Evolution: more on finding jawed fish (Evolution)

by David Turell , Wednesday, September 28, 2022, 22:26 (577 days ago) @ David Turell

A huge find in China:

https://www.sciencenews.org/article/fish-fossils-china-jaws-vertebrates

"A newfound treasure trove of ancient fish fossils unearthed in southern China is opening a window into the earliest history of jawed vertebrates — a group that encompasses 99 percent of all living vertebrates on Earth, including humans. The fossil site, dated from 439 million to 436 million years ago, includes a revealing variety of never-before-seen small, toothy, bony fish species.

"The diversity of the fossils at this one site not only fills a glaring gap in the fossil record, but also highlights the strangeness that such a gap exists, researchers report September 29 in Nature.

"Genetic analyses had previously pointed to this time period, known as the early Silurian Period, as an era of rapid diversification of jawed vertebrates. But the toothy fishes seemed to have left few traces in the fossil record. Instead, as far as the fossil record was concerned, jawless fishes appeared to rule the waves at the time. And what jawed fishes have been preserved were rarely bony; most have been chondrichthyans, ancient cartilaginous ancestors of modern sharks and rays.

"The Chongqing Lagerstätte — paleontologists’ word for a rich assemblage of diverse species all preserved together at one site — “fundamentally changes that picture,” write paleontologist You-an Zhu of the Chinese Academy of Sciences in Beijing and colleagues in the study. The site is teeming with toothy, bony fishes, particularly armored placoderms, but bears just one chondrichthyan.

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"Here’s a closer look at a few of the newly discovered fishy denizens of the Chongqing Lagerstätte.

"About 20 separate specimens of a little fish that the researchers have called Xiushanosteus mirabilis were found at the Chongqing site. Those finds make the animal the most abundant type of fish in that fossil assemblage.

X. mirabilis was only about 30 millimeters long, about the length of a paper clip, but it bears a strong resemblance to larger armored placoderms to come in the future: It had a broad, bony head shield and a body covered in small, diamond-shaped scales.

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"Two types of jawed fish arose around 450 million years ago — and both make an appearance at the Chongqing site. The new site is remarkable for its diversity of osteichthyans, bony jawed fishes like X. mirabilis. But cartilaginous Shenacanthus vermiformis also spent some time in this environment.

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"Thanks to these close relatives, the researchers pieced together how paired fins in the jawless fish evolved in stages to become separate pectoral and pelvic fins in their jawed cousins. Such fins are the precursors of arms and legs in later tetrapods. "
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Comment: irritating gaps get filled, but the general big picture is clear.