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Palaeontology

Fishing for Origins

For decades, a shroud of mystery surrounded the origins of fragments of fossil fish remains found in the Baltic region dating back 420 million years. If they were, as some argued, the earliest known remains of a kind of vertebrate from which man ultimately evolved, the indisputable proof of this remained buried and missing until now.

Very little is known about the early evolution of the osteichthyan group of species, which include bony fish and all land vertebrates. Until now, the oldest known remains of osteichthyans were discovered in fish fossils from Europe and China dating back 416 million years to the early Devonian period.

 

fishing origin

 

The Baltic fossils were made up of the scales, teeth, and bones from two types of fish encrusted in limestone rock from the late Silurian period, between 423 and 416 million years old. But the shape of the scales and formation of the fin spine of the Lophosteus, uncovered in Estonia and Germany in the 19th century, and those of the Andreolepis, found some 40 years ago in Sweden, allowed for various interpretations as to their species affiliation.

The riddle has finally been solved with the recent discovery and analysis of jawbones from both fish found in the Baltic region by an international team of palaeontologists, including CNRS research director Philippe Janvier.1 Their findings2 write a new page in the story of evolution and further explain how mankind found its teeth–well before its feet.

Vertebrate animals comprise three main groups of species. Primitive lampreys and hagfish, a group of 55 species, have no jaws or teeth. Chondrichthyes, representing 700 species of cartilaginous fish like sharks and rays, have teeth that grow on the inner face of the jaw and move up the edge, row after row, pushing out worn teeth. The aforementioned osteichthyes are a group of 50,000 species whose teeth shed individually, replaced by others that grow in exactly the same place. Osteichthyans carry their teeth on jaw bones whose structure is uniquely different from that of other vertebrates.

The structure of the discovered jaw bones from Andreolepis and from Lophosteus proves beyond doubt that both fish were osteichthyans. What adds to the importance of the findings is that while the bones themselves have the key characteristics of osteichthyans, the organization of their tooth-like denticles are plainly different from the large, conical teeth of crown-group osteichthyans from the later Devonian period.

The two fish appear to be a stem group of osteichthyans, possibly the end-part of an evolutionary splitting of species. The different dentitions of osteichthyes and chondrichthyes, such as sharks, may thus be derived from a single ancestral group.

 

Graham Tearse

 

Notes :

1. Laboratoire paléobiodiversité et paléoenvironnements (CNRS / Muséum National d'Histoire Naturelle / Université Paris VI).
2. H. Botella et al.,”Jaws and teeth of the earliest bony fishes,” Nature. 448: 583-6. 2007.

Contacts :

Philippe Janvier,
MNHN, Paris.
janvier@mnhn.fr


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