Jawed vertebrates date back to 439 million years ago, a study reveals
A newly discovered, well-preserved fish fossils from the early Silurian Period – which is around 439 to 436 million years ago – of southern China sheds light on the first spread and diversity of jawed animals. New fish species have been discovered, and the findings reveal the oldest known teeth from any jawed vertebrate.
Research has been published in the journal Nature.
Today, jawed vertebrates account for more than 99.8% of all modern vertebrates – humans and mammals included. It is estimated that jawed vertebrates, in other words, gnathostomes originated around 450 million years ago. However, scarce evidence from that starting point makes things hard to interpret the evolutionary history of jawed vertebrates. The earliest jawed fish fossils discovered date back to around 425 million years ago.
However, with the new discovery, Min Zhu and colleagues can finally change what we have known about this particular species so far.
The Silurian Period had more diversity than previously known
Fossils of diverse whole-bodied fish include placoderms (an extinct group of armored prehistoric fish, which were the earliest known jawed vertebrates) and chondrichthyans (a group of cartilaginous fish such as sharks and rays).
A species that dominated the ecosystem back then is a nearly 3 cm (1.2 inches) long armored prehistoric fish belonging to the Placodermi class, named Xiushanosteus mirabilis. It has the characteristics of major placoderm subgroups that provide insights into the evolution of the skull of living jawed vertebrates.
Another one is a chondrichthyan named Shenacanthus vermiformis, which has a body shape like other cartilaginous fish. However, this one features armor plates, a main characteristic of placoderms.
These findings unveil the diversity in this period, which hasn't been uncovered until now.
The scientists revealed more details about some of the fish. For example, for the first time, the details of galeaspids, an extinct group of armored jawless fish, have been unearthed through the analysis of an approximately 436 million-year-old specimen. This species has unique head shields; also, they demonstrate the basic state in which paired fins existed before they split into pectoral and pelvic fins.
Another paper mentions a spiny shark-like fish (a chondrichthyan named Fanjingshania renovata), which dates back to around 439 million years ago. This one, an ancient shark, has a similar shoulder girdle to those of stem chondrichthyans, along with evidence of hard tissue resorption.
These findings offer the best evidence to date of the early Silurian spread of jawed vertebrates before they were found in the fossil record in the Lower Devonian Period.
The last paper details the fossil teeth of a previously unknown shark relative called Qianodus duplicis, which dates back to around 439 million years ago as well. This discovery replaces the earliest known example of gnathostome teeth, which also extends the minimum age for the origin of vertebrate jaws and dentitions by nearly 14 million years.
Molecular studies suggest that the origin of jawed vertebrates was no later than the Late Ordovician period (around 450 million years ago (Ma))1,2. Together with disarticulated micro-remains of putative chondrichthyans from the Ordovician and early Silurian period3,4,5,6,7,8, these analyses suggest an evolutionary proliferation of jawed vertebrates before, and immediately after, the end-Ordovician mass extinction. However, until now, the earliest complete fossils of jawed fishes for which a detailed reconstruction of their morphology was possible came from late Silurian assemblages (about 425 Ma)9,10,11,12,13. The dearth of articulated, whole-body fossils from before the late Silurian has long rendered the earliest history of jawed vertebrates obscure. Here we report a newly discovered Konservat-Lagerstätte, which is marked by the presence of diverse, well-preserved jawed fishes with complete bodies, from the early Silurian (Telychian age, around 436 Ma) of Chongqing, South China. The dominant species, a ‘placoderm’ or jawed stem gnathostome, which we name Xiushanosteus mirabilis gen. et sp. nov., combines characters from major placoderm subgroups14,15,16,17 and foreshadows the transformation of the skull roof pattern from the placoderm to the osteichthyan condition10. The chondrichthyan Shenacanthus vermiformis gen. et sp. nov. exhibits extensive thoracic armour plates that were previously unknown in this lineage, and include a large median dorsal plate as in placoderms14,15,16, combined with a conventional chondrichthyan bauplan18,19. Together, these species reveal a previously unseen diversification of jawed vertebrates in the early Silurian, and provide detailed insights into the whole-body morphology of the jawed vertebrates of this period.
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