Fossils: Large catch of Silurian fish reveal the rise of jawed vertebrates

A collection of well-preserved fish fossils, found in two newly discovered fossil beds from the early Silurian (around 439 to 436 million years ago) of southern China, provides new insights into the initial spread and diversification of jawed animals. The findings, presented in four papers in Nature this week, reveal new fish species and describe the oldest known teeth from any jawed vertebrate.

Jawed vertebrates make up more than 99.8% of all modern vertebrates, including humans. These jawed vertebrates, also known as gnathostomes, are thought to have originated around 450 million years ago. However, fossil evidence from this time is scarce, making it hard to reconstruct the early evolutionary history of jawed vertebrates. The earliest articulated jawed fish fossils identified to date are from around 425 million years ago.

Min Zhu and colleagues present a deposit of exceptionally well-preserved fossils from the early Silurian of Chongqing, southern China, containing diverse whole-bodied fish that helps to fill this gap in the record. The find includes placoderms (an extinct group of armoured prehistoric fish, which were the earliest known jawed vertebrates) and chondrichthyans (a group of cartilaginous fish such as sharks and rays). The dominant species (more than 20 specimens) is a roughly 3 cm long placoderm given the name Xiushanosteus mirabilis. This fish displays a combination of features from major placoderm subgroups that sheds light on the evolution of the skull of living jawed vertebrates. A chondrichthyan named Shenacanthus vermiformis has a body shape similar to other cartilaginous fish, but is found to have armour plates more commonly associated with placoderms. These discoveries reveal previously unknown diversification in this period.

In separate papers, the authors describe some of the fish in more detail. Details of the whole body of galeaspids, an extinct group of armoured jawless fish known only from China and northern Vietnam, are revealed for the first time through the analysis of an approximately 436 million-year-old specimen. These ancient fish have distinctive head shields, but the anatomy behind the head has been unclear until now. They also show the primitive condition of paired fins before they separated into pectoral and pelvic fins, the forerunner to arms and legs. Another paper describes a spiny shark-like fish (a chondrichthyan named Fanjingshania renovata) found in sediments dating to around 439 million years ago. The ancient shark has a shoulder girdle similar to those seen in some stem chondrichthyans, but also displays evidence of hard tissue resorption, a feature usually seen in bony fish. These findings provide the strongest support yet for a proposed early Silurian radiation of jawed vertebrates before their widespread appearance in the fossil record in the Lower Devonian, the authors suggest. A fourth paper reports fossil teeth from a previously unknown shark relative named Qianodus duplicis, dating back to around 439 million years ago. These teeth predate other known examples of gnathostome teeth, extending the minimum age for the origin of vertebrate jaws and dentitions back by approximately 14 million years.