When we lose our milk teeth they are replaced by new permanent teeth growing out in exactly the same positions. This is an ancient part of our evolutionary heritage and an identifying characteristic of the largest living group of backboned animals. Now, an international team including two scientists from Uppsala University has uncovered ancient fossil fish jaws that cast light on the origin of this group and its unique dentition.
Together with scientists from Spain, Germany and France, Professor Per Ahlberg and Assistant Professor Henning Blom at the Department of Physiology and Developmental Biology have been studying two of the earliest bony fishes found in Sweden and Germany, managing to show that they belong to the same group of vertebrates as ourselves, the Osteichthyes. Their findings are published in this week's Nature.
The backboned animals or vertebrates comprise three main groups. The primitive lampreys and hagfishes have neither jaws nor teeth. The cartilaginous fishes or Chondrichthyes (sharks and rays) grow new teeth on the inner face of the jaw; these gradually move up to the jaw edge, row upon row, and push out the old worn teeth. The third and much bigger group, the Osteichthyes, includes both bony fishes and all land vertebrates. In nearly all osteichthyans teeth are shed one by one and replaced by new ones growing up in the same place. Osteichthyans also carry their teeth on special jaw bones - the dentary in the lower jaw, maxilla and premaxilla in the upper - that have no equivalents in other vertebrates.
Although the Osteichthyes are a big and important group, we know little about their origin. Their fossil record goes back to early bony fishes some 416 million years ago, but then it cuts out abruptly. However, in rocks of the Silurian period (some 420 million years ago) are found fossils of two fishes, Andreolepis from Gotland in Sweden and Lophosteus from Estonia and Germany, that could be very primitive osteichthyans.
”The problem is that until now no complete specimens have been discovered, only hundreds of scales and small bone fragments. Some researchers have denied that these fossils are anything to do with osteichthyans,” says Per Ahlberg.
Now, the discovery of two jaw bones, a dentary from Andreolepis and a maxilla from Lophosteus, settles the identity of these fishes once and for all: they are the earliest known osteichthyans. Curiously, however, the teeth on these bones are not quite arranged in the osteichthyan fashion. Several rows are present, and it appears that new teeth were added along the inner edge of the jaw without the old teeth being shed. In some respects this is more like a shark dentition - not the modern kind, but the sort seen in the earliest fossil sharks.
”This suggests that Andreolepis and Lophosteus belong to the stem group, or common ancestral stock, of the osteichthyans, and also hints that the superficially very different dentitions of sharks and osteichthyans may ultimately be derived from the same ancestral pattern,”says Per Ahlberg.
Andreolepis and Lophosteus may thus help us to understand how the first osteichthyans, our own remote ancestors, evolved from more primitive vertebrates.
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