My goodness, what small teeth they had.
A new 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. . Dating from 439 million to 436 million years ago, the fossil site includes a revealing array of never-before-seen small, toothy, and bony fish species.
The diversity of fossils at this site not only fills an obvious gap in the fossil record, but it also highlights the strangeness that such a gap exists, researchers report September 29 in Nature.
Genetic analyzes had previously pointed to this time period, known as the Early Silurian Period, as an era of rapid diversification of jawed vertebrates. But toothy fish seemed to have left few traces in the fossil record. Instead, as far as the fossil record goes, jawless fish seemed to rule the waves at the time. And the jawed fish that have survived 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 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 has only one chondrichthyan.
The first creatures in develop a backbone they were fish, and they did so about 480 million years ago (Serial number: 10/25/18). Genetic analyzes have suggested that around 450 million years ago, those fish also evolved jaws, the better for biting each other. But the first complete fossils of such jawed fish appear relatively late in the fossil record, about 425 million years ago. By the Devonian Period, which stretched from 419 million to 359 million years ago, jawed fish were a global phenomenonearning at that time the nickname “Era of the Fishes” (Serial number: 7/17/18).
Here’s a closer look at some of the newly discovered fishy denizens of the Chongqing Lagerstätte.
small but fierce
About 20 separate specimens of a small fish that researchers have named Xiushanosteus mirabilis they were found at the Chongqing site. Those findings make the animal the most abundant type of fish in that fossil assemblage.
X. mirabilis it was only about 30 millimeters long, about the length of a paper clip, but it bears a strong resemblance to the larger armored placoderms to come in the future: it had a broad, bony shield on its head and a body covered in small, shaped scales. of diamond .
The surprising abundance of this type of fish at an early Silurian Period site could be due to lucky fossilization conditions: the small, delicate bones of X. mirabilis and the other jawed fishes found in Chongqing would be more difficult to preserve than the larger jawless specimens of the time, or the more robust toothy bony fishes of the later Devonian Period. But another possibility is that this site is an outlier in its time that turned out to be popular with placoderms.
A tiny heavily armored shark.
Two types of jawed fish arose about 450 million years ago, and both appear at the Chongqing site. The new site is notable for its diversity of osteichthyans, bony-jawed fish such as X. mirabilis. but cartilaginous Shenacanthus vermiformis He also spent some time in this environment.
S. vermiform is represented by a single specimen in Chongqing, but like X. mirabilis, is excellently preserved from head to tail. It was also tiny, only 22 millimeters long. Although it had a similar body plan to other chondrichthyans, it differed in one key way: how X. mirabilis, S. vermiform it was heavily armored, with extensive plating to the bottom and rear.
a time of transition
The Chinese site not only sheds light on ancient jawed fish, but offers a window into the evolutionary transition of body features from jawless to jawed species. A newly discovered jawless creature, dubbed Tujiaaspis vividusturns out to be closely related to a group of jawed fish called galleaspidsthe researchers report in a separate article in the same issue of Nature.
Well-preserved fossils of T. vividus open up new opportunities to learn how their mandibular relatives acquired their fin arrangements, a transition for which there has been little prior evidencewrites Matt Friedman, a paleontologist at the University of Michigan in Ann Arbor, in a commentary in the same issue of Nature. This is because galleaspids have distinctive head shields, but scientists have not been able to look under these fossilized shields to study the hidden anatomy.
Thanks to these close relatives, the researchers pieced together how the paired fins in jawless fishes 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 (Serial number: 05/30/18).