Oh, how tiny were their teeth.
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 is dated between 439 million and 436, million years ago. It contains an amazing variety of tiny, toothy, boney fish species that have never been seen before.
The variety of fossils found at this site is not only remarkable, but also incredibly important. Fills a gapResearchers report September 29 in Nature.
Genetic analysis had previously indicated that this period, called the early Silurian Period was an era of rapid diversification in jawed vertebrates. The fossil record of toothy fishes seems to be sparse. Jawless fishes seemed to have ruled the waves in that time period, according to the fossil records. What jawed fishes were preserved are rarely bony. Most have been chondrichthyans. These ancient cartilaginous ancestors of modern sharks, rays, and other marine life.
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. Although the site is full of toothy, bony fishes and armored placoderms it only has one chondrichthyan.
The first creatures to Develop a backboneThey were fish and did it approximately 480 million years ago.SN: 10/25/18). Genetic analysis has shown that these fish developed jaws around 450 million years old, which allowed them to chew each other. The fossil record shows that the oldest complete fossils of jawed fish are found around 425 million years old. By the Devonian Period which spans from 419 to 359 millions years ago. Jawed fishes have been a global phenomenon, earning that era the nickname “Age of Fishes” (SN: 7/17/18).
Here’s a closer look at a few of the newly discovered fishy denizens of the Chongqing Lagerstätte.
It is fierce, but little.
The researchers collected about 20 specimens of a small fish and called them “The Little Fish”. Xiushanosteus mirabilisThey were discovered at the Chongqing site. These discoveries make this animal the most prevalent type of fish in that fossil group.
X. mirabilisAlthough it was just 30 millimeters in length, roughly the same length as a paperclip, it bears a strong resemblance with larger armored placoderms in the future. It had a large, bony head shield, and a body covered with small, diamond-shaped scales.
The surprising abundance of this type of fish at a site from the early Silurian Period might just be due to lucky fossilization conditions — the small, delicate bones of X. mirabilisChongqing’s jawed fishes would be more difficult to preserve that the jawless specimens of the same time or the tougher toothy bony fishes in the Devonian Period. There is another possibility that this site was an exception in its day and is still popular with placoderms.
A diminutive, heavily armored shark
Two types of jawed fish arose around 450 million years ago — and both make an appearance at the Chongqing site. The new site is unique for its wide variety of osteichthyans as well as bony jawed and other fishes. X. mirabilis. But cartilaginous Shenacanthus vermiformisAlso, I spent some time here.
S. vermiformis It is represented only by one specimen at Chongqing. X. mirabilisIt is well preserved from head to tail. It was also very small, measuring only 22 millimeters. It had a similar body to other chondrichthyans but it did have one major difference: Like X. mirabilis, S. vermiformisIt was heavily armored with large plates on the underside and back.
The time of transition
The Chinese site isn’t just shedding light on ancient jawed fish — it offers a window into the evolutionary transition of body features from jawless to jawed species. A jawless creature has been discovered, and it is called “The Jawless”. Tujiaaspis vividusIt turns out that they are closely related A group of jawed fishes known as galeaspidsResearchers report on their findings in a separate paper that appears in the same issue. Nature.
The fossils of T. vividusOpen up new possibilities to learn more about how its jawed relatives got their fin arrangements. A transition for which there is little evidence beforeMatt Friedman, a University of Michigan Paleontologist in Ann Arbor, wrote this comment in the same issue of Nature. That’s because galeaspids have distinctive head shields, but scientists haven’t previously been able to peer beneath these fossilized shields to study the hidden anatomy.
The researchers were able to reconstruct the evolution of paired fins from jawless fish to separate pectoral or pelvic fins in jawed relatives thanks to the close relationships. These fins are precursors to arms and legs. in later tetrapods (SN: 5/30/18).
