewed from its lingual side (holotype of Qianodus duplicis). The specimen is just over 2 mm in length. (Image by Zhu, et al.)

“Despite their peculiarities, tooth whorls have, in fact, been reported in many extinct chondrichthyan and osteichthyan lineages,”said Plamen Andreev, the lead author of the study. “Some of the early chondrichthyans even built their dentition entirely from closely spaced whorls.”

The researchers claim that this was also the case for Qianodus. They made this conclusion after examining the small (1–2 mm long) whorls of the new species with synchrotron radiation—a CT scanning process that uses high energy X-rays from a particle accelerator.

“We were astonished to discover that the tooth rows of the whorls have a clear left or right offset, which indicates positions on opposing jaw rami,” said Prof. ZHU Min from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences.

Fig. 3 Virtual section along the length of a tooth whorl in side view (holotype of Qianodus duplicis). The specimen is just over 2 mm in length (Image by Zhu, et al.)

These observations are supported by a phylogenetic tree that identifies Qianodus as a close relative to extinct chondrichthyan groups with whorl-based dentitions.

“Our revised timeline for the origin of the major groups of jawed vertebrates agrees with the view that their initial diversification occurred in the early Silurian,” said Prof. ZHU.

The discovery of Qianodus provides tangible proof for the existence of toothed vertebrates and shark-like dentition patterning tens of millions of years earlier than previously thought. The phylogenetic analysis presented in the study identifies Qianodus as a primitive chondrichthyan, implying that jawed fish were already quite diverse in the Lower Silurian and appeared shortly after the evolution of skeletal mineralization in ancestral lineages of jawless vertebrates.

“This puts into question the current evolutionary models for the emergence of key vertebrate innovations such as teeth, jaws, and paired appendages,” said Ivan Sansom, a co-author of the study from the University of Birmingham.

Press release from the Chinese Academy of Sciences about the fossil teeth overturning long-held views on the evolution of vertebrates

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Dawn of Fishes — Early Silurian Jawed Vertebrates Revealed Head to Tail

A newly discovered fossil “treasure hoard” dating back some 436 million years to the early Silurian period reveals, for the first time, the complete body shape and form of some of the first jawed fishes.

The discovery was published in Nature on Sept. 28 by an international team led by Prof. ZHU Min from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences and Prof. Per E. Ahlberg from Uppsala University, as the cover story and one in a series of four papers in the same issue.

The Gnathostomata or jawed vertebrates, which include not only almost all the backboned animals you see in zoos and aquariums but humankind as well, have a mysterious origin. The so-called molecular clock, which deduces the age of the most recent common ancestor of two animals by evaluating the difference between the two sets of DNA, suggests that the most recent common ancestor of all modern jawed vertebrates lived 450 million years ago during the Ordovician period. As a result, the origin of jaws cannot be later than that.

However, the fossil record of jawed vertebrates only becomes abundant from the Early Devonian (~419 million years ago), i.e., the beginning of the “Age of Fishes.” Only in the past 10 years have scientists found several complete jawed fishes from the Late Silurian (~425 million years ago). Even so, these records are still more than 25 million years later than when jaws should have originated. The dearth of earlier fossils means that jawed vertebrates are a “ghost lineage” in the early Silurian.

The remarkable discovery of complete early Silurian jawed fishes is the result of 20 years of continuous effort by the authors searching for fossil fishes in all possible Silurian rock strata in China. The breakthrough was finally made in late 2020, when complete early Silurian fishes were found in Xiushan County, Chongqing.

LI Qiang and CHEN Yang, both co-authors and leaders of the fieldtrips, recalled their research:

“We remember it was a rainy day. We climbed a mountain ghat. At the 38^th turn we found a complete Silurian fish, which initiated an explosion of discoveries in this area in the next two years.”

Fig. 1 Life reconstruction of Xiushanosteus mirabilis (Image by ZHANG Heming)

The authors reported two species. The first one and the most abundant species was named Xiushanosteus mirabilis. It is a tiny, 3-cm-long placoderm or armored jawed fish. The flat and semicircular head, along with the trunk armor, are reminiscent of its jawless ancestors, but its paired fins and powerful tail made Xiushanosteus a much more capable swimmer.

First author ZHU You’an, associate research professor at IVPP and also an Uppsala University alumnus, said,

“As a placoderm expert, I am dazzled by the early age and completeness of Xiushanosteus. It is like a dream. A lot of the anatomical features make perfect sense; it was an ‘Oh, now I know’ moment in my career.”

Fig. 2 Life reconstruction of Shenacanthus vermiformis (Image by ZHANG Heming)

The second fish reported is named Shenacanthus vermiformis. Also very small, it is an early shark relative. However, all the sharks we know are covered in tiny scales, or at most small mosaic plates. Shenacanthus instead has prominent “shoulder armor” made of several large plates that completely encircle its body. This feature, thought to be exclusive in placoderms, provides a strong hint that the first cartilaginous fishes were armored, similar to placoderms.

“Only 20 years ago it was still believed that sharks are primitive and other jawed fish evolved from a shark-like archetype. Now with the discovery of Shenacanthus, we can finally make certain that the opposite is true,” said Prof. ZHU You’an.

“Previously we could only dream of such exceptional and early fossils,” said corresponding author Prof. Ahlberg. “However, they are more than curiosities; they are first and foremost crucial data to test—and either support or confound—our long-held hypotheses regarding the rise of our lineage.”

“The excavation continues to yield remarkable materials,” said Prof. ZHU Min, who led the project and is also a CAS academician. “The Chongqing Lagerstätte, like the Chengjiang and Jehol biotas, will become a world-famous paleontological heritage and will provide key evidence for how the extraordinary diversity of the jawed vertebrates we see today arose.”

Press release from the Chinese Academy of Sciences

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