Ancient fish fossils highlight the strangeness of our vertebrate ancestors

My, what small tooth that they had.

A newfound treasure trove of historic fish fossils unearthed in southern China is opening a window into the earliest historical past of jawed vertebrates — a gaggle that encompasses 99 p.c of all dwelling vertebrates on Earth, together with people. The fossil website, dated from 439 million to 436 million years in the past, features a revealing number of never-before-seen small, toothy, bony fish species.

The range of the fossils at this one website not solely fills a glaring gap within the fossil report, but additionally highlights the strangeness that such a spot exists, researchers report September 29 in Nature.

Genetic analyses had beforehand pointed to this time interval, generally known as the early Silurian Interval, as an period of fast diversification of jawed vertebrates. However the toothy fishes appeared to have left few traces within the fossil report. As a substitute, so far as the fossil report was involved, jawless fishes appeared to rule the waves on the time. And what jawed fishes have been preserved had been not often bony; most have been chondrichthyans, historic cartilaginous ancestors of recent sharks and rays.

The Chongqing Lagerstätte — paleontologists’ phrase for a wealthy assemblage of various species all preserved collectively at one website — “essentially adjustments that image,” write paleontologist You-an Zhu of the Chinese language Academy of Sciences in Beijing and colleagues within the research. The location is teeming with toothy, bony fishes, significantly armored placoderms, however bears only one chondrichthyan.

a rock slab containing fossils of an ancient shark and ray relative and an armored jawed fish with labels to indicate fossil locations
The Chongqing Lagerstätte, dated from 439 million to 436 million years in the past, is a newly found various assemblage of fossils bearing a number of the earliest jawed fishes. This slab from the location incorporates two newly named fish species, the traditional shark and ray relative Shenacanthus vermiformis (1a and 1b) and an armored jawed fish, Xiushanosteus mirabilis (2a and 2b).Y.-a. Zhu et al/Nature 2022

The primary creatures to develop a backbone had been fish, they usually did it round 480 million years in the past (SN: 10/25/18). Genetic analyses have urged that by about 450 million years in the past, these fish additionally developed jaws, the higher to chomp one another with. However the earliest full fossils of such jawed fish seem comparatively late within the fossil report, about 425 million years in the past. By the Devonian Interval, which spanned from 419 million to 359 million years in the past, jawed fishes were a global phenomenon, incomes that period the nickname “Age of Fishes” (SN: 7/17/18).  

Right here’s a more in-depth have a look at a number of of the newly found fishy denizens of the Chongqing Lagerstätte.

Little however fierce

About 20 separate specimens of a bit of fish that the researchers have referred to as Xiushanosteus mirabilis had been discovered on the Chongqing website. These finds make the animal probably the most ample kind of fish in that fossil assemblage.

X. mirabilis was solely about 30 millimeters lengthy, concerning the size of a paper clip, but it surely bears a powerful resemblance to bigger armored placoderms to come back sooner or later: It had a broad, bony head protect and a physique coated in small, diamond-shaped scales.

illustration of Xiushanosteus mirabilis
The top-to-tail preservation of fossils of Xiushanosteus mirabilis (proven in an artist’s rendition) has allowed researchers to reconstruct the fish’s anatomy intimately.Heming Zhang

The shocking abundance of any such fish at a website from the early Silurian Interval would possibly simply be because of fortunate fossilization situations — the small, delicate bones of X. mirabilis and the opposite jawed fishes discovered at Chongqing can be tougher to protect than the bigger jawless specimens of the time, or the extra strong toothy bony fishes of the later Devonian Interval. However one other chance is that this website is an outlier in its time that simply occurred to be fashionable with the placoderms.

A closely armored, diminutive shark

Two forms of jawed fish arose round 450 million years in the past — and each make an look on the Chongqing website. The brand new website is outstanding for its variety of osteichthyans, bony jawed fishes like X. mirabilis. However cartilaginous Shenacanthus vermiformis additionally spent a while on this atmosphere.

reconstruction of Shenacanthus vermiformis
Shenacanthus vermiformis (proven on this reconstruction) was a tiny, however armored cartilaginous fish discovered alongside its bony neighbors on the new Chinese language fossil website.Heming Zhang

S. vermiformis is represented by solely a single specimen at Chongqing, however like X. mirabilis, it’s excellently preserved from head to tail. It was diminutive too, simply 22 millimeters lengthy. Although it had an identical physique plan to different chondrichthyans, it did differ in a single key approach: Like X. mirabilis, S. vermiformis was closely armored, with in depth plates on its underside and again.

A time of transition

The Chinese language website isn’t simply shedding mild on historic jawed fish — it provides a window into the evolutionary transition of physique options from jawless to jawed species. One newly found jawless creature, dubbed Tujiaaspis vividus, seems to be intently associated to a group of jawed fishes called galeaspids, researchers report in a separate paper in the identical problem of Nature.

reconstruction of Tujiaaspis vividus
The wonderful preservation of the jawless fish Tujiaaspis vividus (proven on this reconstruction) is providing new insights into the evolution of fins in its later jawed cousins.Heming Zhang

The well-preserved fossils of T. vividus open up new alternatives to find out how its jawed kinfolk acquired their preparations of fins, a transition for which there has been little previous evidence, writes Matt Friedman, a paleontologist on the College of Michigan in Ann Arbor, in a commentary in the identical problem of Nature. That’s as a result of galeaspids have distinctive head shields, however scientists haven’t beforehand been capable of peer beneath these fossilized shields to review the hidden anatomy.

Thanks to those shut kinfolk, the researchers pieced collectively how paired fins within the jawless fish advanced in phases to develop into separate pectoral and pelvic fins of their jawed cousins. Such fins are the precursors of legs and arms in later tetrapods (SN: 5/30/18).

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