Ancient fish highlight the strangeness of our vertebrate ancestors

Wow, what little teeth they had.

A newly discovered trove of ancient fish fossils found in southern China opens a window into the earliest history of jawed vertebrates, a group that comprises 99 percent of all living vertebrates on Earth, including humans. The fossils, dated to 439 million to 436 million years ago, include a revealing variety of previously unseen small, toothy, bony fish species.

The diversity of fossils at this site not only fills an obvious gap in the fossil record, but also highlights the oddity that such a gap exists, researchers reported Sept. 29 in Nature.

Genetic analyzes previously pointed to this time period, known as the Early Silurian, as an era of rapid diversification of jawed vertebrates. But toothed fish seem to have left little trace in the fossil record. Instead, as far as the fossil record is concerned, jawless fish seem to rule the waves at this time. And what jawed fishes have been preserved have rarely been bony; most were chondrichthyans, ancient cartilaginous ancestors of modern sharks and rays.

The Chongqing Lagerstätte — paleontologists’ word for a rich array of diverse species all preserved together in one place — “fundamentally changes this picture,” paleontologist You-an Zhu of the Chinese Academy of Sciences in Beijing and colleagues wrote in the study. The site teems with toothy, bony fishes, especially armored placoderms, but yields only one chondrichthyan.

rock slab containing fossils of an ancient relative of the shark and stingray and an armored jawed fish with labels indicating the location of the fossils
The Chongqing Lagerstätte, dated to 439 million to 436 million years ago, is a newly discovered diverse assemblage of fossils bearing some of the earliest jawed fish. This plaque from the site contains two newly named species of fish, the ancient relative of the shark and the stingray Shenacanthus vermiformis (1a and 1b) and armored jawed fish, Xiushanosteus mirabilis (2a and 2b).Y.-a. Ju et al/Nature 2022

The first creatures to develop a backbone were fish, and they did so about 480 million years ago (SN: 10/25/18). Genetic analyzes suggest that around 450 million years ago, these fish also developed jaws to better bite each other. But the earliest complete fossils of such jawed fish appear relatively late in the fossil record, about 425 million years ago. By the Devonian period, which spanned from 419 million to 359 million years ago, jawed fishes were a global phenomenon, earning the era the nickname “The Age of Fishes” (SN: 7/17/18).

Here’s a closer look at some of the newly discovered fish inhabitants of the Chongqing Lagerstätte.

A little, but cruel

About 20 individual specimens of a small fish that researchers have named Xiushanosteus mirabilis were discovered at the Chongqing site. These findings make the animal the most common type of fish in this group of fossils.

X. mirabilis it was only about 30 millimeters long, about the length of a paperclip, but it bore a strong resemblance to the larger armored placoderms that would appear in the future: it had a broad, bony head shield and a body covered in small, diamond-shaped scales.

illustration of Xiushanosteus mirabilis
The head-to-tail preservation of fossils of Xiushanosteus mirabilis (shown in an artist’s rendering) allowed the researchers to reconstruct the fish’s anatomy in detail.Hemming Zhang

The surprising abundance of this type of fish at an early Silurian site may simply be due to fortunate fossilization conditions—the small, delicate bones of X. mirabilis and the other jawed fishes found at Chongqing would have been more difficult to preserve than the larger jawless specimens of that time or the more robust toothed bony fishes of the later Devonian period. But another possibility is that this site is an outlier for its time that simply became popular among placoderms.

A heavily armored, miniature shark

Two species of jawed fish appeared about 450 million years ago – both appearing in the Chongqing area. The new site is notable for its variety of osteichthyans, fish with bony jaws X. mirabilis. But cartilaginous Shenacanthus vermiformis also spent some time in this environment.

reconstruction of Shenacanthus vermiformis
Shenacanthus vermiformis (shown in this reconstruction) was a small but armored cartilaginous fish found alongside its bony neighbors in the new Chinese fossil site.Hemming Zhang

S. vermiformis is represented by only one specimen at Chongqing, but as X. mirabilis, is perfectly preserved from head to tail. It was also small, only 22 millimeters long. Although it had a similar body plan to other chondrichthyans, it differed in one key way: as X. mirabilis, S. vermiformis it was heavily armored, with extensive plates on the underside and back.

Transition time

The Chinese site doesn’t just shed light on ancient jawed fish—it offers a window into the evolutionary transition of body features from jawless to jawed species. A newly discovered jawless creature duplicated A living foreignerturns out to be closely related to a group of jawed fish called galeaspids, the researchers report in a separate paper in the same issue of Nature.

reconstruction of Tujiaaspis vividus
Excellent preservation of jawless fish A living foreigner (shown in this reconstruction) offers new insights into fin evolution in its later jawed cousins.Hemming Zhang

The well-preserved fossils of T. lively open up new opportunities to learn how its jaw relatives acquired fin placement, a transition for which there was little previous evidence, writes Matt Friedman, a paleontologist at the University of Michigan in Ann Arbor, in a commentary in the same issue of Nature. This is because galeaspids have distinctive head shields, but scientists have so far been unable to peer beneath these fossilized shields to study the hidden anatomy.

Thanks to these close relatives, the researchers pieced together how paired fins in jawless fish evolved in stages to become separate pectoral and pelvic fins in their jawed cousins. Such fins are the forerunners of arms and legs in later tetrapods (SN: 05/30/18).

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